Environmental Pollution 194 (2014) 203e209
Contents lists available at ScienceDirect
Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Sorption affinities of sulfamethoxazole and carbamazepine to two sorbents under co-sorption systems Chi Wang a, Hao Li a, Shaohua Liao a, Di Zhang a, Min Wu a, Bo Pan a, *, Baoshan Xing b a b
Faculty of Environmental Science & Engineering, Kunming University of Science & Technology, Kunming, 650500, China Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA, 01003, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 May 2014 Received in revised form 26 July 2014 Accepted 28 July 2014 Available online
The Kd of sulfamethoxazole (SMX) on activated carbon (AC) was larger than that of SMX on single-walled carbon nanotubes (SC), but the competition of SMX with carbamazepine (CBZ) for adsorption sites was weaker on AC than SC. Thus, a large Kd value does not necessarily reflect a high affinity. The analysis of the apparent sorption, competition, desorption hysteresis, and the sorption thermodynamics for SMX and CBZ did not provide sufficient information to distinguish their sorption affinities. The release of the adsorbed CBZ was not altered with SMX as the competitor, but SMX release increased significantly after CBZ addition. The higher sorption affinity of CBZ may be explained by the interactions of the CBZ benzene rings with the aromatic structures of the adsorbents. Although the thermodynamic meaning cannot be described, the release ratio of the adsorbed pollutants provides useful information for understanding pollutant sorption strength and associated risks. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Competitive sorption Complementary sorption Desorption hysteresis Sorption strength Thermodynamics
1. Introduction Different sorption mechanisms occur simultaneously and contribute to organic contaminant sorption, and the strength of the interaction between an adsorbate and an adsorbent can differ depending on the sorption mechanism (Mitchell and Simpson, 2013; Zhang et al., 2010). The interaction strength determines the sorption affinity and the extent of desorption, as well as competition, and is thus important for evaluating contaminant behavior and risk (Wu et al., 2012). Although the terms of sorption “affinity” or “strength” have previously been used to describe adsorbateeadsorbent interactions, no proper parameter has been proposed to quantitatively assess their interaction strength. Some investigators have compared Kd values when referring to sorption strength/affinity (Srinivasan et al., 2014; Ahangar et al., 2009). However, using Kd values to represent the sorption affinity may be inappropriate. For example, although 17a-Ethinylestradiol showed the highest Kd values in EPA-21, its competition to phenanthrene sorption in EPA-21 was not as significant as in other adsorbents (Yu and Huang, 2005). The Kd values are dependent on the abundance of sorption sites rather than the sorption strength alone.
* Corresponding author. E-mail addresses: [email protected], [email protected] (B. Pan). http://dx.doi.org/10.1016/j.envpol.2014.07.033 0269-7491/© 2014 Elsevier Ltd. All rights reserved.
We have previously compared the sorption strengths of sulfamethoxazole (SMX) and carbamazepine (CBZ) on graphite and graphene oxide (Wang et al., 2013). Our primary emphasis was to demonstrate that competitive and complementary sorption occurred simultaneously (Zhang et al., 2012; Pan and Xing, 2010b). We proposed the idea of combining analyses of desorption hysteresis and sorption thermodynamics to understand the sorption strength of an interaction (Wang et al., 2013). Although we avoided comparing the sorption strengths based only on the difference in the Kd values among the competitive sorption systems, the competitors, which have very different sorption coefficients, were added at the same initial concentration (Wang et al., 2013). This experiment design may have hindered our comparison of sorption strengths because the thermodynamic status of sorption for different adsorbates may differ for the same initial concentration. In addition, we always observed increased inhibition of the sorption of the primary adsorbate (DQpri) coinciding with decreased sorption of the competitor (Qsec) as the aqueous-phase concentration of primary adsorbate (Cepri ) was increased (Wang et al., 2013; Zhang et al., 2012; Pan and Xing, 2010b). This observation was explained by the competition for the sorption sites. However, if the primary adsorbate was adsorbed with overwhelming strength over the competitor, the decreased DQpri may be associated with a decreased in Qsec as Cepri increases (i.e., the competitor is unable to replace the adsorbed primary adsorbate). Evidence confirming this
204
C. Wang et al. / Environmental Pollution 194 (2014) 203e209
observation would provide important information for understanding the distinct sorption affinities for different adsorbates. Thus, adsorbents with high sorption coefficients for both SMX and CBZ were selected for this study. In addition, as typical pharmaceuticals, SMX and CBZ have attracted a great deal of research attention because of their persistence, high toxicity, and notable concentrations in the environment (Peng et al., 2008; Fenet et al., 2012). Activated carbon (AC) and single-walled carbon nanotubes (SC) were used in this experiment based on preliminary sorption experiments. Both adsorbents have high sorption coefficients to SMX and CBZ, which will facilitate the discussion on their complementary and co-sorption. The competitors were applied at concentrations that resulted in comparable sorption coefficients. Both sorption and desorption were examined in binary sorption systems. 2. Experimental section
dQe E* F E* ¼ dE*
(2)
where, E* (kJ/mol) is the difference between the sorption energy at Ce and Cs. The relationship between E* and Ce can be described by the following equation: Ce ¼ Cs exp
E* RT
(3)
Thus, for the PM model, Equation (4) is obtained by combining Equations (1)e(3): *a F E* ¼ lnð10Þ Q 0 Z a E*a1 10ðZE Þ
(4)
The competition strength (A, %) was calculated by the equation:
2.1. Materials The adsorbents used in the study were AC and SC. The AC was obtained from Aladdin Company and the SC was purchased from Chengdu Organic Chemistry Company, with purities of 95%. The AC and SC were characterized for their surface areas based on N2 adsorption (Autosorb-1C, Quantachrome). The elemental compositions of C, H, N, S (using oxygen as the carrier gas) and O (using helium as the carrier gas) were measured at 1150 C with an elemental analyzer (MicroCube, Elementar, Germany). The AC and SC were also characterized for their functional group analysis by a Fourier transform infrared (FTIR) spectrometer (Varian 640IR).The spectra were collected in the range of 4000e400 cm1 with 16 scans at a resolution of 8 cm1(Fig. S1). The SMX (99.7%) and CBZ (99.0%) were obtained from Bio Basic Inc. All the other chemicals were analytical grade or better. The properties of the adsorbates and adsorbents are listed previously (Wang et al., 2013) and in Table S1, respectively.
