or DTPA ... - ACS Publications

Article pubs.acs.org/IECR

Green Synthesis of Magnetic EDTA- and/or DTPA-Cross-Linked Chitosan Adsorbents for Highly Efficient Removal of Metals Feiping Zhao,*,†,‡ Eveliina Repo,† Mika Sillanpaä ,̈ † Yong Meng,‡ Dulin Yin,*,‡ and Walter Z. Tang† †

Laboratory of Green Chemistry, Department of Chemistry, Faculty of Technology, Lappeenranta University of Technology, Sammonkatu 12, FI-50130 Mikkeli, Finland ‡ National Local Joint Engineering Laboratory of Novel Petrochemical Materials and Fine Resources Processing, College of Chemistry and Chemical Engineering, Hunan Normal University, 410081 Changsha, China S Supporting Information *

ABSTRACT: The present paper describes a green and economic approach to explore EDTA/DTPA-functionalized magnetic chitosan as adsorbents for the removal of aqueous metal ions, such as Cd(II), Pb(II), Co(II), and Ni(II). EDTA and DTPA play roles not only as cross-linkers but also as functional groups in chelating metal ions. The morphology, structure, and property of the magnetic adsorbents were characterized by SEM, TEM, XRD, EDS, FT-IR, TGA, and VSM techniques. Their adsorption properties for the removal of metal ions by varying experimental conditions were also investigated. The kinetic results revealed that the transportation of adsorbates from the bulk phase to the exterior surface of adsorbents was the rate-controlling step. The obtained maximum adsorption capacities of magnetic adsorbents for the metal ions ranged from 0.878 to 1.561 mmol g−1. BiLangmuir and Sips isotherm models fitting well to the experimental data revealed the surface heterogeneity of the adsorbents. More significantly, the resulting EDTA-/DTPA-cross-linked magnetic chitosan adsorbents had selectivity to Cu, Pb, Zn, Fe, and Ni from a practical industrial effluent. Furthermore, their good reusability and convenient magnetic separation makes them viable alternatives for real wastewater treatment.

1. INTRODUCTION Chitosan is a natural polysaccharide with a large number of primary amines presenting many useful features such as antimicrobial ability, biodegradability, biocompatibility, and adsorption activity.1 Due to the reactivity of amine groups and stable chelation, chitosan and its derivatives have been employed as adsorbents exhibiting relatively high sorption capacities and fast removal kinetics for heavy metals.2−4 However, chitosan and its derivative adsorbents have drawbacks such as notable swelling in aqueous media, and their separation by common methods such as filtration and centrifugation is complicated,5 which might lead to the adsorbent loss and secondary pollution.6 Therefore, magnetic separation techniques were introduced, and several kinds of magnetic chitosan micro/nanospheres adsorbents were prepared and proved to be attractive in practical applications.7 Generally, magnetic chitosan adsorbents are prepared via a water/oil (W/O) emulsion cross-linking technique,8,9 which commonly uses glutaraldehyde (GA) or epichlorohydrin (EPI) as the cross-linker.10,11 However, the drawbacks are the potential of both GA and EPA to have high levels of toxicity and immunogenicity and carcinogenicity to human beings and animals.12−15 Thus, it is necessary to find an environmentally friendly cross-linker and green cross-linking technique for the production of magnetic chitosan adsorbents. Furthermore, in order to obtain better adsorption properties, the surface of magnetic chitosan adsorbents have been successfully modified with α-ketoglutaric acid,16 β-cyclodextrin,17 ethylenediamine,18 phenylthiourea,19 and xanthate groups.20 However, all the magnetic chitosan adsorbents mentioned above have been cross-linked by GA or EPI, with © 2014 American Chemical Society

further surface modification on chitosan carried out via carbodiimide activation. Chelating agents such as ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) form very strong chelates with metal ions.21,22 Therefore, their immobilization on the solid supports could produce adsorbents with excellent metal binding properties. In the previous studies, our group has quite extensively studied adsorption of metals such as Cd(II), Pb(II), Co(II), and Ni(II) by EDTA−, EGTA−, and DTPA−chitosan adsorbents.3,5,23 More recently, Ren et al. reported a novel preparation method for one kind of EDTA-modified magnetic chitosan adsorbent for metal recovery. This adsorbent was prepared by surface modification of chitosan/SiO2/Fe3O4 with EDTA via using 3-ethyl-1-(3-(dimethylamino)propyl) carbodiimide hydrochloride (EDAC) as a cross-linker.6 However, in that method, prior to surface modification, chitosan/SiO2/ Fe3O4 gels were also cross-linked by GA, which is as stated above considered a toxic compound. Moreover, the modification involved the use of another kind of carbodiimide derivative, EDAC, which is relatively expensive. Here, we describe a green and economic approach to synthesize magnetic EDTA− and/or DTPA−chitosan adsorbents (MEDCS/MDTCS) via an emulsion cross-linking method by using EDTA or DTPA as cross-linkers (Scheme 1). The presented method involves four benefits: (1) EDTA and DTPA play roles of not only cross-linkers but also functional groups Received: Revised: Accepted: Published: 1271

September 30, 2014 December 21, 2014 December 26, 2014 December 26, 2014 DOI: 10.1021/ie503874x Ind. Eng. Chem. Res. 2015, 54, 1271−1281

Article

Industrial & Engineering Chemistry Research Scheme 1. Synthesis of MGACS and MEDCS/MDTCS

2.2. Methods. 2.2.1. Preparation of Magnetic Chitosan Adsorbents. Synthesis of MGACS. As a blank control, a magnetic glutaraldehyde cross-linked chitosan (MGACS) was synthesized via a reversed-phase water/oil emulsion crosslinking method by using glutaraldehyde as a cross-linker (Scheme 1).8 In a typical procedure, 0.7 g of chitosan was dissolved in 50 mL of 5 wt % acetic acid solution. After dissolving, 0.3 g of Fe3O4 MNPs was added, and the mixture was ultrasonicated for 15 min. The W/O emulsion was prepared by dropwise addition of the as-prepared chitosan solution containing Fe3O4 MNPs into the dispersion medium, which was composed of 100 mL of isohexane and emulsifiers (4 mL span-80 and 2 mL of butanol). The W/O ratio of the microemulsion was 1:2 (v/v). During this process, the dispersion medium was stirred mechanically at 1000 rpm until the mixed emulsion became brightly colored. After dropwise adding of 0.5 mL 50 wt % glutaraldehyde and refluxing the reaction system at 60 °C for 6 h, the magnetic chitosan microspheres were collected using a magnet, rinsed with ethanol and deionized water three times, and finally dried at 60 °C in vacuum for 24 h. Synthesis of MEDCS/MDTCS. The W/O emulsion was prepared by the same procedures as MGACS. The major modification of the synthesis was a cross-linking agent. As a cross-linking agent, 0.5 g of EDTA or DTPA anhydride were synthesized according to Pizarro and Geckeler,27 suspended in methanol, and dropwise added into the emulsion; the system was kept under reflux and stirred at 60 °C for 6 h. Afterward,

available for binding metal ions. (2) EDTA and DTPA are cheaper and possess lower toxicity to environment24,25 compared to common cross-linkers such as GA, EPI ,and EDAC. (3) It is an easy process, that is, a two-step procedure (cross-linking and modification) was simplified into a facile one-step procedure (EDTA-/DTPA-cross-linking). (4) Not only the surface but also the inner parts of the adsorbents could be functionalized by EDTA/DTPA groups. The prepared magnetic EDTA- and/or DTPA-cross-linked chitosan adsorbents were used to remove Cd(II), Pb(II), Co(II), and Ni(II) from the aqueous solution. Furthermore, to assess the removal of metals in practical applications, the as-prepared adsorbents were used with a real wastewater effluent.

