TiO2 hybrids for simultaneous removal of dyes and heavy metal ions under visible light

TiO2 hybrids for simultaneous removal of dyes and heavy metal ions under visible light

Journal of Photochemistry & Photobiology A: Chemistry 389 (2020) 112292 Contents lists available at ScienceDirect Journal of Photochemistry & Photob...

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Journal of Photochemistry & Photobiology A: Chemistry 389 (2020) 112292

Contents lists available at ScienceDirect

Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem

Natural melanin/TiO2 hybrids for simultaneous removal of dyes and heavy metal ions under visible light

T

Wanjie Xie, Esfandiar Pakdel, Yujia Liang, Dan Liu, Lu Sun*, Xungai Wang* Deakin University, Institute for Frontier Materials, Geelong, Australian Future Fibers Research and Innovation Center, Victoria 3220, Australia

A R T I C LE I N FO

A B S T R A C T

Keywords: Melanin Visible light Photocatalysts Methyl Orange Cr (VI)

Melanin has attracted researchers’ attention in recent years due to its intriguing properties and various applications in different fields. Here, we fabricated a highly efficient photocatalytic material through utilizing the natural melanin (NM) isolated from textile scraps like yak hair wastes. A simple one-step process was employed to synthesize titanium dioxide (TiO2) nanoparticles on the surface of NM. The synthesized NM/TiO2 hybrids exhibited prominent photocatalytic activity for the simultaneous removal of methyl orange (MO) and hexavalent chromium ions (Cr (VI)) under visible light irradiations, which constitute ∼45 % of the solar light. Our results revealed that NM served as a base material with abundant catechol groups to form C-O-Ti bonds with TiO2 nanoparticles, which narrowed the band gap and broadened the light response of the hybrids. The working mechanism of NM/TiO2 hybrids for the simultaneous removal of dyes (MO) and heavy metal ions (Cr (VI)) was thoroughly explored and proposed. This work highlighted the potential of the NM/TiO2 hybrids for textile dyeing effluent treatment with more efficient utilization of solar spectrum. Our work also opens a pathway for the design of advanced NM-based functional materials from various NM-enriched wastes such as human hair wastes, squid ink, etc.

1. Introduction Textile wastewater accounts for 17–20 % of the total industry water pollution and contains a large quantity of different types of harmful pollutants such as non-degradable organic dyes and toxic heavy metals [1,2]. Most of these impurities are carcinogenic and mutagenic to humans and can endanger the aquatic life after being discharged to the environment [3]. To alleviate the negative effects of the textile dyeing effluent on the environment, various methods have been developed to eliminate these dyes and heavy metal ions prior to their discharge into the drainage system [4–6]. Heterogeneous photocatalysis using TiO2based photocatalysts has attracted the most attention due to the outstanding chemical stability and commercial availability of TiO2, as well as the solar driven-nature of the process [7,8]. It is known that the incoming solar light constitutes ∼5 % ultraviolet (UV) light, ∼45 % visible light and ∼50 % infrared (IR) light [9]. However, TiO2 can only be excited by UV light due to its wide band gap (3.2 eV, anatase) [10,11]. Therefore, researchers have tried to develop visible light-responsive TiO2-based photocatalysts to utilize a larger proportion of the solar spectrum for more efficient dye degradation. For instance, visible light-driven TiO2-based materials were successfully prepared by doping TiO2 with metal/non-metal ions, dyes, ⁎

or combining it with two dimensional (2D) nanomaterials [12–15]. Among these methods, the dye sensitizing strategy is normally restricted to the expensiveness and instability of the dyes [16]. Therefore, it is of paramount importance to use cost-effective and stable materials, as well as simple methods to design the visible light-driven TiO2-based photocatalysts. Natural melanin is an important class of natural pigments which can be derived from flora and fauna [17]. The availability of abundant natural sources highlights its cost-effectiveness and environmental benign nature. Natural melanin can be classified as eumelanin, pheomelanin, neuromelanin, allomelanin, and pyomelanin [18–21]. Eumelanin is the common type of natural melanin, and is composed of disordered heteroaromatic networks based on 5,6-dihydroxyindole (DHI) and 5,6dihydroxyindole carboxylic acid (DHICA) building blocks, which are catechol-derivatives [22,23]. From now on, natural eumelanin will be referred as natural melanin (NM) in this study. NM possesses unique properties including broadband light absorption, photoconductivity, and free radical scavenging, among others [24–27]. This has recently ignited the popular trend of applying NM, its synthetic analogouspolydopamine (PDA), and their hybrid materials in bio-inspired hightech materials for bioelectronics [22,28–32], environment sensors [33,34], polymers [35–38], photothermal agents [39–42], and

Corresponding authors. E-mail addresses: [email protected] (L. Sun), [email protected] (X. Wang).

https://doi.org/10.1016/j.jphotochem.2019.112292 Received 16 October 2019; Received in revised form 25 November 2019; Accepted 4 December 2019 Available online 05 December 2019 1010-6030/ © 2019 Published by Elsevier B.V.

