Preparation of Negatively Charged Hybrid Adsorbents and Their

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Preparation of Negatively Charged Hybrid Adsorbents and Their Applications for Pb2þ Removal Junsheng Liu,* Jingyu Si, Quan Zhang, Jiuhan Zheng, Chengliang Han, and Guoquan Shao Key Laboratory of Membrane Materials & Processes, Department of Chemical and Materials Engineering, Hefei University, 373 Huangshan Road, Hefei 230022, China ABSTRACT: A novel approach to the negatively charged hybrid adsorbents was proposed. These hybrid adsorbents were prepared via the ring opening of pyromellitic acid dianhydride (PMDA) and N-[3-(trimethoxysilyl)propyl]ethylenediamine and a subsequent solgel process. Fourier transform infrared spectra confirmed the step products. Their adsorption kinetics, isotherm, and thermodynamic parameters ΔG, ΔH, and ΔS for Pb2þ removal from aqueous solution were examined. Differential scanning calorimetry and thermogravimetric analysis thermal amylases showed that these samples had high thermal stability and the crystallization temperature increased with an increase in the PMDA content because of cross-linking between organic and inorganic moieties and the further formation of a hybrid matrix. Adsorption experiments indicated that Pb2þ adsorption on these hybrid adsorbents followed the Lagergren second-order kinetic and Langmuir isotherm models. The negative values of ΔG and the positive values of ΔH showed that Pb2þ adsorption on these hybrid adsorbents was spontaneous and an endothermic process in nature. Scanning electron microscopy and energy-dispersive spectrometry images further confirmed Pb2þ ions on the surfaces of samples AD. Moreover, a desorption experiment indicated that they can be regenerated. These findings suggest that the prepared hybrid adsorbents are promising in the separation and recovery of Pb2þ ions from contaminated water.

1. INTRODUCTION Water pollution caused by toxic heavy-metal ions has become a serious environmental problem. These toxic metal ions, such as Pb2þ, Cu2þ, Cd2þ, etc., have deteriorated water resources and drinking water.13 Especially with the wide application of lead acid rechargeable batteries for mineral fuel as one of the most promising new power sources, water pollution caused by Pb2þ ions is becoming worse and worse; it thus captures much public attention. To restrain or delete contamination from Pb2þ ions, an extremely severe maximum contaminant level (MCL) in water resources has been established by many countries. For example, the MCL of Pb2þ has decreased to 0.01 mg L1 from 0.05 mg L1 in Chinese standards for drinking water quality.4,5 The MCL public health goal of Pb2þ stipulated by the U.S. Environmental Protection Agency (EPA) has been set as zero.6 Consequently, Pb2þ removal from aqueous solution has become significantly important and highly needed. Presently, various innovative techniques are developed to capture Pb2þ ions.79 Among these, adsorption using functionalized hybrid polymeric materials as an adsorbent is regarded as one of the most effective techniques because metal ions can be chemically bonded by the inorganic/ organic polymer hybrids. As one important type of functionalized hybrid material, negatively charged (i.e., cation exchange) hybrids have attracted much attention.1014 This charged hybrid material not only combines the advantages of organic and inorganic materials but also exhibits some distinguished properties, such as structural flexibility and thermal and mechanical stability. In particular, the cation-exchange property of negatively charged hybrid material allows its application in the separation and recovery of toxic heavymetal ions from contaminated water via the electrostatic effect. r 2011 American Chemical Society

Therefore, it can be potentially used as an adsorbent for the removal of heavy metals from an aqueous solution. Recently, much effort has been made to prepare the negatively charged hybrid polymers and examine their adsorption performances for Pb2þ ions.15,16 Our continuing interest in such types of negatively charged hybrids stimulates us to make a further effort. Consequently, to develop a new approach to preparing the negatively charged hybrids used as new hybrid adsorbents and examining their adsorption properties for toxic heavy-metal ions, herein, a novel route to negatively charged hybrid adsorbents based on the ring-opening polymerization of pyromellitic acid dianhydride (PMDA) and N-[3-(trimethoxysilyl)propyl]ethylenediamine (TMSPEDA) and a subsequent solgel process will be reported. Compared with the previous articles,15,16 the novelty of this new route is that (1) the silicone was incorporated into the hybrid matrix via the ring-opening polymerization of PMDA and TMSPEDA monomers and (2) the COOH groups were produced by preventing cyclodehydration between the NH and COOH groups in the adjacent polymer chains in low curing temperature. Moreover, the adsorption and desorption behaviors for Pb2þ ions will be examined as a model for the removal of heavy-metal ions from an aqueous solution.

2. EXPERIMENTAL SECTION 2.1. Materials. N-[3-(Trimethoxysilyl)propyl]ethylenediamine (TMSPEDA; purity g 95.0%) was purchased from the Received: January 5, 2011 Accepted: May 23, 2011 Revised: May 16, 2011 Published: June 06, 2011 8645

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Industrial & Engineering Chemistry Research

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Table 1. Composition and Surface Area of Samples AD PMDA (g)

TMSPEDA (mL)

surface area (m2 g1)

