Investigation into adsorption mechanisms of sulfonamides onto porous adsorbents

Journal of Colloid and Interface Science 362 (2011) 503–509

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Investigation into adsorption mechanisms of sulfonamides onto porous adsorbents Weiben Yang ⇑, Fangfang Zheng, Xiaoxu Xue, Yiping Lu College of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210097, China

a r t i c l e

i n f o

Article history: Received 13 May 2011 Accepted 26 June 2011 Available online 8 July 2011 Keywords: Antibiotics Sulfonamide Hypercrosslinked Surface Temperature

a b s t r a c t The presence of sulfonamide antibiotics in aquatic environments poses potential ecological risks and dangers to human health. In this study, porous resins as adsorbents for the removal of two sulfonamides, sulfadiazine and sulfadimidine, from aqueous solutions were evaluated. Activated carbon F-400 was included as a comparative adsorbent. Despite the different surface properties and pore structures of the three resins, similar patterns of pH-dependent adsorption were observed, implying the importance of sulfonamide molecular forms to the adsorption process on the resins. Sulfonamide adsorption to the three resins exhibited different ionic strengths and temperature dependence consistent with sulfonamide speciation and the corresponding adsorption mechanism. Adsorption of sulfadiazine to F-400 was relatively insensitive to pH and ionic strength as micropore-filling mainly contributed to adsorption. The adsorption mechanism of sulfadiazine to the hypercrosslinked resin MN-200 was similar to that of the macroporous resin XAD-4 at lower pH values, whereas it was almost identical to the aminated resin MN-150 at higher pH. This work provided an understanding of adsorption behavior and mechanism of sulfonamide antibiotics on different adsorbents and should result in more effective applications of porous resin for antibiotics removal from industrial wastewater. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction Sulfonamides have been widely used in human and veterinary medicine to treat and prevent infectious bacterial diseases. These sulfonamides are often discharged into aquatic environments via domestic wastewater effluents, disposal of expired pharmaceuticals, and excretion in its original or metabolized form [1]. Exposure to these causes a variety of adverse effects, including acute and chronic toxicity and microorganism antibiotic resistance [2,3]. The removal of pharmaceutical antibiotics using conventional water and wastewater treatment technologies is generally inadequate because such techniques only offer partial removal efficiency [4]. Hence, there is an increasing demand for developing more effective treatment technologies to remove sulfonamides from water [5]. Adsorption of sulfonamide is a complex process, given that it can exist as cation, zwitterion, and anion at environmentally relevant pH values. Some researchers have demonstrated the adsorption of sulfonamides with humic substances [6], organic materials [7], clay minerals [8–10], soils [11,12], among others. In these studies, adsorption is primarily driven by specific mechanisms of cation exchange [6] and surface complexation reactions [7–9] (H-bonding and other polar interactions) between the multi-functional sulfonamide molecules and the corresponding

⇑ Corresponding author. Fax: +86 25 85572627. E-mail address: [email protected] (W. Yang). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.06.071

interactive sites of the adsorbents. Sulfonamide speciation and the surface charge density of the adsorbent are the most important determinants of sulfonamides adsorption. Thiele-Bruhn et al. [11] found that adsorption increases with the aromaticity and electronegativity of the functional groups attached to the sulfonylphenyl-amine core. Phenolic and carboxylic groups, N-heterocyclic compounds, and lignin decomposition products are the preferred binding sites. Moreover, adsorption is pH-dependent and involves diverse mechanisms that respond differently to solution chemistry [6,9]. For instance, Kurwadkar et al. [12] found a significant increase in the sorption coefficient as the sulfonamides were converted from anionic forms at higher pH to neutral/cationic forms at lower pH. It was also deduced that the effect of speciation on sorption is a function of the pH of the soil and the pKa of the sulfonamides. In addition to natural adsorbents, zeolite, carbon nanotubes, chitosan derivative, and activated carbon have also been employed for adsorbing sulfonamide antibiotics [13–18]. The adsorbed amount of sulfonamide changes with the pH and ionic strength of the aqueous solution; this is attributed to the change in dominant species and hydrophobic and electron donor–acceptor (EDA) interactions with the surface of the adsorbent [14,15]. Adsorption is stronger on activated carbons than on the other carbonaceous adsorbents resulting from the pore-filling effect [16]. The underlying mechanism that controls the adsorption of sulfonamides to porous resins remains largely unknown. Porous resin, one of the most effective and widely used adsorbents for purifying water, has shown great promise in the recovery of organic

