Arsenic sorption by humic acids

An investigation of the sorption of As(III) and As(V) species by two humic acids (HA I and HA II) has shown that As uptake varies with pH, adsorbate concentration and ash content of the substrate. At fixed pH, the amount sorbed conformed to a Langmuir relationship, with calculated capacities in the region of maximum uptake (∼pH 5·5) being of the order of 70 (90) mmol kg⁻¹ for As(III) and 90 (110) mmol kg⁻¹ for As(V). The higher values (in parentheses) reflect the higher ash and Ca contents of HA II. Uptake decreased when the ash content was reduced by acid-washing or sample purification, or when F⁻ or EDTA was added, indicating that Ca and polyvalent cations can be involved in As retention. The dominant solution species involved appear to be H₃AsO₃ and H₂AsO₄⁻ and retention of these was subject to competition from other anions, in particular H₂PO₄⁻, and, to a lesser extent, CO₃²⁻ and SO₄²⁻. The environmental significance of the results has been considered.