Rapid Method for the Separation and Recovery of Endocrine

Article pubs.acs.org/Langmuir

Rapid Method for the Separation and Recovery of EndocrineDisrupting Compound Bisphenol AP from Wastewater Lei Zhang,* Pan Fang, Lijun Yang, Jing Zhang, and Xin Wang College of Chemistry, Liaoning University, 66 Chongshan Middle Road, Shenyang 110036, People’s Republic of China ABSTRACT: The removal of bisphenol AP (BPAP) by a multiwalled carbon nanotube (MWCNT) was investigated. BPAP, representing typical scenarios for the BPAP−MWCNT interactions, was employed as a probe molecule. It was found that BPAP exhibited great adsorptive affinity to MWCNT, and the adsorption kinetics equilibrium was arrived within 4.0 min following the pseudo-second-order model. The overall rate process was mainly controlled by the external mass transfer. The hydrogen bond, hydrophobic, and π−π stacking interactions were dominant factors for the strong adsorption of BPAP, instead of the pH ionic strength and other ionic species in contaminated water. The MWCNT has higher stability within 8 removal−regeneration recycles, and up to 95% of recovery could be obtained by eluting the adsorbed BPAP on MWCNT adsorbent using ethanol/sodium hydrate solution. The results of the experiment on real samples verified the effectiveness for the recovery and removal of BPAP from wastewater samples.

1. INTRODUCTION The presence of endocrine-disrupting compounds (EDCs) in a wide range of natural and engineered environments, including surface water, groundwater supplies, wastewater effluents, seawater, and sediment, has emerged as a serious issue worldwide.1−5 EDCs have been closely studied in recent years because they have shown estrogenic effects in fish, avian, and mammalian cells. The adverse health effects of exposure to EDCs have been reported, such as a decreased sperm count, reduced fertility, and increased incidence of breast, ovarian, and testicular cancers.6−8 Bisphenol AP [4,4′-(1-phenylethylidene)bisphenol] (BPAP), is commonly used in polymer materials, the fine chemical industry, and the medicine industry. Especially as an indispensable plasticizer and flame retardant, it is widely used in synthesizing plastic, rubber, and other industrial products. Its excessive applications also generate a lot of problems and have brought harm to the health of people and the environment. BPAP has been confirmed to be one of the EDCs by the United States Environmental Protection Agency (U.S. EPA). Many researchers have identified BPAP in water and soil,9 which may also cause potentially environmental problems. Therefore, the development of a simple, rapid, and reliable method for the removal and recovery of BPAP from wastewater is of great importance. Many investigators have presented some separation processes for the removal of EDCs from wastewater, such as chemical and biological conversion techniques. However, conventional separation techniques, such as coagulation,1 flocculation,10 and precipitation processes,11 are not effective in removing the low-molecular-weight EDCs. Conventional biological processes, such as activated sludge12 and biofiltration13 have shown limited removal of EDCs, which were mostly © 2013 American Chemical Society

derived from biodegradable and/or other compounds readily attached to particles.14,15 Conclusively, conventional treatment techniques are not highly effective on the removal of EDCs, while chemical and advanced treatment processes [e.g., granular activated carbon (GAC), membrane separation, chlorination, and ozonation] have shown satisfying results.16,17 Advanced separation processes, such as adsorption, membrane filtration, and ion exchange, normally show superior removal efficiencies, depending upon the compounds tested. The adsorptive removal methods have been considered as potential treatment options for the effective removal of EDCs from wastewater. Adsorption with GAC generally removed most organic contaminants, including EDCs.1,18 Modified mesoporous silica and molecularly imprinted polymer (MIP) particles are also another important field of research regarding EDC treatment.19,20 Currently, the limited study of BPAP is mainly concentrated on the mechanism of its endocrine-disrupting effect in vitro, and there are no published reports on the method for removing and recovering of BPAP from wastewater. Therefore, very little is known about the kinetic and thermodynamic characteristics of BPAP adsorption on a multiwalled carbon nanotube (MWCNT). Carbon nanotubes (CNTs), ever since their discovery, have attracted extensive attention because of their unique physicochemical and electrical properties. CNTs, which are considered to be extremely superior adsorbents because of their high specific surface area and large micropore volume, have Received: December 4, 2012 Revised: January 17, 2013 Published: February 27, 2013 3968