A ¼ Kd Kd0 Kd 100%
(5)
where, Kd and Kd0 are the sorption coefficients for the primary solute in the singlesolute and bi-solute systems, respectively. The absolute amount of the inhibition of sorption of primary sorbate (DQpri) was calculated as: 0
DQ pri ¼ Qepri Qepri
(6) 0
pri where, Qe
pri and Qe are the solid-phase concentrations without or with competitor, 0 pri pri respectively. Qe is calculated at the Ce values corresponding to those for Qe based
on isotherm fitting. To describe the extent of desorption, the release ratio (RR) was calculated as: . RR ¼ Qe Qe3 Qe
(7)
Qe3
2.2. Sorption experiment SMX (100 mg/L) and CBZ (100 mg/L) were separately dissolved in a background solution as stock solutions. The adsorption experiments were conducted in 15-mL glass vials with teflon-lined screw caps, as described previously (Pan and Xing, 2010a). Briefly, the background solution contained 0.01 mol/L CaCl2 and 200 mg/L NaN3 as a biocide. The solid/water (w/w) ratio was 1:10000 for both AC and SC stock solutions. AC and SC were introduced as dry powders. The stock solutions of SMX or CBZ were diluted to a series of 7 concentrations spanning a range from 1 to 50 mg/L. The same concentration series of primary sorbate with competitor was run under the identical reciprocal conditions. According to our preliminary study, adsorption reached equilibrium within 7 days. Thus, the vials were stored in the dark and rotated vertically using an air-bath shaker at 25 C for 7 days. After equilibration, all the vials were centrifuged at 1000 g for 15 min, and the solute concentrations in the supernatants were quantified. The separation of AC and SC from the aqueous phase was confirmed by aqueous detection using a TOC analyzer. The solution pH at equilibrium was also measured. All experiments including the blanks were run in duplicate. For desorption, more than 14 mL of the supernatant was removed from and immediately re-filled with the same volume of background solution in the vials. The vials were resealed and placed in the shaker for 7 days. After equilibrium, the vials were centrifuged, and the concentrations of SMX and CBZ were measured. The desorption process was repeated for another two cycles. For the sorption thermodynamics experiment, the equilibration time was also 7 days, with the same concentration series of SMX or CBZ, and the equilibration temperatures were 5 and 35 C. The rest of the experimental procedure was the same as that for the adsorption experiment. 2.3. Determination of SMX and CBZ The supernatants were placed in 1.5-mL vials and quantified using an HPLC (Agilent Technologies 1200) equipped with a reverse-phase C18 column (5 mm, 4.6 250 mm) and a UV detector at 280 nm. The mobile phase was 60:40 (v:v) of methanol:deionized water with a flow rate of 1 mL/min. The retention time of SMX and CBZ are 1.9 and 3.8 min, respectively, and the peaks for SMX and CBZ could be reliably separated. The detection limits were 0.05 mg/L for SMX and 0.03 mg/L for CBZ. 2.4. Data analysis All the adsorption isotherms were fit using the Polanyi-Mane (PM) model in SigmaPlot 10.0, and the model expression was: log Qe ¼ log Q 0 þ Z½ðRT lnðCs =Ce ÞÞa
where, Qe (mg/kg) and Ce (mg/L) are the equilibrium solid-phase and aqueous-phase concentrations, respectively. Q0 (mg/kg) is the adsorption capacity from the PM model. R is the universal gas constant (8.314 103 kJ/mol K), and T is the absolute temperature (K). Cs represents the adsorbate solubility in water. The site energy distribution, F (E*), is obtained by differentiating Qe (c) to E* Carter et al., 1995:
(1)
(mg/kg) are the solid-phase concentrations at the beginning of the where, Qe and desorption experiment and after three cycles of desorption, respectively. The thermodynamic parameters were calculated using the following equations (Wang et al., 2010): DG ¼ RT ln Kd
(8)
DG ¼ DH TDS
(9)
where, DG, DH, and DS are the standard Gibbs free energy, standard enthalpy, and standard entropy change, respectively. DH and DS were obtained from the slope and intercept of the linear plot of T against DG, respectively. R is the universal gas constant (8.314 J/mol K). T is the temperature (K), and Kd is the single-point adsorption coefficient.
3. Results and discussion 3.1. Competitive sorption between CBZ and SMX The single-solute isotherms and bi-solute isotherms for CBZ and SMX by AC and SC are presented in Fig. 1. Isotherms were fit by applying the PM model, and the resultant parameters are summarized in Table 1. The PM model fits most of the adsorption data very well (R2adj > 0:98). The calculated single-point sorption coefficients were all greater than 104 L/kg, suggesting high sorption for both chemicals. The relatively greater sorption of CBZ could be attributed to its higher hydrophobicity, as indicated by its higher KOW and lower solubility than SMX (Li et al., 2013a; Sun et al., 2010). In addition, CBZ may have stronger pep interactions with AC and SC than SMX (Wang et al., 2013; Pan et al., 2008) because of its two benzene rings. Both chemicals exhibited a greater sorption capacity (Q0) on AC than SC (Table 1), which may be associated with the higher SSA of AC (Table S1). The co-adsorbates were applied at the same thermodynamic state for an adsorbent, with comparable E* values. For example, both SMX and CBZ were applied at E* ¼ 13.2 ± 0.5 kJ mol1 on AC, and 10.3 ± 0.5 kJ mol1 on SC. Competitive sorption was observed
C. Wang et al. / Environmental Pollution 194 (2014) 203e209
205
Fig. 1. Adsorption of SMX in the absence (C) and presence (B) of CBZ on AC (A) and SC (B), and the adsorption of CBZ in the absence (C) and presence (B) of SMX on AC (C) and SC (D). For the adsorption competition experiments, the initial concentration of the competitor was 20 mg/L.
in the four sorption systems, with obvious decreases in the sorption of the primary adsorbates after the addition of co-adsorbate (Fig. 1). The A values (competition strength calculated using Equation (5)) are presented in Table 1. The first observation is the concentration-dependence of the competition. For example, the A values at 0.01Cs of the primary adsorbates were up to 4 times greater than those at 0.1 Cs. This phenomenon may be explained by the sorption potential. It should be noted that the primary adsorbates were applied in varying concentrations, while the competitors were applied at a fixed concentration. The sorption potential of the competitor is relatively higher at low concentrations of the primary adsorbates, which results in stronger competition (larger A values). The second observation is that CBZ is a stronger competitor to SMX than SMX to CBZ, as suggested by the 2.6e6.4 fold higher A values at the same Ce. This observation is most likely associated with higher sorption (both higher Q0 and Kd) of CBZ than SMX. However, when the different types of adsorbents were considered,
the statement of “higher sorption results in stronger competition” is not always valid. For example, the sorption coefficients of SMX on AC were larger than those of SMX on SC, but the sorption competition of SMX to CBZ was weaker on AC than SC. A similar argument could also be observed for CBZ. In other words, the competition was stronger on SC than on AC, despite the higher sorption of both adsorbates on AC. Clearly, the strength of sorption and level of competition were not determined solely by the sorption coefficients. The following experiments and data analysis were thus carefully conducted to further address this point. The sorption energy distributions were calculated based on Equation (4) and are presented in Fig. S2. For both AC and SC, the site energy distributions exhibited an overall narrowing after the addition of the competitor. According to previous studies, this type of narrowing in site energy distribution could reflect a more even distribution of site energy (Wang et al., 2013) or a decrease in surface heterogeneity (Carter et al., 1995). According to Equation (3), the applied adsorbate occupies the high-energy sorption sites
Table 1 Fitting and calculating results of SMX and CBZ adsorption/competition on AC and SC based on Polanyi-Manes model. Sorbents
SMX
AC SC
CBZ
AC SC
a
Competitor
e CBZ e CBZ e SMX e SMX
Polanyi-Manes model
Kd (L/kg)
Competition strength (%)a
logQ0 mg/kg
Z
a
R2adj
0.01Cs
0.1Cs
5.82 6.16 5.50 5.33 5.46 5.49 5.16 5.14
8.78E02 2.79E01 1.07E01 5.53E02 4.80E03 1.04E02 2.90E03 2.00E03
0.90 0.67 0.79 1.24 1.70 1.53 1.76 2.18
0.994 0.988 0.995 0.995 0.996 0.997 0.997 0.996
2.94Eþ04 1.43Eþ04 1.64Eþ04 4.22Eþ03 1.28Eþ05 1.02Eþ05 6.75Eþ04 4.93Eþ04
6.82Eþ03 4.93Eþ03 3.31Eþ03 1.92Eþ03 2.06Eþ04 1.94Eþ04 1.07Eþ04 1.00Eþ04
0
Competition strength was calculated by ðKd Kd Þ=Kd 100%. A1 was for Ce ¼ 0.01Cs, and A2 was for Ce ¼ 0.1Cs.