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Fig. 1. Molecular structure and selected physicochemical properties of the sulfonamide antibiotics studied (data from Ref. [11]).

compounds from wastewater [19–21]. The surfaces of porous resins have high adsorption capacities, surface areas, and stable chemical structures. One of the main advantages of porous resins is that they can be regenerated more easily than other materials using organic solvents [21]. The main objective of this study was to investigate the mechanism and predominant factors that control the adsorption of sulfonamides to porous resin. Two commonly used sulfonamide antibiotics, sulfadiazine (SDZ) and sulfadimidine (SDM) (Fig. 1), were examined as the adsorbates in batch experiments. Hypercrosslinked resin MN-200, aminated polystyrene resin MN-150, and macroporous resin XAD-4 were selected as adsorbents. Commercial coal-based granular activated carbon Filtrasorb-400 (F-400) was also used for comparison. The impacts of solution chemistry conditions (salt, pH, and temperature) on adsorption were evaluated. 2. Materials and methods 2.1. Adsorbates Sulfadiazine sodium (SDZ, 99%) and sulfadimidine sodium (SDM, 99%) were obtained from Sigma-Aldrich Chemical Co. (Shanghai, China) and used as received. Water used in the study was purified by distillation, and the electrical conductivity is not more than 6 lS cm1.

20, 40, 60, 80, and 100 mg L1. Flasks were then completely sealed and placed in an incubator shaker (New Brunswick, Model G25) at 288, 303, and 318 K with a shaking speed of 130 rpm. The pH of the solution was adjusted using 0.01 mol L1HCl or NaOH. In the ionic strength experiments, adsorption was performed using background solutions of 0%, 1%, 3%, 5%, 7%, 9%, and 11% NaCl (mass concentration). Kinetic experiment was conducted to determine the time required for equilibrium adsorption. The initial concentration of sulfonamide was 100 mg L1; this was shaken with 0.05 g adsorbent and sampled at different time intervals at 303 K. Concentrations of sulfonamide in the solutions were quantified after equilibrium by HPLC with an ultraviolet (UV) detector. SDZ was quantified at 264 nm, while SDM was quantified at 241 nm. The mobile phase was composed of a phosphoric acid/acetonitrile mixture (83:17, v:v), and the flow rate was 1 mL min1. Detection limits were set to 0.05 mg L1. Adsorption data were collected in triplicate for the pH and salt concentration experiments and in duplicate for all other experiments. The adsorption capacity, qe (mg g1), was calculated using:

qe ¼ VðC 0  C e Þ=m

ð1Þ 1

where C 0 is the initial sulfonamide concentration (mg L ), C e is the residual sulfonamide concentration at equilibrium (mg L1), V is the volume of solution (L), and m is the mass of dry resin (g). 3. Results and discussion 3.1. Characteristics of the adsorbents BET surface area of MN-200 is 1155.8 m2 g1, while those of MN-150, F-400 and XAD-4 are between 815–880 m2 g1 (Supplementary Table S1). The three resins show similar matrix (Supplementary Fig. S1) and average pore diameters (5.19–5.80 nm), but rather different chemical compositions and pore size distributions (Fig. 2). MN-200 and MN-150 present bimodal pore size distributions at 0.5–3.5 nm and diameters larger than 30 nm. F-400 dominates the microporous region, while XAD-4 dominates the mesoporous region. Streat and Sweetland [22] found that the surface of MN-150 contains tertiary amines and amine functional groups, while those of MN-200 contain carbonyl (C@O) and hydroxyl groups.

