Microstructural and mechanical effects of biochar particle size on clay compressibility and lead immobilization

The demand for sustainable soil stabilizers has driven interest in soil-biochar mixtures for geotechnical applications such as landfill liners and subgrade systems. However, the role of biochar particle size in governing compressibility, shear strength, and heavy metal immobilization remains unclear. This study examines the influence of fine (FB) and coarse biochar (CB) derived from bamboo on the compressibility, shear strength, and Pb²⁺ immobilization in clayey soil at dosages of 1-10%. One-dimensional consolidation tests reveal that FB at 10% increased the void ratio by 31.7%, whereas CB reduced it by 22.7% relative to untreated soil. Both biochar types decreased compression and swell indices at higher dosages, with FB-treated soils exhibiting greater stiffness under elevated stress. Void index analysis indicates the formation of a stable and stress-resistant soil-biochar matrix. Microstructural observations show denser particle packing in FB mixtures, while CB mixtures exhibit particle fragmentation at higher dosages. Toxicity characteristic leaching procedure (TCLP) tests demonstrate that 5% FB reduces Pb²⁺ concentrations below regulatory limits within 14 days, whereas CB requires higher dosages or longer curing periods. These findings highlight the critical role of biochar particle size in optimizing soil performance and contaminant immobilization for barrier applications.