Removal and recovery of toxic silver ion using deep-sea bacterial generated biogenic manganese oxides

Abstract

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 Mn(2+) 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 Ea 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.

Figure 1. The influence of pH, Ag⁺ concentration and adsorbent type on Ag⁺ adsorption efficiency.

(A), The pH values after the adsorption at different initial concentrations of Ag+ (i) and the adsorption efficiencies of Ag+ by BMO (□) and CMO (▪) at different initial concentrations of Ag+ (ii); (B), the adsorption efficiencies of 20 mmol/L Ag+ by BMO (□), CMO (▪) and the MnI7-9 bacterium only (▴); (C), The pH values after adsorption (i) and the adsorption efficiencies of 0.5 mmol/L Ag+ at different initial pH values (ii); (D), The pH values after adsorption (i) and the adsorption efficiencies of 20 mmol/L Ag+ at different initial pH values (ii).

Figure 2. The Ag+ adsorption kinetics and fitting models.

The adsorption kinetics of 0.5/L Ag+ (A) and 20 mmol/L Ag+ (B) by BMO (□) and CMO (▪). The pseudo-first order model and pseudo-second order model for adsorption of 0.5 mmol/L Ag+ by BMO (C) and CMO (D). The pseudo-first order model and pseudo-second-order model for adsorption of 20 mmol/L Ag+ by BMO (E) and CMO (F).

Figure 3. The Ag⁺ adsorption isotherms at different temperatures for BMO (A) and CMO (B).

Figure 4. The fitting lines of isothermal models of Ag⁺ adsorption.

The fitting lines of different isothermal models at 28°C for BMO (A) and CMO (B), and the Langmuir model at different temperatures for BMO (C) and CMO (D).

Figure 5. The XPS spectra of Ag (3d_5/2) of BMO after absorbing Ag⁺.

The upper black curve represents the observed data. The red curve is the best fit model for the data. The green curve represents the Ag(I) peak, while the pink curve is the Ag(0) peak.