Manganese Oxide-Modified Serpentine for Manganese Removal from Groundwater: Performance and Optimization

Manganese (Mn) concentrations in many groundwater supplies exceed the national drinking water limit of 0.05 mg/L, exacerbating regional water scarcity and potential ecological issues. Adsorption, as an efficient process for wastewater treatment. For the adsorption process, the adsorbent was the key parameter. Manganese oxides have strong specific adsorption and catalytic oxidation capacity for Mn(II). Serpentine loading was an effective way to improve the dispersion of manganese oxides and the adsorption efficiency of heavy metals. This study aimed to remove dissolved Mn(II) from groundwater using a raw serpentine modified with manganese oxide (Srp/MO). SEM, EDS, XRD, BET and XPS were used to characterize the physicochemical properties and characteristic groups of the serpentine before and after loading. Response surface methodology (RSM) based on Box-Behnken method was used to design the experiment to optimize the effects of dosage, oscillation rate and reaction time on manganese removal. The results show that after modification, the pores of serpentine become larger, the surface becomes smooth and loose, the basic skeleton has no obvious change, the specific surface area increases, and the total pore volume decreases. The adsorption behavior of Mn(II) by Srp/MO follows Langmuir isotherm model and pseudo-second-order kinetic model. The maximum adsorption rate of Mn(II) in water by Srp/MO was 34.88 mg/g, which was 7.6 times that of the raw serpentine. The primary mechanism for Mn(II) removal by Srp/MO was based on physical adsorption and mediated by Mn³⁺/Mn⁴⁺ species. The optimal manganese removal conditions were as follows: Srp/MO dosage 80 mg/L, oscillation rate 180 r/min, reaction time 104 min. At this time, the manganese removal rate was 99.3%, which was basically consistent with 100% predicted by the model. A regeneration study over three cycles indicated that Srp/MO possessed promising reusability potential. In conclusion, Srp/MO was an excellent adsorption material for the removal of Mn(II) in groundwater, which provides a new solution for the purification treatment of manganese containing groundwater.