Bioremediation of Arsenic-Contaminated Groundwater Using Arsenite-Oxidizing Bacteria from the Upper Gangetic plains of Laksar region of Haridwar

Arsenic (As), a naturally occurring metalloid in the Earth's crust, poses a significant environmental and public health concern due to its mobilization into groundwater. In India, As contamination primarily originates from the Himalayan region, where As is bound to different minerals like pyrite and iron oxy(hydr)oxide. Inorganic As predominantly exists in two forms—arsenite [As(III)] and arsenate [As(V)]—with As(III) being more mobile in a dynamic system. This process leads to elevated arsenic (As) levels in groundwater, particularly in the low-lying Indo-Gangetic delta plains. A seasonal investigation was done in the Laksar region to evaluate geomicrobiological parameters in groundwater. As concentrations frequently increased in all aquifers (3.5-78 µg/L), surpassing the WHO permissible limits. Hydrochemical analysis revealed that the groundwater was predominantly of Ca-HCO₃ type. As concentration showed a significant correlation with Mn and Fe, suggesting their importance in influencing As mobilisation. Arsenite-oxidizing bacteria (AOB) in groundwater play a critical role in the biogeochemical cycling of As by oxidizing As(III) to the less toxic As(V), thereby holding a potential for bioremediation. Out of ~158 bacterial strains isolated, 28 isolates demonstrated the ability to oxidise As(III). These isolates efficiently oxidised ~1.13 mM As(III) in cultured conditions, with biomass-normalised oxidation rates ranging from 0.2 to 1.13 mM As(III) mg⁻¹ d⁻¹. The major isolated bacteria belong to the genera of Acinetobacter, Stenotrophomonas, Brevundimonas, and Pseudomonas.  These strains were further evaluated in a microcosm setup to determine their efficacy in As bioremediation under simulated groundwater conditions. The results highlight the potential of AOB as a sustainable and cost-effective alternative to conventional As remediation methods, which are often expensive and generate secondary waste in groundwater. The application of such bacteria could significantly mitigate As contamination in affected regions, providing a sustainable and eco-friendly solution to a pressing global issue.