Mobilization of arsenic by dissolved organic matter from iron oxides, soils and sediments

The arsenic contamination of aquifers has been linked to the input of dissolved organic matter (DOM). In light of this suggestion, the aim of this study was to quantify chemical effects of DOM on desorption and redox transformations of arsenic bound to synthetic iron oxide and natural samples from different geochemical environments (soils, shallow aquifer, lake sediment). In batch experiments, solutions containing 25–50 mg/L of two different types of DOM (purified peat humic acid and DOM from a peat drainage) were used as extractants in comparison to inorganic solutions. DOM solution was able to mobilize arsenic from all solid phases. Mobilization from iron oxides (maximum: 53.3%) was larger than from natural samples (maximum: 2.9%). The mobilization effect of extractants decreased in the order HCl > NaH2PO4 > DOM > NaNO3. DOM solutions, therefore, mainly targeted weakly sorbed arsenic. Mobilization was complete within 24–36 h and DOM was sorbed during incubation indicating competition for sorption sites. The same patterns were observed for different DOM types and concentrations. Addition of DOM lead to (a) enhanced reduction (maximum 7.8%) and oxidation (6.4%) of arsenic in aqueous solution and (b) the appearance of arsenite in aqueous phase of soil samples (5.5%). As the primary mechanism for the arsenic release from solid phases we identified the competition between arsenic and organic anions for sorption sites, whereas redox reactions were probably of minor importance. The results of this study demonstrate that sorption of DOM has a strong potential to mobilize arsenic from soils and sediments.