Thermal condensation drives molecular evolution of biochar-derived dissolved organic matter for heavy metal immobilization

ABSTRACT Biochar-derived dissolved organic matter (BDOM), the soluble fraction of biochar, plays an important yet underrecognized role in environmental chemistry, here exemplified by heavy metal mobilization/immobilization. Current studies largely emphasize biochar as a beneficial additive to soil, but neglect the specific role of the highly functional BDOM. In an integrated approach combining spectroscopic analysis, ultrahigh-resolution mass spectrometry, and reaction network modeling, we show that BDOM is the overlooked reactive fraction in biochar application. The complex condensation-derived molecular structures of BDOM across a range of typical pyrolysis temperatures (300-700℃) was analyzed, and molecular transformations changed by metal coordination were elucidated. We demonstrate that higher pyrolysis temperatures facilitate decomposition of humic aggregates and form tyrosine-like phenolic ligands. Using cadmium (Cd) binding as a model metal-interaction process, we show that low temperature pyrolysis produces soluble condensation products which mobilize Cd, while Cd mobility is strongly reduced by the high temperature pendants. This transformation occurs via coordinative mechanisms involving rearrangement of functional groups, conformational changes in polymer structures, and hydrophobic microencapsulation. Finally, we demonstrate that the optimized coordination structure significantly suppresses Cd mobility in the environment and especially minimizes plant uptake. Our thermally controlled synthesis sheds a new light on the role of BDOM and provides a rationale for the controversial reports on different biochars in soil applications.