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 (t_1/2, time to reach 50% isotopic equilibrium) and the minimum time for detectable isotope exchange (t_min) are ≥440,000 and ≥18,000 years, respectively. In the modern oceans, t_1/2 and t_min 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.

Introduction

Selenium is a redox sensitive element with four main valences in nature: -II (Se²⁻, HSe⁻, and organic Se), 0 (Se0), IV (SeO₃²⁻, HSeO₃⁻, and H₂SeO₃) and VI (SeO₄²⁻, HSeO₄⁻ and H₂SeO₄) (Robberecht and Grieken, 1982, Cutter, 1982, Brookins, 1988, Johnson, 2004, Zhu et al., 2004). Se(IV) and Se(VI) are the dominant species in oxic environments (McNeal and Balistrieri, 1989; Johnson et al., 2000, Herbel et al., 2002, Clark and Johnson, 2010, Qin et al., 2017a, Qin et al., 2017b). However, in reducing environments, Se oxyanions (Se(IV) + Se(VI)) can be reduced to elemental Se and the metal selenides (Oremland et al., 1989, Frankanberger and Benson, 1994, Shaker, 1996, Zhu et al., 2004, Zhu et al., 2012, Clark and Johnson, 2010). Elemental Se and selenides are almost insoluble in water and thus Se is nearly immobile in reducing environments. In contrast, Se oxyanions are soluble and mobile, and they can be adsorbed to iron-manganese oxides and organic matter (Balistrieri and Chao, 1990, Zhang and Sparks, 1990, Saeki and Matsumoto, 1994, Saeki et al., 1995, Pezzarossa et al., 1999, Catalano et al., 2006, Duc et al., 2003, Duc et al., 2006, Rovira et al., 2008). Since Se(IV) is more easily adsorbed than Se(VI) (Balistrieri and Chao, 1990, Duc et al., 2006, Mitchell et al., 2013), a large proportion of Se(IV) is absorbed onto sediments, which makes Se (VI) predominating in lakes and oceans. The ratio of Se (IV)/Se (VI) is generally less than 1 in rivers and surface seawater (Measures and Burton, 1980, Measures et al., 1984, Cutter and Bruland, 1984, Aono et al., 1991, Cutter and Cutter, 1995, Cutter and Cutter, 1998, Cutter and Cutter, 2001, Cutter and Cutter, 2004, Hung and Shy, 1995, Yao and Zhang, 2005, Chang, 2017).

Selenium has six stable isotopes: ⁷⁴Se (0.89%), ⁷⁶Se (9.37%), ⁷⁷Se (7.64%), ⁷⁸Se (23.77%), ⁸⁰Se (49.61%) and ⁸²Se (8.73%). The study of Se isotope fractionation can be dated back to 1962, when Krouse and Thode (1962) first measured Se isotope fractionations (δ^82/76Se_reactant - δ^82/76Se_product = 15‰) during reduction of Se(VI) to Se(0) using NH₂OH at room temperature. Johnson (2004) systematically summarized Se isotope fractionation induced by oxidation, reduction, adsorption, methylation and biological assimilation. The reviewed studies found Se isotope fractionations caused by abiotic and/or biotic reduction of Se(VI) to Se(IV) and Se(IV) to Se(0) to be 3.90–11.04‰ and 8.25–12.6‰ (relative to NIST SRM 3149), respectively (Johnson et al., 1999, Johnson and Bullen, 2003, Herbel et al., 2000, Ellis et al., 2003), and by oxidation of elemental Se and Se(IV) to Se(VI) to be less than 1‰ (Johnson et al., 1999, Schilling et al., 2015). The fractionation of Se isotopes during adsorption processes is relatively small, generally less than 1‰ (Johnson et al., 1999, Mitchell et al., 2013). The △^82/76Se (δ^82/76Se_solution-δ^82/76Se_solid) of Se(IV) adsorbed onto iron oxide is less than 1.0‰, while △^82/76Se for the Se(VI) adsorbed onto iron oxide is less than 0.2‰ (Mitchell et al., 2013, Xu et al., 2020). Selenium isotope fractionation in assimilation and methylation for Se oxyanions by algae, phytoplankton and large plants is small (△^82/76Se < 1.5‰) (Johnson et al., 2000, Herbel et al., 2002, Clark and Johnson, 2010), but it is larger (<4.5‰) during the methylation and volatilization processes by fungi (Schilling et al., 2011, Schilling et al., 2013).

