Phosphorus recovery using magnesium-enriched biochar and its potential use as fertilizer

Phosphorus (P) recovery from wastewater using biochar is an alternative to P recycle and reuse. However, the impregnation of cations for biochar production is needed to enhance P removal. We aimed at studying the P adsorption and release capacity of biochar enriched with magnesium (Mg²⁺) and its potential use as fertilizer. Biochars were produced from poultry litter (PLB), pig manure (PMB), and sewage sludge (SSB) impregnated with MgCl₂. Characterization comprised pH, Electrical conductivity, total nutrient content, moisture and ash contents, FTIR, XRD, Raman, and SEM-EDX. Phosphorus adsorption isotherms by biochars were obtained and P desorption was also studied. A pot experiment with maize in an Oxisol was performed with the P-loaded biochars, triple superphosphate – TSP (positive control) and no P application (negative control). PMB showed the highest P adsorption (68.0 mg g⁻¹). The main mechanisms of P removal were through precipitation with cations, ligand exchange reactions and electrostatic attraction on the biochar surface. The P-loaded biochars promoted plant growth and accumulation of P in maize, which was similar or greater than TSP. In conclusion, Mg-impregnated biochar is a sustainable option to add value to organic residues and to recycle P to be reused as enhanced efficiency fertilizer.