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2.3. Preparation of natural melanin/TiO2 hybrid

pollutants (dyes or heavy metal ions) removal [43–47], molecular imaging [48] and cell labelling [49]. A few studies have focused on developing visible-light responsive TiO2-based photocatalysts using PDA, which is a “eumelanin-like” material [16,50,51], but some limitations still exist in this area. For example, the synthesized method used by Wang et al. to prepare PDA/ TiO2 hybrids was template- and carbonization process-required, which was a complicated process [50]. Mao et al. and Zhou et al. did not thoroughly discuss the origin of the visible light-induced behavior and the associated working mechanisms of the synthesized hybrids [16,51]. Additionally, it is still unclear if the existing hybrid systems would be suitable for removing heavy metal ions, or even for a practical textile dyeing effluent treatment. There are some fundamental differences between the features of PDA and NM. For instance, PDA is a synthetic material that has superior adhesive property and can easily form a layer to bind to the outer surface of materials. The thickness of this layer can be tuned by controlling the substrate concentration, the reaction medium and so on [52]. But NM particles are biosynthesized and are polymerized particles with certain shapes (ovoid/spherical/ellipsoidal) and sizes [53]. Moreover, NM has limited adhesive property and therefore, NM-based hybrids can only be fabricated by anchoring other materials on NM particles using its functional groups. To this end, our work isolated the NM granules from textile scraps such as yak hair wastes and then, through a simple one hot pot method, TiO2 nanoparticles were in-situ synthesized on the isolated NM to obtain NM/TiO2 hybrids. Our results revealed that the catechol groups of NM played an important role in forming CeOeTi bonds between NM and TiO2 nanoparticles, which narrowed the band gap of NM/TiO2 hybrids. This band gap narrowing phenomenon led to the excellent photocatalytic activity under visible light for simultaneous methyl orange (MO) and hexavalent chromium ions Cr (VI) removal. Kinetics of the simultaneous MO and Cr (VI) removal were thoroughly investigated to understand the working function of the NM/TiO2 hybrids. A practical wool dyeing effluent was also simulated to evaluate the practical use of the NM/TiO2 hybrids. In brief, our work develops advanced NM-based hybrids for the potential treatment of the textile dyeing effluent with a better exploitation of the sunlight. This also can bring significant implications to the further development of NM-based functional materials using other NM-enriched wastes such as human hair wastes, squid ink, etc.

NM/TiO2 hybrids were prepared using a hydrothermal method. First of all, TiO2 precursor was prepared through the same method reported in our previous study [3,14,56,57]. In short, 2 mL of HNO3 (70 %) was added into 5 mL of ethanol with vortex mixing, then 1 mL of TBT was added to the mixture and then diluted to 10 mL with ethanol (suspension A). NM (40 mg) was dispersed into ethanol (5 mL) and sonicated for around 15 min (suspension B). Afterwards, a certain volumes of suspension A (1, 2, 3, 4 mL) and suspension B were mixed uniformly, followed by dilution with ethanol (75 mL). The as-prepared suspension was then transferred into a 200 mL stainless autoclave for in-situ synthesis at 180 °C for 17 h. Finally, the NM/TiO2 hybrids were separated by centrifugation and washed twice with distilled water to obtain the final products. Synthesis of TiO2 was based on the same method in the absence of NM, while the NM-HNO3-180 °C was prepared in the absence of TiO2 precursor. 2.4. Physical characterizations Morphologies and structures of NM/TiO2 hybrids were characterized using scanning electron microscopy (SEM, Zeiss Supra 55 V P), and transmission electron microscopy (TEM, JEOL 2100 LaB6). X-ray diffraction (XRD) patterns of the samples were obtained using a PANalytical’s X’Pert Powder X-ray diffractometer (40 kV, 30 mA) with Cu-Ka radiation from 10° to 90°. The crystalline size of the nanoparticles was calculated based on XRD spectra using the Scherrer equation (Eq. (1)) [8]:

crystallie size =

0. 94λ × 180 FWHM × π × cosθ

(1)

