Adsorptive removal of pharmaceuticals from water by commercial and waste-based carbons

Journal of Environmental Management 152 (2015) 83e90

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Adsorptive removal of pharmaceuticals from water by commercial and waste-based carbons ^nia Calisto a, *, Catarina I.A. Ferreira a, Joa ~o A.B.P. Oliveira a, Marta Otero b, Va Valdemar I. Esteves a a b

Department of Chemistry and CESAM (Centre for Environmental and Marine Studies), University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal Department of Applied Chemistry and Physics, IMARENABIO, University of L eon, Campus de Vegazana, L eon, Spain

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 October 2014 Received in revised form 15 December 2014 Accepted 14 January 2015 Available online

This work describes the single adsorption of seven pharmaceuticals (carbamazepine, oxazepam, sulfamethoxazole, piroxicam, cetirizine, venlafaxine and paroxetine) from water onto a commercially available activated carbon and a non-activated carbon produced by pyrolysis of primary paper mill sludge. Kinetics and equilibrium adsorption studies were performed using a batch experimental approach. For all pharmaceuticals, both carbons presented fast kinetics (equilibrium times varying from less than 5 min to 120 min), mainly described by a pseudo-second order model. Equilibrium data were appropriately described by the Langmuir and Freundlich isotherm models, the last one giving slightly higher correlation coefficients. The fitted parameters obtained for both models were quite different for the seven pharmaceuticals under study. In order to evaluate the influence of water solubility, log Kow, pKa, polar surface area and number of hydrogen bond acceptors of pharmaceuticals on the adsorption parameters, multiple linear regression analysis was performed. The variability is mainly due to log Kow followed by water solubility, in the case of the waste-based carbon, and due to water solubility in the case of the commercial activated carbon. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Remediation Environment Pyrolysis Paper mill sludge Activated carbon

1. Introduction The current knowledge about the contamination levels of water resources with anthropogenic organic pollutants alerts for the huge necessity of developing economically and environmentally sustainable remediation methods. Within organic contaminants, pharmaceutically active ingredients are a vast class of compounds with enormous impact in the environment, mainly due to their resistance and persistence in aquatic ecosystems (Calisto and Esteves, 2009; Kümmerer, 2009; Calisto et al., 2011b) and the biological effects they might exert on non-target organisms (Calisto and Esteves, 2009; Guler and Ford, 2010; Brodin et al., 2013). During the last decade, there was a significant amount of research focusing on the removal of pharmaceuticals from wastewaters revealing that the most commonly available treatment options (such as flocculation, filtration, activated sludge, chlorination) are not effective in the elimination of these compounds (Ternes

* Corresponding author. E-mail address: [email protected] (V. Calisto). http://dx.doi.org/10.1016/j.jenvman.2015.01.019 0301-4797/© 2015 Elsevier Ltd. All rights reserved.

et al., 2005; Rivera-Utrilla et al., 2013). In this context, removal of organic contaminants by adsorption is a very interesting solution due to its versatility and efficiency (Yu et al., 2008, 2009). Activated carbons (AC) are the most commonly used adsorbents which can be either in granular or powdered form (GAC or PAC, respectively) (Kyriakopoulos and Doulia, 2006). GAC and PAC are both applicable to wastewater treatment; however, PAC is usually more efficient with faster adsorption kinetics (due to smaller particle size) and GAC has the main advantage of regeneration/reuse after saturation (Altmann et al., 2014). Although there is a high availability of ACs in the market, a considerable amount of research has been published concerning the production of carbons using alternative starting materials (such as industrial and agriculture residues) with a view of lowering the production costs and of promoting waste's valorization (Kyriakopoulos and Doulia, 2006; Antunes et al., 2012; Yao et al., 2012; Jung et al., 2013; Calisto et al., 2014; Mestre et al., 2014). Some examples of residues used to produce carbons include sewage sludge, peanut shells, rice husk and pine wood (Ahmad et al., 2014; Mohan et al., 2014). Very few examples describe the production of carbons using paper mill sludge (Li et al., 2011). In fact, the use of paper mill sludge in this context has the

