还原性含水层原位反应生成纳米磁铁矿固砷机理

Elevated arsenic (As) levels in groundwater unsafe for human consumption have been found in many countries including China, threatening the health of > 100 million people. Most of the exposed population are affected by low to moderate levels of arsenic (<100 ug/L) with low occurrence rate (<10% in a village). For the dispersed rural population, communal water supply is too expensive to be practical despite its success in addressing highly exposed populations in villages with high occurrence rates. Very often household arsenic water treatment units have been used among the dispersed population. Yet there are many problems with household treatment, with frequent treatment failures. Recent advances in nano-material water treatment research have shown that several iron oxides based materials are capable of treating arsenic in water and waste water, and have the advantage of being cost effective with low re-contamination risks and high efficiency for arsenic removal. However, the removal mechanisms for arsenic are not always well understood. ..Recently nano-particulate magnetite has been shown to immobilize arsenic in laboratory microcosm and column studies by the project team. Although this recent advance represents a crucial first step towards successful in situ arsenic treatment in reducing groundwater that may be applicable to household water treatment, it is still necessary to evaluate the mechanism of arsenic immobilization through in situ experiments primarily because of hydrogeological and biogeochemical heterogeneities that are hallmarks of high arsenic aquifers, and questions remain whether nano magnetite can be formed in situ, and whether they are effective at arsenic removal, and if so, what the mechanisms are. ...The in situ experiments will be conducted at these wells using a single-well push-pull test strategy. Yinchuan Plain is selected as our study site because both groundwater As and Fe, as well as sediment (HCl-leachable) As and Fe, are correlated, suggesting that this is a system in which As mobility may be primarily controlled by Fe mineralogy. To ascertain whether magnetite forms and immobilizes arsenic during multiple push (injection of ferrous iron and nitrate) then pull (pumping out residual ferrous iron and nitrate) phases, groundwater and sediment samples will be collected before and after experiments for geochemical and mineralogical measurements. The mineralogical measurements will include X-ray absorption spectroscopy (XAS), chemical extractions, magnetic separation etc, to characterize the iron minerals including magnetite in the wells as they change, and to characterize how arsenic is immobilized by the changes in iron mineralogy. Before the field experiments, reactive transport modeling will be conducted to optimize experimental design. After the field experiments, the model will be refined based on the geochemical and mineralogical collected data from the experiments. Modeling results will provide a quantitative in-depth analysis of the experimental data, and can be used to evaluate the potential of in situ remediation at future sites. Modelling can and will also be used to optimize the efficiency and longevity of the scheme by simulating and comparing different variants of operational modes and to study how well the variants work under the imapct of hydrogeological and geochemical heterogeneity...If successful, this represents a breakthrough of arsenic in situ remediation that addresses a common contaminant of concern in aquifers from both anthropogenic and geogenic sources.

高砷地下水威胁到1亿人的健康。但成本低且可靠、适合单井使用除砷技术在国内外都不成熟。纳米材料是近年来重金属水处理研究的热点，其中各类氧化铁纳米材料具有成本低、除砷效果好、环境再次污染风险低等优点。项目组最新实验室研究发现纳米磁铁矿可以在还原条件下固定砷，但固砷机理不详。同时，高砷含水层的还原性和空间异质性对纳米磁铁矿形成会有影响，原位实验很有必要。原位实验拟以中国地质调查局银川平原5孔监测井(井深8–30米，砷浓度35–164微克/升)为对象，单井重复注入和泵出亚铁和硝酸盐，围绕井下滤水段形成约一米直径纳米磁铁矿反应带,并对试验前后地下水和沉积物样品进行深入细致化学及矿物学分析。结合反应性传输模型，阐明原位形成的纳米磁铁矿固砷微观机理，揭示含水层空间异质性对纳米磁铁矿形成的影响规律，为原位除砷提供理论依据。

背景：高砷地下水威胁到1亿人的健康。不论是发达国家还是发展中国家，都亟需成本低、可靠、适合分散型砷暴露人群使用的除砷技术，但成本低且可靠、适合单井使用除砷技术在国内外都不成熟。纳米材料中各类氧化铁纳米材料具有成本低、除砷效果好、环境再次污染风险低等优点。项目组基于实验室研究发现，纳米磁铁矿可以在还原条件下固定砷，但固砷机理不详。同时，高砷含水层的还原性和空间异质性对纳米磁铁矿形成会有影响，原位实验很有必要。..内容：项目组首先在银川平原前进农场施工完成两组YCA（2口24米和3口30米深）和YCB实验井（6口40米深）实验井的试验场地，进行了三方面研究：(1)开展单井重复注入-泵出一系列不同条件下亚铁和硝酸盐溶液的原位现场实验三次，对纳米磁铁矿原位形成及除砷条件实现了优化；（2）采集试验前后数百组地下水和近百份沉积物样品进行高时空分辨率的地球化学及矿物学分析，用EXAFS等现代光谱技术，获取了砷于纳米磁铁矿微观界面结构，查明了微观砷固定机理;（3）建立并完善了反应性传输模型，模拟纳米磁铁矿物形成和转化的主要地球化学和水文物理过程，识别高砷含水层空间异质性对砷的迁移和固定的影响机制。..结果：项目初步证实了在还原性含水层中注入亚铁和硝酸根原位固砷的有效性，揭示了含水层空间非均质性对次生矿物包括黄铁矿、磁铁矿形成的影响机制。基于单井抽注的三次原位实验研究发现，处理后地下水砷在一段时间范围内维持浓度<10 μg/L，并随抽水量的增加呈现出非平衡吸附模式；构建的结合经典表面络合模型与双域质量转换方法相的反应性传输模型模拟发现，形成的磁铁矿在沉积物颗粒表面和颗粒之间的分布存在非均质性：颗粒表面上的磁铁矿可以直接与地下水中的砷反应，除砷效率高；而颗粒之间分布的磁铁矿对砷的吸附则受限于砷从颗粒周围孔隙空间扩散到颗粒内部的动力学过程，因此效率低。..意义：成果对砷迁移转化过程的解析和地下水砷修复方法的设计具有较强的科学理论价值和实际指导意义。