Activated carbons for the adsorption of ibuprofen

Activated carbons for the adsorption of ibuprofen

Carbon 45 (2007) 1979–1988 www.elsevier.com/locate/carbon Activated carbons for the adsorption of ibuprofen A.S. Mestre a,b , J. Pires a,b , J.M...

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Carbon 45 (2007) 1979–1988 www.elsevier.com/locate/carbon

Activated carbons for the adsorption of ibuprofen A.S. Mestre

a,b

, J. Pires

a,b

, J.M.F. Nogueira

a,c

, A.P. Carvalho

a,b,*

a

c

Departamento de Quı´mica e Bioquı´mica, Universidade de Lisboa, Faculdade de Cieˆncias, Campo Grande Ed. C8, 1749-016 Lisboa, Portugal b Centro de Quı´mica e Bioquı´mica, Universidade de Lisboa, Faculdade de Cieˆncias, Campo Grande Ed. C8, 1749-016 Lisboa, Portugal Centro de Cieˆncias Moleculares e Materiais, Universidade de Lisboa, Faculdade de Cieˆncias, Campo Grande Ed. C8, 1749-016 Lisboa, Portugal Received 22 February 2007; accepted 5 June 2007 Available online 12 June 2007

Abstract Powdered activated carbons prepared from cork waste were studied for the ibuprofen removal from liquid phase. Two carbons were used: CAC obtained by chemical activation with K2CO3, and CPAC prepared by a two-step method, chemical activation with K2CO3 followed by steam activation. The ash content analysis showed that, for this raw material, the previous acid treatment can be omitted. The textural properties of the samples, evaluated by low temperature N2 adsorption, show that the main difference is related with the volume of the larger micropores (supermicropores), which is more developed for CPAC. The surface chemistry characterization, made by the determination of the point of zero charge (PZC) and Boehm’s titration, show that the second activation step led to an activated carbon with less acidic groups, associated with the absence of the strongest acidic groups. Kinetic and equilibrium adsorption data show that the process obeys to the pseudo-second order kinetic equation and Langmuir adsorption model. Between 25 and 40 C no significant influence of the temperature on ibuprofen adsorption was observed. Results indicate that the removal efficiency is higher than 90% between pH 2 and 4 and decreases as pH values increase to a value of 11. The results show that both samples are suitable for ibuprofen removal, although CPAC has advantages, namely, high initial adsorption rate, high adsorption capacity and high removal efficiency, in some cases 100%, for a large range of pH.  2007 Elsevier Ltd. All rights reserved.

1. Introduction Pharmaceuticals and personal care products (PPCPs) are important and indispensable elements of modern life. This group of substances describes a large class of chemical contaminants that can arise from human usage and excretions and also veterinary applications of a variety of products, such as over-the-counter and prescription medicines, fungicides and disinfectants used for industrial, domestic and agricultural practices [1]. PPCPs are an environmental emerging concern since this class of products comprises not only new compounds commercially available, but also many substances that were recently detected, due to the development of powerful * Corresponding author. Address: Departamento de Quı´mica e Bioquı´mica, Universidade de Lisboa, Faculdade de Cieˆncias, Campo Grande Ed. C8, 1749-016 Lisboa, Portugal. Fax: +351 217500088. E-mail address: [email protected] (A.P. Carvalho).

0008-6223/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.06.005

analytical strategies that decrease the detection limits at the sub-ppt (ng dm3) level [2]. The maximum concentration admissible for PPCPs in the environment are, in most cases, still unregulated, most probably because there is a lack of knowledge about the long-term effects of continuous exposure to these compounds and their metabolites, even if they are present in the environment at trace level [1,3]. On the other hand, these contaminants do not need to persist long time in the environment to cause negative effects. In fact, even if they could have high transformation/removal rates, the levels in the environment must remain almost constant due to a continuous release [3]. Ibuprofen is one of the most consumed medicines all over the world. For instance, in the United Kingdom it is one of the top five most consumed drugs, having an estimated annual production of several kilotons [4]. It is a nonsteroidal anti-inflammatory (NSAID), analgesic and antipyretic drug widely used in the treatment of rheumatic disorders, pain and fever. Being slightly soluble in water it

