THE RATE OF OXYGEN ISOTOPE EXCHANGE BETWEEN SELENATE AND WATER

Selenium is an essential trace nutrient, but can be toxic when present in higher concentrations. Selenium isotopes of selenate have been used as a powerful tool for understanding the biogeochemical behavior of selenium, and oxygen isotopes also have the potential to be a useful tool, if the rate of oxygen isotope exchange between selenate and water is sufficiently slow to allow for preservation of isotope signatures produced by biogeochemical processes, as is the case for sulfate-water oxygen isotope exchange. This study has measured oxygen isotope exchange rates and equilibrium fractionation factors between selenate and water at various temperatures (10 to 80°C) and pH conditions (0 to 3), and calculated isotope exchange rates at environmental conditions.

Results to date indicate that the oxygen isotope exchange rate between selenate and water increases with decreasing pH and increasing temperature, as observed for sulfate-water isotope exchange. The half-lives (expressed as logt_1/2, hours) of isotope exchange at 50 and 25° and pH1 are 1.6 and 2.7 respectively, and increase linearly with increasing pH, with a slope of 1.5 at 50°C and 1.4 at 25°C (i.e. sub-parallel slopes). The half-lives at pH 1 decrease linearly with increasing temperature. Using these correlations, an estimated half-life under example environmental conditions (25°C, pH7) is 11.3, which is essentially identical to that of sulfate-water isotope exchange (11.3) at the same conditions (e.g. Seal et al., 2000). The equilibrium fractionation factor at 50°C, pH1 is 0.9967 which is lower than that estimated for sulfate (1.0259, Seal et al., 2000). Hence, our results indicate that the selenate-water oxygen isotope exchange rate under environmental conditions is sufficiently slow to facilitate use of the oxygen isotope composition of selenate as a tool to investigate the biogeochemical behavior of selenate.

[Seal et al., (2000), Rev. Mineral.&Geochem., MSA, vol.40, 541-602.]