Structured photocatalysts for the removal of emerging contaminants under visible or solar light

Structured photocatalysts for the removal of emerging contaminants under visible or solar light

3 Structured photocatalysts for the removal of emerging contaminants under visible or solar light Carolina Belver, Jorge Bedia, Manuel Peñas-Garzón, V...

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3 Structured photocatalysts for the removal of emerging contaminants under visible or solar light Carolina Belver, Jorge Bedia, Manuel Peñas-Garzón, Virginia Muelas-Ramos, Almudena Gómez-Avil es, Juan J. Rodriguez Chemical Engineering Department, Universidad Autonoma de Madrid, Madrid, Spain

Chapter outline 1. Introduction 42 2. TiO2 and its modifications 44 2.1 TiO2 doping 46 2.2 Metal deposition 48 2.3 Dye sensitizers 50 3. ZnO and other oxides 50 4. Ternary oxides 54 5. Metal organic frameworks 56 6. Semiconductor heterojunctions 60 6.1 Semiconductoresemiconductor (SeS) 60 6.2 Semiconductoremetal (SeM) 65 6.3 Semiconductorecarbon materials (SeC) 68 6.4 Multicomponent heterojunctions 70 7. Semiconductor anchored on porous solids 73 7.1 Carbon-based supports 73 7.2 Clay supports 75 7.3 Zeolite supports 77 7.4 Silica supports 79 8. Conclusions and outlook 80 References 81

Visible Light Active Structured Photocatalysts for the Removal of Emerging Contaminants. https://doi.org/10.1016/B978-0-12-818334-2.00003-1 Copyright © 2020 Elsevier Inc. All rights reserved.

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Chapter 3 Structured photocatalysts for the removal of emerging contaminants

1. Introduction The quality of water is a worldwide current issue of major importance, with the presence of contaminants of emerging concern (CECs) in wastewater effluents being a rising challenge. These compounds are defined as those anthropogenic chemicals recently detected in water bodies (at very low concentration, from ng/L to mg/L), currently unregulated (except in some countries, such as Switzerland) and whose toxicity can produce negative effects over the metabolism of living beings. CECs include pharmaceuticals and personal care products, hormones, and other industrial chemicals [1e4]. The effluents of the wastewater treatment plants (WWTPs) are considered the primary sources of CECs because the conventional secondary treatment (activated sludge) in these plants is not enough for complete removal of those pollutants, many of them with a remarkable recalcitrant character [5e7]. The treatment of wastewater for the removal of CECs is a decisive strategy for the management of water quality within the water cycle, as established by international agreements and legal mandates (United Nations 2030 Agenda for Sustainable Development, Clean Water Act in USA or Decision 2018/840/EU in Europe [8e10]). Consolidated technologies for wastewater treatment such as adsorption or membrane processes can effectively remove those pollutants, but they are not broken down but only phase-concentrated. Advanced oxidation processes (AOPs) are a wide range of technologies investigated to degrade CECs, on the generation of reactive oxidant species (ROS), such as hydroxyl radical, HO, or superoxide radical, O2 with highly oxidant capacity [11]. The treatment of CECs via heterogeneous photocatalysis, a well-investigated AOP, has been widely explored so far [12]. In this technology, the production of ROS can occur after the irradiation of a semiconductor with light with enough energy to promote a separation of charges. Briefly, electrons in the valence band (VB) of the semiconductor can absorb photons with sufficient energy to reach the conduction band (CB), leaving a positive hole in the first band. The quantized difference of energy between both bands is the band gap energy (Eg), and it is a parameter usually used for the comparison of photocatalysts. Therefore, this photogenerated electronehole pair can allow the generation of ROS by redox reactions on the surface of the semiconductor. In aquatic medium, HO is obtained by the oxidation of water molecules, whereas dissolved oxygen is reduced to $O2  [13]. Thus, in addition of the catalyst, photocatalysis only

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

needs a source of light and oxygen as primary reagent. Moreover, it can be performed under ambient temperature and pressure. Among the different photocatalysts, titanium dioxide (TiO2) is the most investigated owing to its high chemical stability and photoactivity [14]. It is also important to remark that there is a current trend for the use of solar energy for photocatalytic applications [15]. Sunlight is the renewable energy source with the highest feasible potential (about 60 TW) [15,16], owing to its wide radiation spectrum. Fig. 3.1 shows the spectral irradiance of sunlight. Solar radiation comprises a wide range of wavelengths from the ultraviolet (UV) to the infrared (IR) region of the electromagnetic spectrum, with a maximum absorbance within the visible (Vis) range. It can also be observed that, as sunlight crosses the atmosphere, part of that radiation is absorbed by gas molecules (O2, CO2, O3, and water vapor) and aerosols [17]. However, the solar-driven application of TiO2 to the treatment of wastewater by photocatalysis has the main drawback that the photoactive phase of the semiconductor, anatase, is only activated under UV radiation (light absorption for l  387 nm, due to its Eg of 3.2 eV). This means that only about 5% of the solar spectrum can be used for that purpose [18]. In contrast, c. 43% of that spectrum corresponds to visible light [19] and thus growing efforts have been done during the last decade to improve the photocatalytic response of TiO2 and other semiconductors to visible light. These approaches include electronic and morphological modifications, primarily searching a displacement in the absorption region through the solar range and the development of the porous texture for increasing the

Spectral Irradiance (W.m-2.nm-1)

2.50 2.25 UV Vis IR 2.00 1.75

Irradiation at top of the atmosphere

1.50 1.25 1.00 0.75

Irradiation at sea level

0.50 0.25 0 400

800

1200 1600 2000 Wavelength (nm)

2400 2800

Figure 3.1 Spectral solar irradiance in the wavelength range of 200e2800 nm.

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contact surface between the semiconductor and the pollutant. Some of the most conventional electronic modifications are based on doping with metal or nonmetal compounds, sensitization with dyes, deposition of noble metals, or building binary and ternary heterojunctions with different compounds [20,21]. In addition, using micro- and mesoporous solids (carbon-based materials, zeolites and silicas) as supports of the semiconductor are useful solutions for improving both the porous texture and the recovery of the photocatalyst from the reaction medium [14,22]. It is also important that these materials are photochemically stable and have low toxicity and cost. These required features have promoted the development of different methods (thermal treatment, solvo- and hydrothermal, metal and atomic layer deposition, photoreduction, microwave, etc.) to obtain the photocatalyst that combines both a simple synthesis route and an effective performance under solar light. This chapter compiles some of the most relevant knowledge about the suitable photocatalytic materials used for the treatment of CECs under solar and visible light, considering the most important aspects from their synthesis and properties. Special attention will be paid to TiO2 and its modifications (Section 2), but also other semiconductor oxides (binary oxides in Section 3 and ternary oxides in Section 4) and metal-organic frameworks (Section 5) will be reviewed, as well as other photocatalytic structures, such as semiconductors in heterojunctions and supported in porous solids (Sections 6 and 7, respectively).

2. TiO2 and its modifications The application of TiO2 for environmental remediation by photocatalysis has been established since the 1970s. However, it was not until 2000s that researchers systematically studied the photodegradation of emerging contaminants with different TiO2 materials, including the well-known nano-sized P25. Even, interesting works have summarized the use of TiO2 and TiO2-immobilized for the degradation of different CECs under solar light using compound parabolic collector plants [23e25]. Nevertheless, TiO2 has a large band gap of 3.2 eV that restricts its light-harvesting to UV light only [26], thus using no more than 5% of solar irradiation. Moreover, it has been reported that about 90% of the photogenerated electrons and holes undergo recombination, which reduces its photocatalytic efficiency [27]. Thus, the strategies reported for improving TiO2 activity are devoted to both shift its optical response to the

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

visible range and prevent the electronehole recombination. An important option to enhance the photoresponse of TiO2 consists of favoring the creation of imperfections within the crystal structure, which significantly affect its phase stability and its electronic structure [28]. The creation of surface defects, such as oxygen vacancies and Ti3þ sites, has been proved as an efficient procedure to enhance the visible light absorption [28], although some contradictory results exist in literature about the working mechanism of these surface defects [29]. Nevertheless, the application of this self-modified TiO2 for CECs degradation under visible light has not been studied in detail, mainly because the results obtained with dyes and other pollutants revealed that the quantum efficiencies were not as high as expected because the defects favored electronehole recombination [30,31]. More recently, it has been described a shift on the TiO2 absorption to the visible region through hydrogenation of TiO2, resulting in the so-called black TiO2 [32], characterized by a disordered lattice where some TieO bonds are broken down while TieH and OeH bonds are created. Through theoretical calculations, it has been determined that the disordered lattice causes the formation of midgap states that lie above the VB maximum narrowing the band gap [33]. The photocatalytic activity of black-TiO2 has been proved in different applications, mainly H2 production and degradation of dyes [28], while its ability for CECs removal has not been studied in great detail. Although the main method is used for the preparation of black TiO2 if treated thermally under hydrogen atmosphere, recent works describe more interesting approaches. Katal et al. [34] obtained black TiO2 from P25 pellets under vacuum conditions. The characterization revealed that this black TiO2 had a disorder layered structure with oxygen vacancies and Ti3þ species in its structure, which were formed thanks to the vacuum conditions. The band gap was considerably lower, 2.5 eV, than that of P25 due to the presence of defect levels below the CB. These levels enhanced photogenerated electronehole separation and favored acetaminophen degradation under solar light. Complete conversion of acetaminophen was achieved after 3 h with an apparent kinetic constant considerably higher than that of the P25. The photocatalytic activity was related not only to the narrower band gap and midgap levels but also to the density of oxygen vacancies generated, as detected by electron paramagnetic resonance. This black TiO2 prepared under vacuum showed a high stability and reusability after several cycles of use. Later, Zhang et al. [35] prepared macroemesoporous black TiO2 foams using a freeze-drying method combined with cast

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molding to obtain a stable foam that was further hydrogenated under heat treatment. These foams have a higher lightharvesting than TiO2, thanks to the creation of Ti3þ and surface defects in the TiO2 framework and allowed complete mineralization of several CECs (hexadecane and pesticides) under solar light at low concentrations. These foams can float on the water surface thanks to their amphiphilic 3D macroemesoporous networks, and therefore, they can be easily recovered from the solution even showing a long-term stability [35]. Regarding the use of TiO2 for CECs removal under visible light, although some works are focused on the self-structural modification of TiO2, most attempts are mainly addressed to TiO2 modification [27,36e38]. The literature describes as the most feasible modifications doping (metal and nonmetal), metal deposition, and surface sensitization, which will be described in the following sections.

