Investigation on the Efficiency and Mechanism of Iron and Manganese Removal from Acid Mine Drainage Using Serpentine-Loaded Manganese Oxide

Iron and Manganese pollution were widespread in the water of northern Chinese mines, exacerbating regional water scarcity and potential ecological issues. This study aimed to remove dissolved iron and manganese from acid mine water using serpentine-loaded manganese oxide (Serp-MO) prepared by a simple coprecipitation-loaded metal method. Serp-MO was microscopically characterized, and the adsorption performance and adsorption mechanism of the Serp-MO composite adsorbent for Fe²⁺ and Mn²⁺ were analyzed. After loading, the specific surface area and pore volume of the particles significantly increased, and the surface pore structure improved, which was conducive to the simultaneous adsorption and removal of iron and manganese. The optimal reaction conditions for Serp-MO treatment of composite water samples with Fe²⁺ and Mn²⁺ mass concentrations of 20 mg/L and 5 mg/L respectively, were as follows: dosage of Serp-MO 550 mg/L, temperature 35 ℃, oscillation rate 180 r/min, and reaction time 140 min. Under these conditions, the removal rates of Fe²⁺ and Mn²⁺ were 99.8% and 99.6%, respectively. The presence of coexistence cations Zn²⁺ and Cu²⁺ can inhibit the removal of Fe²⁺ and Mn²⁺ by Serp-MO, while SO₄^2- can promote the removal. The pseudo-second-order kinetic model and Langmuir isothermal adsorption model well described the adsorption process of Fe²⁺ and Mn²⁺ by the Serp-MO. The maximum adsorption capacities of Serp-MO for Fe²⁺ and Mn²⁺ were 14.31 mg/g and 24.04 mg/g, representing improvements of 470.12% and 626.28%, respectively, compared to Serp-MO. The adsorption thermodynamics showed that ΔG was <0 and ΔH and ΔS were greater than 0 at all test temperatures, which was an entropy-increasing reaction, and that increasing the temperature was favorable for the removal of Fe²⁺ and Mn²⁺. Based on the analysis of adsorption products, Serp-MO primarily facilitates the precipitation of iron in water, while the removal of manganese was mainly by adsorption. A regeneration study over five cycles indicated that Serp-MO possessed promising reusability potential. Furthermore, the safety leaching test indicated that the material caused minimal secondary pollution.