Comparison of Competitive and Synergistic Adsorption of

Article Cite This: J. Chem. Eng. Data 2019, 64, 2780−2790

pubs.acs.org/jced

Comparison of Competitive and Synergistic Adsorption of Tetrabromobisphenol‑A and Its Metabolites on Two Different Organic-Modified Clays Xiang Li,† Zhongzhen Liu,† Lan Wei,† Lianxi Huang,† Qing Huang,† and Xiaoshan Jia*,‡

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†

Key Laboratory of Plant Nutrition and Fertilizer in South Region, Ministry of Agriculture, Guangdong Key Laboratory of Nutrient Cycling and Farmland Conservation, Institute of Agricultural Resources and Environment, Guangdong Academy of Agricultural Sciences, Guangzhou 510640, China ‡ School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China S Supporting Information *

ABSTRACT: Experimental data of the adsorption of tetrabromobisphenol-A (TBBPA) and its metabolites [bisphenol-A (BPA) and 2,6-dibromophenols (2,6-DBP)] on two different organic-modified clays were determined in single, binary, and ternary solute systems. In a single solute system, both the adsorbents showed high adsorption selectivity for TBBPA; the logarithms of adsorption coefficients (Kd’s) for three organics are nonlinearly related to the logarithm of their octanol−water partition coefficients (Kow’s). In the binary solute system, the uptake of TBBPA was reduced by the presence of BPA and the reduction depended on the initial concentration of BPA. Meanwhile, the uptake of TBBPA was enhanced by the presence of 2,6-DBP at high initial concentrations because of synergetic adsorption. The Freundlich model and the Sheindorf−Rebuhn− Sheintuch model provided the best fit for the adsorption of TBBPA and its metabolites in the binary solute system. On the basis of the comparison between the single and multiple solute systems, the KF value of TBBPA decreased in the order: KF of TBBPA (6.0 μmol/L-2,6-DBP) > KF of TBBPA > KF of TBBPA (6.0 μmol/L-BPA); the KF value of BPA and 2,6-DBP decreased in the order: KF of a single solute system > KF of a binary solute system > KF of a ternary solute system; and no enhancement for BPA and 2,6-DBP adsorption was observed. The adsorption kinetic of TBBPA and its metabolites followed the pseudo second-order equation in single and binary solute systems. The adsorption capacity of TBBPA and its metabolites was reduced compared with that in the single system. However, the total adsorption capacity in the binary system was increased.

1. INTRODUCTION

compounds that must be monitored and removed from the environment. Various physicochemical and biological methods have been proposed for the treatment of waters containing TBBPA.11−15 However, some implementations of these methods are energy intensive because of the level of conversions required or exhibit residual activity that may bring other potential risks. It is now widely recognized that the adsorption process provides a feasible and safety method for the removal of TBBPA from waters.16−20 Fasfous et al.21 chose the multiwalled carbon nanotubes (MWCNTs) as adsorbents for removing TBBPA

Brominated flame retardants (BFRs) comprise a diverse variety of brominated organic compounds used to ensure that manufactured goods comply with fire safety regulations.1 More than 200 000 metric tons of BFRs are produced each year.2 Tetrabromobisphenol-A (TBBPA) is the largest BFRs in terms of production.3 Because of the extensive use and persistence, TBBPA has been detected in a wide range of materials including soil,2,4 plants,2 sediments,5 human serum,6 house dust,7 and clothes dryer lint.8 Toxicity studies indicated that TBBPA induced disruption of thyroid homeostasis and estrogen signal.9,10 Because of its negative effects to human health and organisms and its ubiquitous nature in the environment, TBBPA is considered as one of the high priority © 2019 American Chemical Society

Received: February 18, 2019 Accepted: May 27, 2019 Published: June 4, 2019 2780

DOI: 10.1021/acs.jced.9b00164 J. Chem. Eng. Data 2019, 64, 2780−2790

Journal of Chemical & Engineering Data

Article

from water and found that the equilibrium between TBBPA and MWCNTs was achieved in approximately 60 min with the removal of 96% of the TBBPA. Zhang et al.22 reported adsorption enhancement of TBBPA from water by fly ashsupported nanostructured MnO2 (FA@nM). They found that the equilibrium between TBBPA and FA@nM was achieved in approximately 40 min with the removal of 98% of the TBBPA. Zhang et al.23 investigated the adsorption of TBBPA on graphene oxide. The results showed that the adsorption equilibrium could be well fitted by the Langmuir model with a maximum adsorption capacity of 115.77 mg/g. Although works have been directed toward the application of different adsorbents for TBBPA removal in a single component system, however, there may be rarely a situation in which only a single solute would be adsorbed in the treatment process. In environment water, the biological degradation of TBBPA has been investigated by bacterium in an activated sludge in sewage plants or in natural river waters.24 Two typical degradation products, bisphenol-A (BPA) and 2,6dibromophenol (2,6-DBP), have been widely found in the water environment in the past research.4,24 The BPA and 2,6DBP potential risk to environment have been accessed for additive flame retardant uses.25 Because of the biological degradation of TBBPA, this is usually considered to be a competitive or synthetic situation for TBBPA adsorption. In the multiple solute systems, the TBBPA adsorption time to reach equilibrium may become longer or slower, the adsorption capacity may be reduced or increased, and the adsorption mechanism may be changed. Therefore, it is necessary to study TBBPA adsorption in multiple solute systems. In this paper, TBBPA adsorption on organoclays in multiple solute systems was evaluated. Organoclays were chosen as the adsorbent because of easy availability and low cost. Our past studies had proved that organic-modified montmorillonite (Mt) and kaolinite (Ka) are efficient adsorbents for removing TBBPA from water.26 However, the characterization and mechanism of binary and ternary adsorption systems are still unclear. In order to characterize the adsorption process, the competitive and synergetic adsorption on TBBPA and its metabolites was investigated through the isotherm equilibrium and kinetics techniques. The objective of this work is to compare their adsorption in single, binary, and ternary adsorption systems to study the adsorption process and determine the adsorption, the equilibrium, and kinetics. In addition, the adsorption mechanism of TBBPA in the binary solute system was also performed.