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is readily soluble in organic solvents, and so it has high mobility in the aquatic environment. Measurements made on wastewaters and rivers of Germany showed that ibuprofen is present in concentrations in the order of ppb (lg dm3) [5]. Recent studies concerning drinking or wastewater treatments for PPCPs removal revealed that conventional processes (coagulation, flocculation and sedimentation) are not totally effective [6]. On the contrary, for instance, oxidation with chlorine and ozone, and activated carbon based processes show to be more efficient for the abatement of PPCPs [6,7]. Then the use of activated carbons based technologies is a possible option to eliminate PPCPs from aqueous medium, or act as concentrators of those pollutants for analysis purposes. In fact, activated carbons may, virtually, remove any PPCP since the existence of benzene rings or amine groups in the structure of the majority of these compounds enhance their ability to be adsorbed by the activated carbon. In the literature there is a considerable number of works concerning the use of activated carbons in the treatment of liquid effluents. These studies are mainly focused on the adsorption of metals [8,9], dyes [10–12], phenolic compounds [10,12–14] and endocrine-disrupting compounds [15,16]. To our knowledge, in what concerns the removal of PPCPs by activated carbons there are only few studies, and in all the cases commercially available carbons were employed [6,7,15,16]. It should be stressed also that these studies are usually made in an analytical chemistry point of view, so there is no detailed textural or surface chemistry characterization of the activated carbons, what is essential to optimize the removal performance. Concerning the quantities of wastewaters that have to be treated in industrialized societies, and the fact that environmental regulations are becoming more stringent, it is important to develop low cost technologies for decontamination processes, keeping high removal efficiencies. Activated carbons obtained from agricultural or industrial sub-products, i.e. wastes, are good alternatives to be used in adsorption based technologies. In Portugal, cork processing is an important industrial sector giving rise to large amounts of cork waste powder. In the last years, we have used this environmental friendly and low cost material as precursor to prepare activated carbons that presented high apparent specific surface areas and good performance when tested as adsorbents for noxious volatile organic compounds [17–19]. These results justify the evaluation of the potentialities of cork based activated carbons in the removal of pollutants from solution and, in the case of the present study, a PPCP was selected, namely, the ibuprofen. In this way, kinetic and equilibrium adsorption data were obtained and the effect of several experimental parameters (temperature, pH and initial ibuprofen concentration) in the adsorption process was evaluated. Textural and surface chemistry characterization of the adsorbents was also considered and its influence on the liquid phase adsorption performance of the materials is discussed.

2. Experimental 2.1. Materials Two activated carbon samples – CAC and CPAC – were prepared using as raw material cork powder waste, as supplied. The CAC sample was obtained by chemical activation of cork with K2CO3, according with the procedure previously optimized [19]. Briefly, cork powder (fraction < 0.297 mm) was mixed with ground K2CO3, in a 1:1 weight proportion, and calcined in a horizontal furnace (Thermolyne, model 21100) under nitrogen flow (5 cm3 s1). Temperature was raised (10 C min1) up to 700 C and kept for 1 h. After cooling, under nitrogen flow, the sample was washed with distilled water until pH 7 and dried at 100 C. The CPAC sample was prepared by physical activation of CAC with steam using nitrogen (5 cm3 s1) as carrier flow. The steam was generated from the water vapour pressure at ambient temperature; therefore the water partial pressure was around 0.03. To perform the steam activation CAC was placed in a quartz boat, heated (20 C min1) up to 750 C and kept for 1 h, after what it was cooled to ambient temperature.

2.2. Methods The ash content of the activated carbons was estimated by the mass residue left after the combustion of the samples in air, according to a procedure adapted from Ref. [20]. Initially ca. 1 g of activated carbon was placed in a quartz boat and dried overnight at 105 C. After weighting the dried sample in a Mettler AE 240 analytical balance, it was introduced in a horizontal furnace equipped with a Eurotherm 2416 controller. The temperature was first raised from ambient to 500 C in 10 min, kept for 30 min and then raised to 815 C in 15 min and kept for 2 h 30 min. After cooling to a temperature near 150 C, the sample was placed in a desiccator to reach the ambient temperature and then weighted. The ash content (mean of three essays) was expressed by dried mass of activated carbon. The point of zero charge (PZC) of the samples was determined by reverse mass titration, following the method proposed by Noh and Schwarz [21]. Slurries of (in %) 1, 2, 6 and 10 were prepared by mixing the powdered activated carbon with ultra-pure water in a glass bottle, bubbled and sealed under N2 (to eliminate CO2). Ultra-pure water was obtained from Milli-Q water purification systems (Millipore, Bedfore, MA, USA). The pH of the slurry was measured after shaking for, at least, 24 h. Plotting the equilibrium pH as a function of solid weight fraction a curve is obtained. The equilibrium pH at the plateau of the curve corresponds to the PZC. Surface functional groups such as carboxyl (R–COOH), lactone (R– OCO), phenol (R–OH), carbonyl or quinone ([email protected]) were quantified using Boehm’s method [22,23]. Four basic reagents: sodium ethoxide (NaOC2H5, Merck, 95%), sodium hydroxide (NaOH, Riedel-de Hae¨n, Fixanal), sodium carbonate (Na2CO3, Riedel-de Hae¨n, 99.5100%) and sodium hydrogen carbonate (NaHCO3, Panreac PA) were used. Surface functional groups were quantified by assuming the reactivity of each reagent as follows [22]: NaOC2H5 reacts with all groups; NaOH does not react with the [email protected] group; Na2CO3 does not react with [email protected] and R–OH; NaHCO3 only reacts with R–COOH. Experimentally, about 0.5 g of each sample was placed in a 100 cm3 flask. After adding 25 cm3 of a 0.05 N solution of each basic reagent, prepared with ultra-pure water, the mixture was vigorously agitated for 48 h, at room temperature. After recovering the solution by filtration, four aliquots of 5 cm3 were backtitrated with 0.1 N of HCl (Riedel-de Hae¨n, Fixanal). Nitrogen adsorption isotherms at 196 C were determined in a conventional volumetric apparatus equipped with an MKS-Baratron (310BHS-1000) pressure transducer (0–133 kPa). Before the isotherms acquisition the samples of activated carbon (50 mg) were outgassed for 2 h at 300 C, under vacuum better than 102 Pa. Ibuprofen was synthesized by Shasun Chemicals and Drugs Ltd. (lot IBU0307598). It has a molecular weight of 206.28 g mol1 and its molecular structure is depicted in Fig. 1. The dimensions of the ibuprofen were estimated by molecular modelization with Gaussian-03 [24] employing the

