Removal and Recovery of Zn2+ and Pb2+ by Imine-Functionalized

Nov 29, 2011 - This research investigated the adsorption of zinc and lead from binary metal solution with tunable selectivity. A nano adsorbent was pr...
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Removal and Recovery of Zn2+ and Pb2+ by Imine-Functionalized Magnetic Nanoparticles with Tunable Selectivity Guangming Zeng,*,†,‡ Ya Pang,†,‡ Zhuotong Zeng,§ Lin Tang,*,†,‡ Yi Zhang,†,‡ Yuanyuan Liu,†,‡ Jiachao Zhang,†,‡ Xiaoxia Lei,†,‡ Zhen Li,†,‡ Yiqun Xiong,†,‡ and Gengxin Xie †,‡ †

College of Environmental Science and Engineering, Hunan University, Changsha, 410082 China Key Laboratory of Environmental Biology and Pollution Control, Hunan University, Ministry of Education, Changsha, 410082 China § State Key Laboratory of Medical Genetics, Central South University, Changsha, 410078 China ‡

bS Supporting Information ABSTRACT: This research investigated the adsorption of zinc and lead from binary metal solution with tunable selectivity. A nano adsorbent was prepared by introducing imine groups onto the surface of stability enhanced magnetic nanoparticles and then characterized by TEM and FTIR. Binary metal components adsorption was carried out in different concentration of metal and EDTA solution. Due to the interaction between metals and adsorbent in the presence of EDTA, the selective adsorption of zinc and lead could be achieved with 100% selectivity. To only remove zinc from binary metals, the solution condition was [EDTA]/[M2+] = 0.7 with pH of 6, and its saturated adsorption capacity was 1.25 mmol/g. For selective adsorption of lead, an equilibrium adsorption capacity of 0.81 mmol/g was obtained under the condition of [EDTA]/[M2+] = 0.7 and pH of 2. The exhausted adsorbent could be regenerated by simple acid or alkali wash, and high purity lead and zinc salt solutions were recovered and concentrated.

1. INTRODUCTION Heavy metal pollution in water has been a serious threat to public health and ecosystems, due to their properties of nonbiodegradation, potential toxicity and carcinogenesis even at very low concentrations.1 Industrial effluents are the main sources of heavy metal pollution, in which several metal ions coexist. For example, zinc and lead commonly coexist in the industry of mineral separation, as the two share similar geochemical behaviors, which resulted in the zinc and lead combined pollution in wastewater.2 Besides, industries such as chemical, electroplating, fertilizer, and battery manufacturing also produce various amounts of wastewater containing zinc and lead ions.3 Therefore, it is necessary to effectively remove these heavy metal ions from the wastewater before their discharging and selectively separate them for resource recovery from the viewpoints of health and economy. As an alternative for heavy metal pollution remediation, adsorption has been attracting wide interests due to its convenient operation, low-energy consumption, high capacity, and easy regeneration. Various natural and synthetic adsorbents have been developed to remove heavy metals from water.4,5 Some of them exhibited relatively high affinity toward a certain heavy metal; however, the selective adsorption of metal ions for recovery and reuse is still difficult. Recently, great efforts have been made to selectively remove heavy metal ions from multicomponent solution. On the basis of r 2011 American Chemical Society

the molecular imprinted technology, Cu, Pb, Uranyl, and Cr(III) could be highly selectively separated from wastewater by developing corresponding ionic imprinted polymer.6 9 The selfassembled monolayers on mesoporous supports, whose interfacial chemistry can be fine-tuned, were able to selectively sequester specific target species, such as heavy metals, tetrahedral oxometalate anions, and radionuclides.10,11 Besides, Lam et al. revealed that amine groups grafted on MCM-41 were an effective adsorbent for uptake of noble metal gold, and the adsorbed gold was completely recovered with a purity of 99% by a simple acid wash.12 Mureseanu and Chen both reported the selective removal of Cu from wastewater using thiol groups modified mesoporous hybrid silica and imine groups functionalized cation exchange resin, respectively.13,14 Among the functional group modified adsorbents, the introduced amino, imine, and thiol have strong affinity with target heavy metals, resulting in the selective adsorption performance. In particular, adsorbents modified with amino or imine groups have been widely explored, because it not only chelates cation such as Cu2+, Zn2+, and Cd2+14 16 but also adsorbs negative charged metal complex.17 Much attention has been paid to the selective adsorption of heavy metals using these functional adsorbents.13 15 Despite these devoted efforts, the Received: September 29, 2011 Revised: November 25, 2011 Published: November 29, 2011 468