Where λ is the X-ray wavelength (1.540598 Å), FWHM is the full width at half maximum of diffraction peak, and θ is the diffraction angle of the diffraction peak. Diffuse reflectance ultraviolet-visible-near infrared (UV–vis-NIR) spectra of the samples were determined using a Carry 5000 UV–vis-IR spectrophotometer at room temperature using BaSO4 as the reference. X-ray photoelectron spectroscopy (XPS) analysis were performed on the AXIS Nova (Kratos Analytical in Manchester, UK). Survey scan and high resolution of the 1 s orbitals of carbon (C), oxygen (O), nitrogen (N) and 2p orbital of titanium (Ti) were obtained. The XPS spectra were analyzed on the CasaXPS.

2. Experimental 2.5. Photocatalytic experiments and the detection of active species 2.1. Reagents and chemicals Photocatalytic performance of the NM/TiO2 hybrids was evaluated based on the photocatalytic degradation of methyl orange (MO) solution under visible light irradiation (420−800 nm). Typically, MO solution (50 mL, 10 mg L−1) was firstly prepared in a quartz beaker, followed by adjusting the pH of the solution to 2.5 using H2SO4. The synthesized TiO2 and NM/TiO2 hybrids were added to the solution. The weight of TiO2 content in different NM/TiO2 hybrids was kept at 5.4 mg for MO solution degradation test. The obtained suspensions were stirred in dark for 1 h to reach the adsorption-desorption equilibrium and were then irradiated in the Atlas Suntest CPS1 instrument (Ameteck, United States) equipped with air cooled xenon arc lamp (300−800 nm). The visible light (420−800 nm) was obtained by covering the quartz beaker with a GG420 filter box, which filtered the UV light with wavelength shorter than 420 nm. The temperature in the instrument was 35 °C and the power density was 350 W/m2. While stirring, 3 mL of the suspension was taken out every 1 h and was centrifuged to obtain the clear supernatant. In order to detect the active species involved in photocatalysis, 1,4-benzoquinone (BQ), disodium ethyelenediaminetetraacetate (Na2EDTA), and isopropanol were applied as scavengers for the superoxide radicals (%O2-), holes (h+), and hydroxyl radicals (%OH), respectively. The concentrations for BQ, Na2EDTA, and isopropanol were kept at 1 mM while other conditions were kept unchanged.

Hydrochloric acid (HCl, 32 %) was obtained from RCI Labscan Limited. The yak hairs were kindly provided by Zhongfuda Textile Co., Ltd. Tetrabutyl titanate (TBT, Ti(OC4H9)4, 97 %), nitric acid (HNO3, 70 %), sulfuric acid (H2SO4, 98 %), ethanol (100 %), commercial TiO2 (P25) (80 % anatase and 20 % rutile TiO2, 21 nm particle size), methyl orange (MO), 1,4-benzoquinone (BQ), disodium ethyelenediaminetetraacetate (NA2EDTA), isopropanol, 1,5-diphenyl-carbazide (DPC), Sepia officinalis melanin and synthetic melanin were purchased from Sigma-Aldrich (Australia). Potassium dichromate (K2Cr2O7) was provided by MERCK Pty Ltd. Albegal FFA and Albegal A were purchased from CIBA and sodium acetate was obtained from Chem-Supply. 2.2. Preparation of natural melanin NM was extracted from yak hair wastes using the acidic isolation method as previously reported [54,55]. In brief, yak hairs waste (2 g) was firstly washed with acetone and water, followed by drying at 60 °C overnight. The as-prepared yak hair were then immersed into HCl (32 %, 80 mL) at 100 °C for 3 h. Finally, the NM was collected by centrifugation and washed with distilled water until the supernatant showed a neutral pH level. 2

