Oxidation and Mobilization of Selenium by Nitrate in Irrigation Drainage

Abstract

AbstractSelenium (Se) can be oxidized by nitrate (NO−3) from irrigation on Cretaceous marine shale in western Colorado. Dissolved Se concentrations are positively correlated with dissolved NO−3 concentrations in surface water and ground water samples from irrigated areas. Redox conditions dominate in the mobilization of Se in marine shale hydrogeologic settings; dissolved Se concentrations increase with increasing platinum‐electrode potentials. Theoretical calculations for the oxidation of Se by NO−3 and oxygen show favorable Gibbs free energies for the oxidation of Se by NO−3, indicating NO−3 can act as an electron acceptor for the oxidation of Se. Laboratory batch experiments were performed by adding Mancos Shale samples to zero‐dissolved‐oxygen water containing 0, 5, 50, and 100 mg/L NO−3 as N (mg N/L). Samples were incubated in airtight bottles at 25°C for 188 d; samples collected from the batch experiment bottles show increased Se concentrations over time with increased NO−3 concentrations. Pseudo first‐order rate constants for NO−3 oxidation of Se ranged from 0.0007 to 0.0048/d for 0 to 100 mg N/L NO−3 concentrations, respectively. Management of N fertilizer applications in Cretaceous shale settings might help to control the oxidation and mobilization of Se and other trace constituents into the environment.