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design, synthesis, and application of selective adsorbent are far from perfection. EDTA is a commonly used chelator for separation and recovery of metal ions due to its strong metal-complexing property. The organic chelating agent had a significant effect on metal removal, as confirmed by previous researches.18 20 However, little was reported about the influence on the multimetal solution adsorption process in the presence of chelators. In this study, an imine polymer modified with magnetic nanoparticles with good stability was prepared and characterized. The selective adsorption of zinc and lead from binary metal solutions using the adsorbent was directly regulated by judicious application of EDTA, achieving 100% selectivity for either zinc or lead uptake. The adsorbed heavy metals were recovered as a high purity metal salt by simple acid or alkali wash, and the exhausted adsorbent could be regenerated for reuse.

2. MATERIALS AND METHODS

Figure 1. FTIR spectra for Fe3O4 and MNPs.

2.1. Materials. Polyethylenimine (PEI) (Mw = 20 000) and EDTA were purchased from Xiya Chemical Company. Heavy metal salts of FeCl3 3 6H2O, FeCl2 3 4H2O, Zn (NO3)2 3 6H2O, and Pb(NO3)2 were of reagent grade and used without further purification. Zinc and lead standard solutions (1000 mg/L) were applied to prepare standard curves. Ultrapure water was used throughout the whole experiments. 2.2. Preparation and Characterization of Magnetic Adsorbent. The Fe3O4 nanoparticles were prepared according to previous method21 with some modifications. Briefly, FeCl3 3 6H2O and FeCl2 3 4H2O with the molar ratio of 2:1 were dissolved in N2 gas bubbled ultrapure water. Then, 4 mol/L ammonia solution was added under vigorous mechanical stirring to adjust the solution pH to ∼11. The reaction was maintained for 30 min at 60 oC. The produced nanoparticles were separated by an external magnetic field, followed by repeated washing with ultrapure water to neutrality. Finally, they were vacuum-desiccated at 55 °C. The black Fe3O4 nanoparticles were calcined at 300 °C for 1 h and then ground to gain red-brown magnetic nanoparticles (MNPs). To introduce imine groups onto the surface of the MNPs, a methanol solution with pH of 10 was prepared to dissolve PEI. A certain amount of MNPs were added into the PEI solution and stirred for 24 h at 45 °C. Subsequently, the nanoparticles were separated and transferred to 0.5% glutaraldehyde solution for 20 min cross-link.22 Finally, the modified magnetic nanoparticles (abbreviated as NH-MNPs) were thoroughly rinsed and then dried and ground for further use. Transmission electron microscopy (TEM, JEOL JEM-1230) and X-ray diffraction (Rigaku D/max-II B) were used to examine the morphology and structure of the magnetic nanoadsorbent. FTIR spectrometer (WQF-410) was applied to characterize the preparation process of imine functionalized magnetic nanoparticle. The zeta potential and magnetization of the adsorbent were determined, respectively, by Zetasizer Nano (ZEN3600, Malvern) and vibrating sample magnetometer (VSM Nanjing University Instrument Plant). 2.3. Single and Binary Metals Adsorption. All the adsorption experiments were conducted in 100 mL glass conical flask under 150 rpm shaking at room temperature (23 to 25 °C). An amount of 0.05 g prepared adsorbent was added into 50 mL of metal ions solutions for each treatment. Single Zn(NO3)2 3 6H2O or Pb(NO3)2 solution was utilized to investigate the effect

Figure 2. TEM images for MNPs (a) and PEI-MNPs (b).

