Phosphorus Removal from Wastewater and Stormwater Using Steel Slag-Based Hydrogel Composites

Anthropogenic phosphorus (P) release from human activities continues to degrade freshwater systems, underscoring the need for effective and sustainable approaches to P removal and management. This study investigates steel slag–chitosan–nanoclay hydrogel composites as a waste-derived alternative to metal-doped biopolymer hydrogels for P removal from wastewater and stormwater. Steel slag aggregates (SSAs), a by-product of steel manufacturing, were incorporated into chitosan-based hydrogel matrices to produce composite sorbents derived from waste materials with potential for cost-effective application. Two formulations (SSA20 and SSA40) were synthesized and compared with a chitosan–nanoclay–iron (CH/NC/Fe) reference hydrogel. Phosphorus adsorption affinity was evaluated using a standardized 24 h batch protocol at environmentally relevant concentrations representative of municipal wastewater (10 mg P L−1) and stormwater or agricultural runoff (1 mg P L−1). The SSA40 composite exhibited the highest P adsorption affinity (Rd = 2.39 ± 0.22 L g−1), outperforming both standalone SSA and the Fe-based hydrogel reference. Scanning Electron Microscopy and Energy-Dispersive X-Ray Spectroscopy (SEM–EDX) analyses revealed strong polymer–slag interactions and metal–phosphate associations, consistent with coupled adsorption and precipitation mechanisms. The SSA-based hydrogels also exhibited self-induced acidification (pH 3.3–4.2), enhancing phosphate uptake while providing intrinsic pH buffering. This study introduces a stable, waste-derived hydrogel composite and demonstrates a reproducible, environmentally relevant batch-testing approach for comparative evaluation of hydrogel phosphorus sorbents, supporting evidence-based strategies for sustainable phosphorus pollution control.