A1
A2
51.2
27.8
74.4
42.0
20.0
5.65
27.0
6.58
206
C. Wang et al. / Environmental Pollution 194 (2014) 203e209
first and then spreads to low-energy sorption sites. Thus, the gray areas indicated by an arrow indicate the energy range that is occupied by the competitor. The observed overlap of potential sorption sites could be adopted to explain the competitive sorption dynamics, but could not explain the stronger competition on SC, likely because of the comparable E* used for the experimental design. 3.2. Complementary sorption of CBZ and SMX Comparison between sorption inhibition of the primary sorbate (DQpri) and the sorption of the competitor (Qsec) provides useful information for understanding co-sorption in binary sorption systems (Pan and Xing, 2010b). These parameters were calculated for CBZ and SMX, and the results (based on molar concentrations) are presented in Fig. 2. The calculated DQpri values were generally lower than the Qsec values, suggesting that CBZ and SMX may have different preferences for sorption sites and, thus, complementarily occupy their preferred sorption sites. In previous studies, chargeassisted H-bonding (CAHB) was discussed as a specific mechanism for binding between negatively charged organic molecules and negatively charged solid particles (Li et al., 2013b; Teixido et al., 2011). In this work, both AC and SC were negatively charged (Table S1), and contain oxygen-containing functional groups as indicated by FTIR characterization (Fig. S1). The spectra are characterized by C]O stretching at 1656 cm1 (Wen et al., 2007), and eCeOe stretching at 1090 cm1 (Yu et al., 2008). In the experimental pH range (6.7 ± 0.5) of this study, SMX was negatively charged and CBZ was not ionized. Thus, the negatively charged oxygen-containing functional groups could be the specific sorption sites of SMX through CAHB, while the hydrophobic surface of SC and AC could be the unique sorption sites for CBZ.
pri
In general, the DQpri increased with an increase in Ce , while Qsec increased with an increase in Cepri (Fig. 2). The increased sorption of the primary adsorbates resulted in increased overlap of potential sorption sites, which subsequently led to increased DQpri and decreased Qsec. However, this is not the only competitive dynamics observed in current study. As indicated in Fig. 2D, an increase in the CBZ concentration resulted in a decrease in DQpri. An increase in the extent of overlap for potential sorption sites could not explain this phenomenon. The positive values of DQpri suggest that CBZ sorption was inhibited by SMX (as also indicated in Fig. 2B). It is easy to understand that the increase in CBZ concentration caused competition for both sites for CBZ sorption (which resulted in nonlinear sorption) and for SMX sorption (which resulted in competition). The decrease in the inhibition of CBZ sorption may suggest the overwhelmingly greater strength of CBZ sorption on SC compared to that of SMX because of the much stronger sorption affinity between CBZ and SC. The stronger sorption affinity of CBZ compared to SMX may also be explained by their level of surface coverage on both adsorbents. We first assume that both chemicals sorbed with a monolayer coverage over the majority of the projected area. The calculations based on Q0 suggest that CBZ covered 69.7% and 112% of the SSA of AC and SC, respectively, while SMX covered 151% and 232% of the SSA of AC and SC, respectively. SMX sorption exceeded the maximum coverage, likely indicating the invalidity of the assumption of “monolayer coverage over the majority of the projected area”. SMX molecules may interact with these adsorbents with an angle of intersection through their functional groups and, thus, have a limited contact area with the solid particles. The CBZ molecules may be adsorbed with a better match to the aromatic surface of both adsorbents. Clearly, the lower coverage that results from better surface contact (CBZ) will be associated with a stronger
Fig. 2. The comparison of competitor sorption (Qesec , C) and the sorption inhibition of the primary sorbate (DQepri , B). Panels A and B present SMX as the primary sorbate and CBZ as the competitor on AC and SC, respectively. Panels C and D present CBZ as the primary sorbate and SMX as the competitor, respectively. DQe (mmol/kg) is the adsorption inhibition of primary adsorbate in the presence of the competitor.
C. Wang et al. / Environmental Pollution 194 (2014) 203e209
affinity compared to binding with a limited contact area (SMX). The surface coverage of both chemicals was lower on AC, suggesting that some of the inner pores accessed by N2 molecules were not accessible to CBZ and SMX.
207
It should be emphasized that, although the competition strength (A values) was high for SMX to CBZ at low concentrations, the RR values were hardly affected. The A values reflect the change in the sorption coefficient, but are not necessarily related to the sorption affinities.
3.3. Desorption hysteresis of CBZ and SMX in the coadsorption system
3.4. Adsorption thermodynamics
The desorption hysteresis, as indicated by TII (Sander et al., 2005), decreased in the binary sorption system in comparison to the single-solute sorption system (data not shown). This observation is consistent with a more even distribution of sorption energy (Fig. S2). However, the TII values did not measure the extent of desorption. In addition, the re-equilibration from a state (at the sorption point) in which the level of competition is relatively high to a state (at the desorption point) in which the level of competition is relatively low could also result in desorption hysteresis (Sander and Pignatello, 2007). The RR values were calculated and are presented in Fig. 3. A lower RR at the same Qe may suggest that the adsorbed chemicals interact more strongly with solid particles. In all the Qe vs. RR plots, the RR values increased with an increase in the adsorbed concentration, indicating that both adsorbates first occupy sorption sites with higher sorption affinities. This result is consistent with the analysis of the sorption energy distributions. The effect of the addition of the competitor on the RR values differed distinctly for CBZ and SMX. The SMX RR values increased by up to 5 folds with the addition of CBZ, while the CBZ RR values barely changed after SMX addition. This difference likely indicates that the sites with high sorption energies have a higher affinity for CBZ than SMX. This statement is also verified by the plots of the RR values of the competitors against Qepri (Fig. 4C and D).
The thermodynamic parameters, such as free energy change (DG), enthalpy (DH) and entropy change (DS), were determined using Equations (8) and (9). The negative DG values (Fig. 4) suggest that the sorption of both SMX and CBZ on AC or SC are spontaneous (Bekçi et al., 2007). The less negative the DG, the weaker the driving force of sorption (Wang et al., 2009). As shown in Fig. 4, the negative DG value decreased with an increase in temperature, indicating that the driving force of sorption increased with an increase in temperature. The sorption of SMX and CBZ were more favorable on AC than on SC, as the DG values are more negative (absolute value of DG is greater) for sorption on AC. The high adsorption capacity of AC and the low RR values of SMX and CBZ sorbed on AC also support the same conclusion. It should be noted that DG was calculated based on sorption coefficients, and thus the calculated free energy change does not provide any new information beyond that gained from the sorption isotherms. The DH and DS were based on the changes in sorption at different temperatures, and thus new information is expected. The DH values of CBZ (Fig. 4) were higher than those of SMX, indicating that the sorption of CBZ was stronger than that of SMX. The positive values of DS demonstrate an increase in randomness in the system during the adsorption process. An increase in entropy (positive DS) can also be associated with the hydrophobic effect that arises from the distribution of the cohesive energy and H-bonds among water
Fig. 3. RR values of primary adsorbate with or without competitor when SMX was the primary adsorbate (A) and CBZ was the primary adsorbate (B). Solid circles (C and :) represent the primary adsorbate without competitor, and open circles (B and D) indicate the primary adsorbate with competitor. The RR values of the competitor when CBZ was the competitor (C) and when SMX was the competitor (D).