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high resolution (HR)-TEM image, where TiO2 nanoparticles were clearly observed on the surface of NM (Fig. 1d and e). When the TiO2 loadings were 70 wt%, 80 wt% and 82 wt%, surplus TiO2 nanoparticles aggregated to form a 40–60 nm TiO2 layer on the hybrid particles (Fig. S3c–S3f). XRD pattern (Fig. 2a) of NM showed a dominant peak centred at 2θ ∼21.4°. A similar peak at ∼21.4° had already been reported in the synthetic melanin thin films as the result of the X-ray diffraction from parallel planar layers of the nano-aggregates in melanin [58]. The XRD pattern of the synthesized TiO2 showed several distinct peaks representing the formation of the anatase TiO2 (JCPDS, no. 21-1272) [3], which is consistent with the observed results from TEM results (Fig. 1f). Remarkably, these characteristic peaks were more visible on the XRD patterns of the NM/TiO2 hybrids with the increased TiO2 amount, confirming the successful loading of TiO2 nanoparticles on the NM particles. Furthermore, the average size of the synthesized TiO2 was calculated as around 6.5 nm, which is consistent with the size of TiO2 particles observed from their TEM and HR-TEM images as shown in Fig. 1c, f. Further insights to the chemical interactions between NM and TiO2 nanoparticles were provided by the XPS analysis on NM/TiO2-53 wt%. It should be noted that the TiO2 nanoparticles were synthesized in the presence of a strong acid (HNO3, 70 wt %) from the TiO2 precursor (TBT) at high temperature (180 °C). To accurately understand the influences of the TiO2 loaded on NM, it is important to rule out the effects of acid and temperature. In this regard, the XPS results of NM/TiO253 wt% were compared with those of NM treated with acid at high temperature, which was denoted as NM-HNO3-180 °C. The XPS survey of the NM-HNO3-180 °C and NM/TiO2-53 wt% and Ti 2p spectra are shown in Fig. S4. In Fig. 2b, the O 1s spectra of NM-HNO3-180 °C had two peaks at 532.458 and 533.152 eV, corresponding to the CeOH and eCOOH, respectively. In the O 1s of the NM/TiO2-53 wt%, two new peaks at 529.983 and 530.989 eV were observed, which were due to the presence of TieO and TieOH, respectively. Notably, the CeOH peak showed a very significant shift (0.59 eV) that might be induced by the formation of C-O-Ti bonds between a titanium atom of TiO2 and an oxygen atom linked to the aromatic protomolecules of NM. In contrast, the peak representing eCOOH showed invisible changes, suggesting that eCOOH did not participate in the formation of TiO2 nanoparticles. Furthermore, the N 1s spectra of NM-HNO3-180 °C had two characteristic peaks locating at 398.211 and 400.058 eV, ascribing to –NH and CeN, respectively. These peaks also did not show observable changes in the N 1s spectra of NM/TiO2-53 wt% (Fig. 2c). Similarly, in the C 1s spectra of NM-HNO3-180 °C, three peaks could be deconvoluted, locating at 284.8, 286.285 and 288.709 eV (Fig. 2d), which were attributed to CeC/C]C, CeN/CeO and C]O bonds, respectively. After loading TiO2, these peaks did not present observable shifts, indicating that the TiO2 loading barely affected these groups. All the XPS analysis results revealed that the CeOH from the catechol groups of NM played a major role in forming the CeOeTi bonds in NM/TiO2 hybrids. This finding is consistent with the work from Karthik et al., who confirmed the Ti-OeC bonds in the catechol-TiO2 complexes, and claimed that these bonds were the result of orbital hybridization between 2p(O), π(C) orbitals of catechol (ligand) and 3d(Ti) orbital of TiO2 [59].

Simultaneous removal of the MO and Cr (VI) was investigated at different pH of the solutions (pH = 1.5, 2.5, 5) and ratios between MO and Cr (VI) (1:1, 1:2, 1:3). The removal trends of MO and Cr (VI) were monitored by recording the absorbance values of their UV–vis. absorption spectra at λmax=507 nm and λmax=540 nm, respectively, using the 1,5-diphenylcarbazide (DPC) colorimetric method [3]. Typically, 2 mL of the supernatant was added with 7.5 mL distilled water and 0.2 mL DPC, followed by being adjusted to pH = 2 using the H2SO4. The DPC solution containing Cr (VI) was then left still for 10 min and the absorbance value at 540 nm was recorded from its UV–vis. absorption spectra. The removal of MO and Cr (VI) was calculated using Eq. (2):

MO/Cr (VI) removal =

C Co

(2)

The pseudo-first-order kinetic model was used to evaluate the removal kinetic of the MO and Cr (VI) removal, and its equation can be expressed as Eq. (3):

ln (

C ) = −kt C0

(3)

