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• Freezing concentration effect facilitates the reaction between LMWOA and Cr(VI). • -OH/-COOH/=O located on carbon atoms at α positions play a crucial role. • Five-membered ring complex of Cr(VI)-LMWOA is the key intermediate. • The unique hydrolysis process in the liquid-like layer affects Cr(VI) reduction. • The formation of Cr(III)-LMWOA will have an impact on the stoichiometry. Low molecular weight organic acids (LMWOA) are commonly present in natural water and play a pivotal role in the reduction of Cr(VI). In frozen solutions, the efficiency of Cr(VI) reduction is significantly enhanced due to the freezing concentration effect. However, this facilitation is found to be contingent upon the functional groups of LMWOA in this study. To be specific, LMWOA and Cr(VI) can form five-membered ring complexes, which greatly enhance electron transfer efficiency through Ligand-to-Metal Charge Transfer (LMCT). DFT calculations indicate that oxygen-containing groups located on carbon atoms at α positions play a crucial role in forming these complexes, ultimately determining the kinetics of Cr(VI) reduction. Moreover, freezing not only increases proton concentrations but also reduces free water molecule content in the liquid-like layer (LLL), thereby affecting LMWOA species through regulation of protonation and hydrolysis, and subsequently impacting reaction mechanisms. The stoichiometric ratios between LMWOA and Cr(VI) exceed theoretical values due to complexation with Cr(III). The reduction of Cr(VI) by LMWOA in frozen solutions is inhibited by soil solution, while the degree of inhibition varies among different types of LMWOA. [Display omitted] [ABSTRACT FROM AUTHOR]