208
C. Wang et al. / Environmental Pollution 194 (2014) 203e209
Fig. 4. Thermodynamic analysis of the sorption isotherms. Gibbs free energy change (DG) of SMX/CBZ sorption on AC and SC at 5 C (A) and 25 C (B). The standard enthalpy change (DH) (C) and standard entropy change (DS) (D) of SMX/CBZ sorption on AC and SC. Circles (C for AC and B for SC) indicate data for SMX, and triangles (: for AC and D for SC) indicate data for CBZ.
molecules (Pignatello, 2011). The sorption of CBZ on AC results in a higher DS in comparison to CBZ sorption on SC because AC has more amorphous carbon and the surface functional groups are less ordered. Desorption is more difficult for systems with higher DS values because more energy is needed to return to the original entropy state (Pan et al., 2012). In this study, competition on AC was less obvious than on SC, and desorption from AC is more likely to occur. Based on the foregoing discussion, we conclude that, for systems with a higher DS, the competition is less intense. 4. Conclusions Although both SMX and CBZ exhibited high sorption to AC and SC, the adsorbed SMX could be easily desorbed, and the release ratio of SMX was much greater than that of CBZ. The sorption coefficients of SMX on AC were greater than those of SMX on SC, but the level of competition to CBZ was weaker for sorption on AC than on SC. The introduction of CBZ into the SMX sorption system increased the release of SMX, but not vice versa. For example, a decrease in DQpri (CBZ) coincided with a decrease in Qsec (SMX) as the Cepri (CBZ) was increased for SC, suggesting that SMX could not replace the adsorbed CBZ. The more negative DG values, higher DH and DS are all correlated with the strong sorption and competition of CBZ. This study demonstrates that a high sorption coefficient does not necessarily result in stronger competition. Releasing ratio in combination of thermodynamic parameters could much better facilitate our understanding on sorption strength. Acknowledgments This research was supported by the National Natural Science Foundation of China (41222025, 41303092), Program for New Century Excellent Talents in University, Chinese Ministry of
Education (NCET-10-0971), Recruitment Program of Highly Qualified Scholars in Yunnan (2010CI109), and the USDA AFRI Hatch program (MAS 00978). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.envpol.2014.07.033. References Ahangar, A.G., Smernik, R.J., Kookana, R.S., Chittleborough, D.J., 2009. The effect of solvent-conditioning on soil organic matter sorption affinity for diuron and phenanthrene. Chemosphere 76, 1062e1066. Bekçi, Z., Seki, Y., Kadir Yurdakoç, M., 2007. A study of equilibrium and FTIR, SEM/ EDS analysis of trimethoprim adsorption onto K10. J. Mol. Struct. 827, 67e74. Carter, M.C., Kilduff, J.E., Weber, W.J., 1995. Site energy distribution analysis of preloaded adsorbents. Environ. Sci. Technol. 29, 1773e1780. Fenet, H., Mathieu, O., Mahjoub, O., Li, Z., Hillaire-Buys, D., Casellas, C., Gomez, E., 2012. Carbamazepine, carbamazepine epoxide and dihydroxycarbamazepine sorption to soil and occurrence in a wastewater reuse site in Tunisia. Chemosphere 88, 49e54. Li, J., Jiang, L., Xiang, X., Xu, S., Wen, R., Liu, X., 2013a. Competitive sorption between 17a-ethinyl estradiol and bisphenol A/4-n-nonylphenol by soils. J. Environ. Sci. 25, 1154e1163. Li, X.Y., Pignatello, J.J., Wang, Y.Q., Xing, B.S., 2013b. New insight into adsorption mechanism of ionizable compounds on carbon nanotubes. Environ. Sci. Technol. 47, 8334e8341. Mitchell, P.J., Simpson, M.J., 2013. High affinity sorption domains in soil are blocked by polar soil organic matter components. Environ. Sci. Technol. 47, 412e419. Pan, B., Huang, P., Wu, M., Wang, Z., Wang, P., Jiao, X., Xing, B., 2012. Physicochemical and sorption properties of thermally-treated sediments with high organic matter content. Bioresour. Technol. 103, 367e373. Pan, B., Lin, D., Mashayekhi, H., Xing, B., 2008. Adsorption and hysteresis of bisphenol a and 17a-Ethinyl estradiol on carbon nanomaterials. Environ. Sci. Technol. 42, 5480e5485. Pan, B., Xing, B., 2010a. Adsorption kinetics of 17 alpha-ethinyl estradiol and bisphenol A on carbon nanomaterials. I. Several concerns regarding pseudofirst order and pseudo-second order models. J. Soils Sediments 10, 838e844.
C. Wang et al. / Environmental Pollution 194 (2014) 203e209 Pan, B., Xing, B.S., 2010b. Competitive and complementary adsorption of bisphenol a and 17 alpha-ethinyl estradiol on carbon nanomaterials. J. Agric. Food Chem. 58, 8338e8343. Peng, X., Tan, J., Tang, C., Yu, Y., Wang, Z., 2008. Multiresidue determination of fluoroquinolone, sulfonamide, trimethoprim, and chloramphenicol antibiotics in urban waters in China. Environ. Toxicol. Chem. 27, 73e79. Pignatello, J.J., 2011. Interactions of Anthropogenic Organic Chemicals with Natural Organic Matter and Black Carbon in Environmental Particles. IUPAC. Sander, M., Lu, Y.F., Pignatello, J.J., 2005. A thermodynamically based method to quantify true sorption hysteresis. J. Environ. Qual. 34, 1063e1072. Sander, M., Pignatello, J.J., 2007. On the reversibility of sorption to black carbon: distinguishing true hysteresis from artificial hysteresis caused by dilution of a competing adsorbate. Environ. Sci. Technol. 41, 843e849. Srinivasan, P., Sarmah, A.K., Manley-Harris, M., 2014. Sorption of selected veterinary antibiotics onto dairy farming soils of contrasting nature. Sci. Total Environ. 472, 695e703. Sun, K., Gao, B., Zhang, Z., Zhang, G., Liu, X., Zhao, Y., Xing, B., 2010. Sorption of endocrine disrupting chemicals by condensed organic matter in soils and sediments. Chemosphere 80, 709e715. Teixido, M., Pignatello, J.J., Beltran, J.L., Granados, M., Peccia, J., 2011. Speciation of the ionizable antibiotic sulfamethazine on black carbon (Biochar). Environ. Sci. Technol. 45, 10020e10027. Wang, C., Li, H., Liao, S., Zheng, H., Wang, Z., Pan, B., Xing, B., 2013. Coadsorption, desorption hysteresis and sorption thermodynamics of sulfamethoxazole and carbamazepine on graphene oxide and graphite. Carbon 65, 243e251.
209
Wang, L., Zhang, J., Zhao, R., Li, C., Li, Y., Zhang, C., 2010. Adsorption of basic dyes on activated carbon prepared from polygonum orientale Linn: equilibrium, kinetic and thermodynamic studies. Desalination 254, 68e74. Wang, Z., Yu, X., Pan, B., Xing, B., 2009. Norfloxacin sorption and its thermodynamics on surface-modified carbon nanotubes. Environ. Sci. Technol. 44, 978e984. Wen, B., Zhang, J.-j., Zhang, S.-z., Shan, X.-q., Khan, S.U., Xing, B., 2007. Phenanthrene sorption to soil humic acid and different humin fractions. Environ. Sci. Technol. 41, 3165e3171. Wu, W.H., Chen, W., Lin, D.H., Yang, K., 2012. Influence of surface oxidation of multiwalled carbon nanotubes on the adsorption affinity and capacity of polar and nonpolar organic compounds in aqueous phase. Environ. Sci. Technol. 46, 5446e5454. Yu, J., Wang, N., Ma, X., 2008. Fabrication and characterization of Poly(lactic acid)/ Acetyl Tributyl Citrate/Carbon black as conductive polymer composites. Biomacromolecules 9, 1050e1057. Yu, Z.Q., Huang, W.L., 2005. Competitive sorption between 17 alpha-ethinyl estradiol and naphthalene/phenanthrene by sediments. Environ. Sci. Technol. 39, 4878e4885. Zhang, D., Pan, B., Wu, M., Zhang, H., Peng, H.B., Ning, P., Xing, B.S., 2012. Cosorption of organic chemicals with different properties: their shared and different sorption sites. Environ. Pollut. 160, 178e184. Zhang, D., Pan, B., Zhang, H., Ning, P., Xing, B., 2010. Contribution of different sulfamethoxazole species to their overall adsorption on functionalized carbon nanotubes. Environ. Sci. Technol. 44, 3806e3811.
