Speciation-dependent studies on removal of arsenic by iron-doped calcium alginate beads

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Abstract

This work aims to study the differential attitude of Fe-doped calcium alginate (Fe-CA) beads towards As(III) and As(V) compounds so that speciation-dependent environmentally sustainable methodologies can be developed for removal of arsenic from contaminated water. Throughout the experiment, 76As has been used as precursor of stable arsenic. The affinity of As(V) towards the Fe-CA beads is greater than that of As(III). Removal efficiency of Fe-CA beads for As(V) increases with increasing number of beads and longer shaking times. At pH 3, 30 Fe-CA beads remove As(V) completely from a solution containing 20 mg kg−1 As(V). The technique has been successfully applied to the ground water collected from an arsenic-contaminated area.

Introduction

Several methods of treatment of water for arsenic removal have been proposed. The conventional methods, such as chemical precipitation (Harper and Kingham, 1992), membrane separation (Kosutic et al., 2005), and ion exchange (Balaji et al., 2005) are regularly used for the removal of arsenic from water. The other noteworthy methods include alum coagulation, iron coagulation, and lime softening (Ernest and Christopher, 1995). Adsorption of arsenic onto fly-ash (Diamadopoulos et al., 1993) also helps to get arsenic-free water. Common methods of As-removal from wastewater in metallurgical operations involve Fe(III) arsenate precipitation or arsenate co-precipitation with an excess of iron (Twidwell et al., 2005).

The techniques like reverse osmosis used to remove arsenic by membrane separation method are expensive from the socio-economic point of view. Chemical precipitation is incapable of removing trace levels of metal ions from large volumes of water. The ion exchange technique has high maintenance and operation costs (Gotoh et al., 2004). In alum coagulation method, the water contains the alum sludge after removal of As and this method requires lots of chemicals to separate As (Ernest and Christopher, 1995). Efforts are being made to develop socio-economically sustainable and environmentally favorable technologies. Also small-scale techniques and know-how are important along with development of central facilities. The use of biosorbents for removal of toxic heavy metal from water is an alternative to the existing methods.

The biosorbents like alginic acid originating from brown seaweeds have high affinity towards divalent cations (Nayak and Lahiri, 2006; Nayak, 2005). Each divalent metal ion binds to two carboxyl groups on adjacent alginate molecules (Nestle and Kimmich, 1996). Pretreatment or doping of alginic acid with cations like Ba2+ or Fe2+ allows interaction of anionic species with the alginate moiety. Earlier, for the removal of anionic arsenic from water, calcium alginate (CA) beads were placed in a column to form a fixed bed and were treated with hydrous ferric oxide. Microgram levels (50 μg L−1) of arsenic were removed from water by Zouboulis and Katsoyiannis (2002). Min and Hering (1998) suggested a method for arsenic removal using iron-doped calcium alginate beads (Fe-CA). They showed that up to 94% removal of As(V) from solution of concentration of 400 μg L−1 can be achieved by equilibration of this solution with 20 Fe–CA beads at pH 4 for 120 h. However, the very high equilibration time (120 h) is not at all practical and also both the above-mentioned methods are applicable only to arsenic concentrations of μg L−1 level. Sometimes the contamination of As in drinking water is much more than this level (Kamala et al., 2005). Considering the importance and severity of arsenic contamination in drinking water, we see that the method needs to be improved.

Again, speciation of any element profoundly influences both the toxicity and bioavailability of the element. Exposures to some forms of an element may be harmless, while other species of the same element can be toxic, carcinogenic or even mutagenic. The toxicity of arsenic compounds depends on its chemical forms. For example, elemental arsenic and arsenic (III) sulfides are inert or nearly so, while arsine (AsH3) is toxic. Arsenic compounds are in general hazardous because of their irritant effects on the skin. Inorganic As(V) and As(III) are the most toxic forms of arsenic and As(III) is more toxic than As(V). Therefore as a part of continuous endeavor from our laboratory to develop low-cost green methodologies for removal of toxic elements from water, a project has been undertaken to explore environmentally benign methodology for the removal of various toxic species of arsenic from water. Arsenic removal has been studied in laboratory conditions as well as in environmental samples, by Fe-CA beads. 76As has been used as radiotracer of arsenic throughout the experiment.

Section snippets

Chemicals used

A weighed amount of sodium arsenate (Na2HAsO4·7H2O) and sodium arsenite (NaAsO2) salts were irradiated with thermal neutrons in the CIRUS reactor, Bhabha Atomic Research Centre (BARC), Mumbai, with a neutron flux 9×1012 n cm−2 s−1 to produce 76As through 75As(n, γ) reaction in the corresponding matrix. CaCl2·2H2O and FeCl3 were obtained from Merck, India. Sodium alginate powder was procured from Sigma Aldrich.

Preparation of Fe-doped Ca-alginate (Fe-Ca) beads

Fe-doped Ca-alginate (Fe-CA) beads were prepared by mixing 0.1 M FeCl3 with 0.1 M CaCl2·2H2

pH variation

The pH is the predominating factor affecting the removal of As(V). Experiments were carried out with 30 Fe-CA beads to study the effect of pH on accumulation of As(V) and of As(III) (Fig. 1). Adsorption of As(V) increases with increasing pH in the acidic pH range, showing a maximum 96.6% adsorption at pH 3. In calcium alginate beads, Ca acts as a primary gel-forming cation, providing a stable structure for the polymer network containing carboxyl groups (COO), which serves as a binding site of

Conclusion

From the experimental results we conclude that adsorption of arsenic by Fe-CA beads is highly dependent on the oxidation states of arsenic as well as on the pH of the solution. Fe-CA beads are highly effective in removing As(V) over a wide concentration range (20–250 mg kg−1) with 30 min exposure time at pH 3. Quantitative adsorption of 20 mg kg−1 As(V) by 30 Fe-CA beads is found at pH 3, whereas adsorption of As(III) is negligible at the same experimental condition. This property can be exploited

Acknowledgement

AB sincerely acknowledges Board of Research in Nuclear Science, DAE, Govt. of India for providing a necessary fellowship.

References (18)

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