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Bisphenol A sorption by organo-montmorillonite: Implications for the removal of organic contaminants from water
Chemosphere xxx (2014) xxx–xxx
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Bisphenol A sorption by organo-montmorillonite: Implications for the removal of organic contaminants from water Yuri Park a, Zhiming Sun a,b, Godwin A. Ayoko a,⇑, Ray L. Frost a a b
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, GPO 2434, Brisbane, Queensland 4001, Australia School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing 100083, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Bisphenol A (BPA) is an emerging
pollutant that has attracted immense interest. Different types of organoclays were prepared from different types of surfactants. The loaded surfactants created hydrophobic phases via partitioning mechanism. Thus the organoclays were effective in removing BPA from aqueous solutions.
a r t i c l e
i n f o
Article history: Received 14 May 2013 Received in revised form 9 December 2013 Accepted 18 December 2013 Available online xxxx Keywords: BPA Organoclays Thermodynamics Mechanism Application Environment
a b s t r a c t Remediation of bisphenol A (BPA) from aqueous solutions by adsorption using organoclays synthesized from montmorillonite (MMT) with different types of organic surfactant molecules was demonstrated. High adsorption capacities of the organoclays for the uptake of BPA were observed and these demonstrated their potential application as strong adsorbents for noxious organic water contaminants. The adsorption of BPA was significantly influenced by pH, with increased adsorption of BPA in acidic pH range. However, the organoclays intercalated with highly loaded surfactants and/or large surfactant molecules were less influenced by the pH of the environment and this was thought to be due to the shielding the negative charge from surfactant molecules and the development of more positive charge on the clay surface, which leads to the attraction of anionic BPA even at alkaline pH. The hydrophobic phase created by loaded surfactant molecules contributed to a partitioning phase, interacting with BPA molecules strongly through hydrophobic interaction. Pseudo-second order kinetic model and Langmuir isotherm provided the best fit for the adsorption of BPA onto the organoclays. In addition, the adsorption process was spontaneous and exothermic with lower temperature facilitating the adsorption of BPA onto the organoclays. The described process provides a potential pathway for the removal of BPA from contaminated waters. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
⇑ Corresponding author. Address: 2 George Street, Brisbane, Queensland 4001, Australia. Tel.: +61 731382586. E-mail address: [email protected] (G.A. Ayoko).
The growing population and urbanization of society have resulted in increased production and dispersion of toxic chemicals, including the endocrine-disrupting chemicals (EDCs) into the environment (Mohapatra et al., 2010). Many EDCs including
0045-6535/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.12.050
Please cite this article in press as: Park, Y., et al. Bisphenol A sorption by organo-montmorillonite: Implications for the removal of organic contaminants from water. Chemosphere (2014), http://dx.doi.org/10.1016/j.chemosphere.2013.12.050
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natural estrogens (e.g. estrone (E1), 17b-strodial (E2), and estriol (E3)), synthesized estrogen (e.g. ethinylestradiol (EE2)), and industrial compounds (e.g. bisphenol A (BPA) and nonylphenol) are organic compounds with one or more phenolic groups. Due to their structures and physicochemical properties, these molecules can naturally occupy the high affinity binding acceptor sites of the hormone receptors. These results in disrupted endocrine systems and contribute to environmental risks for animals and humans (Staples et al., 1998; Suzuki et al., 2004; Jiang et al., 2005). EDCs are removed from sewage through wastewater treatment plants (WWTPs) (Rudder et al., 2004; Dong et al., 2010). However, due to their complex structures and low biodegradability (Petrovic´ et al., 2003; Esperanza et al., 2007; Pan et al., 2009), they are not removed completely through biological treatment and can still be detected in the discharge of WWTPS. Since these molecules have been detected in wastewater, groundwater, sediments, and even drinking water, EDCs are of great public and environmental concerns (Petrovic´ et al., 2001; Petrovic et al., 2002; Auriol et al., 2006; Benotti et al., 2009). BPA is widely used as a monomer for the manufacture of polycarbonates and epoxy resin in the plastic industry. Because of its ubiquitous nature in the environment and its negative effects to human health and organisms (Kang et al., 2006; Klecˇka et al., 2009), it is considered as one of high priority compounds that must be monitored and removed from the environment. Hence, BPA is selected as the test molecule in this study. There is a clear need for the development of simple and highly efficient methods for the safe remediation of organic substrates. Some techniques, including adsorption (Dong et al., 2010; Sui et al., 2011; Xu et al., 2012), membrane filtration (Urase et al., 2005; Yoon et al., 2007), electrochemical technique (Gözmen et al., 2003), and catalysis/photocatalysis (Ohko et al., 2001; Fukahori et al., 2003; Chiang et al., 2004; Torres et al., 2006; Torres-Palma et al., 2010) have been proposed for the removal of these contaminants. The approach reported in this study is adsorption process by organoclays which has some advantages over the other techniques. These include their relatively low operational cost, ease of operation, and production of less secondary products (Ali and Gupta, 2007). In recent years, adsorption by organoclays is widely used for the removal of organic and inorganic contaminants in waters (Zhu et al., 1997, 2011; Wu et al., 2001; Bhatnagar and Jain, 2005; Chen et al., 2007; Su et al., 2011) because the use of such low-cost adsorbents which utilizes materials that naturally available is economically feasible. Among many materials that are naturally abundant, clay minerals are potential adsorbents due to their low cost, environmental stability, high adsorption/absorption, and ion exchange properties (Babel and Kurniawan, 2003; Al-Degs et al., 2006; Park et al., 2011). In particular, montmorillonite (MMT) is widely used because of its high cation exchange capacity (CEC), swelling property, and high surface area (Park et al., 2011). However, one of the drawbacks of clay minerals is their hydrophilic properties and this structural property makes them ineffective for the removal of organic pollutants. The permanent negatively charged clay surface can be modified by cationic surfactant molecules, and through ion exchange, hydrophilic clays can be converted to hydrophobic organoclays. We previously demonstrated that these organoclays are effective adsorbents for the removal of phenolic compounds. Given that BPA has two phenolic groups, we reasoned that the organoclays will be good adsorbents for removing BPA from aqueous media. Thus, the aims of this work are to: (a) investigate the adsorption properties of organoclays toward BPA, (b) demonstrate their potential for the uptake of BPA from aqueous solutions under different experimental conditions such as the agitation time, pH, temperature, and different BPA concentrations, (c) evaluate the mechanisms involved in the adsorption of BPA by the organoclays,
and (d) compare the results with those obtained when different adsorbents and other methods were used. To the best of our knowledge, the removal of BPA by organoclays has not been investigated previously, and this work demonstrates the potential performance of the organoclays as adsorbent for the uptake of the industrial by-products such as BPA from aqueous solutions.