A

5

2.5

185.8

B

10

2.5

86.3

C

10

1.6

77.9

D

10

1.2

158.3

sample

Silicone New Material Co., Ltd., of Wuhan University (Wuhan, China) and used without further purification. Pyromellitic acid dianhydride (PMDA; purity g 99.5%) was purchased from the National Pharmaceutical Group Corp. of China (Shanghai, China) and used as received. Other reagents were of analytical grade. 2.2. Preparation of the Negatively Charged Hybrid Adsorbents. The negatively charged hybrid adsorbents (labeled as samples AD) were prepared by synthesizing different hybrid precursors, which were obtained through reacting different amounts of PMDA with TMSPEDA at room temperature in a N,N-dimethylformamide (DMF) solution (the compositions of samples AD are listed in Table 1). As an example, the procedure for sample A was described briefly as follows. First, 5 g of PMDA was dissolved in 20 mL of a DMF solution and stirred vigorously for 1 h at room temperature; then 2.5 mL of a TMSPEDA solution was added dropwise within 1 h into the above-prepared DMF mixed solution. Second, the DMF mixed solution was stirred vigorously for an additional 0.5 h to conduct the ring-opening polymerization of PMDA and TMSPEDA. In this step, the COOH groups (i.e., negatively charged groups) located on the polymer chains were created. Subsequently, a homogeneous sol (i.e., hybrid precursor) could be observed. During this process, the hydrolysis and condensation of the hybrid precursor by the solgel process occurs between the Si and O to produce SiOSi bonds in the hybrid matrix. Finally, the obtained product was washed with ethanol and dried at 80 °C for 24 h to acquire the final negatively charged hybrid adsorbent. 2.3. Sample Characterizations. Fourier transform infrared (FTIR) spectra of the prepared products were obtained using a Shimadzu FTIR-8400S FTIR spectrometer in the region of 4000400 cm1 at a resolution of 0.85 cm1. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) thermal analyses of the prepared samples were investigated with a Netzsch STA 409 PC/PG thermogravimetry analyzer, under a nitrogen flow using a heating rate of 20 °C min1 from 20 to 400 °C. The ion-exchange capacity (IEC) was determined by titrimetric analysis and described in detail in an earlier article.15 The surface areas of the prepared samples were measured using a JW-004 dynamic nitrogen adsorption specific surface area analyzer (Beijing JWGB Science & Technology Co., Ltd., Beijing, China). Surface field-emission scanning electron microscopy (FE-SEM) images of the prepared samples were observed using a fieldemission scanning electron microscope (Sirion 200) equipped with an energy-dispersive X-ray analyzer (EDS), operated with an accelerating voltage of 10.00 kV. Oxford INCA software was used to determine the elemental composition of the detected surface. 2.4. Adsorption Experiments. The adsorption experiments of these hybrid adsorbents for heavy-metal ions were conducted in a manner similar to that of our previous studies, in which Pb2þ ions were used as the adsorption medium.15

Scheme 1. Preparation of Negatively Charged Hybrid Adsorbentsa

a

Step 1 is the ring-opening polymerization of PMDA and TMSPEDA to produce the carboxylic acidic groups in the backbone of polymer chains. Step 2 is hydrolysis and condensation of the hybrid precursor by the solgel process.

The adsorption capacity (qPb2þ) of Pb2þ ions can be calculated using eq 1 qPb2þ ¼

ðC0 CR ÞV W

ð1Þ

where V is the volume of an aqueous Pb(NO3)2 solution (mL), C0 and CR are the concentrations of the initial and remaining Pb(NO3)2, respectively (mol dm3), and W is the weight of the prepared negatively charged hybrid adsorbent (g). The effect of the pH on Pb2þ adsorption was first investigated to determine the optimal pH scale. On the basis of such a pH value, for adsorption kinetic studies in this case, the sample was immersed in a 0.1 mol dm3 aqueous Pb(NO3)2 solution for different adsorption times at pH 4. Meanwhile, the adsorption isotherm was conducted by changing the solution concentration ranging from (0, 0.01, to 0.4) mol dm3 at room temperature for 24 h at pH 4. 2.5. Desorption Experiment. The sample-adsorbed heavymetal ions Pb2þ were immersed into various acidic adsorbents, HNO3, H2SO4, and HCl acidic solutions (0.01 mol dm3). The solutions were filtered, and the residual capacity of Pb2þ ions in filtrate was determined by titrimetric analysis using an ethylenediaminetetraacetic acid (EDTA) solution (0.1 mol dm3). The desorption efficiency (DE, %) of Pb2þ ions can be calculated using eq 2 DE ð%Þ ¼

amount of Pb2þ desorbed into the desorbent amount of Pb2þ adsorbed onto the sample 100 ð2Þ

3. RESULTS AND DISCUSSION 3.1. Preparation of Negatively Charged Hybrid Adsorbents. As mentioned above, the negatively charged hybrid

adsorbents were prepared via the ring-opening polymerization of PMDA and TMSPEDA and a subsequent solgel reaction. 8646

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Figure 1. FTIR spectra of ad for samples AD, respectively.

Because PMDA is an acid anhydride and TMSPEDA is a primary amine compound, ring-opening polymerization between the epoxy and amino groups is easily performed. Scheme 1 presents the reaction steps. As shown in Scheme 1, it can be seen that two steps were involved: step 1 was the ring-opening polymerization of PMDA and TMSPEDA, and the reaction product was the hybrid precursor. The product of this step was mainly determined by the ratio of PMDA and TMSPEDA. In this step, the COOH groups (i.e., negatively charged groups) were created in the backbone of polymer chains. Step 2 was hydrolysis and condensation of alkoxysilane in the hybrid precursor. Alkoxysilane hydrolyzed and condensed and produced the SiOSi bonds via the solgel process. The negatively charged hybrid adsorbent was thus prepared. Because the prepared hybrid polymer contains cation-exchange groups, i.e., COOH groups, it is therefore a negatively charged hybrid adsorbent because of the fact that the net charge difference of ionic groups is negative (as discussed in section 3.5, hereinafter). Moreover, it should be emphasized that, during preparation of the negatively charged hybrid adsorbent, cyclodehydration in the neighboring NH and COOH groups will occur because the secondary amine contains one active hydrogen; thus, the polyimide chains are capable of being fabricated in the step product (step 2); i.e., CONH chains can be created via cyclodehydration between the secondary amine and carboxylic groups in the polymer backbone,1719 which will reduce the amount of COOH groups in the hybrid adsorbent produced and impact its adsorption performance. Consequently, to avoid the occurrence of cyclodehydration in the neighboring NH and COOH groups, both the reaction and curing temperatures were limited in the temperature range of below 100 °C. Thus, the amounts of CONH groups in the polymer chains are decreased to a low level, which can be confirmed by FTIR spectra (cf. Figure 1, hereinafter). 3.2. FTIR Spectra. To confirm the functionalized groups in the polymer chains described in Scheme 1, FTIR spectroscopy was conducted and is shown in Figure 1. It can be noted in Figure 1 that curves ad have similar change trends except the intensity of the main peaks. The strong absorption peak near 1714 cm1 was the stretching vibration of the carbonyl group from the COOH groups. The large band at ∼3440 cm1 was in the range of the stretching vibrations from NH and OH groups. The absorption peaks at ∼2920 and ∼1435 cm1 could be ascribed to the CH stretching and CH bending vibrations of the CH3 and CH2 groups, respectively. The peak at ∼1110 cm1 can be ascribed to the SiOSi,

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Figure 2. TGA curves of samples A (solid line), B (dashed line), C (dash-dotted line), and D (short dash-dotted line).