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extra advantage of contributing to implement new strategies for the management of such residues which is a critical issue for paper mills. The adsorption process is a rather complex spontaneous phenomenon which is mostly governed by electrostatic interactions (relevant when the adsorbate is an electrolyte and can be of attractive or repulsive nature) and by non-electrostatic interactions (such as van der Waals or hydrogen bonding, always of attractive nature) (Moreno-Castilla, 2004). The influence of these interactions on the adsorption process is directly dictated by the adsorbent's and adsorbate's characteristics. Concerning the adsorbent, the two main features to consider are the porosity and pore morphology (Kyriakopoulos and Doulia, 2007) (it is desirable to have a well developed microporosity, enhancing its surface area, but also to have some meso and macroporosity which function as channels to allow easy access to the micropores) and the surface chemistry (which defines the nature of the attractive/repulsive interactions between the functional groups of the adsorbate and the adsorbent and its largely influenced by the heteroatom content of the carbon). Relatively to the adsorbate, the most relevant characteristics are the octanolewater partition coefficient (log Kow), water solubility (closely related to the adsorbate's hydrofobicity), pKa (which defines the charged/neutral speciation of the adsorbate) and molecn, 2008; Baccar et al., 2012). ular size (Moreno-Castilla, 2004; Tasco Inferring about the effectiveness of adsorption for a given adsorbent/adsorbate system is not always straightforward due to the high complexity involved in balancing all of these variables. This manuscript describes the single adsorption of seven pharmaceuticals with distinct physico-chemical characteristics onto a commercially available powdered activated carbon and onto a nonactivated carbon produced from primary paper mill sludge. With the aim of relating the adsorption coefficients with the pharmaceuticals' properties, the following parameters were considered: water solubility, log Kow and polar surface area (all related to the hydrophobicity of the compounds), number of H-bond acceptors (describing the ability of the adsorbate to establish H-bonding with water or with functional groups of the carbon surface) and pKa (related to the neutral/charged speciation). The selected pharmaceuticals for this study were: carbamazepine (CBZ, anti-epileptic), oxazepam (OXZ, anxiolytic), sulfamethoxazole (SMX, antibiotic), piroxicam (PIR, non-steroidal anti-inflammatory), cetirizine (CTRZ, antihistaminic), venlafaxine (VEN, antidepressant, serotoninnorepinephrine reuptake inhibitor) and paroxetine (PAR, antidepressant, selective serotonin re-uptake inhibitor). All these pharmaceuticals have high consumption patterns and have already been found in the environment (Calisto and Esteves, 2009; Loos et al., 2009; Bahlmann et al., 2012). However, information on their removal from water is quite scarce. Although there are some published works on the adsorption of CBZ (Jung et al., 2013; Piel et al., 2013; Altmann et al., 2014) and SMX (Caliskan and Gokturk, 2010; Yao et al., 2012; Jung et al., 2013; Piel et al., 2013; Altmann et al., 2014) from water, very few studies approach the adsorption  mez et al., 2012) and OXZ (Kosjek et al., 2012) and of VEN (Rúa-Go no data were found concerning PIR, CTRZ and PAR.

PS800-150 can be found in previous work (Calisto et al., 2014). A summary of the main chemical and textural properties of these carbons is displayed in Tables S1 and S2 of the Supporting Information (SI). 2.2. Pharmaceuticals The adsorption experiments were carried out with seven pharmaceuticals: CBZ (Sigma Aldrich, 99%); OXZ (TCI, >98%); SMX (TCI, >98%); PIR (Sigma Aldrich, 99%); CTRZ (TCI, >98%); VEN (TCI, >98%); PAR (TCI, >98%). Physico-chemical parameters relevant to this work are summarized in Table 1. 2.3. Micellar electrokinetic chromatography (MEKC) analyses The quantification of the pharmaceuticals in the aqueous phase was carried out by MEKC analyses using a Beckman P/ACE MDQ (Fullerton, CA, USA) instrument, equipped with a UVeVis detection system. A dynamically coated silica capillary was used as described in previous work (Calisto et al., 2011a). Briefly, a bare silica capillary of 40 cm (30 cm to the detection window) was coated with a cationic polymer (hexadimethrine bromide) followed by sodium dodecyl sulfate (SDS). The electrophoretic separation was performed at 25
C, in direct polarity mode at 25 kV, for time intervals ranging from 2.5 to 4.5 min (depending on the pharmaceutical under analysis). Ethylvanilin was used as internal standard, spiked in all samples and standard solutions at a final concentration of 3.34 mg L1. Detection was monitored at 200 nm for SMX, PIR, CTRZ, VEN and PAR, at 214 nm for CBZ and at 230 nm for OXZ, according to their absorption spectra. The separation buffer consisted of 15 mM of sodium tetraborate and 30 mM of SDS for CBZ, OXZ, SMX and PIR and of 15 mM of sodium tetraborate and 20 mM of SDS for CTRZ, VEN and PAR. Separation buffer was renewed every six runs. Capillary washing between runs consisted on 1 min of ultra-pure water and 1.5 min of separation buffer at 20 psi. All the analyses were performed in triplicate. Calibration was carried out for each pharmaceutical using standard solutions of 0.25, 0.50, 1.00, 2.00, 3.00, 4.00 and 5.00 mg L1. 2.4. Adsorption batch experiments

