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, Mg-3(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.

Automated summary

In this study, biochar-based fertilizers (BBFs) were prepared by microwave co-pyrolysis of K3PO4 with and without MgO premixed with cotton stalk. The BBFs had a multi-level nutrient structure with water-soluble P, precipitated P, and organic P, and the content could be regulated by controlling the ratio of K3PO4 and MgO. Addition of MgO not only increased the content of precipitated P and organic P in BBFs, but also improved their pore structure and zeta potential to extend the P release period.