Table 2. Thermal Analysis Data of Samples AD Obtained from the TGA Curves sample

Td5 (°C)

Td10 (°C)

R400 (wt %)

A B

136.74 108.01

184.83 172.34

55.54 36.28

C

157.31

205.98

28.11

D

162.35

211.11

24.57

SiOC, and COC stretching vibrations.14 The band at 1280 cm1 was the stretching vibration of PhN therein. From comparison of curves ad in Figure 1, it can be found that the intensity of the band at ∼1110 cm1 from the COC stretching vibration increases when the PMDA content in the samples increases, suggesting an increase of the organic ingredient in the prepared samples. In addition, the new band at 1928 cm1, which corresponds to the stretching vibration of CdO in acid anhydride,17 clearly was found in curves c and d, indicating that there exists excess PMDA in the prepared hybrid polymers C and D. In contrast, no such peak was observed in curves a and b. This changing trend in the adsorption peak evidences the accomplishment of the ring-opening polymerization reaction in step 1. In addition, it can be noted in Figure 1 that no noticeable band at 1680 cm1 corresponding to the stretching vibration of the CONH chains and CN stretching in an imide ring at 1378 cm1 were observed.20 This finding demonstrates that cyclodehydration between the adjacent secondary amine with carboxylic group in the polymer backbone was not occurring during preparation of the hybrid adsorbent, corroborating the reactions presented in Scheme 1. 3.3. TGA. To examine the thermal stability of the prepared negatively charged hybrid adsorbents, TGA thermal analysis was performed and the related curves are presented in Figure 2. Meanwhile, the thermal analysis data in the TGA curves are listed in Table 2. As shown in Figure 2, it is interesting to find that, for samples AD, their change trends in weight loss (%) are similar and three main degradation steps (i.e., below 160 °C, in the range of 160305 °C, and beyond 305 °C) can be observed. Corresponding to these degradation steps, several exothermic peaks can be found in the DSC curves (as indicated in Figure 3, hereinafter). The first weight loss below 160 °C was attributed to the removal of bonded water and solvent in samples AD. The second weight loss in the temperature range of 160305 °C was ascribed to the decomposition of organic ingredients and the breakage of 8647



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Table 3. CIECs of the Negatively Charged Hybrid Adsorbents sample

CIEC (mmol g1)

Figure 3. DSC curves of samples A (solid line), B (dashed line), C (dash-dotted line), and D (short dash-dotted line).

functionalized groups. The third weight loss beyond 305 °C was the further degradation of polymer chains and the formation of a hybrid matrix. Moreover, it can be noted that their degradation temperatures (Td) at 5 and 10% weight loss (i.e., Td5 and Td10) exhibit the same upward change trend: increasing from samples B, A, C, to D. Among them, sample B has the lowest value in Td5 and Td10. This change trend is in disagreement with that in the composition of samples AD (see Table 1). However, such a change trend in weight loss (%) follows the same change trend as that in the composition of samples AD when the degradation temperature is beyond 305 °C (near 45% weight loss for samples BD). This finding suggests the change in the thermal stability of these samples. In addition, it can be also noted that the weight loss (%) of these samples increases rapidly when the Td value exceeds Td10, suggesting that the thermal decomposition has been accelerated. Furthermore, it can be observed that the residual weight (wt %) values at 400 °C (R400) are 55.54, 36.28, 28.11, and 24.57% for samples AD (cf. Table 2), which is consistent with the theoretical expectation; i.e., the higher the organic component in a polymer, the lower the residue in the sintered product. The reason can be ascribed to an increase in the organic ingredient in samples AD, leading to a decrease in the weight percent of the residual. This outcome demonstrates that the incorporation of organic composition into the hybrid matrix can be used to adjust the thermal stability of samples AD. 3.4. DSC Analysis. To investigate the crystallization transformation behavior of these negatively charged hybrid adsorbents from rigid to flexible, a DSC study was conducted and the related curves are presented in Figure 3. From the DSC curves in Figure 3, it can be noted that, for samples AD, there exist one endothermic peak and several exothermic peaks. Chang et al.21 reported that, in the DSC curves, the single or multiple endothermic peaks are the crystal melting, whereas the exothermic peak can be considered the crystallization. Consequently, it can be deduced that, for samples AD, the melting point temperature (Tm) is near 144.7, 140.0, 134.9, and 127.6 °C, which indicates a downward trend as the PMDA content in these samples increases. Such a declining trend in Tm implies an increase in the flexibility of these hybrid adsorbent samples because of extra organic species being incorporated into the hybrid matrix. Meanwhile, for samples AD, the first crystallization temperature (Tc1) (i.e., the first exothermic peak) is located at 182.2, 182.5, 184.9, and 185.1 °C, which reveals a change trend opposite to that of Tm as the PMDA content in these samples increases. Such an upward trend in Tc suggests the

A

B

C

D

2.14

4.73

6.40

7.32

transformation of crystallization from amorphous to crystalline. In addition, it can be observed that the second crystallization temperature (Tc2; i.e., the second exothermic peak) decreases from samples A to D (i.e., a broader peak for sample A and 237.5, 209.9, and 210.1 °C for samples BD, respectively) and approaches a fixed value, demonstrating the further accomplishment of crystallization. This observation clearly confirms the influence of the PMDA content on the behavior of the crystallization transformation of the hybrid adsorbents. The reason can be ascribed to the cross-linking between organic and inorganic moieties and the further formation and completion of the hybrid matrix. 3.5. IEC. To determine the ion-exchange ability of the hybrid adsorbents, the IEC was examined and values are listed in Table 3. It can be seen that the cation-exchange capacities (CIECs) were in the range of 2.147.32 mmol g1, revealing an upward trend with an increase in the PMDA content, suggesting that the incorporation of PMDA into the polymer matrix can increase the ion-exchange abilities of these negatively charged hybrid adsorbents. Such a change trend can be theoretically explained as follows. One reason is related to the increasing amount of COOH in the polymer chains as the PMDA content increases, which will accordingly increase the CIEC values. As listed in Table 1, from samples AD, larger amounts of COOH were produced with the elevating PMDA content in the polymers as ring-opening polymerization between the epoxy and amino groups occurred, leading to an increase in the CIECs. Another reason can be attributed to the existence of epoxy groups in the PMDA moiety, which are easily converted to COOH in the presence of water because of its acid anhydride property;21 thus, the amount of COOH groups will increase when the PMDA content increases, leading to an increase in the CIECs from samples AD. This finding suggests that the adsorption ability for heavymetal cations, such as Pb2þ, will be increased from samples A to D because of the elevating electrostatic attraction of COOH for counterions, which can be confirmed by adsorption experiments, as discussed later. Consequently, these negatively charged hybrid polymers expect to be used as adsorbents for the removal of heavy-metal ions from aqueous solution; revealing their potential applications in the treatment of contaminated water. Moreover, it should be emphasized that, theoretically, NH groups have the ability to conduct ion exchange with the available free anions. Meanwhile, the point of zero charge (pHPZC) can be estimated according to the charge difference of CIECs and anionexchange capacities (AIECs). For such a purpose, much effort was made to measure the AIECs of NH groups in these samples. Unfortunately, no satisfactory results could be obtained. Consequently, the net charge difference of these samples is equal to CIECs and indicates a negative value. pHPZC of these samples is thus cannot be determined. On the basis of this outcome, it can be concluded that these hybrid materials chiefly reveal the 8648