2. Materials and experimental methods 2.1. Materials Pure MMT was purchased from Sigma–Aldrich and used without further purification. Its cation exchange capacity (CEC) is 76.4 meq/100 g. Different types of cationic surfactants were used as examples of mono- and di-alkyl cationic surfactants in this study and these are: (i) dodecyltrimethylammonium bromide (denoted as DDTMA, C15H34NBr, FW: 308.34), (ii) hexadecyltrimethylammonium bromide (denoted as HDTMA, C19H42NBr, FW: 364.46), and (iii) didodecyldimethylammonium bromide (denoted as DDDMA, C26H56NBr, FW: 462.65) and all of which were purchased from Sigma–Aldrich and used without further purification. 2.2. Synthesis and characterization of organoclays The organoclays were prepared and characterized by various techniques including powder X-ray diffraction (XRD), surface area measurement (BET method), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA), and their structural properties and characteristics were already described in the previous studies by Park et al. (2012, 2013). 2.3. Batch adsorption experiments BPA [2,2-bis(4-hydroxyphenyl)propane] was purchased from Sigma–Aldrich and used to investigate the adsorption capacities of MMT and the modified organoclays. BPA solution was initially prepared as a stock solution (5000 mg L1) in ethanol due to its low solubility in water, and it was further diluted in water for the preparation of different BPA concentrations. From the initial experiment to determine the amount of adsorbent required, 0.2–0.3 g of the adsorbents were placed in sealed 50 mL centrifuge tubes with 40 mL of a BPA solution in the appropriate concentration (which ranged from 5 to 500 mg L1). The bottles were placed in a rotary shaker and/or a shaking water bath for 12 h at the appropriate temperature. The initial pH of BPA was close to pH 4–5. After shaking, the mixture was filtered through 0.22 lm filters (hydrophilic PTFE membrane) and the supernatants were analyzed by a high-performance liquid chromatography (HPLC, Agilent HP 1100) system coupled with a Luna 5l C18 column (Phenomenex Pty Ltd.) and a UV detector. A mobile phase mixture containing 70% methanol and 30% water at a flow rate of 0.1 mL min1 and 40 lL of injection volume was used for this experiment and the detector was operated at 278 nm. To undertake the adsorption kinetics, the adsorbents were added with an initial BPA concentration (100 mg L1) at room temperature in order to determine the minimum time to reach the equilibrium concentration. The concentrations of BPA were measured at different time intervals from 10 min to 240 min. To evaluate the thermodynamic properties, the adsorbed concentrations of BPA solution by the organoclays were obtained at 296.15, 308.15, and 318.15 K, respectively. Similarly, the effect of the pH on the adsorption of BPA (with initial BPA concentration of 100 mg L1) was studied by adjusting the pH of BPA solution (using either diluted 0.1 M
Please cite this article in press as: Park, Y., et al. Bisphenol A sorption by organo-montmorillonite: Implications for the removal of organic contaminants from water. Chemosphere (2014), http://dx.doi.org/10.1016/j.chemosphere.2013.12.050
Y. Park et al. / Chemosphere xxx (2014) xxx–xxx
HCl or NaOH). The quantity of the adsorbed BPA was calculated using the following equation.
qe ¼
VðC i C e Þ M 1000
where qe is the amount of solute adsorbed on the adsorbent (mg g1), Ci is the initial concentration of the solute (mg L1), Ce is the equilibrium concentration of the solute (mg L1), V is the volume (mL), and M is the mass of the adsorbent (g). 3. Results and discussion 3.1. Initial adsorption of BPA by MMT and organoclays The characterized MMT and organoclays intercalated with DDTMA, HDTMA, and DDDMA which were already described in the previous studies (Park et al., 2013) were evaluated for their adsorption capacities for the uptake of BPA from aqueous solutions (see Fig. S1 in Supporting Information). Maximum removal of BPA was obtained after 12 h agitation time while the amount of adsorbed BPA by unmodified MMT was 6.3%, which is negligible. At 1.0 CEC level, the adsorption of BPA was 96.3% and 98.9% by the organoclays intercalated with DDTMA and HDTMA, respectively. In particular, the organoclays prepared from the di-cationic surfactant, DDDMA, adsorbed about 99.5% of BPA solution at 1.0 CEC. The amount of BPA adsorbed by the organoclays intercalated with higher surfactants loadings slightly increased up to 2.0 CEC level. This indicates that the organoclays are more effective than the unmodified MMT for the uptake of BPA and that the efficiency in the adsorption of BPA by the organoclays was relatively influenced by the size of the surfactant molecules as well as the level of surfactant loadings. The initial adsorption result indicates that adsorption by the organoclays is effective for the removal of BPA and the next investigation was the determination of the phase for the uptake of BPA. 3.2. BPA adsorption kinetics – effect of agitation time To obtain a better understanding of the mechanisms in the adsorption of BPA, the relationship between the agitation time and the amount of BPA adsorbed onto the MMT and organoclays was investigated as presented in Fig. 1.
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It appears that the agitation time affected the amount of adsorbed BPA. The adsorption of BPA by both MMT and the organoclays increased quickly in the first 20 min and rose slowly up to equilibrium concentration within 1 h. To investigate the mechanisms involved in the adsorption process, two conventional kinetic models (pseudo-first order and pseudo-second order) were applied to analyze the experimental data. The pseudo-first order and pseudo-second order models can be expressed using the equation (Lagergren, 1898) described in the Supporting Information 2. The diffusion mechanism can also be explained using intra-particle diffusion (Weber and Morris, 1963) and the equation presented in the Supporting Information 2. Table 1 summarizes the kinetic parameters and correlation coefficients for the adsorption of BPA by MMT and organoclays. It is shown that the pseudo-second order kinetic model fitted better than the pseudo-first order model in this study (see Fig. 2). A similar finding was reported in the adsorption of BPA on graphene (Xu et al., 2012). This suggests that the rate-limiting step may be chemical sorption or chemisorption and the model provides the best correlation of the experimental data. In addition, the lower than 1 coefficients (R2 < 0.99) observed in the linear relationship from the intra-particle diffusion modeling may suggest that intra-particle diffusion was not involved and it is not a controlling step during the adsorption of BPA in this study. 3.3. BPA adsorption isotherms Several adsorption isotherm models are usually applied in order to have a better understanding in the adsorption of adsorbates. In this study, the two classic physical isotherm models: Langmuir and Freundlich isotherms were applied and discussed as follows. The equations used for the Langmuir and the Freundlich models are described in Supporting Information 3. The Langmuir isotherm describes the interaction between the adsorption of the adsorbate and the surface of the adsorbent. It assumes that a monolayer adsorption occurs on a homogeneous surface. Once the adsorption occurs at the specific sites in the adsorbent, no further adsorption occurs at the sites. Thus, the adsorption on the surface is strongly related to the driving force and surface area (Langmuir, 1918). On the other hand, the Freundlich model is based on the use of an empirical expression to describe the adsorption theory and this model assumes that a multilayer adsorption occurs on the heterogeneous surface or surface supporting sites of varied
Fig. 1. Effect of agitation time on the adsorption of BPA by MMT and organoclays.
