Ultrasound-Induced Embedded-Silica Migration to Biochar Surface: Applications in Agriculture and Environmental Sustainability

Abstract

Silicon (Si)–containing compounds, such as silica (SiO2), play a crucial role as fillers, binding phases, and linking agents in sustainable materials. Coating biochar with SiO2 can enhance its performance as a carbon-negative filler in composites such as bioplastics, rubber, asphalt, and cement, making it more competitive with conventional fillers. Biochar, derived from biomass pyrolysis, contains a high concentration of biogenic SiO2—typically 50–80% of its total inorganic content. However, conventional extraction methods such as solvent extraction or gasification detach SiO2 from the biochar matrix, leading to energy-intensive and environmentally unfavorable processes. The objective of this study was to develop an environmentally friendly and energy-efficient approach to induce the migration of embedded biogenic SiO2 from within biochar to its surface—without detachment—using ultrasonic treatment. Fifteen biochar samples were produced by pyrolyzing five biomass types (sugarcane bagasse, miscanthus, wheat straw, corn stover, and railroad ties) at 650, 750, and 850 °C. Each sample was subsequently subjected to ultrasonic irradiation in an isopropanol–water mixture for 1 and 2 min. Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) analyses confirmed that ultrasound treatment significantly enhanced SiO2 migration to the biochar surface, with up to a 2.5-fold increase in surface Si and O concentrations after 2 min of sonication. The effect was most pronounced for biochar synthesized at 850 °C, corresponding to higher surface porosity and structural stability. Fourier Transform Infrared (FTIR) spectroscopy revealed an increased intensity of the Si–O–Si asymmetric stretching band at 1030 cm−1, indicating surface enrichment of siloxane networks and rearrangement of Si-containing functional groups. Overall, the results demonstrate that ultrasound-assisted treatment is a viable and sustainable technique for enhancing SiO2 surface concentration and modifying the surface chemistry of biochar. This SiO2-enriched biochar shows potential for advanced applications in soil amendment, CO2 capture, water purification, and as a reactive additive in cementitious and asphalt composites.