Differential regulation mechanisms of Low-molecular-weight organic acids on Cd/As adsorption by Manganese-modified biochar: Multiscale interfacial processes in aqueous and soil environments

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 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 the changes in the ecological risks of Cd and As under the influence of LMWOAs. The results showed that in aqueous solutions, chemical adsorption was the core mechanism for Cd and As adsorption by MBC, which can be described by the pseudo-second-order kinetic model and the Langmuir model (for Cd) / heterogeneous model (for As), respectively. Lower concentrations of LMWOAs (0.1–1 mM) significantly enhanced As adsorption capacity (10.8%-62.3%) by promoting As oxidation and MBC protonation, while LMWOAs primarily inhibited Cd adsorption. In soil environments, LMWOAsdecreased soil pH, increased electrical conductivity, and promoted the relative abundance of Firmicutes and Bacteroidetes bacteria, thereby enhancing the availability of Cd (29.0%–84.6%) and As (9.0%–37.7%).Ultimately, this led to the conversion of Cd and As from stable forms to bioavailable forms, significantly elevating Cd ecological risks 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, while their effects in aqueous environments depend on the concentration and pollutant type.