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Figure 4. pH versus the adsorption capacity of Pb2þ on samples A (solid square), B (half-filled diamond), C (center triangle), and D (halffilled inverted triangle). The concentration of the aqueous Pb(NO3)2 solution was 0.1 mol dm3 for 24 h.

cation-exchange performances, i.e., they are the negatively charged hybrid adsorbents and mainly conduct cation exchange between the COOH groups and metal cations. As a result, they have an affinity for Pb2þ ions in aqueous solution. Several factors might be responsible for such a trend. One can be ascribed to the weak ion-exchange ability of the NH groups. Another factor might be attributed to the inaccessibility of cationic and anionic ions to the NH groups because of the adjacent structure of NH and COOH groups; thus, it will block the AIECs from being detected precisely by a titration technique, resulting in its disappearance. Notice that the undetermined AIECs and pHPZC values have little impact on the adsorption of heavy-metal ions on the prepared samples AD. This is because the adsorption of heavy-metal ions on samples AD is mainly determined by the amount of COOH and NH groups rather than the accurate measurement of NH groups in aqueous solution. Meanwhile, the adsorption of metal ions on an adsorbent is its intrinsic property rather than the precise testing of functional groups in the polymer backbone. 3.6. Adsorption of Pb2þ. To explore the adsorption behaviors of the negatively charged hybrid adsorbents, the adsorption experiment was performed using Pb2þ as a model of heavy-metal ions. The influencing factors examined include the pH, adsorption temperature, and adsorption time. The adsorption data were analyzed using Lagergren first- and second-order kinetics, intraparticle diffusion, and Langmuir and Freundlich isotherm models to study the adsorption kinetic properties of Pb2þ ions on these hybrid adsorbents. On the basis of the adsorption data, the thermodynamic parameters ΔG, ΔH, and ΔS at different temperatures for Pb2þ removal were also calculated. For comparison, a blank experiment using pure PMDA as an adsorbent was conducted, and the related curve is presented in the same plot (as presented in Figures 6 and 9; see later). It needs to be pointed out that an attempt was made to measure the adsorption capacity of pure TMSPEDA as a blank experiment. However, the color of the aqueous Pb(NO3)2 solution became milky white when the pure TMSPEDA was immersed; thus, it is difficult to precisely determine the adsorption capacity of Pb2þ on pure TMSPEDA. Consequently, the data of Pb2þ adsorption on TMSPEDA were not included. 3.6.1. Effect of the pH. It is well-known that the pH has an effect on the adsorption behavior of heavy-metal ions in aqueous solution.2224 For example, Chen et al.22 reported that it is hard to perform adsorption of Pb2þ ions at pH > 8 because of the occurrence of hydroxide precipitation. Consequently, to

Figure 5. Proposed adsorption mechanism of divalent metal ions (Me2þ) on samples AD.

determine the optimal pH for Pb2þ adsorption, the effect of the pH on Pb2þ adsorption from 1 to 5 at 0.1 mol dm3 for 24 h was examined and is presented in Figure 4. It is interesting to find that the adsorption capacity of Pb2þ ions increases with the elevating pH value in the range of 13 and reaches a peak at pH 4. Subsequently, it decreases slightly at pH 5. Clearly, pH 4 is more suitable for Pb2þ adsorption. The aqueous Pb(NO3)2 solution at pH 4 is thus selected as the adsorption medium to study the adsorption behaviors of Pb2þ ions on the negatively charged hybrid adsorbents, hereafter. 3.6.2. Adsorption Mechanism. To explain the effect of the pH on the adsorption behaviors of these negatively charged hybrid adsorbents for metal ions, the possible adsorption mechanism between ionic groups and divalent metal ions (Me2þ) at different pH ranges can be proposed as follows. Theoretically, COOH groups are partially protonated in aqueous solution and the partition degree increases as the pH value of solution is enhanced. The amount of COOH groups transformed into COO groups will thus be elevated, resulting in an increase in the electrostatic attraction of COO groups for metal ions, such as Pb2þ ions. As a result, the adsorption capacity of Pb2þ ions on these samples increases with an increase in the pH value of the solution (cf. Figure 4). However, because of the existence of NH groups in the backbone of the hybrid adsorbents, such groups will give rise to some effects on Pb2þ adsorption. This is because NH groups are easily coordinated with Pb2þ ions to produce the metal complex. Such coordination effects of the NH groups can be adjusted as the pH of the aqueous solution is changed. For example, at low pH (4), the amount of Hþ is relatively lower and the amount of OH is larger; NH groups will thus 8649



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Figure 6. Adsorption capacity of Pb2þ versus the adsorption time for samples A (solid square), B (half-filled diamond), C (center triangle), and D (half-filled inverted triangle) and pure PMDA (half-filled pentagon). The concentration of an aqueous Pb(NO3)2 solution was 0.1 mol dm3 at pH 4.

