From Solid Waste to Slow-Release Fertilizer: Sustainable Phosphorus Recovery Using Dual-Function Engineered Fly Ash Ceramsite

The large amount of fly ash generated from coal-fired power plants not only occupies land when stockpiled but also poses potential environmental risks, making its resource utilization highly significant. In this study, a novel Mg–La layered double hydroxide (LDH)-modified ceramsite was prepared using fly ash as the primary raw material, and its phosphate adsorption and recovery performance in wastewater was systematically evaluated. The ceramsite was produced by high-temperature sintering and subsequently loaded with Mg–La LDH, and its performance was assessed through adsorption isotherms, kinetics, and coexist-ion competition experiments. The results showed that the modified ceramsite exhibited a maximum adsorption capacity of 1496.52 mg/kg and maintained high phosphate removal efficiency even under coexisting ion conditions. XRD, XPS, and FTIR analyses confirmed the successful loading of Mg–La LDH and revealed that phosphate adsorption occurred mainly through the formation of La–P and Mg–O–P complexes on the surface. Furthermore, P-enriched ceramsite obtained after adsorption was applied in pak choi pot experiments, where the addition of 1% P-enriched ceramsite increased plant dry weight by 106.8% and chlorophyll content by 25.4%, indicating its slow-release fertilizer potential. Therefore, the fly ash-based Mg–La LDH-modified ceramsite not only achieves high-value utilization of solid waste but also provides dual benefits of phosphate removal from wastewater and nutrient recycling. In addition, the use of ceramsite as a robust carrier facilitates material recovery and reuse, enhancing its potential for large-scale application.