Macrocystis pyrifera Biochar Activated with FeCl2: An Effective and Sustainable Adsorbent for As(V) Removal at Drinking-Water-Relevant Concentrations

Arsenic contamination of drinking water threatens more than 100 million people worldwide, particularly in regions where decentralized, low-cost treatment solutions are urgently needed. Although biochar-based adsorbents have shown promise, the performance of seaweed-derived biochars—especially under environmentally relevant, low arsenic concentrations—remains poorly characterized. Here, we developed and evaluated a Fe-activated biochar derived from the kelp Macrocystis pyrifera for As(V) removal under drinking-water-relevant conditions. Fe activation with FeCl2—rather than textural modification via KOH—was critical for introducing Fe–O surface complexes that governed chemisorption and drove high removal efficiency at low concentrations. The Fe-activated biochar exhibited a BET surface area of 22.23 m2 g−1, a point of zero charge near neutral pH (pHPZC ≈ 6.36), and low ash content (<20%), all favoring arsenate adsorption. Adsorption kinetics were best described by the pseudo-second-order model (R2 > 0.99), consistent with a strong contribution of surface chemical interactions under the tested conditions. Fe-activated biochar removed 53–67% of As(V) across initial concentrations of 0.01–0.20 mg L−1, approaching an adsorption plateau within 12–20 h, positioning this sustainable, kelp-derived material as a promising candidate for point-of-use arsenic treatment in affected regions.