Microbial processes and iron-mineral formation in household sand filters used to remove arsenic from drinking water in Vietnam

砷污染的地下水和饮用水是一个全球性的环境问题，约有1-2%的世界人口受到影响。砷的饮用水上限（10微克/升）经常被超过，特别是在亚洲国家，如越南。家用砂滤器已经被用作一种非常简单和具有成本效益的处理方法，用于去除水中的砷。存在于地下水中的溶解铁（Fe（II））的氧化导致在砂滤器中形成疏溶性氧化铁（氢）颗粒（Fe（III）OOH），其结合带负电荷的砷物种并降低水中的砷浓度。亚砷酸盐（As（III）; H3 AsO 3）与金属氧化物的结合强度通常低于砷酸盐（As（V）; H2 AsO 4-/HAsO 4 2-），因此As（V）比As（III）更有效地去除。这就是为什么As（III）氧化为As（V）对于从饮用水中去除砷特别感兴趣的原因。铁和亚砷酸盐氧化细菌的活性是否以及如何有助于在家用砂滤器中有效地去除砷，目前尚不清楚。因此，本研究的目标之一是分离，鉴定和量化的Fe（II）-和As（III）-氧化微生物的过滤器，并研究其铁和砷的氧化还原活性。基于培养的工作将辅之以分子的、不依赖于培养的技术，以表征和量化在过滤器操作期间（在现场和人工实验室过滤系统）的不同时间点从不同过滤器位置采集的样品中的微生物群落。将研究分离的铁和亚砷酸盐氧化细菌沉淀铁矿物（在存在或不存在砷的情况下）和氧化亚砷酸盐的能力。生物成因和非生物成因的铁矿物形成的分离菌株在实验室中，在越南的砂过滤材料和人工实验室过滤系统将被识别和表征，也相对于砷吸附。我们将确定如何生物和非生物过程，有助于砷动员从砷负载铁矿物阶段影响过滤器的性能随着时间的推移。这项研究的长期目标是更好地了解微生物氧化还原转化过程，这些过程驱动自然和工程系统中的砷/铁矿物相互作用，例如家用砂滤器，并为改进过滤器使用和过滤材料处理提供建议。

Arsenic-contaminated ground- and drinking water is a global environmental problem with about 1-2% of the world’s population being affected. The upper drinking water limit for arsenic (10 μg/L) is often exceeded, especially in Asian countries, such as Vietnam. Household sand filters are already used as one very simple and cost-efficient treatment to remove arsenic from water. Oxidation of dissolved iron (Fe(II)) present in the groundwater leads to the formation of sparsely soluble iron(hydr)oxide particles (Fe(III)OOH) in the sand filter, which bind negatively charged arsenic species and reduce arsenic concentrations in the water. Arsenite (As(III); H3AsO3) binds generally less strong to metal oxides than arsenate (As(V); H2AsO4 -/HAsO4 2-), therefore As(V) is removed much more effectively than As(III). This is why As(III) oxidation to As(V) is of special interest for arsenic removal from drinking water. Whether and how the activity of iron- and arsenite-oxidizing bacteria contributes to effective arsenic removal in household sand filters is currently not known. One of the goals of this study therefore is to isolate, identify, and quantify Fe(II)- and As(III)-oxidizing microorganisms from filters and to study their iron and arsenic redox activities. Cultivation-based work will be complemented by molecular, cultivation-independent techniques to characterize and quantify the microbial communities in samples from different filter locations taken at various time points during filter operation (both at field sites and in artificial laboratory filter systems). The isolated iron- and arsenite-oxidizing bacteria will be studied with respect to their abilities to precipitate iron minerals (in the presence or absence of arsenic) and oxidize arsenite. Biogenic and abiogenic iron minerals formed by the isolated strains in the lab, on the sand filter material in Vietnam and in artificial laboratory filter systems will be identified and characterized, also with respect to arsenic sorption. And we will determine how biotic and abiotic processes that contribute to arsenic mobilization from arsenic-loaded iron mineral phases affect filter performance over time. The long-term goal of this research is to better understand the microbial redox transformation processes that drive arsenic/iron mineral interactions in natural and engineered systems, such as household sand filters and to give recommendations for improved filter use and filter material disposal.