combine with OH to fabricate the Nþ groups. Similarly, the Nþ groups will block Pb2þ adsorption on these samples, resulting in a lower adsorption capacity of the Pb2þ ions (cf. Figure 5b). Consequently, the partition or the deprotonation adsorption mechanism will be the main control step and can be used to explain the effect of the pH on Pb2þ adsorption. Moreover, ion exchange and the complex effect of functionalized groups will also make some contributions to Pb2þ adsorption (cf. Figure 5c). For example, Pb2þ may directly conduct ion exchange with COOH groups, which will result in an improvement in the adsorption capacity of metal ions. In contrast, NH groups can coordinate with metal ions to produce the metal complex to impact Pb2þ adsorption on these samples. In this case, they will follow the ion-exchange mechanism as reported by Liu et al.14 The proposed adsorption mechanism of divalent metal ions (Me2þ) on these hybrid adsorbents are described in Figure 5ac. Notice that the pH effect on the adsorption of metal ions on the prepared samples can be further explained by the pHPZC of these hybrid adsorbents. Ramesh et al.24 proposed that when the pH is below pHPZC, the surface of the samples is positively charged; in this case, the adsorption of metal cations on the sample will be prevented, leading to a decrease in the adsorption capacity of metal ions. In contrast, when the pH is above pHPZC, the surface of the samples will be negatively charged; in this case, the adsorption of metal cations on the sample will be elevated, resulting in an increase in the adsorption capacity of metal ions. Taking the prepared samples into consideration, it can be speculated that at low pH; i.e., at pH < pHPZC, the amount of COOH groups in the polymer chains is higher, causing a decrease in the amount of Pb2þ adsorption, whereas at pH > pHPZC, the amount of COO groups in the polymer chains is increased as the pH increases, leading to a high amount of Pb2þ adsorption. 3.6.3. Adsorption Kinetics. Figure 6 illustrates the adsorption kinetic curves of Pb2þ in noncompetitive adsorption on samples AD, i.e., the relationship between the adsorption capacity and adsorption time. For comparison, the adsorption capacity of Pb2þ on pure PMDA versus the adsorption time is also presented in the same plot. As shown in Figure 6, it is interesting to find that the adsorption capacities of Pb2þ ions on samples AD and pure PMDA all increase with the adsorption time. However, for the individual sample, different trends are observed. For example, the adsorption capacities of Pb2þ ions on samples AD are compared with that on pure PMDA, it can be found that the

Figure 7. Lagergren kinetic model for Pb2þ adsorption on samples A (solid square), B (half-filled diamond), C (center triangle), and D (halffilled inverted triangle): (a) first-order model; (b) second-order model.

adsorption capacity of Pb2þ ions on these hybrid adsorbents is larger than that on pure PMDA except that on sample A, which has higher TMSPEDA content than the others. This finding suggests that the proper addition of inorganic composition TMSPEDA into a hybrid polymer will favor Pb2þ adsorption. However, excess addition of TMSPEDA into a hybrid adsorbent will decrease its adsorption for Pb2þ ions. The cross-linking between organic and inorganic moieties and the formation of a hybrid matrix will be responsible for such a trend, as discussed before. In addition, it can also be observed in Figure 6 that the adsorption of Pb2þ ions on samples AD obtained the equilibrium state at an earlier time. However, for pure PMDA, such an equilibrium state needed more time because of the presence of epoxy groups in PMDA. Moreover, it can be noted in Figure 6 that the adsorption capacity of Pb2þ ions on samples AD increases with an increase in the CIECs, suggesting that Pb2þ adsorption on these samples is related to the content of the ionic groups and higher CIECs will favor Pb2þ adsorption on these hybrid adsorbents. The theoretical explanation to this phenomenon can be ascribed to the electrostatic attraction between Pb2þ ions and carboxylic groups on the polymer chains. In addition, the protonation effect of amino groups on metal ions will also be responsible for such a trend. It is well accepted that a Lagergren adsorption kinetic model is a useful method in describing the adsorption property of a species.23,24 Its first- and second-order kinetic equations can be linearly expressed as eqs 3b and 4b, respectively. qt ¼ qe ð1 ek1 t Þ logðqe qt Þ ¼ log qe 8650

k1 t 2:303

ð3aÞ ð3bÞ



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Table 4. Lagergren Second-Order Kinetic Model Parameters for Pb2þ Adsorption on Samples AD ha

k2

qexp

qcal

1

sample (h g mmol ) (h g mmol1) (mmol g1) (mmol g1) A B

a

R2

0.906 1.049

1.683 6.775

1.335 2.508

1.362 2.541

0.997 0.999

C

1.157

11.189

3.091

3.109

0.999

D

1.601

15.845

3.130

3.145

0.999

The initial adsorption rate (h) = k2qe2.

2

qe k2 t 1 þ qe k2 t

ð4aÞ

t 1 t ¼ þ qt k2 qe 2 qe

ð4bÞ

qt ¼

where k1 and k2 are the first- and second-order rate constants, respectively, and qt and qe are the adsorption capacities of metal ions (Me2þ) at time t and at the equilibrium state, respectively. The Lagergren adsorption kinetic models for Pb2þ adsorption on samples AD were calculated and are presented in Figure 7a,b. It can be noted that the linear regression coefficient (R) of the Lagergren second-order model fitted well for Pb2þ adsorption on samples AD (cf. Table 4). However, the Lagergren first-order model for Pb2þ adsorption exhibited a poor linear regression coefficients (R2 = 0.849, 0.964, 0.790, and 0.875 for samples AD, respectively). This finding suggests that Pb2þ adsorption on samples AD followed the Lagergren second-order kinetic model.24 Moreover, it can be noted in Table 4 that both the secondorder rate constant (k2) and the initial adsorption rate (h) of Pb2þ ions all increase from samples A to D, which indicates the same change trend as that of CIECs, implying the influence of IECs of samples AD on Pb2þ adsorption. It should be pointed out that the adsorption of pure PMDA for Pb2þ ions was not obtained at equilibrium state within 24 h. Thus, the Lagergren adsorption kinetic model for Pb2þ adsorption on pure PMDA cannot be calculated. 3.6.4. Effect of Intraparticle Diffusion. The effect of intraparticle diffusion on the adsorption rate can be calculated based on the relationship of the adsorption capacity and time, which can be expressed as eq 525 qt ¼ xi þ kp t 0:5

ð5Þ

where qt is the adsorbed amount (mmol/g) at time t, kp is the intraparticle diffusion rate constant, and xi is the intercept of the straight line, which is related to the boundary layer thickness. Commonly, when metal ions are adsorbed by an adsorbent, the metal ions transport from the solution through the interface between the solution and adsorbent into the pores of the particles. It is now well accepted that if the plot of qt vs t0.5 gives a straight line, the adsorption process is solely controlled by intraparticle diffusion. If the data exhibit multilinear plots, however, two or more steps will influence the adsorption process.25,26 Figure 8 illustrates the intraparticle diffusion curves of Pb2þ adsorption on samples AD. As shown in Figure 8, it can be found that, for samples AD, the intraparticle diffusion curves are similar and three steps are involved: the rapid interface diffusion from 0 to 3 h; subsequently, the intraparticle diffusion

Figure 8. Intraparticle diffusion curves of Pb2þ adsorption on samples A (solid square), B (half-filled diamond), C (center triangle), and D (half-filled inverted triangle).

