The role of silica in biomass for calcium-modified biochar: Phosphorus removal mechanism and potential as a phosphate fertilizer application.

The interaction mechanism between eggshell calcium and endogenous silica in biomass during biochar modification, and its impact on phosphate adsorption performance and slow-release fertilizer characteristics, remains unexplored. This study investigates that high silica content in biomass (> 6 %) inhibits the decomposition of CaCO in eggshells during pyrolysis, reducing the formation of active calcium species (CaO and Ca(OH)₂), while moderate silica levels (4 %-5 %) promote the formation of CaSiO₃, enhancing phosphorus adsorption without hindering Ca²⁺ activation. Adsorption studies reveal that the precipitation of Ca(PO)(OH) resulting from the combination of CaO and Ca(OH) with phosphate is the primary and effective form for phosphorus removal in calcium-modified adsorbents, accompanied by Ca(PO)·2HO precipitation formed by CaSiO. Eggshell calcium-modified corn straw biochar (ECS) exhibited the highest adsorption capacity, reaching 123.3 mg/g, outperforming materials in previous studies. ECS also demonstrated excellent pH adaptability and selective phosphate removal. As a biochar-based phosphorus fertilizer, ECS-P exhibits high phosphorus extractability in formic acid (93.92 %) but low water solubility (0.62 %), with phosphorus release during the seven-day intermittent leaching experiment remaining between 0.53 to 0.875 mg/L. These results confirm its potential as a phosphorus cycling fertilizer. This study provides fundamental insights into optimizing biomass selection based on silica content for calcium modification, offering an efficient strategy for both phosphate recovery and slow-release fertilizer development.