Please cite this article in press as: Park, Y., et al. Bisphenol A sorption by organo-montmorillonite: Implications for the removal of organic contaminants from water. Chemosphere (2014), http://dx.doi.org/10.1016/j.chemosphere.2013.12.050
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Table 1 Kinetic parameters for the adsorption BPA by MMT and organoclays. Sample
Pseudo-first order
Pseudo-second order
1
k1 (min MMT 1.0 CEC–DDTMA 2.0 CEC–DDTMA 1.0 CEC–HDTMA 2.0 CEC–HDTMA 1.0 CEC–DDDMA 2.0 CEC–DDDMA
1
)
qe1 (mg g
0.096 0.0103 0.0101 0.0097 0.0002 0.0195 0.0009
)
R
1.061 14.408 14.486 14.729 18.746 15.941 28.746
2
0.9687 0.9733 0.9742 0.9757 0.1400 0.4557 0.5660
k2 (g mg1 min)
qe2 (mg g1)
R2
0.027 1.819 4.338 0.266 0.368 0.862 2.083
2.413 13.32 13.32 13.35 13.33 20.00 20.00
0.996 1.000 1.000 1.000 1.000 1.000 1.000
Fig. 2. Test of pseudo-second order model for sorption of BPA by MMT and organoclays.
Table 2 Isotherm parameters for the adsorption of BPA by organoclays. Temp (°C)
Langmuir
Freundlich qm (mg g1)
R2
KF (L g1)
n
R2
1.0 CEC–DDTMA 23 0.29 35 0.10
105.26 86.957
0.9126 0.9738
21.55 8.226
1.021 2.843
0.9712 0.7101
2.0 CEC–DDTMA 23 0.18 35 0.13
102.04 90.909
0.7771 0.9868
18.97 12.26
1.032 2.625
0.9106 0.7288
1.0 CEC–HDTMA 23 0.62 35 0.36
114.94 111.11
0.9663 0.9891
21.20 20.04
1.896 1.896
0.9106 0.7252
2.0 CEC–HDTMA 23 0.46 35 0.29
129.87 128.21
0.9803 0.9925
30.57 22.00
2.366 2.366
0.6954 0.5610
1.0 CEC–DDDMA 23 0.37 35 0.11
181.82 156.25
0.9328 0.9471
23.24 10.96
1.721 1.530
0.9384 0.7393
2.0 CEC–DDDMA 23 0.83 35 0.43
256.41 212.77
0.9206 0.9639
70.50 54.24
1.146 2.178
0.8829 0.8710
KL (L mg1)
affinities (Freundlich, 1906). This model is capable of predicting the infinite surface coverage involving the multilayer adsorption of the surface. Based on the above models, the relative isotherm parameters for the adsorption of BPA at different temperatures were calculated
and summarized in Table 2. The obtained correlation coefficient (R2) values suggest that the adsorption of BPA by the organoclays intercalated with longer chain surfactants (HDTMA and DDDMA) were better fitted into the Langmuir isotherm model than the Freundlich isotherm while the Freundlich isotherm fitted the adsorption data for the organoclays intercalated with DDTMA at the lower temperature better. Higher values of qm were generally observed at the lower temperature which indicates that the adsorption is more favorable at the lower temperature. 3.4. Adsorption thermodynamics The adsorbed amounts of BPA by the organoclays were measured in a range of temperature 296.15–318.15 K and the thermodynamic data obtained provides more in-depth information about the changes of internal energy that are associated with adsorption of BPA. Details of the equations used for calculation of the thermodynamic parameters are presented in the Supporting Information 4 and the obtained parameters including the standard free energy (DG ), the standard enthalpy change (DH ), and the standard entropy change (DS ) were summarized in Table 3. The enthalpy changes obtained are negative and this suggests that the adsorption of BPA is an exothermic process; this also supports the observed decrease in the adsorption capacity of BPA with increasing temperature. In particular, the value of adsorption enthalpy of BPA by the organoclays prepared at 2.0 CEC level became more negative than those prepared at 1.0 CEC, and this implied that the organoclays intercalated with higher surfactant loadings interacted with BPA more strongly. The negative change in free energy (DG ) observed showed that the adsorption was a spontane-
Please cite this article in press as: Park, Y., et al. Bisphenol A sorption by organo-montmorillonite: Implications for the removal of organic contaminants from water. Chemosphere (2014), http://dx.doi.org/10.1016/j.chemosphere.2013.12.050
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Y. Park et al. / Chemosphere xxx (2014) xxx–xxx Table 3 Thermodynamic parameters for the adsorption of BPA by the organoclays. Sample
Thermodynamic constant Temp (K)
ln K
DG (kJ mol1)
DH (kJ mol1)
DS (J mol1 K)
1.0 CEC–DDTMA
296.15 308.15 318.15
2.7233 2.6758 2.0847
6.705 6.855 5.514
21.81
50.24
2.0 CEC–DDTMA
296.15 308.15 318.15
2.5822 2.1937 1.2514
6.358 5.620 3.310
46.31
134.0
1.0 CEC–HDTMA
296.15 308.15 318.15
3.6318 3.5937 3.0592
8.942 9.207 8.092
19.54
35.10
2.0 CEC–HDTMA
296.15 308.15 318.15
4.0187 3.9580 2.6179
9.895 9.745 6.446
47.71
125.92
1.0 CEC–DDDMA
296.15 308.15 318.15
3.5112 2.7732 3.0110
8.645 7.105 7.964
19.17
36.64
2.0 CEC–DDDMA
296.15 296.15 318.15
4.5546 4.2801 3.3517
11.21 10.97 8.866
41.63
101.73
Fig. 3. Effect of the initial pH on the adsorption of BPA by MMT and organoclays.
ous process. It is also noteworthy that the value of free energy became more negative with decreasing temperature. This indicates that the adsorption of BPA by organoclays is a more favorable and spontaneous process at a lower temperature. The negative DS values suggest a decrease in randomness or disorder at the solid–liquid interface during the adsorption of BPA by the organoclays, while the negative values of DS and DH indicate that the adsorption process is more spontaneous at lower temperatures. 3.5. Effect of pH The pH of the reaction mixture is one of the important factors that determines the adsorption properties of an adsorbent and the optimum condition for the adsorption process. The obtained result in Fig. 3 shows the change in the amount of adsorbed BPA by MMT and organoclays with the initial pH ranging from 3 to 11. It was clearly seen that under acidic pH conditions the adsorption of BPA showed little change and remained fairly constant.
When the pH rose beyond around 7, the adsorption of BPA gradually declined and even sharply decreased. These trends can be explained by the surface charge of the clays and the degree of dissociation of BPA solution at different pH values (Dong et al., 2010; Xu et al., 2012; Park et al., 2013). The siloxane (Si–O) group in the tetrahedral sheets on the external structure of MMT becomes Si–O which is further converted into Si–OH at the different pH environments employed. At an alkaline pH, the surface group is fully or partially deprotonated and these increases the negative charges on the surface of the clays. MMT was always negatively charged over the whole range of pH scales, as shown in the zeta potential from some studies (Ijagbemi et al., 2009; Sarkar et al., 2011). However, BPA remains as a neutral molecule when the pH of the solution goes below its pKa (i.e. pH < pKa) and prevailed as phenolate anions when the pH goes beyond the pKa (pH > pKa). In the current study, BPA has its molecular structure when pH < 7, started to deprotonate at around pH 8 and tended to have the second deprotonation form at the pH 10. A similar result was observed in the previous studies by Xu et al.
