Ozonation as a Scalable Method to Tune Biochar Cation Exchange Capacity and pH for Amending Alkaline Sandy Soils

Biochar is increasingly being recognized as soil amendment that can enhance nutrient and water retention while sequestering carbon. However, biochars derived from locally available feedstocks in arid regions, e.g., date palm fronds, often exhibit high intrinsic alkalinity, which limits their application in sandy soils despite their high cation exchange capacity (CEC), the property governing nutrient retention in soil. Here, we investigate ozonation as a scalable post-treatment strategy to tune the CEC and pH of biochar produced from date palm fronds, a major organic waste stream in the Middle East. Ball-milled biochar was ozonated over a wide range of temperatures (25–190 ℃) and reaction durations (24–96 h), and changes in CEC, pH, surface area, functional groups, and mass were quantified. Ozonation increased CEC by up to 220 % and reduced biochar pH from 8.6 to as low as 3.9. Biochars ozonated at higher temperatures (≥110 ℃) exhibited significant mass loss, due to CO₂ evolution, and higher concentrations of adsorbed base cations, whereas lower-temperature ozonation (<80 ℃) resulted in net mass gain, associated with surface oxidation, and increased acidity. Although ozonation increased the O/C ratio, proximate analysis indicated that chemical stability was not compromised, and H/C _org remained largely unchanged. A preliminary cost analysis further indicates that this approach is scalable. Therefore, these results establish ozonation as a sustainable method for tuning biochar properties for amending alkaline sandy soils, with implications for desert rehabilitation, urban greening, food production, and carbon sequestration.