The Effects of Nitrate on the Oxidation and Mobilization of Selenium

Winfield G. Wright, U.S. Geological Survey

Peter B. McMahon, U.S. Geological Survey

The influence of nitrate on the oxidation and mobilization of selenium in 
ground water presently is under investigation by the Department of the 
Interior National Irrigation Water Quality Program at the Gunnison/Grand 
Valley study area in western Colorado. Environmental data, thermodynamic 
relations, and preliminary laboratory experiments indicate that nitrate in 
ground water can oxidize and mobilize selenium. Ground-water-quality data from 
irrigated land underlain by seleniferous marine shale in the study area (and 
from other irrigation studies in Colorado and Wyoming) indicate that 
selenium concentrations are positively correlated with nitrate; selenium and 
nitrate also are positively correlated in surface water. Thermodynamic 
relations for selenium oxidation indicate that elemental selenium could be 
oxidized to selenite (SeIVO3) and selenate (SeVIO4) by nitrate and nitrite, 
and theoretical energies for some of the reactions nearly equal the energy 
available for oxidation of selenium by oxygen.

Preliminary laboratory-batch experiments were performed on shale-residuum 
samples using different concentrations of nitrate with near-zero-dissolved-
oxygen concentrations; these preliminary results are inconclusive yet 
promising. Rate data indicate an increase of SeVIO4 with a concomitant 
decrease of SeIVO3 over a two-week period. Batch-sediment samples that were 
autoclaved exhibited oxidation-rate curves similar to unautoclaved samples 
possibly indicating an abiotic oxidation process. The selenium oxidized by the 
experiments (about 50 micrograms per liter) comprised only 2 to 5 percent of 
the total selenium in the solid-phase samples (ranging from 2.5 to 4.8 
micrograms per gram determined by total digestion) indicating that most of the 
selenium occurs as insoluble, coprecipitated clay minerals or oxyhydroxides. 
It was difficult to completely sparge dissolved oxygen from control samples 
(dissolved-oxygen concentrations were about 0.35 milligrams per liter); 
these control samples contained selenium concentrations ranging from 15 to 
50 micrograms per liter. Work is continuing to determine the importance of 
selenium oxidation by nitrate relative to selenium oxidation by dissolved-
oxygen concentrations in ground water.