Fast colloidal and dissolved release of trace elements in a carbonatic soil after experimental flooding

- Some metals (Ba, Co, Cr, Ni, V) are released mainly in dissolved form, from anoxic soil.
- We explain the minor release (< 50%) in colloidal form by association with FeCO3/MnCO3.
- Another group of metals (Ag, Cd, Cu, Pb) shows rapid colloidal (> 80% colloids) release.
- We explain this release by microbial effects and metal sulfide formation.
- Partitioning of most metals changed during flooding to more bioavailable fractions.

Many floodplains worldwide are contaminated with a variety of potentially toxic trace metals. The flooding of such plains causes decreases of the soil redox potential and strongly affects the mobility of stored metals. We conducted a 40-day incubation experiment by flooding a carbonatic soil in a microcosm and determining Eh, pH, H2S, organic and inorganic C, ion composition, and metal concentrations in bulk soil solution (< 10 μm) and in the dissolved fraction (< 0.02 μm). Furthermore, we partitioned metals in the soil solid phase four times during the incubation. When Eh decreased, Group I metals (Ba, Co, Cr, Ni, and V) were constantly released (< 50% as colloids) closely correlated with the release of Fe and Mn by reductive dissolution of (oxyhydr)oxides. Colloidal release of Group I metals is probably attributable to coprecipitation with and sorption to colloidal Fe and Mn minerals. Group II metals (Ag, Cd, Cu and Pb) showed a peak in metal concentrations after 2-6 days, strongly dominated by the colloidal fraction (> 80%; 0.02-10 μm). With continued flooding, partitioning of metals in the soil solid phase changed and a larger fraction of the total metal concentrations became exchangeable, i.e. bioavailable. We demonstrate markedly different release patterns for two different groups of trace metals and significant changes in metal partitioning in the soil solid phase in response to artificial flooding.