Please cite this article in press as: Park, Y., et al. Bisphenol A sorption by organo-montmorillonite: Implications for the removal of organic contaminants from water. Chemosphere (2014), http://dx.doi.org/10.1016/j.chemosphere.2013.12.050
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Fig. 4. Schematic diagram of the mechanism for the adsorption of BPA onto organoclays.
(2012) and Dong et al. (2010). However, such an explanation tends to overlook the fact that BPA ionizes to mono- and divalent anions. Thus, the decreased adsorption of BPA in alkaline might be explained by the repulsive electrostatic interactions which occurred between the negatively charged surface of the clays and the biphenolate anions. It is also found that the organoclays with higher surfactant loadings and those prepared with larger surfactant molecules (e.g. DDDMA) were less influenced by the pH of the environment and consistently attracted anionic species of BPA even under the alkaline conditions. 3.6. Adsorption mechanism The limited adsorption of BPA by the unmodified MMT was greatly improved when the MMT was converted to organoclays with organic surfactant molecules. It is clearly seen that the modified clays act as an effective adsorbents and the increase in the total organic carbon contents contributed to the increased hydrophobic lateral interactions. The loaded surfactants created the organic solvent-like hydrophobic phase and the interaction can be described as partition mechanism, resulting in a strong attraction of BPA molecules with the organoclays through van der Waals interaction. It was also found that the adsorption of the anion formed by BPA was greatly reduced under alkaline condition and that the main interaction expected between the BPA and the organoclays is the hydrophobic interaction (Fig. 4a). Moreover, the longer alkyl trains of the surfactant molecules tend to develop more positive charge on the clay surface, resulting in strong interaction between the positively charged surfactant species and the anionic forms of BPA through hydrophobic interaction. Thus organoclays prepared with the longer or two alkyl chained surfactant (DDDMA) had less influence on the adsorption of BPA at different pH environment. In addition, there are several possible mechanisms for the adsorption of
BPA, including electrostatic interaction, donor–acceptor complex formation, and hydrogen bonding. The loaded organic surfactants tended to develop more positive charges on the clay surfaces by shielding the negative charge, which leads to greater attraction of the anionic forms of BPA. The dissociated BPA anions tended to associate with the positively charged head of surfactants on both inner and outer layer of the clays via electrostatic interaction (Fig. 4b). Another mechanism suggested in this adsorption system may be donor–acceptor complex formation between the solute and adsorbent. Although this is less energetic, as an electron acceptor, the positively charged head group of the surfactants interacted with the oxygen atoms of a phenol group in neutral BPA which acts an electron donator (Dong et al., 2010). Hydrogen bonding was a potential mechanism suggested between the phenol groups of BPA and Si–O on clay surfaces. However, this binding may not contribute significantly to the adsorption of BPA by the unmodified MMT since less adsorption occurred. Thus, several possible mechanisms are proposed in this study, and the most likely mechanism appears to be a partitioning process into the hydrophobic phase created by the surfactant through van der Waals and hydrophobic interaction. 3.7. Comparison the results with those obtained by other methods Once the batch experiments were concluded, the optimized conditions and the mechanisms associated with the adsorption of BPA by the organoclays were determined. It was evident from the results that the adsorption by the organoclays is an effective method for the removal of BPA from aqueous medium. Although the obtained results cannot be directly compared with those obtained by the use other carbonaceous adsorbents such as graphene and zeolites, it appears that the adsorption by the organoclays offers a good pathway for the removal of BPA at ppm levels from
Please cite this article in press as: Park, Y., et al. Bisphenol A sorption by organo-montmorillonite: Implications for the removal of organic contaminants from water. Chemosphere (2014), http://dx.doi.org/10.1016/j.chemosphere.2013.12.050
Y. Park et al. / Chemosphere xxx (2014) xxx–xxx
water. In particular, the organoclays intercalated with the larger size of surfactants, DDDMA, are effective adsorbents for BPA. The prepared organoclays afforded 256.41 mg g1, which was the maximum adsorption values obtained from the Langmuir model at 302.15 K compared with values of 181.6 and 114.9 mg g1 by graphene and zeolites reported in the previous study, respectively. Additionally, the uptake of BPA by the organoclays was little affected by varied pH environment up to pH 9. From the obtained results, an optimum adsorption parameter was achieved when the pH of the solution in acidic pH range (3– 6) and the use of organoclays can be advantageous over the use of graphene in this pH range. However, under the alkaline conditions, better adsorption of BPA by the zeolites was achieved (Dong et al., 2010). Therefore, the performance of the organoclays and other materials such as graphene and zeolites towards to the adsorption of BPA solution appeared to be limited by the influence of different pH ranges, suggesting that different mechanisms (e.g. hydrophobic interaction or surface charge of material) are involved. While the adsorption of BPA by the organoclays and graphene occurred over a similar time scale, given the relatively low cost of production of the organoclays they may offer significant advantages in the removal of industrial pollutants from the environment (Chen et al., 2013). The fate of the organoclays loaded with the adsorbed organic contaminants (e.g. BPA) will require further monitoring and investigations of the possible re-use of the adsorbents will be necessary in order to fully assess the practical implications of results described in this paper. 4. Conclusions The investigation showed that the organoclays were found to be effective adsorbents for the removal of BPA from aqueous solutions. The adsorption of BPA was relatively influenced by pH and temperature. The adsorption of BPA was relatively decreased when the pH rose beyond 7 and the phenomenon can be explained due to the surface charge of the clays and the degree of dissociation of BPA solution. The organoclays intercalated with highly loaded surfactants and/or large surfactant molecules tended to be less influenced by the pH and this was due to the development of positive charges on the clay surface from surfactant molecules. The loaded surfactants acted as a partitioning phase and strongly attract to BPA molecules through hydrophobic interaction. The adsorption process occurred spontaneously and exothermic reaction was involved. The described results would support the use of organoclays as potential adsorbents for the removal of BPA from aqueous solutions. Acknowledgements The authors gratefully acknowledge and express gratitude for the financial and infrastructural assistance provided by Queensland University of Technology (QUT). The Australian Research Council (ARC) is thanked for funding some of the instrumentation. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2013.12.050. References Al-Degs, Y.S., El-Barghouthi, M.I., Issa, A.A., Khraisheh, M.A., Walker, G.M., 2006. Sorption of Zn(II), Pb(II), and Co(II) using natural sorbents: equilibrium and kinetic studies. Water Res. 40, 2645–2658.