2.1. Adsorbents

Batch experiments were carried out in order to study the adsorption of the seven pharmaceuticals onto PBFG4 e PS800-150. For this purpose, single solutions of each pharmaceutical with an initial concentration of 5 mg L1 were prepared in ultra-pure water, obtained from a Milli-Q Millipore system (Milli-Q plus 185). These solutions were placed in 15 or 50 mL polypropylene tubes together with the corresponding adsorbent and shaken in an overhead shaker (Heidolph, Reax 2) at 80 rpm under controlled temperature (25.0 ± 0.1
C). All the adsorption experiments were done in triplicate. In parallel with each experiment, a control (pharmaceutical solution without the adsorbent) and a blank (adsorbent in water without the presence of the pharmaceuticals) were run in order to test adsorption onto polypropylene tubes or thermo-degradation and matrix effects during MEKC analysis, respectively. After shaking, the polypropylene tubes content was filtered through 0.22 mm PVDF filters (Millipore) and immediately analyzed by MEKC as described in Section 2.3.

Two powdered carbons were used as adsorbents for the pharmaceuticals under study: a commercially available activated carbon (PBFG4, provided by ChemViron Carbon) and a non-activated carbon produced by pyrolysis (under N2 atmosphere, at 800
C for 150 min) of primary paper mill sludge (PS800-150). Detailed information concerning the production and characterization of

2.4.1. Adsorption kinetics Prior to the adsorption equilibrium tests, the time needed for attaining the equilibrium between each pharmaceutical and the two carbons was determined. For that purpose, a 5 mg L1 solution of each pharmaceutical was shaken together with PS800-150 or PBFG4 during different time intervals (5, 15, 30, 60, 240 and

2. Experimental section

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Table 1 Physico-chemical properties of the pharmaceuticals under study. Mw/(g mol1)

Sw/(mg L1)

pka

Log Kowk

Carbamazepine C15H12N2O

236.27

18 (25
C)a

2.3e 13.9f

2.67

46.33

3

Oxazepam C15H11ClN2O2

286.72

20 (22
C)b

1.7g 11.6g

2.31

61.69

4

Sulfamethoxazole C10H11N3O3S

253.28

500 (25
C)b

5.7h,i 1.8h

0.89

106.6

6

Piroxicam C15H13N3O4S

331.37

23 (22
C)b

6.3g

1.71

107.98

7

Cetirizine C21H25ClN2O3$2HCl

461.81

69570

2.12j 2.90j 7.98j

2.16

53.01

5

Venlafaxine C17H27NO2$HCl

313.87

572000c

8.91c 14.42c

2.91

32.7

3

Paroxetine C19H20FNO3$HCl$1/2H2O

374.80

5400d

9.9k

3.89

39.72

4

Structure

Polar surface area/Å2

Number of H bond acceptors

References. a Aga, 2008. b Yalkowsky et al., 2010. c Drugbank database. d Glaxo Smith Kline. e Nghiem et al., 2005. f Jones et al., 2002. g Moffat et al., 2005. h  , 2007. Petrovic and Barcelo i Cairns, 2008. j Tam et al., 2001. k Chemspider database.

