Abstract Tungsten is an emerging contaminant of interest with serious health implications whose environmental transport and fate have scarcely been studied. Sorption to mineral particles is likely the primary process controlling tungsten's mobility in oxidizing soils and aquifers, but the few papers published thus far have not yet resolved the relationships between aqueous speciation, substrate phases, and sorption complexes at conditions relevant to these settings. In many cases, these relationships are best investigated with techniques such as X-ray spectroscopy, but the low concentrations of tungsten in typical contaminated settings limit the applicability of that approach. Here, we lay initial groundwork for eventual use of the stable isotope geochemistry of tungsten to infer sorption mechanisms at field-relevant conditions, because metal stable isotope systematics are sometimes very sensitive to speciation and surface complexation geometry. We report the first tungsten stable isotope fractionations for soil-relevant conditions using simple batch experiments, in which tungsten interacted with either birnessite (Mn oxyhydroxide) or ferrihydrite (Fe oxyhydroxide) nanoparticles, at pH 5 or 8 and very low ionic strength. In 120-h experiments, in which proportions of sorbate and sorbent were varied, lighter isotopes of W preferentially sorbed on ferrihydrite. On a plot of δ183/182W against % W sorbed, parallel lines fit the pH 8 data from ferrihydrite experiments well, with a best fit Δ183/182Wdissolved-sorbed of +0.39‰ (R2 = 0.95), indicating an equilibrium fractionation (reversible sorption in a closed system). Parallel, linear trends also fit the data from ferrihydrite experiments at pH 5 well, with a best-fit fractionation of +0.32‰ (R2 = 0.93). When error bars are propagated rigorously, these two Δ183/182Wdissolved-sorbed values are indistinguishable, indicating that fractionation is insensitive to pH in the W-ferrihydrite system over the relevant range of pH and suggesting that sorption mechanism may also not vary with pH. In every birnessite experiment, lighter isotopes preferentially sorbed, but fractionation magnitude varied systematically with fraction of W sorbed, with larger fractionations at larger fraction of total W sorbed. At pH 8, the range in Δ183/182Wdissolved-sorbed was +0.12 to +0.40‰, with an average of 0.31‰, and at pH 5, the range was +0.29 to +0.67‰, and the average was +0.47 ± 0.10‰. Isotope behavior and sorption mechanisms appear to be different for birnessite and more complicated compared to ferrihydrite; neither parallel lines (equilibrium trends) nor Rayleigh trends obviously fit all the data well. This observation led us to consider the possibility of a kinetic isotope effect superimposed on equilibrium fractionation, where the expressed mixture of kinetic and equilibrium fractionation varies with both surface loading and time. To explore further, we conducted experiments at pH 5 with fixed W/birnessite proportions and durations up to 504h. Results indicate that the amount of W sorbed to birnessite continues to increase gradually for at least 3 weeks. The magnitude of isotope fractionation also appears to increase gradually over time, from ~0.3‰ at ≤48 h. to ~0.6‰ at >100 h. We use multiple lines of indirect evidence to hypothesize that W initially sorbs directly to birnessite surfaces, but gradually a polymeric W-O surface precipitate phase assembles and grows, as observed in previous publications for W sorption on other substrates. In future work, we will attempt to confirm this hypothesis with X-ray spectroscopic analysis. Once the relationship between isotopic fractionation and sorption mechanism(s) are constrained, use of tungsten isotopes to probe sorption mechanisms at field-relevant concentrations of tungsten may be possible. In addition, inclusion of sorption-driven isotope systematics in reactive transport models may be useful in tracing W mobility in contaminated soils and sediments.

中文翻译：

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钨稳定同位素在低离子强度下吸附到 Fe 和 Mn 羟基氧化物时的差异行为