Table 1. CAS Registry Number, Mass Fraction Purity, and Analysis Method component

CAS reg. no.

TBBPA

79-94-7

BPA

80-05-7

2,6-DBP

608-33-3

suppliers Sigma Chemical Co., Ltd (USA) Sigma Chemical Co., Ltd (USA) Sigma Chemical Co., Ltd (USA)

mass fraction

analysis method

≥0.98

HPLC

≥0.97

HPLC

≥0.97

HPLC

system, Barnstead/Thermolyne Co., Ltd. (Dubuque, IA, USA). All other chemicals were of analytical grade unless stated otherwise. 2.2. Preparation and Characterization of Adsorbents. The CTMAB-modified Mt (CMt) and CTMAB-modified Ka (CKa) were prepared in ion exchange reactions. A typical experimental procedure was as follows: 5.0 g of Mt or Ka was dispensed in the 0.1 L of distilled water. A certain amount of CTMAB was dissolved in 0.2 L of distilled water and then slowly added to the clay dispersion. The amounts of CTMAB are equivalent to 0.7 times the CEC of Mt and Ka, respectively. The suspension was stirred for 24 h at 25 °C and filtered by vacuum filtration. The treated clays were washed several times with distilled water until the Br− was not detected by the AgSO4 solution. The obtained clays were dried at 65 °C and activated for 1 h at 105 °C. The characteristics of CMt and CKa were investigated by Fourier transform infrared (FTIR), X-ray diffraction (XRD), Brunauer−Emmett−Teller (BET), and total organic carbon analysis. The parameters are listed in Table 3. X-ray powder diffraction patterns were obtained using a Rigaku D/MAX2200 diffractometer operated at 30 kV and 30 mA with Cu Kα radiation. The XRD patterns were recorded from 4° to 60° of 2θ with a scan speed of 4°/min. FTIR spectra were recorded with a KBr pellet on a Thermo NicoletIS10 FTIR spectrometer in the spectral range 4000−400 cm−1. Nitrogen sorption−desorption experiments were carried out at 77 K on a Micromeritics ASAP 2020 surface area and porosity analyzer (Quantachrome, United states). The samples were outgassed for 6 h at 200 °C before the adsorption measurements. The specific surface area was calculated on the basis of the multipoint BET equation. The organic carbon content was detected by the TOC (multi N/C 3100, Analytikjena Jena). The TOC results are shown in Figure S1 for the table of contents only. 2.3. Adsorption Experiments. The experimental condition of adsorption tests was set as follows: initial concentration range of main solutes: 0.4−4.0 μmol/L; mass of organoclays: 0.08 g/L; volume of solution: 20 mL; and initial pH of solution: 7. After mixing solutes and organoclays, the reactors were kept shaking at 150 rpm, 298 K for 24 h to make sure the adsorption reached equilibrium. Then, the mixed solution was centrifuged at 3500 rpm for 15 min, and the supernatant was used for analysis by high-performance liquid chromatography (HPLC). In the TBBPA binary solute system, the initial concentrations of TBBPA were ranging from 0.4 to 4.0 μmol/L and the initial concentration of the coexisting solute was controlled at three different levels, which were 2.0, 6.0, and 10.0 μmol/L. In BPA−2,6-DBP binary and trinary systems, the initial concentrations of BPA−2,6-DBP were ranging from 0.4 to 4.0 μmol/L and the initial

2. MATERIALS AND METHODS 2.1. Materials. TBBPA (99% purity, CAS number 79-947), BPA (99.8% purity, CAS number 80-05-7), and 2,6-DBP (97% purity, CAS number 608-33-3) were purchased from Sigma Chemical Co., Ltd. (USA). The selected physicochemical properties of these chemicals are obtained from the article of Sun et al.27 and Toxicology DATA NETWORK,28 which is given in Tables 1 and 2. Mt and Ka were obtained from Aladdin Chemistry Co., Ltd. (Shanghai, China). Cetyltrimethyl ammonium bromide (CTMAB) was obtained from Bio Science & Technology. Co., Ltd. (Shanghai, China). The cation exchange capacities (CECs) were 44.3 and 3.9 mequiv/100 g. Methanol is of HPLC grade and purchased from Fisher Co., Ltd. (ShangHai, China). The ultrapure water was obtained directly from a Nanopure UV deionisation 2781