0.3 MN-200

0.2

2.2. Adsorbents

0.1 0.0 0.3

dV/dD(cm3/g/nm)

Commercially available resins MN-200 and MN-150 were supplied by the Shanghai Office, Purolite International Co., Ltd. Commercial resin XAD-4 (Rohm–Haas Co., USA) and granular activated carbon Filtrasorb-400 (F-400) (Calgon Carbon Co., USA) were purchased from the Shanghai reagent station (Shanghai, China). Nitrogen adsorption and desorption experiments were carried out at 77 K to determine the surface properties of the adsorbents. BET surface area was calculated using the standard Brunauer– Emmert–Teller equation. Pore size distribution was determined from desorption isotherms using the Barrett, Joyner, and Halenda (BJH) method. All calculations were performed automatically by an Accelerated Surface Area and Porosimeter system (ASAP 2010, Micromeritics, USA).

0.5

1

2

4

0.2

8

16

32

64

32

64

32

64

32

64

MN-150

0.1 0.0 0.3 0.25

0.5

1

2

4

0.2

8

16

XAD-4

0.1 0.0 0.3

0.5

1

2

4

8

0.2

16

F-400

0.1

2.3. Adsorption assay

0.0 0.25

Adsorbent (0.05 g) was introduced to a series of 150-mL conical flasks, 100 mL of sulfonamide in aqueous solution was added to each flask, and the initial concentrations (C0) of the solutions were

0.5

1

2

4

8

16

D (nm) Fig. 2. Pore size distributions of the four adsorbents.

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W. Yang et al. / Journal of Colloid and Interface Science 362 (2011) 503–509

3.2. Adsorption kinetics Fig. 3 compares the adsorption kinetics of sulfonamides between the four adsorbents at pH 1.3 and 7.8. For both adsorbates, much slower adsorption is shown on F-400 than on the rest of the resins tested. Adsorption of sulfonamides reaches equilibrium after 50 h for F-400, whereas equilibrium times for MN-200, MN-150, and XAD-4 are 38, 28, and 22 h, respectively. Sulfonamides cannot enter small micropores smoothly, and adsorption diffusion of the molecules within micropores is expected to be very slow because of the high microporosity of F-400. Mesoporous XAD-4 has ordered, open, and interconnected three-dimensional pore structures, thus allowing rapid solute diffusion into them. The equilibrium time for MN-200 and MN-150 are between those for F-400 and XAD-4 because of their bimodal pore size distribution. Considering the adsorbed amounts of SDZ and SDM after equilibrium on the three resins, the predominance of bimodal pore size distribution, especially in the micropore area, can be speculated from Figs. 2 to 3. Higher adsorbed amounts on MN-200 compared with MN-150 may be ascribed to the increased microporosity of the former, since its mesoporous and macroporous areas are roughly the same size. Similarly, although the specific surface area of XAD-4 is slightly higher than that of MN-150, the adsorbed amounts on MN-150 are obviously greater than on XDA-4 since MN-150 presents bimodal pore size distribution. In addition to the importance of bimodal pore size distribution, the observed adsorbed amounts of the two sulfonamides at pH 1.3 are remarkably greater than their respective values at pH 7.8 on the three resins, whereas their adsorbed amounts on activated carbon F-400 are almost identical. This indicates that the adsorption mechanism of sulfonamides on the resins is different from that on F-400. Interestingly, the adsorbed amounts of SDZ on the three resins are all greater than those of SDM at pH 1.3, whereas the opposite trends are observed at pH 7.8. This may be ascribed to the different contributions of cationic, neutral, and anionic sulfonamides. The two tested sulfonamide antibiotics each possess one benzene ring and one aromatic heterocyclic group. The sulfonamide group between them has a strong electron-withdrawing ability, causing the two aromatic rings to be electron-depleted and hence serve as effective p-electron acceptors. The benzenoid rings of the resin surface (polystyrene resin) contain sp2-hybridized carbon atoms with high electronic polarizability and form electron-rich

