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• BDOC promoted the dehydrochlorination of 1,1,2,2-tetrachloroethane in basic solution. • Synergies of dipole-dipole and conjugate bases was the dominant catalysis mechanism. • The promoting effect was enhanced at a higher pH by forming more conjugate bases. • >1 kDa fraction of BDOC presented greater promoting effect than <1 kDa fraction. • N-containing species and aromatic protein-/polyphenol-like matters played key roles. The environmental effects of biochar-derived organic carbon (BDOC) have attracted increasing attention. Nevertheless, it is unknown how BDOC might affect the natural attenuation of widely distributed chloroalkanes (e.g., 1,1,2,2-tetrachloroethane (TeCA)) in aqueous environments. We firstly observed that the kinetic constants (k e) of TeCA dehydrochlorination in the presence of BDOC samples or their different molecular size fractions (<1 kDa, 1∼10 kDa, and >10 kDa) ranged from 9.16×103 to 26.63×103 M−1h−1, which was significantly greater than the k e (3.53×103 M−1h−1) of TeCA dehydrochlorination in the aqueous solution at pH 8.0, indicating that BDOC samples and their different molecular size fractions all could promote TeCA dehydrochlorination. For a given BDOC sample, the kinetic constants (k e) of TeCA dehydrochlorination in the initial pH 9.0 solution was 2∼3 times greater than that in the initial pH 8.0 solution due to more formation of conjugate bases. Interestingly, their DOC concentration normalized kinetic constants (k e /[DOC]) were negatively correlated with SUVA 254 , and positively correlated with A 220 /A 254 and the abundance of aromatic protein-like/polyphenol-like matters. A novel mechanism was proposed that the C H dipole of BDOC aliphatic structure first bound with the C Cl dipole of TeCA to capture the TeCA molecule, then the conjugate bases (-NH-/-NH 2 and deprotonated phenol-OH of BDOC) could attack the H atom attached to the β-C atom of bound TeCA, causing a C Cl bond breaking and the trichloroethylene formation. Furthermore, a fraction of >1 kDa had significantly greater k e /[DOC] values of TeCA dehydrochlorination than the fraction of <1 kDa because >1 kDa fraction had higher aliphiticity (more dipole-dipole sites) as well as more N-containing species and aromatic protein-like/polyphenol-like matters (more conjugate bases). The results are helpful for profoundly understanding the BDOC-mediated natural attenuation and fate change of chloroalkanes in the environment. [Display omitted] [ABSTRACT FROM AUTHOR]