Transformations in physicochemical properties and pore structure of biochar derived from rice straw revealed by synchrotron techniques

Rice straw (RS), a major agricultural byproduct in Thailand, holds significant potential for biochar production. This study investigated the physicochemical properties, pore structure, and yield of rice straw-derived biochar (RSBC) produced using a traditional drum kiln, compared to raw RS. Synchrotron-based X-ray tomographic microscopy (XTM) was employed to characterize changes in pore type, size, and volume. The pyrolysis of RS resulted in a 70% mass reduction; however, RSBC exhibited significantly higher fixed carbon content, cation exchange capacity (CEC), and water-holding capacity (WHC), attributed to increased aromatic functional groups following lignin enrichment and the decomposition of cellulose and hemicellulose. Synchrotron-based XTM analysis revealed an 18% increase in total porosity, driven by a 19% rise in open-pore porosity and a 91% reduction in closed-pore porosity. Additionally, macropore volume (> 100 μm) expanded by 271%, leading to a 267% increase in total pore volume. These structural modifications, particularly the enhanced open-pore porosity and macropore expansion, along with the formation of aromatic and phenolic functional groups, are key contributors to the improved WHC and CEC observed in RSBC. Overall, these findings highlight the potential of rice straw-derived biochar produced via traditional drum kiln pyrolysis as an effective soil amendment, offering a viable alternative to agricultural waste management in rice cultivation systems.