of pH on the removal of heavy metals. The binary component adsorption was carried out using equimolar of zinc and lead with or without EDTA. Adsorption of Zn2+ or Pb2+ reached equilibrium within 30 min. For the adsorption experiments of binary metals system in the presence of EDTA, the contact time was increased to 12 h to ensure the saturated adsorption, taking into the possible effect of EDTA on adsorption equilibrium time. The final concentrations of the metals in the solution were determined by AAS (Hitachi Z-8100, Japan). Three measurements were conducted for each sample with the mean values reported. 2.4. Regeneration and recovery. The solutions of 0.05 mol/L HCl and 0.04 mol/L NaOH were utilized, respectively, to regenerate the zinc and lead loaded adsorbent. In short, the heavy metals loaded adsorbent was separated from solution by external magnetic field, followed by incubating in desorption agent for 2 h under 150 rpm shaking. After that, it was separated and washed to neutrality for reuse. Besides, the desorption agent was used to wash the metal loaded adsorbent repeatedly for heavy metals enrichment.

3. RESULTS AND DISCUSSION 3.1. Characterization of Imine Functionalized Magnetic Nanoadsorbent. The red-brown of MNPs was the characteristic

color of γ-Fe2O3, which can be formed by calcining Fe3O4 in air condition.23 The XRD patterns for MNPs and Fe3O4 had no obvious differences (Figure S1). Additionally, in the FTIR 469

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Figure 4. Adsorption isotherms of Zn and Pb in binary components solution (equimolar metal ions, adsorbent dosage: 0.05 g, contact time: 0.5 h, pH 6, curves are Langmuir model).

Figure 3. Effect of pH on the adsorption of Zn and Pb (initial concentration: 1 mmol/L, adsorbent dosage: 0.05 g, contact time: 0.5 h).

pattern (Figure 1) of wavenumbers ranging from 600 to 800 cm 1, a characteristic peak of 623 cm 1 was observed, which meant the existence of typical spectra of γ-Fe2O3.23 However, no characteristic spectra of Fe3O4 were detected in this wavenumber range. These indicated that only surface Fe3O4 transformed into γ-Fe2O3 to form a shell core structure of γ-Fe2O3@Fe3O4, which was confirmed by previous studies.24,25 The MNPs were able to resist the high concentration HCl of 1 mol/L; however, the Fe3O4 was dissolved completely in 1 mol/L HCl. The enhanced stability was attributed to the formed γ-Fe2O3, which had better chemical stability to prevent the inner Fe3O4 from oxidation. The TEM images of MNPs before and after the PEI modification were presented in Figure 2. Both of them had a spherical shape with their average diameters ∼14 and 21 nm, which indicated that the two were super paramagnetic, since the size was less than 30 nm.26 This property permits the particles to separate from liquid under external magnetic field, more importantly, prevents magnetic aggregation and benefits their dispersion in solution.27,28 In addition, the pH of the zero-point charge (pHPZC) of NH-MNPs increased from 5.8 (for naked MNPs) to 11(Figure S2). The high pHPZC suggested that the adsorbent was positively charged at the entire environmentally relevant acidity (pH 3 9), which also facilitated its dispersion in liquid, and adsorption of anionic metal complex in acid environment.17 Besides, the respective magnetization for MNPs and NH-MNPs were 68.2 and 52.7 emu/g. Separation of them from aqueous dispersions can be easily finished in 5 min by permanent handheld magnet. 3.2. Single and Binary Components Adsorption. 3.2.1. Effect of pH on Single Adsorption. The pH not only influences the surface charge and the protonation degree of adsorbent, but also affects the speciation of the sorbate.3 Figure 3 showed the single component of Pb2+ or Zn2+ adsorption by NH-MNPs at pH range of 3 to 7. Adsorption of Zn2+ or Pb2+ was not observed below pH 3.0. The phenomenon could be attributed to the complete protonation of the unpaired electrons on nitrogen atom at this pH value, since divalent metal ions was adsorbed through forming dative bond with imines. Afterward, the adsorbed metal ions increased with the increase of pH value, and the qe (mmol of adsorbed metal ions per gram of adsorbent) was 0.24 and 0.48 mmol/g, respectively, for Pb2+ and Zn2+ at pH of

Figure 5. Adsorption of Zn and Pb in binary components solution as a function of EDTA concentration (initial concentration: 1 mmol/L, adsorbent dosage: 0.05 g, contact time: 12 h, pH 6).