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Application of organoclays for the adsorption of recalcitrant organic molecules from aqueous media. J. Colloid Interface Sci. 354, 292–305. Park, Y., Ayoko, G., Kristof, J., Horváth, E., Frost, R., 2012. Thermal stability of organoclays with mono- and di-alkyl cationic surfactants. J. Therm. Anal. Calorim. 110, 1087–1093. Park, Y., Ayoko, G.A., Horváth, E., Kurdi, R., Kristof, J., Frost, R.L., 2013. Structural characterisation and environmental application of organoclays for the removal of phenolic compounds. J. Colloid Interface Sci. 393, 319–334. Petrovic´, M., Eljarrat, E., López de Alda, M.J., Barceló, D., 2001. Analysis and environmental levels of endocrine-disrupting compounds in freshwater sediments. TrAC Trends Anal. Chem. 20, 637–648. Petrovic, M., Solé, M., López De Alda, M.J., Barceló, D., 2002. Endocrine disruptors in sewage treatment plants, receiving river waters, and sediments: integration of chemical analysis and biological effects on feral carp. Environ. Toxicol. 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Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Bisphenol A sorption by organo-montmorillonite: Implications for the removal of organic contaminants from water Yuri Park a, Zhiming Sun a,b, Godwin A. Ayoko a,⇑, Ray L. Frost a a b
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, GPO 2434, Brisbane, Queensland 4001, Australia School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing 100083, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Bisphenol A (BPA) is an emerging
pollutant that has attracted immense interest. Different types of organoclays were prepared from different types of surfactants. The loaded surfactants created hydrophobic phases via partitioning mechanism. Thus the organoclays were effective in removing BPA from aqueous solutions.
a r t i c l e
i n f o
Article history: Received 14 May 2013 Received in revised form 9 December 2013 Accepted 18 December 2013 Available online xxxx Keywords: BPA Organoclays Thermodynamics Mechanism Application Environment
a b s t r a c t Remediation of bisphenol A (BPA) from aqueous solutions by adsorption using organoclays synthesized from montmorillonite (MMT) with different types of organic surfactant molecules was demonstrated. High adsorption capacities of the organoclays for the uptake of BPA were observed and these demonstrated their potential application as strong adsorbents for noxious organic water contaminants. The adsorption of BPA was significantly influenced by pH, with increased adsorption of BPA in acidic pH range. However, the organoclays intercalated with highly loaded surfactants and/or large surfactant molecules were less influenced by the pH of the environment and this was thought to be due to the shielding the negative charge from surfactant molecules and the development of more positive charge on the clay surface, which leads to the attraction of anionic BPA even at alkaline pH. The hydrophobic phase created by loaded surfactant molecules contributed to a partitioning phase, interacting with BPA molecules strongly through hydrophobic interaction. Pseudo-second order kinetic model and Langmuir isotherm provided the best fit for the adsorption of BPA onto the organoclays. In addition, the adsorption process was spontaneous and exothermic with lower temperature facilitating the adsorption of BPA onto the organoclays. The described process provides a potential pathway for the removal of BPA from contaminated waters. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
⇑ Corresponding author. Address: 2 George Street, Brisbane, Queensland 4001, Australia. Tel.: +61 731382586. E-mail address: [email protected] (G.A. Ayoko).
The growing population and urbanization of society have resulted in increased production and dispersion of toxic chemicals, including the endocrine-disrupting chemicals (EDCs) into the environment (Mohapatra et al., 2010). Many EDCs including
0045-6535/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.12.050
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natural estrogens (e.g. estrone (E1), 17b-strodial (E2), and estriol (E3)), synthesized estrogen (e.g. ethinylestradiol (EE2)), and industrial compounds (e.g. bisphenol A (BPA) and nonylphenol) are organic compounds with one or more phenolic groups. Due to their structures and physicochemical properties, these molecules can naturally occupy the high affinity binding acceptor sites of the hormone receptors. These results in disrupted endocrine systems and contribute to environmental risks for animals and humans (Staples et al., 1998; Suzuki et al., 2004; Jiang et al., 2005). EDCs are removed from sewage through wastewater treatment plants (WWTPs) (Rudder et al., 2004; Dong et al., 2010). However, due to their complex structures and low biodegradability (Petrovic´ et al., 2003; Esperanza et al., 2007; Pan et al., 2009), they are not removed completely through biological treatment and can still be detected in the discharge of WWTPS. Since these molecules have been detected in wastewater, groundwater, sediments, and even drinking water, EDCs are of great public and environmental concerns (Petrovic´ et al., 2001; Petrovic et al., 2002; Auriol et al., 2006; Benotti et al., 2009). BPA is widely used as a monomer for the manufacture of polycarbonates and epoxy resin in the plastic industry. Because of its ubiquitous nature in the environment and its negative effects to human health and organisms (Kang et al., 2006; Klecˇka et al., 2009), it is considered as one of high priority compounds that must be monitored and removed from the environment. Hence, BPA is selected as the test molecule in this study. There is a clear need for the development of simple and highly efficient methods for the safe remediation of organic substrates. Some techniques, including adsorption (Dong et al., 2010; Sui et al., 2011; Xu et al., 2012), membrane filtration (Urase et al., 2005; Yoon et al., 2007), electrochemical technique (Gözmen et al., 2003), and catalysis/photocatalysis (Ohko et al., 2001; Fukahori et al., 2003; Chiang et al., 2004; Torres et al., 2006; Torres-Palma et al., 2010) have been proposed for the removal of these contaminants. The approach reported in this study is adsorption process by organoclays which has some advantages over the other techniques. These include their relatively low operational cost, ease of operation, and production of less secondary products (Ali and Gupta, 2007). In recent years, adsorption by organoclays is widely used for the removal of organic and inorganic contaminants in waters (Zhu et al., 1997, 2011; Wu et al., 2001; Bhatnagar and Jain, 2005; Chen et al., 2007; Su et al., 2011) because the use of such low-cost adsorbents which utilizes materials that naturally available is economically feasible. Among many materials that are naturally abundant, clay minerals are potential adsorbents due to their low cost, environmental stability, high adsorption/absorption, and ion exchange properties (Babel and Kurniawan, 2003; Al-Degs et al., 2006; Park et al., 2011). In particular, montmorillonite (MMT) is widely used because of its high cation exchange capacity (CEC), swelling property, and high surface area (Park et al., 2011). However, one of the drawbacks of clay minerals is their hydrophilic properties and this structural property makes them ineffective for the removal of organic pollutants. The permanent negatively charged clay surface can be modified by cationic surfactant molecules, and through ion exchange, hydrophilic clays can be converted to hydrophobic organoclays. We previously demonstrated that these organoclays are effective adsorbents for the removal of phenolic compounds. Given that BPA has two phenolic groups, we reasoned that the organoclays will be good adsorbents for removing BPA from aqueous media. Thus, the aims of this work are to: (a) investigate the adsorption properties of organoclays toward BPA, (b) demonstrate their potential for the uptake of BPA from aqueous solutions under different experimental conditions such as the agitation time, pH, temperature, and different BPA concentrations, (c) evaluate the mechanisms involved in the adsorption of BPA by the organoclays,
and (d) compare the results with those obtained when different adsorbents and other methods were used. To the best of our knowledge, the removal of BPA by organoclays has not been investigated previously, and this work demonstrates the potential performance of the organoclays as adsorbent for the uptake of the industrial by-products such as BPA from aqueous solutions.
