Enhanced simultaneous recovery of phosphate and nitrate from municipal wastewater using sewage sludge–derived biochars

The transformation of municipal sewage sludge into biochars via pyrolysis provides an innovative solution for sustainable sludge management and wastewater treatment. This study examines the influence of pyrolysis temperature (300–700°C) on the physicochemical properties of sewage sludge-derived biochar and its ability to adsorb PO43− and NO3− from municipal wastewater. The biochars were synthesized under a nitrogen atmosphere to maintain oxygen-free conditions, and their structural and chemical characteristics were analyzed via SEM, XRD, FTIR, and CHNOS techniques. Synthesized biochars at increasing pyrolysis temperatures show Type IV isotherms and H3-type hysteresis loops, reflecting the development of meso/micropores and slit-like pore geometry. BC700 achieved the highest PO43− removal efficiency of 65% at an optimal dose of 2 g L−1 and a contact time of 2 h. For NO3− removal, BC400 showed superior performance (14%), which increased significantly to 62% when modified with iron (Fe-BC400). The Langmuir isotherm model best described PO43− adsorption, whereas NO3− adsorption followed the Freundlich model. BC700 displayed maximum PO43− and NO3− adsorption capacities of 85.74 and 56.50 mg g−1, respectively. The percentage of PO43− removal with Fe-BC400 was 83.13. The adsorption efficiency for both PO43− and NO3− was pH dependent, with BC700 and Fe-BC400 performing optimally at pH values below 6 and 4, respectively. Kinetic analysis confirmed that the pseudo-second-order model accurately described the chemisorption mechanism. The overall selectivity order can be described as SO42− > Cl− ≈ HCO3− for both PO43− and NO3− adsorption on the BCs. Thermodynamic analysis indicated that compared with BC700, Fe-BC400 had stronger and more favorable adsorption and recovery affinities for PO43− and NO3−. These findings demonstrate the potential of Fe-BC400 as an effective adsorbent for nutrient recovery and pollution control in wastewater treatment systems. Implications: The findings demonstrate the potential of sewage sludge-derived biochars as effective adsorbents for recovering phosphate and nitrate from municipal wastewater. BC700 exhibited superior phosphate removal, whereas Fe-BC400 showed enhanced nitrate adsorption and selectivity under competitive multi-ion conditions. Iron modification improved surface reactivity and adsorption performance. The rapid adsorption kinetics and high adsorption capacities highlight their suitability for practical wastewater treatment applications. Nutrient recovery can reduce eutrophication risks, protect aquatic ecosystems, and provide opportunities for nutrient reuse in agriculture. Furthermore, converting sewage sludge into valuable adsorbents supports circular economy principles, reduces disposal costs, and promotes sustainable wastewater management.