2.1 TiO2 doping The most important effect of dopants is modifying the TiO2 electronic structure to extend its effective range from the UV to visible light. Both nonmetal and metal ions have been used for TiO2 doping. Their effect, behavior, and photocatalytic performance is very diverse, depending on the synthesis method, amount of dopants, contaminant concentration, type of irradiation, etc. The main objective of doping TiO2 with nonmetal ions, such as N, C, S, and halides (F, Cl, Br, I), consists on the substitution of the oxygen lattice of TiO2 by these species, altering both the electronic and structural properties of TiO2, narrowing the band gap, and creating oxygen-defect sites. Asahi et al. [39] reported the enhanced visible light activity of N-doped TiO2 for dyes degradation. Then, several comprehensive reviews have been reported summarizing representative results on the efficiency of nonmetal doping [20,21,29,40]. Although most works are focused on the degradation of dyes and priority pollutants, in the last years more attention has been paid to CEC [14,22]. N has been the most used nonmetal species in TiO2 doping, as it is easily introduced in the TiO2 matrix thanks to its similar atomic size to oxygen and its stability [20]. However, the origin of the visible light activity of nonmetal-doped TiO2 is still under debate [29,41]. For instance, some authors state that mixing the N 2p and O 2p states narrows the band gap [39,42], whereas Irie et al. [43] propose that the new N 2p levels are

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

separated (not mixed) from the VB (formed by O 2p states), giving rise to localized midgap states responsible for visible light harvesting. Nevertheless, other authors contend that the TiO2 band gap does not change but rather the doping leads the generation of defect sites, such as oxygen vacancies [44,45], interstitial lattice sites [14], or N-atom impurities (NOx, Nx, and NOx) [14,46]. It is also noteworthy that the photocatalytic activity of N-doped TiO2 varies depending on the method of synthesis [47e49]. The bibliography collects examples of both physical and chemical methods [21,22]. For example, an N-doped TiO2 with 2.5 eV of band gap was prepared via solegel by incorporating ammonia in the hydrolytic solution [50]. This photocatalyst yielded high mineralization (close to 90%) of the antibiotic spiramycin within 4 h under visible light in real pharmaceutical wastewater. Worse results were obtained by Barndök et al. [37] with NFeTiO2 films prepared by solegel. These materials showed excellent activity for the photodegradation of several CECs, atrazine, carbamazepine, and caffeine, under solar light, but under visible radiation, the conversion rates were considerably lower (decreasing from 100% to less than 10%). Moreover, other nonmetal-doping approaches have been studied to enhance the photocatalytic activity of TiO2 under visible light, as carbon, boride, sulfur, and some halides [20,21,29], although not so many have been tested for the degradation of CEC. As an example, B-doped TiO2 (5% w/w boron [51]) was prepared by solegel adding boric acid as boron source in the aqueous solution (in a similar way than that reported for NeTiO2 materials [50]). This B-doped TiO2 had mesoporous network where boron occupied interstitial TiO2 positions forming new BeOeTi bonds. Although its band gap was similar to that of TiO2, it allowed higher conversion rates for metoprolol (b-blocker) degradation under solar light, which was attributed to a lower recombination of the photogenerated charges. The incorporation of multiple dopants as a strategy to obtain visible light-driven photocatalysts has received more interest [20e22]. N/S-doped TiO2 was prepared by solegel using amino sulfate as precursor of both N and S dopants [52]. The photoactivity for the degradation of the anticancer pharmaceutical 5-fluorouracil was tested. The catalyst with an equimolar Ti:N/S ratio showed the best performance under visible light, whereas under solar light the best was that with a Ti:N/S molar ratio equal to 0.5. This difference was related to a different contribution by the radicals generated. Although under solar light the main contribution was due to HO and hþ, under visible light the major radical was $O2  . The authors also observed this

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behavior for the degradation of an organophosphorus flame retardant (tris (1-chloro-2-propyl) phosphate [TCPP]) [36]. Doped materials allowed higher degradation rates under visible light than the undoped TiO2, although their activity increased under solar light. Likewise, CeNeS tridoped TiO2 has been also reported as an efficient photocatalyst for the degradation of tetracycline antibiotic under visible light [53]. Using a solegel route and urea as precursor, the authors found C substitution forming TieC bonds, N at substitutional and interstitial sites yielding TieNeO, TieOeN, and OeTieN bonds, and S6þ substituted the Ti4þ lattice. The photocatalytic performance was associated with band gap narrowing, the presence of carbonaceous species acting as sensitizers, and adequate structural and textural properties. Although doping can be used to enhance the overall efficiency of the photocatalysts under visible light, many factors need to be considered, such as the dopant concentration, type and number of dopants, and preparation method. The combined effects of such factors are likely to play a crucial role on the enhancement of the overall efficiency of photocatalysis. Doping with transition metal ions has been extensively studied. The literature contains examples using Cu, Zr, Zn, Co, Ni, Cr, Mn, Mo, Nb, V, Fe, Ru, W, Sb, Sn, and even some lanthanides, such as La, Ce, Er, Pr, Gd, Nd, or Sm [38,54,55]. Ti substitution by metal ions introduces new energy states close to the CB or VB edge, inducing visible light absorption at subeband gap energies. Moreover, some transition ions could also act as electronehole trappers, leading to an increase of the charge carriers lifetimes and therefore inhibiting the recombination processes [21]. However, doping with metals has created great controversy among researchers, especially when comparing activity for pollutants other than dyes [20].

2.2 Metal deposition Noble metal nanoparticles (NPs) have been deposited onto the TiO2 surface as an approach to develop highly efficient photocatalysts with visible and solar light [20,29,40,41]. The mechanism consists of the migration of the photogenerated electron from the VB of the TiO2 to the metal NPs via an interfacial charge transfer, acting as an electron trap and avoiding the recombination of charges. Particularly important is the deposition of NPs (e.g., Au, Ag) capable to absorb visible light via surface plasmon resonance (SPR) [56,57]. They can be excited itself by the light injecting electrons to the CB of the TiO2 and promoting charge separation.

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

SPR is an inherent property of NPs, characterized by the redistribution of the charge density when the NP is irradiated by light with a wavelength larger than its size. In this sense, Ag and Au NPs receive special attention because they can absorb in the visible region of the solar spectrum, and thereby scatter photons due to the charge distribution [58]. The photocatalytic performance of these materials depends on the nature of the metal, but also on the method used to deposit the NPs and on the interaction between the NPs and the TiO2 surface. A high number of synthesis approaches have been reported in literature, including solegel, hydrothermal, depositioneprecipitation, encapsulation, and photodeposition [29,40,57]. An interesting work was reported by Nalbandian et al. [59] where Au/TiO2 nanofiber composites were synthesized and tested. The authors firstly compared conventional impregnationereduction with a direct synthesis embedding Au in the TiO2 precursor sol. Opposite to the expected, the best photocatalytic performance was associated with the impregnation method. These Au/TiO2 nanofibers were tested for the degradation of several emerging contaminants (20 mM), including atrazine, sulfamethoxazole, carbamazepine, and DEET (N,N-diethyl-m-toluamide). In all cases, these nanofibers yielded higher degradation rates than the bare TiO2 and even higher than an Au/P25 catalyst used as reference. To extend the use of these Au/TiO2 nanofibers for water treatment, the authors also worked with different water matrixes. Unfortunately, the activity of these nanofibers in WWTP effluent decreased more than 20-fold with respect to synthetic samples prepared with deionized water. This activity loss was related to the presence of radical inhibitors in the WWTP effluent (carbonate or organic matter), which act as scavengers reducing the amount of active oxidizing species in the reaction medium. Although M/TiO2 materials present some benefits: (i) separation of photogenerated electronehole pairs thanks to the metal NPseTiO2 interface; (ii) absorption of light in the visible region of the solar spectrum because of SPR; and (iii) modification of the TiO2 surface reducing the number of defects that can act as recombination centers [21,60], they also present important drawbacks, derived from the formation of negatively charged NPs, which can act as recombination centers reducing the photoefficiency. Moreover, the use of high concentrations of NPs can block the TiO2 surface, decreasing the absorption of light and thus the photocatalytic activity. Likewise, other problems are related to the oxidation, corrosion, and leaching of NPs [40,61]. Therefore, efforts have been devoted to explore new

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metal-decorated photocatalysts with performance and good stability [40].

better

photocatalytic

2.3 Dye sensitizers Another approach to improve the photocatalytic efficiency of TiO2 under visible light uses dye sensitizers on its surface. The dye absorbs visible light from the highest occupied molecular orbital (HOMO) to its lowest unoccupied molecular orbital (LUMO). The excited dye injects electrons into the CB of TiO2, thereby giving rise to the cationic radical species. In this process, the TiO2 acts as mediator, receiving the photogenerated electrons from the dye. The LUMO of the dye needs to be located above the bottom of the semiconductor CB. A strong interaction between dye and TiO2 is also required to have a fast and efficient electron injection. One of the advantages of the dye-sensitized TiO2 is that the electron injection is very fast, occurring from femto to pico seconds, compared to the nanosecond scale of the recombination process [29]. Consequently, sensitization enhances the separation of photogenerated charges and expands the light absorption to the visible region. The dyes used are mainly transition metal complexes derived from polypyridines (Ru), porphyrins, or phthalocyanines (Zn, Mg, Al) as ligands [30,62]. The use of dyes requires overcoming certain inconveniences. Sometimes, the dye itself suffers degradation under the light, changing its structure and needing regeneration to recover its initial properties [14]. The synthesis of dyes can be also an important issue in this approach because it can involve complex organic pathway with low yields, which increase their cost [63]. Lu et al. [64] have recently prepared a new dye [Pt(3,30 -dicarboxy-2,20 -bpy) (1,2-benzenedithiolate)] with absorbance peaks in the visible range, at 372, 420, and 621 nm. This Pt complex in contact with TiO2 allowed complete conversion of sulfamethoxazole, a popular sulphonamide antibiotic used worldwide, under visible light (420 nm). The reaction rate constant increased from 0.22$102 to 1.68$102 min1 with respect to bare TiO2. The ring-containing intermediates of sulfamethoxazole degradation were destroyed on 3 h of reaction.