A.S. Mestre et al. / Carbon 45 (2007) 1979–1988

1981

≈ 1.30 nm

COOH

≈ 0.34 mn

b

≈ 0.52 nm

≈ 1.03 nm

Fig. 1. Molecular structure of ibuprofen. (a) General structure, (b) and (c) arrangement of the atoms in space presenting interatomic distances estimated according with the molecular modelization made with Gaussian-03 employing the semi-empirical method PM3. semi-empirical method PM3 [25,26] for the optimization. From this model the dimensions of the ibuprofen molecule are (in nm) 1.03 (lenght) · 0.52 (width) · 0.43 (thickness), as displayed in Fig. 1 for the lowest energy configuration. All the ibuprofen solutions contained 10% of methanol (Chromasolv for HPLC, 99.9%, Sigma–Aldrich) and were prepared with ultra-pure water. The solutions used in kinetic and equilibrium essays were prepared without pH adjustment, presenting values around 4. The initial and residual concentrations of ibuprofen were analysed by high performance liquid chromatography (HPLC). The analysis were carried out on a benchtop Agilent 1100 series LC chromatographic system (Agilent Technologies, Waldbronn, Germany) equipped with a vacuum degasser (G1322A), autosampler (G1313A), thermostated column compartment (G1316A), quaternary pump (G1311A) and a diode array detector (G1315A). The analyses were preformed on a Tracer excel 120 OctaDecilSilica-A column, 150 mm · 4.0 mm, 5 lm particle size (120 ODS-A, Teknokroma, Spain). The mobile phase consisted on a mixture of 75% (v/v) methanol solution (solvent A) and ultra-pure water with 0.1% H3PO4 (solvent B), with flow of 1 ml min1. The detector was set at 220 nm, the column temperature was maintained at 25 C and the injection volume was 40 ll with a draw speed of 200 ll min1. For quantification purposes a calibration plot was performed under the instrumental conditions used. To study the effect of the initial concentration of ibuprofen (20, 40 and 60 mg dm3) on adsorption kinetics, 15 cm3 of ibuprofen solution were mixed with ca. 10 mg of activated carbon in glass vials (Macherey-Nagel, Du¨ren). After introducing a magnetic stir bar, the vials were sealed using a hand crimper. The sealed vials were placed in a water bath at 30 C (controlled with a Eurotherm 2216L device) and stirred at 700 rpm in a multipoint agitation plate (Variomag Multipoint). The time recording was started when the stirring began and several samples were collected between 5 min and 6 h. After filtration, using cellulose acetate filters with a pre-filter of glass fiber (Minisart Plus 0.45 lm CA/GF), the amount of ibuprofen was determined. Ibuprofen uptake was calculated according to the following equation: qt ¼

ðC 0  C t Þ V W

ð1Þ

where qt is the amount (mg g1) of ibuprofen adsorbed at time t, C0 is the ibuprofen initial concentration (mg dm3), Ct is the ibuprofen concentration at time t (mg dm3), V is the volume (dm3) of the adsorbate solution and W is the weight (g) of dried carbon. Equilibrium adsorption studies were made varying the adsorbent doses (2.5–10 mg), the solution volumes (15 and 30 cm3) and the ibuprofen concentrations (20–120 mg dm3). After stirring for 4 h the concentration of

ibuprofen remaining in solution at equilibrium (Ce) was determined and the uptake (qe) was calculated using Eq. (1). The stirring time of 4 h was selected for the equilibrium essays since, according with the results of the kinetic essays; between 2 and 6 h the ibuprofen uptake was practically the same and did not change even after 24 h of stirring. Equilibrium adsorption isotherms were defined at different temperatures: 25, 30 and 40 C. The influence of initial pH, at 30 C, was studied over a pH range between 2 and 11. For this, hydrochloric acid or sodium hydroxide were added to the ibuprofen solution to adjust the desired pH. The measurements were made in a pH meter from Metrohm (model 744). It must be noted that prior to use, the activated carbons were dried overnight in a ventilated oven at 110 C and all the solution adsorption essays were made in triplicate.