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Table 2. Selected Physico-Chemical Data for TBBPA, BPA, and 2,6-DBPa

a

MW: molecule weight (g/mol); Sw: aqueous solubility (μmol/L); density (g/cm3); MV: molar volume (cm3/mol); Kow: octanol−water partition coefficient.27,28

where n is a positive integer and λ is the wavelength of the incident wave.29 As can be seen from Figure S3, d001 values of CMt (70% CEC) and CKa (70% CEC) were 1.01 and 0.72 nm, respectively. The values are similar to those of original clays, suggesting that the surfactant did not increase the original clays’ interlayer distance of any further. The N2 adsorption and desorption isotherms were utilized to calculate the BET surface area of CMt and CKa (Figure S5). The surface area for CMt was significantly decreased after CTAMB loading. This is because of that for the organo-bentonite with relatively high surfactant loading amounts, their surface areas are extremely small because of filling of pores by CTMAB.26 The main characterizations of organic modified clays are given in Table 3. 3.2. Adsorption Equilibrium Isotherms. 3.2.1. Single Solute System. First, the removal efficiency was investigated. At the initial concentration of 4.0 μmol/L, the removal efficiency of CMt is about 90, 38, and 13%, respectively, for TBBPA, BPA, and 2,6-DBP and the removal efficiency of CKa is about 63, 25, and 8%, respectively, for TBBPA, BPA, and 2,6-DBP. The removal efficiency followed the order: TBBPA > BPA > 2,6-DBP. The adsorption isotherms of TBBPA, BPA, and 2,6-DBP were investigated in the single solute system, separately. The results are shown in Figure 1. It can be seen that the organic-modified clays exhibited much larger affinity to TBBPA than to other two compounds in all range of equilibrium concentration. To further investigate the adsorption characters, Langmuir and Freundlich models were used to fit the adsorption isotherm data

Table 3. Characteristics of CMt and CKa sample Mt CMt Ka CKa

CEC (%) 70 70

BET surface area (m2/g)

basal spacing (d001 nm)

organic carbon content (%)

252.4 128.3 25.7 19.7

1.01 1.01 0.71 0.72

0.68 11.66 0.48 1.25

concentration of the coexisting solute was ranging from 0.4 to 4.0 μmol/L. 2.4. Analytical Methods. The content of each sample was determined by HPLC (Shimadzu LC-20AT, Kyoto, Japan) equipped with a photodiode array detector (SPD-M2OAV) and a VP-ODS column (150 × 4.6 mm, 5 μm) under the following conditions: 80% methanol/20% water with a wavelength of 209 nm for the analysis of TBBPA; 70% methanol/30% water with a wavelength of 280 nm for the analysis of BPA; and 75% methanol/25% water with a wavelength of 279 nm for the analysis of 2,6-DBP. The injection volume was 20 μL for all of the solutions. The column was operated at 308 K. All of the solutions were injected at a flow of 0.8 mL/min. The chromatogram of TBBPA, BPA, and 2,6-DBP by using HPLC is shown in Figure S2.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Adsorbent. The FTIR spectrum of CMt and CKa is shown in Figure S3. Compared to the original clays, new peaks (2917−2926 and 2852−2857 cm−1) were observed in organic-modified clays. A previous research had reported that the new peaks at 2917−2926 and 2852−2857 cm−1 were corresponded to the CH2 asymmetric stretching mode and the symmetric stretching mode, respectively,26 which come from the CTAMB. The organic carbon content was also investigated. It can be seen from Table 3 that the organic carbon content was increased from 0.68 to 11.66% for CMt and from 0.48 to 1.25% for CKa. This proved that the surfactant molecules were successfully loaded onto two different inorganic clays. Figure S4 presents XRD patterns of CMt and CKa. Basal spacing values of the samples were calculated from 2θ values of the peaks in the patterns based on the Bragg’s law. The Bragg’s law equation was as follows (1) 2d sin θ = nλ

Ce C 1 = + e qe qmax KL qmax

(2)

ln qe = n ln Ce + ln KF

(3)

where qe is the equilibrium-adsorbed concentration and Ce is the equilibrium solute concentration. KL and qmax are the Langmuir affinity coefficient and maximum adsorption capacity, respectively; KF and n are the Freundlich affinity coefficient and exponential coefficient, respectively. The obtained adsorption parameters are collected in Table 4. The discrimination between the two models has been performed by means of the statistical criteria reported by Sciascia.30 The Langmuir model failed to describe the isotherms. Inadequate fits were reflected in sum squared error (SSE), 2782



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was affected by the underlying mineral type and mass fraction of carbon.31 3.2.2. Relationship between the Octanol−Water Partition Coefficient and Adsorption Capacity in the Single Solute System. Previous scholars reported that the hydrophobic property of the target solute has a significant effect on the adsorption coefficient in the organoclay adsorption process.32−34 To reveal the relationship between the hydrophobic property and adsorption affinity, the octanol−water partition coefficient (Kow) was employed to describe the organic hydrophobic property. The single point adsorption coefficients (Kd) and carbon-normalized coefficients (Koc) (Koc = Kd/foc) derived from the equilibrium solute concentration of 1.5 μmol/L due to isotherm nonlinearity were plotted to Kow (Figure 2a). The results showed that the TBBPA adsorption

Figure 1. Adsorption isotherms of TBBPA, BPA, and 2,6-DBP in a single solute system (a) CMt and (b) CKa. The lines were obtained from the Freundlich model.