3. ZnO and other oxides ZnO is after TiO2 one of the most studied simple oxides for photocatalytic removal of water pollutants under visible or solar light. It is also a functional n-type semiconductor with a similar band gap

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

(3.37 eV) than TiO2. Besides this, ZnO also presents low cost and toxicity and more light quanta absorption than TiO2 [65,66]. However, ZnO can suffer photocorrosion under light radiation, which decreases its photocatalytic activity in aqueous media under acidic conditions [67]. The use of ZnO and other oxides in the degradation of emerging contaminants under visible or solar light have been summarized in Table 3.1. Commercial ZnO (Degussa) was tested versus commercial TiO2 (P25) in the photocatalytic removal of cefotaxime (an antiinflammatory antibiotic) under simulated solar light [68]. The results were slightly better than that obtained with TiO2 (P25), yielding a complete conversion of the contaminant after a few minutes. Pardeshi and Patil [69] tested synthesized ZnO for the photocatalytic degradation of resorcinol, a target endocrine disruptor, under natural solar light. The photocatalytic activity was affected by the crystallite size and morphology of ZnO, favored by the smallest crystallite size (c. 32 nm). However, other authors described the rapid recombination of electronehole pairs for ZnO materials that decreased its photocatalytic [82]. As previously described for TiO2, the ZnO performance can be enhanced by doping approach with transition metals, rare earth, or noble metals, and also by coupling with other semiconductors oxides. The literature collects several examples on the use of doped ZnO in the solar- or even visible-driven photocatalytic degradation of emerging contaminants. Lanthanum-doped ZnO NPs were used for the removal of acetaminophen [70]. La load (from 1% to 5%) caused a red-shift of the absorption spectrum respect to bare ZnO, allowing the band gap reduction to 2.9 eV with La 3% loading. Moreover, it was found that La3þ can act as electron trapper thanks to the presence of incomplete orbitals, thus partially avoiding the recombination of charges and improving the activity. Calza et al. [71] synthesized CeeZnO materials by hydrothermal treatment and tested their activity for the degradation of acesulfame K. The best performance was achieved with 1% Ce load, yielding higher removal than commercial P25 under visible light. This activity was attributed to the transfer of electrons from the upper orbitals of cerium to the ZnO. Sin and Lam [72] prepared Nd-ZnO with ZnO nanorods that were tested for the degradation of resorcinol under natural light. Photoluminescence results demonstrated that 2% Nd/ZnO (band gap of 3.22 eV) achieved the highest rate of HO formation and together with the best separation rate of charge carriers. Furthermore, a higher Nd load increased the recombination of charges due to a narrower separation between bands.

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Table 3.1 Representative metal oxides tested for the photocatalytic degradation of emerging contaminants under visible and solar light. Metal oxides

Synthesis

ZnO (Degussa) Commercial ZnO

Heat treatment

LaeZnO

Gel-combustion

CeeZnO

Hydrothermal treatment Solvothermal treatment Heat treatment

NdeZnO WO3 Bi2O3

Bi2O3

b-Bi2O3 Sr-b-Bi2O3

BiOBr BiOBr BiOBr Bi24O31Br10

Ultrasoundassisted precipitation Hydrothermal and calcination treatment Solvothermal treatment Solvothermal treatment Hydrothermal treatment Hydrothermal treatment Solvothermal treatment Hydrothermal treatment

TOC, total organic carbon.

Contaminant (initial concent.)

Light source

% Removal (time)

References

Cefotaxime (20 mg/L) Resorcinol (100 mg/L) Acetaminophen (5 mg/L) Acesulfame K (20 mg/L) Resorcinol (20 mg/L) Lidocaine (50 mg/L) Tetracycline hydrochloride (20 mg/L) Sulfamethoxazole (10 mg/L)

Simulated solar light Natural solar light Visible light (l > 420 nm) Simulated solar light Natural solar light Natural solar visible light Simulated solar light

>80 (8 min)

[68]

TOC: 100 (7 h)

[69]

80 (180 min)

[70]

70 (90 min)

[71]

98 (60 min) TOC: 79 98 (60 min) 96 (60 min) 65 (180 min) TOC: 72 (24 h)

[72]

Simulated solar light

72 (60 min)

[75]

Acetaminophen (10 mg/L) Tetracycline hydrochloride (20 mg/L) Ciprofloxacin (5 mg/L) Acetochlor (10 mg/L) Ibuprofen (20 mg/L) Tetracycline hydrochloride (20 mg/L)

Visible light (l > 420 nm) Visible light (l > 420 nm)

94 (240 min) TOC: 90 90 (120 min)

[76]

Visible light (l > 420 nm) Visible light (l > 420 nm) Simulated solar light Visible light (l > 420 nm)

100 (140 min)

[78]

97 (10 h) TOC: 73 100 (20 min) TOC: 65 >95 (90 min)

[79]

[73] [74]

[77]

[80] [81]

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

WO3 is another interesting semiconductor used in photocatalysis for the degradation of emerging pollutants under visible or solar light because of its small band gap (2.5 eV) and its absorbance in the blue part of the visible spectrum (c. 500 nm) [83]. Fakhri and Behrouz [73] tested WO3 NPs for the degradation of lidocaine (anesthetic drug), achieving mineralization close to 80% under natural solar light and 70% under visible light. Other examples found in the literature about the improved performance of WO3 (by depositing platinum or silver on the oxide) will be later discussed in Section 6. Table 3.1 also collects examples of different bismuth oxides tested for the removal of CECs. Bi2O3 has received special attention thanks to its low cost and band gap (2.8 eV) [14]. Bao et al. [75] were the first to synthesize 3D flower-like Bi2O3 materials by hydrothermal methodology followed by thermal treatment and test them for the solar photodegradation of sulfamethoxazole (antibiotic). The flower-like framework yielded a higher antibiotic removal than bare Bi2O3 and commercial P25. On the other hand, Xiao et al. [76] synthesized b-Bi2O3 nanospheres by solvothermal method for the degradation of acetaminophen under visible light. The performance of these b-Bi2O3 nanospheres was considerably higher (93%) than the commercial b-Bi2O3 (53%). Bismuth oxyhalides, BiOX (where X ¼ Cl, Br or I), have been also studied as novel photocatalysts for the abatement of contaminants under visible light (Table 3.1). The layered structure of these compounds provides a unique polarization effect that promotes the separation of photogenerated electronehole pairs [84], a suitable energy band position, and the possibility to generate oxygen vacancies [85,86]. Among oxyhalides, BiOBr was the most used under visible and solar light. Zhang et al. [78] synthesized BiOBr (2.55 eV) by solvothermal method and tested it for the degradation of ciprofloxacin (a fluoroquinolones antibiotic). Complete removal was achieved on 140 min under visible light. More complex BiOBr compounds have been also studied. Wang et al. [81] synthesized Bi24O31Br10 nanosheets for tetracycline degradation. Depending on the synthesis parameters, it was possible to tune the thickness of the nanosheets, yielding to distributions from w40 to w130 nm. The sheet thickness controlled the band gap; resulting materials with low values close to 2.30 eV Bi24O31Br10 achieved higher removal than commercial TiO2 P25 under visible light. To conclude this section, it can be affirmed that the use of synthesized metal oxides as photocatalysts (such as ZnO and Bi2O3) yielded better photocatalytic activity than the commercial one. The use of dopants is an affordable approach to reduce the

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band gap of the metal oxides besides the detriment in the recombination rate of the electronehole pairs. Regarding the BiOBr compounds, it is noteworthy the high influence of the morphology and structure of the photocatalysts to improve their photocatalytic performances. Nevertheless, the main drawback of these nanostructures is their difficult recovery and reuse owing to their small particle size.

4. Ternary oxides Ternary oxides have been also tested as photocatalysts for the degradation of different CECs. For instance, Bi6WO12 shows higher light absorption above 440 nm than bare Bi2O3 or WO3, thus improving the absorption light response [87,88]. Table 3.2 summarizes some of the most relevant examples, mainly tungstates and vanadates, tested under solar and visible light. Ternary oxides are characterized by excellent physicochemical properties and a promising photocatalytic performance [102]. As occurred with titania and other simple oxides, doping and other approaches are also used to improve the charges separation and light absorption. Bi2WO6 prepared by hydrothermal method was tested for the degradation of some antibiotics under visible light: fluoroquinolones (ofloxacin and levofloxacin) [89] and cephalosporins (ceftriaxone) [91]. The removal achieved was similar in all cases, although the morphology of Bi2WO6 was different. On the contrary, Zhao et al. [90] tested Bi2WO6 with different morphologies (granulate, flake, rodlike, knot shape, and nanoflowers microspheres) for the removal of ceftriaxone and concluded that Bi2WO6 3D-nanoflower obtained the best removal because of its better developed porous network. Bi2MoO6 has also received special attention because of its 2D-layered structure (Table 3.2). Gupta et al. [92] studied the photodegradation of ofloxacin with ultra-thin B2MoO6 nanosheets, allowing similar removal but in less reaction time (90 min) than Bi2WO6 [89]. BiVO4 is other Bi-based ternary oxide tested because of its low band gap (2.3e2.4 eV), high visible-light harvesting, and very low toxicity and resistance to chemical corrosion [103,104]. Despite these good characteristics, BiVO4 has low efficiency in separating the photogenerated eehþ pairs [105]. Thus, some works report metal-doped BiVO4 as promising catalysts allowing better charges separation than bare BiVO4. Regmi et al. synthesized FeeBiVO4 [94], NieBiVO4 [95], and CueBiVO4 [96] by a combined microwave-hydrothermal

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

55

Table 3.2 Ternary oxides tested for the photocatalytic degradation of emerging contaminants under visible and solar light. Ternary oxides Synthesis

Contaminant Light (initial concent.) source

% Removal (time)

References

Bi2WO6

Ofloxacin

w73 (120 min)

[89]

70 (240 min)

[90]

w80 (150 min)

[91]

w71 (90 min) TOC: 44.9 w40 (60 min)

[92]

80 (180 min)

[94]

Hydrothemal treatment Bi2WO6 Hydrothemal treatment Bi2WO6 Hydrothemal treatment Bi2MoO6 Hydrothemal treatment Fee Hydrothemal Bi2MoO6 treatment FeeBiVO4 Microwavehydrothermal treatment NieBiVO4 Microwavehydrothermal treatment CueBiVO4 Microwavehydrothermal treatment AgVO3 Hydrothermal treatment SrV2O6 Hydrothermal treatment PeCeVO4 Hydrothermal treatment Bi2Ti2O7

NiTiO3

Solegel and hydrothermal treatment Solvo-combustion treatment

TOC, total organic carbon.

