Removal of selenate from irrigation drainage water using zero -valent iron

Reuse of drainage waters originating from irrigated agriculture is gaining in popularity due to greater demands for limited water resources. Elevated levels of selenium in irrigation drainage waters of the San Joaquin Valley in California resulted in the death and reproductive failure of birds during the early 1980. Selenium treatment methods have included biological systems, wetlands, chemical and physical removal and source reduction. Most studies conducted to reduce the selenate concentration in drainage waters have focused primarily on the use of microbiological systems. In contrast, this research involved the use of zero-valent iron as the electron source in the reduction of selenate.;The first objective was to establish and characterize a laboratory system to study the reduction of selenate with zero-valent iron. The second objective was to determine the effect of ionic strength and ionic composition, O 2, and pH on the rate of selenate reduction. The third objective was to determine a mass balance and a partitioning of the reacted Se in solution and on the solid phases.;The zero-valent iron used in the reaction was acid washed prior to use to remove oxide coatings. Stirred batch reactors were selected to study the reduction reaction. The pH, ionic strength, ionic composition and Eh were varied to encompass the typical water qualities found in irrigation drainage water. X-ray absorption near edge structure spectroscopy was used to identify the oxidation state of Se following reactions with zero-valent iron.;Selenate was rapidly reduced by zero-valent iron, exhibiting pseudo first-order kinetics under anaerobic conditions. The chemical parameters studied in these experiments affected the rate of selenate reduction and indicated that a decrease in pH increased the first-order reduction rate constant under anaerobic conditions. Similar behavior was seen in aerobic systems but the first-order rate constant was greater under aerobic than anaerobic conditions. The rate of Se reduction in irrigation and drainage water from the Imperial Valley of California was slower than with simpler laboratory solutions. A mixture of Se oxidation states is the likely end product of the short-term reactions between zero-valent iron and selenate. Additional studies to determine the end products after long-term exposure to selenate-containing drainage water are recommended.