360 min). The remaining concentration of pharmaceutical in the aqueous phase was then determined by MEKC, as previously described in Section 2.3. The adsorbent concentration was selected so as to have an adsorption percentage between (approximately) 40% and 60%, allowing to properly measure the remaining concentration of pharmaceutical in solution and, simultaneously, to have a significant adsorption percentage. In this context, the concentration of adsorbents for the kinetics with PS800-150 was 0.10 g L1 for PAR, 0.15 g L1 for OXZ, 0.25 g L1 for CBZ, CTRZ and VEN, 0.4 g L1 for PIR and 2.0 g L1 for SMX. For the studies with PBFG4 all the kinetics were performed with an adsorbent dose of 0.025 g L1, with the exception of VEN which was performed with 0.060 g L1. Fittings of the experimental data to pseudo-first (Eq. (1)) and pseudo-second (Eq. (2)) kinetic models (Lagergren, 1898) were determined using GraphPad Prism, version 5:

  qt ¼ qe 1  ek1 t

(1)

qt ¼

q2e k2 t 1 þ qe k2 t

(2)

with t (min) representing the adsorbent/solution contact time, qt (mg g1) the amount of solute adsorbed by mass unit of adsorbent at time t, qe the amount of solute adsorbed when the equilibrium is attained (mg g1) and k1 (min1) and k2 (mg g1 min) the pseudofirst and pseudo-second order rate constant, respectively. 2.4.2. Adsorption equilibrium Solutions of each pharmaceutical (5 mg L1) were shaken with different concentrations of PS800-150 and PBFG4 for the time needed to attain the equilibrium, as determined in the previous section. The concentration of adsorbent varied from 0.02 to 0.12 g L1 for PBFG4 and from 0.15 to 4.0 g L1 for PS800-150, depending on the pharmaceutical under test. Fittings of the experimental data to two non-linear isothermal models commonly used to describe the adsorption process, namely Freundlich (Eq.

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(3)

first or pseudo-second kinetics models, depending on the pharmaceutical. The results obtained for the adsorption kinetics of the studied pharmaceuticals onto the waste-based and commercial carbons show that both adsorbents are kinetically adequate for wastewater treatment as the equilibrium is quickly reached.

(4)

3.2. Adsorption equilibrium

(3)) (Freundlich, 1906) and Langmuir (Eq. (4)) (Langmuir, 1918) isotherm models, were determined using GraphPad Prism, version 5: 1=N

qe ¼ Kf Ce qe ¼

q m Kl C e 1 þ Kl Ce

with qe representing the amount of solute adsorbed at equilibrium (mg g1), Ce the amount of solute in the aqueous phase at equilibrium (mg L1), Kf the Freundlich adsorption constant (mg g1 (mg L1)N), N the degree of non-linearity, qm the Langmuir maximum adsorption capacity (mg g1) and Kl (L mg1) the Langmuir affinity coefficient, respectively. 3. Results and discussion 3.1. Adsorption kinetics The amount of each pharmaceutical adsorbed onto PS800-150 or PBFG4 at time t (qt, mg g1) versus time and the fittings of the experimental data to pseudo-first and second orders kinetic models are represented in Fig. 1. The fitting parameters are summarized in Table 2. Considering the adsorption of pharmaceuticals onto PS800-150, one can verify that the equilibrium is very quickly attained: the slowest kinetics was obtained for SMX and VEN and took only between 15 and 30 min until equilibrium; all the other pharmaceuticals reached the equilibrium in 5 or less minutes of contact with the adsorbent. This is a major positive characteristic of this adsorbent which, furthermore, was proven to be valid for pharmaceuticals with different physico-chemical properties. This fact points to a key role of the adsorbent properties on the adsorption process, which is consequently less dependent on the adsorbate. The experimental data are, in general, best described by the pseudo-second order model. On the other hand, adsorption onto PBFG4 took from 30 to 120 min to reach the equilibrium. In this case, the experimental data are described either by the pseudo-