2. Materials and experimental methods 2.1. Materials Pure MMT was purchased from Sigma–Aldrich and used without further purification. Its cation exchange capacity (CEC) is 76.4 meq/100 g. Different types of cationic surfactants were used as examples of mono- and di-alkyl cationic surfactants in this study and these are: (i) dodecyltrimethylammonium bromide (denoted as DDTMA, C15H34NBr, FW: 308.34), (ii) hexadecyltrimethylammonium bromide (denoted as HDTMA, C19H42NBr, FW: 364.46), and (iii) didodecyldimethylammonium bromide (denoted as DDDMA, C26H56NBr, FW: 462.65) and all of which were purchased from Sigma–Aldrich and used without further purification. 2.2. Synthesis and characterization of organoclays The organoclays were prepared and characterized by various techniques including powder X-ray diffraction (XRD), surface area measurement (BET method), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA), and their structural properties and characteristics were already described in the previous studies by Park et al. (2012, 2013). 2.3. Batch adsorption experiments BPA [2,2-bis(4-hydroxyphenyl)propane] was purchased from Sigma–Aldrich and used to investigate the adsorption capacities of MMT and the modified organoclays. BPA solution was initially prepared as a stock solution (5000 mg L1) in ethanol due to its low solubility in water, and it was further diluted in water for the preparation of different BPA concentrations. From the initial experiment to determine the amount of adsorbent required, 0.2–0.3 g of the adsorbents were placed in sealed 50 mL centrifuge tubes with 40 mL of a BPA solution in the appropriate concentration (which ranged from 5 to 500 mg L1). The bottles were placed in a rotary shaker and/or a shaking water bath for 12 h at the appropriate temperature. The initial pH of BPA was close to pH 4–5. After shaking, the mixture was filtered through 0.22 lm filters (hydrophilic PTFE membrane) and the supernatants were analyzed by a high-performance liquid chromatography (HPLC, Agilent HP 1100) system coupled with a Luna 5l C18 column (Phenomenex Pty Ltd.) and a UV detector. A mobile phase mixture containing 70% methanol and 30% water at a flow rate of 0.1 mL min1 and 40 lL of injection volume was used for this experiment and the detector was operated at 278 nm. To undertake the adsorption kinetics, the adsorbents were added with an initial BPA concentration (100 mg L1) at room temperature in order to determine the minimum time to reach the equilibrium concentration. The concentrations of BPA were measured at different time intervals from 10 min to 240 min. To evaluate the thermodynamic properties, the adsorbed concentrations of BPA solution by the organoclays were obtained at 296.15, 308.15, and 318.15 K, respectively. Similarly, the effect of the pH on the adsorption of BPA (with initial BPA concentration of 100 mg L1) was studied by adjusting the pH of BPA solution (using either diluted 0.1 M
Please cite this article in press as: Park, Y., et al. Bisphenol A sorption by organo-montmorillonite: Implications for the removal of organic contaminants from water. Chemosphere (2014), http://dx.doi.org/10.1016/j.chemosphere.2013.12.050
Y. Park et al. / Chemosphere xxx (2014) xxx–xxx
HCl or NaOH). The quantity of the adsorbed BPA was calculated using the following equation.
qe ¼
VðC i C e Þ M 1000
where qe is the amount of solute adsorbed on the adsorbent (mg g1), Ci is the initial concentration of the solute (mg L1), Ce is the equilibrium concentration of the solute (mg L1), V is the volume (mL), and M is the mass of the adsorbent (g). 3. Results and discussion 3.1. Initial adsorption of BPA by MMT and organoclays The characterized MMT and organoclays intercalated with DDTMA, HDTMA, and DDDMA which were already described in the previous studies (Park et al., 2013) were evaluated for their adsorption capacities for the uptake of BPA from aqueous solutions (see Fig. S1 in Supporting Information). Maximum removal of BPA was obtained after 12 h agitation time while the amount of adsorbed BPA by unmodified MMT was 6.3%, which is negligible. At 1.0 CEC level, the adsorption of BPA was 96.3% and 98.9% by the organoclays intercalated with DDTMA and HDTMA, respectively. In particular, the organoclays prepared from the di-cationic surfactant, DDDMA, adsorbed about 99.5% of BPA solution at 1.0 CEC. The amount of BPA adsorbed by the organoclays intercalated with higher surfactants loadings slightly increased up to 2.0 CEC level. This indicates that the organoclays are more effective than the unmodified MMT for the uptake of BPA and that the efficiency in the adsorption of BPA by the organoclays was relatively influenced by the size of the surfactant molecules as well as the level of surfactant loadings. The initial adsorption result indicates that adsorption by the organoclays is effective for the removal of BPA and the next investigation was the determination of the phase for the uptake of BPA. 3.2. BPA adsorption kinetics – effect of agitation time To obtain a better understanding of the mechanisms in the adsorption of BPA, the relationship between the agitation time and the amount of BPA adsorbed onto the MMT and organoclays was investigated as presented in Fig. 1.
3
It appears that the agitation time affected the amount of adsorbed BPA. The adsorption of BPA by both MMT and the organoclays increased quickly in the first 20 min and rose slowly up to equilibrium concentration within 1 h. To investigate the mechanisms involved in the adsorption process, two conventional kinetic models (pseudo-first order and pseudo-second order) were applied to analyze the experimental data. The pseudo-first order and pseudo-second order models can be expressed using the equation (Lagergren, 1898) described in the Supporting Information 2. The diffusion mechanism can also be explained using intra-particle diffusion (Weber and Morris, 1963) and the equation presented in the Supporting Information 2. Table 1 summarizes the kinetic parameters and correlation coefficients for the adsorption of BPA by MMT and organoclays. It is shown that the pseudo-second order kinetic model fitted better than the pseudo-first order model in this study (see Fig. 2). A similar finding was reported in the adsorption of BPA on graphene (Xu et al., 2012). This suggests that the rate-limiting step may be chemical sorption or chemisorption and the model provides the best correlation of the experimental data. In addition, the lower than 1 coefficients (R2 < 0.99) observed in the linear relationship from the intra-particle diffusion modeling may suggest that intra-particle diffusion was not involved and it is not a controlling step during the adsorption of BPA in this study. 3.3. BPA adsorption isotherms Several adsorption isotherm models are usually applied in order to have a better understanding in the adsorption of adsorbates. In this study, the two classic physical isotherm models: Langmuir and Freundlich isotherms were applied and discussed as follows. The equations used for the Langmuir and the Freundlich models are described in Supporting Information 3. The Langmuir isotherm describes the interaction between the adsorption of the adsorbate and the surface of the adsorbent. It assumes that a monolayer adsorption occurs on a homogeneous surface. Once the adsorption occurs at the specific sites in the adsorbent, no further adsorption occurs at the sites. Thus, the adsorption on the surface is strongly related to the driving force and surface area (Langmuir, 1918). On the other hand, the Freundlich model is based on the use of an empirical expression to describe the adsorption theory and this model assumes that a multilayer adsorption occurs on the heterogeneous surface or surface supporting sites of varied
Fig. 1. Effect of agitation time on the adsorption of BPA by MMT and organoclays.