3. Results and discussion 3.1. Ash content Ash content is one characteristic of the activated carbon that can influence its adsorption performance. To reduce the ash content an acid treatment of the raw material is usually the first step in the methodology of the preparation of activated carbons. Besides being time-consuming this step is not an environmental friendly procedure, since it uses large quantities of, usually, 10% solution of H2SO4, what implies the consumption of large volumes of water to leach all the acid. To check the importance of this preliminary treatment when cork is used as carbon precursor, the ash content of acid treated and untreated cork samples was determined. The results reported in Table 1 show that the acid treatment partially removes inorganic compounds from cork reducing, consequently, its ash content. However, it must be noted that untreated cork has a very low ash content comparing with the literature data for activated carbons [10,11,27,28]. From these results one could predict that in this case the previous acid treatment would not be necessary. The results of sample CAC prepared from untreated and treated cork show that, in fact, the use of acid

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Table 1 Results of ash content analysis of cork, CAC and CPAC samples, obtained with and without preliminary treatment with H2SO4

18 16

Ash (%) CAC

CPAC

0.3 0.1

4.1 3.0

12.8 –

treatment leads to a decrease of only 1% in the ash content. Accordingly with these results, in the case of CPAC sample only untreated cork was used as raw material. The ash content of this sample, although higher than that of CAC also obtained from untreated cork, compares favourably with the literature data [10,11,27,28]. Therefore, both samples used in this study were prepared from untreated cork. 3.2. Surface chemistry characterization of the activated carbon samples The results of the PZC measurements and Boehm’s titration are quoted in Table 2. From these results it is clear that the preparation procedures led to activated carbon samples with different surface chemistry properties. CAC sample has a more acidic surface than that of CPAC material, since it presents the lowest PZC value. The surface oxygen group quantification by Boehm’s titration corroborates this conclusion since the total number of oxygen functional groups present in CPAC sample is considerably lower than that obtained for CAC and, on the other hand, the more acidic groups, R–COOH and R–OCO, were not detected on the sample submitted to the steam activation step (CPAC). 3.3. Nitrogen adsorption The N2 adsorption isotherms at 196 C, presented in Fig. 2, disclose the essential microporous nature of both activated carbons associated with a mesoporous structure and/or external surface area. The CPAC sample presents higher adsorption capacity and the adsorption isotherm has a rounder off knee, than CAC, revealing that the physical activation step led to a more pronounced development of porosity, related with the widening of the microporous size distribution due to the formation of larger micropores (supermicropores). Apparent specific surface area, ABET, assessed applying the BET equation (in the range 0.05 < p/p < 0.15) [29], Table 2 Points of zero charge (PZC) and surface functional groups of the samples Sample

PZC

Surface functional groups R–COOH

R–OCO

R–OH

[email protected]

Total

26 43

163 110

meq/100 g activated carbon CAC CPAC

7.5 9.9

15 0

42 0

80 67

n ads / mmol g-1

Untreated Treated

14

Cork

12 10 8 6 CPAC 4

CAC

2 0 0

0.2

0.4

0.6

0.8

1

p/p0 Fig. 2. Nitrogen adsorption–desorption isotherms at 196 C in the mentioned samples (closed symbols are desorption points).

microporous volume, VDR, from the Dubinin–Radushkevich (DR) equation [29], mean micropore half width, L0, evaluated from the characteristic adsorption energy and E0, obtained from the slope of the DR plots and using the empirical equation L0 ¼ ð13:028  1:53  105 E3:5 0 Þ=E 0 [30] are quoted in Table 3. The microporosity characterization was complemented applying as method, taking as reference the isotherm reported in Ref. [31]. With this method the values of total, Vatotal, ultra (width less than 0.7 nm), Vaultra, and supermicropore (width between 0.7 and 2 nm), Vasuper, volumes were obtained [29]. The values are displayed in Table 3. From the values of apparent specific surface area and porous volumes, the difference in the porous structure characteristics of the two samples is evident; CPAC sample presenting the highest values. However, the main textural difference between the two samples is related with the supermicroporous volume, that is, the widest micropores, which almost triple after the physical activation with steam. In parallel, the volume correspondent to the smallest micropores decreases, which indicates that, as a consequence of the steam activation, at least a fraction of the ultramicropores is converted into larger micropores. In line with these results an increase of L0 values (the pores half width) is observed. Considering the molecular dimensions of ibuprofen presented before, it is clear that this adsorbate can access to the microporosity of both solids. 3.4. Adsorption from solution 3.4.1. Adsorption kinetics The adsorption kinetics describes the solute uptake rate and its knowledge is of great importance to design appropriate adsorption technologies. Lagergren’s first order and Ho’s second order kinetic equations [32] were tested to fit the experimental data obtained from the batch experiments. These equations are based on the adsorption of an adsorbate from solution onto solid adsorbents, so they

