Antimony leaching from contaminated soil under changing redox conditions

Contamination of shooting range soils by antimony (Sb) released from corroding ammunition has become an issue of public concern in various countries. Many of these soils are at least occasionally subject to waterlogging; yet mechanisms controlling Sb mobility under anaerobic conditions are still poorly understood. This thesis investigated Sb leaching in contaminated soil in terms of changing redox conditions using microcosm batch, column and lysimeter experiments. The same calcareous shooting range soil was used throughout all experiments, and the level of complexity increased with each set of experiments according to the progress in mechanistic process understanding. In the microcosm batch experiment, we investigated Sb concentration and speciation dynamics in the shooting range soil by incubating soil batches under anaerobic conditions for up to 64 days. Lactate was added to some of the microcosms as an external electron donor to promote reducing conditions by enhancing indigenous microbial activity. The transition to reducing conditions at first led to the immobilization of Sb, as Sb(V) was converted to Sb(III), which has a higher sorption affinity to iron (Fe) (hydr)oxides. When reducing conditions continued, previously sorbed Sb(III) was gradually released into solution due to reductive dissolution of Fe (hydr)oxides. Speciation measurements in the solid phase by X-ray absorption near edge structure (XANES) spectroscopy and in the soil solution by liquid chromatography (LC) ICP-MS provided clear evidence that Sb(III) predominated at low redox conditions (Eh<0.05 V) in both phases (>90%). The objective of the second experiment was to investigate how changes in soil reducing conditions affect Sb release and retention under advective transport, with a special focus on the interplay of reductive Fe and manganese (Mn) mineral dissolution and Sb sorption processes. After eluting columns packed with the study soil for 5 days with 15 mM sodium lactate solution at a flow rate of 33 mm d−1, the flow was interrupted for 37 days, and then resumed for another 4 days. With the transition to moderately reducing conditions after 1 day of flow, effluent Sb (identified as Sb(V) by LC-ICP-MS) and Mn concentrations showed a concomitant increase, providing clear evidence that sorbed Sb(V) was released under moderately reducing conditions by reductive dissolution of Mn minerals. The release of Sb(V) was counteracted by the reduction to Sb(III), which was first scavenged by Fe (hydr)oxides and then slowly liberated again when the redox potential further decreased to Fe-reducing conditions. Laser ablation-ICP-MS revealed the presence of an initial pool of Sb associated with Mn-containing, Fe-free phases, underscoring the important role of the latter in addition to Fe (hydr)oxides as Sb sorbents. In last experiment, we compared Sb leaching from a calcareous shooting range soil under drained and waterlogged conditions in an outdoor experiment using four large lysimeters. Leachate samples were taken at bi-weekly intervals and supplemented by soil solution samples to link Sb leaching to the chemical processes occurring in the soil. After monitoring the leachate for >1.5 years