The influence of As on the biogenic schwertmannite formation and its potential regulatory mechanism in acid mine drainage

• Low concentrations (2–10 mg L⁻¹) promote, while high concentrations inhibit, Fe²⁺ bio-oxidation and mineral formation by A. ferrooxidans . • As enhances iron-oxidase activity and EPS secretion, which together drive accelerated Fe²⁺ oxidation, Fe³⁺ hydrolysis, and biogenic schwertmannite formation • The exclusive occurrence of As‑induced effects in biotic systems highlights the critical role of microbial activity in regulating mineralogical outcomes in AMD environments. Coal mining activities generate large quantities of arsenic (As)-rich Acid Mine Drainage (AMD), which represents one of the most severe environmental challenges associated with coal extraction. In As-rich AMD, the co-occurrence of schwertmannite and As(V) is frequently observed. However, the regulatory relationship between biogenic schwertmannite formation and As(V) in AMD remains poorly understood. Therefore, this study investigates the influence of Acidithiobacillus ferrooxidans ( A. ferrooxidans ) and As interactions on schwertmannite formation and the underlying regulatory mechanisms in AMD systems. The results demonstrate that in AMD systems, low As concentrations (2–10 mg L⁻¹) promote Fe 2+ bio-oxidation and schwertmannite formation. Notably, 10 mg L⁻¹ As exhibited the most significant enhancement ( p < 0.05), achieving a Fe 2+ oxidation rate of 91.6% at 36 h—a 39.9% increase compared to the control. In contrast, a high As concentration (50 mg L⁻¹) inhibited Fe 2+ bio-oxidation and led to the formation of a mixed mineral phase comprising both schwertmannite and jarosite, rather than pure schwertmannite. Under abiotic conditions, As had no effect on Fe 2+ oxidation or Fe 3+ hydrolysis and subsequent mineral precipitation. Instead, As primarily facilitated Fe 2+ oxidation and subsequent iron mineral formation by modulating the Fe 2+ -oxidizing enzyme activity and extracellular polymeric substance (EPS) secretion in A. ferrooxidans. Specifically, As significantly enhanced the iron-oxidizing enzyme activity ( p < 0.05) and increased the secretion of cell-surface-associated polysaccharides and proteins (particularly in LB-EPS and TB-EPS). Biogenic schwertmannite (regardless of As treatment) maintained its mineralogical stability throughout the 144-hour incubation period. Over time, small iron mineral particles progressively aggregated into larger clusters while retaining the characteristic features of schwertmannite. These findings provide critical insights into the mechanisms governing As-mediated biogenic schwertmannite formation in AMD systems.