Speciation of Arsenic under Dynamic Conditions

In periodically flooded soils, reductive conditions can occur, which favor the dissolution of Fe (hydr)oxides. Fe (hydr)oxides such as goethite are important sorbents for arsenate (As-V), which is the dominant As species in Soils under aerobic conditions. Hence, the dissolution of Fe (hydr)oxides under reductive conditions can result in the mobilization and reduction of As-V and, thus, in an increase in the bioavailability of arsenic. The temporal dynamics of these processes and possible re-sorption or precipitation of arsenite (As-III) formed are poorly understood. Under controlled laboratory conditions, the temporal change in the redox potential and arsenic speciation with time after a simulated flooding event ill a quartz-goethite organic matter substrate, spiked with As-V, was examined. During a period of 6 weeks, substrate solutions were sampled weekly using micro-suction CLIPS and analyzed for pH, As-III and As-V, Fe, Mn and P concentrations. Redox potentials and matric potentials were determined in situ in the substrate-bearing cylinders. The redox potential and the ratio between As-III and As-V concentrations remained unchanged during the experiment without organic matter application. With organic matter applied, the redox potential decreased and the As-III concentrations in the Substrate Solution increased while the total As concentrations in the substrate solution strongly decreased. An addition of goethite (1 g/kg) per se led to a decrease of the total As in the substrate Solution (almost 50%). In respect to the potential As availability for plants, and consequently, the transfer into the food chain, the results are difficult to evaluate. The lower the total As concentrations in the Substrate solution, determined with decreasing redox potential, the least plant AS uptake will occur. This effect may however be compensated by a shift of the molar P/As-V ratio in the Solution in favor of As-V which is expected to increase the As uptake.