Mechanistic insights into Mn(II) retention and release at soil-water interfaces: Contrasting natural soils and hydrochemical controls.

The retention and release of manganese (Mn) at soil-water interfaces control its subsurface mobility and secondary-remobilization potential, yet their dependence on the combined physicochemical and mineralogical properties of natural soils remains poorly resolved. Here, three compositionally contrasting natural soils, designated as sandy (S), clayey (N), and loamy (R) soils, were evaluated under a common batch framework using adsorption-desorption experiments, hydrochemical perturbations, and complementary interfacial characterization. The clayey soil exhibited the highest Mn(II) retention, with a Langmuir-estimated apparent maximum adsorption capacity of 2.0214 mg g-1, followed by the loamy (1.2825 mg g-1) and sandy (0.8231 mg g-1) soils. This retention hierarchy coincided with differences in specific surface area, cation-exchange capacity, organic matter content, and crystalline mineral assemblage. Increasing pH enhanced dissolved Mn attenuation at pH 3-6, whereas soil-free controls showed that abiotic Mn loss contributed substantially to the apparent removal observed at pH 7-9. PHREEQC calculations predicted Mn2+ dominance and undersaturation with respect to Mn(OH)2(s) throughout the investigated pH range. Coexisting Ca2+ suppressed Mn adsorption and promoted the release of retained Mn through competitive ion exchange and inhibition of readsorption. Zeta-potential shifts, FTIR and XPS responses, and Mn enrichment detected by SEM-EDS collectively supported charge compensation, exchange-related retention, and interactions with oxygen-containing surface sites. The Mn 2p envelopes were also compatible with a possible contribution from partial surface-associated oxidation. The preservation of the dominant XRD patterns further indicated that Mn stabilization occurred primarily at mineral interfaces without detectable transformation of the bulk crystalline framework. These results establish a soil-specific retention-release framework for evaluating Mn attenuation and Ca2+-induced remobilization in heterogeneous subsurface environments.