Combustion ash addition promotes the production of K-enriched biochar and K release characteristics

Biogas residues (BR) and maize silage (MS) from biogas plants were mixed with biomass combustion ash (ASH) for K-enriched biochar preparation. The characteristics and nutrient contents of biochar were determined by thermo-gravimetric apparatus, Fourier-transform infrared spectroscopy, X-ray diffraction, scanning electron microscope, and chemical extraction methods, respectively. The desorption kinetics and slow release mechanism of K in biochar were evaluated after an incubation experiment for 28 days. Results showed that biochar yields were improved from 36.6% to 49.0% for BR-based biochars (BRCs) and 24.9%–38.2% for MS-based biochars (MSCs) with the addition of ASH during pyrolysis. The total potassium (TK) content of composite biochar increased to the highest TK content of 37.29 mg/g in biochar derived from MS with 20 wt% ash (MSCA20%). Furthermore, slow-available K content increased from 0.86 mg/g and 1.62 mg/g of primary biochar derived from BR and MS to 6.88 mg/g and 7.8 mg/g in composite biochar with 20 wt% ash respectively. The formation of K–Si–Al–O species involved unstable clay minerals and stable insoluble minerals during the catalytic pyrolysis improved slow release performance. The composite biochar showed a slower and continuable K⁺ release property in the incubation experiment, releasing less than 80% of the total available K into water in 28 days. The K⁺ desorption kinetics fitted the Power function well, indicating release of K⁺ was a complex reaction process. This lab-scale work showed that the pyrolytic composite biochar from wastes in biogas plants with combustion ash addition has the potential to be slow release fertilizer. The combined thermal conversion of solid wastes provides a sustainable strategy to simultaneously achieve the reclamation of nutrients and generation of environmental benefits via the production of biochar fertilizer.