6 and 7. At low pH value, there was significant protonation on adsorbent, large amount of H+ might compete with the metal ions for the binding sites. With the increase of pH, the competition from H+ lessened, thus, increased qe was observed. Investigation at pH over 7 was avoided to prevent precipitation of metal hydroxide complexes and hydrolytic action of metal ions. 3.2.2. Binary Metal Adsorption. Figure 4 presented the binary component of Pb2+ and Zn2+ adsorption as a function of Ce (equilibrium metal ions concentration), which was carried out in solution with equimolar metal ions at pH 6. The binary adsorption data were modeled by the Langmuir equation with the correlation coefficients r2 > 0.95 for both Pb2+ and Zn2+. The saturated uptake capacities were approximately 1.04 and 0.81 mmol/g for Zn2+ and Pb2+, respectively (The MNPs showed less than 0.2 mmol/g saturated adsorption capacity for both Zn2+ and Pb2+). The exhibited capacity difference might be relevant to the interactions between heavy metal ions and NH-MNPs. In general, the functionalized adsorbent showed preferential affinity 470

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Figure 7. Selective adsorption of Zn in binary components solution (equimolar metal ions, adsorbent dosage: 0.05 g, contact time: 12 h, pH 6).

Figure 6. Speciation diagrams for equimolar zinc and lead at pH 6 as a function of EDTA concentration.

for binding metal ions based on the type of soft or hard donor. Nitrogen had been considered as a harder donor atom compared with sulfur, indicating that the NH-MNPs exhibited stronger complexation affinity with the harder metal ion Zn2+ than relatively softer metal ion Pb2+.29,30 Although there was a difference in adsorption capacity, adsorption of Pb2+ and Zn2+ were coinstantaneous in the process. Hence, no selective adsorption occurred for the binary component solution under this condition. 3.3. Effect of EDTA on Binary Metal Adsorption. Figure 5 displayed the zinc and lead adsorption in the presence of different concentration of EDTA, which was carried out using equimolar Zn(NO3)2 and Pb(NO3)2 salts (1 mmol/L) at pH of 6. The adsorption capacity was approximate 0.54 and 0.28 mmol/g, respectively, for zinc and lead, when the EDTA concentration was less than 1.0 mmol/L (i.e., [EDTA]/[M2+] < 0.5). Subsequently, the amount of lead adsorbed almost decreased to zero as the EDTA concentration increased to 1.2 mmol/L (i.e., [EDTA]/ [M2+] g 0.6). In contrast, the zinc uptake was unchanged and responded insensitively to EDTA concentration until it increased to 1.6 mmol/L (i.e., [EDTA]/[M2+] < 0.8). In the presence of 1.2 to 2 mmol/L EDTA (i.e., 0.6 < [EDTA]/[M2+] e 1), the NHMNPs only adsorbed zinc, and exhibited 100% selectivity for zinc and lead binary component solution. The presented selective adsorption behavior was mainly attributed to the speciation of Zn (NO3)2 3 6H2O, and Pb(NO3)2 in the presence of different concentrations of EDTA (Figure 6). The speciation was calculated by Visual MINTEQ231 as a function of EDTA concentration with equimolar (1 mmol/L) Zn (NO3)2 and Pb(NO3)2 and fixed pH of 6. It could be seen that the EDTA preferentially bound to Pb2+ forming the PbEDTA2‑ complex, and binding to Zn2+ was not notable until the concentration of EDTA increased to 1.2 mmol/L. A half of Zn2+ changed into ZnEDTA2‑ when the EDTA concentration was 1.4 mmol/L (i.e., [EDTA]/[M2+] = 0.7). However, almost 100% PbEDTA2‑ appeared under this concentration. The significant speciation differences were due to the stability constant of metal ion-EDTA complexes, which were log KZnEDTA = 16.50 and log KPbEDTA = 18.04, respectively. In addition, no ZnHEDTA , PbHEDTA , ZnH2EDTA, and PbH2EDTA

