High-temporal-resolution tracking of manganese(III/IV) (hydr)oxide dissolution kinetics induced by low-molecular-weight organic compounds

The dissolution of manganese (Mn) (hydr)oxides induced by low-molecular-weight organic compounds (LMWOCs) critically influences Mn and carbon cycling, yet limited high-resolution characterization hinders mechanistic understanding. Here, we investigated the dissolution kinetics of Mn(III/IV) (hydr)oxides mediated by citric acid, pyruvic acid, oxalic acid, and hydroquinone, as well as the controlling factors (LMWOC type and concentration, pH, and coexisting ions), by real-time and in-situ electrochemical quartz crystal microbalance (EQCM), complemented by grazing-incidence wide-angle X-ray scattering (GIWAXS) and kinetic modeling. Hydroquinone and citric acid promoted significant dissolution at pH 5 and 6, with hydroquinone dominating initially but slowing later; at pH 7, hydroquinone remained highly effective, far surpassing citric acid. In contrast, pyruvic acid and oxalic acid at pH 5, 6, and 7 showed negligible dissolution. Citric acid– and hydroquinone–mediated dissolution exhibited characteristic S-shaped kinetics with induction, acceleration, and deceleration stages, indicating an autocatalytic pathway, whereas pyruvic acid and oxalic acid showed no such pattern. Analysis of controlling factors showed that citric acid–mediated dissolution increased with increasing citric acid concentration and decreasing pH. Coexisting ions, including NO 3 − , Cl − , SO 4 2− (5–50 mM), and Mg 2+ , Ca 2+ , Zn 2+ (0.5–4 mM), inhibited the dissolution. Notably, HPO 4 2− and P 2 O 7 4− (0.1–1 mM) and Mn 2+ (0.5–4 mM) exhibited concentration-dependent dual effects on dissolution, arising from competing promotive and inhibitory processes. GIWAXS analysis revealed that citric acid–mediated dissolution of Mn(III/IV) (hydr)oxides was facet-dependent, whereas exogenous Mn 2+ promoted non-selective citric acid–mediated dissolution. This study provides insights into complex interfacial geochemical processes on redox-active minerals.