SORPTION OF ARSENIC BY IRON-OXIDES AND OXYHYDROXIDES IN SOILS

The concentration of arsenic has been determined in natural iron oxyhydroxides and oxide minerals extracted from soils at the Ashanti mine, Ghana, and was found to vary from 2 to 35,600 mg/kg. The highest concentration of arsenic in these phases occurred in neutral-pH oxidized clay-rich soils, up to 35,600 mg/kg, and in the oxidized surface portion of mine tailings. Where highly acidic soils occurred or reducing conditions were prevalent, lower concentrations of arsenic were measured in the iron oxyhydroxide and oxide fraction (up to 433 mg/kg). Lower concentrations of arsenic in these phases were also recorded from minerals in the organic-rich soils. The proportion of arsenic associated with amorphous iron oxyhydroxides was much greater than that associated with crystalline iron oxyhydroxide and oxide minerals within the studied sample set. In order to study the effects of pH and soil organic acids on arsenic sorption, the sorbed concentrations of As(V), As(III), monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) on goethite, hematite and lepidocrocite were measured. The sorption decreases in the order as (V) > DMAA = MMAA > As(III) below pH 7 and As(V) > As(III) > MMAA = DMAA above pH 7. Sorption decreased in the order goethite > lepidocrocite > hematite. Increased fulvic acid concentration reduced arsenic sorption on all the iron minerals. Therefore, both a drop in pH and an increase in fulvic acid increases arsenic leaching while greater sorption occurs in solutions in the pH range 4-8. From the agreement between field observations and laboratory experiments, arsenic sorption by iron oxyhydroxide and oxide minerals would appear to be controlled by Eh and pH and in the presence of other potential sorbents such as organic acids. This has important consequences for the disposal of arsenic in mine tailings where reducing conditions are common and influence the stability of As(III) in place of As(V). This leads to lower sorption and greater leaching of arsenic. Additionally, in organic-rich soils, the competition for binding sites on mineral surfaces between organic acids and arsenic species may also lead to lower levels of arsenic retention.