Selenium Oxidation and Release during Washing of Agricultural Seleniferous Soil Samples.

Selenium (Se) has emerged as an environmental contaminant, released from geological sources by human activity, such as mining or agricultural irrigation. Selenium exists as four oxidation states (-II, 0, IV and VI), each characterized by unique toxicity, mobility and environmental impact. The most soluble and toxic Se species are the oxyanions selenate (Se(VI) as SeO ₄ ^2- ) a nd (hydro)selenite (Se(IV) as SeO ₃ ^2- ). Under reducing conditions elemental selenium (Se(0)) and selenide (Se(-II)) are the main Se species, both of which are insoluble at near-neutral pH. Oxidation and leaching of Se from selenium-rich soils has been thought to be an important selenium-mobilizing process. Here, we present initial findings on Se oxidation and mobilization by c hanges in speciation during washing of agricultural seleniferous soil samples from Punjab, India (Se concentrations ranging from 0.3 to 3.6 mg/kg Se). Release of metals is also investigated. Washing experiments were performed on ten soils from two soil profiles with and without 850 mM H ₂ O ₂ as an oxidizing agent at 28°C. Selenium species were monitored in the liquid phase over 5 days. The results show a logarithmic increase of total Se release with stabilization after 12 hours if washed with a synthetic groundwater, and after 48h in the same groundwater in presence of oxidizing agent. The rate of release Se ranges from 0.19 to 1.64 ‰ of the total Se depending on the soil horizon and time. Selenium concentrations in solution are positively related with initial concentrations in soils and Se oxidation is clearly visible with a higher release rate in presence of H ₂ O ₂ for all soil horizons. The results will provide the first quantification of Se release and change of speciation depending of the soil horizon with and without H ₂ O ₂ as an oxidizing agent during washing of seleniferous soils in Punjab.