The adsorption isotherms, represented as the amount of each pharmaceutical adsorbed onto PS800-150 and PBFG4 at equilibrium (qe, mg g1) versus the amount of pharmaceutical remaining in solution (Ce, mg L1), are shown in Fig. 2. Fitting parameters to Freundlich and Langmuir isothermal models are summarized in Table 2. For both carbons, experimental data are, in general, well described either for Freundlich or Langmuir model, with very satisfactory correlation coefficients. However, overall, the Freundlich model presents the highest correlation coefficients (between 0.974 and 0.998) and the lowest Syx residuals being, for this reason, the Freundlich fitting parameters the ones selected for the subsequent analysis. A first comparison between PBFG4 and PS800-150 reveals that, in terms of adsorption affinity of the pharmaceuticals, PBFG4 has the best results with the highest adsorption coefficient (Kf) obtained for PIR (128 ± 2 mg g1 (mg L1)N) and the lowest obtained for VEN (39.7 ± 0.6 mg g1 (mg L1)N). Considering the surface area as one of the decisive factors for the adsorption process, this result was expectable if we take into account that, despite both adsorbents having a microporous structure, the surface area of PBFG4 (848.22 m2 g1) is 4 higher than those of PS800-150 (209.12 m2 g1) e see Table 2 of SI. In any case, although microporosity and surface area are two main physical characteristics of the adsorbent with high relevance on its performance, differences between the adsorption of the seven pharmaceuticals under study evidence the influence of further variables. Moreover, this process should not only be influenced by the adsorbate chemical structure but also by the adsorbent chemical characteristics since the highest Kfs were not obtained for the same pharmaceuticals, when comparing the two carbons.

Fig. 1. Kinetic study of the adsorption of the selected pharmaceuticals onto PS800-150 and PBFG4. The results were fitted to pseudo-first and pseudo-second order kinetic models. Each point (±standard deviation) is the average of three replicates. Note that the y-axis scale is not the same in all graphs to allow a better visualization of the results.

2.3 ± 0.2 0.991 0.877 38 ± 3 0.7 ± 0.1 0.988 1.034 15 ± 8 0.988 3.429 90 ± 3 8±4 0.991 3.081 5.6 ± 0.7 0.987 0.296 8.5 ± 0.2 3.3 ± 0.3 0.995 0.184 21 ± 8 0.987 1.579 42.5 ± 0.9 17 ± 7 0.988 1.542 10 ± 2 0.980 0.372 8.2 ± 0.1 11 ± 1 0.994 0.205 5.0 ± 0.4 0.992 3.02 101 ± 2 6.2 ± 0.6 0.996 2.192 13 ± 2 0.991 0.185 5.82 ± 0.09 30 ± 5 0.992 0.181 9±1 0.994 3.718 138 ± 2 18 ± 2 0.996 3.024 4.4 ± 0.7 0.974 0.077 1.69 ± 0.09 2.9 ± 0.7 0.954 0.102 3.1 ± 0.2 0.987 4.031 118 ± 5 2.3 ± 0.4 0.982 4.744 23 ± 6 0.994 0.221 7.8 ± 0.8 34 ± 8 0.995 0.195 6.0 ± 0.5 0.997 2.186 119 ± 4 4.8 ± 0.8 0.991 3.726 5.0 ± 0.3 0.994 0.264 12.6 ± 0.4 2.7 ± 0.4 0.980 0.483 9.1 ± 0.7 0.998 2.011 116 ± 3 10 ± 2 0.991 3.972

PS800-150

0.992 0.936 16.3 ± 0.6 0.984 1.819 39.7 ± 0.6 0.995 0.224 7.2 ± 0.1

0.997 0.201 6.4 ± 0.1

59 ± 4 0.05 ± 0.01 0.952 6.281 67 ± 4 0.0009 ± 0.0003 0.977 4.299 80 ± 2 8.9 ± 0.1 0.34 ± 0.04 0.994 0.285 9.2 ± 0.1 0.09 ± 0.02 34.5 ± 0.5 0.16 ± 0.01 0.995 1.019 37 ± 1 0.007 ± 0.002 7.8 ± 0.5 0.45 ± 0.04 0.998 0.146 7.9 ± 0.1 0.24 ± 0.09

Not converged 94 ± 4 0.08 ± 0.01 0.964 7.505 5.03 ± 0.07 102 ± 3 ~1.8eþ09 0.0011 ± 0.0002 0.993 0.990 0.178 3.878 5.53 ± 0.07 84 ± 1 0.04 90 ± 4 0.03 0.051 ± 0.007 0.981 5.547 0.05 100 ± 9 0.07 0.0007 ± 0.0003 0.990 0.946 0.064 9.493 1.22 ± 0.03 128 ± 2 1.39 ± 0.12 ± 0.983 0.082 1.45 ± 0.19 ±

