Selenium isotope fractionation during adsorption onto montmorillonite and kaolinite
Selenium isotope fractionation during adsorption onto montmorillonite and kaolinite
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DOI：
10.1016/j.clay.2021.106189
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发表时间：
2021-09
影响因子：
5.6
通讯作者：
Wenpo Xu;H. Qin;Jian‐Ming Zhu;T. Johnson;Decan Tan;Chengshuai Liu;Y. Takahashi
中科院分区：
文献类型：
--
作者：
Wenpo Xu;H. Qin;Jian‐Ming Zhu;T. Johnson;Decan Tan;Chengshuai Liu;Y. Takahashi
吸附作用是制约硒在环境中全球循环的重要地球化学过程。然而，硒在粘土矿物上吸附过程中的同位素分馏却鲜有报道。在这项研究中，硒同位素分馏过程中吸附到蒙脱石和高岭石的吸附实验和扩展X射线吸收精细结构（EXAFS）光谱相结合的研究。结果表明，在pH 4.5时，硒（VI）在蒙脱石和高岭石上的吸附量很小，同位素分馏可忽略不计。而Se（IV）在蒙脱石和高岭石上吸附时的同位素分馏（Δ82/76 Se溶解-吸附）均≤ 0.23‰，表明较轻的Se（IV）同位素优先吸附在粘土矿物上。此外，离子强度对Se（IV）同位素分馏的影响在高岭石和蒙脱石之间存在差异，这可能是由于两种粘土矿物之间不同的表面电荷和结构所致。这些很少或没有硒同位素分馏可能与硒氧阴离子主要通过外层络合作用吸附在蒙脱石和高岭石上有关，硒K边EXAFS分析表明。该研究结果有助于进一步识别和约束导致Se同位素变化的关键地球化学过程，为Se同位素作为自然环境中Se地球化学循环的代用指标奠定了基础。
Adsorption is an important geochemical process constraining the global cycling of selenium (Se) in the environment. However, Se isotope fractionation during adsorption onto clay minerals has been rarely reported. In this study, Se isotope fractionation during adsorption onto montmorillonite and kaolinite was investigated by a combination of adsorption experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy. Results showed that a small adsorption and negligible isotope fractionation were observed during Se(VI) adsorption on montmorillonite and kaolinite at pH 4.5. By contrast, Se isotope fractionations (Δ82/76Sedissolved-adsorbed) during the adsorption of Se(IV) on montmorillonite and kaolinite were ≤ 0.23‰, suggesting that lighter Se(IV) isotopes are preferentially adsorbed onto clay minerals. In addition, different effect of ionic strength on Se(IV) isotope fractionation was found between kaolinite and montmorillonite, which is likely ascribed to distinct surface charge and structure between the two clay minerals. These little or no Se isotope fractionations could be related to the fact that Se oxyanions are mainly adsorbed on montmorillonite and kaolinite via the outer-sphere complexation, as revealed by Se K-edge EXAFS analysis. The findings from this study would help further identify and constrain the key geochemical processes causing Se isotope variations, providing a foundation to develop Se isotope as a proxy for biogeochemical cycling of Se in the natural environment.