The Co and C in both Eqs. (2) and (3) are the initial absorbance of the original solution and the solution at different irradiation time intervals, respectively. k is the pseudo-first-order rate constant and t is the irradiation time in Eq. (3). 2.6. Simulated wool dyeing effluent A wool dyeing effluent was simulated by including dyes, heavy metal ions (Cr (VI)) as well as auxiliaries (Table 1) [3]. 3. Results and discussion 3.1. Characterizations In this study, natural melanin (NM) obtained from yak hair wastes were mainly ellipsoidal eumelanin particles (Figs. S1 and S2) with the longitudinal and transversal length at around 822 and 409 nm, respectively [53]. The basic units of natural eumelanin are catechol derivatives, dihydroxyindole (DHI) and dihydroxyindole-carboxylic acid (DHICA), which assembled through π-π stacking and hydrogen bonding, forming small aggregations which are arranged in the concentric rings (Fig. 1a and b) [17,28]. The size of the synthesized TiO2 nanoparticles was around 5−7 nm, which is consistent with our previous results (Fig. 1c) [3]. The crystallite lattice (0.35 nm) of the TiO2 nanoparticles corresponded to the (101) plane of the anatase TiO2, indicating that the synthesized TiO2 is in the anatase phase (Fig. 1f). Different NM/TiO2 hybrids were obtained by adding different volumes of TiO2 precursor. Specific TiO2 weight percentage was determined as 53 ± 8 wt%, 70 ± 3 wt%, 80 ± 4 wt%, and 82 ± 1 wt% with the addition of 1, 2, 3, 4 mL TiO2 precursor, respectively (Fig. S3a–S3b). The TEM image of NM/TiO2-53 wt% showed that the periphery of NM was slightly covered by TiO2 nanoparticles, which was confirmed by Table 1 Components of the simulated wool dyeing effluent.

3.2. Formation mechanism

Targeted pollutants

Concentration (mg L−1)

MO Cr (VI)

2.5 5

Auxiliary chemical

Function in the dyeing and /or rinsing process

Concentration (g L−1)

Albegal FFA Albegal A Sodium acetate

Wetting agent Levelling agent pH buffer

0.5 1 % o.w.f 1 % o.w.f

Based on the above characterization results, the formation mechanism of NM/TiO2 hybrid can be proposed. Catechol moieties have been reported to show affinities towards multivalent cations [22]. Therefore, by adding NM into the TiO2 synthesis system, the Ti4+ in TBT is prone to binding to the periphery of NM, where catechol moieties are presented. The adsorbed TBT was then hydrolyzed into TieOeC4H9 or TieOH, followed by the self-condensations to form TiO2 nanoparticles [60]. The hydroxyl groups (CeOH) of the catechol moieties in NM would compete with those hydrolyzed products to form 3

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Fig. 1. (a) TEM image of one NM particle; (b) HR-TEM image of the rectangular area from (a), the circle area represents the nano-aggregations of basic units of NM indicating they are composed of stacked planes arranged in the concentric ring; (c) TEM image of the synthesized TiO2 nanoparticles, and (f) is the HR-TEM image of the rectangular area in (c); (d) TEM image of the NM/TiO2-53 wt% particle; (e) HR-TEM image of the rectangular area in (d); (g) Scheme illustration of isolation and synthesis processes for NM and NM/TiO2 hybrids, respectively.

concentrations of MO solutions after mixing with different catalysts in dark showed neligible differences (Fig. S7). In the next step, through exposing the suspensions to visible light irradiation for 6 h, NM/TiO280 wt% hybrids induced ∼90 % degradation of MO (Fig. 3a). On the other hand, bare TiO2 (synthesized TiO2 and commercial TiO2-P25), and NM did not show any photocatalytic performance (Fig. 3a). A number of research studies have shown that the photocatalytic degradation of MO occurs by the cleavage of azo bonds (λmax) and degrading it into colourless smaller molecules [61]. Based on the UV–vis absorption of the MO solution, the characteristic absorption peak locating at 507 nm gradually decreased indicating the successful cleavage of the azo bonds (Fig. 3b). This would then trigger the successive demethylation and addition of hydroxyl groups, followed by the cleavage of the sulphonate group from the ring, finally producing CO2, H2O and NO32− [62]. The MO degradation by the NM/TiO2-80 wt%, and the XRD pattern of NM/TiO2-80 wt% showed insignificant change after several cycles of use under the visible light (Fig. S8 and Fig. 2a), indicating the excellent photostability of the NM/TiO2 hybrids. Typically, a semiconductor can be excited to produce electrons (e−) and holes (h+) under the illumination of light with wavelength shorter than its absorption edge. These e- and h+ can react with the surrounding absorbents to initiate redox reactions, representing the photocatalytic process [63,64]. TiO2 can only be excited by the UV light

CeOeTi bonds between NM and TiO2. As the catechol groups played the most important role in forming the CeOeTi bonds, NM is an excellent candidate to serve as a catechol-rich base that can be easily derived from abundant natural sources. On the basis of the above-discussed characterization results, the molecular structures of NM and NM/TiO2 hybrids were depicted in Fig. 2(e–f). Fig. 2e presents the monomer of NM (DHI or DHICA) before and after being loaded with TiO2 nanoparticles, while Fig. 2f shows the nano-aggregations formed in NM after linking to TiO2 nanoparticles. DHI/DHICA tetramer was presented as a representative because natural eumelanin was demonstrated to contain porphyrin-like protomolecules previously [28].

