<p>Low-molecular-weight organic acids (LMWOAs) can influence the adsorption of cadmium (Cd) and arsenic (As) on manganese-modified biochar (MBC); however, their differential mechanisms of action remain unclear. This study investigated the dominant mechanisms by which LMWOAs affect the adsorption of Cd and As by MBC through kinetic and isothermal adsorption experiments combined with soil incubation experiments to reveal changes in the ecological risks of Cd and As under the influence of LMWOAs. The results showed that chemisorption dominated the adsorption of Cd and As by MBC in aqueous solution, which was well described by the pseudo-second-order kinetic model. The adsorption isotherms of Cd fit better with the Langmuir model, suggesting monolayer adsorption on energetically homogeneous surfaces, whereas As adsorption was better described by the Freundlich model, indicating adsorption on heterogeneous surfaces with non-uniform site energies. Low concentrations of LMWOAs significantly enhanced As adsorption by promoting As oxidation and MBC protonation, whereas LMWOAs exhibited an inhibitory effect on Cd adsorption overall. In soil environments, LMWOAs decreased soil pH, increased electrical conductivity, and promoted the relative abundance of Firmicutes and Bacteroidetes, thereby enhancing the availability of Cd and As. Ultimately, this led to the conversion of Cd and As from stable to bioavailable forms, thereby significantly elevating the ecological risks associated with Cd and As activation. Among the LMWOAs, citric acid exhibited the strongest risk-driving effects, owing to its greatest impact on environmental parameters and microorganisms. This study revealed the environment-dependent regulatory mechanisms of LMWOAs on MBC remediation efficiency, significantly enhancing Cd/As mobility in soil, whereas their effects in aqueous environments depend on concentration and pollutant type.</p> Graphical abstract <p></p>