Fe3O4/cyclodextrin polymer nanocomposites for selective heavy metals removal from industrial wastewater
Highlights
► CD polymer grafted magnetic nanoparticles are used for selective heavy metals removal. ► CD polymer grafted on MNP enhances the metal uptake due to its complexation ability. ► The adsorption affinity is in the order of Pb2+ > Cd2+ > Ni2+ in single-metal system. ► CDpoly-MNPs can preferentially adsorb Pb2+ ions in multi-metal system. ► Adsorption mechanism, desorption and reusability of CDpoly-MNPs has been explored.
Introduction
Water contamination due to toxic heavy metals has attracted significant attention because of their detrimental effects on the environment and human health. Heavy metal ions such as lead (Pb2+), cadmium (Cd2+), and nickel (Ni2+) are toxic and carcinogenic at even relatively low concentrations. Heavy metals are non-biodegradable and they can accumulate in living organisms. They are generally discharged to the environment from various industrial activities such as smelting, electroplating, painting, mining, leather tanning, alloy and battery manufacturing, etc., posing a significant threat to the environment and public health (Ozay, Ekici, Baran, Aktas, & Sahiner, 2009). Therefore, reliable methods are necessary for the removal of heavy metals from aquatic environment.
A great deal of effort has been devoted to the effective removal of heavy metal ions from environmental matrices. Various treatment techniques available for the removal of toxic metals are adsorption, chemical precipitation, ion exchange, coagulation, reverse osmosis, electrolysis and membrane process, etc. (Gupta and Suhas, 2009, Gupta et al., 2006a, Heidari et al., 2009, Xu et al., 2011). However, among all these methods, adsorption is considered an effective, efficient and economic method for the removal of pollutants from wastewater (Ali and Gupta, 2007, Feng et al., 2010, Gupta and Nayak, 2012, Gupta and Rastogi, 2009, Gupta et al., 2006b, Gupta et al., 2007c, Gupta et al., 2007a, Gupta et al., 2007d, Gupta et al., 2010a, Gupta et al., 2010b, Gupta et al., 2011a, Mittal et al., 2008). This technique can be applied frequently on large scale, as it can handle fairly large flow rates, producing a high quality of water without producing notorious sludge, residual contaminants, etc. (Gupta et al., 2007b, Gupta et al., 2011b). Moreover, adsorption is universal and fast in nature and applicable for the removal of organic and inorganic pollutants even at low concentration. Recent research focused on adsorbents such as activated carbons, zeolites, clays, biomass and polymeric materials for the removal of heavy metals (Crini, 2005). In wastewater treatment, especially in the removal of heavy metals, natural polymers, mainly polysaccharides such as chitosan, starch and its derivatives, cyclodextrins, have attracted particular attentions, due to their physico-chemical characteristics, low cost, availability and the presence of various reactive groups on the backbone chain (Crini, 2005). Natural polymers present, however some disadvantages that limit their use in practical wastewater treatment applications, such as their low surface area and difficult separation from the liquid phase. Magnetic sorbents on the other hand, have a relatively high surface area and are easy to separate and manipulate in complex multiphase systems with an external magnetic field. Their advantages prevail over the difficulties normally associated with other polymeric powdered adsorbents. A step further in developing adsorbents with superior properties would be the inclusion of magnetic particles into natural polymers, thus combining the advantages of both materials. It was reported that magnetic nanomaterials functionalized with biopolymers such as chitosan (Chang and Chen, 2005, Tran et al., 2010, Zhou et al., 2009), alginate (Bée et al., 2011, Lim et al., 2009), gum arabic (Banerjee & Chen, 2007), cellulose (Zhu et al., 2011), etc. have been used for the removal of toxic metals from aqueous solution. Recently, we have synthesized carboxymethylated β-cyclodextrin modified magnetic nanoparticles which could remove Cu2+ ions effectively from water based on complexation reactions between metal ions and carboxyl groups (Badruddoza, Tay, Tan, Hidajat, & Uddin, 2011). Cyclodextrins (CDs) are cyclic oligosaccharides consisting of 6 (α), 7 (β), 8 (γ) glucopyranose units linked together via α (1–4) linkages. They form a torus-shaped ring structure which contains an apolar cavity with primary hydroxyl groups lying on the outside and secondary hydroxyl groups inside (Szejtli, 1998). This renewable and biodegradable compound has the ability to complex various metal ions and this complexation ability can be improved by modifying CDs with suitable functional groups through esterification, oxidation reactions and cross-linking of hydroxyls outside the interior cavity (Norkus, 2009). Though β-CD/metal complexation has been used in water decontamination technique, particularly in the removal of heavy metals (Crini and Peindy, 2006, Mahlambi et al., 2010), very little attention has been paid to the adsorption selectivity of heavy metal ions on these cyclodextrin based adsorbents.
In this work, with an effort to improve the metal complexing ability and selectivity of cyclodextrin, carboxymethyl-β-cyclodextrin (CM-β-CD) polymer was synthesized and used to graft on the Fe3O4 nanoparticles surface (Scheme 1). The ligands on the crosslinked β-CD polymers were predominantly carboxyl groups, along with hydroxyl groups. This polymer grafted magnetic nanoparticles (CDpoly-MNPs) were used as easily separable, recyclable and highly selective nanoadsorbent for the effective removal of metal ions, i.e., Pb2+, Cd2+ and Ni2+ ions from contaminated water. The adsorption characteristics of CDpoly-MNPs for the removal of heavy metal ions from aqueous solution were studied in non-competitive and competitive adsorption modes. Efforts were devoted to elucidate adsorption selectivity and mechanism and to explore the recyclability of these nanoadsorbents.
Section snippets
Chemicals
Iron (II) chloride tetrahydrate (99%), iron (III) chloride hexahydrate (98%), chloroacetic acid (99%) was purchased from Alfa Aesar (MA, USA). Ammonium hydroxide (25%), lead(II) nitrate, cadmium(II) nitrate and nickel(II) nitrate were purchased from Merck (MA, USA). β-Cyclodextrin (99%) was obtained from Tokyo Chemical Industry Co., Ltd., Tokyo, Japan. All other chemicals were of analytical grade and used as received without further purification. The water in this work was Milli-Q ultrapure
Synthesis and characterization of magnetic nanoparticles
CDpoly-MNPs were synthesized by simple co-precipitation where iron precursors (Fe2+ and Fe3+) and CM-β-CD polymer were mixed together in the reaction medium. The step-by-step reaction procedures to synthesize CM-β-CD polymer modified magnetic nanoparticles are shown in Scheme 1.
The functionalization of CM-β-CD polymer on magnetic nanoparticles was confirmed by FTIR spectroscopy. Fig. 1(A) shows the FTIR spectra of CM-β-CD polymer, bare and polymer coated Fe3O4 nanoparticles in the 4000–400 cm−1
Conclusions
Superparamagnetic magnetite (Fe3O4) nanoparticles coated with CM-β-CD polymer were synthesized, which could be used as an efficient adsorbent for the removal of heavy metal ions from wastewater. The adsorption capacities of Fe3O4 nanoparticles were enhanced upon the surface modification with CM-β-CD polymer. The CM-β-CD polymer provides numerous surface carboxyl and hydroxyl groups, and hence, generates the strong affinity for metal ions. The solution pH, temperature and ionic strength affected
Acknowledgments
AZMB acknowledges the Research Scholarship of National University of Singapore. This work was financially supported by the National University of Singapore Research Fund (C279-000-003-001).
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