Dynamics of Fe, Mn, and Al Liberated from Contaminated Soil by Low-Molecular-Weight Organic Acids and Their Effects on the Release of Soil-Borne Trace Elements

Abstract

A 15-day batch experiment was conducted to investigate the behaviours of Fe, Mn, and Al oxides upon attack by three common low-molecular-weight organic acids, and their effects on liberation of trace elements from a multi-contaminated soil. While the capacity of malic acid to mobilize soil-borne Fe, Mn, and Al was weaker compared to citric and oxalic acids, a similar trend was observed, showing that the concentration of dissolved Fe, Mn, and Al increased with increasing duration of the experiment. Marked increase in metal concentrations only took place after 5 or 7 days of the experiment. For the same organic acid treatment, Fe, Mn, and Al all showed a very similar temporal variation pattern. The concentration of dissolved Fe, Mn, and Al was markedly controlled by the total Fe, Mn, and Al contained in the soil, respectively. It appears that manganese oxides were more reactive to the organic acids, as compared to their Fe and Al counterparts. However, when multiple organic acids were present, the soil-borne Fe, Mn, and Al were mobilized rapidly within the first 5 or 7 days of the experiment and then tended to decrease. The formation of insoluble Fe, Mn, and Al organic complexes tended to be enhanced due to co-existence of multiple organic acids, resulting in the re-immobilization of the dissolved Fe, Mn, and Al. The organic acid-driven dissolution of Fe, Mn, or Al had a major control on the mobilization of As, Cr, Zn, Ni, Cu, and Cd that were bound to these oxides with a correlation coefficient being frequently greater than 0.9 for As, Cr, Zn, and Ni.