3.3. Effect of pH The adsorption of sulfonamides is related to the ionization of amphoteric sulfonamides [27]. When the pH increases from 1 to 10, the SDZ species change from 79.9% cationic form (SDZ+) and 20.1% neutral form (SDZ0) to a dominant neutral form at pH 35 and then to 99.98% anionic form (SDZ) at pH 10 (Supplementary Table S2). Evidently, SDZ and SDM adsorption on the three resins exhibit very similar patterns of pH-dependence, suggesting the importance of species in adsorption (Fig. 4). The obvious transformation of adsorbed amounts can be observed after pH 5, where the percentages of neutral SDZ and SDM are all greater than 96%. To quantitatively identify the contribution of different species to the overall sulfonamide adsorption, the following empirical models could be used to calculate sorption coefficients for individual SDZ and SDM species using SPSS 17.0 [15]: þ

0

0



Kd ¼ Kd dþ þ Kd d0 Kd ¼ Kd d0 þ Kd d

100 80 60

MN-200 pH=7.8 MN-150 pH=7.8 F-400 pH=7.8 XAD-4 pH=7.8

40 20

Amounts of Adsorption (mg/g)

120

ð2Þ

5 6 pH 6 10

ð3Þ

where Kd (L kg ) is the overall adsorption coefficient and Kd , Kd0, and Kd are the adsorption coefficients for cationic, neutral (including zwitterionic), and anionic sulfonamides, respectively. The terms d+, d0, and d are the percentages of cationic, neutral, and anionic sulfonamides at a certain pH, respectively. The contributions of negatively charged sulfonamide species at pH 6 5 and positively charged sulfonamide species at pH P 5 were neglected because of their much lower percentage. The calculated results are listed in Table 1.

+

B

140

MN-200 pH=1.3 MN-150 pH=1.3 F-400 pH=1.3 XAD-4 pH=1.3

1 6 pH 6 5

1

A

140

Amounts of Adsorption (mg/g)

regions on the resin surfaces. So, the existence of p–p EDA interactions and/or cation–p bonding (cationic sulfonamide with the resin) is reasonable between the sulfonamide molecules and the surface of the resin [14–16]. Sulfonamide contains several moieties capable of engaging in hydrogen bonding as solely H acceptors (ASO 2 , pyrimidine N) or as H acceptors and H donors (anilinic N, sulfonamidic N). The additive effect of multiple H bonds between adsorbates and adsorbents could improve adsorption affinity [23,24]. Charge transfer complexes between the pyrimidine ring and/or anilinium moieties of sulfonamide molecules and functional groups on the surface of MN-200 and MN-150 are also conceivable [25,26]. The main impetus in the adsorption may come from the comprehensive contributions of the above-mentioned interactions.

120

MN-200 pH=1.3 MN-150 pH=1.3 F-400 pH=1.3 XAD-4 pH=1.3

100 80

MN-200 pH=7.8 MN-150 pH=7.8 F-400 pH=7.8 XAD-4 pH=7.8

60 40 20

0

0 0

10

20

30

40

Time (h)

50

60

70

0

10

20

30

40

50

60

Time (h)

Fig. 3. Adsorption kinetics of (A) SDZ and (B) SDM on the four adsorbents at 303 K. Initial concentrations = 100 mg L1.

70

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W. Yang et al. / Journal of Colloid and Interface Science 362 (2011) 503–509

Amounts of Adsorption (mg/g)

160

SDZ on

140

3.4. Effect of ionic strength

MN-200 MN-150 F-400 XAD-4

120 100 80 60

MN-200 MN-150 F-400 XAD-4

40

SDM on 20 1

2

3

4

5

6

7

8

9

10

pH Fig. 4. The relation of adsorbed amounts of SDZ and SDM to the pH values of the four adsorbents at 303 K. Initial concentrations = 100 mg L1.

