Alginate-Sludge Derived Biochar-Calcium Hydrogel for Phosphate Removal and Slow-Release Fertilizer: A Sustainable and Multifunctional Solution

Phosphorus (P) pollution and depletion pose significant environmental and agricultural challenges. In this study, a multifunctional alginate-biochar-calcium (ABC) hydrogel for efficient phosphate removal and slow-release fertilization is developed. The hydrogel is synthesized by pyrolyzing biological sewage sludge to produce biochar, followed by calcium chloride modification and cross-linking with sodium alginate. The ABC-hydrogel achieves a high phosphate adsorption capacity, with maximum adsorption of 252.15 mg g⁻¹, and enables controlled phosphate release, with 48% gradually released over 37 days, supporting sustained plant growth. Pot experiments demonstrate its agricultural benefits, with the P-loaded hydrogel enhancing the wet weight of lettuce by 110% compared to the control group. Additionally, the treatment methods, including pyrolysis, calcium modification, and crosslinking in hydrogel, significantly reduce the ecological risk of heavy metals in biological sewage sludge, lowering the ecological risk index (ERI) from 533.13 to 5.79, effectively transforming sludge from waste into a safe and valuable resource for agricultural and environmental applications. With a water absorption capacity of 439.22%, the hydrogel improves soil moisture retention, making it particularly beneficial for drought-prone regions. This work significantly contributes to sustainable phosphorus management by converting waste sludge into a valuable resource, offering promising applications in agriculture, phosphate removal, and waste reutilization.