2. EXPERIMENTAL SECTION 2.1. Materials. Fe3O4 magnetic nanoparticles (MNPs) were synthesized using a coprecipitation method.26 Chitosan flakes >85% deacetylated supplied by Sigma-Aldrich had a molecular weight ranging from 190 000 to 375 000 g mol−1 and a viscosity of 200−2000 MPa. All other chemicals used in this study were of analytical grade and supplied by Merck (Finland). Stock solutions of 1000 mg L−1 were prepared by dissolving appropriate amounts of Cd(II), Pb(II), Co(II), and Ni(II) nitrate salts in deionized water. Working solutions ranging from 1 to 500 mg L−1 of heavy metals were prepared by diluting the stock solutions. Adjustment of pH was undertaken using 0.1 M NaOH and 0.1 M HNO3. 1272

DOI: 10.1021/ie503874x Ind. Eng. Chem. Res. 2015, 54, 1271−1281

Article

Industrial & Engineering Chemistry Research

Figure 1. SEM (top) and TEM (bottom) images of MNPs, MGACS, MEDCS, and MDTCS (left to right).

The adsorption capacities (mmol g−1) of adsorbents were calculated as follows

the magnetic EDTA- or DTPA-cross-linked chitosan microparticles were collected using a magnet, mixed with ethanol, and subsequently stirred for another 6 h. Then, the adsorbents were rinsed with 0.1 M NaOH, deionized water, 0.1 M HCl, again deionized water, and ethanol. The final products were dried at 60 °C in vacuum for 24 h. 2.2.2. Characterization. The morphologies of the samples were investigated using a Hitachi S-4100 scanning electron microscope (SEM) and high-resolution transmission electron microscopy (HRTEM), which was performed using a doubleaberration corrected JEOL 2200FS (Japan) microscope equipped with a field emission gun operated at 200 kV. The energy dispersive X-ray spectroscopy (EDS) analysis, simultaneously performed during the SEM examinations, was conducted to confirm the element distribution on the surface of adsorbents. The crystal lattice structure of the particles was determined by X-ray diffraction analysis (XRD) PANalytical Xray diffractometer (The Netherlands). Fourier transform infrared (FTIR) spectroscopy of the type Nicolet Nexus 8700 (U.S.A.) was used to identify the surface groups of the synthesized adsorbents before and after bivalent metal binding. The content of the polymer on the magnetic particles was measured by thermogravimetric analysis (TGA) NETZSCH STA 409 TG-DTA (Germany). The magnetic behavior was examined using a vibrating sample magnetometer (VSM) Lake Shore 7407 (U.S.A.). The concentrations of metals in solution were measured by an inductively coupled plasma optical atomic emission spectrometer (ICP-OES) Model Icap 6300 (Thermo Electron Corporation, U.S.A.). Surface charge and a point of zero charge of all the adsorbents were determined by isoelectric point titration as a function of pH by using a Zetasizer Nano ZEN3500 (Malvern, U.K.). The zeta potential measurements were performed in 0.1 M NaCl. 2.2.3. Batch Adsorption Tests. The batch experiments of Cd(II), Pb(II), Co(II), and Ni(II) sorption on adsorbents were undertaken by mixing 10 mg of adsorbent with 5 mL of metal solution at concentrations ranging from 0.02 to 8.0 mmol L−1. The effect of pH was investigated at a metal concentration of 0.8 mmol L−1 in the pH range of 1−6. The effect of treatment time was studied at metal concentrations of 3.4 mmol L−1. At designated contact times, the adsorbent was separated from the solution using a magnet. After dilution with 2% HNO3, the residual metal concentrations in the solutions were analyzed by ICP-OES. All the adsorption tests were carried out in duplicate.

qe =

(C i − Ce) V M

(1)

where Ci and Ce are the initial and equilibrium concentrations (mmol L−1), respectively, while M (g) and V (L) represent the weight of the adsorbent and volume of the solution, respectively. 2.2.4. Adsorption in Multimetal Systems. The competitive adsorption of Cd(II), Pb(II), Co(II), and Ni(II) on MEDCS/ MDTCS was conducted in multimetal systems, containing the same initial concentration of each kind of metal ranging from 0.1 to 1.8 mmol L−1. To maximize the removal of metal ions by the adsorbents, pH of the multimetal solution was adjusted to the value of 3.5, which was based on the batch tests carried out in a monometal system.28 2.2.5. Application of MEDCS/MDTCS in Practical Effluent. The real metal wastewater used in this study was taken from the effluent of a nonferrous metal smelter plant in Suxian district, Chenzhou city (Hunan province, China) producing approximately 200 m3 of wastewater per hour. Prior to treatment, all the practical effluent solutions were allowed to settle for 24 h. Then the supernatants were used for the analysis of pH, chemical oxygen demand (COD), suspended solids (SS), color, turbidity, and further adsorption experiments. The measurements of COD, SS, and color followed a standard method.29 Turbidity was measured using a turbidity meter (Hach 1900C, U.S.A.), and it was expressed in nephelometric turbidity units (NTU). After a 16 h contact at ambient temperature, the adsorbents were separated by an external magnetic field. The concentrations of metal ions in the practical effluents before and after adsorption were analyzed by ICP-OES. 2.2.6. Regeneration Studies. To evaluate their desorption ability and reusability, regeneration of the spent magnetic adsorbents were carried out in acidic conditions according to the methods reported before.3,23 At first, 0.1 g of the adsorbent was mixed with 0.03 L of 2.0 mmol L−1 heavy metal solution. After attaining equilibrium, the spent adsorbent was separated from the solution using an external magnet. Subsequently, metal ions were eluted by using 0.01 L of 2 M HNO3. Then the adsorbent was treated with deionized water to neutralize and recondition the sorbent for adsorption in succeeding cycles. 1273