(increased slowly and approached equilibrium), suggesting that Pb2þ adsorption on samples AD is not governed by intraparticle diffusion; diffusion-controlled adsorption mechanisms might be the major process, as reported in other articles.25,27 Two chief factors might be responsible for the above trend. One can be ascribed to the formation of a hybrid matrix. The other can be attributed to the electrostatic effect from the COOH and NH groups in the polymer chains. By a comparison of the change trend of CIECs and the surface area with that of the adsorption capacity, it can be observed that CIECs exhibit the same upward trend as that in the adsorption capacity of samples AD, suggesting that electrostatic attraction from the ionic groups favors the adsorption of heavy-metal ions on these samples. However, the surface areas of samples AD indicate a change trend opposite to that in their adsorption capacities except that of sample D (cf. Table 1), demonstrating that the surface areas of the pores have little impact on the adsorption of heavy-metal ions on these samples. On the basis of these findings, it can be deduced that the adsorption process mainly dominates by the electrostatic interactions from the ionic groups in the polymer chains. 3.6.5. Adsorption Isotherms. Langmuir and Freundlich isotherm models are useful tools for analyzing the tested adsorption data of heavy metals. The Langmuir isotherm equation is based on monolayer adsorption on the active sites of the adsorbent; which can be expressed as eq 624,28 ce ce 1 ¼ þ qe Qm Qm b

ð6Þ

where qe and ce are the equilibrium concentrations of metal ions in the adsorbed and liquid phases, respectively. Qm and b are the Langmuir constants, which can be calculated from the intercept and slope of the linear plot based on ce/qe versus ce. Different from the Langmuir isotherm model, the Freundlich isotherm model is considered to be adsorption occurring on a heterogeneous surface with uniform energy, which can be expressed as eqs 7a and 7b24,28 qe ¼ kF ce 1=n

ð7aÞ

1 log qe ¼ log kF þ log ce n

ð7bÞ

where qe and ce are the equilibrium concentrations of metal ions in the adsorbed and liquid phases, respectively. kF and n are the Freundlich constants, which can be calculated from the slope and intercept of the linear plot according to log(qe) versus log(ce). 8651



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Table 5. Langmuir and Freundlich Isotherm Parameters for Pb2þ Adsorption on Samples AD Langmuir 1

Freundlich 1

3

2

sample Qm (mmol g ) b (dm mol )

Figure 9. Adsorption capacity of Pb2þ versus the initial solution concentration at pH 4 for samples A (solid square), B (half-filled diamond), C (center triangle), and D (half-filled inverted triangle) and pure PMDA (half-filled pentagon). The sample was immersed in different concentrations of aqueous Pb(NO3)2 solution for 24 h.

R

kF

n

R2

A

3.928

21.265

0.818

4.776 2.767 0.964

B

5.229

56.247

0.960

9.141 2.608 0.949

C D

5.860 6.379

82.834 86.607

0.982 11.940 2.485 0.945 0.980 13.293 2.471 0.940

Table 6. Calculated RL Values Based on the Langmuir Isotherm Parameter RL value C0 (mol dm3)

A

B

C

D

0.01

0.824

0.640

0.546

0.535

0.05

0.484

0.262

0.194

0.187

0.1

0.319

0.150

0.107

0.103

0.2 0.4

0.190 0.105

0.0816 0.0425

0.0569 0.0293

0.0545 0.0280

they fitted worse with the Freundlich isotherm model. This result evidences that the adsorption process of Pb2þ is a Langmuir monolayer rather than a heterogeneous surface one. Moreover, for the Langmuir isotherm model, the separation factor or equilibrium parameter (RL) can be used to predict the favorability of adsorption, which can be expressed as eq 824,28 RL ¼

Figure 10. Adsorption isotherm of Pb2þ ions on samples A (solid square), B (half-filled diamond), C (center triangle) and D (half-filled inverted triangle): (a) Langmuir model; (b) Freundlich model.

The adsorption isotherms of Pb2þ on samples AD and pure PMDA (i.e., the dependence of the adsorption capacity on the initial solution concentration) are illustrated in Figure 9. As expected, the adsorption capacity of Pb2þ increases when the initial solution concentration is elevated. When the adsorption capacities of Pb2þ ions on samples AD are compared with that on pure PMDA, it can be seen that, at a low concentration of the aqueous Pb(NO3)2 solution, the adsorption capacity of Pb2þ ions on these hybrid adsorbents is higher than that on pure PMDA except that on sample A, suggesting that these hybrid adsorbents can be used to remove Pb2þ ions from the dilute solution. Parts a and b of Figure 10 present the Langmuir and Freundlich adsorption isotherms of Pb2þ on samples AD. The Langmuir and Freundlich isotherm parameters are listed in Table 5. It can be found that the experimental data fitted well with the Langmuir isotherm model except that of sample A. In contrast,

1 1 þ bC0

ð8Þ

where C0 is the initial solution concentration and b is the Langmuir adsorption equilibrium constant. When the RL value is within 0 < RL < 1, it is favorable adsorption. Otherwise, it is unfavorable adsorption. Table 6 lists the calculated RL values based on the Langmuir isotherm parameters. It can be seen that RL values are all in the range of 0 < RL < 1, suggesting that Pb2þ adsorption on samples AD is favorable. Consequently, these negatively charged hybrid adsorbents are promising in the separation and recycle of heavymetal ions from contaminated water because of the existence of carboxylic groups and secondary amino groups in the polymer backbone. Table 7 lists the maximum capacity, Qm, obtained from these samples compared with those of other adsorbents reported in the literature.22,25,2833 Clearly, Pb2þ adsorption on these negatively charged hybrid adsorbents was efficient; i.e., the hybrid adsorbents prepared in our case have advantages over other different types of sorbents reported in the literature for lead removal. Consequently, it can be concluded that these samples are excellent candidates for Pb2þ removal. To gain further insight into the adsorption performances of samples AD, the adsorption capacity of per unit charge from COO groups can be calculated by the ratio Qm/CIEC. It was found that the amount of Pb2þ adsorbed per amount of CIEC in samples AD is 1.84, 1.11, 0.92, and 0.87. From these data, it can be reasoned that the complex effects between the metal ions and the COO groups are increased as the amount 8652



