Effects of synthesis temperature on ε-MnO2 microstructures and performance: Selective adsorption of heavy metals and the mechanism onto (100) facet compared with (001)

The heavy-metal adsorbent ε-MnO2 was produced through a simple, one-step oxidation-reduction reaction at three different synthesis temperatures (25 °C, 50 °C and 75 °C) and their morphology and chemical-physical properties were compared. Of the three materials, MnO2-25 had the largest specific surface area and the highest surface hydroxyl concentration. Its optimal performance was demonstrated by batch adsorption experiments with Pb2+, Cd2+ and Cu2+. Of the three metals, Pb2+ was adsorbed best (339.15 mg/g), followed by Cd2+ (107.50 mg/g) and Cu2+ (86.30 mg/g). When all three metals were present, Pb2+ was still absorbed best but now more Cu2+ was adsorbed than Cd2+. In order to explore the mechanism for the inconsistent adsorption order of Cd2+ and Cu2+ in single and competitive adsorption, we combined experimental data with density functional theory (DFT) calculations to elucidate the distinct adsorption nature of MnO2-25 towards these three metals. This revealed that the adsorption affinity of the (100) facet was superior to (001), and since the surface complexes were also more stable on (100), this facet was most likely determining the adsorption order for the single metals. When the metals were present in combination, Pb2+ preferentially occupied the active adsorption sites of (100), forcing Cu2+ to be adsorbed on the (001) facet where Cd2+ was only poorly bound. Thus, the adsorption behavior was affected by MnO2-25 surface chemistry at a molecular scale. This study provides an in-depth understanding of the adsorption mechanisms of the heavy metals on this adsorbent and offers theoretical guidance for production of adsorbent with improved removal efficiency.