Phosphorus-enriched biochar from biogas residue of Eichhornia crassipes: transformation and release of phosphorus

Abstract

Pyrolysis is an effective technology for treating and utilizing biogas residue. To explore the phosphorus (P) supply capacity of the biochar generated from biogas residue of Eichhornia Crassipes, the P speciation of E. crassipes biogas residue and biomass during pyrolysis (300–700 °C) was analyzed by combining sequential chemical extraction, ³¹P nuclear magnetic resonance (NMR) and P K-edge X-ray absorption near edge structure (XANES) spectroscopy. Pyrolysis treatment promoted the conversion of amorphous Ca-P phases in biogas residue and biomass into crystalline hydroxyapatite (HAP) phase, which matched the formation of stable HCl-P pools in the biochar derived from biogas residue (AEBs, 22.65–82.04%) and biomass (EBs, 13.08–33.52%) in the process of pyrolysis. Moreover, the total P contents in AEBs (19.43–28.92 mg g⁻¹) were higher than that of EBs (3.41–5.26 mg g⁻¹), indicating that AEBs had a great P reclamation potential. The P release kinetics from AEBs and EBs in water were evaluated via an incubation experiment for 360 h. The P release from both AEBs and EBs conformed to the pseudo-second order kinetics model (R² > 0.93), but their P release behaviors were different. The P release of AEBs conformed to the diffusion-re-adsorption model, while that of EBs accorded with the diffusion-dissolution model. The diffusive gradients in thin-films (DGT) analysis showed that AEBs could significantly increase soil available P content as compared with EBs. Hence, the biochar produced from biogas residue of E. crassipes via pyrolysis has a good application potential as a P fertilizer.

Graphical Abstract

Highlight

•	Pyrolysis promoted the conversion of amorphous Ca-P phases in biogas residue into crystalline HAP phase.
•	The pseudo-second order kinetic model well fitted the experimental data of phosphorus release.
•	Biogas residue-derived biochar greatly increased  available P content in the soil based on the DGT test.

Funding

National Natural Science Foundation of China(42107237)

the key research and development program of Zhejiang Province(2021C03164)