Table 4. Results of Model Fitting to the Single Solute System of TBBPA, BPA, and 2,6-DBPa CMt parameters

TBBPA

BPA

CKa DBP

TBBPA

BPA

DBP

2.42 33.56 0.89 45.21 −11.41 84.53

0.19 17.42 0.94 1.21 13.93 1.38

0.47 8.17 0.82 1.99 10.44 2.49

0.44 22.02 1.24

0.76 2.78 0.40

0.57 2.61 0.34

KL qmax R2 SSE AIC PRESS

0.43 212.76 0.84 79.32 79.32 133.05

n KF log Kd (Ce = 1.5 μmol/L) log Koc (Ce = 1.5 μmol/L) R2 SSE AIC PRESS

0.66 80.94 1.85

Langmuir Model 0.12 0.09 111.11 34.72 0.42 0.91 9.73 6.06 −0.66 2.65 19.62 17.23 Freundlich Model 0.88 0.81 11.76 3.02 1.05 0.45

2.78

1.98

1.38

3.14

2.30

2.24

0.95 37.09 −10.02 107.64

0.98 10.99 −1.51 20.46

0.96 3.77 5.97 13.72

0.98 14.48 −3.44 25.92

0.99 0.32 23.24 0.65

0.95 1.13 14.41 1.87

Figure 2. (a) Relationships between log Kow vs log Kd of TBBPA, BPA, and 2,6-DBP (CMt). (b) Relationship between log Kow vs log Koc of TBBPA, BPA, and 2,6-DBP(CKa). ▲ and △, TBBPA; ● and ○, BPA; ■ and □, 2,6-DBP.

was much stronger than BPA and 2,6-DBP because of its high hydrophobic property. Moreover, BPA exhibited stronger adsorption affinity than 2,6-DBP in spite of its lower hydrophobic property (Figure 2a). The higher BPA adsorption may be contributed from its molecule structure. A previous study indicated that BPA has a unique butterfly chemical structure and could wedge into high-energy sites of heterogeneous surface adsorption sites, which could enhance the adsorption capacity.32 Therefore, the affinity of BPA to organic-modified clays was stronger than that of 2,6-DBP. Consequently, results obtained from Kow values of three solutes in the single solute system demonstrated that the adsorption affinity was dependent on not only the hydrophobic property but also molecule structure and properties of adsorbed solutes.

a

High sum squared error (SSE); prediction error sum (PRESS).

prediction error sum (PRESS), and R2. The better fitting results were obtained by the Freundlich model. All adsorption isotherms were nonlinear with n values ranging from 0.44 to 0.88. The adsorption affinity coefficients (KF) followed the order: TBBPA > BPA > 2,6-DBP. In addition, the KF value for CMt was higher than that for CKa. These different affinity coefficients might be resulting from the different amount of adsorbed surfactants, which depended on the CEC of clay minerals (Table 3). This indicated that the adsorption capacity 2783



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3.2.3. Relationship between the Mineral Type and Adsorption Capacity in the Single Solute System. Figure 2b also shows that the Kd values of CMt were generally higher than that of CKa, whereas Koc values’ trend was opposite. This indicated that the CKa adsorption efficiency was higher than that of CMt. The observation was consistent with previous studies on chlorobenzene adsorption on organoclays.31 Mt was a kind of swelling layer silicates, and Ka was a nonswelling one. The preselected solutes adsorbed in swelling clays such as Mt may adopt very loose and disordered flat-lying arrangement in the clay interlayers in the wet state.35 This indicated that CKa showed an advantage for TBBPA, BPA, and 2,6-DBP adsorption than CMt in the single solute system. 3.2.4. Adsorption Isotherms of TBBPA in the Binary Solute System. The effects of the coexisting solute systems on target solute adsorption are examined in Figures 3 and 4. Here,

Figure 4. (a) Adsorption isotherm of TBBPA on CMt with 2,6-DBP; (b) adsorption isotherm of TBBPA on CKa with 2,6-DBP.

