Influence of potassium addition on phosphorus availability and heavy metals immobility of biochar derived from swine manure

Pyrolysis of animal manure at high temperature is necessary to effectively immobilize heavy metals, while the available phosphorus (P) level in biochar is relatively low, rendering it unsuitable for use as fertilizer. In this study, the pretreatment of swine manure with different potassium (K) sources (KOH, K₂CO₃, CH₃COOK and C₆H₅K₃O₇) was conducted to produce a biochar with enhanced P availability and heavy metals immobility. The addition of all K compounds lowered the peak temperature of decomposition of cellulose in swine manure. The percentage of ammonium citrate and formic acid extractable P in biochar increased with K addition compared to undoped biochar, with CH₃COOK and C₆H₅K₃O₇ showing greater effectiveness than KOH and K₂CO₃, however, water- extractable P did not exhibit significant changes. Additionally, the available and dissolved Si increased due to the doping of K, with KOH and K₂CO₃ having a stronger effect than CH₃COOK and C₆H₅K₃O₇. X-Ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) analysis revealed that K addition led to the formation of soluble CaKPO₄ and silicate. In addition, the incorporation of K promoted the transformation of labile copper (Cu) and znic (Zn) into the stable fraction while simultaneously reducing their environmental risk. Our study suggest that the co-pyrolysis of swine manure and organic K represents an effective and valuable method for producing biochar with optimized P availability and heavy metals immobility.