Acid-resistant iron sludge-based fly ash geopolymer foam for efficient arsenic removal with pH self-regulation

Acidic arsenic-containing wastewater requires efficient treatment technologies due to its high toxicity and environmental risks. This study developed a novel adsorbent (Iron Sludge-Based Fly Ash Geopolymer Foam) by combining fly ash-based geopolymer foam as a carrier with iron-containing sludge, aiming to synergistically enhance arsenic adsorption performance and pH self-regulation under acidic conditions. By adjusting the iron sludge content to optimize the porous structure and mechanical properties, the optimal fly ash/iron sludge ratio was determined to be 4:1. Characterization analysis revealed that iron oxides were uniformly dispersed as nanoparticles within the silica-alumina matrix, forming abundant three-coordinate iron hydroxyl groups (≡Fe-OH) as active sites along with a gradient pore structure. The adsorption isotherm conformed to the Sips model with a fitted adsorption capacity of 77 ± 9 mg/g and achieved 98.34 % arsenic removal at an initial concentration of As(V) of C₀ = 1 g/L, the initial pH of 2.5, and a dosing rate of 0.6 g/L. The adsorption process followed a pseudo-second-order kinetic model, dominated by coordination interactions, where the chemical bonding between iron hydroxyl groups and arsenic effectively overcame the surface protonation inhibition under acidic conditions. Adsorption-desorption cycle experiments confirmed the material's excellent regeneration ability (removal efficiency >95 % after five cycles). Dynamic column experiments demonstrated that the material could continuously treat 2340 bed volumes of wastewater while maintaining an effluent arsenic concentration below 10 μg/L. A dual-column series process optimization was proposed to enhance the system's pH regulation performance. This study provides theoretical support and a technical pathway for the resource utilization of industrial solid waste and the "waste-to-treat-waste" approach for acidic wastewater treatment.