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Table 7. Comparison of Qm Values Obtained from Samples AD with Those of Different Types of Sorbents Reported in the Literature Qm (mg g1)

sorbent type

literature

metal-complexed chitosans

105.26

22

magnetic chelating resin fructose-mediated [poly(ethylene glycol)/chitosan] membrane

596.71 (2.88 mmol g1) 185.18

25 28

EMMB

333.0 (1.607 mmol g1)

29

Cassia grandis seed gum-graft-poly(methyl methacrylate)

126.58

30

poly(methyl methacrylatemethacryloylamidoglutamic acid) beads

65.2

31

nanoscale manganese oxide powder

326.74

32

two-dimensional coordination polymer cadmium phosphate

1139.55 (5.50 mmol g1)

33

negatively charged hybrid adsorbents

813.84 (3.928 mmol g1)

this work

Table 8. Relationship of the Amount of Pb2þ Adsorbed per Amount of CIEC in Samples AD Compared with Those of Other Sorbents Reported in the Literature sorbent type

Qm

CIEC

Qm/

(mmol g1)

(mmol g1)

CIEC

literature

EMMB

1.607

1.04

1.55

29

AAm-AMPSNa/

1.03

2.40

0.43

34

1.11 (230 mg g1)

3.8

0.29

35

1.99 (414 mg g1)

3.2

0.62

36

2.14

1.84

this work

clay hydrogel nanocomposites cross-linked AA-g-PVA polymer-based

Figure 11. Adsorption capacity of Pb2þ versus the solution temperature. The concentration of an aqueous Pb(NO3)2 solution was 0.1 mol dm3.

zirconium phosphate negatively charged 3.928

Table 9. Thermodynamic Data for Pb2þ Adsorption on Samples AD

hybrid adsorbents

of CIECs increases. For example, for sample A, around one metal ion might be adsorbed by two COO groups. For samples B and C, nearly one metal ion is adsorbed by one COO group, whereas for sample D, about five metal ions are adsorbed by four COO groups. This finding is similar to the result reported by Junior et al.,29 in which it was found that approximately three metal ions were adsorbed by the two EDTA incorporated. Moreover, considering the electroneutrality principle, one divalent metal ion is attracted by two monovalent functionalized groups. Thus the residual charge should be ascribed to the coordination effect of NH group with the available Pb2þ ion, i.e, the adsorption capacity of per unit charge from NH group is equal to the charge difference of 1 Qm/CIEC, which indicates an upward trend from samples A to D. Such phenomenon suggests that the NH groups also make contribution to the adsorption of Pb2þ ions on the prepared samples. These results demonstrate that with an increase in the amount of COO groups in the prepared samples; their adsorptive actions for metal ions will be adjusted, resulting in the formation of network structure between the functionalized group and metal ion. Table 8 presents the relationship of the amount of Pb2þ adsorbed per amount of CIEC in samples AD with those of other various sorbents reported in the literature.29,3436 As shown in Table 8, it can noted that, compared with other sorbents, these negatively charged hybrid adsorbents can adsorb larger amounts of metal ions per unit charge because of the multiple effects of

temperature

ΔG

ΔS

ΔH

sample

(°C)

(kJ mol1)

(J mol1 K1)

(kJ mol1)

R2

A

25

2.209

54.129

18.356

0.997

35 45

1.721 1.124

25

0.917

30.289

8.096

0.992

35

1.259

45

1.521

25

2.423

49.353

12.295

0.997

35

2.881

45

3.412

25 35

2.818 3.478

54.040

13.247

0.973

45

3.894

B

C

D

functionalized groups, demonstrating an obvious advantage of using these adsorbents for lead removal. 3.6.6. Thermodynamic Parameters. Adsorption experiments at different temperatures for Pb2þ removal were conducted and are presented in Figure 11. Meanwhile, the thermodynamic parameters for Pb2þ adsorption on samples AD were calculated and are listed in Table 9. As shown in Figure 11, it can be seen that the adsorption capacity of Pb2þ ions increases with an increase in the solution temperature, implying that Pb2þ adsorption on samples AD is endothermic in nature.24 On the basis of the dependence of 8653


Industrial & Engineering Chemistry Research the adsorption capacity of Pb2þ ions on the solution temperature, the thermodynamic parameters, such as free energy (ΔG),

ARTICLE

enthalpy (ΔH), and entropy (ΔS), can be calculated from eqs 9 and 1024 ΔG ¼ RT lnðKc Þ

ð9Þ

ΔS ΔH R RT

ð10Þ

lnðKc Þ ¼

in which Kc is the equilibrium partition coefficient and can be calculated from eq 1124 Kc ¼ Figure 12. van’t Hoff plot of Pb2þ adsorption on samples A (solid square), B (half-filled diamond), C (center triangle), and D (half-filled inverted triangle). The concentration of the aqueous Pb(NO3)2 solution was 0.1 mol dm3 at 25, 35, and 45 °C for 24 h.

Cs Ce

ð11Þ

where R is the gas constant (8.314 J mol1 K1), Cs and Ce are the equilibrium concentrations of Pb2þ ions in the adsorbent and solution, respectively, and T is the solution temperature. Both ΔH and ΔS values can thus be calculated from the slope and intercept

Figure 13. SEM images (ae) for the adsorbed samples AD and pure PMDA. SEM image (f) for the original sample A. 8654



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Table 10. DE (%) of Pb2þ Ions in Various Desorbents

Figure 14. EDS images (ac) for the adsorbed samples A and C and PMDA.

of the linear plot according to ln(Kc) vs 1/T (cf. Figure 12). The calculated thermodynamic data are tabulated in Table 9. As shown in Table 9, it can be noted that the ΔG values decrease from positive to negative from samples A to D. Meanwhile, both ΔH and ΔS values for samples AD are positive. These results suggest that Pb2þ adsorption on samples AD is spontaneous and endothermic as the PMDA content increases in these samples. Moreover, it can also be noted that, for the respective samples, these ΔG values become smaller and smaller with the elevated solution temperature, whereas at the same temperature, the ΔG values also decrease remarkably from samples A to D, implying that the Pb2þ adsorption ability increases with the elevating content of ionic groups, which is consistent with the upward trend in CIECs (cf. Table 3), demonstrating that the solution temperature and the content of ionic groups in the polymer chains have some effects on Pb2þ adsorption. The above trend can be ascribed to the electrostatic attraction between Pb2þ ions and ionic groups in the negatively charged hybrid adsorbents. Moreover, the existence of NH groups in the polymer chains will also be responsible for such a trend. 3.6.7. SEM and EDS Images. To gain insight into Pb2þ adsorption on the surface of the prepared samples, SEM images

desorbent

desorption time (h)

