氧化铁铝取代效应影响土壤砷/镉活性的化学机制

Ferrosols is a special soil orders in Chinese Soil Taxonomic Classification, which contain a large amount of amorphous iron (hydr)oxides (IOx) and is widely distributed in China, especially in tropical and subtropical zones in Southern China. Al-substituted IOx is one of the most important feature under the effect of soil desilicification and allitization; and the isomorphous substitution of Al for Fe in the IOx crystal structures influence the soil iron biogeochemical cycles as well as the transformation of heavy metals (i.e As/Cd) in soils. The reactivity of As/Cd in soils are dominated by their speciation, which have been proved to be closely correlated with soil iron redox cycles, especially the adsorption/desorption activity as well as their bonding structures with function groups on IOx. This proposal will choose various synthesized Al-substituted IOx as the research objects, and the reactivity of As/Cd as the model probe reaction. Synchrotron radiation cooperated with various advanced analysis methods will be applied in the experiments of this proposal, whereas experiments are designed and conducted to investigate the chemical mechanism of Al-substituted IOx on the reactivity of As/Cd. In detail, (1) to study the impact of Al-substitution effect on IOx crystal structures, surface properties, adsorption sites, surface function groups, as well as the kinetic and thermodynamic adsorption/desorption trends of As/Cd on various Al-substituted IOx; (2) to illustrate the extends of As/Cd interaction on the Al-substituted IOx; (3) to investigate the effect of usual co-existential cations/anions, soil organic matters on the reactivity of As/Cd on various Al-substituted IOx and their chemical correlations; (4) to illustrate the critical biogeochemical factors like Eh, pH on the reactivity of As/Cd on various Al-substituted IOx and their chemical correlations. The expected results of this proposal will improve our general understanding in Al-substitution effects on IOx and its biogeochemical impacts on typical elements’ cycling processes; meanwhile, it will provide valuable information on soil iron redox chemistry as well as the remediation research on the polluted subtropical soils by heavy metals.

红壤是我国华南地区广泛分布的一种典型地带性土壤，剧烈的人为活动导致红壤的加速酸化，加剧土壤重金属的环境风险。红壤富含氧化铁，铝取代氧化铁是其典型成土特征，影响土壤铁的生物地球化学活性，紧密关联土壤砷、镉的环境行为。本项目以土壤典型铝取代氧化铁为模型矿物，以铝取代氧化铁影响砷/镉活性的化学作用机制为核心科学问题，综合运用同步辐射结合矿物表面物理化学、结构化学的研究方法，主要研究包括：氧化铁的铝取代效应对氧化铁晶体结构、表面性质、吸附活性位点、表面络合模型、成键结构等特征理化因子的影响；氧化铁界面砷-镉相互作用的微观界面化学过程及其与铝取代效应的定量关系；常见土壤共存阴阳离子、氧化还原电位、pH对该相互作用过程的影响机制；铝取代氧化铁调控砷/镉环境行为的关键理化因子及其矿物学结构特征；土壤氧化铁铝取代效应影响砷/镉活性的化学作用机制及矿物学机制。对丰富土壤铁循环过程的研究具有重要的科学意义。

本项目以土壤典型铝取代氧化铁为模型矿物，以铝取代氧化铁影响砷/镉活性的化学作用机制为核心科学问题，探寻氧化铁铝取代效应影响土壤砷/镉活性的化学机制。研究结果表明：①不同矿物界面(非铁矿物、铁氧化物及铝取代氧化铁矿物)上砷和镉的吸附动力学均符合准二级动力学模型，化学吸附为其控速步骤，共存阴阳离子不改变其吸附动力学模型；②镉及砷的吸附效率（吸附量/比表面积）在不同矿物界面上均呈现出含铁矿物界面大于非铁矿物界面；③铝同晶替代会改变铁矿的晶粒、结晶度、形貌、Zeta电位、表面官能团和比表面积等性质，随着铝取代率增加，铝取代针铁矿的结晶度变高、尺寸变小，比表面积减小，而铝取代赤铁矿的结晶度变弱、尺寸增大，比表面积增大，氧化铁发生铝取代后，其对镉及砷的吸附更加稳定，主要归因于铝取代氧化铁表面羟基相对含量的增加；④证实静电吸附、形成界面-砷-镉三元络合物以及形成表面沉淀是调控界面镉-砷相互作用的关键机制，阴阳离子通过改变矿物界面氧化还原电位、竞争吸附以及形成共沉淀等途径影响界面镉-砷相互作用过程，在排除pH干扰情况下，矿物界面镉及砷共吸附的调控机制主要包括竞争效应和协同效应。本研究为深入研究重金属元素在土壤矿物界面的微观化学作用过程及其调控措施研究提供借鉴。项目共发表研究论文16篇（第一标注8篇，第二标注7篇，第三标注1篇），SCI收录期刊论文10篇，中文核心期刊6篇，会议论文2篇；已投稿SCI研究论文1篇（第一标注）；项目核心成员获评环境科学副研究员资格；独立指导硕士毕业生5名，1名获“研究生国家奖学金”。