The adsorption coefficients of anionic sulfonamides (Kd) on the three resins are much lower than those of neutral (Kd0) and cationic (Kd+) species, and the values of Kd0 are all greater than those of Kd+. This is reasonable because the anionic or cationic form is apparently much less hydrophobic than the neutral counterpart. Additionally, deprotonation on the sulfonamide group could significantly decrease the p-withdrawing ability and hydrogen bonds of the group, suppressing the interactions with the surface of resin [14]. Considering the higher percentages of neutral species, the contribution of neutral sulfonamides to the overall adsorption is evident. On resin MN-200, the contribution of neutral SDZ is always higher than 50% and mostly higher than 80% from pH 1.3 to 7.8. This is supported by the fact that the adsorbed amounts of SDZ and SDM increase with increasing pH values from 1 to 5 and then decrease with increasing pH values from 5 to 10 (Fig. 4). Compared with SDM, SDZ exhibits stronger adsorption on the resins at pH 1.3 because of their higher percentage of neutral species (33.4%) and obviously weaker adsorption at pH 7.8 (3.8% neutral species). Moreover, the adsorption affinities of the two sulfonamides on the resins decrease drastically from pH 1.3 to 7.8 because of the transformation of the cationic species to anionic ones. This confirms that the contribution of cationic species of sulfonamide is greater than that of anionic species on the resins. For activated carbon F-400, the effects of pH on adsorbed amounts of sulfonamides are insignificant, and adsorption remained nearly constant over the pH range of 1–8. Obvious decreases were observed when the pH increased from 8 to 10. This illustrates again that the adsorption mechanism of SDZ on F-400 was different from that on the resins.

The effects of ionic strength on adsorption was investigated and depicted in Fig. 5. For F-400, little change in adsorption is apparent under the experimental conditions, consistent with observations of the pH effect. Our findings further confirm that the adsorption affinity of sulfonamide onto F-400 has no clear relationship with their species form. The special adsorption behavior of sulfonamide on F-400 could largely result from the microporous structure of activated carbon F-400, and micropore-filling may be the dominating factor for sulfonamide adsorption onto the micropores of F-400 [28,29]. Results depicted in Fig. 5 for the three resins show that SDZadsorbed amounts increase with increasing NaCl concentration at pH 1.3 and 7.8. This could largely result from the well-known salting out effect [30], where the solubility of SDZ in water is decreased when NaCl is added to it. The decrease in solubility of SDZ in water facilitates the diffusion of more SDZ to the surface of the resins and an increase in adsorption amount. In addition to the salting out effect, hydrophobic interactions are among the main factors that influence SDZ adsorption onto XAD-4 resin. Resin XAD-4 is composed of styrene divinylbenzene copolymers, which are hydrophobic in character and possess no ion-exchange capacity [31]. Although several interactions occur during the adsorption of sulfonamide onto XAD-4 resin, the important contribution of hydrophobic interactions to sulfonamide adsorption is incontrovertible, and the increase in adsorption affinity of organic compounds onto XAD-4 with increasing salt content in the solution is widely accepted in the literature [32,33]. At a pH value of 1.3, the adsorbed amounts of SDZ onto MN-200 and MN-150 slightly increase (8% and 4%, respectively) with increasing salt concentration, whereas those at pH 7.8 have obvious improvements (22% and 43%, respectively). The large disparity of the salting out effect on the adsorbed amounts by MN-200 and MN-150 at different pH values may result from changes in the adsorption mechanism because of the change in solution chemistry. The hydrophobic interactions, p–p interactions, hydrogen bonding, and electrostatic interactions are the most representative interactions present during adsorption [34]. Multiple hydrogen bonds may contribute much more to the adsorbed amounts on MN-150 than the other interactions at pH 1.3 because of the existence of tertiary amine and amine functional groups on the surface. However, the larger improvement in adsorbed amounts on MN150 at pH 7.8 indicates that the main interaction may differ from that at pH 1.3. The main interactions on MN-200 and MN-150 at pH 7.8 will be discussed below. At a pH value of 7.8, the surface charge of MN-200 and MN-150 are negative, as their respective zero crossover points are observed at pH 4.8 and 7.1 [22]. Electrostatic screening could be a factor that influenced the adsorption process. Furthermore, under this condition, the density of the negative charge on MN-200 is greater than that on MN-150, leading to much stronger electrostatic repulsion and less improvement in adsorbed amounts on MN-200. Previous studies have also reported

