The interaction of selenite and selenate with selected freshwater sediments
Lipinski, Norman George
The objective of this study was to investigate the interactions of selenate and selenite with sediments from Buffalo Pound and Katepwa Lakes in the upper Qu'Appelle River basin in southern Saskatchewan Canada. Selenite and selenate at submicrogram concentrations were added to suspensions of the lake sediments. The studies of the reactions were conducted at constant temperature. The sediment from Buffalo Pound Lake was found to rapidly oxidize selenite to selenate; the oxidation followed the Arrhenius type reaction and obeyed multiple first order kinetics. The sediment from Katepwa Lake also oxidized selenite, but at a much slower rate. Selenite was strongly adsorbed by both sediments; the adsorption decreased with increasing temperature. The oxidation of selenite by these sediments was an abiotic process. Selenate was rapidly reduced to selenite by the Katepwa Lake sediment at 25ËšC. The reduction slowed considerably at 4ËšC and was not observed at 60ËšC. The Buffalo Pound Lake sediment did not reduce selenate. Selenate was not adsorbed by the Buffalo Pound Lake sediment- but was adsorbed by the Katepwa Lake sediment after reduction to selenite. The reduction of selenate by the Katepwa Lake sediment was likely mediated by biotic activity, associated biochemical processes, and/or organic matter. Selenium transformations and its subsequent bioavailability would vary with seasonal fluctuations of temperature in freshwater ecosystems. When the sediments were treated with sodium hypochlorite to remove the organic components the redox reactions of selenite and selenate were halted. The retention of selenite by the treated sediments was decreased and the retention of selenate by the sediments was not observed. The suspension of the reduction of selenate by Katepwa Lake sediment after the treatment was attributed to the elimination of microbial activities and the loss of the organic components of the sediment. The removal of active inorganic materials by the treatment was responsible for the cessation of oxidative activity by both sediments. The sodium hypochlorite treated Katepwa Lake sediment lost more of its retentive capacity for selenite than did treated Buffalo Pound Lake sediment. These results indicate that the Katepwa Lake sediment is more dependent on the organo-mineral complexes for selenite retention than the Buffalo Pound Lake sediment. When treated with acetylacetone-benzene for the removal of sesquioxidic components- the Buffalo Pound Lake sediment was unable to oxidize selenite to selenate and selenite retention was decreased. This indicates that the Buffalo Pound Lake sediment depended on the sesquioxidic components for its oxidative ability. The capacity of the Katepwa Lake sediment to retain selenite was also decreased by the acetylacetone-benzene treatment, but the sediment was not as severely affected as when treated with sodium hypochlorite. This observation again indicates that organic matter and/or organo-mineral complexes were largely responsible for selenite retention in the Katepwa Lake sediment. The acetylacetone-benzene treatment halted the reduction of selenate to selenite by this sediment. This was attributed to the inhibition of biotic activity and the associated biochemical processes by the treatment. The Buffalo Pound and Katepwa Lakes have similar basic mineralogy, climatic conditions and natural and anthropogenic inputs. Their differences in the redox behaviors and retention of selenium compounds may be explained by the chemical and biological milieu and the associated sediment properties created by differing physical characteristics of the lakes.