微生物胞外聚合物-锰氧化物微界面上铈分配异常的分子机制

The micro-interface between microbe extracellular polymeric substances (EPS)，Mn oxide, and soil solution is one of the most reactive sites in soil, which affect the accumulation and migration behavior of rare earth element (REE). As a redox sensitive REE, Ce(III) could be oxidized by Mn oxide to less soluble Ce(IV). Therefore, a positive Ce anomaly is commonly observed on Mn oxide. In previous studies, we found that EPS could induce the formation of a negative Ce anomaly on Mn oxide. However, the coupled effect of EPS and Mn oxide on Ce anomaly formation is still unclear. In this work, the EPS and Mn oxide are used as research objects, the main chemical compounds in EPS are used as model materials. A combination of SEC-UV-ICP-MS and XAFS spectroscopy is used to clarify 1) the binding characteristics and the complex capacity between EPS-Mn oxide and Ce-EPS in a binary system; and 2) the effect of EPS as well as its key chemical compounds on chemical species and distribution coefficient of Ce in a ternary system. The final goal is to explore the coupled effect of EPS and Mn oxide on Ce anomaly formation. This knowledge is vital for understanding the diversity of Ce anomaly distribution in aquatic and terrestrial environments, also helpful for the remediation and risk assessment of Ce and nuclear waste contaminated soil.

微生物胞外聚合物(EPS)-锰氧化物与土壤溶液交界处形成的微界面是土壤中最活跃的反应位点之一，影响着稀土元素的富集和迁移。作为变价稀土元素，铈(Ce)在氧化锰表面的吸附氧化作用使其分布相对相邻稀土元素呈现正异常。前期研究发现，EPS可使Ce(IV)从氧化锰表面溶出，导致其在氧化锰表面呈现负异常。然而EPS与氧化锰耦合作用对Ce分配异常影响的分子机制仍不清楚。本项目选取EPS和氧化锰为研究对象，以EPS中常见的化学组分为对照，采用SEC-UV-ICP-MS联用技术、XAFS光谱技术等现代仪器分析手段，先获取二元体系下EPS分别与氧化锰和Ce的配位方式及络合容量；再分析三元体系下EPS及其关键组分对Ce化学形态及固液分配的影响；探究EPS与氧化锰的耦合作用对Ce异常值变化的影响机理。为合理解释表生环境中Ce异常分布变化的多样性提供理论依据，也为稀土及放射性核素污染土壤的修复和风险评价提供参考。

Ce的异常程度常常被作为指示环境氧化还原条件的指标，并对放射性物质的分布有很好的指示性。然而，环境中普遍存在的微生物及其代谢产物对Ce分配异常影响的分子机制仍不清楚。本项目采用SEC-UV-ICP-MS联用技术，分析了三元体系下EPS及其关键组分对Ce化学形态及固液分配的影响。研究发现，即便在锰氧化物的形成过程（强氧化条件），如果存在微生物产生的生物弱配体（如糖类物质），它们可以通过与锰氧化物等天然电子受体的耦合作用，使Ce呈现显著的异常的变化，从而不能有效指示环境的氧化还原条件。研究结果表明，自然界中普遍存在的生物弱配体（如糖类物质）可以通过与锰氧化物等天然电子受体的耦合作用，显著影响Ce异常的变化。该结果提醒人们在利用该Ce异常指标进行环境条件监测的时候要考虑这种特殊情况。相关研究成果发表在Nature Index期刊Geochimica et Cosmochimica Acta上。