3.3. Photocatalytic performance under visible light (420−800 nm) The photocatalytic performance of four NM/TiO2 hybrids with different TiO2 contents (53 %, 70 %, 80 %, and 82 %) were evaluated under visible light irradiation. Here, we selected the NM/TiO2-80 wt% as the representative example for the detailed discussion because it showed the excellent rate for photocatalytic degradation of methyl orange (MO) under visible light (420−800 nm) irradiation (Fig. S5). The MO solutions with NM/TiO2-80 wt%, synthesized TiO2, P25 and NM were initially stirred in dark for 1 h to achieve the dye adsorption-desorption equilibrium (Fig. S6). It was observed that the 4

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Fig. 2. (a) XRD patterns of NM/TiO2 hybrids; O 1s (b), N 1s (c) and C 1s (d) spectra of NM-HNO3-180 °C and NM/TiO2-53 wt%; (e–f) Scheme of molecular structure of NM and NM/TiO2 hybrids.

The photo-generated h+ and e− can react with water and oxygen, respectively, to produce superoxide radicals (%O2-), and hydroxyl radicals (%OH), which are the main species involved in photocatalytic degradation of organic molecules [66,67]. To understand which species are responsible for degrading the MO by the NM/TiO2 hybrids, trapping experiments were conducted by adding 1,4-benzoquinone (BQ), isopropanol and disodium ethyelenediaminetetraacetate (Na2EDTA) in MO solutions, performing as the %O2-, %OH, and h+ scavengers, respectively. Fig. 4a shows the MO degradation trends in the presence of the above-mentioned scavengers whereas Fig. 4b presents their degradation rates calculated based on the pseudo-first-order model. It can be seen that the degradation rates for MO solutions were sequenced as: isopropanol (0.3598 h−1) > no scavenger (0.355 h−1) > Na2EDTA (0.16277 h−1) > BQ (0.01368 h−1). The significant inhibitions of the Na2EDTA and BQ suggested that both %O2- and h+ played very important roles in the MO degradation. Conversely, the addition of %OH scavenger (isopropanol) slightly increased the MO degradation rate. It should be mentioned that NM is known to be a scavenger for %OH and % O2-radicals [68]. As the scavenge rate for the %OH was 150–1500 times higher than that for the %O2-, the produced %OH was preferably

due to its wide band gap. However, the UV light only accounts for ∼5 % of the sunlight, and visible constitutes ∼45 %. It is known that narrowing the band gap is a key to extend the light response region of TiO2. The above results suggested that loading TiO2 on the NM successfully broadened its response to visible light. To understand the origin of the visible light-driven photocatalytic activity of the NM/TiO2 hybrids, we investigated the optical absorption of the NM/TiO2 hybrids. It can be seen that compared with the synthesized TiO2, the NM/TiO280 wt% hybrid showed greatly enhanced absorption in the visible and infrared regions (Fig. 3c). The band gap of the NM/TiO2-80 wt% hybrid was evaluated by the Kubelka-Munk function as shown in Fig. 3d. The band gap of the synthesized TiO2 was estimated as 3.1 ± 0.02 eV, very close to the band gap of the anatase TiO2 [65]. Here we found that the band gap of the prepared NM/TiO2 hybrids was 1.26 ± 0.2 eV after loading TiO2 on the NM particles. This might be attributed to the formation of CeOeTi bonds, which would induce the energy rearrangement between TiO2 and NM. The narrowed band gap led to a higher absorption edge for the NM/TiO2 hybrids, and therefore extended the excited light wavelength. It thus can be concluded that the NM/TiO2 hybrids can be excited by visible light (420−800 nm) to produce h+ and e−. 5

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Fig. 3. (a) Photocatalytic degradation of MO under visible light irradiation; (b) UV–vis. absorption spectra of MO solution under visible light irradiation; (c) Absorbance spectra of NM, synthesized TiO2 and NM/TiO2-80 wt% at (I) visible and (II) NIR light region; (d) KubelkaMunk function of the synthesized TiO2 and NM/ TiO2-80 wt%.

scavenged by the NM. In this regard, very low amount or even no %OH could participate in the MO degradation. However, the addition of isopropanol facilitated the consumption of %OH, which could promote the production of e- and h+. This further increased the population of % O2- and h+, benefiting to the MO degradation.