Table 5. Freundlich Model Fitting Results for TBBPA Adsorption on CMt and CKa in Multiple Solute System CMt

CKa

main solute

coexisting solute

KF

n

KF

n

TBBPA TBBPA TBBPA TBBPA TBBPA TBBPA TBBPA

TBBPA single BPA (2.0 μmol/L) BPA (6.0 μmol/L) BPA (10.0 μmol/L) 2,6-DBP (2.0 μmol/L) 2,6-DBP (6.0 μmol/L) 2,6-DBP (10.0 μmol/L)

80.94 55.88 47.41 41.46 52.82 96.45 82.63

0.66 0.58 0.48 0.35 0.47 0.63 0.44

22.02 19.21 17.14 11.53 15.81 21.19 24.47

0.44 0.78 0.95 1.02 0.81 0.67 1.36

The competitive adsorption could be explained by the greater overlap in the affinity of the two solutes for the same adsorption sites.32 As shown in Table 2, BPA had a molecule structure similar to that of TBBPA. When BPA was introduced into the TBBPA solution, BPA competed with TBBPA for the same adsorption sites, thereby resulting in decreased adsorption of TBBPA. For CMt, the value of n in single and binary solute systems was smaller than 1, indicating a normal Freundlich isotherm (a beneficial adsorption occurring at a heterogeneous surface of CMt). For CKa, the value of n in the binary solute system was ranged from 0.78 to 1.02, which indicated a cooperative adsorption.37 The adsorption isotherm of TBBPA in the TBBPA−2,6DBP binary system is shown in Figure 4a,b. It can be seen that the effect of 2,6-DBP was different from that of BPA. The presence of 2,6-DBP with high initial concentration appeared to yield increased adsorption capacity for TBBPA. The KF values (Table 5) obtained from CMt followed the order: TBBPA−2,6-DBP (6.0 μmol/L) > TBBPA−2,6-DBP (10.0 μmol/L) > TBBPA > TBBPA−2,6-DBP (2.0 μmol/L). This indicated that with the higher initial concentration of 2,6-DBP

Figure 3. (a) Adsorption isotherm of TBBPA on CMt with BPA; (b) adsorption isotherm of TBBPA on CKa with BPA.

TBBPA was chosen as the target solute. The adsorption isotherms of TBBPA were first measured in binary solute systems using BPA and 2,6-DBP as coexisting solutes separately. The Freundlich model yields a good fit to the multiple solute isotherms (R2 > 0.948) (Tables S1 and S2). The parameters obtained from the Freundlich model are listed in Table 5. The adsorption isotherm of TBBPA in the TBBPA−BPA binary system is shown in Figure 3a,b. For both adsorbents, it is observed that the adsorption capacity for TBBPA was decreased by the presence of BPA. The KF values listed in Table 5 followed the order: TBBPA > TBBPA/BPA (2.0 μmol/L) > TBBPA/BPA (6.0 μmol/L) > TBBPA/BPA (10.0 μmol/L). This indicated that BPA exhibited a competitive effect on TBBPA adsorption. Generally, the more similar the molecular structure and properties between the competitive and the primary solutes, the greater the competition was.36 2784



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competing with TBBPA for the same adsorption sites. When the BPA molecule access into the TBBPA system, some of the TBBPA molecule will be desorbed from the surface of CMT. Thus, the removal of TBBPA was decreased with the increase of initial concentration of BPA. However, the adsorption of TBBPA was increased by 2,6-DBP with initial concentrations of 6.0 and 10.0 μmol/L. This is because of the interaction between the 2,6-DBP and CTMAB surfactants which make more interlayer region accessible for TBBPA adsorption. Thus, TBBPA could more easily access into the interlayer region of clays, and the removal of TBBPA by CMt was increased in the 2,6-DBP binary system. 3.2.5. Multicomponent Adsorption Model for TBBPA Adsorption in the Binary Solute System. The multicomponent Freundlich-type adsorption isotherm [Sheindorf− Rebuhn−Sheintuch (SRS) model] was applied to study the adsorption behavior in binary systems (the description of this model is listed in the Supporting Information). Figure S7 shows the fitting between measured and predicted adsorption capacities of TBBPA and its composition products in a binary solute system (various initial concentrations of the interferential component), which indicated that the SRS model,39 an extended Freundlich model, gave acceptable results in the range 0−60 μmol/g to describe the simultaneous adsorption of TBBPA and its metabolites from a binary mixture onto CMt, but it failed to fit bisolute adsorption isotherms on CKa. The competition coefficients obtained from the SRS model are listed in Table S3. The competition coefficients obtained indicated the relative adsorption capacity of each solute in the binary system. As shown in Table S3, the α12 values are corresponded to the selectivity of solute 1 in the presence of solute 2. In the binary system of TBBPA (solute 1) + BPA (solute 2) and TBBPA (solute 1) + 2,6-DBP (solute 2), the α12 values were high, indicating high affinity of TBBPA to CMt. These results could be explained by the difference of their hydrophobicity; that is, the solubility of TBBPA in water was much lower than those of degradation products. Meanwhile, the α21 values, corresponding to the selectivity of solute 2 (BPA and 2,6-DBP) in the presence of solute 1 (BPA), were not very high. The results were rationalized by that the solubilities of BPA and 2,6-DBP were of the same magnitude. Therefore, it can be deduced that the SRS model was an efficient method for predicting competitive adsorption of TBBPA on CMt. 3.2.6. Adsorption Isotherms of BPA and 2,6-DBP in Binary and Ternary Solute Systems. The adsorption mechanism of BPA and 2,6-DBP in multiple solute systems was also studied by measuring the isotherms (Figure 5). According to the comparison among single, binary, and ternary solute systems, it is suggested that both BPA and 2,6-DBP adsorption coefficients decreased in the order: KF of single solute system > KF of binary solute system > KF of ternary solute system (Table 6). The reason for the KF decrease is that the TBBPA had the biggest molecule volume and could occupy a larger CMt/CKa surface, which decreased the BPA and 2,6-DBP adsorption site amount and eventually resulted in the decrease of BPA and 2,6-DBP adsorption. Moreover, the BPA and 2,6DBP in the ternary solute system were compared to those in the binary solute system. As for BPA adsorption isotherms, the 2,6-DBP and TBBPA binary systems depressed the BPA adsorption. When BPA was in the 2,6-DBP and TBBPA ternary systems, it is found that the reduction of BPA in the ternary system is higher than that in the binary solute system.