dx.doi.org/10.1021/la304792m | Langmuir 2013, 29, 3968−3975

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and the injection volume was 20 μL. The mobile phase was 65:35 (volume ratio) acetonitrile/phosphate-buffered saline (pH 4.5), and the flow rate was 1.0 mL min−1. The detection wavelengths were 278 nm. The retention time of BPAP was 2.604 min. The adsorption percentage (ads %) was calculated on the basis of the following equation:

been used for the sorption of a number of organic and inorganic pollutants.21−30 To develop a simple method for the separation BPAP from wastewater and understand the interaction between BPAP and MWCNT, MWCNT was used as a sorbent for the isolation of a trace amount of BPAP from wastewater samples. The adsorption type, dynamics, and thermodynamics of BPAP on MWCNT were investigated in detail. The best conditions for recovery of BPAP were discussed and optimized.

Ads. % =

C i − Ca × 100 Ci

(1)

where Ci and Ca are the initial and final concentrations of BPAP in solution phase, respectively. Kinetic experiments were performed using a series of 50 mL flasks containing 45.0 mg of MWCNT and 25 mL of 20.0 mg L−1 BPAP solution in a temperature range of 286−313 K. On regular time intervals, suitable aliquots were taken, whereupon the BPAP concentration was determined. The rate constants were calculated using the conventional rate expression. Adsorption isotherm studies were carried out with the concentrations of BPAP ranging from 30.0 to 120.0 mg L−1; the amount of adsorbent was kept constant (10.0 mg); and the experimental temperatures were controlled at 286, 298, and 313 K. The thermodynamic parameters for the adsorption process were determined at each temperature. 2.4. Desorption and Reuse Study. The BPAP absorbed by MWCNT was eluted with 10 mL of ethanol/1.0 M sodium hydrate solution (7:3, v/v) (stirring for 5 min before separated), and the recovery BPAP was determined. The regenerated MWCNT was treated with deionized water to neutralize, vacuum-dried at 333 K overnight, and then explored for BPAP removal in succeeding recycles. 2.5. Water Samples. Water samples were collected from drinking water, river water, synthetic water, and effluents. The effluents used in this study were collected from a municipal wastewater treatment plant (Shenyang, China). The samples were stored at 4 °C in low-density polyethylene (LDPE) bottles. The synthetic water samples spiked with 10 mg/L BPAP were used in the experiments. A total of 1 mM each of K+, NH4+, Mg2+, Cu2+, Zn2+, Ni2+, Pb2+, NO3−, HCO3−, CO32−, SO42−, SO32−, and PO43− ions was separately added to the above-mentioned synthetic water.

2. MATERIALS AND METHODS 2.1. Chemicals. Three CNTs (95% purity) [pristine (MWCNT), hydroxyl functionalized (MWCNT−OH), and carboxyl functionalized (MWCNT−COOH)] were purchased from Chengdu Organic Chemicals Co., Ltd., China. Each of the three CNTs has a specific surface area above 500 m2 g−1 and an outer diameter below 8 nm. Pore distributions (pore volume with the pore diameter in parentheses) of MWCNT used as an adsorbent throughout the experiment are as follows: 0.085 m3 g−1 (0−20 nm) and 1.839 m3 g−1 (20−50 nm). All of these physical parameters of CNTs were provided by the manufacturer. The analytical standard BPAP (chemical reference substance, 99.0% purity) was purchased from HEOWNS Biochemical Technology Co., Ltd. (Tianjin, China). The molecular structure of BPAP is showed in Figure 1.