A.S. Mestre et al. / Carbon 45 (2007) 1979–1988

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Table 3 Apparent surface area, ABET, total, Vtotal, and mesoporous, Vmeso, volumes Sample

CAC CPAC

ABET (m2 g1)

Vtotal (cm3 g1)a

Vmeso (cm3 g1)b

891 1060

0.42 0.57

0.03 0.13

DR equation

as method

VDR (cm3 g1)

E0 (kJ mol1)

L0 (nm)

Vatotal (cm3 g1)

Vaultra (cm3 g1)

Vasuper (cm3 g1)

0.39 0.44

25.8 22.2

0.45 0.55

0.37 0.44

0.25 0.16

0.12 0.28

Microporous volumes evaluated by DR equation and as method. Characteristic adsorption energy, E0, and mean micropore half width, L0, estimated from the empirical equation reported in Ref. [30]. a Vtotal – volume adsorbed at p/p = 0.95. b Vmeso – difference between Vtotal and VDR.

are usually referred as pseudo-first and pseudo-second order kinetic models [33]. The pseudo-first order equation (2) is given as follows: ð2Þ

where k1 is the pseudo-first order rate constant (h1), qe and qt are the adsorbate uptake (mg g1) at equilibrium and at time t, respectively. Integrating this equation for the boundary conditions t = 0 to t = t and qt = 0 to qt = qt, it may be rearranged for linearized data plotting, as shown by the following equation:   k1 logðqe  qt Þ ¼ logðqe Þ  t ð3Þ 2:303 The pseudo-first order rate constant, k1, can then be directly obtained from the slope of the plot of log(qe  qt) versus t. The pseudo-second order equation is expressed as: dqt ¼ k 2 ðqe  qt Þ2 dt

ð4Þ

where k2 is the pseudo-second order rate constant (g mg1 h1), qe and qt are the adsorbate uptake (mg g1) at equilibrium and at time t, respectively. Separating the variables in Eq. (4) gives: dqt ðqe  qt Þ

2

¼ k 2 dt

ð5Þ 100

Integrating Eq. (5) for the boundary conditions abovementioned, the following linearized form is obtained:   t 1 1 ¼ þ t ð6Þ qt k 2 q2e qe The values of qe and k2 can be estimated from the slope and the intercept, respectively, of the plot (t/qt) versus t. The product k 2 q2e represents the initial adsorption rate and, in this study, will be designated by h. The half-life time, t1/2, that is, the time required for the adsorbent to uptake half of the adsorbate amount that will be retained at equilibrium, is often used as a measure of adsorption rate and is determined by the following equation: t1=2 ¼

1 k 2 qe

ð7Þ

80

qe / mg g-1

dqt ¼ k 1 ðqe  qt Þ dt

This equation is obtained by the rearrangement of Eq. (6) considering t = t1/2 as qt = qe/2. Both kinetic models were applied to the experimental data (Fig. 3) and the fitting results, presented in Table 4, clearly show that the adsorption of ibuprofen onto the tested activated carbons obeys to the pseudo-second order equation (R2 P 0.997). The coefficients of determination (R2) of the pseudo-first order fitting were very unfavourable (as low as 0.5 in some cases) showing that this model does not adjust to the experimental data. This can be confirmed in Fig. 4, where the values of qe,calc, ibuprofen uptake at equilibrium calculated using pseudo-first and pseudo-second order rate equations, are confronted with the experimental values, qe. From the results presented in Table 3 it is possible to analyse the effect of initial concentration, C0, in the ibuprofen uptake In both activated carbons, the equilibrium uptake, qe,calc, increased as C0 increased from 20 to 60 mg dm3, showing that the initial concentration provides a powerful driving force to overcome the mass transfer resistance between de solution and solid phases [15]. On the other hand, in the range of initial concentrations studied, no significant differences between the equilibrium uptake of ibuprofen were noticed, although CPAC sample always presents slightly higher qe,calc values (see Fig. 3 and Table 4).

60

40

20 20ppm

40ppm

60ppm

0 0

1

2

3

4

5

6

t/h Fig. 3. Kinetic results of ibuprofen adsorption at 30 C. Empty and filled symbols for CPAC and CAC, respectively.

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Table 4 Pseudo-second order ibuprofen adsorption parameters for the CAC and CPAC samples under different initial concentrations, at 30 C: qe,calc and Ce,calc, respectively, the ibuprofen uptake and that remaining in solution at equilibrium, both calculated by the pseudo-second order kinetic model and removal efficiency (re) Sample

C0 (mg dm3)

k2 (g mg1 h1)

R2

h (mg g1 h1)

t1/2 (h)

qe,calc (mg g1)

Ce,calc (mg dm3)

CAC

20 40 60

0.96 0.34 0.14

0.999 0.997 0.999

830 1110 1023

0.035 0.051 0.084

29.4 57.1 85.5

0.4 1.9 3.0

98.0 95.3 95.0

CPAC

20 40 60

1.10 0.55 0.31

1.000 1.000 1.000

997 1993 2472

0.030 0.030 0.036

30.1 60.2 89.3

0.0 0.0 0.5

100.0 100.0 99.2

a

re ¼

ðC 0 C e;calc Þ C0

 100.

