Synergistic ultra-high adsorption and oxidation of arsenic in groundwater by iron-modified biochar: Mechanisms and potential application

• Low-cost iron-modified biochar enhances As adsorption and oxidation . • Synergistic adsorption and oxidation yield ultra-high As removal efficiency. • H 2 O 2 aids full As(III) oxidation with iron-modified biochar. • Iron-modified biochar remediates high-concentration As-polluted groundwater . This study explores the potential of low-cost biochar modified with ferrihydrite and goethite for the remediation of heavily As-polluted groundwater. Combining batch experiments and synchrotron radiation-based characterization, we investigated the mechanisms of As(III) adsorption and oxidation. The modified biochars exhibited significant adsorption capacities, achieving fitted q max of 45.7 ± 3.9 mg/g for ferrihydrite-modified biochar (FhGM) and 20.2 ± 1.5 mg/g for goethite-modified biochar (GtGM) at a dosage of 1.0 g/L and an initial concentration of 10 mg/L As(III) over 24 h. Biochar facilitated approximately 30 % As(V) generation from As(III) due to enhanced electron transfer. Moreover, over 90 % of As(III) was oxidized to As(V) following the addition of H 2 O 2 , with scanning transmission X-ray microscopy revealing the distribution of As(III), and As(V) on the solid phase. In the GtGM-H 2 O 2 system, reactive oxidation species, primarily O 2 •– , served as the main oxidant. In the FhGM-H 2 O 2 system, Fe(IV) likely acted as the primary oxidizing agent. Column experiments with 0.64 g of FhGM demonstrated that, when treating 50 mg/L As-contaminated groundwater, the effluent total As concentration remained below the maximum allowable limit of 10 μg/L after 21 h. Furthermore, in the presence of H 2 O 2 , the system managed to treat 200 mg/L As-contaminated groundwater, and the effluent total As concentration exceeded 10 μg/L after 11 h. This study provides valuable insights and practical results for the remediation of high-concentration arsenic-contaminated groundwater, highlighting the effectiveness of iron-modified biochar in this application.