simultaneous removal of MO and Cr (VI) was achieved regardless of the MO/Cr (VI) ratio. Fig. 5a–b suggested that when the MO/Cr (VI) ratio increased from 1:1 to 1:2 and 1:3, the MO removal rate increased accordingly: 0.18088 h−1 (1:1) < 0.28506 h-1 (1:2) < 0.40573 (1:3). It is known that the reduction process of Cr (VI) consumed the e- [69], therefore the addition of Cr (VI) facilitated the generation of e- and h+, which contributed to the MO removal. Furthermore, increasing ratio of MO/Cr (VI) led to a decreased Cr (VI) removal rate: 1:1 (0.39978 h−1) > 1:2 (0.32911 h−1) > 1:3 (0.11514 h−1) (Fig. 5c–d). This is because the increased Cr (VI) concentration induced more Cr (VI) ions to compete with each other for the consumption of e-, thereby lowering the photocatalytic reduction rate of Cr (VI).

3.4. Simultaneous removal of MO and Cr (VI) under visible light (420−800 nm) It is known that textile dyeing effluent contains both organic dyes and inorganic heavy metal ions. Particularly, wool dyeing industry commonly uses mordant (i.e. K2Cr2O7) for improving the wet and light fastness of the dyed wool fibers [3]. It is therefore of significance to investigate the performance of simultaneous removal of MO and Cr (VI) using the NM/TiO2 hybrids and the role of influential factors.

3.4.2. Influence of pH The influence of pH on simultaneous removal of MO and Cr (VI) was investigated at different pH from 1.5 to 5 (Fig. 6). It can be seen that through decreasing pH, both MO and Cr (VI) removal rates increased. This suggested that acidic condition favored in boosting the simultaneous removal of MO and Cr (VI) with NM/TiO2 hybrids. It was noted

3.4.1. Influence of MO/Cr (VI) ratio The Cr (VI) was added to the MO solution (10 mgL−1) at MO/Cr (VI) concentration ratios of 1:1, 1:2 and 1:3. It was realized that

Fig. 4. (a) Dynamic curve of MO degradation in trapping experiments for NM/TiO2-80 wt%; (b) Plots of –ln(C/Co) versus time for MO degradation in trapping experiments. 6

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Fig. 5. Dynamic curve of (a) MO degradation and (c) Cr (VI) reduction; Plots of –ln(C/Co) versus time for (b) MO degradation and (d) Cr (VI) reduction in mixed solutions (50 mL, pH = 2.5) of MO (10 mg L−1) and Cr (VI) (10 mg L−1, 20 mg L−1 and 30 mg L−1) by NM/TiO2 hybrids (6.75 mg).

consuming e-. The associated mechanism in simultaneous removal of MO and Cr (VI) by NM/TiO2 hybrids favored by an acidic condition can be summarized as follows. Firstly, the reduction of Cr (VI) in acidic solutions consumes the available H+, therefore, having a more acidic condition can provide a larger amount of H+, contributing to the Cr (VI)

that at pH 1.5, Cr (VI) removal process underwent two stages. In the first three hours (stage 1: 0−3 h), MO removal reached ∼91 % (Fig. 6a), resulting in only Cr (VI) solution in the next three hours (stage 2: 4−6 h). In this regard, removal rate of Cr (VI) at stage 2 (1.74918 h−1) was much higher than that of stage 1 (0.16157 h−1), because no other pollutants (MO) was available to compete with Cr (VI) ions in

Fig. 6. Dynamic curve of (a) MO degradation and (c) Cr (VI) reduction; Plots of –ln(C/Co) versus time for (b) MO degradation and (d) Cr (VI) reduction in mixed solutions (50 mL, pH = 1.5, 2.5, 5) of MO (10 mg L−1) and Cr (VI) (30 mg L−1) by NM/TiO2 hybrids (6.75 mg). 7

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Fig. 7. Dynamic curve of (a) MO degradation and (c) Cr (VI) reduction in the simulated wool dyeing effluent; (b) Simultaneous removal of MO and Cr (VI) by NM/ TiO2 hybrids (6.75 mg) in the simulated wool dyeing effluent for four consecutive cycles.

reduction [70]. Furthermore, the MO molecule structure is converted to quinone structure which is much easier to be oxidized than the azo structure [71]. Secondly, the zero charge point of the TiO2 (pzc) is between 6 and 7, and a lower pH leads to a more positively-charged surface of the TiO2 [71]. Since the NM/TiO2 hybrids were covered by the TiO2 nanoparticles as revealed by Fig. S3e, it can be concluded that a lower pH leads to a more positively-charged surface of the NM/TiO2 hybrids accordingly. MO molecule is anionic in nature and Cr (VI) distributes in the solution mainly in the form of HCrO4−, which is also an anionic specie [72]. Therefore, both MO and Cr (VI) have higher affinities towards the NM/TiO2 hybrids due to the electrostatic attractions in a more acidic condition. This favorable adsorption is beneficial for the contact between the pollutants and the produced reactive species such as e−, h+, %OH and O2, thereby promoting the MO and Cr (VI) removal.

