Comparative Phosphate Recovery from Synthetic Solution and Anaerobic Digestion Centrate Using Calcium- and Magnesium-Modified Commercial Biochar: Performance, Adsorption Behavior, and Preliminary Agronomic Evaluation

Phosphorus recovery from wastewater can improve resource circularity while reducing eutrophication risks. This study directly compared calcium- and magnesium-modified commercial biochar for phosphate capture from a controlled synthetic solution and real anaerobic digestion centrate. Biochar was modified with Ca and Mg salts at biochar:salt mass ratios of 1:1–1:20 and characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) before and after adsorption. In synthetic solution, 24-h experimental adsorption capacities reached 55.8 and 24.6 mg P gBC−1 for Ca- and Mg-modified biochar, respectively. In centrate, the selected 1:5 Ca-BC and Mg-BC conditions achieved 83.6 and 54.5 mg P gBC−1, corresponding to calculated phosphate-P removals of 69.6% and 45.4%, respectively. Ca-modified biochar showed stronger phosphate-associated surface changes and higher measured phosphate uptake under the tested conditions, while FTIR, SEM-EDS, pH behavior, and kinetic modeling were interpreted as supporting evidence of adsorption behavior rather than definitive identification of specific mineral phases or molecular-scale mechanisms. The 1:5 biochar:salt ratio was selected because higher modifier loadings did not provide proportional gains in phosphate capture. Phosphate-loaded Ca- and Mg-modified biochars achieved 100% germination for wheat and lettuce, with no evidence of early phytotoxicity. Overall, Ca-modified biochar showed higher phosphate-capture performance, while both phosphate-loaded materials demonstrated preliminary germination compatibility; phosphorus release, leaching, and plant phosphorus uptake were not evaluated.