Not converged 110 ± 3 0.15 ± 0.02 0.978 6.674 12.4 ± 0.2 117 ± 3 ~2.502eþ017 0.0021 ± 0.0004 0.994 0.991 0.382 4.387 7.42 ± 0.09 78 ± 2 Not converged 93 ± 2 0.13 ± 0.02 0.984 4.961 10.1 ± 0.1 99 ± 4 ~4.747eþ013 0.0022 ± 0.0006 0.992 0.979 0.336 5.729 9.0 ± 0.1 95.8 ± 0.9 121.6 ± 2.9 0.10 ± 0.01 0.989 5.349 132 ± 3 0.0011 ± 0.0002 0.993 4.253 102.7 ± 0.8

qt (mg g1) k1 (min1) R2 Syx Pseudo 2nd qt (mg g1) order k2 (mg g1 min) R2 Syx Freunlich Kf (mg g1 (mg L1)N) N R2 Syx Langmuir Qm (mg g1) Kl (L mg1) R2 Syx Pseudo 1st order

PS800-150 PBFG4

PAR VEN

PS800-150 PBFG4 PBFG4

CTRZ

PS800-150 PS800-150 PBFG4

PIR SMX

PBFG4 PS800-150

OXZ

PBFG4 PBFG4

PS800-150 CBZ

Table 2 Fitting parameters of pseudo-first and pseudo-second order kinetic models and of Freundlich and Langmuir equilibrium models to the experimental data.

24.7 ± 0.4 0.6 ± 0.2 0.977 0.977 24.9 ± 0.5 0.12 ± 0.09

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87

Focusing on the Freundlich adsorption coefficients achieved by PS800-150, they vary from 1.22 ± 0.03 to 16.3 ± 0.6 mg g1 (mg L1)N for SMX and PAR, respectively, following the order SMX < PIR < VEN < CTRZ < OXZ < CBZ < PAR. Taking the chemical structure of the pharmaceuticals as the starting point to understand the variability on the adsorption coefficients some conclusions can be withdrawn. PAR is the only studied pharmaceutical with one fluorine atom which is the most electronegative halogen; thus, its presence might potentiate strong hydrogen bonds with the PS800-150 functional groups (this carbon has still a high percentage of oxygen, 9.7%, indicating the possible high prevalence of hydroxyl and carboxyl groups compatible with hydrogen bonding), justifying the affinity of PAR to adsorb onto PS800-150. Following this reasoning, CTRZ and OXZ are the only pharmaceuticals with one chlorine atom, the second most electronegative halogen, and they present a lower Kf than PAR but higher than the other adsorbates (which do not have halogens on their structure), reinforcing this argument. Moreover, both SMX and PIR, the pharmaceuticals with lowest affinity to PS800-150, have a sulfur dioxide group which enhances their hydrophilicity and they are, in fact, the ones with the lowest Kf. As it was referred above, the same order is not followed for the adsorption coefficients onto PBFG4 suggesting that the possible interaction with surface functional groups might not be the decisive factor in this case. In general, the same qualitative tendency is not perceptible for PBFG4. For the adsorption of the pharmaceuticals under study onto this carbon, the porous structure of the adsorbent probably plays a more important role than its surface chemistry. Another variable to take into account when interpreting differences on the adsorption coefficients is the pH. All the adsorption tests were performed in unbuffered solutions and the initial concentration of pharmaceuticals is not high enough to alter the solution pH (which was experimentally confirmed); thus, the working pH corresponds to the point of zero charge (pzc) of the carbons. The pzc of both carbons was previously determined by Jaria et al. (unpublished work) being 10.5 for PS800-150 and 7.0 for PBFG4, meaning that PS800-150 has a surface dominated by basic functional groups while PBFG4 is neutral. Given that the studied pharmaceuticals have very distinct pKas (please see Table 1), it is important to assess the main protonation state of the pharmaceuticals during the adsorption experiments in order to better understand if electrostatic interactions play an important role in the adsorption process. Thus, when in contact with PS800-150 (at pH 10.5), CBZ, OXZ and VEN should be mainly in the neutral form; SMX, PIR and CTRZ in the anionic form; and PAR mainly in the cationic form. On the other hand, when in contact with PBFG4 (at pH 7.0), CBZ, OXZ and CTRZ should be neutral; SMX and PIR anionic and finally VEN and PAR cationic. In this context, it is observable that, for adsorption onto PS800-150, the highest adsorption coefficients were obtained for cations, followed by neutrals and, then anions. Overall, electrostatic interactions appear to be decisive for the adsorption process onto the produced carbon as there is a defined tendency related with the protonation state of the pharmaceuticals. The obtained tendency is in line with the pzc of PS800-150: a pzc of 10.5 should imply that the main functional groups are deprotonated, negatively charged, justifying the higher adsorption coefficients obtained for pharmaceuticals in the cationic form. These conclusions indicate the pH as a decisive variable to consider in the adsorption onto PS800-150. Similarly to the analysis presented above, no tendency is observable for the adsorption onto PBFG4 underlying that this adsorption process might be highly defined by the microporous structure of the adsorbent with less influence of its surface chemistry. In order to systematize this qualitative analysis, the effects of quantitative chemical properties of the pharmaceuticals under