MO + %O2− → degradation products (path 3) MO + h+ → degradation products (path 3) 4. Conclusions In summary, we have designed a visible light-driven photocatalytic material based on natural melanin (NM) and TiO2 nanoparticles. The results revealed that the catechol groups in NM played a dominant role in obtaining NM/TiO2 hybrids with narrowed band gap. The working mechanism of this photocatalyst for the simultaneous removal of dyes and heavy metal ions was then comprehensively investigated. The NM/ TiO2 hybrids showed excellent simultaneous removal of MO and Cr (VI) in a simulated wool dyeing effluent. Its performance under visible light was well-maintained after several consecutive cycles of use, indicating its high stability. The present study offers new insights of developing novel photocatalytic materials based on NM, which is a potential material for dyeing effluent treatment with a better solar light utilization.

3.4.3. Simulated wool dyeing effluent In order to evaluate the performance of the NM/TiO2 hybrids for an industrial dyeing effluent, a wool dyeing effluent with the presence of auxiliaries was simulated, containing wetting agent, levelling agent and a pH buffer (Table 1). Fig. 7 shows the simultaneous MO and Cr (VI) removal in this wool dyeing effluent using the NM/TiO2 hybrids under visible light. It can be seen that the MO and Cr (VI) were removed simultaneously in the simulated wool dyeing effluent, suggesting that the complex compositions of the wool dyeing effluent did not hamper the performance of NM/TiO2 hybrids under visible light. Most importantly, it was shown that the simultaneous MO and Cr (VI) removal changed insignificantly after four consecutive cycles, indicating the prominent stability of the NM/TiO2 hybrids for the pollutants removal under visible light irradiation.

CRediT authorship contribution statement Wanjie Xie: Conceptualization, Methodology, Investigation, Writing - original draft, Data curation. Esfandiar Pakdel: Writing review & editing, Data curation. Yujia Liang: Writing - review & editing. Dan Liu: Supervision, Writing - review & editing. Lu Sun: Supervision, Resources, Writing - review & editing, Funding acquisition, Project administration. Xungai Wang: Supervision, Writing - review & editing. Declaration of Competing Interest

3.5. Working function of the NM/TiO2 hybrids The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

On the basis of the above-discussed results, a working function of the NM/TiO2 hybrids for simultaneous removal of MO and Cr (VI) under visible light has been illustrated in Scheme 1. When NM/TiO2 hybrids are irradiated by visible light, electrons and holes are produced. The generated electrons reduce oxygen to produce %O2− (path 1a), or to reduce the Cr (VI) to Cr (III) (path 1b). While the h+ can react with water to produce %OH (path 2), and then be scavenged by the NM (path 2). The %O2− and h+ would participate in the MO degradation (path 3).

Acknowledgements The authors gratefully acknowledge financial support from the Australian Research Council Discovery Early Career Research Award Scheme (DE150101617) and the Research Excellence Grant Scheme of Deakin University. The 1st author also would like to acknowledge the Australian Government Research Training Program Fees Offset Scholarship and Deakin University Postgraduate Research Scholarship. The 2nd author acknowledges the support of the Alfred Deakin Postdoctoral Research Fellowship. Deakin University’s Advanced Characterization Facility is acknowledged for the use of the JEOL

NM/TiO2 + hν → NM/TiO2 (e− + h+) e- + O2 → %O2− (path 1a) HCrO4− + 7+ + 3e-→ Cr3+ + 4H2O (path 1b) h+ + H2O → %OH + H++ NM/TiO2 → NM/TiO2 (%OH) (path 2) 8

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Scheme 1. Schematic diagram of the working function of NM/TiO2 hybrids as photocatalyst under visible light.

2100F and the assistance from Rosey Squire. The Australian National Fabrication Facility (ANFF), a company established under the National Collaborative Research Infrastructure Strategy, through the La Trobe University Centre for Materials and Surface Science is acknowledged for the XPS analysis.

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