Ceftriaxone sodium (10 mg/L) Levofloxacin (10 mg/L) Ofloxacin (10 mg/L) Salicylic acid (10 mg/L) Ibuprofen (20 mg/L)

Visible light (l > 420 nm) Visible light (l > 420 nm) Visible light (l > 420 nm) Natural solar light Simulated solar light Visible light (l > 420 nm)

[93]

Ibuprofen (20 mg/L)

Visible light (l > 420 nm)

80 (90 min)

[95]

Ibuprofen (20 mg/L)

Visible light (l > 420 nm)

75 (90 min)

[96]

Atrazine (10 mg/L) Metronidazole (20 mg/L) Tetracycline hydrochloride (20 mg/L) Acetaminophen (0.7 mg/L)

Visible light (l > 420 nm) Visible light (l > 420 nm) Simulated solar light

97 (72 h)

[97]

>98 (60 min)

[98]

w70 (180 min)

[99]

Visible light (l > 420 nm)

88 (180 min) TOC: 100

[100]

Tetracycline hydrochloride (20 mg/L)

Simulated solar light

61 (300 min) TOC: 15 (5 h)

[101]

56

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

methodology. These photocatalysts were tested for the visibledriven degradation of ibuprofen. Authors observed that by increasing the Fe load (with maximum at 1%), 70% removal was achieved on 60 min, being w35% lower for bare BiVO4. Doping with other metals (Ni and Cu, up to 1%) showed similar yields. Moreover, all doped catalysts did not suffer photocorrosion and showed high stability after three successive cycles. Other vanadates are currently under study, comparing their performance with that of BiVO4. Table 3.2 collects some examples, such as AgVO3, tested for the degradation of atrazine (pesticide) [97], or 0.5% PeCeVO4 nanorods [99] used for the removal of tetracycline. Ilmenite-structured titanates (MTiO3) have received increasing attention in the last years [106] (Table 3.2). Bismuth-modified titanates are interesting because of their low band gap (2.5e2.8 eV). Fan et al. [100] synthesized Bi-(MTiO3) with different morphologies, ribbons, nano-bulks and nanosheets, which were tested for the acetaminophen photodegradation on visible light. Nanoribbons allow highest removal because of their high surface area and the inhibition of charges recombination [107], due to the formation of a hybrid VB that facilitates the migration of the photogenerated electrons. Elvira et al. [101] synthesized other ilmenite-type MTiO3 (M ¼ Fe, Co, and Ni) for the removal of tetracycline. The highest removal (61%) was reached with NiTiO3 on 300 min, thanks to its higher crystallinity and large surface area. To sum up, all reported ternary oxides have lower band gap and better performances under visible light than TiO2. Generally, doping ternary oxides reduces the band gap favoring the light harvesting and allowing higher removal of CECs. Commonly, the size and morphology of the ternary oxides have a high influence in the removal of the pollutant.

5. Metal organic frameworks A metal organic framework (MOF) is a new class of porous material constituted by metal ion nodes or clusters, also called secondary building units (SBUs), and organic linkers between the metal ions. The type of network will depend on the choice of these two elements where organic linkers are generally polytypic structures, such as carboxylates, sulfates, phosphonates, azole, and heterocyclic compounds. They have a wide range of applications such as gas storage, sensors, or catalysis because of their structural diversity and tailored properties [108]. After many studies on adsorptive removal of contaminants with

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

different types of materials, such as zeolites, carbons, or chitosan beads, MOFs have been recognized as useful candidates for water purification, including the removal of CECs. Nevertheless, the stability of these materials in aqueous media plays an important role and finally a few of them can be used in water [109]. Owing to their semiconductor properties, MOFs are currently investigated as photocatalysts for water treatment using visible or solar light. MOFs can be modified to improve their properties by different routes such as change of organic linkers, partial substitution of the metal cluster, combination with semiconductors, metal nanoparticles (MNPs) loading, sensitization, or decoration by rGO [110]. Despite its promising properties as adsorbents and semiconductors, few MOFs have been able to deal with CECs, but further research will certainly expand their potential application in this field. Table 3.3 collects MOFs used for emerging contaminants in photodegradation, specifying the metal of the SBUs, and the type of modification. As can be observed, the most used for water purification are constituted by iron clusters. Different modifications allow obtaining different MOFs with useful properties to remove emerging pollutants from water. Huang et al. [113] have recently reported a novel Ag/[email protected](Fe) composite for the degradation of ibuprofen under visible light. This novel photocatalyst allowed

Table 3.3 Metal organic frameworks (MOFs) tested for the removal of CECs. CEC

MOF

Metal

Modification

References

Acetaminophen

Ag/[email protected] NH2-MIL-125

Zn Ti/Zr

[111] [112]

Ibuprofen

Ag/[email protected] [email protected] [email protected] [email protected] [email protected] g-C3N4/ZIF-8 Fe-MOFs MIL-68(In)-NH2/GrO

Fe Fe Fe Fe Ti Zn Fe In

Combination with semiconductor Change of organic linker and partial metal substitution Combination with semiconductor Combination with semiconductor MNPs loading MNPs loading Combination with semiconductor Combination with semiconductor

Theophylline Tetracycline

Amoxicillin

Decoration by rGO

[113] [114] [115] [115] [116] [117] [118] [119]

57

58

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

complete removal of ibuprofen after 3 h of reaction, with a TOC reduction above 90%. This MOF showed a remarkable stability on four successive cycles. The authors described the importance of holes (hþ), electrons (e), and $O2  in the degradation of the pollutant. Even higher removal of this nonsteroidal antiinflammatory drug were achieved by Liu et al. [114] with a novel MOF, named [email protected](Fe), where 1T is the metallic phase of molybdenum disulfides. The authors tested different molar ratios 1T-MoS2:MIL53(Fe), and the best results were obtained with 5% of 1T-MoS2. The photocatalyst dose and the initial pH were studied, being the optimal conditions 400 mg/L of catalyst and pH ¼ 7. After 2 h, the contaminant was completely removed from the solution with 95% of mineralization under visible light. This MOF maintained a high photocatalytic activity after five cycles, and its structure did not change thanks to the absence of metal leaching. The photocatalytic mechanism indicated that electrons (e), OH, and $O2  radicals were the main active radicals in the degradation of ibuprofen. Liang et al. [115] studied the degradation of two pharmaceutical products such as ibuprofen and theophylline under visible light with [email protected](Fe). This photocatalyst was synthetized for the first time via facile alcohol reduction, but its activity was quite low after 150 min, and then an oxidant reagent such as H2O2 was added and the activity was increased to 99.5%, through a photo-Fenton type process. The TOC reduction was of 69.2% for ibuprofen and 45.2% for theophylline. Different noble metals (Au, Pd, and Pt) were evaluated to accomplish the process and the best performance was obtained with Pd [115]. The stability of the MOF was maintained for four cycles but with a small account of Fe3þ leaching. The optimal amount of Pd deposited in MIL-100(Fe) was 1%. The pH value and the addition of H2O2 played an important role in the photocatalytic reaction. The study of the mechanism concluded that electrons and OH radicals were involved in pollutants degradation. Wang et al. [118] evaluated three different Fe-MOFs with no modifications where Fe-MIL-101 yielded the best result (almost 100% removal of tetracycline after 3 h under visible light). It must be pointed out that nearly 60% was due to adsorption. Meanwhile, Fe-MIL-100 and Fe-MIL-53 allowed only 57% and 41% removal. Fe-MIL-101 showed a high stability in four cycles of adsorption and photocatalytic. The study of radical mechanism indicated that hþ, $O2  , and OH were the main active species in the decay of that antibiotic. Other interesting works have been recently reported with titas et al. [112] have investigated nium as metal cluster. Gómez-Avile some new mixed ZreTi MOFs synthetized by partial substitution

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

59

of Ti by Zr. The highest activity was achieved with a Ti:Zr molar ratio of 85:15, higher than that of the bare Ti-MOF. After 90 min, complete removal of acetaminophen was achieved under visible light. No structural changes were observed after three successive cycles and more than 90% acetaminophen conversion was maintained. The main contributing radical was O2 $ , but electrons and OH were also involved. Wang et al. [116] prepared novel coreeshell [email protected](Ti) photocatalyst, which was tested for tetracycline breakdown under visible light. Removal reached 63% after 60 min, quite higher than that achieved with Fe-based MOFs [118]. Nevertheless, the stability of [email protected] MIL-125(Ti) was affected after three successive cycles, decreasing the tetracycline removal to 40%, with only 17% mineralization. Fan et al. [111] have recently evaluated the degradation of acetaminophen with ZIF-8, a Zn-based MOF, combined with other semiconductors (Ag/[email protected]). The effects of contaminant concentration, MOF dosage or pH were evaluated. After optimization, 98.5% of conversion was reached after 60 min under visible light, with a good stability on three  successive cycles. Superoxide radicals $O2  were the main responsible of the photocatalytic process (Fig. 3.2). Other zinc heterostructure, g-C3N4/ZIF-8, yielded good results for the degradation of tetracycline [117]. After 60 min under sunlight, more than 90% removal occurred but 45% correspond to adsorption. Although there were no data on mineralization, the stability of this MOF remained fairly good after five runs.

Figure 3.2 Scheme of acetaminophen degradation mechanism with by Ag/[email protected] Reproduced with permission from G. Fan, X. Zheng, J. Luo, H. Peng, H. Lin, M. Bao, L. Hong, J. Zhou, Rapid synthesis of Ag/[email protected] as a highly efficient photocatalyst for degradation of acetaminophen under visible light, Chem. Eng. J. 351 (2018) 782e790. https://doi.org/ 10.1016/j.cej.2018.06.119.

60

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

Superoxide anions and holes controlled the photocatalytic process. Amoxicillin, a broad-spectrum and semisynthetic b-lactam antibiotic, was removed with a novel heterostructure based on In-based MOF decorated with rGO (MIL-68(In)-NH2/rGO). Removal reached 93% after 2 h with TOC reduction of 80% after 210 min under visible light. Thus, after three successive cycles the structure remained intact. Regarding the photocatalytic mechanism, hþ and $O2  played a dominant role [119]. From the aforementioned data, the modification of MOFs by doping and metal NPs decoration yields to active photocatalysts for CECs removal. Other interesting ways to enhance the photocatalytic performance are the linker modification [112], incorporating functional groups that increase the light absorption, and the combination with other semiconductors [116]. The potential application of MOFs in photocatalysis for water purification need to overcome some important limitations so far associated to cost, stability, band gap compatibility, and photon efficiency, but, as described in this section, there are promising chances to improve their current relative position with respect to other materials.

6. Semiconductor heterojunctions A useful approach for avoiding the recombination between charges and, thus, improving the efficiency of the photocatalytic process, is the coupling of a semiconductor with a second material to form a heterojunction. These materials have shown improved activity in visible- and solar-driven photocatalytic processes for the degradation of emerging contaminants. Moreover, the lower recombination ascribed to heterojunctions can be qualitatively measured through the increased transient photocurrent response and decreased photoluminescence with respect to the individual semiconductor components [120]. The following classification of heterojunctions into four main categories was suggested by Wang et al. [121]: (i) semiconductoresemiconductor; (ii) semiconductoremetal; (iii) semiconductorecarbon materials; and (iv) multicomponent heterojunction.