The selenium isotope system has been applied to tracing the biogeochemical transformations of Se in various environmental settings (Johnson et al., 1999, Johnson et al., 2000, Herbel et al., 2002, Rouxel et al., 2004, Clark and Johnson, 2010, Layton-Matthews et al., 2013, Zhu et al., 2014, Schilling et al., 2015), to reconstructing the redox conditions of the atmosphere-ocean system over various spatial and temporal scales (Mitchell et al., 2012, Mitchell et al., 2016, Wen et al., 2014, Pogge von Strandmann et al., 2015, Stüeken et al., 2015a, Stüeken et al., 2015b, Stüeken et al., 2015c, Stüeken, 2017, Kipp et al., 2017, König et al., 2019), and to investigating the origin and evolution of Earth’s volatiles (Labidi et al., 2018, Yierpan et al., 2018, Yierpan et al., 2019, Varas-Reus et al., 2019). However, as one of the fundamental prerequisites for understanding the application of Se isotopes, isotope exchange between different valence states of Se has received little attention. Studies on isotope exchange for other elements such as Cr, U, Mg and Fe has found significant equilibrium isotope fractionations (Wang et al., 2015a, Wang et al., 2015b, Zink et al., 2010, Johnson et al., 2002, Welch et al., 2003, Skulan et al., 2002, Wiesli et al., 2004, Beard et al., 2010, Wu et al., 2010, Wu et al., 2011, Gorski et al., 2012, Frierdich et al., 2014a, Frierdich et al., 2014b, Reddy et al., 2015, Li et al., 2011, Li et al., 2015, Shahar et al., 2008, Macris et al., 2013). Meanwhile, in the past 60 years, equilibrium isotope exchange and kinetic effects between different speciation of sulfur have been extensively studied and applied (Mcdonald, 1961, Igumnov, 1976, Sakai and Dickson, 1978, Sakai, 1983, Fossing et al., 1992, Eldridge et al., 2016). However, studies on equilibrium isotope fractionation between different Se speciation have been limited to theoretical calculations (Krouse and Thode, 1962, Schauble, 2004, Li and Liu, 2011).

Selenium (IV) and Se(VI) are the two most predominant species in the surface environment, and they can coexist for a long time. For example, the residence times of Se in one lake was reported to 2.4 years (Clark and Johnson, 2010) whereas in the ocean it was estimated at 26,000 years (Large et al., 2015). First-principles calculations showed that equilibrium Se isotope fractionation between Se(IV) and Se(VI) at 25 °C is 13.78‰ (for HSeO₃⁻- SeO₄²⁻) or 13.28‰ (for SeO₃²⁻- SeO₄²⁻) (Li and Liu, 2011), but the timescales for detectable isotope exchange are unknown. Therefore, experimental determination of the equilibrium fractionation and isotope exchange kinetics between Se (IV) and Se (VI) are of great significance to the application of Se isotope system to tracking geochemical processes. In this work, we investigated Se isotope exchange kinetics between HSeO₃⁻ and SeO₄²⁻ at pH = 7 under different experimental temperatures and at different Se concentrations. An electronic shuttle (Anthraquinone-2, 6-disulfonate, AQDS) was also tested for its capability of promoting electron transfer between Se(VI) and Se(IV) and thus enhancing isotope exchange rates. A ⁸²Se tracer was used in some of the experiments to increase the ability to detect slow isotope exchange.