Contents lists available at ScienceDirect
Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Sorption affinities of sulfamethoxazole and carbamazepine to two sorbents under co-sorption systems Chi Wang a, Hao Li a, Shaohua Liao a, Di Zhang a, Min Wu a, Bo Pan a, *, Baoshan Xing b a b
Faculty of Environmental Science & Engineering, Kunming University of Science & Technology, Kunming, 650500, China Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA, 01003, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 May 2014 Received in revised form 26 July 2014 Accepted 28 July 2014 Available online
The Kd of sulfamethoxazole (SMX) on activated carbon (AC) was larger than that of SMX on single-walled carbon nanotubes (SC), but the competition of SMX with carbamazepine (CBZ) for adsorption sites was weaker on AC than SC. Thus, a large Kd value does not necessarily reflect a high affinity. The analysis of the apparent sorption, competition, desorption hysteresis, and the sorption thermodynamics for SMX and CBZ did not provide sufficient information to distinguish their sorption affinities. The release of the adsorbed CBZ was not altered with SMX as the competitor, but SMX release increased significantly after CBZ addition. The higher sorption affinity of CBZ may be explained by the interactions of the CBZ benzene rings with the aromatic structures of the adsorbents. Although the thermodynamic meaning cannot be described, the release ratio of the adsorbed pollutants provides useful information for understanding pollutant sorption strength and associated risks. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Competitive sorption Complementary sorption Desorption hysteresis Sorption strength Thermodynamics
1. Introduction Different sorption mechanisms occur simultaneously and contribute to organic contaminant sorption, and the strength of the interaction between an adsorbate and an adsorbent can differ depending on the sorption mechanism (Mitchell and Simpson, 2013; Zhang et al., 2010). The interaction strength determines the sorption affinity and the extent of desorption, as well as competition, and is thus important for evaluating contaminant behavior and risk (Wu et al., 2012). Although the terms of sorption “affinity” or “strength” have previously been used to describe adsorbateeadsorbent interactions, no proper parameter has been proposed to quantitatively assess their interaction strength. Some investigators have compared Kd values when referring to sorption strength/affinity (Srinivasan et al., 2014; Ahangar et al., 2009). However, using Kd values to represent the sorption affinity may be inappropriate. For example, although 17a-Ethinylestradiol showed the highest Kd values in EPA-21, its competition to phenanthrene sorption in EPA-21 was not as significant as in other adsorbents (Yu and Huang, 2005). The Kd values are dependent on the abundance of sorption sites rather than the sorption strength alone.
* Corresponding author. E-mail addresses: [email protected], [email protected] (B. Pan). http://dx.doi.org/10.1016/j.envpol.2014.07.033 0269-7491/© 2014 Elsevier Ltd. All rights reserved.
We have previously compared the sorption strengths of sulfamethoxazole (SMX) and carbamazepine (CBZ) on graphite and graphene oxide (Wang et al., 2013). Our primary emphasis was to demonstrate that competitive and complementary sorption occurred simultaneously (Zhang et al., 2012; Pan and Xing, 2010b). We proposed the idea of combining analyses of desorption hysteresis and sorption thermodynamics to understand the sorption strength of an interaction (Wang et al., 2013). Although we avoided comparing the sorption strengths based only on the difference in the Kd values among the competitive sorption systems, the competitors, which have very different sorption coefficients, were added at the same initial concentration (Wang et al., 2013). This experiment design may have hindered our comparison of sorption strengths because the thermodynamic status of sorption for different adsorbates may differ for the same initial concentration. In addition, we always observed increased inhibition of the sorption of the primary adsorbate (DQpri) coinciding with decreased sorption of the competitor (Qsec) as the aqueous-phase concentration of primary adsorbate (Cepri ) was increased (Wang et al., 2013; Zhang et al., 2012; Pan and Xing, 2010b). This observation was explained by the competition for the sorption sites. However, if the primary adsorbate was adsorbed with overwhelming strength over the competitor, the decreased DQpri may be associated with a decreased in Qsec as Cepri increases (i.e., the competitor is unable to replace the adsorbed primary adsorbate). Evidence confirming this
204
C. Wang et al. / Environmental Pollution 194 (2014) 203e209
observation would provide important information for understanding the distinct sorption affinities for different adsorbates. Thus, adsorbents with high sorption coefficients for both SMX and CBZ were selected for this study. In addition, as typical pharmaceuticals, SMX and CBZ have attracted a great deal of research attention because of their persistence, high toxicity, and notable concentrations in the environment (Peng et al., 2008; Fenet et al., 2012). Activated carbon (AC) and single-walled carbon nanotubes (SC) were used in this experiment based on preliminary sorption experiments. Both adsorbents have high sorption coefficients to SMX and CBZ, which will facilitate the discussion on their complementary and co-sorption. The competitors were applied at concentrations that resulted in comparable sorption coefficients. Both sorption and desorption were examined in binary sorption systems. 2. Experimental section
dQe E* F E* ¼ dE*
(2)
where, E* (kJ/mol) is the difference between the sorption energy at Ce and Cs. The relationship between E* and Ce can be described by the following equation: Ce ¼ Cs exp
E* RT
(3)
Thus, for the PM model, Equation (4) is obtained by combining Equations (1)e(3): *a F E* ¼ lnð10Þ Q 0 Z a E*a1 10ðZE Þ
(4)
The competition strength (A, %) was calculated by the equation:
2.1. Materials The adsorbents used in the study were AC and SC. The AC was obtained from Aladdin Company and the SC was purchased from Chengdu Organic Chemistry Company, with purities of 95%. The AC and SC were characterized for their surface areas based on N2 adsorption (Autosorb-1C, Quantachrome). The elemental compositions of C, H, N, S (using oxygen as the carrier gas) and O (using helium as the carrier gas) were measured at 1150 C with an elemental analyzer (MicroCube, Elementar, Germany). The AC and SC were also characterized for their functional group analysis by a Fourier transform infrared (FTIR) spectrometer (Varian 640IR).The spectra were collected in the range of 4000e400 cm1 with 16 scans at a resolution of 8 cm1(Fig. S1). The SMX (99.7%) and CBZ (99.0%) were obtained from Bio Basic Inc. All the other chemicals were analytical grade or better. The properties of the adsorbates and adsorbents are listed previously (Wang et al., 2013) and in Table S1, respectively.