Article


3. RESULTS AND DISCUSSION 3.1. Characterization. Morphology of the samples was observed by SEM and HRTEM. Figure 1 shows that Fe3O4 MNPs with diameters of approximate 30 nm were well encapsulated in cross-linked chitosan. Glutaraldehyde crosslinked magnetic chitosan shows a microsphere structure (Figure 1b and f), while EDTA-/DTPA-cross-linked magnetic chitosans are cube-shaped (Figure 1c, d, g, and h). This might be attributed to the fact that EDTA/DTPA has four to five −COOH groups available for cross-linking, while glutaraldehyde only has two −CHO groups (Scheme 1). This results in a high degree of cross-linking and higher crystalline structures for MEDCS and MDTCS than that of MGACS. The high crystallinity and high degree of cross-linking might cause the advantages of poor solubility in water and common organic solutions,1 which could protect magnetic cores from leaching. As shown in Figure 2, all peaks of the prepared samples could

originated from not only chitosan but also the cross-linker (EDTA/DTPA). This confirmed the presence of EDTA/ DTPA functional groups on the surface of MEDCS/MDTCS. Surface coverage of EDTA and DTPA was calculated based on the difference between the mass amounts of nitrogen on unmodified (42.1 g kg−1, reported by our group before)3 and modified chitosans (104.8 g kg−1 for MEDCS and 110.3 g kg−1 for MDTCS, see Figure S1c and d, Supporting Information) obtained from EDS analysis. Coverages were calculated to be 2.24 and 1.62 mmol g−1 for MEDCS and MDTCS, respectively. The surface coverages of functional groups obtained in this study were well associated with the amount of metals absorbed, which is discussed in Sections 3.2.4 and 3.5. When compared with the FTIR spectrum of MGACS, the FTIR spectra of both MEDCS and MDTCS show two new peaks at around 1640 and 1740 cm−1 associated with the carbonyl groups vibration of −CONH− and −COOH, respectively (Figure 3). Furthermore, a broad band between

Figure 2. XRD of MNPs, MGACS, MEDCS, and MDTCS.

Figure 3. FTIR spectra of MNPs, MGACS, MEDCS, and MDTCS before and after Ni(II) adsorption.

be clearly indexed as a magnetite phase (JCPDS 19-0629), confirming unequivocally the presence of Fe3O4 in all the adsorbents. For MEDCS and MDTCS, the presence of a new peak of the (020) reflection at 2θ = 22.5° could be attributed to the crystalline structure of EDTA-/DTPA-cross-linked chitosans.30 According to the peak height method that has been reported by Katsutoshi group,31 the value of the crystallinity index was calculated to be 71% for MEDCS and 88% for MDTCS. The relatively higher crystalline structure of MDTCS than that of MEDCS, which could also be observed from SEM and TEM images (Figure 1c, d, g, and h), might be ascribed directly to the amount of carboxyl terminals of the used crosslinking agents (five for DTPA and four for EDTA). EDS was carried out to confirm the composition of the surface of adsorbents. The EDS spectrum and quantitative elemental composition are shown in Figure S1 of the Supporting Information, confirming the presence of C, N, O, and Fe on the surface of adsorbents. The Fe signal is originated from the Fe3O4 MNPs, confirming the existence of Fe3O4, consistent with the results of TEM and XRD. The quantity of N on the surface of MEDCS (10.00 atom %) and MDTCS (10.82 atom %) was much higher than that of MGACS (2.76 atom %), which could be explained by the fact that the N signal only originated from chitosan for MGACS (because of no N element in GA), while for MEDCS/MDTCS, the N signal

3160 and 3700 cm−1 was assigned to the stretching vibration of −OH from −COOH and overlapping stretching of the residual −NH2 of chitosan, while for MGACS, a corresponding narrow peak at 3430 cm−1 was associated with the residual −NH2 after glutaraldehyde cross-linking. The thermal stabilities of MNPs, MGACS, MEDCS, and MDTCS were measured by thermogravimetric analysis (Figure S2, Supporting Information). For all samples, the weight loss at temperatures below 200 °C could be attributed to the release of moisture on the surface of samples.32 Obviously, both MEDCS and MDTCS adsorbed physically a little bit more water than MGACS. At a temperature higher than 220 °C, all adsorbents rapidly lost part of their masses due to the decomposition of the organic polymers or nitrogen-containing functional groups. This is similar to the polymers decomposition period of CMS and EDCMS (GA cross-linked magnetic−silica chitosan and further surface modified by EDTA) reported by Ren et al.6 Finally, the organic polymers were completely decomposed at the temperature of 650 °C for MGACS and 700 °C for MEDCS/MDTCS. On the basis of the weight loss at this stage, the weight proportions of cross-linked chitosan on MGACS, MEDCS, and MDTCS were calculated to be 67.8, 72.3, and 66.4 wt % (excluding moisture), respectively. This result 1274


Article


Figure 4. Effects of pH on adsorption of (a) Cd(II), Pb(II) and (b) Co(II), Ni(II) by magnetic cross-linked chitosan adsorbents.

(54.8%), and H5EDTA+ (2.8%) for EDTA and H3DTPA2− (14.93%), H4DTPA− (54.39%), and H6DTPA+ (3.51%) for DTPA suggesting negligible protonation and ability of both EDTA and DTPA to form stable complexes with metal ions. The maximum adsorption of metal ions occurred at pH ranging from 3.5 to 5. It is well known that most of the practical effluents containing heavy metals are strongly acidic;37 therefore, pH 3.5 was selected for the subsequent adsorption experiments as the optimum pH. It is worthwhile to note that both MEDCS and MDTCS had substantially higher adsorption efficiency than that of MGACS at pH from 2.5 to 6. This result absolutely illuminates that both EDTA and DTPA groups play roles not only as cross-linkers but also as functional groups available for binding metal ions. 3.2.2. Comparison of Adsorbents. Figure 4 shows that EDTA-/DTPA-cross-linked magnetic chitosans had substantially higher metal removal than glutaraldehyde cross-linked magnetic chitosan. The removal of both Cd(II) and Ni(II) by MGACS at 0.8 mmol L−1 of initial concentration was less than 20%, while MEDCS and MDTCS could almost completely remove the metal at the same operational conditions, suggesting that the chelating groups played predominant roles in adsorption process. As presented in Figure 5, the maximum uptakes of Cd(II), Co(II), Pb(II), and Ni(II) by MEDCS were 1.50, 1.24, 1.03, and 1.38 mmol g−1 and by MDTCS were 1.56, 1.12, 0.88, and 1.19 mmol g−1, respectively. The higher maximum adsorption capacity of most metals except Cd(II) by MEDCS could be attributed to the much higher surface coverage of EDTA (2.24 mmol g−1) compared to DTPA (1.62 mmol g−1). The higher uptake of Cd(II) by MDTCS compared to that of MEDCS was probably due to the much higher stability constant of Cd(II)DTPA3− (21.15) than that of Cd(II)EDTA2− (18.1).38 In addition, according to the surface coverages of EDTA and DTPA, it was calculated that 46−67% of EDTA surface groups and 54−96% of DTPA surface groups were occupied by metal ions. The reason for unoccupied surface groups could be explained by the cross-linking effect, which makes part of the functionalities unable for metal binding.3 The higher utilization of functional groups for MDTCS than that for MEDCS might also be ascribed to the relatively higher amount of carboxyl terminals of DTPA compared to EDTA, resulting in less of a cross-linking effect on MDTCS compared to MEDCS. Table 1 lists the maximum adsorption capacities (qm) of metal ions on as-prepared and also some commonly used adsorbents. The higher qm of

