Mechanistic understanding of copper (Cu) mobility in aged biochar: Influence of dissolved organic matters release and oxygen-containing groups

The long-term behavior of copper (Cu) in biochar-amended soils is not well understood, particularly concerning the role of dissolved organic matter (DOM) and oxygen-containing functional groups formed during biochar aging. This study hypothesizes that aging-induced release of DOM and surface oxidation, both influenced by pyrolysis temperature, differentially affect Cu mobility in soil systems. To test this, biochars were produced from granular anaerobic sludge, coffee grounds, and rice straw at 300°C and 700°C, and applied as both biochar solid and aqueous extracts to Cu-contaminated soil and synthetic model soils with Cu-adsorbed ferrihydrite or hematite. Low-temperature biochar extracts (LDOM) significantly increased Cu mobility, with pore water concentrations rising from 0.7–2.8 mg/L (1 day) to 11.9–19.6 mg/L (28 days), while high-temperature biochar extracts (HDOM) showed no Cu solubilization. LDOM mobilized Cu from crystalline hematite but not from ferrihydrite, while HDOM transformed Cu into more stable oxidizable forms. Aged biochars exhibited enhanced Cu adsorption due to the formation of surface O-functional groups, although DOM released from biochar decreased adsorption efficiency. Surface complexation modeling supported these trends, indicating increased binding affinity after aging. This study demonstrates that biochar aging significantly alters Cu behavior in soil, depending on the interplay between DOM release and surface chemistry, which are both dictated by the original pyrolysis temperature. This has potential applications in heavy metal remediation and sustainable soil management practices, offering a strategy for mitigating Cu contamination in agricultural and environmental settings.