Removal and Recovery of Toxic Silver Ion Using Deep-Sea Bacterial Generated Biogenic Manganese Oxides

Products containing silver ion (Ag⁺) are widely used, leading to a large amount of Ag⁺-containing waste. The deep-sea manganese-oxidizing bacterium Marinobacter sp. MnI7-9 efficiently oxidizes Mn²⁺ to generate biogenic Mn oxide (BMO). The potential of BMO for recovering metal ions by adsorption has been investigated for some ions but not for Ag⁺. The main aim of this study was to develop effective methods for adsorbing and recovering Ag using BMO produced by Marinobacter sp. MnI7-9. In addition, the adsorption mechanism was determined using X-ray photoelectron spectroscopy analysis, specific surface area analysis, adsorption kinetics and thermodynamics. The results showed that BMO had a higher adsorption capacity for Ag⁺ compared to the chemical synthesized MnO₂ (CMO). The isothermal absorption curves of BMO and CMO both fit the Langmuir model well and the maximum adsorption capacities at 28°C were 8.097 mmol/g and 0.787 mmol/g, for BMO and CMO, respectively. The change in enthalpy (ΔH^θ) for BMO was 59.69 kJ/mol indicating that it acts primarily by chemical adsorption. The change in free energy (ΔG^θ) for BMO was negative, which suggests that the adsorption occurs spontaneously. Ag⁺ adsorption by BMO was driven by entropy based on the positive ΔS^θ values. The Ag⁺ adsorption kinetics by BMO fit the pseudo-second order model and the apparent activation energy of E_a is 21.72 kJ/mol. X-ray photoelectron spectroscopy analysis showed that 15.29% Ag⁺ adsorbed by BMO was transferred to Ag(0) and meant that redox reaction had happened during the adsorption. Desorption using nitric acid and Na₂S completely recovered the Ag. The results show that BMO produced by strain MnI7-9 has potential for bioremediation and reutilization of Ag⁺-containing waste.