indicated that the cross-linking ability for magnetic chitosan in the emulsion system is EDTA > GA > DTPA. The higher cross-linking value of MEDCS than that of MDTCS was consistent with the functional group coverage results obtained from EDS analysis (more cross-linking, more functional groups on surface). The magnetization curves of samples were measured by VSM as illustrated in Figure S3a of the Supporting Information. The saturation magnetizations of MNPs, MGACS, MEDCS, and MDTCS were 55.57, 12.21, 14.98, and 16.76 emu g−1, respectively. The much lower saturation magnetization of magnetic adsorbents than that of MNPs could be attributed to the diamagnetic matrixes of the cross-linked chitosan.33 However, the magnetic adsorbents MEDCS and MDTCS could still be rapidly separated from their well-dispersed aqueous solutions by an external magnet within tens of seconds (Figure S3b, Supporting Information), which endowed a potential to use magnetic separation in practical applications. 3.2. Adsorption Study. 3.2.1. Effects of pH. It is well known that the adsorption of metal ions from aqueous solutions significantly depends on the solution pH because pH affects both the protonation of the surface groups of the adsorbent and the degree of ionization of the metal ions.34 Therefore, the knowledge of an optimum pH is important for maximum removal of the target metals.35 The effect of pH was studied at a metal concentration of 0.8 mmol L−1 under pH ranging from 1 to 6, and the results are presented in Figure 4. Alkaline solutions were not used in order to avoid the formation of metal hydroxides (Visual MINTEQ ver. 3.0). It is apparent from the figure that the removal of heavy metals by both MEDCS and MDTCS was dependent on pH value. The adsorption efficiencies were found to be low, pH < 2, due to the high competition between protons and metal ions on the available adsorption sites in strongly acidic media.23 Metal removal soared with increasing pH from 1 to 2.5 and gradually reached a plateau at pH 2.5. This could be attributed to the species distribution of EDTA and DTPA at different pHs. As shown in Figure S4a and b of the Supporting Information, along with increasing pH, the amount of the protonated species decreases while that of the negative species increases, resulting in more and more electrostatic attraction between the positively charged metal ions and the negatively charged adsorbents. It is noted that the plateau of pH 2.5 is lower when compared with some other available adsorbents.10,36 A reason for this could be that at pH 2.5 the species are H2EDTA2− (15.5%), H3EDTA− 1275


Article


was undertaken in this study, which bring advantages to acidic practical wastewater treatment. 3.2.3. Adsorption Kinetics. The adsorption behavior of heavy metals on MEDCS and MDTCS in relation to contact time is depicted in Figure 6. The adsorption was very fast and

Figure 5. Adsorption isotherms for Cd(II), Co(II), Pb(II), and Ni(II) adsorption by MEDCS and MDTCS. Figure 6. Effect of contact time on Cd(II), Pb(II), Co(II), and Ni(II) adsorption by (a) MEDCS and (b) MDTCS.

MEDCS/MDTCS than those of most of the presented sorbents indicates that MEDCS and MDTCS are efficient heavy metal adsorbents. It is worth noting that the qm values of MEDCS and MDTCS were much higher than that of EDCMS, which also involves EDTA groups.6 This verifies that the preparation method utilizing EDTA-/DTPA-cross-linking in this study resulted in more functional terminals than the literature method of EDTA surface modification after glutaraldehyde cross-linking. More significantly, a lower pH

could attain great than 60% of maximum adsorption capacity within 5 min. Especially, the qt of Cd(II) at 5 min reached 1.28 and 1.04 mmol g−1 for MEDCS and MDTCS, respectively. These values are even higher than the qm of some adsorbents listed in Table 1, indicating that MEDCS and MDTACS are fast and efficient adsorbents for heavy metals. Overall, MEDCS presented a shorter adsorption equilibrium time (120 min for

Table 1. Comparison of Adsorption Capacities of Heavy Metals by Different Adsorbents maximum metal adsorption capacity (mmol g−1)

a

Sorbent

Cd(II)

Co(II)

Pb(II)

Ni(II)

pH

ref

commercially available carbon nanotube activated carbon cloths low-cost biosorbent chitosan supported on porous glass beads modified magnetic chitosan chelating resin chitosan/magnetite composite beads magnetic EDTA-modified chitosan/SiO2/Fe3O4 (EDCMS) MEDCS MDTCS

− − 0.217 4.29 − − 0.563 1.499 1.561

− − − − 0.908a − − 1.236 1.119

a

− 0.152 0.202 2.47 0.684a 0.895a − 1.381 1.187

5.0 5.0 − − 5.0 6.0 5.0 3.5 3.5

39 40 41 42 10 36 6 this work this work

0.132 0.147 0.356 8.84 − 0.306a 0.596 1.030 0.878

Conversed from the original unit of mg g−1 presented in the literatures. 1276


Article


Figure 7. Kinetic modeling of metals adsorption by MEDCS and MDTCS.

surface coverage and the activation energy for the chemisorption. As presented in Table S2 of the Supporting Information, the Elovich model described the adsorption kinetics better than pseudo-first-order model for each system with relatively higher correlation coefficient values. This is probably because MEDCS and MDTCS process heterogeneous surface active sites,43 which will be further discussed in Section 3.2.4 using bi-Langmuir and Sips isotherms. However, the Elovich equation was still not enough to describe the adsorption kinetics, and the pseudo-second-order model was applied44

Cd; 240 min for Pb, Co, and Ni) compared to MDTCS (240 min for Cd; 360 min for Pb and Ni; 600 min for Co). This is consistent with our previous studies on EDTA- and DTPAmodified chitosan3 and might be due to the higher functional group coverage of MEDCS than that of MDTCS. For the purpose of investigating the kinetic mechanism of the adsorption process, various kinetic models were applied, and the calculated results for each system after linear fitting are provided in Tables S1 and S2 of the Supporting Information. In the pseudo-first-order model, the rate constant of metal adsorption is given as23 ln(qe − qt) = ln(qe) − k1t

t 1 1 = + t qt qe k 2qe2

(2)

where qt and qe (mmol g−1) are the adsorption capacity at time t and at equilibrium, respectively, while k1 (min−1) is the pseudo-first-order rate constant. As shown in Table S1 of the Supporting Information, the pseudo-first-order model was not able to predict the adsorption capacities of metal ions on MEDCS and MDTCS at an equilibrium time. It is reported that an Elovich equation, which characterizes the kinetic law of chemisorption and its rate equation based on the adsorption capacity, might be used to describe the adsorption kinetics while a simple pseudo-first-order model fails.43 The Elovich model is generally described as37 qt =

1 1 ln(αβ) + ln(t ) β β

−1

where k2 (g mmol min ) is the pseudo-second-order rate constant. As shown in Figure 7 and Table S1 of the Supporting Information, the pseudo-second-order model gave the perfect fit to the experimental plots of t/qt vs t, while qe,exp and qe,model were quite close to each other and all the correlation coefficient (R2) values higher than 0.999. The values of k2 showed faster adsorption kinetics for MEDCS compared to MDTCS, consistent with the results of adsorption equilibrium time. Furthermore, to investigate if film or pore diffusion was the controlling step in the adsorption, the intraparticle diffusion model was tested as follows45

qt = k idt 0.5 + C id

(3) −1

(4) −1

−1

−1

where the Elovich coefficients, α (mmol g min ) is the initial adsorption rate, and β (g mmol−1) is related to the extent of

