Pollution to resource: Converting phosphorus eutrophication to calcium–phosphorus slow‐release fertilizer

Abstract Phosphorus‐containing wastewater and waste river sludge both pose significant environmental hazards, yet simultaneously possess resource attributes. Improper disposal not only leads to severe environmental pollution but also results in resources loss. Therefore, it is imperative to develop an integrated strategy that addresses the environmental challenges posed by phosphorus‐containing wastewater and waste river sludge while simultaneously enabling resource recovery and utilization. This study proposes a “waste‐to‐resource” strategy, utilizing river sludge as a raw material modified with calcium to enhance phosphorus adsorption, thereby fabricating calcium‐modified biochar (RSB‐Ca) for phosphorus recovery from phosphorus‐containing wastewater. The recovered product is subsequently applied as a sludge‐derived biochar phosphorus fertilizer. The results demonstrate that RSB‐Ca exhibits a hierarchically porous structure and abundant oxygen‐containing functional groups. Through the synergistic effects of electrostatic attraction, complexation, chemical precipitation, and pore adsorption, it achieves excellent phosphate adsorption performance, with a maximum adsorption capacity of 45.43 mg/g. Furthermore, the phosphate release behavior of the phosphorus‐laden sediment biochar fertilizer (P‐RSB) was investigated under various conditions. The results indicate that P‐RSB possesses stable and favorable release characteristics, with the highest release observed under high‐temperature and alkaline conditions. Notably, P‐RSB significantly promoted the germination and growth of water spinach. This work provides a low‐cost, sustainable, and environmentally friendly approach for the integrated treatment of phosphorus‐containing wastewater and waste river sludge. By converting waste sludge into a value‐added phosphorus recovery material and agricultural fertilizer, the proposed strategy reduces secondary pollution and promotes resource recycling and circular economy development.