100

qe,calc / mg g-1

80

60

40

20

0 0

re (%)a

20

40

60

80

100

-1

qe / mg g

Fig. 4. Correlation between the amounts of ibuprofen adsorbed: qe (experimental data) and qe,calc (calculated using the pseudo-first () and the pseudo-second () order kinetic models). Error bars are included.

Concerning the pseudo-second order rate constant, k2, the values quoted in Table 3 decrease with the increase of the initial ibuprofen concentration, for both absorbents. However, for the same C0, the k2 value obtained for CPAC sample is always higher than for the CAC sample. This means that, to adsorb a given amount of ibuprofen a high quantity of CPAC sample is need. So, considering only the rate of the overall process, one could conclude that CAC sample has the best adsorption performance for ibuprofen. Nevertheless, when the initial adsorption rate, h, or half-life time, t1/2, are considered the CPAC sample is the one that presents best characteristics to remove ibuprofen from solution. In fact, considering, for instance, the half-life time determined in the essays with C0 = 60 mg dm3 it is obvious that CAC sample needs twice the time requested by CPAC sample for retain half of the ibuprofen that will be adsorbed at equilibrium. In the same essays, the initial adsorption rate presented by CPAC is 2.5 times higher than that of CAC. It must be also mentioned that in the range of initial concentrations studied, CPAC sample presented t1/2 values that are almost independent of C0 and always smaller than those obtained with CAC (Table 4). On the other

hand, the values of the initial adsorption rate obtained in the essays with CPAC are much more sensitive to initial concentration and always higher than those found with CAC sample. The different behaviour of the two adsorbents is most probably a direct consequence of the intrinsic characteristics of their microporous structure, namely the supermicroporous volume. Actually, as already discussed, CPAC sample has a much higher volume of largest micropores than CAC which certainly favours a quicker initial adsorption uptake. In fact, according with the molecular dimensions of ibuprofen presented before it is clear that to enter into the ultramicropores the benzene ring of the ibuprofen has to be parallel to the pore entrance, considering the slit-shape pore that usually models the structure of activated carbons [34]. On the contrary, in the case of adsorption onto supermicropores, no specific orientation of the molecule is necessary. The amount of ibuprofen that remains in solution at equilibrium, Ce,calc, was determined by the mass balance principle using the qe,calc value obtained from the fitting of the linearized pseudo-second order kinetic equation. The values quoted in Table 4 show that, for both adsorbents, Ce,calc slightly increases with the increase of C0. However, it must be emphasized, that the removal efficiency is always at least 95% and with CPAC a complete removal of ibuprofen is obtained for the two lowest C0 values. 3.4.2. Adsorption isotherms The ibuprofen adsorption isotherms for both activated carbons at 30 C, presented in Fig. 5, are of Langmuir type, characterized by a steep initial rise that approaches a plateau attributed to the formation of a complete monolayer. From the various isotherm equations that may be used to analyse adsorption from solution experimental data, the Langmuir (theoretical) and the Freundlich (empirical) models are the most widely used [9,11,13,28]. The non-linear and linear forms of these equations are displayed in Table 5. The Langmuir constant, b, is a direct measure of the adsorption affinity and qm is the monolayer adsorption capacity [35]. The Freundlich constant, KF, measures the relative adsorption capacity of the adsorbent and the slope

A.S. Mestre et al. / Carbon 45 (2007) 1979–1988

where the fitting of the two isotherm models are displayed along with the experimental values. In Table 6 the results of the non-linear chi-square test analysis (v2) are also presented. In a recent study Ho [37] discussed the advantages of using this analysis to compare the fitting of experimental data to isotherm models. v2 is determined using the following equation:

500 CPAC CAC

qe / mg g-1

400

1985

300

200

v2 ¼

X ðqe  qe;m Þ2

ð8Þ

qe;m

100

where qe is the experimental equilibrium uptake and qe,m is the equilibrium uptake calculated from the model. From this equation it is clear that if the values calculated from the model are similar to experimental data, v2 should be a small number and vice versa. The v2 values displayed in Table 6 corroborate Ho’s conclusions since from v2 values the worst fitting of the Freundlich model is much clear than what is observed when R2 values are considered. On the other hand, in what concerns Langmuir model the better fitting for the isotherms obtained with CAC sample is highlight when v2 values are considered. In fact, the difference between the qm values and the maximum experimental uptakes (showed in Fig. 4 for the essays at 30 C) is much more pronounced for the essays with CPAC. From Table 6 it is evident that the values of the monolayer adsorption capacity for CPAC are always higher than those of CAC, which is in aggrement with the porous volumes determined by N2 adsorption for these adsorbents. However, besides porous volume other parameters must determine the samples adsorption capacities for ibuprofen, since the difference between qm values of the two activated

0 0

20

40

60

Ce / mg dm

80

-3

Fig. 5. Experimental ibuprofen adsorption isotherms at 30 C presenting the fitting of Langmuir (broken lines) and Freundlich (lines) models to the experimental data (activated carbon dosage = 10 mg/15 cm3 solution).