(5) −0.5

where Kid (mmol g min ) and Cid are the rate constant and the intercept of intraparticle diffusion at different stages, 1277


Article

Industrial & Engineering Chemistry Research respectively. In this study, three linear portions in the plots of qt vs t1/2 (Figure 7c and d) indicated that the adsorption of metals by adsorbents occurred via three steps, which successively presented external surface adsorption, mesopore diffusion, and micropore diffusion.3 As shown in Table S2 of the Supporting Information, the Kid,1 values were much higher than Kid,2 and Kid,3 values, indicating the adsorption process from the bulk phase to the exterior surface of adsorbents should be the ratecontrolling step.45 The relatively low values of Kid,2 and Kid,3 (close to zero), which presented mesopore and micropore diffusions, respectively, denoted that pore diffusion was not apparent in this system. The Cid values present the boundary layer thickness, where the larger the Cid value is, the thicker the boundary layer is.35 As shown in Table S2 of the Supporting Information, MEDCS had relatively higher Cid values than MDTCS, indicating the greater boundary layer thickness on MEDCS. This might also be attributed to the higher functional group coverage of MEDCS than MDTCS. 3.2.4. Adsorption Isotherms. Langmuir, Freundlich, Sips, and bi-Langmuir (two-site Langmuir) isotherms were tested to describe the adsorption behavior of metals on MEDCS and MDTCS. The Langmuir isotherm is widely applicable to a homogeneous adsorption surface where each of the adsorption sites can only bind one adsorbate23 qe =

Figure 8. Adsorption isotherms of Cd(II) on MEDCS.

Freundlich model at low adsorbate concentrations, while at sufficiently high concentrations, it possess a finite saturation limit as Langmuir isotherm.4 As shown in Table S4 of the Supporting Information, all the resulting exponent nS values differed from unity, which means that metal sorption onto the prepared adsorbents was heterogeneous. The bi-Langmuir model, which assumes that the surface contains two different active sites with different affinities toward target compounds, was also employed for equilibrium approach23

qmKLCe 1 + KLCe

(6)

where qe (mmol g−1) and Ce (mmol L−1) are the adsorption capacity and the equilibrium concentration of the adsorbate from experimental data, while qm (mmol g−1) and KL (L mmol−1) present the maximum adsorption capacity of adsorbents and the energy of adsorption obtained after nonlinear fitting, respectively. The Freundlich isotherm assumes a heterogeneous surface and nonuniform distribution of adsorption heat over the surface without a saturation of adsorption sites37

qe = KFCe1/ nF

qe =

(7)

qm(KSCe)nS 1 + (KSCe)nS

1 + K1Ce

+

qm,2K 2Ce 1 + K 2Ce

(9)

where qm,1 and qm,2 are the maximum adsorption capacities of two different adsorption sites, while K1 and K2 are adsorption energies corresponding to adsorption sites 1 and 2. Obviously, the curves of Sips and bi-Langmuir models gave much better fit to the experimental data compared to those of both Langmuir and Freundlich models (Figure 8). According to the coefficient parameters (Table S4, Supporting Information), the bi-Langmuir model fitted better for Cd(II) and Co(II) on MEDCS and Pb(II) on MDTCS isotherms, indicating the coexistence of two kinds of active sites on the surface correlated to the species distribution of EDTA at pH 3.5: R-NHHEDTA2− and R-NH-EDTA3−, and DTPA: R-NH-HDTPA3− and R-NH-DTPA4− (appendix IA and IB of ref 38). The Sips model fitted better to the isotherms of Pb(II) and Ni(II) on MEDCS and Cd(II), Co(II), and Ni(II) on MDTCS. The surface affinity KS values obtained by the Sips model followed the order of stability constants of aqueous metal EDTA chelates (Ni(II) > Pb(II) > Co(II) > Cd(II)).38 However, the KS parameters did not totally follow the order of stability constants of metal DTPA chelates (Ni(II) > Cd(II) > Co(II) = Pb(II)),38 indicating that not only stability constants of chelates but also the properties of metals, especially the hydration number, play a role in the adsorption process in the case of immobilized EDTA and DTPA ligands. Overall, on the basis of curve fitting and R2 values, the experimental data yielded excellent fitting following the isotherm order Sips ≈ bi-Langmuir > Langmuir > Freundlich. Furthermore, the predicted maximum adsorption capacities (qm ) of the Sips model were more identical to the corresponding experimental data (qm,exp) than those of bi-

where KF ((mmol g−1)/ (L mmol−1)nF) is a unit capacity coefficient, and nF is the Freundlich constant related to the degree of system heterogeneity. The larger the nF value is, the more heterogeneous the system is. Figure 8 shows that both Langmuir and Freundlich curves did not fit well with the experimental data. Moreover, the correlation coefficients (R2 values, Table S3, Supporting Information) were relatively low. All these suggest that neither Langmuir nor Freundlich are enough representative for the adsorption equilibrium of metals adsorption on MEDCS and MDTCS. The adsorption plateau at high concentration and high nF values indicate that both maximum adsorption capacity and heterogeneity should be involved in the isotherm modeling. The Sips isotherm, which is a combination of the Langmuir and Freundlich isotherms and takes heterogeneity into account, is usually applicable where both Langmuir and Freundlich models fail46 qe =

qm,1K1Ce

(8)

−1

where KS (L mmol ) is the Langmuir equilibrium parameter, and nS is comparable to the Freundlich heterogeneity factor (nS = 1/nF). The Sips model behavior is the same as that of 1278


Article


Table 2. Concentrations of Metal Ions in Smelter Effluents before (Ci) and after (Ce) Adsorption by MEDCS and MDTCS metals Ci (mg L−1) MEDCSa MDTCSa a

Ce (mg L−1) R% Ce (mg L−1) R%

Cu

Pb

Zn

Cd

Fe

Co

Ni

Cr

As

Hg

13.52 0.75 94.45 1.06 92.16

32.86 0.22 99.33 0.43 98.69

26.13 2.09 92.00 2.14 91.81

6.85 1.66 75.77 0.97 85.84

9.21 0.08 99.13 0.05 99.46

0.32 0.09 71.88 0.13 59.38

0.74 98.6 98.6

0.11 0.09 18.18 0.09 18.18

0.28 0.26 7.14 0.21 25.00

0.06 0.03 50.00 0.04 33.33

The pH was adjusted to 3.5 before adsorption; adsorbent dosage of 2 g L−1; contact time of 16 h.

