Adsorption behavior and pH adjustment effect of iron sludge-based fly ash geopolymer toward Sb(V) removal in acidic wastewater: From wastes to water cleanup

Antimony, a toxic metalloid posing ecological and human health risks, remains difficult to remove efficiently from aquatic environments, particularly under acidic conditions. In this study, fly ash-based geopolymers were employed as the matrix, and a series of iron sludge-modified composite adsorbents were prepared by incorporating different proportions of iron sludge. Their adsorption performance toward Sb(V) and applicability in acidic wastewater were systematically evaluated. With increasing iron sludge content, the Sb(V) removal efficiency improved from 67% to 94%. Among the composites, the material with a fly ash-to-iron sludge mass ratio of 4:1 exhibited the best overall performance in terms of adsorption capacity, kinetic response, and applicability under acidic conditions. Characterization results revealed that Sb adsorption was governed not only by coordination with surface Fe active sites but also by localized acid-induced corrosion and reconstruction of Si-O-Al bonds within the geopolymer framework, accompanied by the formation of a new Sb-doped sillimanite phase, indicating distinctive structural evolution during adsorption. The adsorption kinetics were well described by the Elovich model, suggesting heterogeneous surface energy distribution, while intraparticle diffusion analysis suggested that iron sludge incorporation facilitated Sb(V) mass transfer and improved access to internal adsorption sites. The isotherm data fitted the Sips model best, and the maximum adsorption capacity of IS-FAGF 4:1 reached 74 mg/g, representing a 23.3% increase compared with the pristine geopolymer. Lower pH favored Sb removal, and iron sludge modification broadened the effective working pH range. The increase in final pH observed after adsorption suggests a pH adjustment effect under acidic conditions, which is likely related to the release of alkaline species from the geopolymer matrix. Although phosphate exerted competitive effects, the composite still maintained relatively stable Sb(V) removal, indicating favorable anti-interference performance under acidic conditions. Regeneration experiments further confirmed the good cyclic stability of the composite. Overall, this study elucidates the synergistic multi-mechanism adsorption behavior of Sb(V) on iron sludge-fly ash geopolymers, providing theoretical support for the design of geopolymer-based composite adsorbents and the treatment of acidic Sb-containing wastewater.