Equilibrium fractionation and isotope exchange kinetics between aqueous Se(IV) and Se(VI)

Abstract

The selenium (Se) isotope system has been proposed as a redox proxy in environmental and paleoceanographic studies. However, Se isotope exchange among various Se species can potentially interfere with redox-related isotope signatures, and is still poorly understood. In this work, we investigated Se isotope exchange kinetics and equilibrium fractionations between aqueous Se(IV) and Se(VI) under various experimental conditions. At pH = 7, low-Se concentration experiments (0.026 mM Se(IV) and 0.026 mM Se(VI)) at 25 °C, 38 °C and 60 °C were conducted for 900 days, while high-Se concentrations (0.13 mM Se (IV) and 0.14 mM Se(VI); 1.3 mM Se(IV) and 1.4 mM Se(VI)) at 60 °C were conducted for 1547 days. All experiments did not reach isotopic equilibrium, with observed Se isotope fractionations <0.20‰. Adding an electronic shuttle (Anthraquinone-2, 6-disulfonate) did not increase the isotope exchange rate. These results show that under the experimental conditions examined, the isotope exchange reaction between aqueous Se(IV) and Se(VI) is extremely slow.The exchange kinetics between Se(IV) and Se(VI) were also investigated using a 82Se tracer. The exchange rates (R) at 0.13 mM Se(IV) and 0.13 mM Se(VI) at 25 °C, 38 °C and 60 °C were determined to be ≤6.34 × 10−10 M day−1, ≤1.12 × 10−09 M day−1 and ≤1.17 × 10−09 M day−1, respectively. Using the upper bound for the isotope exchange rate at 25 °C and theoretically calculated equilibrium fractionations, and assuming a first order isotope exchange reaction between Se(IV) and Se(VI) by analogy to the sulfur system, the timescale of isotope exchange between aqueous Se (IV) and Se (VI) in a natural lake (Sweitzer Lake, Colorado, USA) was estimated. The minimum half-time (t1/2, time to reach 50% isotopic equilibrium) and the minimum time for detectable isotope exchange (tmin) are ≥440,000 and ≥18,000 years, respectively. In the modern oceans, t1/2 and tmin are ≥51 million and ≥3.6 million years, respectively. These timescales are much longer than the residence time of Se in Sweizer Lake (2.4 years) and the modern ocean (26,000 years). Therefore, when using Se isotopes to trace the biogeochemical cycle of Se in lakes and oceans, the effect caused by isotope exchange between aqueous Se(IV) -Se(VI) systems is insignificant.