1/n, ranging between 0 and 1, is a measure for the adsorption affinity or surface heterogeneity [36]. The linear Langmuir and Freundlich isotherms for the adsorption of ibuprofen onto CAC and CPAC, at different temperatures, were fitted to the experimental data. The Langmuir and Freundlich parameters, along with the coefficients of determination (R2) of the linear plots, are presented in Table 6. In all the cases, the experimental data fit better to the Langmuir model since the coefficient of determination of the Langmuir plots (R2 P 0.997) are always higher than those obtained for the fitting of the Freundlich model. These results are clearly shown in Fig. 5, Table 5 Langmuir and Freundlich isotherms and their linear forms Isotherm

Non-linear form bqm C e 1þbC e

Langmuir

qe ¼

Freundlich

qe = KF(Ce)1/n

Linear form

Plot

Ref.

Ce qe

Ce qe

[35]

¼

1 bqm

þ

1 qm

Ce

lnðqe Þ ¼ lnðK F Þ þ 1n lnðC e Þ 1

3

versus Ce

ln(qe) versus ln(Ce)

1

[36] 1

Lagmuir parameters: qe – uptake at equilibrium (mg g ), b – Langmuir constant (dm mg ), qm – monolayer adsorption capacity (mg g ), Ce – solution concentration at equilibrium (mg g1). Freundlich parameters: qe – uptake at equilibrium (mg g1), KF – Freundlich constant (mg11/n (dm3)1/n g1), Ce – solution concentration at equilibrium (mg g1), n – Freundlich exponent.

Table 6 Langmuir and Freundlich isotherm parameters for the adsorption of ibuprofen onto CAC and CPAC, linear regression coefficient of determination, R2, and non-linear chi-square test analysis, v2 Sample

T (C)

Langmuir equation 1

Freundlich equation 3

1

qm (mg g )

b (dm mg )

R

2

2

v

1/n

KFa

R2

v2

CAC

25 30 40

139.2 145.2 153.2

0.356 0.238 0.262

0.997 1.000 0.999

1.95 1.02 0.76

0.303 0.356 0.382

45.8 36.6 39.1

0.969 0.898 0.890

4.27 17.27 16.24

CPAC

25 30 40

393.4 378.1 416.7

0.123 0.104 0.112

0.995 0.997 0.998

3.94 3.09 1.82

0.596 0.560 0.478

44.4 40.5 56.9

0.917 0.911 0.919

61.35 63.03 58.93

a

KF in mg11/n (dm3)1/n g1.

A.S. Mestre et al. / Carbon 45 (2007) 1979–1988

carbons is much higher than the difference of their micro or total porous volumes. In fact, one can always admit that this difference is a consequence of the packing of ibuprofen molecules in the porous structure that, most probably is not completely filled with the adsorbate species. Nevertheless, considering the nature of the surface chemistry groups of the two activated carbons, another justification for this result can be considered. Actually, experimental results [38,39] and theoretical studies [40] point out that the number and location of oxygenated groups on the pore structure of activated carbons determine their adsorption capacities when they are used as adsorbents in aqueous solutions. With molecular simulation Mu¨ller and Gubbins [40] showed that the water molecules are adsorbed on the hydrophilic polar oxygen groups (particularly carboxylic groups located at the entrance of the pores) forming three-dimensional clusters that can totally block the pore entrances. They also report that the water adsorption is greatly enhanced when such clusters may interconnect, due to the proximity of other similar structures, which results in the formation of dense regions of water in the pores. Considering the nature of the oxygenated groups present in CAC and CPAC samples (Table 2) one can expect that the formation of the above-mentioned water clusters is favoured in CAC, because only in this sample the strongest acid groups (R–COOH and R–OCO) are detected. So, a possible pore blockage can be envisage in this case, enabling the access of ibuprofen to all the extension of the microporous structure. As it was already mentioned, the Langmuir constant (b) is a measure of the adsorption affinity or heterogeneity of the surface. So, according to the b values quoted in Table 6, the CAC sample has higher adsorption affinity to the ibuprofen than the CPAC sample. This conclusion is in line with the surface chemistry characterization results which revealed that CAC is the sample that presents the largest number of acidic centres and also the strongest ones. However, other reasons can also explain this result. In fact, the conclusions recently presented by Moura˜o et al. [13] point out that the micropore narrowing of the adsorbent strongly influences the adsorption process from liquid phase, enhancing the adsorption affinity. So, considering the values of L0 (Table 3) it is evident that the sample with the small mean micropore half width (CAC) is the one that presents high Langmuir constant values and consequently high adsorption affinity. It is interesting to notice that although the Freundlich equation do not fit to the experimental data as well as the Langmuir model, the 1/n values also show that CAC sample has higher adsorption affinity than the CPAC sample. So, it seems that both models are consistent in indicating that the adsorption affinity of the activated carbons is different, being highest for the CAC sample.

