纳米级零价铁负载六价铬/五价砷在地下水的迁移转化

Chromium and arsenic have been among the most commonly detected environmental contaminants in soil and groundwater. In the past decade, permeable reactive barriers have been extensively studied for in-situ site remediation (i.e., in original place without extraction), exploiting the advantages of granular zero-valent iron being readily available, inexpensive, and non-toxic. In view of the observed limitations.of the granular zero-valent iron (e.g., slow reaction rates and rapid surface passivation), recent development of nano-scale zero-valent iron (NZVI) has emerged as an appealing option for in-situ site remediation. The high surface area and small particle size of NZVI present a significantly higher removal capacities and faster kinetics. However, as arsenic and chromium are rapidly adsorbed or (co-)precipitated on the NZVI surfaces, the fate and transport of chromium/arsenic-loaded NZVI presents an environmental concern. We hypothesize that under certain geochemical conditions NZVI will deposit on soil media and minimize the mobility and bioavailability of chromium/arsenic, whereas under other conditions NZVI will travel long distances as mobile colloids facilitating chromium/arsenic spreading and endangering water.supplies. Therefore, this project aims to elucidate the roles of varying geochemical conditions and heterogeneous surface characteristics in the fate and transport of chromium/arsenic-loaded NZVI. We will conduct batch and column experiments to determine the aggregation, deposition, mobility, remobilization and release of chromium/arsenic-loaded NZVI in different soil media under environmentally relevant conditions. Based on the experimental results, we will develop and calibrate a mathematical transport model for predicting chromium/arsenic environmental risk as related to NZVI behaviors in various soil media. The results of this project are expected to advance our fundamental understanding of NZVI applications and provide a tool for prudent management of the associated risks.

铬和砷普遍存在于受污染的土壤和地下水中。近年来，粒状零价铁可渗透反应格栅被广泛应用于污染场地原位修复中。鉴于粒状零价铁存在较慢的反应速率及相对快的表面钝化等限制，近期发展的纳米级零价铁（NZVI）因其较小的粒径和高比表面积的特性逐渐成为一种颇具潜力的新选择。然而，由于铬和砷可以迅速吸附或沉淀于NZVI 的表面，所以NZVI 负载铬/砷的迁移转化成为潜在的环境风险。假定在不同的地球化学条件和土壤表面特性下，NZVI 可能沉积在土壤中并且最大限度地降低铬/砷的移动性和生物有效性，或相反地移动较长的距离而导致铬/砷的扩散。因此，本项目拟通过序批实验和土壤柱实验及光谱分析来阐明NZVI 负载铬/砷在不同土壤介质与环境条件下的聚集、沉积、迁移、转运和释放，并建立和验证用于预测相关环境风险的数学运移模型。研究结果将有助于提供NZVI 应用的理论基础及可靠的风险管理工具。

本项目研究废水中主要污染物（砷等金属化合物及有机氯化物）在环境中的迁移、转化机制，以及自然地质条件和废水特点（高盐度、成分复杂、多种污染物共存、性质多变）对其环境行为的影响。评估污染物在周边土壤-水环境中的长期扩散分布情况。本项目对应用纳米级零价铁去除废水中多种金属与有机氯化物的效率及原理进行了探究。对废水性质如离子强度、酸碱度、竞争性阴离子等因素对零价铁去除污染物的影响进行深入分析。利用聚合物对纳米级零价铁表面进行改性，研究对比改性后的纳米零价铁处理废水金属污染物及有机氯化物的效果及机理。.研究结果表明，高离子强度的废水能加快金属砷和硒在土壤-水环境中的扩散迁移。其迁移率、生物可利用性、土壤微生物脱氢酶活性和磷酸单酯酶活性均随着废水离子强度的增加而显着降低。零价铁能有效去除废水中的金属化合物，然而反应所需时间较长。纳米零价铁与废水反应8小时后，随着离子强度升高，铜的去除率逐渐升高，锌的去除率逐渐降低，铬的去除率先升高后降低，砷的去除率基本不受离子强度的影响。光谱分析结果表明铜的去除原理为共沉淀和还原，提高溶液离子强度，铁溶出增加，从而促进了与铜的共沉淀过程，提高铜的去除效率。铬的去除原理为吸附、还原、共沉淀。锌和砷的去除原理为吸附。随着溶液离子强度增加，锌离子逐渐由正电性金属阳离子转化为负电性金属阴离子，从而增加了与负电性纳米零价铁表面之间的静电斥力，降低去除效率。由于砷与纳米零价铁表面形成了稳定的双键内旋结构，溶液离子强度基本不影响砷的去除效率。经聚合物改性的纳米零价铁显著提高金属化合物的去除效率，并通过吸附和化学还原去除废水中的1,1,2-三氯乙烷。.本项目研究成果阐明了废水主要污染物在环境中的迁移、转化机制，并探究了纳米零价铁处理废水的可行性和原理，对废水原位修复提供了一种安全可靠技术参考。