Please cite this article in press as: Park, Y., et al. Bisphenol A sorption by organo-montmorillonite: Implications for the removal of organic contaminants from water. Chemosphere (2014), http://dx.doi.org/10.1016/j.chemosphere.2013.12.050
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Table 1 Kinetic parameters for the adsorption BPA by MMT and organoclays. Sample
Pseudo-first order
Pseudo-second order
1
k1 (min MMT 1.0 CEC–DDTMA 2.0 CEC–DDTMA 1.0 CEC–HDTMA 2.0 CEC–HDTMA 1.0 CEC–DDDMA 2.0 CEC–DDDMA
1
)
qe1 (mg g
0.096 0.0103 0.0101 0.0097 0.0002 0.0195 0.0009
)
R
1.061 14.408 14.486 14.729 18.746 15.941 28.746
2
0.9687 0.9733 0.9742 0.9757 0.1400 0.4557 0.5660
k2 (g mg1 min)
qe2 (mg g1)
R2
0.027 1.819 4.338 0.266 0.368 0.862 2.083
2.413 13.32 13.32 13.35 13.33 20.00 20.00
0.996 1.000 1.000 1.000 1.000 1.000 1.000
Fig. 2. Test of pseudo-second order model for sorption of BPA by MMT and organoclays.
Table 2 Isotherm parameters for the adsorption of BPA by organoclays. Temp (°C)
Langmuir
Freundlich qm (mg g1)
R2
KF (L g1)
n
R2
1.0 CEC–DDTMA 23 0.29 35 0.10
105.26 86.957
0.9126 0.9738
21.55 8.226
1.021 2.843
0.9712 0.7101
2.0 CEC–DDTMA 23 0.18 35 0.13
102.04 90.909
0.7771 0.9868
18.97 12.26
1.032 2.625
0.9106 0.7288
1.0 CEC–HDTMA 23 0.62 35 0.36
114.94 111.11
0.9663 0.9891
21.20 20.04
1.896 1.896
0.9106 0.7252
2.0 CEC–HDTMA 23 0.46 35 0.29
129.87 128.21
0.9803 0.9925
30.57 22.00
2.366 2.366
0.6954 0.5610
1.0 CEC–DDDMA 23 0.37 35 0.11
181.82 156.25
0.9328 0.9471
23.24 10.96
1.721 1.530
0.9384 0.7393
2.0 CEC–DDDMA 23 0.83 35 0.43
256.41 212.77
0.9206 0.9639
70.50 54.24
1.146 2.178
0.8829 0.8710
KL (L mg1)
affinities (Freundlich, 1906). This model is capable of predicting the infinite surface coverage involving the multilayer adsorption of the surface. Based on the above models, the relative isotherm parameters for the adsorption of BPA at different temperatures were calculated
and summarized in Table 2. The obtained correlation coefficient (R2) values suggest that the adsorption of BPA by the organoclays intercalated with longer chain surfactants (HDTMA and DDDMA) were better fitted into the Langmuir isotherm model than the Freundlich isotherm while the Freundlich isotherm fitted the adsorption data for the organoclays intercalated with DDTMA at the lower temperature better. Higher values of qm were generally observed at the lower temperature which indicates that the adsorption is more favorable at the lower temperature. 3.4. Adsorption thermodynamics The adsorbed amounts of BPA by the organoclays were measured in a range of temperature 296.15–318.15 K and the thermodynamic data obtained provides more in-depth information about the changes of internal energy that are associated with adsorption of BPA. Details of the equations used for calculation of the thermodynamic parameters are presented in the Supporting Information 4 and the obtained parameters including the standard free energy (DG ), the standard enthalpy change (DH ), and the standard entropy change (DS ) were summarized in Table 3. The enthalpy changes obtained are negative and this suggests that the adsorption of BPA is an exothermic process; this also supports the observed decrease in the adsorption capacity of BPA with increasing temperature. In particular, the value of adsorption enthalpy of BPA by the organoclays prepared at 2.0 CEC level became more negative than those prepared at 1.0 CEC, and this implied that the organoclays intercalated with higher surfactant loadings interacted with BPA more strongly. The negative change in free energy (DG ) observed showed that the adsorption was a spontane-
Please cite this article in press as: Park, Y., et al. Bisphenol A sorption by organo-montmorillonite: Implications for the removal of organic contaminants from water. Chemosphere (2014), http://dx.doi.org/10.1016/j.chemosphere.2013.12.050
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Y. Park et al. / Chemosphere xxx (2014) xxx–xxx Table 3 Thermodynamic parameters for the adsorption of BPA by the organoclays. Sample
Thermodynamic constant Temp (K)
ln K
DG (kJ mol1)
DH (kJ mol1)
DS (J mol1 K)
1.0 CEC–DDTMA
296.15 308.15 318.15
2.7233 2.6758 2.0847
6.705 6.855 5.514
21.81
50.24
2.0 CEC–DDTMA
296.15 308.15 318.15
2.5822 2.1937 1.2514
6.358 5.620 3.310
46.31
134.0
1.0 CEC–HDTMA
296.15 308.15 318.15
3.6318 3.5937 3.0592
8.942 9.207 8.092
19.54
35.10
2.0 CEC–HDTMA
296.15 308.15 318.15
4.0187 3.9580 2.6179
9.895 9.745 6.446
47.71
125.92
1.0 CEC–DDDMA
296.15 308.15 318.15
3.5112 2.7732 3.0110
8.645 7.105 7.964
19.17
36.64
2.0 CEC–DDDMA
296.15 296.15 318.15
4.5546 4.2801 3.3517
11.21 10.97 8.866
41.63
101.73
Fig. 3. Effect of the initial pH on the adsorption of BPA by MMT and organoclays.
ous process. It is also noteworthy that the value of free energy became more negative with decreasing temperature. This indicates that the adsorption of BPA by organoclays is a more favorable and spontaneous process at a lower temperature. The negative DS values suggest a decrease in randomness or disorder at the solid–liquid interface during the adsorption of BPA by the organoclays, while the negative values of DS and DH indicate that the adsorption process is more spontaneous at lower temperatures. 3.5. Effect of pH The pH of the reaction mixture is one of the important factors that determines the adsorption properties of an adsorbent and the optimum condition for the adsorption process. The obtained result in Fig. 3 shows the change in the amount of adsorbed BPA by MMT and organoclays with the initial pH ranging from 3 to 11. It was clearly seen that under acidic pH conditions the adsorption of BPA showed little change and remained fairly constant.
When the pH rose beyond around 7, the adsorption of BPA gradually declined and even sharply decreased. These trends can be explained by the surface charge of the clays and the degree of dissociation of BPA solution at different pH values (Dong et al., 2010; Xu et al., 2012; Park et al., 2013). The siloxane (Si–O) group in the tetrahedral sheets on the external structure of MMT becomes Si–O which is further converted into Si–OH at the different pH environments employed. At an alkaline pH, the surface group is fully or partially deprotonated and these increases the negative charges on the surface of the clays. MMT was always negatively charged over the whole range of pH scales, as shown in the zeta potential from some studies (Ijagbemi et al., 2009; Sarkar et al., 2011). However, BPA remains as a neutral molecule when the pH of the solution goes below its pKa (i.e. pH < pKa) and prevailed as phenolate anions when the pH goes beyond the pKa (pH > pKa). In the current study, BPA has its molecular structure when pH < 7, started to deprotonate at around pH 8 and tended to have the second deprotonation form at the pH 10. A similar result was observed in the previous studies by Xu et al.
