BiocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More is a management tool in environmental science, lauded for its ability to stabilize carbon, adsorb contaminants, and modulate soil pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More. The secret to its effectiveness lies in its unique surface chemistry, which is heavily influenced by the feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More used and the production process. A recent study published in the journal Fuels by Paul C. Ani, Hasan J. Al-Abedi, Joseph D. Smith, and Zeyad Zeitoun, explored how adding refuse-derived fuel (RDF) to a traditional hardwood and softwood mix affects the resulting biochar. The research provides critical insights into the opportunities and challenges of using waste materials to create a valuable environmental tool.

This study set out to analyze the surface chemistry of two biochar types: one made from 100% oak (hardwood, HW) and another from a ternary blend of 50% oak, 30% pine, and 20% RDF (HW/SW/RDF). Both were created using downdraft gasificationGasification is a high-temperature, thermochemical process that converts carbon-based materials into a gaseous fuel called syngas and solid by-products. It takes place in an oxygen-deficient environment at temperatures typically above 750°C. Unlike combustion, which fully burns material to produce heat and carbon dioxide (CO2​), gasification More at 850°C. The researchers used advanced techniques, including X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR), to examine the biochar’s surface at a molecular level. They also measured zeta potential and pH to understand the material’s electrostatic behavior and colloidal stability in water.

The results were fascinating. XPS analysis revealed significant elemental changes in the RDF-containing biochar. While the carbon content remained high, the RDF blend caused a substantial increase in surface nitrogen and calcium. Specifically, the nitrogen content in the biochar surged from 0.24% in the RDF-free blend to 0.90% in the RDF-containing one, a remarkable 43.3% increase. Calcium content also rose from 2.07% to 2.27%. These enrichments could be beneficial, as nitrogen-bearing compounds may enhance nutrient exchange, and calcium can improve a biochar’s ability to buffer acidity and immobilize heavy metals.

However, the inclusion of RDF also introduced some less desirable elements. The HW/SW/RDF biochar was the only sample to contain chlorine (0.20%) and showed elevated levels of silicon (0.69%) compared to the other samples. The presence of chlorine, a component of plastics and chlorinated waste in RDF, could lead to the formation of harmful dioxins and furans during high-temperature processing. Furthermore, the study noted a troubling absence of potassium in the RDF blend, likely due to its volatilization as potassium chloride (KCl) during gasification. Potassium is a key nutrient for soil fertility, so its loss could diminish the biochar’s agricultural value.

Beyond elemental composition, the study also showed how RDF affects the biochar’s electrostatic properties. Zeta potential, a measure of a particle’s surface charge, is a critical indicator of its colloidal stability in aqueous environments. The HW biochar had a robust negative zeta potential of -31.5 mV, suggesting high stability and a strong capacity for electrostatic repulsion. In contrast, the zeta potential of the RDF-containing biochar plummeted to -24.2 mV. This 23% decrease in magnitude indicates a significant reduction in surface charge and a weaker ability to remain dispersed in water, making it more prone to aggregation or sedimentation. The decline in zeta potential was directly correlated with a reduction in oxygen-containing functional groups, which are vital for attracting positively charged contaminants and ions.

The study’s findings highlight a crucial trade-off. While incorporating RDF into biochar production offers a sustainable way to valorize municipal waste, it also compromises some of the material’s most valuable properties. The chemical heterogeneity of RDF enriches the biochar with elements like nitrogen and calcium but at the cost of reduced electrostatic stability and the introduction of potentially hazardous elements like chlorine. For biochar to be effective in applications like water remediation or soil amendmentA soil amendment is any material added to the soil to enhance its physical or chemical properties, improving its suitability for plant growth. Biochar is considered a soil amendment as it can improve soil structure, water retention, nutrient availability, and microbial activity.   More, a strong negative surface charge is often essential. Therefore, the study concludes that RDF-modified biochar may require additional post-treatment strategies, such as oxidative activation or acid washing, to restore its surface polarity and functional performance before it can be deployed for high-performance environmental applications.

Source: Ani, P.C.; Al-Abedi, H.J.; Smith, J.D.; Zeitoun, Z. Biochar Surface Chemistry Modification by Blending Hardwood, Softwood, and Refuse-Derived Fuel: Insights from XPS, FTIR, and Zeta Potential Analysis. Fuels 2025, 6, 71.