Table 1 Calculated adsorption coefficients for three species of SDZ and SDM. pH

1 6 pH 6 5

Adsorption coefficients (L g1)

Sulfadiazine (SDZ)

5 6 pH 6 10

Kd+

Kd0

R2adj

Kd+

Kd0

R2adj

Kd0

Kd

R2adj

Kd0

Kd

R2adj

MN-200 MN-150 F-400 XAD-4

3.27 ± 1.13 2.15 ± 0.27 1.16 ± 0.07 1.01 ± 0.05

7.47 ± 0.52 3.02 ± 0.12 1.36 ± 0.03 1.36 ± 0.02

0.98 0.99 0.99 0.99

3.23 ± 0.65 1.97 ± 0.18 1.25 ± 0.09 0.97 ± 0.05

7.76 ± 0.59 3.10 ± 0.17 1.31 ± 0.08 1.37 ± 0.04

0.98 0.99 0.99 0.99

7.56 ± 0.63 2.99 ± 0.17 1.34 ± 0.05 1.29 ± 0.07

0.24 ± 0.60 0.27 ± 0.16 1.11 ± 0.05 0.16 ± 0.06

0.96 0.98 0.99 0.99

6.18 ± 1.19 2.75 ± 0.37 1.31 ± 0.04 1.17 ± 0.13

0.234 ± 1.85 0.01 ± 0.57 0.97 ± 0.06 0.19 ± 0.20

0.81 0.90 0.99 0.94

Sulfadimidine SDM)

Sulfadiazine (SDZ)

Sulfadimidine SDM)

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160

160

MN-200 MN-150 F-400 XAD-4

B

140

pH=7.8

120

120

100

100

80

80

3.5. Adsorption isotherms

60

60

MN-200 MN-150 F-400 XAD-4

pH=1.3 40

A

20

0

2

4

6

8

10 12

40 20 0

3

6

9

12

Concentration of Salt (mass %) Fig. 5. Influence of ionic concentration on adsorbed amounts of sulfadiazine (SDZ) at 303 K; A (pH = 1.3) and B (pH = 7.8). Initial concentration = 100 mg L1.

160

160

A

140

Amounts of Adsorption (mg/g)

Adsorption isotherms plotted as adsorbed concentration (q, mg g1 adsorbent) versus aqueous-phase concentration (Ce, mg L1) at equilibrium are shown in Fig. 6. The shapes of the isotherms for the three resins are similar and obviously different from those of F-400. The adsorbed amounts on MN-150 and F-400 increase with increasing temperature, while those adsorbed on XAD-4 present opposite trends, i.e., decreasing with increasing temperature. The adsorbed amounts on MN-200 decrease at pH 1.3 and increase at pH 7.8 with elevated temperatures. The adsorption data were fitted to the Freundlich model, q = KFC ne , by nonlinear regression (Supplementary Table S3). The Freundlich model fits most of the adsorption data very well (R2 > 0.99), which is in agreement with the literature [9,14,15].

T

288 K 303 K 318 K 288 K 303 K 318 K

120 100

T

80 60 40

B

120

288 K 303 K 318 K

T

100 80 60

T

40 20

20 0

5

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

10 15 20 25 30 35 40 45 50 55 60 65 70

Equilibrium Concentration (mg/L)

Equilibrium Concentration (mg/L) 160

C

288 K 303 K 318 K

140

288 K 303 K 318 K

120 100 80

T

60 40

Amounts of Adsorption (mg/g)

160

Amounts of Adsorption (mg/g)

288 K 303 K 318 K

140

Amounts of Adsorption (mg/g)

Amounts of Adsorption (mg/g)

140

that increasing ionic strength facilitates the adsorption of ionic compounds on carbonaceous adsorbents because of electrostatic screening of the surface charge brought about by the counterion species added [35,36].

