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 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.

中文翻译：

---

水相 Se(IV) 和 Se(VI) 之间的平衡分馏和同位素交换动力学

摘要 硒 (Se) 同位素系统已被提议作为环境和古海洋学研究中的氧化还原代理。然而，各种硒物种之间的硒同位素交换可能会干扰氧化还原相关的同位素特征，并且仍然知之甚少。在这项工作中，我们研究了各种实验条件下 Se(IV) 和 Se(VI) 水溶液之间的 Se 同位素交换动力学和平衡分馏。在 pH = 7 时，在 25 °C、38 °C 和 60 °C 下进行了 900 天的低 Se 浓度实验（0.026 mM Se(IV) 和 0.026 mM Se(VI)），而高 Se 浓度（0.13 mM Se (IV) 和 0.14 mM Se(VI)；1.3 mM Se(IV) 和 1.4 mM Se(VI)) 在 60 °C 下进行 1547 天。所有的实验都没有达到同位素平衡，观察到的 Se 同位素分馏 还使用 82Se 示踪剂研究了 Se(IV) 和 Se(VI) 之间的交换动力学。0.13 mM Se(IV) 和 0.13 mM Se(VI) 在 25 °C、38 °C 和 60 °C 下的交换率 (R) 被确定为≤6.34 × 10−10 M day−1，≤1.12 ×分别为 10−09 M day−1 和 ≤1.17 × 10−09 M day−1。使用 25 °C 时同位素交换率的上限和理论计算的平衡分馏，并通过类比硫系统假设 Se(IV) 和 Se(VI) 之间的一级同位素交换反应，之间同位素交换的时间尺度估计了天然湖泊（美国科罗拉多州斯威策湖）中的硒 (IV) 和硒 (VI)。最小半衰期（t1/2，达到 50% 同位素平衡的时间）和可检测同位素交换的最小时间（tmin）≥440,000 和 ≥18,000 年，分别。在现代海洋中，t1/2和tmin分别≥5100万年和≥360万年。这些时间尺度比硒在斯威泽湖的停留时间（2.4年）和现代海洋（26000年）要长得多。因此，当使用Se同位素来追踪湖泊和海洋中Se的生物地球化学循环时，Se(IV)-Se(VI)水溶液系统之间的同位素交换所造成的影响是微不足道的。