Biochar Catalysis for the Enhanced Abiotic Humification of Polyphenols: An Important Mechanism Helping Sequester Carbon

Abiotic humification, dominated by catalytic oxidation, is one of the critical mechanisms for organic carbon preservation in nature. However, the effects of biochar catalysis on abiotic humification have not yet been elucidated. This study investigated the catalytic power of biochar from walnut shells at different temperatures (300 °C, 600 °C, and 900 °C) for the abiotic transformation of hydroquinone (HQ) as a representative polyphenol. All the biochar samples catalyzed HQ polymerization, resulting in the formation of humic polymers such as fulvic acids (FAs) and humic acids (HAs). Light and oxygen promoted HA formation. HO• was detected in the BC600–HQ reaction system, and HO• quenching resulted in a 41.22% decrease in HA production, indicating that HO• plays a major role in the oxidative polymerization. In the proposed pathway for the abiotic humification, biochar active sites and generated reactive oxygen species accept an electron from HQ, resulting in oxidation to (semi)quinone radicals, which subsequently undergo cleavage or a coupling reaction to form the oligomerized products. Under BC600 catalysis, the weight-average molecular weight (Mw) of the reaction products of HQ, glucose, and glycine reached 14,449 Da. These findings provide new insights into the application potential of biochar for promoting soil carbon sequestration.