the monolayer adsorption capacities and the Langmuir constants presents an unexpected variation with the increase of temperature. In fact, the increase of temperature from 25 to 40 C leads to a slight increase of qm values. We believe that these results are the consequence of the slight increment of the error bars associated with the values correspondent to the isotherm plateau. Nevertheless, some interaction of chemical nature, in the overall process of ibuprofen adsorption, cannot be entirely ruled out. In this situation the effect of temperature would be opposite and the net effect would be difficult to detect as it seems to be the case. So, it is possible to admit that, in the experimental conditions used, no significant influence of this experimental parameter in ibuprofen adsorption process was noticed. 3.4.4. Effect of initial pH The removal efficiencies (re in %) results displayed in Fig. 6 show that, as expected, pH plays an important role in the adsorption process. The nature of the activated carbon surface groups modifies with pH, leading to a net negatively charged carbon surface for pH values higher than PZC of the sample. Simultaneously, for pH values higher than the pKa of the ibuprofen (4.91 [41]) the molecule will be deprotonated. Consequently, as the pH increases for values higher than 5, the adsorption of ibuprofen will be less favourable due to the electrostatic repulsion between the adsorptive anion and the surface of the activated carbon that gradually becomes more negatively charged. However, as the two activated carbons have distinct PZC values the influence of the pH will be different. The experimental results show that, for both activated carbons, the removal efficiency is higher than 90% between pH 2 and 4. This similar behaviour of the two samples is in agreement with their PZC values that indicate that for this pH range both carbons have positively charged surfaces. For pH > 4 a decrease of the removal efficiency is observed, being more accentuated in the case of CAC sample. In fact, this carbon has a PZC of 7.5 which means that, in the 100

80

60

re / %

1986

40 CPAC

20

CAC 0

3.4.3. Effect of temperature In the range of temperatures essayed no significant differences were observed for both activated carbons since

0

2

4

6

8

10

12

pH Fig. 6. Effect of pH on the adsorption of ibuprofen on CAC and CPAC.

A.S. Mestre et al. / Carbon 45 (2007) 1979–1988

experiments made in the pH basic region the surface presents a net negative charge. So, the interaction with ibuprofen (that is also negatively charged for these pH values) becomes progressively unfavourable due to electrostatic repulsions. The same effect is observed for CPAC only for higher pH values since this sample has a PZC of 9.9. On the other hand, it must be noted that even at pH 11 CPAC sample still presents removal efficiency around 70%, against the 15% obtained with CAC. Most probably, the smallest dependence of the pH presented by CPAC also results from the fact that this sample has weaker acidic groups than CAC sample. From these results one can conclude that, concerning pH effect, CPAC sample presents advantages when compared with CAC sample. The removal efficiency of CPAC for ibuprofen is always higher than the one presented by CAC and less dependent of pH. 4. Conclusions In this study, the potentialities of activated carbons prepared from cork waste for ibuprofen removal from liquid phase were evaluated. Two samples of carbon were used: CAC prepared by chemical activation with K2CO3 and CPAC prepared by a two-step activation methodology (chemical activation with K2CO3 followed by steam activation). The preparation procedures led to samples with only small textural differences, but with distinct surface chemistry properties. The adsorption of ibuprofen onto these samples obeys to the pseudo-second order kinetic equation and equilibrium data presents a better fitting to the Langmuir model. The sample CPAC presents higher initial ibuprofen adsorption rates and adsorption capacity than CAC. This different behaviour is most probably due to a more developed supermicroporous structure of the sample CPAC. In the range of essayed temperatures (25–40 C) no significant influence of this experimental parameter in ibuprofen adsorption process was noticed. In what concerns the influence of initial pH, CPAC also presented advantages since with pH increase between 2 and 11 its removal efficiency decreases only 30%, although CAC presents a decrease of around 85%. These results are in accordance with the surface characteristics of the samples, namely their PZC values. Although the results obtained show that the two activated carbons prepared from cork waste are suitable for ibuprofen removal, CPAC presents some advantages. Besides the advantages already mentioned, this sample has high removal efficiency (in some cases 100%), associated with lower adsorption affinity than CAC. These characteristics permit to envisage the possibility of its use, for instance, as novel adsorbent material for enrichment of trace levels of priority pollutants from environmental matrices (e. g. solid phase extraction) prior to chemical analysis.

1987

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