Microwave co-pyrolysis of biomass, phosphorus, and magnesium for the preparation of biochar-based fertilizer: Fast synthesis, regulable structure, and graded-release

Biochar-based fertilizers (BBFs) have gradually become a promising alternative to chemical fertilizers. In this study, K3PO4 with and without MgO were premixed with cotton stalk (CS) to prepare BBFs via microwave co-pyrolysis. The synthesis process and structure of BBFs and P release kinetics in soil were evaluated. The co-pyrolysis of CS and K3PO4 with or without MgO was finished within 15 min because K3PO4 and generated Mg2P2O7, Mg3(PO4)2, and MgHPO4 reduced the activation energy. The synthetic BBFs had a multi-level nutrient structure that included water-soluble P, precipitated P, and organic P due to the reactions of K3PO4 to CS and MgO. The content of these three forms of P could be regulated by controlling the ratio of K3PO4 and MgO. In particular, the addition of MgO at 18% (w/w) significantly increased the content of precipitated P (108.76 mg/g) and organic P (71.93 mg/g). The multi-level nutrient was graded-released in the order water-soluble P > precipitated P > organic P, and the addition of MgO also improved BBFs’ pore structure and zeta potential to trapping precipitated P and adsorbing water-soluble P, thereby extending the P release period. Our results provided insights into a new and efficient method of preparing BBFs by microwave co-pyrolysis of biomass, phosphorus, and magnesium.