Potential of Fe-Mn wastes produced by a water treatment plant for arsenic immobilization in contaminated soils

This work was aimed to examine the capability of Fe-Mn wastes produced by water treatment plant for arsenic sorption and immobilization in highly contaminated soils. In a batch experiment, As (III) and As (V) sorption on Fe-Mn wastes was examined at various initial concentrations of arsenic (200-800 mg L− 1), various pH conditions (pH in the range 4.5-9.5), and different contact time (1 − 20 h). Arsenic sorption depended on As species; and the amounts of As (III) adsorbed were by several fold higher compared to As (V). The maximum sorption capacity of As on Fe-Mn material was over 40 mg kg− 1 at initial As (III) concentration 800 mg L− 1 and shaking time 2 h (22 °C). Corresponding sorption capacity of As (V) in the same conditions was only 12.3 mg kg− 1. The waste material was used in an incubation experiment with two highly contaminated soils collected from the area affected by former arsenic mining and processing. Total As concentrations in silt loam (soil I) and sandy loam (II) were 3619 and 1836 mg As kg− 1, respectively. The Fe-Mn-rich wastes were applied to soils at the rates: 0.2, 1.0 and 5.0 g d.m. per 100 g (treatments: Fe1, Fe2, and Fe3, respectively), corresponding to 7.2, 36 and 180 Mg ha− 1, respectively. Additionally, the effects of simultaneous sewage sludge application to soils were examined. Sewage sludge was applied at two rates, equivalent of 45 and 100 Mg d.m.·ha− 1 (SS1 and SS2). Soils were incubated for 5 months under changing water conditions (altering watering and drying), and thereafter As solubility in soils was determined in the extraction with 0.05 M (NH4)2SO4. Application of increasing doses of Fe-Mn wastes to both soils resulted in substantial decrease of As extractability. The presence of SS had apparently reverse impact on As solubility, and reduced the effect of immobilization particularly at lower rates of Fe-Mn wastes.