Figure 1. Molecular structure of BPAP. A 500 mg L−1 stock standard solution of PBAP was prepared by dissolving 252.5 mg of BPAP in 100 mL of ethanol and diluted by adding deionized water up to a 500 mL brown volumetric flask. The standard stock solutions of BPAP were diluted successively to the required concentration with deionized water in the experiment and kept in the dark below 4 °C. The inorganic salts used in the synthetic water were KCl, NH4Cl, MgCl2, Ni(NO3)2, Cu(NO3)2, Zn(NO3)2, Pb(NO3)2, NaCl, NaHCO3, Na2CO3, Na2SO4, Na2SO3, and Na3PO4. All other chemicals were of analytical reagent grade and purchased from Shenyang Chemical Company, China. 2.2. Characterization of MWCNT. The functional groups of the MWCNT surface were detected by Avatar 330 Fourier transform infrared (FTIR) spectroscopy (Nicolet Co., USA). A Malvern Zetasizer Nano-ZS particle analyzer (Malvern, U.K.) was used to determine the ζ potential of adsorbent. 2.3. General Adsorption Procedure. Duplicate and triplicate samples were used for the experiments of adsorption isotherm and conditions (pH and ion strength), respectively. The pH values of test solution were set changed over a range of 3.0−12.0 using a small amount of diluted HCl and NaOH, which decrease the effects of ion strength led by other buffer solutions. The ionic strength of the test solutions was adjusted with NaCl. Batch adsorption experiments were conducted using 50 mL flasks containing 45.0 mg of MWCNT and 25 mL of 20.0 mg L−1 BPAP solution. After stirring at a constant rate at 293 K for 4.0 min, the solid/liquid phases were separated by centrifuging at 5000 rpm for 5 min. Then, the concentration of BPAP in suspensions was determined by the HPLC system Agilent 1100 equipped with an autoinjector and diode array detector and a SB-C18 reversed-phase column (150 × 4.6 mm inner diameter, 5 μm). The column temperature was set at 35 °C,

3. RESULTS AND DISCUSSION 3.1. Comparison of Removal Efficiency. Three MWCNT sorbents were chosen to test their adsorbability of BPAP. To obtain higher adsorption efficiencies for BPAP, the adsorption conditions (solution pH and sorbent amount) of each sorbent were optimized. MWCNT has the highest adsorption efficiency for BPAP among the selected MWCNT, MWCNT−COOH, and MWCNT−OH. Functional groups can change the wettability of CNT surfaces and, consequently, make CNT more hydrophilic and helpful for the adsorption of relatively low-molecular-weight and polar compounds. Therefore, the functional groups of MWCNT have, in general, a deleterious effect on the adsorption of weak polar or nonpolar compounds because they can block the porosity of the carbon matrix.31−33 In addition, water molecules could form hydrogen bonds with functional groups on MWCNT, which will either compete with organic chemicals for adsorption sites or form a three-dimensional cluster and block the sorption sites nearby. Thus, the hydrogen bond formation between water and MWCNT functional groups could effectively decrease the sorption of organic chemicals. As a result, MWCNT was used as an adsorbent for BPAP in the following experiments. 3.2. BPAP Adsorptions on MWCNT. A detailed study on the adsorption process was performed by varying the sorbent amount and sorption time. A total of 25 mL of 20 mg L−1 BPAP solutions was applied to test the sorption behavior at different conditions. It was found that the adsorption 3969


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Figure 2. (A) Effect of pH and (B) effect of ionic strength on the adsorption of BPAP on MWCNT.

Figure 3. (A) Zeta potential under various pH and (B) equilibrium pH under various initial pH.

strength has no impact on the adsorption of BPAP onto MWCNT. Generally, NaCl has a “salting-out” effect on the adsorption of hydrophobic compounds.34 The increase of ionic strength may also alter the aggregation state of MWCNT. The aggregates of MWCNT would be more compact (squeezingout).35 A highly compacted aggregation structure of MWCNT was unfavorable for BPAP adsorption. As shown in Figure 2B, an increased ionic strength by the addition of NaCl had little effect on the adsorption of BPAP by the MWCNT. This can be explained that the contribution of the salting-out effect to BPAP is equivalent to that of the squeezing-out effect to MWCNT. 3.4. Mechanistic Aspects. The pH variation can not only affect the protonation−deprotonation transition of functional groups on CNTs but also result in a change in chemical speciation for ionizable organic compounds. The values of ζ potential of MWCNT suspensions were determined under various pH values, and the point of zero charge (PCZ) of the MWCNT was found to be about 3.0 in Figure 3A. At pHPCZ 3.0, MWCNT carried no charges and the repulsive energy between MWCNT was the smallest, resulting in a highly compacted aggregation structure of MWCNT, which was unfavorable for BPAP adsorption. Furthermore, BPAP could have different charges on different sites depending upon solution pH. With a pH greater than 10.0 (the pKa of BPAP is 9.0−10.0), BPAP existed as an anion and the adsorption was significantly impeded. The ionized form of BPAP was the predominant fraction at pH > pKa, and the hydrogen bonds and

