Waste-Derived Functional Materials for Coupled Nitrogen and Phosphorus Removal from Eutrophic Waters: A Critical...

Waste-derived materials can supply phosphorus-binding surfaces, mineral precursors, electron donors, alkalinity, and porous carriers for nitrogen (N) and phosphorus (P) removal from eutrophic waters. However, reported performance is difficult to compare because studies differ in nutrient forms, influent loads, hydraulic conditions, test duration, and removal endpoints. This review evaluates steel slag, red mud and bauxite residue, ferruginous and sulfur-bearing mining residuals, magnesian wastes, agricultural ash, and biochar, focusing on composition, modification, removal mechanisms, process configuration, and long-term operation. Three findings emerge. First, compositions favorable for N removal may not favor P removal: sulfur-siderite media showed the highest reported N-removal rate at 25% siderite, but the highest P-removal rate at 75%. Second, beneficial interaction is clearest when one material function removes a measurable limitation on another process. For example, alkaline slag mitigates acidification during sulfur-autotrophic denitrification, while Ca, Mg, and Fe phases retain phosphate. Third, dominant limitations shift during operation from initial dissolution and reaction kinetics to intraparticle transport, surface passivation, clogging, breakthrough, and possible nutrient re-release. Organic-matter and bloom-derived metabolite loading can reverse iron-based phosphorus retention and therefore belongs in any realistic test protocol. Material formulation should therefore address electron supply, proton balance, oxygen distribution, and accessible P-reactive phases rather than rely on a fixed waste-mass ratio. Simultaneous concentration decreases should be distinguished from true process interaction using equal-load single-function controls and measurements of net N₂ production and stable or exported P. Continuous-flow studies in real waters should report breakthrough, head loss, leaching, greenhouse-gas emissions, and spent-media behavior to establish whether short-term removal persists throughout material service life under realistic hydraulic conditions and variable water quality.