Role of microbial processes in arsenic cycling

Abstract

Arsenic (As) is a toxic metalloid of geogenic origins that can lead to groundwater contamination and serious health consequences. South and Southeast Asia, the most populated regions of the world, are particularly affected as the access to water treatment facilities is limited and many people, mainly in rural areas still rely on shallow groundwater wells. Various mechanisms of As release to the groundwater have been suggested to date. Yet, the most commonly accepted one is that As is released from aquifer sediments during microbially-mediated reductive dissolution of As-bearing Fe(III) (oxyhydr)oxide minerals. This process, however, requires the presence of bioavailable carbon (C) that Fe(III)-reducing microorganisms need as energy, electron and carbon sources for their activity. Most laboratory studies, however, used simple fatty acids or sugars, often at high and not environmentally relevant concentrations, instead of naturally-occurring organic matter (OM). Therefore, in this work extracted in-situ OM was characterized (FTIR, NMR, EEM and Pyrolysis GC/MS), and used in 100-day microcosm experiments to determine Fe(III) mineral reduction, As mobilization and the microbial community composition compared to easily bioavailable fatty acids. Furthermore, this work explored the potential of single C compound such as methane (CH4) as the electron donor and driver of reductive dissolution of Fe- and As-bearing sediments. This work for the first time demonstrated that CH4, widely abundant in many As contaminated aquifers across South and Southeast Asia, can be efficiently used by methanotrophic archaea such as Candidatus Methanoperedens to reduce Fe(III) and consequently mobilize As to groundwater. Finally, this PhD thesis revealed main microbial processes occurring in As contaminated aquifer in Vietnam. Some of these process such as SO42- reduction, Fe(III) reduction, Fe2+ oxidation or methanotrophy can directly affect the fate of As in groundwater. Other processes that were found dominant in situ such as methanogenesis and fermentation indirectly favor As mobilization by providing wide range of electron donors. Overall this PhD thesis broadened our understanding of As biogeochemical cycling and filled some knowledge gaps about microbial contribution to this cycle.