Controlled variation of redox conditions in a floodplain soil: Impact on metal mobilization and biomethylation of arsenic and antimony

An automated biogeochemical microcosm system allowing the control of redox potential (EH) in soil suspensions was used to assess the effect of EH on the mobility of cadmium (Cd), copper (Cu), nickel (Ni), zinc (Zn), iron (Fe), and manganese (Mn) as well as on the methylation of arsenic (As) and antimony (Sb) in a contaminated and slightly acidic floodplain soil. The experiment was conducted under stepwise variation from reducing (approximately −300 mV at pH 5) to oxidizing (+ 600 mV at pH 5) conditions. The EH was found to be an important factor controlling the dynamics of studied compounds and elements. Concentrations of Cd, Cu, Mn, Ni, and Zn in solution were low at low EH and increased with rising EH what might be attributed to the interaction with dissolved organic carbon (DOC), Mn, and precipitation as sulphides. Redox potential and pH correlate significantly with Cd, Ni, Cu, Zn, and Mn. Total Fe concentrations in solution were high at low EH and dropped sharply at EH > 350 mV at pH 5 to lower values due to the formation of Fe (hydr)oxides. Other metals did not adsorb to or co-precipitate with Fe, which may be attributed to the low pH (between 4.4 and 5.3) amongst other factors. Concentrations of inorganic arsenic (Asi) and antimony (Sbi), momomethyl arsenic (MMAs), monomethyl antimony (MMSb), and dimethyl arsenic (DMAs) in solution decreased significantly with rising EH, indicating that low EH promotes the mobility of these compounds.

Research Highlights
► The redox potential (EH) was found to be an important factor controlling the dynamics of metals and biomethylation of arsenic (As) and antimony (Sb) in a floodplain soil.
► Concentrations of heavy metals were low at low EH and increased with rising EH due to possible interactions with manganese (Mn), dissolved organic carbon (DOC), and precipitation as sulphides. Interactions between heavy metals and Fe (hydr)oxides are less pronounced in this study due to the prevailing acidic conditions.
► The mobility of As and Sb increases under reducing conditions. Low EH promotes the formation of monomethyl arsenic (MMAs), monomethyl antimony (MMSb), and dimethyl arsenic (DMAs).
► The biogeochemical microcosm system suits well to assess the mobility of heavy metals and the biomethylation of As and Sb in frequently flooded soils.