Advancing phosphorus recovery from secondary resources through speciation, process chemistry, and product valorisation

Phosphorus (P) recovery from secondary resources is increasingly important for reducing dependence on phosphate rock and closing material cycles. However, the diversity of P-bearing wastes prevents a universally effective recovery route. This review critically evaluates P recovery from industrial and metallurgical residues, wastewater, agricultural wastes, and emerging battery and lithium-processing residues by linking P speciation and mineralogy with process chemistry, product formation, and process viability. Evidence shows that P concentration alone is insufficient to determine recovery potential. Instead, calcium phosphate, iron phosphate, and aluminium phosphate associations govern dissolution, impurity co-extraction, and achievable products. Selective leaching followed by controlled precipitation is the most established pathway for solid P-rich residues, although reagent consumption, impurity dissolution, and precipitation control remain key limitations. Adsorption and crystallization are more applicable to dilute aqueous streams, but high P removal does not necessarily yield recoverable products. Thermodynamic analysis highlights ion activity, complexation, supersaturation, and metastable phases as critical determinants of product selectivity. Life-cycle and techno-economic evidence further shows that maximum P recovery does not necessarily maximize sustainability or profitability. The findings support feedstock-specific, product-driven strategies in which P speciation guides process chemistry and target product quality determines separation intensity.