A ¼ Kd Kd0 Kd 100%
(5)
where, Kd and Kd0 are the sorption coefficients for the primary solute in the singlesolute and bi-solute systems, respectively. The absolute amount of the inhibition of sorption of primary sorbate (DQpri) was calculated as: 0
DQ pri ¼ Qepri Qepri
(6) 0
pri where, Qe
pri and Qe are the solid-phase concentrations without or with competitor, 0 pri pri respectively. Qe is calculated at the Ce values corresponding to those for Qe based
on isotherm fitting. To describe the extent of desorption, the release ratio (RR) was calculated as: . RR ¼ Qe Qe3 Qe
(7)
Qe3
2.2. Sorption experiment SMX (100 mg/L) and CBZ (100 mg/L) were separately dissolved in a background solution as stock solutions. The adsorption experiments were conducted in 15-mL glass vials with teflon-lined screw caps, as described previously (Pan and Xing, 2010a). Briefly, the background solution contained 0.01 mol/L CaCl2 and 200 mg/L NaN3 as a biocide. The solid/water (w/w) ratio was 1:10000 for both AC and SC stock solutions. AC and SC were introduced as dry powders. The stock solutions of SMX or CBZ were diluted to a series of 7 concentrations spanning a range from 1 to 50 mg/L. The same concentration series of primary sorbate with competitor was run under the identical reciprocal conditions. According to our preliminary study, adsorption reached equilibrium within 7 days. Thus, the vials were stored in the dark and rotated vertically using an air-bath shaker at 25 C for 7 days. After equilibration, all the vials were centrifuged at 1000 g for 15 min, and the solute concentrations in the supernatants were quantified. The separation of AC and SC from the aqueous phase was confirmed by aqueous detection using a TOC analyzer. The solution pH at equilibrium was also measured. All experiments including the blanks were run in duplicate. For desorption, more than 14 mL of the supernatant was removed from and immediately re-filled with the same volume of background solution in the vials. The vials were resealed and placed in the shaker for 7 days. After equilibrium, the vials were centrifuged, and the concentrations of SMX and CBZ were measured. The desorption process was repeated for another two cycles. For the sorption thermodynamics experiment, the equilibration time was also 7 days, with the same concentration series of SMX or CBZ, and the equilibration temperatures were 5 and 35 C. The rest of the experimental procedure was the same as that for the adsorption experiment. 2.3. Determination of SMX and CBZ The supernatants were placed in 1.5-mL vials and quantified using an HPLC (Agilent Technologies 1200) equipped with a reverse-phase C18 column (5 mm, 4.6 250 mm) and a UV detector at 280 nm. The mobile phase was 60:40 (v:v) of methanol:deionized water with a flow rate of 1 mL/min. The retention time of SMX and CBZ are 1.9 and 3.8 min, respectively, and the peaks for SMX and CBZ could be reliably separated. The detection limits were 0.05 mg/L for SMX and 0.03 mg/L for CBZ. 2.4. Data analysis All the adsorption isotherms were fit using the Polanyi-Mane (PM) model in SigmaPlot 10.0, and the model expression was: log Qe ¼ log Q 0 þ Z½ðRT lnðCs =Ce ÞÞa
where, Qe (mg/kg) and Ce (mg/L) are the equilibrium solid-phase and aqueous-phase concentrations, respectively. Q0 (mg/kg) is the adsorption capacity from the PM model. R is the universal gas constant (8.314 103 kJ/mol K), and T is the absolute temperature (K). Cs represents the adsorbate solubility in water. The site energy distribution, F (E*), is obtained by differentiating Qe (c) to E* Carter et al., 1995:
(1)
(mg/kg) are the solid-phase concentrations at the beginning of the where, Qe and desorption experiment and after three cycles of desorption, respectively. The thermodynamic parameters were calculated using the following equations (Wang et al., 2010): DG ¼ RT ln Kd
(8)
DG ¼ DH TDS
(9)
where, DG, DH, and DS are the standard Gibbs free energy, standard enthalpy, and standard entropy change, respectively. DH and DS were obtained from the slope and intercept of the linear plot of T against DG, respectively. R is the universal gas constant (8.314 J/mol K). T is the temperature (K), and Kd is the single-point adsorption coefficient.
3. Results and discussion 3.1. Competitive sorption between CBZ and SMX The single-solute isotherms and bi-solute isotherms for CBZ and SMX by AC and SC are presented in Fig. 1. Isotherms were fit by applying the PM model, and the resultant parameters are summarized in Table 1. The PM model fits most of the adsorption data very well (R2adj > 0:98). The calculated single-point sorption coefficients were all greater than 104 L/kg, suggesting high sorption for both chemicals. The relatively greater sorption of CBZ could be attributed to its higher hydrophobicity, as indicated by its higher KOW and lower solubility than SMX (Li et al., 2013a; Sun et al., 2010). In addition, CBZ may have stronger pep interactions with AC and SC than SMX (Wang et al., 2013; Pan et al., 2008) because of its two benzene rings. Both chemicals exhibited a greater sorption capacity (Q0) on AC than SC (Table 1), which may be associated with the higher SSA of AC (Table S1). The co-adsorbates were applied at the same thermodynamic state for an adsorbent, with comparable E* values. For example, both SMX and CBZ were applied at E* ¼ 13.2 ± 0.5 kJ mol1 on AC, and 10.3 ± 0.5 kJ mol1 on SC. Competitive sorption was observed
C. Wang et al. / Environmental Pollution 194 (2014) 203e209
205
Fig. 1. Adsorption of SMX in the absence (C) and presence (B) of CBZ on AC (A) and SC (B), and the adsorption of CBZ in the absence (C) and presence (B) of SMX on AC (C) and SC (D). For the adsorption competition experiments, the initial concentration of the competitor was 20 mg/L.
in the four sorption systems, with obvious decreases in the sorption of the primary adsorbates after the addition of co-adsorbate (Fig. 1). The A values (competition strength calculated using Equation (5)) are presented in Table 1. The first observation is the concentration-dependence of the competition. For example, the A values at 0.01Cs of the primary adsorbates were up to 4 times greater than those at 0.1 Cs. This phenomenon may be explained by the sorption potential. It should be noted that the primary adsorbates were applied in varying concentrations, while the competitors were applied at a fixed concentration. The sorption potential of the competitor is relatively higher at low concentrations of the primary adsorbates, which results in stronger competition (larger A values). The second observation is that CBZ is a stronger competitor to SMX than SMX to CBZ, as suggested by the 2.6e6.4 fold higher A values at the same Ce. This observation is most likely associated with higher sorption (both higher Q0 and Kd) of CBZ than SMX. However, when the different types of adsorbents were considered,
the statement of “higher sorption results in stronger competition” is not always valid. For example, the sorption coefficients of SMX on AC were larger than those of SMX on SC, but the sorption competition of SMX to CBZ was weaker on AC than SC. A similar argument could also be observed for CBZ. In other words, the competition was stronger on SC than on AC, despite the higher sorption of both adsorbates on AC. Clearly, the strength of sorption and level of competition were not determined solely by the sorption coefficients. The following experiments and data analysis were thus carefully conducted to further address this point. The sorption energy distributions were calculated based on Equation (4) and are presented in Fig. S2. For both AC and SC, the site energy distributions exhibited an overall narrowing after the addition of the competitor. According to previous studies, this type of narrowing in site energy distribution could reflect a more even distribution of site energy (Wang et al., 2013) or a decrease in surface heterogeneity (Carter et al., 1995). According to Equation (3), the applied adsorbate occupies the high-energy sorption sites
Table 1 Fitting and calculating results of SMX and CBZ adsorption/competition on AC and SC based on Polanyi-Manes model. Sorbents
SMX
AC SC
CBZ
AC SC
a
Competitor
e CBZ e CBZ e SMX e SMX
Polanyi-Manes model
Kd (L/kg)
Competition strength (%)a
logQ0 mg/kg
Z
a
R2adj
0.01Cs
0.1Cs
5.82 6.16 5.50 5.33 5.46 5.49 5.16 5.14
8.78E02 2.79E01 1.07E01 5.53E02 4.80E03 1.04E02 2.90E03 2.00E03
0.90 0.67 0.79 1.24 1.70 1.53 1.76 2.18
0.994 0.988 0.995 0.995 0.996 0.997 0.997 0.996
2.94Eþ04 1.43Eþ04 1.64Eþ04 4.22Eþ03 1.28Eþ05 1.02Eþ05 6.75Eþ04 4.93Eþ04
6.82Eþ03 4.93Eþ03 3.31Eþ03 1.92Eþ03 2.06Eþ04 1.94Eþ04 1.07Eþ04 1.00Eþ04
0
Competition strength was calculated by ðKd Kd Þ=Kd 100%. A1 was for Ce ¼ 0.01Cs, and A2 was for Ce ¼ 0.1Cs.