percentages increased with the increasing amount of MWCNT, and when the amount exceeded 45.0 mg, the adsorption percentages attained 92.0% with no obvious change. Therefore, a constant adsorbent concentration of 1.8 g L−1 was selected for the adsorption of BPAP, and the system could reach equilibrium around 4.0 min. 3.3. Effect of pH and Ionic Strength. The effect of pH on BPAP adsorption was examined at different initial BPAP concentrations ranging from 10.0 to 70.0 mg L−1 and a constant adsorbent concentration of 1.8 g L−1. The adsorption maxima generally occurred between pH 4.0 and 10.0. For different BPAP concentrations, the differences in adsorption efficiencies may be attributed to the solubility and hydrophilicity of BPAP. A lower concentration of BPAP shows relatively high solubility and hydrophilicity, resulting in a lower adsorption percentage. Conversely, a higher concentration of BPAP has good hydrophobicity and a higher adsorption percentage. As seen in Figure 2A, there was no remarkable effect on the adsorption of BPAP on MWCNT from pH 4.0 to 10.0. However, the adsorption percentage will decrease in the strong acidic (pH < 4.0) and base (pH > 10.0) solutions. Generally, the pH of natural BPAP solution was close to 5.0. In this work, the BPAP solution was used directly. The ionic strength of test solution with 20.0 or 60.0 mg L−1 was controlled using sodium chloride with the concentrations of 0, 0.02, and 0.2 mol L−1. The variations of adsorption efficiencies were given in Figure 2B. It can be seen that the ionic strength has no effect on the BPAP adsorption percentages for each concentration, indicating that ionic 3970


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hydrophobic interactions between MWCNT and ionized BPAP were much weaker than that between MWCNT and nonionized BPAP. Besides, both BPAP and MWCNT are negatively charged, and the electrostatic repulsion between them can also weaken their adsorption to some extent at pH > 10.00. While BPAP showed high adsorption under conditions between 4.0 and 10.0, nearly all BPAP molecules carry no net electrical charges, which makes them hardly have electrostatic attraction or repulsion with MWCNT. The increase of pH from 4.0 to 10.0 had no significant effect on the adsorptive affinity of BPAP in the experiment. Therefore, the adsorption mechanism is probably the π−π stacking interactions between the bulk π system on the MWCNT surface and BPAP molecules. In addition, the strength of hydrogen bonds was the primary cause of the enhanced adsorption for a reason. The hydrogen bond interaction may be formed between the −OH groups of BPAP and the O-containing groups of MWCNT, and the benzene ring on MWCNT surface may also act as a hydrogen bond donor and form hydrogen bonds with −OH functional groups on BPAP. The hydrophobic effect is often associated with the adsorption of MWCNT.36 The outer surface of individual MWCNT provided evenly distributed hydrophobic sites for organic molecules, because of their hydrophobic surfaces and strong interactions between MWCNT and BPAP. Different mechanisms (hydrogen bond, hydrophobic, and π−π stacking interactions) may act simultaneously and respond differently to the change of the pH value; thus, the prediction of BPAP adsorption on MWCNT is not straightforward. The change of the pH between before and after adsorption is the result of the interaction between MWCNT and BPAP solution. Hence, the variations of solution pH before (pHin) and after (pHeq) BPAP adsorption were investigated. Figure 3B presented the pHeq under various pHin. The symbols above the solid line indicated a rise in solution pH during the adsorption process at pHin 3.0−8.0. The ΔpH was greater at pHin 3.0−8.0, where a larger amount of BPAP were adsorbed onto MWCNT (Figure 3B) because of the release of much more OH− into solution. The symbols below the solid line reflected a decrease in solution pH during the adsorption process at pHin 9.0−12.0. It could be explained by the ionization of more BPAP phenolic groups at a higher pHin,37 because of the release of more H+ into solution. 3.5. Adsorption Kinetic. The variation of the adsorption amount with adsorption time at different temperatures was described in Figure 4. The adsorption could reach equilibrium in less than 4.0 min, indicating that it was a very fast adsorption process. To investigate the adsorption process of BPAP on MWCNT, the pseudo-first-order and pseudo-second-order models were used. The pseudo-first-order equation is given as eq 2 log(q1 − qt ) = log q1 −

k1t 2.303

Figure 4. Adsorption capacity of BPAP on MWCNT versus time at different temperatures (45.0 mg MWCNT; CBPAP, 20.0 mg L−1; pH 5.0).

