Soil moisture influences performance of selected stabilizing amendments in soil remediation

Abstract

Various stabilizing amendments are used and tested for immobilization of metals and metalloids in contaminated soils. Although the soil moisture level may fundamentally drive these stabilizing agents' performance, most laboratory studies do not provide consistent data for different moisture regimes. In addition, the possible influence of applied stabilizing materials on the soil's physical and hydraulic properties and associated changes in soil chemistry is virtually unknown. For this reason, this study aimed to investigate the stabilizing potential of two immobilizing agents (amorphous Mn oxide – AMO, and AMO-biochar composite – AMOchar) toward As, Cd, Pb, and Zn in two contaminated soils at two different soil moisture regimes (unsaturated and saturated conditions). At the same time, changes in saturated hydraulic conductivity (Ks), soil swelling, and saturated water content were evaluated. The soil moisture conditions essentially affected all the studied parameters, including both physical and chemical properties. Under the saturated condition, the contents of As, Cd, Pb and Zn in soil solution were elevated compared to the unsaturated condition. The stabilizing effectiveness of the AMO/AMOchar strongly depended on the moisture regime (saturated vs. unsaturated conditions) and type of contaminant based on its binding mechanism in soil. The release of Cd and Zn was reduced by AMO and AMOchar after 10 weeks under both moisture regimes due to an induced pH increase, decreasing the contents of Cd and Zn in soil solution in maximum by 87% (AMOchar, saturated conditions) and 74% (AMO, unsaturated conditions), respectively. Under the saturated conditions, only AMO was effective at immobilizing Pb and As, decreasing their release by 58% and 53%. The application of AMOchar led under full saturation to a significant redox (Eh) drop (minimum value of 181 mV), followed by the dissolution of soil Fe oxides and the associated release of bound Pb and As. As a result, the contents of Pb and As in soil solution increased compared to control by 78% and 248%, respectively. This decline in Eh may be a consequence of the observed AMOchar swelling. However, additional factors, such as changes in the soil microbiology, probably contributed to the overall effect. The study demonstrates the fundamental role of soil moisture content in the stabilizing agents' performance and highlights the need for future interdisciplinary studies aiming at physical/geochemical/biological aspects of soil systems together.