A1
A2
51.2
27.8
74.4
42.0
20.0
5.65
27.0
6.58
206
C. Wang et al. / Environmental Pollution 194 (2014) 203e209
first and then spreads to low-energy sorption sites. Thus, the gray areas indicated by an arrow indicate the energy range that is occupied by the competitor. The observed overlap of potential sorption sites could be adopted to explain the competitive sorption dynamics, but could not explain the stronger competition on SC, likely because of the comparable E* used for the experimental design. 3.2. Complementary sorption of CBZ and SMX Comparison between sorption inhibition of the primary sorbate (DQpri) and the sorption of the competitor (Qsec) provides useful information for understanding co-sorption in binary sorption systems (Pan and Xing, 2010b). These parameters were calculated for CBZ and SMX, and the results (based on molar concentrations) are presented in Fig. 2. The calculated DQpri values were generally lower than the Qsec values, suggesting that CBZ and SMX may have different preferences for sorption sites and, thus, complementarily occupy their preferred sorption sites. In previous studies, chargeassisted H-bonding (CAHB) was discussed as a specific mechanism for binding between negatively charged organic molecules and negatively charged solid particles (Li et al., 2013b; Teixido et al., 2011). In this work, both AC and SC were negatively charged (Table S1), and contain oxygen-containing functional groups as indicated by FTIR characterization (Fig. S1). The spectra are characterized by C]O stretching at 1656 cm1 (Wen et al., 2007), and eCeOe stretching at 1090 cm1 (Yu et al., 2008). In the experimental pH range (6.7 ± 0.5) of this study, SMX was negatively charged and CBZ was not ionized. Thus, the negatively charged oxygen-containing functional groups could be the specific sorption sites of SMX through CAHB, while the hydrophobic surface of SC and AC could be the unique sorption sites for CBZ.
pri
In general, the DQpri increased with an increase in Ce , while Qsec increased with an increase in Cepri (Fig. 2). The increased sorption of the primary adsorbates resulted in increased overlap of potential sorption sites, which subsequently led to increased DQpri and decreased Qsec. However, this is not the only competitive dynamics observed in current study. As indicated in Fig. 2D, an increase in the CBZ concentration resulted in a decrease in DQpri. An increase in the extent of overlap for potential sorption sites could not explain this phenomenon. The positive values of DQpri suggest that CBZ sorption was inhibited by SMX (as also indicated in Fig. 2B). It is easy to understand that the increase in CBZ concentration caused competition for both sites for CBZ sorption (which resulted in nonlinear sorption) and for SMX sorption (which resulted in competition). The decrease in the inhibition of CBZ sorption may suggest the overwhelmingly greater strength of CBZ sorption on SC compared to that of SMX because of the much stronger sorption affinity between CBZ and SC. The stronger sorption affinity of CBZ compared to SMX may also be explained by their level of surface coverage on both adsorbents. We first assume that both chemicals sorbed with a monolayer coverage over the majority of the projected area. The calculations based on Q0 suggest that CBZ covered 69.7% and 112% of the SSA of AC and SC, respectively, while SMX covered 151% and 232% of the SSA of AC and SC, respectively. SMX sorption exceeded the maximum coverage, likely indicating the invalidity of the assumption of “monolayer coverage over the majority of the projected area”. SMX molecules may interact with these adsorbents with an angle of intersection through their functional groups and, thus, have a limited contact area with the solid particles. The CBZ molecules may be adsorbed with a better match to the aromatic surface of both adsorbents. Clearly, the lower coverage that results from better surface contact (CBZ) will be associated with a stronger
Fig. 2. The comparison of competitor sorption (Qesec , C) and the sorption inhibition of the primary sorbate (DQepri , B). Panels A and B present SMX as the primary sorbate and CBZ as the competitor on AC and SC, respectively. Panels C and D present CBZ as the primary sorbate and SMX as the competitor, respectively. DQe (mmol/kg) is the adsorption inhibition of primary adsorbate in the presence of the competitor.
C. Wang et al. / Environmental Pollution 194 (2014) 203e209
affinity compared to binding with a limited contact area (SMX). The surface coverage of both chemicals was lower on AC, suggesting that some of the inner pores accessed by N2 molecules were not accessible to CBZ and SMX.
207
It should be emphasized that, although the competition strength (A values) was high for SMX to CBZ at low concentrations, the RR values were hardly affected. The A values reflect the change in the sorption coefficient, but are not necessarily related to the sorption affinities.
3.3. Desorption hysteresis of CBZ and SMX in the coadsorption system
3.4. Adsorption thermodynamics
The desorption hysteresis, as indicated by TII (Sander et al., 2005), decreased in the binary sorption system in comparison to the single-solute sorption system (data not shown). This observation is consistent with a more even distribution of sorption energy (Fig. S2). However, the TII values did not measure the extent of desorption. In addition, the re-equilibration from a state (at the sorption point) in which the level of competition is relatively high to a state (at the desorption point) in which the level of competition is relatively low could also result in desorption hysteresis (Sander and Pignatello, 2007). The RR values were calculated and are presented in Fig. 3. A lower RR at the same Qe may suggest that the adsorbed chemicals interact more strongly with solid particles. In all the Qe vs. RR plots, the RR values increased with an increase in the adsorbed concentration, indicating that both adsorbates first occupy sorption sites with higher sorption affinities. This result is consistent with the analysis of the sorption energy distributions. The effect of the addition of the competitor on the RR values differed distinctly for CBZ and SMX. The SMX RR values increased by up to 5 folds with the addition of CBZ, while the CBZ RR values barely changed after SMX addition. This difference likely indicates that the sites with high sorption energies have a higher affinity for CBZ than SMX. This statement is also verified by the plots of the RR values of the competitors against Qepri (Fig. 4C and D).
The thermodynamic parameters, such as free energy change (DG), enthalpy (DH) and entropy change (DS), were determined using Equations (8) and (9). The negative DG values (Fig. 4) suggest that the sorption of both SMX and CBZ on AC or SC are spontaneous (Bekçi et al., 2007). The less negative the DG, the weaker the driving force of sorption (Wang et al., 2009). As shown in Fig. 4, the negative DG value decreased with an increase in temperature, indicating that the driving force of sorption increased with an increase in temperature. The sorption of SMX and CBZ were more favorable on AC than on SC, as the DG values are more negative (absolute value of DG is greater) for sorption on AC. The high adsorption capacity of AC and the low RR values of SMX and CBZ sorbed on AC also support the same conclusion. It should be noted that DG was calculated based on sorption coefficients, and thus the calculated free energy change does not provide any new information beyond that gained from the sorption isotherms. The DH and DS were based on the changes in sorption at different temperatures, and thus new information is expected. The DH values of CBZ (Fig. 4) were higher than those of SMX, indicating that the sorption of CBZ was stronger than that of SMX. The positive values of DS demonstrate an increase in randomness in the system during the adsorption process. An increase in entropy (positive DS) can also be associated with the hydrophobic effect that arises from the distribution of the cohesive energy and H-bonds among water
Fig. 3. RR values of primary adsorbate with or without competitor when SMX was the primary adsorbate (A) and CBZ was the primary adsorbate (B). Solid circles (C and :) represent the primary adsorbate without competitor, and open circles (B and D) indicate the primary adsorbate with competitor. The RR values of the competitor when CBZ was the competitor (C) and when SMX was the competitor (D).