摘要 钨是一种新兴的污染物，对健康具有严重的影响，其环境迁移和归宿鲜有研究。矿物颗粒的吸附可能是控制氧化土壤和含水层中钨迁移率的主要过程，但迄今为止发表的少数论文尚未解决在与这些环境相关的条件下水相、基质相和吸附复合物之间的关系。在许多情况下，最好使用 X 射线光谱等技术研究这些关系，但在典型污染环境中钨的低浓度限制了该方法的适用性。在这里，我们为最终使用钨的稳定同位素地球化学来推断现场相关条件下的吸附机制奠定了初步基础，因为金属稳定同位素系统有时对形态和表面络合几何非常敏感。我们使用简单的批量实验报告了第一个用于土壤相关条件的钨稳定同位素分馏，其中钨在 pH 5 或 8 和非常低的离子强度下与水钠锰矿（羟基氧化锰）或水铁矿（羟基氧化铁）纳米颗粒相互作用。在 120 小时的实验中，吸附剂和吸附剂的比例不同，较轻的 W 同位素优先吸附在水铁矿上。在 δ183/182W 对 % W 吸附的图上，平行线很好地拟合了来自水铁矿实验的 pH 8 数据，最佳拟合 Δ183/182W 溶解吸附为 +0.39‰ (R2 = 0.95)，表明平衡分馏（可逆吸附）在封闭系统中）。平行，线性趋势也很好地拟合了来自 pH 5 的水铁矿实验数据，最佳拟合分馏为 +0.32‰ (R2 = 0.93)。当误差线严格传播时，这两个 Δ183/182W 溶解吸附值无法区分，表明在相关 pH 范围内，W-铁水体系中的分馏对 pH 不敏感，并表明吸附机制也可能不随 pH 变化。在每个水钠锰矿实验中，较轻的同位素优先被吸附，但分馏量级随吸附的 W 分数而系统地变化，更大的分馏在总 W 吸附的比例较大时。pH 8时，Δ183/182W溶解吸附范围为+0.12～+0.40‰，平均值为0.31‰，pH 5时，范围为+0.29～+0.67‰，平均值为+0.47±0.10 ‰。水钠锰矿的同位素行为和吸附机制似乎不同，而且与水铁矿相比更为复杂；平行线（均衡趋势）和瑞利趋势显然都不能很好地拟合所有数据。这一观察结果使我们考虑了动力学同位素效应叠加在平衡分馏上的可能性，其中表达的动力学和平衡分馏混合物随表面载荷和时间而变化。为了进一步探索，我们在 pH 5 下进行了实验，其中固定的 W/水钠锰矿比例和持续时间长达 504 小时。结果表明吸附到水钠锰矿上的 W 量继续逐渐增加至少 3 周。同位素分馏的幅度似乎也随着时间的推移逐渐增加，从 ≤ 48 小时时的 ~0.3‰。在 >100 小时时达到 ~0.6‰。我们使用多条间接证据来假设 W 最初直接吸附到水钠锰矿表面，但逐渐聚合 WO 表面沉淀相组装和生长，如先前出版物中观察到的 W 在其他基材上的吸附。在未来的工作中，我们将尝试通过 X 射线光谱分析来证实这一假设。一旦同位素分馏和吸附机制之间的关系受到限制，就可以使用钨同位素来探测与现场相关的钨浓度下的吸附机制。此外，将吸附驱动的同位素系统学纳入反应迁移模型可能有助于追踪受污染土壤和沉积物中的 W 迁移率。但逐渐地，聚合物 WO 表面沉淀相聚集并生长，如先前出版物中观察到的 W 在其他基材上的吸附。在未来的工作中，我们将尝试通过 X 射线光谱分析来证实这一假设。一旦同位素分馏和吸附机制之间的关系受到限制，就可以使用钨同位素来探测与现场相关的钨浓度下的吸附机制。此外，将吸附驱动的同位素系统学纳入反应迁移模型可能有助于追踪受污染土壤和沉积物中的 W 迁移率。但逐渐地，聚合物 WO 表面沉淀相聚集并生长，如先前出版物中观察到的 W 在其他基材上的吸附。在未来的工作中，我们将尝试通过 X 射线光谱分析来证实这一假设。一旦同位素分馏和吸附机制之间的关系受到限制，就可以使用钨同位素来探测与现场相关的钨浓度下的吸附机制。此外，将吸附驱动的同位素系统学纳入反应迁移模型可能有助于追踪受污染土壤和沉积物中的 W 迁移率。一旦同位素分馏和吸附机制之间的关系受到限制，就可以使用钨同位素来探测与现场相关的钨浓度下的吸附机制。此外，将吸附驱动的同位素系统学纳入反应迁移模型可能有助于追踪受污染土壤和沉积物中的 W 迁移率。一旦同位素分馏和吸附机制之间的关系受到限制，就可以使用钨同位素来探测与现场相关的钨浓度下的吸附机制。此外，将吸附驱动的同位素系统学纳入反应迁移模型可能有助于追踪受污染土壤和沉积物中的 W 迁移率。