Arsenic(V) adsorption by immobilized iron mediation. Modeling of the adsorption process and influence of interfering anions

Development of techniques for the removal of As(V) from aqueous matrixes is an important issue due to the high toxicity of this element and its falling World demand that converts As-containing by-products into an important problem for many producers. In this paper, iron(III)-chelated iminodiacetate resins have been prepared and studied for the adsorption of As(V) from aqueous solution in concentrations up to 1000 ppm. Adsorption capacity for As(V) has been determined for adsorbents with different amounts of iron in the range 0.5 > pH > 7.0. A maximum in adsorption has been found in the pH range 1.30-1.80. In this range, adsorption of As(III) is negligible indicating the possibility of separating these two species with the studied sorbents. Sorption of As(V) has been found to increase with the amount of iron chelated in the resin up to a certain capacity of the resin to load Fe(III). For the higher content of iron in the resin, the adsorption of As(V), expressed in mmol As/mmol Fe, reaches a value close to 0.5. This result indicates an adsorption stoichiometry of 2:1 (Fe:As). A ligand-exchange model based on this stoichiometry has been satisfactorily applied, attributing to the compound (R-Fe)(2)HAsO4 as the moiety responsible for As(V) adsorption, with a log K = 4.8+/-0.6. Interference of other anions on the adsorption of As(V) has shown to follow the affinity pattern of these anions for Fe(III): phosphate much greater than sulphate > chloride congruent tonitrate. This interference produces a decrease of the adsorption of As(V) but, except for the case of phosphate, the use of this adsorbent in presence of the mentioned anions is effective for the removal of As(V). (C) 2002 Elsevier Science B.V. All rights reserved.