6.1 Semiconductoresemiconductor (SeS) In an SeS heterojunction (as depicted in Fig. 3.3), photogenerated electrons can migrate from the CB of one semiconductor to the same band of the second semiconductor if, thermodynamically, the minimum CB potential of the former is higher than that

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

Figure 3.3 Schematic separation of charges via electron transfer in an SeS heterojunction.

of the second. In contrast, the effective migration of electrons from the VB of the first semiconductor to the second one, with lower maximum VB potential to occupy the holes generated there, induces an excess of positive charge in the VB of the first semiconductor. Thus, the number of available photogenerated charges increases, allowing a higher velocity of generation of ROS that can oxidize the contaminant [122]. Table 3.4 summarizes some relevant examples on the investigation of semiconductoresemiconductor heterojunctions in the visible and solar photocatalytic oxidation of emerging contaminants. It must be remarked that it is not possible to establish an easy comparison of the performance of the different heterojunctions because the photocatalytic behavior of each system depends of several issues including both the catalyst textural and electronic properties and the reaction conditions. In this sense, the type of radiation, concentration of contaminant and photocatalyst, temperature, pH, and even the reactor configuration are important aspects to consider in the application of the photocatalyst. The synthesis of the SeS heterojunctions collected in Table 3.4 includes hydro- and solvothermal methods, heat treatment, solegel, coprecipitation, and deposition techniques. Among them, hydrothermal synthesis appears as the most used procedure, in many cases followed by other steps. This is the case of Fe2O3/ZnO, a coreeshell heterojunction integrating the high quantum efficiency of ZnO with the magnetic properties of g-Fe2O3 to facilitate recovery from the reaction medium [123]. The synthetic way consists of a hydrothermal treatment followed by the deposition of ZnO, allowing the formation of a threedimensional layer of ZnO over the Fe2O3. Its application in the

61

62

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

Table 3.4 SeS heterojunctions tested in the photocatalytic degradation of emerging contaminants under visible and solar light. Heterojunction Synthesis

Contaminant (initial concent.)

Light source

Fe2O3/ZnO

CIP (10 mg/L)

Simulated sunlight 92 (60 min)

CdS/SnIn4S8 CdS/Bi2MoO6

CuS/BiFeO3 AgI/Bi5O7I

AgI/WO3 AgI/BiVO4 Ag3PO4/AgBr Ag3PO4/WO3 Ag3PO4/BiOBr g-C3N4/ZnIn2S4 Bi2WO6/g-C3N4 Nb2O5/g-C3N4

V2O5/g-C3N4 ZnO/g-C3N4

Hydrothermalatomic layer deposition Hydrothermal Solvothermal eprecipitation ecalcination Combustion ehydrothermal Deposition eprecipitation

% Contaminant removal (time)

References [123]

Real wastewater Visible light (l > 420 nm) LEV (20 mg/L) Visible light (l > 400 nm)

35 (TOC) (18 h) 80 (60 min)

[124]

AL (5 mg/L)

90 (240 min)

[126]

TC: 93; DTC: 87 OTC: 79; CIP: 82 (40 min)

[127]

75 (60 min)

[128]

80 (60 min)

[129]

96 (120 min) 90 (2 min)

[130] [131]

95 (30 min)

[132]

100 (120 min)

[133]

TC: 80 CIP: 20 (120 min) TC: 90 CIP: 60 LEV: 75 (60 min) 75 (120 min)

[134]

100 (60 min)

[137]

Visible light (l > 400 nm) Visible light TC (20 mg/L), (l > 400 nm) DTC (10 mg/L), OTC (10 mg/L) CIP (10 mg/L) Precipitation TC (35 mg/L) Visible light (l > 420 nm) Deposition OTC (20 mg/L) Visible light eprecipitation (l > 420 nm) Precipitation TC (20 mg/L) Visible light Deposition SMX (0.525 Simulated eprecipitation mg/L) sunlight Precipitation CFZ (10 mg/L) Visible light (l > 420 nm) Hydrothermal TC (20 mg/L) Visible light (l > 420 nm) Hydrothermal TC and CIP Visible light ecalcination (both 10 mg/L) Calcination TC (20 mg/L) Simulated CIP (20 mg/L) sunlight LEV (20 mg/L) Calcination TC (10 mg/L) Visible light (l > 420 nm) Thermal atomic CEF (10 mg/L) Simulated layer deposition sunlight

[125]

[135]

[136]

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

63

Table 3.4 SeS heterojunctions tested in the photocatalytic degradation of emerging contaminants under visible and solar light.dcontinued Heterojunction Synthesis

Contaminant (initial concent.)

InVO4/g-C3N4

TC (15 mg/L)

WO3/TiO2

Hydrothermal ecalcination Solegel

Bi2MoO6/NiTiO3

Solvothermal

TC (15 mg/L)

Bi2WO6/NieAl LDH

Coprecipitation ehydrothermal

TC (30 mg/L)

MA (12 mg/L)

Light source Simulated sunlight Natural solar light Visible light (l > 400 nm) Visible light

% Contaminant removal (time)

References

50 (180 min)

[138]

100 (120 min)

[139]

90 (90 min)

[140]

85 (11 h)

[141]

AL, alachlor; CEF, cephalexin; CFZ, cefazolin; CIP, ciprofloxacin; DTC, deoxytetracycline; LEV, levofloxacin; MA, malathion; OTC, oxytetracycline; SMX, sulfamethoxazole; TC, tetracycline; TOC, total organic carbon.

photodegradation of ciprofloxacin (antibiotic) under solar light improved the removal percent from 55% to 93% with respect to bare ZnO. Another hydrothermal synthesis, in this case for CdS/SnIn4S8, was evaluated by Wang et al. [124]. CdS is a commonly used photocatalyst for other visible-driven applications due to its low band gap (2.3 eV). However, its low stability, due to photocorrosion, together with low lighteharvesting efficiency are main drawbacks of this semiconductor [142]. Different approaches have been developed to overcome these inconveniences, which include coupling with other semiconductors. The application of CdS/ SnIn4S8 to the treatment of real pharmaceutical wastewater under visible light allowed 35% TOC reduction, higher than that obtained with the individual semiconductors because of a more efficient separation of charges. The stability with respect to CdS was also improved. Other sulfide-based heterojunction for visible-driven applications is CuS/BiFeO3, consisting of CuS nanorods coupled to nanoplates of BiFeO3. It was tested for the photocatalytic degradation of the herbicide alachlor. BiFeO3 has

64

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

a lower band gap (2.1 eV) and better visible lighteharvesting capacity than other Bi-based semiconductors [126,143]. Besides, CuS (c. 1.7 eV) was selected because of its higher minimum CB potential favoring the separation of electronehole pairs through the semiconductors. Advantages of SeS heterojunctions were also observed with other systems combined with silver compounds. Bi5O7I (2.90 eV) is characterized by a low separation of charges but good photocatalytic performance [144], whereas AgI (2.77 eV) presents a higher ability for harvesting visible light. However, this latter tends to form agglomerates, yielding a poorer separation of photogenerated charges [145]. Chen et al. [127] studied the photocatalytic degradation of several antibiotics with AgI/Bi5O7I heterojunction (prepared on decoration of Bi5O7I microspheres with AgI NPs). An increased photoefficiency was observed compared to bare Bi5O7I, which was attributed to the formation of a Z-scheme mechanism in the heterojunction and the recombination of the electrons of Bi5O7I and the holes of AgI. In contrast with the conventional heterojunction mechanism, in a direct Z-scheme system (Fig. 3.4), the photogenerated electrons from one semiconductor are energetically prone to migrate to the VB of the second semiconductor, occupying the available holes. Therefore, this recombination through semiconductors enhances the photostability of the heterojunction and its performance by means of enabling the electrons from the second semiconductor (and the holes from the first one) for further generation of ROS [146]. This mechanism can also include a charge mediator facilitating the transfer of charges via redox process. This is also the case of the AgI/WO3 heterojunction,

Figure 3.4 Schematic transfer of electrons on a direct Z-scheme in an SeS heterojunction.

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

obtained by deposition of AgI NPs on WO3 nanosheets. This heterojunction was tested for the photocatalytic oxidation of tetracycline (bactericide) under visible light [128], yielding a 15-fold improved performance respect to WO3. Graphitic-carbon nitride (g-C3N4) represents another important type of SeS heterojunction. In 2009, Wang et al. [147] reported for the first time the photocatalytic use of g-C3N4 (in this case, for hydrogen production from water splitting), based on its narrow band gap (2.7 eV) and high stability. In contrast, this material suffers from high recombination rate and low quantum efficiency. In a recent work, Guo et al. [133] used a ternary chalcogenide, ZnIn2S4, for the hydrothermal synthesis of a gC3N4/ZnIn2S4 heterojunction via g-C3N4 sheets incorporation into the microflower-like ZnIn2S4 structure. This material was tested in the photodegradation of tetracycline under visible light, allowing complete conversion of the pollutant after 120 min, with 65% TOC reduction. The results were significantly better than that obtained with the individual components, due to a better separation of electronehole pairs. Another representative g-C3N4-based heterojunction is Bi2WO6/g-C3N4, with a two-dimensional structure derived from the formation of nanosheet/nanosheet hybrids, which increased the surface area of the photocatalyst and allowed better separation of the electronehole pairs [134]. Bi2WO6/g-C3N4 has been tested for the visible-driven photodegradation of tetracycline and ciprofloxacin, yielding two- and threefold higher removal than the uncoupled Bi2WO6 and g-C3N4, respectively.

6.2 Semiconductoremetal (SeM) Another approach to improve the photocatalytic activity of a semiconductor under visible irradiation consists of coupling a semiconductor with metallic particles to form an SeM heterojunction. NPs of noble metals such Ag and Au have a strong absorption in the visible part of the electromagnetic spectrum due to the SPR effect [148,149]. As depicted in Fig. 3.5, SPR is an inherent property of these metals whereby the charge density is redistributed when they are irradiated with light with a wavelength much higher than the NPs size. As a consequence, the electric field near the particle is remarkably intensified, favoring the absorption and scattering of the incident photons, and thus, incrementing the transference of excited electrons from the metal NP to the CB of the adjacent semiconductor [57]. These systems are usually synthesized by the deposition and photoreduction of the metal over the semiconductor [150]. Ag and Au NPs are commonly used to synthesize SeM heterojunctions because of

65

66

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

Figure 3.5 Schematic redistribution of charge density in metal nanoparticles (NPs) as a result of surface plasmon resonance effect.

