The role of electron donors in arsenic-release by redox-transformation of iron oxide minerals – A review

Abstract

The contamination of groundwater by geogenic arsenic affects the health of tens of millions of people, especially in South and Southeast Asia. The primary cause of the contamination in highly reducing alluvial and deltaic aquifers of South and Southeast Asia, is thought to be microbial reduction of Fe(III) and As(V) to (more mobile) Fe(II) and As(III), respectively. Key to understanding the Fe(III)/As(V) reduction process in aquifers, is the identity and source of electron donors driving the system – this has been a key area of research within arsenic biogeochemistry in the last two decades. Studies in this research area can broadly be divided into three approaches: 1) field-based, which include the organic geochemical analysis of aquifer samples to deduce composition and potential sources (by a variety of techniques and proxies), and/or analysis of multiple biogeochemical parameters (microbes, genes, inorganic and organic geochemistry) combined with multivariate statistical analysis; 2) microcosm-based, where the reductive dissolution mechanism can be studied in laboratory-scale incubation experiments, where key parameters (including electron donor type and availability) can be manipulated, and the effects monitored; 3) in-situ experiments using mineral phases deployed into aquifers. Here we review studies of the three aforementioned approaches to address three key questions: (i) What potential electron donors are present in the aquifers? (ii) What are the environmental sources of the electron donors? (iii) Which electron donors have been directly implicated in arsenic release in aquifers? We find that the potential electron donors available (and utilised), depend on a number of site-specific factors, including geomorphological setting, sediment lithology, and the metabolic potential of the microbial communities present. A wide range of electron donors are implicated, including dissolved labile organic matter, methane and ammonium. In conclusion, we highlight the need for further multidisciplinary investigations that combine state-of-the-art organic geochemical analyses, with mineralogical and microbial analyses in future studies, ideally conducted in-situ.