in the binary solute system, the competitive effect disappeared and a synergetic effect may take place. This synergetic effect, as interpreted likely by: (1) the delocalized π-bond systems in 2,6-DBP interact strongly with the cationic ammonium center and alkyl chains of CTMAB. This interaction can result in a reorientation of the alkyl substituent with the revealed mineral surfaces. A more orthogonal position of the alkyl chain expands the CTMAB-modified clays’ interlamellar region, which makes more interlayer region accessible for TBBPA adsorption and eventually increases the TBBPA adsorption capacity. A similar synergetic effect was also observed for phenols adsorbed on clay minerals.38 (2) The adsorption of 2,6-DBP also increased the organic carbon content of CMt. A higher organic carbon content could also increase the CMt adsorption capacity for removing TBBPA from water. Moreover, the KF value in the 10.0 μmol/L 2,6-DBP binary system is lower than that of 6.0 μmol/L. The KF of TBBPA did not increased with the increase of 2,6-DBP. This indicated that the second explanation cannot be responsible for the synergetic effect of 2,6-DBP on TBBPA adsorption. Because the higher the initial concentration of 2,6-DBP can increase the total organic carbon content of CMt, but did not further increase the TBBPA adsorption. It is most possible that in the 6.0 μmol/L 2,6-DBP binary system, the 2,6-DBP molecule interacts with the cationic ammonium center and alkyl chains of CTMAB, which make more interlayer region accessible for TBBPA adsorption. The XRD of CMt before and after 2,6DBP adsorption is performed in Figure S6. The basal spacing values (d001) were slightly increased from 1.01 to 1.52 nm after 2,6-DBP adsorption. Thus, it can be concluded that the presence of 2,6-DBP in the binary system could make the interlayer region more accessible for removing TBBPA. However, with the initial concentration of 2,6-DBP increased to 10.0 μmol/L, the sorbate−sorbate interactions may occur and the steric obstruction probably reduced the interaction between the 2,6-DBP and CTMAB. Thus, the KF value in 10.0 μmol/L 2,6-DBP binary system is lower than that of 6.0 μmol/ L. As a result, the 2,6-DBP at 6.0 and 10.0 μmol/L enhanced the TBBPA adsorption. This phenomenon provided an idea that adding brominated phenols into the system could increase the TBBPA removal efficiency during the organic-modified clay adsorption process. In summary, BPA and 2,6-DBP exhibited different effects on TBBPA adsorption in the binary solute system. The effects were proposed and are summarized in Scheme 1. Specifically, it can be seen from Scheme 1 that the adsorption of TBBPA was decreased by the presence of BPA, attributing to BPA Scheme 1. TBBPA Adsorption in the Binary Solute System

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adsorption, we only listed the data of TBBPA and BPA. It can be seen that the adsorption capacities of TBBPA onto CMt and CKa were higher than that of other adsorbents. However, the CMt and CKa exhibited lower adsorption capacity of BPA than other adsorbents. In particular, the obtained material is an effective adsorbent for the simultaneous removal of TBBPA and its metabolites in the binary solute system. Besides, the current work demonstrated CMt and CKa prepared in a lowcost one-step synthesis, which offered advantage in the removal of combined pollutants from the environment. 3.3. Adsorption Kinetics Study. 3.3.1. Single Solute System. The adsorption kinetics of TBBPA, BPA, and 2,6-DBP in single and binary solute systems are performed in Figure 6.

Figure 5. (a) Adsorption isotherm of BPA in single, binary, and ternary solute system; (b) adsorption isotherm of 2,6-DBP in single, binary, and ternary solute system.

Table 6. Freundlich Model Fitting Results for BPA and 2,6DBP Adsorption on CMt and CKa in the Multiple Solute System CMt main solute BPA BPA BPA BPA 2,6-DBP 2,6-DBP 2,6-DBP 2,6-DBP

coexisting solute single TBBPA (0.4−4 μmol/L) 2,6-DBP (0.4−4 μmol/L) TBBPA + 2,6-DBP (0.4−4 μmol/L) single TBBPA (0.4−4 μmol/L) BPA (0.4−4 μmol/L) TBBPA + BPA (0.4−4 μmol/L)

CKa

KF

n

KF

n

11.92 9.08 8.45 8.08

0.85 0.86 0.87 0.77

2.85 2.18 2.6 1.78

0.73 0.27 0.6 0.51

2.51 2.38 1.96 1.93

1.08 1.19 0.99 0.82

2.72 2.41 2.3 2.13

0.52 0.91 0.87 1.12

Figure 6. Adsorption kinetics of TBBPA (a), BPA, and 2,6-DBP (b) in single and binary solute systems.