208
C. Wang et al. / Environmental Pollution 194 (2014) 203e209
Fig. 4. Thermodynamic analysis of the sorption isotherms. Gibbs free energy change (DG) of SMX/CBZ sorption on AC and SC at 5 C (A) and 25 C (B). The standard enthalpy change (DH) (C) and standard entropy change (DS) (D) of SMX/CBZ sorption on AC and SC. Circles (C for AC and B for SC) indicate data for SMX, and triangles (: for AC and D for SC) indicate data for CBZ.
molecules (Pignatello, 2011). The sorption of CBZ on AC results in a higher DS in comparison to CBZ sorption on SC because AC has more amorphous carbon and the surface functional groups are less ordered. Desorption is more difficult for systems with higher DS values because more energy is needed to return to the original entropy state (Pan et al., 2012). In this study, competition on AC was less obvious than on SC, and desorption from AC is more likely to occur. Based on the foregoing discussion, we conclude that, for systems with a higher DS, the competition is less intense. 4. Conclusions Although both SMX and CBZ exhibited high sorption to AC and SC, the adsorbed SMX could be easily desorbed, and the release ratio of SMX was much greater than that of CBZ. The sorption coefficients of SMX on AC were greater than those of SMX on SC, but the level of competition to CBZ was weaker for sorption on AC than on SC. The introduction of CBZ into the SMX sorption system increased the release of SMX, but not vice versa. For example, a decrease in DQpri (CBZ) coincided with a decrease in Qsec (SMX) as the Cepri (CBZ) was increased for SC, suggesting that SMX could not replace the adsorbed CBZ. The more negative DG values, higher DH and DS are all correlated with the strong sorption and competition of CBZ. This study demonstrates that a high sorption coefficient does not necessarily result in stronger competition. Releasing ratio in combination of thermodynamic parameters could much better facilitate our understanding on sorption strength. Acknowledgments This research was supported by the National Natural Science Foundation of China (41222025, 41303092), Program for New Century Excellent Talents in University, Chinese Ministry of
Education (NCET-10-0971), Recruitment Program of Highly Qualified Scholars in Yunnan (2010CI109), and the USDA AFRI Hatch program (MAS 00978). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.envpol.2014.07.033. References Ahangar, A.G., Smernik, R.J., Kookana, R.S., Chittleborough, D.J., 2009. The effect of solvent-conditioning on soil organic matter sorption affinity for diuron and phenanthrene. Chemosphere 76, 1062e1066. Bekçi, Z., Seki, Y., Kadir Yurdakoç, M., 2007. A study of equilibrium and FTIR, SEM/ EDS analysis of trimethoprim adsorption onto K10. J. Mol. Struct. 827, 67e74. Carter, M.C., Kilduff, J.E., Weber, W.J., 1995. Site energy distribution analysis of preloaded adsorbents. Environ. Sci. Technol. 29, 1773e1780. Fenet, H., Mathieu, O., Mahjoub, O., Li, Z., Hillaire-Buys, D., Casellas, C., Gomez, E., 2012. Carbamazepine, carbamazepine epoxide and dihydroxycarbamazepine sorption to soil and occurrence in a wastewater reuse site in Tunisia. Chemosphere 88, 49e54. Li, J., Jiang, L., Xiang, X., Xu, S., Wen, R., Liu, X., 2013a. Competitive sorption between 17a-ethinyl estradiol and bisphenol A/4-n-nonylphenol by soils. J. Environ. Sci. 25, 1154e1163. Li, X.Y., Pignatello, J.J., Wang, Y.Q., Xing, B.S., 2013b. New insight into adsorption mechanism of ionizable compounds on carbon nanotubes. Environ. Sci. Technol. 47, 8334e8341. Mitchell, P.J., Simpson, M.J., 2013. High affinity sorption domains in soil are blocked by polar soil organic matter components. Environ. Sci. Technol. 47, 412e419. Pan, B., Huang, P., Wu, M., Wang, Z., Wang, P., Jiao, X., Xing, B., 2012. Physicochemical and sorption properties of thermally-treated sediments with high organic matter content. Bioresour. Technol. 103, 367e373. Pan, B., Lin, D., Mashayekhi, H., Xing, B., 2008. Adsorption and hysteresis of bisphenol a and 17a-Ethinyl estradiol on carbon nanomaterials. Environ. Sci. Technol. 42, 5480e5485. Pan, B., Xing, B., 2010a. Adsorption kinetics of 17 alpha-ethinyl estradiol and bisphenol A on carbon nanomaterials. I. Several concerns regarding pseudofirst order and pseudo-second order models. J. Soils Sediments 10, 838e844.
C. Wang et al. / Environmental Pollution 194 (2014) 203e209 Pan, B., Xing, B.S., 2010b. Competitive and complementary adsorption of bisphenol a and 17 alpha-ethinyl estradiol on carbon nanomaterials. J. Agric. Food Chem. 58, 8338e8343. Peng, X., Tan, J., Tang, C., Yu, Y., Wang, Z., 2008. Multiresidue determination of fluoroquinolone, sulfonamide, trimethoprim, and chloramphenicol antibiotics in urban waters in China. Environ. Toxicol. Chem. 27, 73e79. Pignatello, J.J., 2011. Interactions of Anthropogenic Organic Chemicals with Natural Organic Matter and Black Carbon in Environmental Particles. IUPAC. Sander, M., Lu, Y.F., Pignatello, J.J., 2005. A thermodynamically based method to quantify true sorption hysteresis. J. Environ. Qual. 34, 1063e1072. Sander, M., Pignatello, J.J., 2007. On the reversibility of sorption to black carbon: distinguishing true hysteresis from artificial hysteresis caused by dilution of a competing adsorbate. Environ. Sci. Technol. 41, 843e849. Srinivasan, P., Sarmah, A.K., Manley-Harris, M., 2014. Sorption of selected veterinary antibiotics onto dairy farming soils of contrasting nature. Sci. Total Environ. 472, 695e703. Sun, K., Gao, B., Zhang, Z., Zhang, G., Liu, X., Zhao, Y., Xing, B., 2010. Sorption of endocrine disrupting chemicals by condensed organic matter in soils and sediments. Chemosphere 80, 709e715. Teixido, M., Pignatello, J.J., Beltran, J.L., Granados, M., Peccia, J., 2011. Speciation of the ionizable antibiotic sulfamethazine on black carbon (Biochar). Environ. Sci. Technol. 45, 10020e10027. Wang, C., Li, H., Liao, S., Zheng, H., Wang, Z., Pan, B., Xing, B., 2013. Coadsorption, desorption hysteresis and sorption thermodynamics of sulfamethoxazole and carbamazepine on graphene oxide and graphite. Carbon 65, 243e251.
209
Wang, L., Zhang, J., Zhao, R., Li, C., Li, Y., Zhang, C., 2010. Adsorption of basic dyes on activated carbon prepared from polygonum orientale Linn: equilibrium, kinetic and thermodynamic studies. Desalination 254, 68e74. Wang, Z., Yu, X., Pan, B., Xing, B., 2009. Norfloxacin sorption and its thermodynamics on surface-modified carbon nanotubes. Environ. Sci. Technol. 44, 978e984. Wen, B., Zhang, J.-j., Zhang, S.-z., Shan, X.-q., Khan, S.U., Xing, B., 2007. Phenanthrene sorption to soil humic acid and different humin fractions. Environ. Sci. Technol. 41, 3165e3171. Wu, W.H., Chen, W., Lin, D.H., Yang, K., 2012. Influence of surface oxidation of multiwalled carbon nanotubes on the adsorption affinity and capacity of polar and nonpolar organic compounds in aqueous phase. Environ. Sci. Technol. 46, 5446e5454. Yu, J., Wang, N., Ma, X., 2008. Fabrication and characterization of Poly(lactic acid)/ Acetyl Tributyl Citrate/Carbon black as conductive polymer composites. Biomacromolecules 9, 1050e1057. Yu, Z.Q., Huang, W.L., 2005. Competitive sorption between 17 alpha-ethinyl estradiol and naphthalene/phenanthrene by sediments. Environ. Sci. Technol. 39, 4878e4885. Zhang, D., Pan, B., Wu, M., Zhang, H., Peng, H.B., Ning, P., Xing, B.S., 2012. Cosorption of organic chemicals with different properties: their shared and different sorption sites. Environ. Pollut. 160, 178e184. Zhang, D., Pan, B., Zhang, H., Ning, P., Xing, B., 2010. Contribution of different sulfamethoxazole species to their overall adsorption on functionalized carbon nanotubes. Environ. Sci. Technol. 44, 3806e3811.