their absorption capacity in the solar spectrum, high chemical resistance, and even their antimicrobial activity [151]. However, the release of the metallic ions to the medium is an important drawback regarding technological implementation of these heterojunctions. The photocatalytic degradation of organic contaminants under visible light using SeM heterojunctions has been widely investigated, mainly in the case of dyes [57]. However, application to the treatment of water containing emerging contaminants has been much more scarcely studied compared to other type of heterojunctions (see Table 3.5). Ag/WO3 heterojunction, prepared by modifying WO3 with dispersed Ag NPs, showed a twofold higher photoefficiency than bare WO3 in the degradation of sulfonamide (antibiotic) under visible light [152]. This was attributed to the widening of the absorption range due to the SPR caused by Ag NPs and the reduction of charge recombination, the latter observed by photoluminescence. However, it was also stated that an excess of Ag NPs can negatively affect the photocatalytic performance. Other systems include various metal NPs. Peng et al. [153] prepared well-dispersed AueAgeAgI over Al2O3 for the photocatalytic degradation of the pharmaceutical ibuprofen and other organic pollutants. The presence of Au NPs in the AueAge AgI/Al2O3 heterojunction increased the red-shift light absorption due to combined SPR effect and diminished the leaching of metallic ions to the aqueous medium; the latter is due to a more accelerated electron transference to the semiconductor. C3N4 nanorods were decorated with Au NPs by hydrothermal treatment and tested under visible light for tetracycline breakdown showing better performance than C3N4 nanorods alone. This improved

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

67

Table 3.5 Representative SeM and SeC heterojunctions used for visible- and solar-driven photocatalytic treatment of emerging contaminants. Heterojunction Synthesis

Contaminant (initial concent.) Light source

% removal (time)

References

96 (5 h)

[152]

100 (20 min)

[153]

84 (120 min)

[154]

72 (60 min) 100 (90 min) 100 (180 min)

[155] [156] [157]

SeM Heterojunctions

Ag/WO3

Photoreduction

SAM (10 mg/L)

AueAgeAgI/ Al2O3 Au/C3N4 nanorods

Photoreduction

IBU (10 mg/L)

Hydrothermal

TC (20 mg/L)

Visible light (l > 420 nm) Visible light (l > 420 nm) Visible light (l > 400 nm)

SeC heterojunctions

TiO2erGO TiO2erGO WO3erGO

Hydrothermal Hydrothermal Hydrothermal

ATL (25 mg/L) RS (2 mg/L) SMX (10 mg/L)

TiO2/CNT

Sol-gel

TiO2eGO TiO2eCNT TiO2eC60

Liquid phase deposition

Mixture of 20 PPCP in RSWWE DP (100 mg/L)

TiO2/C-dots BiVO4/C-dots TiO2/C-spheres

Calcination Hydrothermal Hydrothermal

GEM (2 mg/L) CBZ (10 mg/L) OP (5 mg/L)

Simulated sunlight Simulated sunlight Visible light (l > 420 nm) Simulated sunlight

Depending of compounds Visible light TiO2eGO: 20 (l > 420 nm) TiO2eCNT: 15 TiO2eC60: 8 (60 min) Simulated sunlight 100 (25 min) Simulated sunlight >95 (180 min) Visible light 40 (360 min) (l > 400 nm)

[158] [159]

[160] [161] [162]

ATL, atenolol; CBZ, carbamazepine; DP, diphenhydramine; GEM, gemfibrozil; IBU, ibuprofen; OP, oseltamivir phosphate; PPCP, pharmaceuticals and personal care products; RS, risperidone; RSWWE, real secondary wastewater effluent; SAM, sulfanilamide; SMX, sulfamethoxazole; TC, tetracycline.

performance was attributed to the occurrence of new absorption peak (c. 550 nm) in the diffuse reflectance spectrum of the Au/C3N4 heterojunction as a consequence of the SPR effect caused by Au NPs [154].

68

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

6.3 Semiconductorecarbon materials (SeC) Heterojunction with carbon-based materials allows increasing the specific surface area, shifting the light absorption to the visible region, and enhancing electron mobility [163]. Semiconductors coupled to carbonaceous materials (SeC heterojunctions) have also been tested for the treatment of emerging contaminants under visible and solar irradiation [164]. Minella et al. [170] and Awfa et al. [165] have recently reviewed this topic, including UV-based photocatalytic degradation of emerging contaminants and other organic species (i.e., dyes and priority pollutants). The coupling between semiconductor and carbon materials can be achieved by different methods, including heat-treatment, hydrothermal, solegel, or deposition in liquid phase as the most representative. The carbonaceous materials used for the SeC heterojunctions include carbon nanotubes (CNTs), graphene-base materials, and carbon quantum dots, among others, as summarized in Table 3.5. The main works found are related to graphene or graphenebased materials [155e157,159], characterized by high surface area (above 2600 m2/g) and electron mobility [166,167], both being useful properties for improving the visible- or solar-driven photocatalytic applications. Better separation of photogenerated charges can be achieved with graphene oxide (GO) or reduced graphene oxide (rGO) as cocatalysts in the heterojunction. Bhatia et al. [155] investigated degradation of atenolol (pharmaceutical) with a TiO2erGO composite under simulated solar light. More than 70% conversion was reached in 1 h and complete TOC removal after 7 h were achieved, higher than that obtained with commercial TiO2. The incorporation of GO increases the mobility of charges and decreases the band gap up to 2.2 eV, as a consequence of the effective bonding between both compounds [168]. Other semiconductors for SeC heterojunctions include WO3. Coupling with rGO, to give WO3erGO heterojunction, reduces the rapid recombination of photogenerated charges with was a main drawback of WO3. Complete conversion of sulfamethoxazole under 3 h of visible light irradiation has been reported [157]. The improved photocatalytic performance was explained as the result of increased surface area, which improves adsorption and a better separation of charges, as shown in Fig. 3.6, owing to the electron flow from the CB of the semiconductor and the electron trap effect of rGO layers. CNTs, both single-walled (SWCNT) and multi-walled (MWCNT), are also suitable materials for SeC heterojunctions [169]. A comparative test with different SeC heterojunctions was carried out by Pastrana-Martínez et al. [159], when TiO2eGO,

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

69

Figure 3.6 Schematic transfer of electrons from the semiconductor to the rGO in an SeC heterojunction. Reprinted with permission from W. Zhu, F. Sun, R. Goei, Y. Zhou, Facile fabrication of rGO-WO3 composites for effective visible light photocatalytic degradation of sulfamethoxazole, Appl. Catal. B Environ. 207 (2017) 93e102. https://doi.org/10.1016/j.apcatb.2017. 02.012.

TiO2eCNT, and TiO2eC60 (fullerene) were synthesized by liquidphase deposition. These materials were tested in the degradation of the antihistaminic diphenhydramine under visible light. The increased performance of TiO2eGO and TiO2eCNT heterojunctions was attributed to the electronic transference from the sp2 states of the carbon material to the CB of TiO2 and further generation of radical species. This sensitizer effect has been previously observed with similar SeC heterojunctions [170]. Furthermore, a practical application of SeC heterojunction using CNTs was carried out by Murgolo et al. [158], who used TiO2/CNTs for the degradation under simulated solar light of a mixture of 20 different pharmaceutical and personal care products in ultrapure water and a real WWTP secondary effluent. The results showed a strong dependence on the contaminant and the water matrix, owing to the presence of other compounds that can act as scavengers of the reactive species. Other carbon materials for SeC heterojunctions include carbon quantum dots (C-dots), which have unique electronic properties, especially due to their property of up conversion photoluminescence (UCPL). This effect means that they can be excited with low-energy light (>600 nm), following with the conversion to higher-energy light (325e650 nm) by means of up-converted emissions of the quantum dots. This higher-energy emission enhances the excitation of charges in the semiconductor, thus allowing more efficient absorption of

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light within the solar spectrum and better photocatalytic performance [171,172]. The heterojunction with TiO2, yielding TiO2/C-dots, has been applied to the conversion of gemfibrozil, a blood lipid-regulator pharmaceutical under solar-simulated light [160]. Compared to bare TiO2, this heterojunction showed higher photocatalytic activity due to an increased generation rate of radical species, observed by electron spin resonance. These heterojunctions have been also synthesized with carbon spheres (C-spheres). A coreeshell TiO2/C-spheres composite prepared by TiO2 NPs deposition over carbon spheres obtained from hydrothermal treatment of sucrose [162] was reported for the degradation of the pharmaceutical oseltamivir phosphate (OP, commercially known as Tamiflu) under visible light. Experimental results showed a twofold increased conversion with respect to bare TiO2 (Degussa P25) as a consequence of the effective assembly between TiO2 and carbon spheres and the effective formation of TieOeC bonds, causing a reduction in the band gap of the heterojunction. GO-based materials appear to be the most promising for semiconductorecarbon heterojunctions. The easy synthesis, mainly by hydrothermal way, and the optimal mobility of electrons because of the effective assembly between the semiconductor and the rGO are the determining factors regarding their application under solar- and visible-driven degradation of water pollutants.

6.4 Multicomponent heterojunctions This type of heterojunctions is obtained by coupling more than two components, usually integrated by at least two components with light absorption in the visible range and other serving charges transfer mediator, i.e., favoring the migration of photogenerated electroneholes through the semiconductors. Table 3.6 collects the most significant multicomponent heterojunctions found in literature used in the visible- and solardriven photocatalytic degradation of emerging contaminants. These heterojunctions include some of the materials previously detailed for binary heterojunctions, to achieve the synergistic effects and improve the photocatalytic efficiency. This is the case of Ag NPs, which are usually incorporated in this type of heterojunction for widening the absorption range by SPR effect [173e180]. Yang et al. [175] synthesized a Z-scheme multicomponent heterojunction (Ag/AgCl/Ag2O) where the plasmonphotogenerated electrons in Ag are transferred to the CB of

Table 3.6 Representative multicomponent heterojunctions for the photocatalytic treatment of emerging contaminants under visible and solar light. Heterojunction

Synthesis

Contaminant (initial concent.)

Light source

% Removal (time)

References

AgeBiPO4/BiOBr/BiFeO3 AgeAgCl/Ti3P4O16

Precipitationeimpregnationephotoreduction Photoreduction

NFN (20 mg/L) CIP (5 mg/L)

>98 (90 min) 48 (180 min)

[173] [174]

Ag/AgCl/Ag2O

Precipitationephotoreduction

CIP (10 mg/L)

91 (100 min)

[175]

Ag/C-dots/g-C3N4

Thermo-polymerization

NPX (4 mg/L)

88 (24 min)

[176]

Ag/C-dots/BiOCl

Solvothermal

TC (30 mg/L)

69 (50 min)

[177]

Ag/AgBr/BiVO4

Hydrothermalephotoreduction

CIP (10 mg/L)

91 (120 min)

[178]

g-C3N4/Ag/AgCl/BiVO4 Ag/g-C3N4/BiVO4

Hydrothermal Wet impregnationephotoreduction

IBU (2 mg/L) TC (20 mg/L)

95 (60 min) 80 (60 min)

[179] [180]

C-dots/g-C3N4/ZnO

Impregnationecalcination

TC (10 mg/L)

100 (30 min)

[181]

ZnO/Fe2O3/g-C3N4

Hydrothermalecalcination

SMZ (5 mg/L)

100 (140 min)

[182]

ZnO/g-C3N4/MoS2

Wet impregnationecalcination

ATR (10 mg/L)

85 (300 min)

[183]

MWCNT/TiO2/SiO2

Solegel

ACE (10 mg/L)

Visible light Visible light (l > 400 nm) Visible light (l > 420 nm) Visible light (l > 420 nm) Visible light (l > 420 nm) Visible light (l > 420 nm) Simulated sunlight Visible light (l > 420 nm) Visible light (l > 420 nm) Visible light (l > 420 nm) Visible light (l > 400 nm) Visible light (l > 500 nm)

82 (60 min)

[184]

ACE, acetaminophen; ATR, atrazine; CIP, ciprofloxacin; IBU, ibuprofen; NFN, norfloxacin; NPX, naproxen; SMZ, sulfamethazine; TC, tetracycline.

