Microbiological Activity and Oxygen Availability Influence Arsenic Mobilization and Redox Transformation in an Arsenic-Rich Aquifer

Groundwater with elevated arsenic levels is a global health concern. In an arsenic-contaminated aquifer, redox processes play a crucial role in the fate and transport of arsenic, affecting both its release and remediation. In this study, how engineering parameters such as oxygen supply, carbon addition, and the presence of arsenic-reducing/oxidizing bacteria may lead to arsenic mobilization and redox transformation in an arsenic-rich aquifer were investigated. Core samples from arsenic-contaminated aquifer were collected and incubated with artificial groundwater medium, nutrient, oxygen, and/or arsenic redox bacteria. Arsenic-related genes and microbial community were analyzed to inform microbial responses during the incubation. The results showed that arsenite (As(III)) was released from bioactive core samples within a week and reached 192.32 μg/L after 8 weeks of anaerobic incubation. About a 1.5-fold increase in As(III) concentration was detected when acetate was added as a carbon source. At the end of the incubation, the major affected microbial community was closely related to Acidiferrobacter thiooxydans, an anaerobic iron oxidizer. The presence of arsenate (As(V))-reducing isolate, Enterobacter sp. AR-8, exhibited a maximum of 19.10 μg/L and 31.86 μg/L As(III) mobilization with acetate and glucose addition, respectively, under an anaerobic environment. Microcosm experiments with both arsenic oxidizers (Bosea sp. AR-11) and reducers (Enterobacter sp. AR-8) further demonstrated that nutrient and oxygen availability are two critical factors that regulate the biotransformation of arsenic. Marker genes (arrA, arsB, arsC, and aioA) involved in the arsenic redox process were analyzed. Moreover, gene-encoded arsenite oxidase (aioA) can be a useful indicator during site investigation and remedy design. In summary, this study presents a series of experiments to evaluate how common engineering factors affect arsenic (bio)transformation and microbial community in the aquifer. Marker genes analysis can further complement the biological details and provide more comprehensive information for arsenic groundwater management.