Mobilization of Zn upon waterlogging riparian Spodosols is related to reductive dissolution of Fe minerals

Contaminated riparian soils can release metals to surface water. Periodic waterlogging affects metal mobility but the processes and soil factors governing net trends are not well understood. Experiments were combined with geochemical modelling to identify processes explaining the dynamics of zinc (Zn) in contaminated soils following waterlogging. Samples were collected from 12 Spodosols near streams in a metal-contaminated area and four similar but uncontaminated soils were sampled in a reference area. Air-dried samples were submerged and incubated under N2. The soil redox potential decreased from 470 mV initially to approximately 30 mV over 2 months. The pore-water Zn concentrations surprisingly increased over the same period by, on average, a factor of 18 (range 0.6–80; immobilization in one soil only) despite an increase in pH of 1.8 units, on average. Dissolved organic matter (DOM) in the soil solution increased during waterlogging but the observed increase in Zn solubility could not be explained by increased complexation with DOM, because the estimated Zn2+ activity also increased by a factor of 18 on average (range 0.2–82). Speciation modelling suggests that Zn mobilization during waterlogging results from Fe2+ displacing sorbed Zn2+ from particulate organic matter and from dissolution of Zn-bearing Fe/Mn oxyhydroxides. This hypothesis is supported by the significant positive correlation (r = 0.87, n = 13) between the factor change in pore-water Zn concentration and the ratio of dithionite-extractable Fe to organic carbon content. These results show that Fe dynamics are important for predicting the fate of trace metals in anoxic soils.L. Van Laer, F. Degryse, K. Leynen and E. Smolder