To investigate the adsorption kinetics of TBBP and its metabolites on organic-modified clays, three kinetic models, such as pseudo-second-order equation, Elovich equation, and intraparticle diffusion model, were employed. The pseudo-second-order kinetic model is given by the following equation43

As for the 2,6-DBP adsorption isotherms, the adsorption amount at equilibrium in the BPA binary system is very close to that in the single system and the TBBPA binary system decreased the 2,6-DBP adsorption. It is interesting that when 2,6-DBP was in the TBBPA and BPA ternary systems, the reduction of 2,6-DBP is still higher than that in the TBBPA binary system. This implied that the adsorption mechanism in the ternary solute system maybe different from that in the binary solute system and the sorbate−sorbate interaction may contribute to the adsorption ability of CMt in the ternary solute system.40 More investigation will be performed in the future to study the adsorption mechanism in the ternary solute system. 3.2.7. Comparison with Other Adsorbents. For further comparison with other adsorbents, additional data obtained from the literature41,42 for different adsorbents are listed in Table S4. Because there is scarcely any data of 2,6-DBP

qt =

k 2qe 2t 1 + k 2qet

(4)

where k2 is the rate constant of pseudo-second order adsorption. The Elovich equation is given as the following equation43 qt = A + B ln t

(5)

where A and B are the Elovich constants. The pseudo-second-order model and Evolich model were used to investigate the overall adsorption kinetics in single and binary solute systems. On the basis of the values of R2 given in Table 7, the pseudo-second-order model showed the good fitting performance for single solute adsorption kinetics (R2 > 2786



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Table 7. Kinetic Parameters for TBBPA, BPA, and 2,6-DBP Adsorption by CMt in Single and Binary Solute Systems pseudo-second order

intra-particle diffusion

Elovich equation

solutes (co-solutes)

1/k2(qe)2

1/qe

R2

kp

c

R2

A

B

R2

TBBPA BPA 2,6-DBP TBBPA (BPA) TBBPA (2,6-DBP) BPA (TBBPA) 2,6-DBP (TBBPA)

0.06 0.11 0.04 0.13 0.09 0.16 0.29

0.59 0.05 0.07 0.03 0.03 0.05 0.08

0.99 1 0.99 0.99 0.99 0.99 0.99

0.61 0.11 0.05 0.41 0.37 0.09 0.01

27.54 19.13 14.60 29.04 32.05 16.62 11.43

0.74 0.59 0.51 0.67 0.71 0.51 0.07

23.48 18.54 14.98 25.84 29.66 16.24 11.78

2.23 0.36 0.20 1.62 1.34 0.28 0.06

0.75 0.59 0.54 0.74 0.71 0.52 0.11

For the BPA−TBBPA and 2,6-DPB−TBBPA systems (Figure 6b), the adsorption capacity of BPA and 2,6-DBP was obviously decreased by the presence of TBBPA. The reductions were about 12.3 and 14.1%, respectively, for BPA and 2,6-DBP. The TBBPA exhibited obvious competitive effect on BPA and 2,6-DBP adsorption. On the basis of the adsorption equilibrium, the individual adsorption of BPA and 2,6-DBP was reduced. However, the total adsorption capacity of BPA + TBBPA or 2,6-DBP + TBBPA is higher than that of any BPA and 2,6-DBP adsorption capacity at equilibrium in the single adsorption system. Under competitive adsorption condition, the fitting results were consistent with that in single solute systems. The pseudosecond-order model showed satisfaction performance for the binary solute adsorption kinetics. Table 7 shows that the 1/ k2qe(cal)2 values of TBBP were increased by the presence of BPA and 2,6-DBP. The simultaneous presence of cosolutes affected the 1/k2qe(cal)2 value in the binary solute system. For the BPA and 2,6-DBP, the 1/k2qe(cal)2 values were also increased in the binary solute system in comparison with that of the single solute system. The adsorption rate factor in this study could be increased by the presence of other cosolute, which could be attributed to the cosolute interaction on the surface of the adsorbent.44 Elovich equations failed to describe the adsorption kinetics, indicating that the adsorption process was not an ion exchange process. The intraparticle diffusion model gave an acceptable fitting performance for TBBPA adsorption kinetics with R2 > 0.7. As can be seen from Table 7, the c value of TBBPA in the binary solute system was above zero, which suggested that pore diffusion was not only the sole rate-limiting step at the beginning of bath adsorption. The relative adsorption of TBBPA and BPA−2,6-DBP in the binary system was obtained by the following equation

0.99), whereas Elovich equations failed to describe the adsorption kinetics (R2 < 0.80). The Elovich model was frequently employed to describe the adsorption kinetics of the ion exchange system, so it could be deduced that the adsorption process was not an ion exchange process. The pseudo-second-order model was usually related to the chemisorption. The estimated exponents of the pseudosecond-order model (1/k2qe(cal)2) for TBBPA, BPA, and 2,6DBP are listed in Table 7. These values could be related to the empirical rate coefficients of the overall adsorption processes over the entire reaction time range.43 As can be seen from Table 7, the 1/k2qe(cal)2 value of TBBPA was smaller than that of BPA and 2,6-DBP, which indicated that TBBPA was initially more rapidly adsorbed on CMt. The intraparticle diffusion model was also used to investigate the adsorption kinetics of single solute system and binary solute system. It is given by the following equation qt = ktt 0.5 + C

