Arsenic species-adaptive removal from groundwater by carbon foam-supported Mn-doped FeOOH with precisely regulated Fe/Mn ratios.

Groundwater arsenic (As) contamination, particularly the widespread occurrence of highly toxic and mobile As(Ⅲ), represents a significant global environmental challenge. The simultaneous presence of As(Ⅲ) and As(V) in natural aquatic systems further complicates remediation strategies. To address this challenge, we developed a carbon foam-supported Mn-doped β-FeOOH composite (CF@FeOOH-Mn) and, moving beyond conventional fixed-ratio designs, employed a response surface methodology (RSM) to precisely optimize the Fe/Mn molar ratio in accordance with the aqueous As(Ⅲ)/As(V) concentration ratio. The optimized adsorbent demonstrated outstanding removal performance in both batch and column experiments, achieving maximum adsorption capacities of 43.70 mg g-1 and 63.83 mg g-1 at As(Ⅲ)/As(V) ratios of 1 and 2, respectively, with corresponding total As removal efficiencies of 87.50 % and 90.00 %, which are in close agreement with model predictions. Systematic characterization combined with density functional theory (DFT) calculations unveiled a redox-coupled mechanism in which Mn sites selectively oxidize As(Ⅲ) to As(V), followed by efficient capture of the generated As(V) by Fe sites. DFT results quantitatively confirmed that Mn doping significantly enhances the adsorption energy of As(Ⅲ), increasing it from -2.02 eV to -5.30 eV. This work provides not only a highly efficient adsorbent but also a predictive framework for the rational design of customizable materials tailored to address complex As speciation in groundwater.