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AgCl, whereas the holes in Ag recombine with the electrons from Ag2O. The improved separation of charges between AgCl and Ag2O improved the degradation of ciprofloxacin with this multicomponent heterojunction. The assembly with quantum carbon dots as ternary component can also improve the photocatalytic performance as a consequence of its upconverted photoluminescence property (as seen for SeC heterojunctions) [176,177,181]. The incorporation of 3% of Ag NPs and 1% of C-dots over C3N4(Ag/C-dots/g-C3N4) increased the conversion of the antiinflammatory naproxen with respect to bare g-C3N4 [176]. Many of these multicomponent heterojunctions include bismuth vanadates (BiVO4) owing to their low band gap (c. 2.4 eV) and high chemical stability. Even though the BiVO4-based binary heterojunctions can inhibit the recombination of electronehole pairs to certain extent, the construction of a Z-schemeebased multicomponent heterojunction with BiVO4 is a useful approach to enhance the performance of this type of systems [178]. On the other hand, although the deposition of Ag NPs is currently used for the synthesis of multicomponent heterojunction, its practical application is unlikely because of the high cost of this metal. Other similar heterojunctions with g-C3N4 can be obtained with less expensive materials, such as carbon dots or zinc oxide. These heterojunctions have been tested for the removal of tetracycline, sulfonamide, or atrazine [181e183], showing higher photoefficiency than the binary heterojunction because of the electron donoreacceptor effect of the mediator in the multicomponent heterojunction. It has to be remarked that comparison between different heterojunctions is not simple because multiple factors are involved, affecting photocatalytic performance. The nature of the pollutant and its concentration, the type of radiation, or the synthesis route followed to prepare the heterojunction are important issues, in addition to the photocatalyst arrangement inside the reactor and its recovery. However, heterojunctions have proved their efficiency at lab scale, yielding higher photoactivity than benchmark TiO2. Heterojunctions of semiconductor and carbon materials deserve particular attention because they combine good performance under visible light and easier recovery from the reaction medium. The wide range of suitable components to build the heterojunctions makes them promising materials for the treatment of real wastewaters containing emerging contaminants under solar light.

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

7. Semiconductor anchored on porous solids The implementation of semiconductor photocatalysts suspended in aqueous phase is hindered by the separation and recovery of the fine photocatalyst particles [185,186]. This difficulty can be overcome by supporting the semiconductor on the surface of some convenient material [187,188]. However, this solution also suffers from unwanted side drawbacks. Because photocatalytic reactions take place predominantly on the surface of the semiconductors, their anchoring onto the support generally reduces the available photocatalytic area and thus the reaction efficiency. One possible solution consists in using porous supports, when the pollutant molecules can adsorb close to photocatalytic active sites, resulting in an overall improvement of photodegradation [189,190]. Furthermore, reaction intermediates can be retained onto the porous support with a higher enhancement of mineralization. An additional advantage of this approach is a better electronehole separation with certain supports, which can enhance the photocatalytic activity. Numerous porous materials have been used as supports for semiconductors in photocatalytic applications, such as activated carbons (ACs), clays, zeolites, and various kinds of silica, among many others. This section summarizes the application of porous adsorbents as supports for semiconductors in simultaneous adsorptione photocatalytic processes for the treatment of polluted water under solar or visible irradiation. The different methods for loading the semiconductor onto the supports and the main issues affecting adsorption are also analyzed. The method used for the synthesis of the semiconductoreporous support arrangement has significant effect on its final properties and, thus, on the photocatalytic activity. The methods most widely used in the literature include chemical vapor deposition, impregnation, precipitation, and solegel. The different types on porous supports more commonly reported in the literature are reviewed in the following.

7.1 Carbon-based supports Carbon-based supports, including ACs and graphene among others, have been extensively studied. In the case of ACs, their high adsorption capacity not only improves the removal of

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organic contaminant by adsorption but also enhances the photocatalytic performance by increasing the concentration of organic molecules close to the semiconductor actives sites. Both, the porosity and surface chemistry of the AC, have shown relevant importance in the photocatalytic degradation of dissolved organic compounds. Peñas-Garzón el al. [191] used several ACs prepared by chemical activation of lignin with different activating agents for synthesizing TiO2/AC heterostructures. The activating agents did not affect the crystalline phase of the supported TiO2, although they gave rise to different porous texture and surface chemistry of the resulting AC. The highest photoactivity under solar irradiation was obtained with the heterostructure using FeCl3-AC, which allowed complete conversion of acetaminophen after 6 h of irradiation. Yap et al. [192] studied the synthesis and characterization of nitrogen-doped TiO2/AC composites for the adsorptionephotocatalytic degradation of aqueous bisphenol-A (BPA) using solar light. They concluded that despite the synergistic effect of adsorptionephotocatalysis that produced a higher BPA photodegradation, it should also be taken into account that the AC-supported NeTiO2 had a smaller average crystallite size (c. 5.0 nm) as compared to the unsupported Ne TiO2 (c. 5.4 nm), which can also affect their photocatalytic activity. The photodegradation of antibiotic sulfamethazine (SMZ) was studied with Bi-doped TiO2 nanocomposites of variable Bi/Ti molar ratios supported on powdered AC by a sol-hydrothermal method [193]. The composites showed higher photocatalytic activity under visible radiation, probably because of their smaller crystal size, larger specific surface area, larger pore size, stronger visible light absorption ability, and lower band gap energy than bare TiO2. Not only TiO2, but other semiconductors have been supported in ACs for the photodegradation of emerging pollutants. ZnO/ZnWO4 supported on AC was synthesized by a hydrothermal method and tested for the degradation of oxytetracycline and ampicillin from aqueous phase under solar light [194]. The authors concluded that a synergistic effect of adsorption and photocatalysis was produced. Graphene and derived materials, GO and reduced graphene oxide (rGO), have been tested in photocatalysis by their well-known high conductivity, stability, and even high surface area [195]. But probably, their main properties for photocatalytic applications are the efficient transport and separation of electrons and holes. Reduced graphene oxideeWO3 composites were synthesized by hydrothermal method and used as visible lighte driven photocatalysts for the degradation of sulfamethoxazole antibiotic [196]. Almost complete conversion was achieved after

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

75

3 h of visible light irradiation. The study concluded that the improvement in photocatalytic activity was due to the fact that the p-n junction enhances the electronehole pair generation and suppresses recombination. Fig. 3.7 shows SEM and TEM images of reduced graphene oxideeWO3 composites with decreasing W content [157]. Cu2O/rGO photocatalysts were prepared by a simple wet-chemical route and tested for the photodegradation of sulfamethoxazole under visible light [197]. The highest photocatalytic activity was ascribed to the bestdispersed and visible lightedriven Cu2O NPs on surface of graphene sheets. The improved behavior was associated to the effect of rGO, which serves not only as support to stabilize and disperse Cu2O but also as adsorbent of the pollutant species and as charge acceptor to promote the separation and transfer of photogenerated carriers.

7.2 Clay supports Clay minerals and clay-derived materials are interesting supports owing to their high availability and low cost in addition to their

(A)

(C)

(E)

(B)

(D)

(F)

Figure 3.7 FESEM and TEM images of RW-400 (A and B), RW-200 (C and D), and RW-100 (E and F). Reproduced with permission of Elsevier W. Zhu, F. Sun, R. Goei, Y. Zhou, Facile fabrication of rGO-WO3 composites for effective visible light photocatalytic degradation of sulfamethoxazole, Appl. Catal. B Environ. 207 (2017) 93e102. https://doi.org/10.1016/j.apcatb.2017. 02.012.

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functional characteristics. Clays can be classified in four different groups, namely kaolinite, smectite, illite, and chlorite. They can be considered good adsorbents because of their relatively high surface area, high chemical stability, and good cation-exchange capacity. The literature contains many studies on the photocatalytic purification of water using clayesemiconductor composites. Akkari et al. [198] prepared ZnO NPs on the surface of sepiolite (Sep) fibrous clay previously modified by magnetite (Fe3O4) NPs (Fig. 3.8). The materials showed high photoactivity for the degradation of emerging pollutants under solar-simulated irradiation. Furthermore, ZnO/Fe3O4-Sep photocatalyst showed superparamagnetic character allowing its easy recovery by application of an external magnetic field. Belver et al. [199e202] synthesized bare and Zr-, Ce-, and W-doped TiO2 NPs successfully immobilized on delaminated clay materials by a one-step solegel route. The most promising results were obtained on Zr doping, which was incorporated into the anatase lattice, giving rise to a slight deformation of the anatase crystal and the reduction of the band gap. These composites were tested in the solar photodegradation of antipyrine, as model emerging pollutant. High degradation rates were

Figure 3.8 TEM (A) and HR-STEM (B) micrograph images of the ZnO/Fe3O4-Sep heterostructures and their corrected STEM-HAADF analysis of Fe3O4 (top) and ZnO (bottom) nanoparticles. Reprinted with permission of Elsevier M. Akkari, P. Aranda, C. Belver, J. Bedia, A. Ben Haj Amara, E. Ruiz-Hitzky, ZnO/sepiolite heterostructured materials for solar photocatalytic degradation of pharmaceuticals in wastewater, Appl. Clay Sci. 156 (2018) 104e109. https://doi.org/10. 1016/j.clay.2018.01.021.

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

achieved at low antipyrine concentrations and high solar irradiation intensities. Atrazine was photodegraded under solar light with WeTiO2/clay photocatalysts with different W loads (0.5, 2, and 5 M%) [202]. The W-substitution of Ti ions produced a moderate reduction of band gap and increase of the photocatalytic activity. The relevance of W doping on the photocatalytic performance was also corroborated by the activity under visible light. Li et al. [203] synthesized g-C3N4/montmorillonite composites by a wet-chemical method followed by calcination. The composites showed high activity toward photodegradation of rhodamine B and tetracycline under visible light. This enhanced photocatalytic activity was attributed to the relatively high surface area and enhanced light-adsorption ability under visible light in addition to a synergistic effect allowing efficient separation of the photo-generated charge carriers. Bansal and Verma [204] analyzed the immobilization of FeeAgeTiO2 composite on clay beads from waste fly ash and foundry sand. The composites were tested for the photodegradation of pentoxifylline, a persistent drug, under natural sunlight and even at pilot plant scale. In other work [205], N-doped TiO2/montmorillonite composites modified with carbon were obtained by mixing a swelling clay solution with an N-doped TiO2 solution prepared by hydrolysis of titanium tetraisopropoxide with urea. The introduction of C greatly extended the absorption edges to the visible light region, improving the photocatalytic activity for the degradation of bisphenol-A under visible light.

