Effects of pre- and post-pyrolysis potassium silicate enrichment on potassium fractionation and release behavior of rice husk and sugarcane press mud biochars

The utilization of agro-industrial residues as value-added biochar products offers a sustainable approach for nutrient recovery, resource recycling, and improved management of potassium resources in agricultural systems. In this study, biochars derived from rice husk and sugarcane press mud were enriched with potassium silicate through pre- and post-pyrolysis approaches to evaluate their physicochemical properties, potassium fractionation, and release behavior. Potassium silicate enrichment significantly influenced the physicochemical characteristics of the biochars, resulting in increased alkalinity, electrical conductivity, ash content, and potassium retention. Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Diffraction (XRD) analyses revealed the presence of silicate-associated functional groups and potassium-bearing mineral phases in the enriched biochars. Potassium fractionation demonstrated that enrichment increased the proportion of readily available potassium fractions, particularly water-soluble and exchangeable forms, while also influencing potassium stabilization within the biochar matrix. Potassium release studies indicated a progressive release pattern over time, with enriched biochars exhibiting enhanced release characteristics compared with the corresponding controls. Kinetic modelling showed that potassium release was best described by the parabolic diffusion model for most treatments, while the Elovich model provided the best description for the unenriched press mud biochar. Among the enrichment strategies, pre-pyrolysis potassium silicate enrichment was generally more effective in improving potassium retention, fractionation, and release characteristics than post-pyrolysis enrichment. These findings highlight the potential of agro-industrial waste-derived potassium-enriched biochars as sustainable materials for potassium management and resource-efficient agricultural applications.