Sustainable dual-mechanism arsenic remediation by Rhodopseudomonas palustris: Synergistic adsorption-methylation and life cycle assessment

Abstract

Arsenic pollution in groundwater is a global health concern that requires long-term cleanup techniques. This study presents Rhodopseudomonas palustris biomass as a unique, eco-friendly option for arsenic detoxification, achieving levels below the WHO/EPA limit of 10 µg L⁻¹ through synergistic adsorption and enzymatic methylation. The biomass can successfully adsorb arsenate [As(V)] and arsenite [As(III)], with capacities of 92 ± 0.5 µg g⁻¹ and 127 ± 0.5 µg g⁻¹, respectively, following pseudo-first-order kinetics at near-neutral pH (6.7). HPLC-ICP-MS analysis showed that As(III) transforms into less dangerous dimethylarsinic acid (DMA) at 1.30 ± 0.28 µg L⁻¹, whereas As(V) displays minor methylation (<0.01 µg L⁻¹ DMA) as estimated by mass balance calculations, thus emphasising As(III)'s higher reactivity. FTIR and FESEM-EDAX confirm that arsenic binds to carboxyl and phosphate groups on the biomass surface. The technique maintains 70 % efficiency after three repetitive adsorption cycles, increasing cost-effectiveness. Life Cycle Assessment reveals an environmental footprint of 2.34 kg CO₂-eq/m³, 0.015 kg PO₄³⁻-eq, and 0.00015 kg 1,4-DCB-eq, offering a significant improvement over reverse osmosis (3.5 kg CO₂-eq/m³) and adsorption-only systems (2.8 kg CO₂-eq/m³) by leveraging biological methylation to minimize emissions and toxicity. This dual-functional strategy, which combines strong adsorption and biotransformation, provides a scalable, long-term sustainable solution for arsenic remediation in resource-constrained areas, aligning with global water safety goals and advancing bioremediation paradigms.