Genesis and distribution of geogenic arsenic-contaminated groundwater under variable reducing conditions: Why bioenergetic constraints matter.

Geogenic arsenic (As)-contaminated groundwater frequently occurs under Fe(III)- and sulfate-reducing aquifer conditions, yet its genesis and distribution cannot be explained by classical redox cascading theory. Spatial heterogeneity in dissolved organic matter (DOM) composition imposes bioenergetic constraints on microbiological electron partitioning, particularly for low-energy yield pathways (e.g., sulfate reduction). Nonetheless, the mechanisms regulating electron partitioning and As mobilization/immobilization dynamics remain elusive. By quantifying microbially available Gibbs free energy change, this research elucidates the bioenergetic control of electron partitioning under variable reducing conditions. When the Gibbs free energy change of sulfate reduction (ΔGr,SO42-) became less negative than -6.5 kJ/mol e⁻ (i.e., less exergonic), sulfate reduction was suppressed despite the presence of sulfate and electron donors, limiting sulfide production and As immobilization. Under lower energy conditions, groundwater As speciation was dominated by As(V), indicating inhibited As reductive transformation. Dynamic S-Fe redox interactions further controlled the balance between As mobilization and immobilization and thus its distribution in groundwater. When the bioenergetic advantage of Fe(III) reduction over sulfate reduction fell below -20.9 kJ/mol e⁻, DOM-donated electrons were preferentially partitioned toward Fe(III) reduction, thereby promoting As release. By grounding redox competition in bioenergetics, this study consolidates a hydrobiogeochemical framework for interpreting and predicting As cycling and distribution in free energy constrained groundwater environments.