Co-pyrolysis of wheat straw, bentonite and NPK for developing slow-release fertilizer: Synergetic effect of blending...

Biochar-based slow-release fertilizers (BSRFs) offer a promising alternative to conventional fertilizers by enhancing nutrient retention and reducing environmental loss. This study aimed to develop a sustainable and cost-effective BSRF through the co-pyrolysis of wheat straw (WS), bentonite and nutrient solution containing KH₂PO₄ and KNO₃. WS and bentonite were blended in 50:50 and 70:30 ratios with fixed doses of nutrients, then co-pyrolyzed (at 350 °C and 500 °C) to produce BSRFs. The resulting materials exhibited biochar yields of 44.6-56.3 %, pH values between 7.6 and 8.6, C/N ratios of 7.9-22.7, H/C values of 0.24-0.65, and O/C ratios of 0.33-0.49. SEM-EDX, XPS and FT-IR analyses confirmed the presence of N-, P-, and SiOSi groups in the BSRFs. A 35-day (d) soil incubation study demonstrated that the respective release of NO₃-N, PO₄^3-, and K⁺ ranged from 43.2 to 59.4 %, 22.7 to 66.8 %, and 52.1 to 69.5 %. Notably, the BSRF prepared with 30 % bentonite at 350 °C pyrolysis demonstrated the best slow-release behaviour in soil compared to the others. The release kinetics were analyzed using pseudo-first-order (PFO), pseudo-second-order (PSO), particle-diffusion model (PDM), Elovich model (EM), and Korsmeyer-Peppas model (KPM), indicating that the release mechanism adhered to quasi-Fickian diffusion and nutrients partially disseminated through pores and the swollen matrix. The influence of BSRFs on soil carbon loading and denitrification could be a possible avenue for future research.