Optimisation of microwave-assisted pyrolysis for biochar production from empty fruit bunches and palm kernel shell for controlled-release urea fertiliser

• Optimising pyrolysis parameters enhance biochar stability and nutrient content. • Pyrolysis temperature impacts biochar yield and O/C ratio, while CO 2 flow rate affects C/N ratio. • Different feedstocks produced distinct biochar characteristics. • Nitrate release varies between urea coated with EFB biochar and PKS biochar. • Urea-to-biochar coating ratio significantly affects nitrate release and efficiency. This study focuses on optimising microwave-assisted pyrolysis of empty fruit bunches (EFB) and palm kernel shells (PKS) to produce biochar as a coating material for urea in the formulation of controlled-release fertilisers. Response surface methodology (RSM) with ANOVA was applied to develop quadratic models predicting biochar yield, O/C, and C/N ratios. Temperature was the dominant factor influencing biochar yield and O/C ratio, while CO 2 flow rate significantly affected the C/N ratio. Optimum conditions of 400 °C and 300 mL/min CO 2 flow produced Opt-EFB-BC (39.07 wt%) and Opt-PKS-BC (56.34 wt%) with favourable O/C (0.4–0.5) and C/N (15–30), demonstrating their suitability as coating materials for urea in controlled-release fertilisers. Physicochemical analysis showed that Opt-EFB-BC had higher volatile matter, whereas Opt-PKS-BC exhibited greater ash content and slightly larger surface area. Both had high fixed carbon content. FTIR analysis revealed abundant oxygenated functional groups, particularly in Opt-EFB-BC, indicating greater potential for nutrient interaction. Nitrate release studies demonstrated staged release of EBU0.75 released 45 mg/L over 30 days in four stages, while PBU0.5 released 40 mg/L within 16 days in three stages. These optimised biochar–urea formulations exhibit strong potential for controlled nutrient delivery, improving nutrient use efficiency and contributing to soil sustainability.