Redox effects on release kinetics of arsenic, cadmium, cobalt, and vanadium in Wax Lake Deltaic freshwater marsh soils

The impact of redox potential (EH), pH, iron (Fe), manganese (Mn), chloride (Cl−), aliphatic and aromatic dissolved organic carbon (DOC), and sulfate (SO42−) on the release of dissolved arsenic (As), cadmium (Cd), cobalt (Co), and vanadium (V) were studied in Louisiana freshwater marsh Wax Lake Delta soil (Mississippi River) using an automated biogeochemical microcosm apparatus. The experiment was conducted from reducing (−60 mV) to stepwise oxidizing (+491 mV) conditions. The initial pH was 7.4 and decreased under reducing conditions to 4.9, and remained constant during the increase of EH. Concentrations of As (1.3-120.5 μg L−1), V (0.9-48.6 μg L−1), Fe, DOC, and the specific UV absorbance increased under reducing conditions and decreased with rising EH. Release of As and V appeared to be related to changes of EH/pH, co-precipitation with Fe oxides, and the release of dissolved aromatic carbon compounds. Concentrations of soluble Cd (4.8-11.2 μg L−1), Mn, SO42−, and Cl− increased under oxidizing conditions. Release of Co (166.6-258.2 μg L−1) was related to the chemistry of Fe, Mn and DOC. Phospholipid fatty acids analysis indicated the potential for the microbial community to be involved in biogeochemical processes such as the formation of sulfides, oxidation and reduction of compounds, and the bio-methylation of elements such as As. Overall, we measured a release of As and V under anoxic conditions, while oxic conditions favored the release of Cd. These results outline concern on the potential risk of mobilization of toxic elements in temporary waterlogged soils for agricultural purposes in deltaic ecosystems.