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Fig. 2. Equilibrium data on the adsorption of pharmaceuticals onto PS800-150 and PBFG4. The results were fitted to Langmuir and Freundlich equilibrium models. Each point (±standard deviation) is the average of three replicates. Note that x and y-axis scales are not the same in all graphs to allow a better visualization of the results.

study on their adsorption were analyzed. The adsorbate characteristics selected as susceptible of influencing their adsorption affinity were: water solubility (Sw), log Kow, pKa, polar surface area (PSA) and number of H-bond acceptors (Hbond) (see Table 1 in Section 2.2). Firstly, a direct correlation between Kf (Eq. (3)) and each of the chosen parameters was investigated. A correlation coefficient (r2) of 0.8565 (0.9875 if we consider VEN as an outlier) was found between the Kf of the seven pharmaceuticals onto PS800-150 and log Kow; no correlation was observed for PBFG4 (r2 ¼ 0.0866). None of the other parameters showed any direct correlation with the Kf, neither for PS800-150 nor for PBFG4. The described analysis was also performed for the non-linearity of the adsorption process (N, Eq. (3)), and no correlation was found between N and any of the parameters, neither for PS800-150 nor PBFG4. Subsequently, a multiple linear regression analysis was performed and the coefficients (a) of the model were determined using Matlab®, as shown in Equation (5):

Kf ¼ a0 þ aPSA  PSA þ alogKow  logKow þ apKa  pKa þ aSw  Sw þ aHbond  Hbond (5) A Student t-test was performed in order to evaluate the statistical significance of the obtained linear regression coefficients for

Table 3 Linear regression analysis parameters and statistical evaluation for Kf ¼ a0 þ aPSA  PSA þ alog Kow  logKow þ apKa  pKa þ aSw  Sw þ aHbond  Hbond. p-Values corresponding to statistically significant coefficients are presented in bold.

PS800-150 Linear regression coefficients t (estimated) p PBFG4 Linear regression coefficients t (estimated) p

a0

aPSA

7.576

0.19 4.99

alog

Kow

apKa

aSw

aHbond

each parameter. The coefficients of the linear regression analysis and p-values are summarized in Table 3. p-values were used to check the significance of the coefficients; p < 0.05 indicates that the coefficient under analysis is statistically significant. For PS800-150, the results revealed that only log Kow and water solubility are significant and thus, they are the two main variables describing the variations observed in the Kf values. Among these two parameters, log Kow is the one that most influences the adsorption coefficients of the pharmaceuticals onto the produced carbon. Note that the positive sign of the coefficient indicates that an increase in log Kow implies an increase on Kf. On the other hand, despite being less significant than log Kow, the water solubility is also responsible for the variability of the results but with the opposite effect (an increase in the water solubility implies a decrease in the Kf values). Regardless of the qualitative conclusions presented above, concerning the presence of high electronegative atoms which might be able to establish strong hydrogen bonding interactions with the oxygen containing functional groups of PS800-150, the number of the H bond acceptors of the adsorbate appears to be not relevant to the variability of Kf. Actually, the presence of H-bond acceptors on the solute might not always function as an enhancer of the Kf as these H-bond acceptors should also be capable of establishing hydrogen bonds with water, increasing their solvation and thus increasing the energy needed by the pharmaceuticals to adsorb on the water/solid interface. Concerning the adsorption onto PBFG4, and contrarily to PS800-150, log Kow does not significantly influences the variability of the Kf. The variability of the results is explained by water solubility and, despite not being (technically) statistically significant, pKa and the number of H-bond acceptors coefficients have p-values which are very close to 0.05, indicating the existence of some influence of these variables.

