Biochar Surface Chemistry Modification by Blending Hardwood, Softwood, and Refuse-Derived Fuel: Insights from XPS, FTIR, and Zeta Potential Analysis

This study investigates how the inclusion of refuse-derived fuel (RDF) alters the surface chemistry and electrostatic behavior of oak-based biochar. Biochars were produced using downdraft gasification at 850 °C from 100% oak (HW) and a ternary blend comprising 50% oak, 30% pine, and 20% RDF (HW/SW/RDF). Characterization using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), zeta potential, pH, and electrophoretic mobility was conducted to assess surface functionality and colloidal behavior. The RDF-containing biochar exhibited a 43.3% increase in surface nitrogen content (from 0.24% to 0.90%) and a 6.6% rise in calcium content (from 2.07% to 2.27%) alongside the introduction of chlorine (0.20%) and elevated silicon levels (0.69%) compared to RDF-free counterparts. A concurrent reduction in oxygen-containing functional groups was observed, as O1s decreased from 15.75% in HW to 13.37% in HW/SW/RDF. Electrokinetic measurements revealed a notable decrease in zeta potential magnitude from −31.5 mV in HW to −24.2 mV in HW/SW/RDF, indicating diminished surface charge and colloidal stability. Moreover, the pH declined from 10.25 to 7.76, suggesting a loss of alkalinity and buffering capacity. These compositional and electrostatic shifts demonstrate that RDF inclusion significantly modifies the surface reactivity of biochar, influencing its performance in catalysis, ion exchange, and nutrient retention. The findings underscore the need for tailored post-treatment strategies to enhance the functionality of RDF-modified biochars in environmental applications.