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** minus **1 for As(III) and 90 (110) mmol kg** minus **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** minus or EDTA was added, indicating II) Ca and polyvalent cations can be involved in As retention. The dominant solution species involved are identified.