DE (%)

HNO3

2

91.5

H2SO4

2

47

HCl

2

33

of the adsorbed samples AD and pure PMDA were observed and are illustrated in Figure 13ae. For comparison, as a typical example, the SEM image of the original sample A is also presented in the same plot as Figure 13f. As shown in Figure 13, it is interesting to find that the surface of the original sample A exhibits a larger pore-size layered structure (cf. Figure 13f). However, the surface of the adsorbed sample A shows the threadlike structure, and the surface becomes tighter (cf. Figure 13a), demonstrating the occurrence of Pb2þ adsorption on this surface. Moreover, when the SEM images of the adsorbed samples AD and pure PMDA are compared, it can be noted that, with an increase in the adsorption capacity of Pb2þ ions from samples A to D, the threadlike structure surface is much clearer. Meanwhile, the size of the thread becomes larger and it is turned into a rodlike macromolecule (cf. Figure 13ad), suggesting larger amounts of Pb2þ ions being deposited on the surface of samples AD. In addition, it can also be noted that the surface of the adsorbed pure PMDA exhibits a rodlike structure (cf. Figure 13e) similar to that of the adsorbed sample D, confirming that pure PMDA can also be used as an adsorbent for Pb2þ removal. To further corroborate the existence of Pb2þ ions on the surface of the prepared samples, as typical examples, EDS images of the adsorbed samples A and C and PMDA were observed and are presented in Figure 14. It is clear that a lead peak dominated the detected surface of the adsorbed samples, revealing the occurrence of Pb2þ adsorption on the surface of the adsorbed samples A and C and PMDA. Clearly, these SEM and DES images provide significant evidence for confirming the existence of Pb2þ ions on the surface of the adsorbed samples. 3.7. Desorption of Pb2þ. Currently, emphasis is given on desorption of the metal ion rather than its simple adsorption and disposal.3 To regenerate and recycle the adsorbent spent, a desorption experiment of Pb2þ ions was conducted and is shown in Table 10. It can be noted that the DE using an aqueous HNO3 solution as the desorbent has the largest value and can arrive at 91.5%, indicating an effective regeneration cycle of heavymetal ions. It should be pointed out that the reuse of an adsorbent is more significant and practical for its application in industry. Several researchers have conducted such an investigation.22,37 For example, Chen et al.22 reported the reuse of cross-linked metalcomplexed chitosans for Pb2þ adsorption three times. It was found that a significant decrease in the adsorption capacity (>30%) occurred during reuse of the particle adsorbents. He et al.37 reported that the Cu2þ adsorption capacity only decreased by 8.2% when using an activated nylon-based membrane as the adsorption elution cycle was repeated three times, indicating excellent reuse of the prepared membranes. From these examples, it can be seen that the membrane structure of an adsorbent might be more available for its reuse than the particle shape. To obtain the long-term reuse of an adsorbent, the shape of the adsorbent will be the major concern. 8655


Industrial & Engineering Chemistry Research Notice that this study mainly focuses on the preparation of the negatively charged hybrid adsorbents and their adsorption properties; little work is done on their reuse. However, this does not mean that it is less important. For their industrial application, further work is needed to optimize the preparation of the related hybrid membrane using these hybrid polymers as membrane materials. When these negatively charged hybrid adsorbents are shaped into membranes, their reuse will be further investigated, which will be our future work.

4. CONCLUSIONS The present study focused on the preparation and adsorption performances of negatively charged hybrid adsorbents, and the following findings were drawn. (1) It was found that Pb2þ adsorption on samples AD followed the Lagergren second-order kinetic and Langmuir isotherm models. (2) The calculated thermodynamic parameters ΔG, ΔH, and ΔS demonstrated that Pb2þ adsorption on these samples was an endothermic and spontaneous process in nature. (3) SEM and EDS images further confirmed the existence of Pb2þ ions on the surface of the prepared samples AD. (4) By comparison with various adsorbents reported in the literature, it can be concluded that these hybrid adsorbents were efficient for Pb2þ removal. (5) The higher DE of Pb2þ ions in a HNO3 solution implies that these hybrid adsorbents can be regenerated in industry. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ86 551 2158439. Fax: þ86 551 2158437. E-mail: jsliu@ hfuu.edu.cn.

’ ACKNOWLEDGMENT This project was financially supported by the Natural Science Foundation of China (Grant 21076055), the Anhui Provincial Natural Science Foundation (Grant 090415211), and the Opening Project of Key Laboratory of Solid Waste Treatment and Resource Recycle, Ministry of Education (Grant 09zxgk03). Special thanks are given to the four anonymous reviewers for their insightful comments and suggestions. ’ REFERENCES (1) Zhuang, Y.; Yang, Y.; Xiang, G.; Wang, X. Magnesium silicate hollow nanostructures as highly efficient absorbents for toxic metal ions. J. Phys. Chem. C 2009, 113, 10441–10445. (2) Wu, Q.; Tian, P. Adsorption of Cu2þ ions with poly(N-isopropylacrylamide-co-methacrylic acid) micro/nanoparticles. J. Appl. Polym. Sci. 2008, 109, 3470–3476. (3) Chatterjee, P. K.; Sengupta, A. K. Sensing of toxic metals through pH changes using a hybrid sorbent material: concept and experimental validation. AIChE J. 2009, 55, 2997–3004. (4) Ministry of Health of P. R. China. Standards for drinking water quality (GB 5749-2006), 2006. (5) Ministry of Health of P. R. China. Sanitary Standard for drinking water (GB5749-85), 1985. (6) U.S. Environmental Protection Agency. National primary drinking water regulations (EPA 816-F-09-004), 2009. (7) Zhang, Q. J.; Pan, B. C.; Zhang, W. M.; Pan, B. J.; Zhang, Q. X.; Ren, H. Q. Arsenate removal from aqueous media by nanosized hydrated

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