Phosphate Removal from Wastewater via Coagulation and Preliminary Evaluation of the Recovered Residues for Lettuce ( Lactuca sativa ) Cultivation

Phosphate ( P O 4 3 − ) discharge from municipal wastewater is a major contributor to eutrophication, highlighting the need for efficient treatment technologies that enable both phosphorus (P) removal and potential reuse. In this study, tin (Sn)- and calcium (Ca)-based coagulants, and their composites with chitosan (CS) (CS@Sn, CS@Ca, CS@Sn/Ca), were developed and evaluated for P O 4 3 − removal and tentative recovery from both aqueous solutions and municipal wastewater. Among the tested materials, SnCl 4 exhibited the highest performance, achieving 97.2–100% P O 4 3 − removal over a wide pH range and at low dosages, demonstrating its high affinity toward P O 4 3 − . The CS@Sn composite material demonstrated satisfactory efficiency, while the Ca-based coagulants exhibited lower efficiency and greater sensitivity to solution chemistry. CaO achieved maximum phosphate removal of 89% under neutral to alkaline conditions, but its efficiency decreased significantly under acidic conditions. CS@Ca and CS@Sn/Ca composites exhibited only moderate removal efficiencies under all experimental conditions tested. SEM and FTIR analyses indicated that P O 4 3 − removal occurred through interactions with hydrolyzed Sn(IV) species. The recovered phosphate-rich residues were subsequently subjected to a preliminary pot experiment using lettuce ( Lactuca sativa ) to explore their potential for agricultural reuse. The results provided an initial indication that some recovered residues may support early plant growth, although further agronomic evaluation under controlled conditions is required. Therefore, S n C l 4 represents a promising approach for simultaneous phosphate removal and potential recovery, while the preliminary plant cultivation results indicate potential for agricultural reuse of the recovered residues, warranting further investigation.