Agricultural Waste–Biochar (Reclaiming Phosphate from Wastewater)–Soil (Slow-Release Phosphate Fertilizer)–Plant (Peanut Growth) System: Economical and Environmentally Sustainable Strategy

Excessive use of traditional phosphate fertilizers has led to excess phosphorus entering the environment through leaching, volatilization, and agricultural runoff, causing a series of problems such as eutrophication, soil degradation, and phosphorus resource exhaustion. Therefore, the reclamation of phosphorus from wastewater is essential for ecological protection and sustainable agricultural development. Herein, an effective strategy for the synthesis of calcium-modified porous biochar (CEBC) was provided and designed for the recovery of phosphorus from phosphorus-containing wastewater, followed by its application as a slow-release phosphorus fertilizer. CEBC exhibited an excellent phosphate recovery performance with a maximum adsorption capacity of 193.93 mg P/g. Furthermore, slow-release experiments demonstrated that the nutrient release rate of phosphorus-loaded biochar-based slow-release fertilizer (CEBC-P) was 8.95% within 24 h. After application of CEBC-P, the total biomass accumulation and net photosynthetic rate of peanuts increased by 2.54-fold and 4.91-fold, respectively. Mechanistic studies revealed that phosphate was retained within the biochar through precipitation, coordination, hydrogen bonding, and electrostatic attraction. Moreover, release kinetics optimally fitted the Peppas–Sahlin model (R² = 0.996), suggesting that the nutrient release from CEBC-P was synergized by diffusion and dissolution. Overall, this study provides an economical and environmentally sustainable strategy for the recovery and recycling of phosphorus resources.