3.4. Selective Adsorption Test. To investigate the selectivity of metal ions on MEDCS and MDTCS, adsorption tests were conducted in the solutions containing the same amount of each metal (ranging from 0.1 to 1.8 mml L−1, Figure S6, Supporting Information). Under competing conditions, metals clearly showed the selectivities of Ni(II) > Pb(II) > Cd(II) > Co(II) for MEDCS and Pb(II) > Ni (II) > Co(II) > Cd(II) for MDTCS. These results fit well with the affinity KS values (Ni(II) > Pb(II) > Co(II) > Cd(II) for MEDCS and Pb(II) > Ni (II) > Co(II) > Cd(II) for MDTCS; Table S4, Supporting Information), indicating the importance of affinity constants in multimetal systems. The sole mismatch of Cd(II) and Co(II) for MEDCS might be due to the smaller hydration number of Cd(II) (0.426 nm) than that of Co(II) (0.423 nm).50 Several other researches also presented similar findings.5,34,37 Overall, the selective adsorption in multimetal systems suggested that Ni(II) and Pb(II) could be separated from Cd(II) and Co(II) using MEDCS and MDTCS. 3.5. Application of MEDCS/MDTCS in Practical Effluent. To investigate their application in practice, MEDCS and MDTCS were tested to remove the metal ions in a real industrial effluent from Suxian nonferrous metal smelter. The initial characteristics of the supernate of the raw effluent used were determined as 2.3 for pH, 210 mg O2 L−1 for COD, 160 mg L−1 for SS, 65 Co−Pt for color, and 48 NTU for turbidity. Ten types of metals (Cu, Pb, Zn, Cd, Fe, Co, Ni, Cr, As, and Hg) were determined in the practical smelter effluent. The initial concentrations of metal ions in the effluent are shown in Table 2. The presence of Cu, Fe, Co, and Ni endowed the wastewater a high value of color (65 Co−Pt). Before adsorption by MEDCS and MDTCS, the pH value of the wastewater was adjusted from 2.3 to 3.5 to maximize the adsorption capacity. As shown in Table 2, the removal of Cu, Pb, Zn, Fe, and Ni was relatively high (>90%) for both MEDCS and MDTCS, indicating that strong coordination happened between these five types of metal ions and the functional groups of adsorbents. The removal of Pb and Ni was much higher than that of Cd and Co, coinciding with the result in multimetal systems (see Section 3.4). This indicates that the data from pure water-simulated system is also valuable for the practical effluent system. The removal of Cr and As was relatively low for both MEDCS and MDTCS because the major series of Cr and As at pH 3.5 are Cr2O72−, H2AsO3−, and H2AsO4−,47 which had electrostatic repulsion toward the negatively charged carboxyl groups on the surface of the adsorbents. This also supports that EDTA and DTPA play the principal roles in the adsorption process. The total adsorption capacity of all metal ions (qe,total) from effluent was calculated as 0.476 mmol g−1 for MEDCS and 0.477 mmol g−1 for MDTCS. According to the coverages of EDTA and DTPA on the surface of the adsorbents, it was calculated that 21.3% of EDTA surface groups and 29.4% of DTPA surface groups were occupied by metal ions. The reason for unoccupied surface groups is most

Langmuir (qm = qm,1 + qm,1, Table S4, Supporting Information). Therefore, the Sips model gave the best representation of the experimental results of the adsorption isotherms in this study, suggesting the surface heterogeneity of the prepared adsorbents. The results of bi-Langmuir isotherm also supported that the adsorbents had heterogeneous binding sites. 3.3. Adsorption Mechanism. Several sorption mechanisms such as electrostatic interaction, chelation, and complexation could be involved in the adsorption processes.37 As shown in Figure S5 of Supporting Information, the isoelectric point was determined as 3.35 for MEDCS and 3.65 for MDTCS, much lower than 6.85 for MGACS, which can be attributed to the introduction of EDTA and DTPA groups on the surface of magnetic chitosan adsorbents during the crosslinking process. This result corroborates with the structural characterizations (FTIR) in which the existence of immobilized carboxylate groups was proposed. Therefore, these groups are deprotonated at pH > 3.5, increasing the negative charge density of the material. Obviously, the electrostatic interactions, which were stimulative to the sorption of positive metal ions, should be considered as one of the adsorption mechanisms. This study also gives promising results about the applicability of these materials at pH 3.5, which was commonly used in this study. On the basis of thermodynamic data,47 EDTA has various species distribution represented by HnEDTAn‑4, where n ranges from 0 to 5. The calculation by MINEQL software (MINTEQ ver. 3.0) showed that the negatively charged H2EDTA2− (81%) and H3EDTA− (19%) are the dominant species at pH 3.5. This supports the results of zeta potential. On the basis of this, it is proposed that the metal adsorption by MEDCS could be expressed as follows3 RNH_HiEDTAi − 3 + M2 + ⇌ M(RNH_HEDTA) + (i − 1)H+

(10)

where RNH represents the magnetic chitosan, and i is the number of hydrogen ions complexed with EDTA ranging from 2 to 3, while M2+ is the target metal. Equation 10 indicates that the chelating groups play important roles in the adsorption process. To further verify this mechanism, FTIR spectra of adsorbents before and after metal ion binding were compared (Figure 3). After Ni(II) adsorption, the band of carboxylic −OH (stretching vibration) between 3160 and 3700 cm−1 obviously was displaced to 3420 and 3280 cm−1, while the peak of the CO (1740 cm−1 for MEDCS and 1730 cm−1 for MDTCS) vibration of the carboxyl groups disappeared and the C−O (1067 cm−1) stretching of the carboxyl groups shifted to 1020−1030 cm−1. This behavior reflects the interaction between carboxyl groups and nickel ions.23 Similar results of metal ions bound to carboxyl groups by chelation or complexation have been reported earlier.46,48,49 1279


Article

Industrial & Engineering Chemistry Research likely the effect of the cations and anions such as Na2+, Ca2+, SO42−, PO43−, and other pollutants presented in real effluent. In conclusion, both MEDCS and MDTCS are effective adsorbents to treat also practical wastewater containing Cu, Pb, Zn, Fe, and Ni. 3.6. Regeneration Studies. For practical uses, reusability is prerequisite for an advance adsorbent. In this study, Cd(II)loaded MEDCS and MDTCS were regenerated using 2 M HNO3 10 times. Figure 9 suggests that the regeneration

a function of pH, adsorption selectivities in multimetal systems, and tables representing parameters of kinetics models and isotherm models. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +358 40 0205 215. Fax: +358 40 0205 215. E-mail: feiping.zhao@lut.fi, [email protected] (F.Z.). *Tel.: +86 731 88872531. Fax: +86 731 88872531. E-mail: [email protected] (D.Y.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

The authors are grateful to the Hunan Industrialization Fostering Projects for Universities (12CY004) and Hunan Provincial Innovation Foundation for Postgraduate (CX2010B220) for financial support.

