Research Note pubs.acs.org/IECR
Enhanced Selectivity for Heavy Metals Using Polyaniline-Modified Hydrogel Yian Zheng,†,‡ Wenbo Wang,† Gong Zhu,† and Aiqin Wang*,† †
Center of Eco-materials and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *
ABSTRACT: With Fenton’s reagent as the redox initiator, a granular hydrogel was prepared under ambient conditions, and then modified with polyaniline (PAN) to develop a composite adsorbent with enhanced adsorption selectivity for the heavy metals, such as the Cu−Ni system in this study. The results indicate that PAN-modified hydrogel shows decreasing adsorption kinetics, but with a comparable adsorption capacity for Cu(II). Moreover, the adsorption selectivity for Cu(II) exhibits an impressive improvement, with the selectivity coefficient (KCu−Ni) being enhanced by a factor of 6−7 (aniline:hydrogel = 1:20) for binary Cu−Ni and quaternary Cu−Ni−Cr−Zn systems. Finally, the adsorption and desorption characteristics were evaluated during multicycle adsorption processes.
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INTRODUCTION Hydrogels are three-dimensional (3D) structured networks of hydrophilic polymers. Because of facile preparation, tailored functionality, fast adsorption kinetics, and high adsorption capacity, hydrogels have been receiving increased attention in recent years as the excellent adsorbents for removing many pollutants from aqueous solutions, such as heavy metals,1,2 dyes,3,4 and ammonium.5,6 As the main drawback, traditional hydrogels are observed to be limited in mechanical strength and adsorption selectivity. To improve the mechanical strength, many attempts have been made over the past few decades to fabricate hydrogels with various structured networks, such as nanocomposite hydrogels,7−9 double network hydrogels,10−12 and topological hydrogels.13,14 Polyaniline (PAN) is one of the most promising conducting polymers, because of its stability, price, and ease of synthesis, and now it is combined with traditional hydrogel to obtain a conductive composite hydrogel. The incorporation of PAN into the hydrogel can not only afford the hydrogel with desired properties, such as appreciable electrical conductivity,15,16 but also result in enhanced mechanical strength.17,18 Furthermore, PAN carries phenylene and quinoid rings joined together by amine-type and/or imine-type nitrogen atoms, and until now, PAN and its composites have been used to remove Pb(II),19 Hg(II),20 Cr(VI),21 As(V),22 and organic dyes.23,24 In this study, a granular hydrogel was first prepared using Fenton’s reagent as the redox initiator under ambient conditions, and then was modified with PAN, using a “swelling-diffusion-interfacial-polymerization” method. When a thin and uniform PAN layer was formed onto the surface of this hydrogel, the resulting composite hydrogel was found to show different affinities for the heavy metals, i.e., enhanced adsorption selectivity for Cu(II) and Ni(II) in this study. As we have known, copper−nickel separation is an important topic in the copper−nickel ore processing, in that copper is generally recognized as the impurity for nickel smelting, while in other © 2013 American Chemical Society
cases, it appears more important for copper separation, recovery, and enrichment. In view of the practical importance of copper−nickel separation, it is necessary to develop such an adsorbent with good adsorption selectivity.
