Study of the Reaction Mechanism of the Excessive Adsorption of Mn2+ from Water by In Situ Synthesis of MnO2@SiO2 Colloid as an Adsorbent

An in situ-generated MnO₂@SiO₂ colloidal (ISMC) composite was used for the adsorption of Mn²⁺ ions in water. The adsorption capacity of ISMC at a concentration of 1 mg/L at 25 °C was as high as 3017.97 mg/g for the original concentration of 50 mg/L Mn²⁺ ions. Material characterization revealed that it is a porous sponge with a fibrous structure with a rough surface, many folds, and abundant pores, and these features provide many adsorption sites, which are conducive to the attachment of Mn²⁺ ions on its surface. ISMC has an isoelectric point of 3.5, indicating a negative surface charge that favors electrostatic attraction of Mn²⁺ ions. The surface hydroxyl groups provide additional active sites that allow for strong complexation with Mn²⁺ ions. Adsorption conformed to the Freundlich isotherm model (R² > 0.98), suggesting multilayer adsorption, followed by pseudo-second-order kinetics (R² > 0.98), with an optimum adsorption time of approximately 12 h. Low temperatures favor physical adsorption, whereas higher temperatures promote chemisorption via hydroxyl group complexation. The adsorption capacity increased with pH, which was attributed to the increased presence of surface hydroxyl groups. These findings highlight the significant potential of ISMCs for cation adsorption in water treatment applications.