0.74 1.393 0.17

183.9 1.0 0.0017 0.24 87 15

50.5 0.0063 3

4.5 22.3 0.069 0.014 54 27

0.78 0.29 66

39.3 1.6 0.0081 0.18

0.50 0.35

6.2 8.25 0.051 0.038

5.4 0.058

3.3. Environmental applications and future work The use of the tested carbons as adsorbents to remove pharmaceuticals from contaminated waters is particularly relevant as an advanced treatment option for waste water treatment plants, both for domestic sewage and for highly polluted industrial effluents.

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Yet, a complete assessment of the performance of the tested carbons in real conditions ought to be considered, namely the effect of competitive conditions in the efficiency of the materials (for example, the presence of complex mixtures of pharmaceuticals). Another point to take into consideration in future studies is the regeneration of the carbons, as their reusability is an important aspect in the cost effectiveness of this method. In the particular case of PS800-150, the high availability and no cost of the starting material is, however, a very relevant point to take into account. In this context, the regeneration of the carbon might not compensate (economically and energetically), and its incineration after saturation could be an interesting alternative e incineration for energy production is, indeed, the main application of the paper mill sludge. 4. Conclusions The results obtained revealed that adsorption onto the commercially available AC (PBFG4) is effective for the removal of pharmaceuticals from water. Comparatively, although the carbon produced from primary paper mill sludge (PS800-150) showed a lower adsorption capacity, the adsorption kinetics was faster. In fact, adsorption equilibrium onto PS800-150 was attained almost instantaneously for all the pharmaceuticals, which constitute an interesting feature of this material. For both carbons, it is important to retain the large differences in the adsorption coefficients between pharmaceuticals which imply that they were not all removed with the same efficiency. A qualitative analysis of the obtained results points out that the adsorption mechanism onto PS800-150 is mostly determined by its surface chemistry while in the case of PBFG4, the adsorption is be mainly defined by its large surface area. Quantitatively, by means of multiple linear regression analysis, the variability of the results was mostly explained by the pharmaceutical's log Kow and water solubility (for PS800-150) and by water solubility (for PBFG4). A step forward was made in this work on the analysis of the observed results, which enabled to shed some light on the adsorption efficiency of custom and alternative carbons for the removal of pharmaceuticals from water. Acknowledgments This work was supported by European Funds through COMPETE and by National Funds through the Portuguese Science Foundation ^nia Calisto and (FCT) within project PEst-C/MAR/LA0017/2013. Va Catarina I. A. Ferreira thank (FCT) for their postdoctoral (SFRH/BPD/ 78645/2011) and PhD grants (SFRH/BD/88965/2012), respectively. Also, Marta Otero acknowledges financial support from the Spanish Ministry of Science and Innovation (RYC-2010-05634). The authors also thank the kind collaboration of Eng
Pedro Sarmento from RAIZ ~o da Floresta e do Papel. e Instituto de Investigaça Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2015.01.019. References Aga, D.S., 2008. Fate of Pharmaceuticals in the Environment and in Water Treatment Systems. CRC Press, Boca Raton. Ahmad, M., Rajapaksha, A.U., Lim, J.E., Zhang, M., Bolan, N., Mohan, D., Vithanage, M., Lee, S.S., Ok, Y.S., 2014. Biochar as a sorbent for contaminant management in soil and water: a review. Chemosphere 99, 19e33. Altmann, J., Ruhl, A.S., Zietzschmann, F., Jekel, M., 2014. Direct comparison of ozonation and adsorption onto powdered activated carbon for micropollutant removal in advanced wastewater treatment. Water Res. 55, 185e193. gan, R., Crespo, J.S., Fernandes, A.N., Giovanela, M., Antunes, M., Esteves, V.I., Gue 2012. Removal of diclofenac sodium from aqueous solution by Isabel grape

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