adsorptions, which were obtained from the plots of log(q1 − qt) against t and t/qt versus t, respectively. The second-order rate constants were also used to calculate the initial sorption rate h (mg g−1 min−1), given by

h = k 2q2 2

The batch kinetic data were fitted to both pseudo-first- and pseudo-second-order models. Both models adequately described the kinetic data at the 95% confidence level. The results of the kinetic parameters and the calculated initial sorption rate values were listed in Table 1. On the basis of the linear regression coefficients, it was found that the adsorption of BPAP could be described by the pseudo-second-order model. The initial sorption rate h was quite large, indicating that the adsorption of BPAP on MWCNT was a fast process. The high initial uptake rate can be an indication that the surfaces of BPAP had a high density of active sites for MWCNT adsorption. 3.6. Adsorption Mechanism. In the adsorption process, only the external mass transfer and intraparticle diffusion play an important role in rate determination. To evaluate the relative importance of the two steps, time-course BPAP sorption data were processed using the intraparticle diffusion and the external mass transfer models. The Weber−Morris intraparticle diffusion model is represented as follows:38

qt = Kdt 1/2 + I

(5)

where qt is the amount of BPAP adsorbed at time t (min) and Kd is the rate constant for intraparticle diffusion. Values of I give an idea about the thickness of the boundary layer; i.e., the larger the intercept, the greater the boundary layer effect will be.39 The plots of qt versus t1/2 for the sorption of BPAP were shown in Figure 5A, and the related parameters were listed in Table 2. Piecewise linear regression of data showed that qt versus t1/2 plots had three distinct regions. The first linear portions included the sorption period of 0.0−1.5 min, which represented external mass transfer and binding of BPAP by those active sites distributed onto the outer surface of MWCNT. The second linear portions included the sorption period of 2.0−3.5 min, representing intraparticle diffusion and binding of BPAP by active sites distributed to macro-, meso-, and micropores of MWCNT. The third linear portions included the time period of 4.0−10.0 min, which denoted

(2)

and the pseudo-second-order model is represented as eq 3 t 1 t = + qt q2 k 2q2 2

(4)

(3)

where q1, q2, and qt are the amount of BPAP adsorbed on the sorbent (mg g−1) at equilibrium and time t, respectively. k1 and k2 are the rate constants of the first- and second-order 3971


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Table 1. Kinetic Parameters for BPAP Adsorption on MWCNT at Different Temperatures pseudo-first-order model

pseudo-second-order model

T (K)

k1 (min−1)

q1 (mg g−1)

r1

k2 (g mg−1 min−1)

q2 (mg g−1)

h (mg g−1 min−1)

r2

286 298 313

1.515 0.997 0.867

3.383 0.649 0.091

0.933 0.812 0.758

0.654 1.125 2.932

10.378 10.238 10.015

70.472 117.925 294.118

0.999 0.9999 0.9999

Figure 5. (A) Weber−Morris plots, (B) external mass transfer plots, and (C) Boyd plots.

Table 2. Kinetic Parameters Calculated from the Weber−Morris Kinetic Model for BPAP Adsorption on MWCNT at Different Temperatures initial linear portion

second linear portion

third linear portion

T (K)

Kd1 (mg g−1 min−1/2)

I1

r1


I2

r2


I3

r3

286 298 313

3.763 3.252 1.647

4.766 5.774 7.827

0.986 0.997 0.976

0.832 0.546 0.291

8.595 9.089 9.446

0.867 0.992 0.811

0.077 0.032 0.014

9.936 10.017 10.001

0.896 0.446 0.609

establishment of the equilibrium.40 The third step was considered to be very fast and, thus, could not be treated as a rate-limiting step. Generally, the adsorption rate was controlled by the external mass transfer, the intraparticle diffusion, or both. Nonetheless, I was not equal to 0 in the test conditions, suggesting that the intraparticle diffusion was not the sole rate-limiting step and the external mass transfer had also played an important role in controlling the adsorption rate. External mass transfer model:41,42 ⎡ d(Ct /C i) ⎤ ⎢ ⎥ = − βS ⎣ dt ⎦t = 0

Bt = −ln(1 − F ) − 0.4977

(7)

where F = qt/qe, with qt and qe being the amounts of BPAP adsorbed on MWCNT (mg g−1) at time t (min) and equilibrium time (min), respectively. Bt values were calculated for BPAP sorption at different time periods (qt). Boyd plots for the sorption of BPAP at three different temperatures were presented in Figure 5C. However, the linear lines for BPAP did not pass through the zero axis, indicating that the adsorption of BPAP on the MWCNT was mainly governed by the external mass transfer.43 3.7. Adsorption Isotherms. The equilibrium adsorption amount of BPAP on MWCNT as a function of the equilibrium concentration of BPAP was depicted in Figure 6. An increased adsorption was observed for BPAP until saturation was attained. To investigate the adsorption isotherm of BPAP on MWCNT, the Langmuir and Freundlich isotherm models were used.