(1) Qian, L.; Zhang, H. Green synthesis of chitosan-based nanofibers and their applications. Green Chem. 2010, 12, 1207−1214. (2) Ali, I. New generation adsorbents for water treatment. Chem. Rev. 2012, 112, 5073−5091. (3) Repo, E.; Warchol, J. K.; Kurniawan, T. A.; Sillanpäa,̈ M. E. T. Adsorption of Co(II) and Ni(II) by EDTA- and/or DTPA-modified chitosan: Kinetic and equilibrium modeling. Chem. Eng. J. 2010, 161, 73−82. (4) Reddy, D. H. K.; Lee, S.-M. Synthesis and characterization of a chitosan ligand for the removal of copper from aqueous media. J. Appl. Polym. Sci. 2013, 130, 4542−4550. (5) Repo, E.; Warchoł, J. K.; Bhatnagar, A.; Sillanpäa,̈ M. Heavy metals adsorption by novel EDTA-modified chitosan−silica hybrid materials. J. Colloid Interface Sci. 2011, 358, 261−267. (6) Ren, Y.; Abbood, H. A.; He, F.; Peng, H.; Huang, K. Magnetic EDTA-modified chitosan/SiO2/Fe3O4 adsorbent: Preparation, characterization, and application in heavy metal adsorption. Chem. Eng. J. 2013, 226, 300−311. (7) Liu, X.; Hu, Q.; Fang, Z.; Zhang, X.; Zhang, B. Magnetic chitosan nanocomposites: A useful recyclable tool for heavy metal ion removal. Langmuir 2008, 25, 3−8. (8) Zhou, L.; Wang, Y.; Liu, Z.; Huang, Q. Characteristics of equilibrium, kinetics studies for adsorption of Hg(II), Cu(II), and Ni(II) ions by thiourea-modified magnetic chitosan microspheres. J. Hazard. Mater. 2009, 161, 995−1002. (9) Fan, L.; Luo, C.; Li, X.; Lu, F.; Qiu, H.; Sun, M. Fabrication of novel magnetic chitosan grafted with graphene oxide to enhance adsorption properties for methyl blue. J. Hazard. Mater. 2012, 215− 216, 272−279. (10) Monier, M.; Ayad, D. M.; Wei, Y.; Sarhan, A. A. Adsorption of Cu(II), Co(II), and Ni(II) ions by modified magnetic chitosan chelating resin. J. Hazard. Mater. 2010, 177, 962−970. (11) Reddy, D. H. K.; Lee, S.-M. Application of magnetic chitosan composites for the removal of toxic metal and dyes from aqueous solutions. Adv. Colloid Interface Sci. 2013, 201−202, 68−93. (12) Zeiger, E.; Gollapudi, B.; Spencer, P. Genetic toxicity and carcinogenicity studies of glutaraldehydeA review. Mutat. Res.-Rev. Mutat. 2005, 589, 136−151. (13) Sano, L. L.; Krueger, A. M.; Landrum, P. F. Chronic toxicity of glutaraldehyde: differential sensitivity of three freshwater organisms. Aquat. Toxicol. 2005, 71, 283−296. (14) Hirakawa, B. Epichlorohydrin. In Encyclopedia of Toxicology, Third ed.; Wexler, P., Ed.; Academic Press: Oxford, 2014; pp 431− 432.

Figure 9. Regeneration of MEDCS and MDTCS for Cd(II) by 2 M HNO3 (dose, 2 g L−1; pH 3.5; contact time, 16 h; initial concentration, 2.0 mM).

efficiency of both adsorbents was almost 100% for the first three cycles, indicating that HNO3 is an effective regenerant in these cases. The regeneration efficiency could still retain above 90% after the eighth cycle for MEDCS and the sixth cycle for MDTCS, suggesting that both MEDCS and MDTCS are qualified for practical application.

4. CONCLUSION Two efficient magnetic adsorbents, MEDCS and MDTCS, were fabricated by a simple and green approach via an emulsion cross-linking method by using EDTA or DTPA as the crosslinker. The adsorbents exhibited high absorptivity toward Cd(II), Pb(II), Co(II), and Ni(II) with maximum adsorption capacities of 1.499, 1.030, 1.236, and 1.381 mmol g−1 for MEDCS and 1.561, 0.878, 1.120, and 1.187 mmol g−1 for MDTCS, respectively. Importantly, the as-prepared adsorbents showed good reusability and fast magnetic separation ability. More significantly, the adsorbents had selectivity to Cu, Pb, Zn, Fe, and Ni from a mixed solution of metal ions, especially from the practical industrial effluent. This study shows that using EDTA and DTPA as the cross-linker to fabricate EDTA-/ DTPA-functional magnetic chitosan is an effective strategy for the development of high performance sorbents for the removal of metal ions from contaminated water. It is believed that this green synthesis method can be extended to prepare a wide variety of functional magnetic composite materials for various applications.

■

ASSOCIATED CONTENT

S Supporting Information *

EDS spectrum of samples before and after metal adsorption, thermogravimetric analysis, magnetization characterization, calculated species distribution, zeta potentials of adsorbents as 1280


Article


(34) Hokkanen, S.; Repo, E.; Sillanpäa,̈ M. Removal of heavy metals from aqueous solutions by succinic anhydride modified mercerized nanocellulose. Chem. Eng. J. 2013, 223, 40−47. (35) Reddy, D. H. K.; Lee, S.-M. Three-Dimensional Porous Spinel Ferrite as an Adsorbent for Pb(II) Removal from Aqueous Solutions. Ind. Eng. Chem. Res. 2013, 52, 15789−15800. (36) Tran, H. V.; Tran, L. D.; Nguyen, T. N. Preparation of chitosan/magnetite composite beads and their application for removal of Pb(II) and Ni(II) from aqueous solution. Mater. Sci. Eng., C 2010, 30, 304−310. (37) Luo, S.; Xu, X.; Zhou, G.; Liu, C.; Tang, Y.; Liu, Y. Amino siloxane oligomer-linked graphene oxide as an efficient adsorbent for removal of Pb(II) from wastewater. J. Hazard. Mater. 2014, 274, 145− 155. (38) Repo, E. EDTA- and DTPA-Functionalized Silica Gel and Chitosan Adsorbents for the Removal of Heavy Metals from Aqueous Solutions; Lappeenranta University of Technology: Mikkeli, Finland, 2011. (39) Tian, X.; Zhou, S.; Zhang, Z.; He, X.; Yu, M.; Lin, D. Metal impurities dominate the sorption of a commercially available carbon nanotube for Pb(II) from water. Environ. Sci. Technol. 2010, 44, 8144− 8149. (40) Kadirvelu, K.; Faur-Brasquet, C.; Cloirec, P. L. Removal of Cu(II), Pb(II), and Ni(II) by adsorption onto activated carbon cloths. Langmuir 2000, 16, 8404−8409. (41) Reddad, Z.; Gerente, C.; Andres, Y.; Le Cloirec, P. Adsorption of several metal ions onto a low-cost biosorbent: Kinetic and equilibrium studies. Environ. Sci. Technol. 2002, 36, 2067−2073. (42) Shen, C.; Wang, Y.; Xu, J.; Luo, G. Chitosan supported on porous glass beads as a new green adsorbent for heavy metal recovery. Chem. Eng. J. 2013, 229, 217−224. (43) Martins, A. C.; Pezoti, O.; Cazetta, A. L.; Bedin, K. C.; Yamazaki, D. A. S.; Bandoch, G. F. G.; Asefa, T.; Visentainer, J. V.; Almeida, V. C. Removal of tetracycline by NaOH-activated carbon produced from macadamia nut shells: Kinetic and equilibrium studies. Chem. Eng. J. 2015, 260, 291−299. (44) Alatalo, S.-M.; Repo, E.; Mäkilä, E.; Salonen, J.; Vakkilainen, E.; Sillanpäa,̈ M. Adsorption behavior of hydrothermally treated municipal sludge & pulp and paper industry sludge. Bioresour. Technol. 2013, 147, 71−76. (45) Repo, E.; Kurniawan, T. A.; Warchol, J. K.; Sillanpäa,̈ M. E. T. Removal of Co(II) and Ni(II) ions from contaminated water using silica gel functionalized with EDTA and/or DTPA as chelating agents. J. Hazard. Mater. 2009, 171, 1071−1080. (46) Repo, E.; Warchoł, J. K.; Bhatnagar, A.; Mudhoo, A.; Sillanpäa,̈ M. Aminopolycarboxylic acid functionalized adsorbents for heavy metals removal from water. Water Res. 2013, 47, 4812−4832. (47) Powell, K. J. IUPAC Stability Constants Database, SC-Database: York, U.K., 2000. (48) Yu, J.; Tong, M.; Sun, X.; Li, B. Enhanced and selective adsorption of Pb2+ and Cu2+ by EDTAD-modified biomass of baker’s yeast. Bioresour. Technol. 2008, 99, 2588−2593. (49) Sun, Z.; Liu, Y.; Huang, Y.; Tan, X.; Zeng, G.; Hu, X.; Yang, Z. Fast adsorption of Cd2+ and Pb2+ by EGTA dianhydride (EGTAD) modified ramie fiber. J. Colloid Interface Sci. 2014, 434, 152−158. (50) Tansel, B. Significance of thermodynamic and physical characteristics on permeation of ions during membrane separation: Hydrated radius, hydration free energy and viscous effects. Sep. Purif. Technol. 2012, 86, 119−126.