Figure 8. Pb and Zn binary components adsorption in the presence of EDTA as a function of pH.

complexes were formed at pH 6. Adsorption of zinc and lead using NH-MNPs was conducted by forming dative bonds with Zn2+ and Pb2+ ions instead of negatively charged ZnEDTA2‑ and PbEDTA2‑ in this solution environment ([EDTA]/[M2+] > 0.6, pH 6). Therefore, selective adsorption of zinc was feasible in the zinc and lead binary component solution by the judicious application of EDTA chelator. 3.4. Tunable Adsorption Selectivity. 3.4.1. Selective Adsorption of Zinc. As indicated in Figure 5, the maximum selective adsorption capacity for Zn2+ was obtained in the presence of 1.4 mmol/L EDTA. Thus, the solution conditions of equimolar of Zn(NO3)2, Pb(NO3)2 and a certain concentration of EDTA (specific, [EDTA]/[M2+] = 0.7) as well as the pH of 6 were applied to selectively remove Zn2+. Figure 7 plotted the metal ions adsorption as a function of equilibrium concentration of zinc in the solution. It showed that zinc was significantly removed and the equilibrium qe of 1.25 mmol/g (calculated by Langmuir model) was slightly more than that of 1.04 mmol/g (Figure 4). Furthermore, the adsorption of zinc was maintained even at very low concentration, indicating that removal of trace amount of 471

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Figure 9. Speciation diagrams for zinc (a) and lead (b) in the presence of EDTA as a function of pH.

lead and zinc in the presence of EDTA (Figure 9, solution concentration conditions: Zn2+ = Pb2+ = 1 mmol/L; EDTA = 1.4 mmol/L). Figure 9a revealed that more than half of the zinc existed in the form of Zn2+ in the whole pH range, and on the other hand, less than 3.5% of Pb2+ existed when pH g 5 (Figure 9b). As it was demonstrated that adsorbing of Zn2+ and Pb2+ by NH-MNPs enhanced with the increase of pH value, thus selective adsorption of Zn2+ was observed above pH 5 in the presence of 1.4 mmol/L EDTA (i.e., [EDTA]/[M2+] = 0.7). This result was in line with the one concluded in section 3.2. Figure 9b showed that the PbEDTA2‑ and PbHEDTA were the dominant species below pH of 3. In particular, the negative charged complex took a proportion as high as 90% at pH of 2, however, under this pH, less than 45% negative charged zinc complex existed. Since the adsorbent possessed a large amount of imine groups on its surface due to the modification of PEI, resulting in its easy protonation at low pH solution. And the zeta potential of the positive charged adsorbent was 32.4 mV at pH of 2. From the viewpoint of electrostatic interaction, low pH was favorable to the adsorption of anionic lead complex (PbEDTA2‑ and PbHEDTA ).34 With the increase of pH, the protonation degree of the adsorbent decreased and the competition influences from negative charged zinc complex (ZnEDTA2‑) and OH groups was unavoidable. These actions resulted in the reduced uptake of lead. Therefore, nearly 100% selective adsorption of lead was observed only at pH below 2.5 under this condition. Figure 10 was the result of the binary component adsorption experiment, which was performed at pH 2 with equimolar Zn(NO3)2, and Pb(NO3)2 as well as EDTA (i.e., [EDTA]/ [M2+] = 0.7). It is clear that NH-MNPs adsorbed lead with excellent selectivity. The equilibrium adsorption capacity was 1.08 mmol/g, which was higher than that in Zn2+ and Pb2+ binary component solution (qe = 0.81, reported in Figure 4) without EDTA; that is, adding EDTA not only enhanced the selectivity significantly but also increased adsorption capacity for lead. The minor removal of zinc might come from the adsorption of ZnHEDTA by NH-MNPs. 3.5. Regeneration and Recovery. Regeneration of the adsorbent and recovery of the adsorbed heavy metals are very important for the practical application of the adsorption method. In this work, desorption of adsorbed zinc was carried out by incubating zinc loaded adsorbent in 0.05 mol/L HCl solution, and 96.1% of zinc was eluted from the adsorbent. After separating

