Enhancing water retention performance of biochar modified by alkali-treated coal fly ash: Pyrolysis behavior, field simulation, and metal leaching assessment

Biochar, produced from biomass pyrolysis, is widely recognized as an effective soil amendment due to its ability to enhance soil water retention capacity. To further improve biochar’s performance, catalytic pyrolysis using coal fly ash (CFA), as a low-cost and promising additive owing to its containing catalytically active minerals, was explored in this study. CFA was expected to enhance pyrolysis efficiency and serve as a soil conditioner. Raw CFA (RCFA) and KOH-modified CFA (KC11) were tested in pyrolysis of rice straw. The addition of KC11 reduced the apparent activation energy (166.19 KJ/mol) of RS pyrolysis and promoted gas production (2.14 ± 0.3 g), while RCFA increased liquid yields. Heavy metal leaching tests suggested minimal environmental risk associated with the application of biochar. The modification increased the potassium content in biochar and enhanced its water retention properties. Notably, while specific surface area (SSA) and hydrophilic functional groups may not be dominant in soil water retention. Soils amended with KC11-derived biochar (B-KC11) extended the first evaporation stage (12.3–19.3 h) and exhibited the highest evaporation mitigation capacity (EMC) (1115.66 hr), which represents the ability to mitigate physical evaporative water loss. A field simulation based on experimental water evaporation profiles confirmed the temperature-dependent benefits of biochar application, and evaporation could be reduced by up to 30.32 % in biochar-amended soil. Overall, this study demonstrates that CFA, particularly in its alkali-modified form, can serve as an effective catalyst and soil amendment, offering a sustainable strategy to enhance biochar’s water retention capacity and agronomic value.