(6)

where kt is the intraparticle diffusion rate and C is a constant related to the thickness of the boundary layer. The parameters are listed in Table 7. As shown in Table 7, the intraparticle diffusion model only gave an acceptable fitting performance for TBBPA adsorption kinetics with R2 value > 0.7. The intercept c was related to the thickness of the boundary layer. If c = 0, the intraparticle diffusion would be the sole rate-limiting step. If c > 0, it implied that intraparticle diffusion was not the sole rate control step and other process may control the adsorption rate. As can be seen from Table 7, the c value of TBBPA was above zero, which suggested that pore diffusion was not only the sole rate-limiting step at the beginning of bath adsorption. Filmdiffusion control may also be occurred in these early stages of the adsorption process. It may also have been the rate-limiting step during the time period of initial adsorption stage. It also suggested that the actual adsorption process may contain both the surface adsorption and intraparticle diffusion. 3.3.2. Binary System. The TBBPA−BPA binary system and TBBPA−2,6-DBP binary system are also shown in Figure 6a. For the TBBPA−BPA binary system, the adsorbed amount of the selected chemical increased with time, similar to the single solute adsorption system. However, the equilibrium adsorption capacity of TBBPA was obviously decreased by the presence of BPA. The reduction of TBBPA uptake in the presence of BPA was about 3.1%. The decreased adsorption capacity indicated competitive adsorption. For the TBBPA−2,6-DBP binary system, the equilibrium adsorption capacity of TBBPA was slightly increased by the presence of 2,6-DBP. The increased adsorption capacity indicated a synergetic adsorption. These kinetic results were consistent with that in the isotherm equilibrium system.

A r = [qe]B /[qe]S

(7)

where [qe]B and [qe]S are the adsorption capacity of specific adsorbate in binary system and single system at time t, respectively.43 Figure 7 shows the variation of relative adsorption of three selected chemicals with time. For BPA and 2,6-DBP, the relative adsorption did not change too much during the overall adsorption process. For TBBPA, the relative adsorption decreased slowly, especially for the BPA binary system. This indicated a competition behavior occurring on the surface of CMt. As for the binary adsorption system, the selectivity of adsorption on CMt was calculated by the following equation S= 2787

A r ‐ TBBPA A r ‐ BPA−2,6 ‐ DBP

(8) DOI: 10.1021/acs.jced.9b00164 J. Chem. Eng. Data 2019, 64, 2780−2790


Article

The chemical structure might influence the adsorption affinity. CKa exhibited a higher adsorption efficient for TBBPA and its degradation products than CMt. In multiple solute systems, the adsorption capacity of TBBPA was decreased by the presence of BPA and the competitive effect was dependent on the initial concentration of BPA. However, 2,6-DBP with high initial concentration enhanced the adsorption capacity of TBBPA. The synergetic effects could be explained by that: (1) 2,6-DBP adsorption made the basal space of the CMt and CKa expanded. Then, the interlayer region became more accessible to TBBPA adsorption. (2) 2,6-DBP increased the organic carbon content. Additionally, BPA and 2,6-DBP adsorption isotherm in multiple solute systems was also investigated. In addition, the kinetic results showed that the competition reduced the adsorption rate. Although the adsorption capacity was decreased in the binary system compared to that of the single system, the total adsorption capacity was increased. Moreover, the selectivity of TBBPA in the 2,6-DBP binary system was obviously higher than that in the BPA binary system. It further confirmed that the 2,6-DBP could enhance the TBBPA adsorption in the binary solute system.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.9b00164.

Figure 7. Relative adsorption of TBBPA (a) and BPA−2,6-DBP (b) in the binary solute system.

■

The result is shown in Figure 8. It was found that the selectivity of adsorption on CMt shows a decreased tendency,

Experimental detail and characterization of organicmodified clays (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 020-85161402. ORCID

Xiang Li: 0000-0002-4155-3256 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported in part by Department of Science and Technology of Guangdong Province project (2016A020210034 and 2017B020203002) and Guangdong Natural Science Foundation project (2016A030313772 and 2015A030313570). Meanwhile, it is also supported by Pearl River S&T Nova Program of Guangzhou, China (201610010131) and President Foundation of Guangdong Academy of Agricultural Sciences, China (201716).

Figure 8. Selectivity of TBBPA in the BPA−2,6-DBP binary system.

whereas the value is still higher than 1. This suggested that CMt had a higher affinity to TBBPA in the binary adsorption system. When the adsorption achieves the equilibrium, the selectivity of adsorption would still be different for TBBPA and its metabolites. Moreover, the selectivity of TBBPA in the 2,6DBP binary system was obviously higher than that in the BPA binary system. It further confirmed that the 2,6-DBP could enhance the TBBPA adsorption in the binary solute system.

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REFERENCES

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Comparison of Competitive and Synergistic Adsorption of

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