D

288 K 303 K 318 K

140

288 K 303 K 318 K

120 100 80

T

60 40

T 20 20 0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

Equilibrium Concentration (mg/L)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

Equilibrium Concentration (mg/L)

Fig. 6. The adsorption isotherms of SDZ by (A) MN-200, (B) MN-150, (C) F-400, and (D) XAD-4 at different temperatures and pH 1.3 (straight line with open shape) and 7.8 (dot line with solid shape). Lines are data fittings with the Freundlich model.

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W. Yang et al. / Journal of Colloid and Interface Science 362 (2011) 503–509

Table 2 Adsorption coefficients of the adsorbents at the two pHs. Resin

MN-200

Temperature (K) Kd (L g1)

pH = 1.3 pH = 7.8

MN-150

F-400

XAD-4

288

303

318

288

303

318

288

303

318

288

303

318

4.31 1.27

3.85 1.45

3.08 1.65

2.34 0.53

2.63 0.71

2.91 0.90

0.78 0.73

1.44 1.37

1.83 1.77

1.71 0.43

1.34 0.36

1.10 0.26

The Freundlich n values are all smaller than 1, reflecting adsorption nonlinearity. In order to better explain the relationship between the adsorption affinity and the adsorbents, single-concentration point adsorption coefficients (Kd, L g1) of SDZ at equilibrium concentrations of 50 mg L1 (Table 2) were calculated to quantitatively compare the adsorption mechanism. For F-400, adsorption coefficients (Kd) increase at pH 1.3 and 7.8 when the temperature increases. There are many examples and explanations for the adsorption by F-400 via micropore-filling in the literature [37–39]. Researchers attribute apparent endothermic adsorption to the effective favoring of pore diffusion in terms of the large percentage of micropores and an increase in the package density of adsorbate molecules in the micropores of activated carbons. F-400 has the largest micropore area (86%) and least average pore diameter (2.27 nm) among the three resins; therefore, microporefilling could play a dominant role in sulfonamide adsorption onto it. For XAD-4, the hydrophobic interaction plays a main role in sulfonamide adsorption. The strength of the hydrophobic interactions decreases with increasing temperature [40], while the adsorption coefficient (Kd) decreases, as expected. The tendency of Kd on MN-200 with elevated temperature is different at pH 1.3 and 7.8. At pH 1.3, adsorption coefficients decrease with increasing temperature, similar to observations for XAD-4. Considering their structural similarity (polystyrene), MN-200 and XAD-4 depend on homologous interactions at pH 1.3 during the adsorption process. As such, the adsorption interactions of SDZ on MN-200 and XAD-4 at acidic to circum-neutral conditions may represent hydrophobic interactions dependent on the aid of other interactions. At pH 7.8, the contribution of hydrophobic interactions is drastically weakened due to speciation effects. The adsorbed amounts of sulfonamide on MN-200 increase with increasing temperature, similar to adsorption onto MN-150. This indicates that the key interactions could be identical during the adsorption of sulfonamides onto MN-200 and MN-150 at pH 7.8. Except for hydrophobic interactions, hydrogen bonding, and electrostatic interactions, p–p interactions are the main contributors to the adsorption process on MN-200 and MN-150 at basic conditions.

4. Conclusions This is the first study on the adsorption of sulfonamide antibiotics by porous resins with different surface properties and pore structures. The results demonstrate the following: (1) Adsorption of sulfonamides to porous resins is mainly controlled by pH and surface interactions. Hydrophobic interactions, hydrogen bonding, electrostatic interactions, and p–p interactions provide different contributions to the resins at different pH. (2) Speciation of sulfonamide will change from hardly adsorbing anions to neutral molecules that interact much stronger with porous resins, which provides a cleaner and more cost-effective alternative method by adjusting the aqueous phase pH, applying weakly acidic conditions in adsorption and basic conditions in desorption.