(15) Hutler Wolkowicz, I. R.; Aronzon, C. M.; Pérez Coll, C. S. Lethal and sublethal toxicity of the industrial chemical epichlorohydrin on Rhinella arenarum (Anura, Bufonidae) embryos and larvae. J. Hazard. Mater. 2013, 263 (Part 2), 784−791. (16) Zhou, Y.-T.; Nie, H.-L.; Branford-White, C.; He, Z.-Y.; Zhu, L.M. Removal of Cu2+ from aqueous solution by chitosan-coated magnetic nanoparticles modified with α-ketoglutaric acid. J. Colloid Interface Sci. 2009, 330, 29−37. (17) Fan, L.; Li, M.; Lv, Z.; Sun, M.; Luo, C.; Lu, F.; Qiu, H. Fabrication of magnetic chitosan nanoparticles grafted with βcyclodextrin as effective adsorbents toward hydroquinol. Colloids Surf., B 2012, 95, 42−49. (18) Zhou, L.; Liu, Z.; Liu, J.; Huang, Q. Adsorption of Hg(II) from aqueous solution by ethylenediamine-modified magnetic crosslinking chitosan microspheres. Desalin. 2010, 258, 41−47. (19) Monier, M.; Abdel-Latif, D. A. Preparation of cross-linked magnetic chitosan−phenylthiourea resin for adsorption of Hg(II), Cd(II) and Zn(II) ions from aqueous solutions. J. Hazard. Mater. 2012, 209−210, 240−249. (20) Zhu, Y.; Hu, J.; Wang, J. Competitive adsorption of Pb(II), Cu(II) and Zn(II) onto xanthate-modified magnetic chitosan. J. Hazard. Mater. 2012, 221−222, 155−161. (21) Sillanpäa,̈ M.; Orama, M.; Rämö, J.; Oikari, A. The importance of ligand speciation in environmental research: a case study. Sci. Total Environ. 2001, 267, 23−31. (22) Koehler, F. M.; Rossier, M.; Waelle, M.; Athanassiou, E. K.; Limbach, L. K.; Grass, R. N.; Gunther, D.; Stark, W. J. Magnetic EDTA: Coupling heavy metal chelators to metal nanomagnets for rapid removal of cadmium, lead and copper from contaminated water. Chem. Commun. 2009, 4862−4864. (23) Zhao, F.; Repo, E.; Yin, D.; Sillanpäa,̈ M. E. T. Adsorption of Cd(II) and Pb(II) by a novel EGTA-modified chitosan material: Kinetics and isotherms. J. Colloid Interface Sci. 2013, 409, 174−182. (24) Sillanpäa,̈ M.; Oikari, A. Assessing the impact of complexation by EDTA and DTPA on heavy metal toxicity using microtox bioassay. Chemosphere 1996, 32, 1485−1497. (25) Sillanpäa,̈ M., Environmental Fate of EDTA and DTPA. In Reviews of Environmental Contamination and Toxicology; Ware, G., Ed.; Springer: New York, 1997; Vol. 152, pp 85−111. (26) Deng, H.; Li, X.; Peng, Q.; Wang, X.; Chen, J.; Li, Y. Monodisperse magnetic single-crystal ferrite microspheres. Angew. Chem., Int. Ed. 2005, 44, 2782−2785. (27) Pizarro, G. d. C.; Marambio, O. G.; O, M. J.; Geckeler, K. E. Synthesis and properties of hydrophilic polymers, 10. Macromol. Chem. Phys. 2003, 204, 922−927. (28) Zhao, F.; Tang, W. Z.; Zhao, D.; Meng, Y.; Yin, D.; Sillanpäa,̈ M. Adsorption kinetics, isotherms and mechanisms of Cd(II), Pb(II), Co(II) and Ni(II) by a modified magnetic polyacrylamide microcomposite adsorbent. J. Water Process Eng. 2014, 4, 47−57. (29) Greenberg, A. E.; Clesceri, L. S.; Eaton, A. D. Standard Methods for the Examination of Water and Wastewater. In Standard Methods Online, 18 ed.; American Public Health Association: Washington, DC, 1992. (30) Costa-Júnior, E. S.; Barbosa-Stancioli, E. F.; Mansur, A. A. P.; Vasconcelos, W. L.; Mansur, H. S. Preparation and characterization of chitosan/poly(vinyl alcohol) chemically crosslinked blends for biomedical applications. Carbohydr. Polym. 2009, 76, 472−481. (31) Pangeni, B.; Paudyal, H.; Abe, M.; Inoue, K.; Kawakita, H.; Ohto, K.; Adhikari, B. B.; Alam, S. Selective recovery of gold using some cross-linked polysaccharide gels. Green Chem. 2012, 14, 1917− 1927. (32) Wang, J.; Zheng, S.; Shao, Y.; Liu, J.; Xu, Z.; Zhu, D. Aminofunctionalized Fe3O4@SiO2 core−shell magnetic nanomaterial as a novel adsorbent for aqueous heavy metals removal. J. Colloid Interface Sci. 2010, 349, 293−299. (33) Shuang, C.; Li, P.; Li, A.; Zhou, Q.; Zhang, M.; Zhou, Y. Quaternized magnetic microspheres for the efficient removal of reactive dyes. Water Res. 2012, 46, 4417−4426. 1281


or DTPA ... - ACS Publications

Recommend Documents