Biogeochemical Factors Governing Cobalt, Nickel, Selenium, and Vanadium Dynamics in Periodically Flooded Egyptian North Nile Delta Rice Soils

The mobility of water soluble cobalt (Co), nickel (Ni), selenium (Se), and vanadium (V) was determined in fluvial (F) and lacustrine (L) soils used for rice (Oryza sativa L.) production in the northern portion of the Egyptian Nile Delta. The impact of redox potential (EH), soil pH, dissolved organic carbon (DOC), dissolved aromatic carbon compounds (DAC), iron (Fe), manganese (Mn), and sulfate (SO42−) on the dynamics of the studied metals was quantified in soil suspensions using an automated biogeochemical microcosm apparatus. The experiment was conducted stepwise from reducing (−307 mV) to oxidizing (+564 mV) soil conditions. We found a significantly positive correlation between soil EH and pH in both soils. Concentrations of Co, Ni, Se, DOC, Fe, and Mn were higher under reducing conditions than under oxidizing conditions. This suggests that the changes of EH/pH, Fe, Mn, and DOC might be linked to the dynamics of Co, Ni, and Se in both soils. The specific UV absorbance (SUVA) and concentrations of V were higher under oxidizing conditions than under reducing conditions especially in the F soil. This result implies that release of aromatic carbon compounds might be related to release of V in this soil. Our findings suggest that a release of Co, Ni, Se, and V in temporally flooded rice soils should be considered due to increased mobility and the potential environmental risks including food security in using metal-enriched soils for flooded agricultural systems.