(6)

where Ci and Ct represent the concentrations of BPAP at the beginning and time t, respectively (mg L−1), β is the external mass transfer coefficient (cm min−1), and S is the specific surface of MWCNT for external mass transfer (cm−1), with the assumption that Ct = Ci at t = 0. Then, βS values were calculated by the slope of the plot of Ct/Ci versus time t (Figure 5B) and found to be 0.226, 0.160, and 0.079 min−1 at 286, 298, and 313 K, respectively. High regression coefficients showed that BPAP adsorption data could be interpreted by the external mass transfer model. The above discussion makes it clear that both intraparticle and external mass transfer processes play an important role in the sorption of BPAP. However, it is unclear as to which one exerted a greater influence on the rate of BPAP sorption. This can be resolved using the Boyd plot, which is obtained by plotting Bt versus time t. Bt is expressed by the following equation:39

Langmuir model: Ce C 1 = e + qe qm bqm

(8)

Freundlich model: log qe = log KF +

1 log Ce n

(9)

where qm is the maximum monolayer adsorption (mg g−1), Ce is the equilibrium concentration of BPAP (mg L−1), qe is the 3972


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The enthalpy change (ΔH°) and entropy change (ΔS°) of adsorption are estimated from the following equation:

ln Kc =

ΔS° ΔH ° − R RT

(12)

According to eq 12, ΔH° and ΔS° parameters can be calculated from the slope and intercept of the plot of ln Kc versus 1/T, respectively. The thermodynamic parameters were summarized in Table 4. The negative values of ΔH° and ΔG° showed the exothermic and spontaneous nature of the sorption process. Table 4. Thermodynamic Parameters for the Adsorption of BPAP on MWCNT ΔG° (kJ mol−1)

Figure 6. Isotherm of BPAP adsorption on MWCNT at different temperatures (10.0 mg of MWCNT; the initial BPAP concentration range was 30.0−120.0 mg L−1).

amount of BPAP adsorbed per unit weight of MWCNT at equilibrium concentration (mg g−1), and b is the Langmuir constant related to the affinity of binding sites (L mg−1). KF and n are Freundlich constants indicating the sorption capacity and intensity, respectively. The isothermal constants and the linear regression coefficients extracted from the experimental data were presented in Table 2. Two isotherm models were statistically significant at a 95% confidence level. As seen in Table 3, the adsorption data

Ci (mg L−1)

ΔH° (kJ mol−1)

ΔS° (kJ mol−1 K−1)

286 K

298 K

313 K

90.0

−7.560

16.794

− 3.811

−3.692

−3.328

3.9. Effect of Major Cations and Anions. Various inorganic ions are often contained in the industrial wastewater; therefore, it is necessary to investigate the effects of common existing ions on the adsorption of BPAP. The mixture solution consists of 10 mg L−1 BPAP and 1 × 10−3 mol L−1 K+, NH4+, Mg2+, Cu2+, Zn2+, Ni2+, Pb2+, SO42−, PO43−, CO32−, SO32−, and NO3−. The results were shown in Figure 7. Pb2+ caused a

Table 3. Langmuir and Freundlich Constants Correlation Coefficients of BPAP Adsorption on MWCNT at Different Temperatures Langmuir

Freundlich

T (K)

qm (mg g−1)

b (L mg−1)

r1

KF (mg g−1)

n

r2

286 298 313

162.338 155.280 136.799

0.277 0.277 0.390

0.999 0.999 0.998

60.628 56.898 57.980

3.927 3.848 4.445

0.947 0.943 0.900

correlated better (r > 0.998) with the Langmuir equation than the Freundlich equation (r > 0.900) under the studied concentration range. The maximum adsorption capacity of BPAP on MWCNT was 162.34, 155.28, and 136.80 mg g−1 at 286, 298, and 313 K, respectively. 3.8. Thermodynamic Studies. The sorption behaviors of different concentrations of BPAP onto MWCNT were critically investigated at 286, 298, and 313 K, respectively. Thermodynamic parameters are calculated from the following equation: ΔG° = −RT ln Kc

