Biofilm Mediated Arsenic Migration and Transformation in Groundwater under the Influence of Dissolved Organic Matter

Biofilms (structured microbial communities), ubiquitous in a variety of aquatic and terrestrial ecosystems, strongly regulate arsenic (As) cycle. Dissolved organic matter (DOM), prevalent in natural environments, can stimulate the development and activity of microbial communities, thus enhancing microbially mediated arsenic biogeochemical processes. However how DOM regulate groundwater biofilms to drive the fate of As migration and transformation remains unclear. In this study, laboratory incubation experiments were integrated with extensive biofilm characterizations, 16S rRNA, qPCR, Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) to explore the behaviors and potential mechanisms of As under the mediation of biofilm and fluvic acid (FA), a representative of DOM in groundwater. The results showed that the regulation of FA induced more As incorporation and subsequent reduction of As(V) after the As(III) oxidation potentially mediated by aoxA/B. The interaction of protein and polysaccharide on the biofilms with As was the dominant adsorption mechanism. FA modification resulted in the secretion of more abundant EPS and provided more binding sites for the organic functional groups, which intensified the adsorption of protein and polysaccharide for As. In parallel, the addition of FA led to the secretion of larger amounts of α-configuration polysaccharide that produced greater steric hindrance promoting the As adsorption. The formation of FA-Ca-As ternary complexes still remained an important way for arsenic sequestration after biofilm-FA modification. The ultimately higher diversity and abundance of N and S cycling associated bacteria (e.g., Desulfitobacterium, Acinetobacter, Sphingobacterium), yielded by the addition of FA, likely contributed to the reduction of As(V) by enhancing arrA. Additionally, the electron shuttle effect of FA accelerated the electron transfer between As(V) and As (III), serving as another mechanism for As transformation. To the best of our knowledge, this study for the first time reveals the importance of DOM on the migration and transformation of As by biofilms. This study enriches the theoretical understanding of biosorption and biotransformation of As and provides new insight into environmental arsenic cycles.