Please cite this article in press as: Park, Y., et al. Bisphenol A sorption by organo-montmorillonite: Implications for the removal of organic contaminants from water. Chemosphere (2014), http://dx.doi.org/10.1016/j.chemosphere.2013.12.050
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Fig. 4. Schematic diagram of the mechanism for the adsorption of BPA onto organoclays.
(2012) and Dong et al. (2010). However, such an explanation tends to overlook the fact that BPA ionizes to mono- and divalent anions. Thus, the decreased adsorption of BPA in alkaline might be explained by the repulsive electrostatic interactions which occurred between the negatively charged surface of the clays and the biphenolate anions. It is also found that the organoclays with higher surfactant loadings and those prepared with larger surfactant molecules (e.g. DDDMA) were less influenced by the pH of the environment and consistently attracted anionic species of BPA even under the alkaline conditions. 3.6. Adsorption mechanism The limited adsorption of BPA by the unmodified MMT was greatly improved when the MMT was converted to organoclays with organic surfactant molecules. It is clearly seen that the modified clays act as an effective adsorbents and the increase in the total organic carbon contents contributed to the increased hydrophobic lateral interactions. The loaded surfactants created the organic solvent-like hydrophobic phase and the interaction can be described as partition mechanism, resulting in a strong attraction of BPA molecules with the organoclays through van der Waals interaction. It was also found that the adsorption of the anion formed by BPA was greatly reduced under alkaline condition and that the main interaction expected between the BPA and the organoclays is the hydrophobic interaction (Fig. 4a). Moreover, the longer alkyl trains of the surfactant molecules tend to develop more positive charge on the clay surface, resulting in strong interaction between the positively charged surfactant species and the anionic forms of BPA through hydrophobic interaction. Thus organoclays prepared with the longer or two alkyl chained surfactant (DDDMA) had less influence on the adsorption of BPA at different pH environment. In addition, there are several possible mechanisms for the adsorption of
BPA, including electrostatic interaction, donor–acceptor complex formation, and hydrogen bonding. The loaded organic surfactants tended to develop more positive charges on the clay surfaces by shielding the negative charge, which leads to greater attraction of the anionic forms of BPA. The dissociated BPA anions tended to associate with the positively charged head of surfactants on both inner and outer layer of the clays via electrostatic interaction (Fig. 4b). Another mechanism suggested in this adsorption system may be donor–acceptor complex formation between the solute and adsorbent. Although this is less energetic, as an electron acceptor, the positively charged head group of the surfactants interacted with the oxygen atoms of a phenol group in neutral BPA which acts an electron donator (Dong et al., 2010). Hydrogen bonding was a potential mechanism suggested between the phenol groups of BPA and Si–O on clay surfaces. However, this binding may not contribute significantly to the adsorption of BPA by the unmodified MMT since less adsorption occurred. Thus, several possible mechanisms are proposed in this study, and the most likely mechanism appears to be a partitioning process into the hydrophobic phase created by the surfactant through van der Waals and hydrophobic interaction. 3.7. Comparison the results with those obtained by other methods Once the batch experiments were concluded, the optimized conditions and the mechanisms associated with the adsorption of BPA by the organoclays were determined. It was evident from the results that the adsorption by the organoclays is an effective method for the removal of BPA from aqueous medium. Although the obtained results cannot be directly compared with those obtained by the use other carbonaceous adsorbents such as graphene and zeolites, it appears that the adsorption by the organoclays offers a good pathway for the removal of BPA at ppm levels from
Please cite this article in press as: Park, Y., et al. Bisphenol A sorption by organo-montmorillonite: Implications for the removal of organic contaminants from water. Chemosphere (2014), http://dx.doi.org/10.1016/j.chemosphere.2013.12.050
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water. In particular, the organoclays intercalated with the larger size of surfactants, DDDMA, are effective adsorbents for BPA. The prepared organoclays afforded 256.41 mg g1, which was the maximum adsorption values obtained from the Langmuir model at 302.15 K compared with values of 181.6 and 114.9 mg g1 by graphene and zeolites reported in the previous study, respectively. Additionally, the uptake of BPA by the organoclays was little affected by varied pH environment up to pH 9. From the obtained results, an optimum adsorption parameter was achieved when the pH of the solution in acidic pH range (3– 6) and the use of organoclays can be advantageous over the use of graphene in this pH range. However, under the alkaline conditions, better adsorption of BPA by the zeolites was achieved (Dong et al., 2010). Therefore, the performance of the organoclays and other materials such as graphene and zeolites towards to the adsorption of BPA solution appeared to be limited by the influence of different pH ranges, suggesting that different mechanisms (e.g. hydrophobic interaction or surface charge of material) are involved. While the adsorption of BPA by the organoclays and graphene occurred over a similar time scale, given the relatively low cost of production of the organoclays they may offer significant advantages in the removal of industrial pollutants from the environment (Chen et al., 2013). The fate of the organoclays loaded with the adsorbed organic contaminants (e.g. BPA) will require further monitoring and investigations of the possible re-use of the adsorbents will be necessary in order to fully assess the practical implications of results described in this paper. 4. Conclusions The investigation showed that the organoclays were found to be effective adsorbents for the removal of BPA from aqueous solutions. The adsorption of BPA was relatively influenced by pH and temperature. The adsorption of BPA was relatively decreased when the pH rose beyond 7 and the phenomenon can be explained due to the surface charge of the clays and the degree of dissociation of BPA solution. The organoclays intercalated with highly loaded surfactants and/or large surfactant molecules tended to be less influenced by the pH and this was due to the development of positive charges on the clay surface from surfactant molecules. The loaded surfactants acted as a partitioning phase and strongly attract to BPA molecules through hydrophobic interaction. The adsorption process occurred spontaneously and exothermic reaction was involved. The described results would support the use of organoclays as potential adsorbents for the removal of BPA from aqueous solutions. Acknowledgements The authors gratefully acknowledge and express gratitude for the financial and infrastructural assistance provided by Queensland University of Technology (QUT). The Australian Research Council (ARC) is thanked for funding some of the instrumentation. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2013.12.050. References Al-Degs, Y.S., El-Barghouthi, M.I., Issa, A.A., Khraisheh, M.A., Walker, G.M., 2006. Sorption of Zn(II), Pb(II), and Co(II) using natural sorbents: equilibrium and kinetic studies. Water Res. 40, 2645–2658.
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Please cite this article in press as: Park, Y., et al. Bisphenol A sorption by organo-montmorillonite: Implications for the removal of organic contaminants from water. Chemosphere (2014), http://dx.doi.org/10.1016/j.chemosphere.2013.12.050