7.3 Zeolite supports Zeolites can be defined as hydrated aluminosilicate minerals that can occur naturally or can be synthesized with controllable pore size and surface characteristics. Zeolites have a well-developed porosity, high number of active sites, good thermal stability and ion exchange capacity, and tailored shape selectivity. Because of this, they are widely used in oil refining, petrochemical industry, water purification, and food processing. Regarding photocatalytic applications, zeolites are considered good adsorbents owing to their high surface area and charged framework with amphoteric properties. Furthermore, the presence of AleO units in their structure can reduce electronehole recombination making them promising candidates as supports for photocatalytic semiconductor composites. CuO semiconductor was incorporated on natural Clinoptilolite zeolite on calcination of Cu(II)-ion exchanged Clinoptilolite [206]. The composite was tested as a photocatalyst to remove

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p-aminophenol from aqueous solution. The zeolite support increased significantly the photocatalytic activity under solar irradiation. TiO2 on H-mordenite zeolite photocatalysts were used for the photocatalytic degradation and mineralization of isoproturon herbicide under solar light [207]. The study analyzed the effect of TiO2, H-MOR support, and different TiO2 loads. Increasing the TiO2 load decreased the adsorption capacity of the zeolite because of high dispersion of TiO2 on the zeolite surface. Besides, high TiO2 loadings were found detrimental because of poor adsorption. Eskandarian et al. [208] prepared zeoliteeTiO2 nanocomposites immobilized onto a low-density polyethylene film to enhance its mechanical stability. The reactivity of zeolitee TiO2 nanocomposite under UV-LED powered by solar radiation was evaluated in the presence of potassium persulfate (K2S2O8) as an oxidant to decompose reactive dyes and pharmaceuticals (reactive black 5, cefiximetrihydrate, and phenazopyridine). The composites improved photodegradation more significantly than any other factors such as addition of persulfate and application of UV-LED. Zeolite-immobilized silver oxideedecorated titanium dioxide (Ag2O/TiO2eZeolite) composites were used as photocatalysts for the degradation of Norfloxacin, an antimicrobial drug frequently detected in municipal plants, under simulated solar light [209]. The study concluded that Ag2O enhances the visible light absorbance and charge separation efficiency, increasing the photocatalytic activity. TiO2 on iron-exchanged ZSM zeolite composites (TiO2eFeZ) were synthesized and tested for the solar photodegradation of the pharmaceutical diclofenac [210]. The degradation mechanism included adsorption of diclofenac onto the photocatalyst surface and consequent degradation. Liu et al. [211] synthesized magnetically separable Ag/AgCl-zero valent iron particles supported on zeolite X as photocatalysts for tetracycline degradation under visible light. The enhanced photocatalytic activities and stabilities of these composite photocatalysts were ascribed to their improved adsorption capacity, the absorbance in the visible light region, and the better separation of photo-induced charge carriers. Recently, Bi2Sn2O7eC3N4 semiconductors were deposited on a Y zeolite by an ultrasoundassisted dispersion method [212]. The photocatalysts were tested for the degradation of tetracycline antibiotic under simulated solar light irradiation. According to this study, the zeolite support avoids the accumulation of Bi2Sn2O7eC3N4 active phase and favors the accessibility to active sites. Furthermore, it enhanced the adsorption capacity of tetracycline improving the photocatalytic oxidation.

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

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7.4 Silica supports Silica and silica-derived materials, such as MCM-41, MCM-48, or SBA-15, have been also explored as potential supports of semiconductors for photocatalytic applications. Their welldeveloped porosity, neutral framework, mild hydrophobicity, transparency, and light diffraction properties make them promising supports for photocatalytic purification of water. Ibuprofen was degraded using a mesoporous hierarchical BiOBr/[email protected] magnetic photocatalyst (Fig. 3.9) under visible irradiation [214]. The photocatalysts showed fast kinetics and allowed almost complete mineralization. After use and magnet

(A)

(B)

(C)

(D)

(E)

(F)

(G)

(H)

(I)

Figure 3.9 (A) TEM micrograph of BiOBr/[email protected]; (B) SEM micrograph of BiOBr/[email protected] wherein the inset shows the particle size distribution, while the rectangularly outlined portion is magnified in (C); and (DeI) SEM-EDS mapping of BiOBr/[email protected] Reprinted with permission of Elsevier M. Khan, C.S.L. Fung, A. Kumar, I.M.C. Lo, Magnetically separable BiOBr/[email protected] for visible-light-driven photocatalytic degradation of ibuprofen: Mechanistic investigation and prototype development, J. Hazard. Mater. 365 (2019) 733e743. https://doi.org/10.1016/j.jhazmat.2018.11.053.

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separation, w80% of its initial activity is maintained even after five consecutive cycles. The scale-up of the process was accomplished by a 5-L photocatalytic reactor with an electromagnetic separation unit, and the results were comparable to that obtained in the lab experiments. Kumar et al. [215] studied the photocatalytic degradation of ibuprofen under visible light using a magnetic g-C3N4/TiO2/[email protected] heterojunction synthesized via solegel. The superparamagnetic character of the photocatalyst allowed easy recovery from the reaction medium. SBA-15 has also been studied as semiconductor support. Phanikrishna Sharma et al. [213] investigated the photodegradation and mineralization of several pesticides (isoproturon, imidacloprid, and phosphamidon) using TiO2 supported on mesoporous SBA-15 using solar light. It was established that the adsorption and electron delocalization properties of SBA-15 are the key factors enhancing photodegradation. SBA-15 was also used as support of CueTiO2 colloids for the photocatalytic degradation of paraquat herbicide under UV and visible light [216]. The inclusion of SBA-15 significantly improved the photocatalytic activity, probably due to the increased surface area thereby enhancing adsorption of paraquat, and also the shift of zero-point charge of TiO2 to very low pH. Another herbicide, 2,4-dichlorophenol, was photodegraded under visible light using immobilized V-doped sensitized TiO2 onto SBA-15 walls [217] and sulphonated cobalt phthalocyanine immobilized onto MCM-41 [218].

8. Conclusions and outlook Many opportunities related to the design of structured photocatalysts with functional properties and the exploration of their potential application in the removal of emerging contaminants is still being open. Current research is mainly focused on the preparation of nanostructured semiconductors able to induce the pollutant degradation under visible light but in general with low full-scale implementation so far. TiO2 modifications, ternary oxides, semiconductor heterojunctions, and novel heterostructures offer a large number of opportunities for the preparation of functional and multifunctional photocatalysts for water treatment. The photoefficiency of these semiconductors has been evaluated for the removal of different compounds, including pharmaceuticals, personal care products, pesticides, industrial additives, among others. Finding ways for full-scale implementation of these photocatalysts is the near-future challenge.

Chapter 3 Structured photocatalysts for the removal of emerging contaminants

Comprehensive understanding of the semiconductors behavior and the mechanism of the photocatalytic reactions will help to scale-up the processes. Main issues to consider are the separation and recovery of the photocatalyst, the design of optimal reactors, the operating conditions for using both the UV and visible components of the sunlight and, of course, the nature of the pollutant.

References [1] S. Sauv e, M. Desrosiers, A review of what is an emerging contaminant, Chem. Cent. J. 8 (2014) 15e21, https://doi.org/10.1186/1752-153X-8-15. [2] L. Rizzo, S. Malato, D. Antakyali, V.G. Beretsou, M.B. Ðolic, W. Gernjak, E. Heath, I. Ivancev-Tumbas, P. Karaolia, A.R. Lado Ribeiro, G. Mascolo, C.S. McArdell, H. Schaar, A.M.T. Silva, D. Fatta-Kassinos, Consolidated vs new advanced treatment methods for the removal of contaminants of emerging concern from urban wastewater, Sci. Total Environ. 655 (2019) 986e1008, https://doi.org/10.1016/j.scitotenv.2018.11.265. n, J. Harmsen, J. Hollender, [3] V. Dulio, B. van Bavel, E. Brorström-Lunde M. Schlabach, J. Slobodnik, K. Thomas, J. Koschorreck, Emerging pollutants in the EU: 10 years of NORMAN in support of environmental policies and regulations, Environ. Sci. Eur. 30 (2018) 5, https://doi.org/ 10.1186/s12302-018-0135-3. [4] Conseil Federal Suisse, Ordonnance sur la protection des eaux, 2015, pp. 1e70. [5] M. Papageorgiou, C. Kosma, D. Lambropoulou, Seasonal occurrence, removal, mass loading and environmental risk assessment of 55 pharmaceuticals and personal care products in a municipal wastewater treatment plant in Central Greece, Sci. Total Environ. 543 (2016) 547e569, https://doi.org/10.1016/j.scitotenv.2015.11.047. [6] Y. Yang, Y.S. Ok, K.H. Kim, E.E. Kwon, Y.F. Tsang, Occurrences and removal of pharmaceuticals and personal care products (PPCPs) in drinking water and water/sewage treatment plants: a review, Sci. Total Environ. 596e597 (2017) 303e320, https://doi.org/10.1016/ j.scitotenv.2017.04.102. [7] J.C.G. Sousa, A.R. Ribeiro, M.O. Barbosa, M.F.R. Pereira, A.M.T. Silva, A review on environmental monitoring of water organic pollutants identified by EU guidelines, J. Hazard. Mater. 344 (2018) 146e162, https:// doi.org/10.1016/j.jhazmat.2017.09.058. [8] WWAP (United Nations World Water Assessment Programme), The United Nations World Water Development Report 2017, UNESCO, Paris, 2017. [9] U.S. EPA Office, Priorities for Water Quality Standards and Criteria Programs, FY 2017-2018, 2018, pp. 1e5, https://doi.org/10.1021/ jm990211i. [10] European Commission, EU Decision 2018/840 of 5 June 2018 establishing a watch list of substances for Union-wide monitoring in the field of water policy pursuant to Directive 2008/105/EC of the European Parliament and of the Council and repealing 495, Off. J. Eur. Union. L141 (2018) 9e12. [11] Y. Deng, R. Zhao, Advanced oxidation processes (AOPs) in wastewater treatment, Curr. Pollut. Rep. 1 (2015) 167e176, https://doi.org/10.1007/ s40726-015-0015-z.

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