Figure 10. Selective adsorption of Pb in binary component solution (equimolar metal ions, adsorbent dosage: 0.05 g, contact time: 12 h, pH 2).

zinc with 100% selectivity was possible. However, the adsorption amount of lead was considerably small (qe = 0.045) in the entire process, thus, it can be concluded that the excellent selective adsorption of zinc in lead and zinc binary metal solution was achieved. 3.4.2. Selective Adsorption of Lead. It is well-known that the physicochemical characteristic of the adsorbates and the surface chemistry of adsorbent are the two specific properties for control adsorption. Figure 8 provided the adsorption of zinc and lead as a function of solution pH values, which was performed using 1 mmol/L Zn(NO3)2, 1 mmol/L Pb(NO3)2 and 1.4 mmol/L EDTA. It was found that the adsorbent was able to highly effectively adsorb lead or zinc at pH < 2.5 or pH > 5, and adsorption of one metal would suppress the uptake of the others. The selectivity performances of the NH-MNPs were attributed to the different adsorption mechanism for the two heavy metals. For the adsorption of zinc, it began at weak acidic condition and gradually increased with the increase of solution pH, exhibiting a typical cationic-type adsorption behavior.32 34 For the uptake of lead in the presence of EDTA, followed ligand-type adsorption, that is, adsorption decreased with the increase of pH.32 34 The different adsorption type was ultimately due to the speciation of 472

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Langmuir and washing to neutrality, the adsorbent was able to effectively adsorb zinc or lead again, suggesting that the regeneration method did not possess any negative influences on its performances. For the reuse of lead loaded adsorbent, 0.04 mol/L of NaOH was applied to regenerate it, and 97.5% of lead was recovered in the first desorption treatment. High purity of zinc or lead (>90%) was recovered and concentrated (as high as 10 mmol/L) from the exhausted adsorbent and could be considered for recycling purpose. In addition, the refreshed NH-MNPs were able to be reused for six consecutive adsorption desorption cycles with the adsorption capacity only losing 10% in the end (Figure S3).

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4. CONCLUSIONS In this work, stability enhanced magnetic nanoparticles with the shell core structure of γ-Fe2O3@Fe3O4 were easily prepared and modified with imine groups. Selective removal of toxic zinc and lead from bimetal solutions by NH-MNPs with tunable selectivity was successfully demonstrated. The adsorption selectivity could be adjusted and achieved 100% for either zinc or lead by just adding EDTA and controlling the solution pH. The adsorbed metals were recovered and concentrated with high purity by a simple acid or alkali solution washing. In addition, the regenerated NH-MNPs retained good adsorption capacity for reuse. In many cases, the complexity and cost of treatment can be dramatically increased for the effluents containing several pollutants. The selective adsorption was an excellent alternative for the removal, recovery, and reuse of raw materials and products that would otherwise be discharged randomly into environment, resulting in a significant cost reduction for wastewater cleanup. ’ ASSOCIATED CONTENT

bS

Supporting Information. XRD spectra for Fe3O4 and MNPs and α-Fe2O3. Zeta potential of MNPs and NH-MNPs under different pH values. Regeneration results of adsorbent. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: +86-731-88822754. Fax: +86-731-88823701. E-mail: [email protected] (G.Z.); [email protected] (L.T.).

’ ACKNOWLEDGMENT This study was financially supported by the Fundamental Research Funds for the Central Universities, Hunan University, the National Natural Science Foundation of China (50608029, 50978088, and 51039001), the Hunan Key Scientific Research Project (2009FJ1010), the Hunan Provincial Natural Science Foundation of China (10JJ7005), the Hunan Provincial Innovation Foundation For Postgraduate (CX2009B080), the New Century Excellent Talents in University (NCET-08-0181), the Research Fund for the Doctoral Program of Higher Education of China (20100161110012), and the Scholarship Award for Excellent Doctoral Student granted by Ministry of Education. 473

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