Adsorption of Co(II) and Ni(II) by EDTA- and/or DTPA-modified chitosan: Kinetic and equilibrium modeling
Introduction
The increasing level of toxic metals such as Co(II) and Ni(II) that are discharged into the environment as industrial wastes, represent a serious threat to human health, living resources, and ecological systems [1]. Co(II) is present in the wastewater of nuclear power plants and many other industries such as mining, metallurgical, electroplating, paints, pigments, and electro-engineering [2]. Ni(II) is widely used in silver refineries, electroplating, zinc base casting, and storage battery industries [3].
Various technologies have been applied to remove Co(II) and Ni(II) from waste streams. These include chemical precipitation [4], chemical oxidation/reduction [5], and electrochemical treatment [6]. However, all of the above methods have disadvantages making them less technically appealing in wastewater treatment. Precipitation is ineffective and produces a lot of sludge, chemical reduction/oxidation requires extra chemicals and electrochemical treatment has high operating costs [7], [8].
One of the most effective methods for the removal of Co(II) and Ni(II) from wastewater streams is adsorption. Activated carbon has been the most popular material in wastewater treatment for heavy metal removal. However, the high cost of this material makes its application less economically attractive in industrial scale [9]. Cation-exchange resins used for Co(II) and Ni(II) removal can produce treated effluents that contain metals less than the required discharge limits [10]. However, commercial resins remain expensive materials [7]. To reduce the operational costs, the search for alternative adsorbents has intensified in recent years. For example, natural bentonite [2], orange peel [11], chitosan [12], [13], [14], [15], and anaerobic granular sludges [16], [17] have been tested for heavy metal removal. However, these materials have usually low adsorption capacities in as-received forms. To improve their performance, non-conventional materials such as chitosan needs to be modified chemically.
Due to the reactivity of amine groups and stable chelation, chitosan can be functionalized to improve its adsorption properties [15]. Chemical modification of chitosan with chelating agents such as ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA), which form very strong chelates with metal ions [18], [19] may produce adsorbents with excellent metal binding properties. The environmental fate of these chelating agents has received attention, but, when immobilized, EDTA and DTPA are not expected to be environmentally critical compounds [20]. Inoue et al. studied quite extensively adsorption of metals such as Cd, Fe, Cu, Ni, Co, and Zn by EDTA- and DTPA-chitosans [21], [22]. However, their work lacked of simulation modeling of adsorption kinetics and isotherms of the adsorbents as well as adsorption mechanism and regeneration studies. Therefore, the aim of this study was to investigate the adsorption properties of these promising materials in more detail.
In the previous study, we investigated the applicability of EDTA- and/or DTPA-modified silica gels to remove Co(II) and Ni(II) from contaminated water at optimized conditions [23]. In this work, EDTA- and DTPA-chitosans were used to adsorb Co(II) and Ni(II) from aqueous solutions. The effects of variables including the type of chelating agent, metal concentration, and pH on the adsorption capacity, selectivity and desorption properties of the modified chitosan were considered. To investigate the mechanism of adsorption the gathered experimental data was fitted to kinetic and equilibrium models. Furthermore, equilibrium behavior of modified chitosans was investigated in Co(II)/Ni(II) two-component systems and obtained data modeled using binary isotherm selected based on the modeling results of one-component systems.
Section snippets
Materials
Chitosan flakes >85% deacetylated supplied by Sigma–Aldrich had molecular weight ranging from 190,000 to 375,000 g mol−1 and 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 Co(II) and Ni(II) nitrate salts in double deionized water. Working solutions ranging from 1 to 200 mg L−1 of Co(II) or Ni(II) were prepared by diluting the stock solutions.
Characterization of modified chitosans
The presence of additional functional groups on the surface of modified chitosans was studied using FTIR spectroscopy. Absorption peaks of the carbonyl groups of amides and carboxylic groups were observed at 1629 and 1729 cm−1, respectively [22]. Surface coverage of EDTA and DTPA was calculated based on the difference between the amount of nitrogen in unmodified (42.1 g kg−1) and modified chitosans (81.2 and 82.6 g kg−1) obtained from elemental analysis. Coverages were 1.4 and 0.96 mmol g−1 for EDTA-
Conclusions
EDTA- and DTPA-modified chitosans were found to effectively adsorb Co(II) and Ni(II) from aqueous solutions. The maximum metal uptake by the EDTA-chitosan was higher (qm = 63.0 mg g−1 for Co(II) and qm = 71.0 mg g−1 for Ni(II)) than that by the DTPA-chitosan (qm = 49.1 mg g−1 for Co(II) and qm = 53.1 mg g−1 for Ni(II)). At metal concentration of 100 mg L−1, the removal of Co(II) and Ni(II) by the modified chitosans ranged from 93.6% to 99.5%. The selectivity sequence of both ions uptake Ni(II)EDTA/DTPA >
Acknowledgements
Authors are grateful to the European Commission (Brussels) for the Marie Curie Research Fellowship for Transfer of Knowledge (No. MTKD-CT-2006-042637) and Finnish Funding Agency for Technology and Innovation (TEKES) for financial support.
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