Ball-milled biochar: Structural transformation and implications for Cu and Pb immobilization in smelter-contaminated soil

Abstract

Ball-milled biochar (BBC) was evaluated for long-term immobilization of copper (Cu) and lead (Pb) in smelter-contaminated soil. In a one-year column leaching test, soil amended with 5 % (w/w) BBC was compared with macro-sized biochar (MBC). Biochars were characterized by scanning electron microscopy, Brunauer–Emmett–Teller surface area, X-ray diffraction, Fourier transform infrared spectroscopy, 3D topography, and Morphologi G3 imaging. Leachates were periodically collected for 356-day and analyzed for heavy metals (HMs) and leachate chemistry. Ball-milling increased biochar reactivity by raising surface area six-fold, introducing microporosity, and enriching oxygen-containing functional groups. BBC particles were 63–66 % smaller, more fragmented, and showed greater surface roughness and crystallinity than MBC. The largest change in leachate chemistry pH, electrical conductivity, water-soluble organic carbon and its spectral indices occurred during the first 50 days, coinciding with the highest HM release. Leaching kinetics followed a first-order model, with lower HM release rate constants in BBC-amended columns. BBC also showed better efficiency of HM immobilization: Cu and Pb leaching reduction was 57.6 % and 22.9 %, Cu and Pb reduction in mobile fractions was 19.0 % and 24.2 %, and Cu and Pb increase in stable fractions was 30.2 % and 10.0 %, respectively. Using nine soil indicators, the soil quality index was highest for BBC-amended soil. These findings demonstrate greater long-term effectiveness of BBC than MBC in immobilizing Cu and Pb. They also provide evidence linking biochar microstructure to HM leaching kinetics, effluent chemistry, and soil quality, supporting BBC as a practical and sustainable amendment for smelter-contaminated soils.