Chemical immobilisation of arsenic in contaminated soils using iron(II)sulphate—advantages and pitfalls

Chemical immobilisation of inorganic contaminants by increasing the sorption capacity of soils and/or promoting the formation of sparingly soluble precipitates may be a cost-effective approach to counteract groundwater pollution. This study focuses on the enhanced retention of arsenic in two contaminated soils by addition of solid iron(II)sulphate. Four lab-scale column experiments were performed under unsaturated conditions with subsoil material sampled at a former timber preservation site and a pigment production plant. Arsenic effluent concentrations indicated 89.9 to 99.8% immobilisation in the treated columns. Sequential extractions showed a shift in contaminant binding forms towards the iron(hydr)oxide and residual fractions. Possible immobilisation mechanisms are the precipitation of FeAs phases, the formation of inner sphere complexes, and/or the occlusion of arsenic in newly formed amorphous/crystalline iron oxides. Bromide breakthrough curves point to the fact that the addition of iron(II)sulphate only moderately affects soil hydraulic properties. In contrast to reduced emissions of arsenic, increased seepage water concentrations were observed for other trace elements (e.g., cobalt, nickel, zinc). Mass balances indicate that this effect is primarily related to the temporary pH-drop caused by the oxidation of ferrous iron. The results show that chemical immobilisation using iron(II)sulphate is a promising way to protect groundwater quality at sites contaminated with timber preservation and pigment production remnants. As a prerequisite, optimum amendment levels need to be established and practical/field tests should be accompanied by a monitoring for a broad range of relevant trace elements.