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

Abstract

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.

Arsenic mobility in the amended mine tailings and its impact on soil enzyme activity

“…They reported that water-extractable As in Feamended soil was reduced from 459 to 40 lg kg -1 . Gemeinhardt et al (2006) reported from a column test that up to 90% of leachable As in Fe-amended soil was immobilized. Also, the addition of Fe-halide salts to contaminated soils decreased the bioaccessibility of As in a physiologically based extraction test (Subacz et al 2007).…”

“…Reducing the mobility of As in soil by the addition of amendments has mostly been evaluated via chemical distribution from sequential extraction tests (Gemeinhardt et al 2006). Although this approach has some advantages, such as simple and rapid decisions, it does not provide useful information on As toxicity with respect to soil biological quality as a result of changes in soil enzyme activities ), which have previously been used as good indicators for predicting the changes in the soil quality and biological toxicity due to As (Dick 1997).…”

An integrated sampling/speciation method for inorganic arsenic in soil solution

“…Arsenic in soils is sorbed mainly onto surfaces of clay minerals and Fe, Al, and Mn oxides (Jain and Loeppert, 2000;Grafe et al, 2001;Goldberg, 2002;Kinniburgh and Cooper, 2004). Solubility-controlling mineral phases are Mn arsenates (Tournassat et al, 2002), Ca arsenates (Bothe and Brown, 1999), and Fe oxides, e.g., scorodite or arsenopyrite (Thornburg and Sahai, 2004;Gemeinhardt et al, 2006).…”

Immobilisation of soil toxic metals by repeated additions of Fe(II) sulphate solution

“…Addition of Fe(II), as single step treatment, has already been proposed to remediate As and Cr contaminated soils (Moore et al, 2000;Warren and Alloway, 2003;Gemeinhardt et al, 2004) showing that FeSO 4 can greatly reduce mobility and plant availability of As and Cr. Hartley et al (2004), testing different forms of Fe treatments added to three As contaminated soils, found that Fe(II) and Fe(III) were the most effective at reducing the concentration of As in leachates.…”

“…No explanation for this phenomenon was given. Also Warren and Alloway (2003) and Gemeinhardt et al (2004) observed a significant increase in toxic metal solubility after Fe(II) additions, but attributed it to insufficient lime application and/or to a temporary drop in soil pH. To date, there have been no studies on the potential of multiple FeSO 4 additions to reduce the availability and mobility of metals in contaminated soils.…”