Figure 7. Effect of the major cations and anions on adsorption.

remarkable decrease in the adsorption of BPAP, and the competitive influence of Pb2+ on BPAP adsorption could not be ignored. However, the complex composition of heavy metal ions and many other ionic species had no obvious negative effect on the adsorption of BPAP, suggesting the potential application of MWCNT as a sorbent for recovery of hazardous BPAP from wastewater. 3.10. Recovery of BPAP. The regeneration of the adsorbent is important to reduce the cost of the adsorption process and possibly recover the pollutant extracted from wastewater. To find a suitable desorbent for BPAP, the various eluent was tested in the batch system. It was found that 10 mL of ethanol/1.0 M sodium hydrate solution (7:3, v/v) could quantitatively extract BPAP (>95%).

(10)

where Kc is the distribution coefficient. Gibbs free energy change of adsorption (ΔG°) was calculated using ln Kc values for different temperatures. The Kc value is calculated using following equation: q Kc = e Ce (11) where Ce is the equilibrium concentration of BPAP and qe is the amount of BPAP adsorbed per unit weight of MWCNT at equilibrium concentration (mg g−1). 3973


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Figure 8. (A) Times of MWCNT recycle and (B) FTIR spectra of MWCNT and BPAP.

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To evaluate the stability and possibility of reuse of MWCNT adsorbent, repeating application of MWCNT experiments has been performed. Removal efficiencies of repeating application of MWCNT were shown in Figure 8A. It was stable for up to 8 adsorption recycles without an obvious decrease in the removal efficiency for BPAP. The adsorption efficiency can still be above 85% in the final recycle, which indicated that there were no irreversible sites on the surface of the adsorbent. In the FTIR spectra of MWCNT (Figure 8B), after adsorption, MWCNT showed some apparent characteristic bands of BPAP, which were aromatic −CC− bonds (1600−1400 cm−1), benzene ring −C−H bonds (900−650 cm−1), phenolic −C−O bonds (1250 cm−1), and linking two aromatic ring −C−C− bonds (1150 cm−1), while MWCNT displayed no significant bands before adsorption. It could also be seen from Figure 8B that the main functional groups of BPAP disappeared after elution. After the eighth recycle, FTIR of MWCNT did not show evident change compared to that of the first recycle. 3.11. Potential Use of MWCNT To Treat BPAPContaining Wastewater. The removal efficiencies of BPAP in different water samples using MWCNT were studied. Removal efficiencies of BPAP from different water sources (wastewater, river water, drinking water, and synthetic water) ranged from 83 to 89%. For each 50 mL of 10 mg L−1 BPAP sample, MWCNT exhibited almost 85% removal efficiencies, suggesting that MWCNT has strong anti-interference ability in different water environments.

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-24-62207809. Fax: +86-24-62202380. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (NSFC51178212), the Natural Science Foundation of Liaoning Province, China (201102082), the Liaoning Provincial Department of Education Innovation Team Projects (LT2012001), the Shenyang Science and Technology Plan (F12-277-1-69), and the Foundation of 211 Project for Innovative Talent Training, Liaoning University. The authors also thank their colleagues and other students who participated in this work.

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REFERENCES

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4. CONCLUSION Overall, this study demonstrated high removal efficiencies of BPAP from the simulated wastewater and municipal wastewater with broad pH values (4.0−10.0) using MWCNT. The adsorption was hardly influenced by the mixed ions. The thermodynamic parameters implied that the adsorption was a spontaneous and exothermic process. Adsorption rate-controlling mechanism studies revealed that the overall rate process was mainly governed by the external mass transfer. Besides, the adsorbed BPAP could be efficiently desorbed in ethanol/ sodium hydrate solution with the recovery percentages above 95.0%, and the MWCNT can be reused at least 8 times without an obvious decrease in the removal efficiency. The test results of kinetics and thermodynamics studied will be useful for designing and operating the treatment of wastewater containing BPAP. 3974


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Rapid Method for the Separation and Recovery of Endocrine

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