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EXPERIMENTAL SECTION Materials. Acrylic acid (AA, chemically pure; Shanghai Shanpu Chemical Factory, Shanghai, PRC) and aniline (AN; Guangdong Xilong Chemical Co., Ltd., PRC) were purified by vacuum distillation before polymerization. Chitosan (CTS, with an degree of deacetylation of 90% and average molecular weight of 3.0 × 105; Zhejiang Yuhuan Ocean Biology Co., Ltd., Zhejiang, PRC) and N,N′-methylene-bisacrylamide (MBA, chemically pure, Shanghai Yuanfan additives plant, Shanghai, PRC) were used as received. Ferrous ammonium sulfate (FAS, analytical grade) and hydrogen peroxide solution (H2O2, analytical grade) were provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, PRC) and Jiangsu Sanmu Group Co. Ltd. (Jiangsu, PRC), respectively. Ammonium persulfate (APS), 2,9-dimethyl-1,10-phenanthroline, and dimethylglyoxime were received from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, PRC). Preparation of Hydrogel. Typically, 0.9 g CTS was dissolved in 135 mL of distilled water containing 10.8 g AA and 0.45 g MBA in a 250-mL three-neck flask equipped with a mechanical stirrer. After stirring for 1 h at room temperature (18 ± 2 °C), 0.3 g FAS and 10 mL 3% H2O2 were added sequentially. Instantaneously, the granular hydrogel was observed, and by then the reaction was carried out for 1 h at room temperature in atmospheric oxygen. The resulting granular hydrogel was neutralized with 4.0 mol/L NaOH Received: Revised: Accepted: Published: 4957
September 20, 2012 March 14, 2013 March 19, 2013 March 19, 2013 dx.doi.org/10.1021/ie302562f | Ind. Eng. Chem. Res. 2013, 52, 4957−4961
Industrial & Engineering Chemistry Research
Research Note
In addition, the adsorption experiments were performed in a binary Cu−Ni system with the respective initial concentration of 100 mg/L and in a real electroplating wastewater collected from an electroplating plant located at Yuyao City in Zhejiang, China, with the concentration of 83.16 mg/L for Cu(II), 419.96 mg/L for Ni(II), 60.33 mg/L for Cr(VI), and 18.04 mg/L for Zn(II). For selective adsorption experiments, the concentrations of each metal ion were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES). Then, the selectivity coefficient KCu−Ni was calculated from KCu−Ni = qeCu(II)/qeNi(II) under competitive conditions. The reusability of the adsorbent was evaluated according to the following procedures: Typically, 25 mg adsorbent was contacted first with 15 mL of a 200 mg/L Cu(II) solution. After the adsorption equilibrium (4 h), the adsorbent was separated from Cu(II) solution and dispersed into 15 mL of 0.1 mol/L HCl solution for 2 h (30 °C/120 rpm) to desorb Cu(II) ions from the adsorbent. Afterward, the adsorbent was separated from the acid solution, neutralized with 0.1 mol/L NaOH solution for 10 min, and washed with distilled water several times. The recovered adsorbent was applied for another adsorption, and a similar procedure was performed in succeeding cycles. Characterization. FTIR spectra were recorded on a Thermo Nicolet NEXUS spectrophotometer, using KBr pellets in the range of 400−4000 cm−1. The morphology of the hydrogel with and without PAN modification was observed by scanning electron microscopy (SEM), using a JSM-5600LV instrument (JEOL, Ltd.) with an acceleration voltage of 20 kV, after the sample was coated with gold film.
solution to neutral pH, dehydrated with industrial alcohol, and dried at room temperature. Preparation of PAN-Modified Hydrogel. The dried hydrogel was placed in a 1.0 mol/L HCl solution containing an appropriate amount of aniline for 4 h to achieve the complete swelling process. The hydrogel was kept constant at 4 wt %, and the weight ratio of AN monomer to the hydrogel was varied (0, 1:20, and 1:10). Afterward, the oxidation of AN was initiated using APS solution with an APS:AN molar ratio of 0.5:1. This process was maintained for 20 h at room temperature. The resulting green PAN/hydrogel composite hydrogel was washed thoroughly with distilled water until the supernatant became colorless. Finally, the product was washed with industrial alcohol and dried at room temperature. As a comparative study, PAN was also prepared according to the above procedure, except for the addition of hydrogel. The samples used for adsorption experiments had a particle size in the range of 40−80 mesh. Determination of Swelling Degree. In 15 mL of solution containing 100 mg/L Cu(II) and 100 mg/L Ni(II), or real electroplating wastewater, an accurately weighed adsorbent (25 mg) was immersed for 6 h. These adsorbents were then separated from the solution by filtrating with a 100-mesh stainless screen and hung up for 30 min, to allow excess water to drain downward, by gravity, from the swollen adsorbent. The swelling degree (SD) was calculated from eq 1: SD =
ms − md md
(1)
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where ms and md are the swollen and dry weight of each adsorbent, respectively. Adsorption Experiments. In a series of 50-mL conical flasks, 25 mg of adsorbent was contacted with 15 mL of Cu(II) or Ni(II) solution. The mixtures were shaken in a thermostatic orbital shaker (Model THZ-98A) at 30 °C/120 rpm for a given time, and then the adsorbents were separated by direct filtration. The concentrations of Cu(II) and Ni(II) in the solution before and after the adsorption were measured spectrophotometrically, using 2,9-dimethyl-1,10-phenanthroline and dimethylglyoxime as the respective complexing agents. The adsorption capacity (qe) was determined according to the differences in the initial and equilibrium concentrations. The analytical study was carried out in triplicate, and the relative standard deviation was
