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Adsorption versus precipitation: Behavior of As(V) in goethite/Pb(II)/ carbonate systems

Abstract:

Recently, it has been found that in certain aerated soils contaminated with mining-metallurgical wastes arsenate ions are not predominantly bound to the surfaces of iron oxides present, as typically occurs, but form highly insoluble lead-copper-calcium metal arsenate compounds. The present work reports the fundamental geochemical conditions required for predominance of adsorption or precipitation processes in relatively simple goethite/As(V)/Pb(II)/carbonate systems. Thermodynamic modeling was performed through the coupling of accurate Triple Layer surface complexation modeling parameters of arsenate and Pb(II) adsorption on goethite, with recently-updated solid arsenate formation thermodynamic constants. Laboratory experiments were performed for different As(V)/Fe molar ratios, and confirmed the accuracy of the equilibrium model pbkp_redictions. It was found that at circumneutral pH As(V) is predominantly adsorbed on goethite only at very low total As/goethite ratios. As the ratio increases precipitation competes favorably with adsorption, provided Pb(II) occurs at similar contents as As(V)-a common situation found in mining-metallurgical wastes. © 2010 Taylor & Francis Group, London.