(3) Hypercrosslinked resin MN-200 with bimodal pore size distribution is effective adsorbent for sulfonamides in wastewater treatment. The hypercrosslinked structure, high surface area, and suitable pore size distribution make MN200 superior antibiotic adsorbents with high adsorption capacity. (4) Despite conspicuous differences between the other porous adsorbents, they are not suitable adsorbents for sulfonamides suffering from their incomplete properties compared to MN-200. Therefore, the adsorption selectivity and efficiency to the hypercrosslinked resin can be improved by adjusting the adsorbent pore size distribution and surface area. Our results highlight the importance of considering adsorbent surface property and pore structure in predicting the influencing factors in adsorption and their potential application in associated wastewater treatment. Acknowledgments We gratefully acknowledge the generous support by the National Nature Science Fund (Grant No. 50978137), the Resources Key Subject of National High Technology Research & Development Project (2009AA06Z315), Nature Science Fund of Jiangsu Province (BK2008436), PR China. Additionally, a project funded by the priority academic program development of Jiangsu Higher Education Institutions and the Program for Excellent Talents in Nanjing Normal University (2008101XGQ0079) are also appreciated. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2011.06.071. Reference [1] W.H. Xu, G. Zhang, X.D. Li, S.C. Zou, P. Li, Z.H. Hu, J. Li, Water Res. 41 (2007) 4526. [2] A.B.A. Boxall, D.W. Kolpin, B. Halling-Sørensen, J. Tolls, Environ. Sci. Technol. 37 (2003) 286A. [3] H. Schmitt, K. Stoob, G. Hamscher, E. Smit, W. Seinen, Microb. Ecol. 51 (2006) 267. [4] S. Pérez, P. Eichhorn, D.S. Aga, Environ. Toxicol. Chem. 24 (2005) 1361. [5] T.A. Ternes, A. Joss, H. Siegrist, Environ. Sci. Technol. 38 (2004) 392A. [6] M.K. Richter, M. Sander, M. Krauss, I. Christl, M.G. Dahinden, M.K. Schneider, R.P. Schwarzenbach, Environ. Sci. Technol. 43 (2009) 6632. [7] M. Kahle, C. Stamm, Environ. Sci. Technol. 41 (2007) 132. [8] M. Kahle, C. Stamm, Chemosphere 68 (2007) 1224. [9] J.A. Gao, J.A. Pedersen, Environ. Sci. Technol. 39 (2005) 9509. [10] D. Avisar, O. Primor, I. Gozlan, H. Mamane, Air. Water, Soil Pollut. 209 (2010) 439. [11] S. Thiele-Bruhn, T. Seibicke, H.R. Schulten, P. Leinweber, J. Environ. Qual. 33 (2004) 1331. [12] S.T. Kurwadkar, C.D. Adams, M.T. Meyer, D.W. Kolpin, J. Agric. Food Chem. 55 (2007) 1370. [13] I. Braschi, S. Blasioli, L. Gigli, C.E. Gessa, A. Alberti, A. Martucci, J. Hazard. Mater. 178 (2010) 218. [14] L. Ji, W. Chen, S. Zheng, Z. Xu, D. Zhu, Langmuir 25 (2009) 11608. [15] D. Zhang, B. Pan, H. Zhang, P. Ning, B. Xing, Environ. Sci. Technol. 44 (2010) 3806. [16] L. Ji, F. Liu, Z. Xu, S. Zheng, D. Zhu, Environ. Sci. Technol. 44 (2010) 3116. [17] E.F.S. Vieira, A.R. Cestari, C.S. Oliveira, P.S. de Lima, L.E. Almeida, Thermochim. Acta 459 (2007) 9.

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