Biochar from Swine Manure: An Alternative for Nutrient Recovery and Slow-Release Fertilization

Abstract

The production of swine manure (SM) biochar was optimized to improve the nutrient recovery. SM was activated with KOH, HCl, or MgCl2 at different ratios (1:1 and 3:1, v/w) and carbonized at 400, 600, and 800 °C. Results showed that MgCl2-activated biochar at 800 °C (3:1) showed enhanced production (84.9%) and the highest phosphate adsorption (2.93 mg g–1) and ammonium adsorption (1.27 mg g–1). Adsorption followed Langmuir isotherm for phosphate (qm = 67.56 mg g–1) and Freundlich for ammonium (qm = 17.48 mg g–1). Adsorption kinetics indicated that phosphate uptake was best described by the Elovich model (R2 = 0.99), while ammonium followed pseudo-second order kinetics (R2 = 0.90), suggesting distinct but predominantly chemisorption-controlled mechanisms. Desorption tests showed a limited solubility of phosphate in water and partial ammonium release, supporting the use of biochar as a slow-release fertilizer. Overall, MgCl2-activated biochar at 800 °C proved to be the most effective condition, combining high adsorption capacity with nutrient release potential, highlighting its value for sustainable nutrient recovery in agriculture.