Technical information

1.    A nanostructured MnO2-based adsorbent and a low-cost method that is capable of extracting gold, in the form of metallic nano-meter-size particles, from down to sub-ppm-level aqueous solutions with high yield, good selectivity and recyclability. For aqueous solutions containing 100 ppm gold, a yield of 70 mg of gold/g of adsorbent was achieved. The method that based on a water splitting by the MnO2 is well suited for gold recovery from very dilute solutions. For example, over 95% of the gold was recovered from seawater samples containing 0.1, 1 ppm and 1 ppt of added gold. At present, conventional activated carbons and ion-exchange reigns are used as the adsorbent of gold, commonly for the collection of gold cyanide in industrial effluents. United Nations Environment Program had declared the policy of abolition of utilization of cyanogens chemicals until 2020 in the Strategic Approach to International Chemicals Management, http://www.saicm.org/ In near future, gold chlorides and sub-sulfuric acid gold will be widely used in industry instead of gold-cyanide. However, conventional adsorbents do not have capability to absorb the gold chloride and the sub-sulfuric acid gold from aqueous solutions at all, unlike the MnO2-based adsorbent.
   

Fig. 1 Yield in units of mg of gold/g adsorbent vs. the gold concentration of the solutions at equilibrium of saturated adsorption. The initial gold concentrations in units of ppm are given next to the data points.

　

 

Fig. 2 SEM image of gold particles (bright) on the surface of MnO2 (dark)

   

1. Mechanism of the electroless deposition of gold
   

1st Anodic reaction

OH- + Mn(IV)O2 (surface) → Mn(III)OO•H (surface) + 1/2O2 + e-  (1)

2nd Anodic reaction

Mn(III)OO•H (surface) → Mn (IV) O2 (surface) + H+ + e- (2)

Cathodic reaction

Au3+ + 3e- → Au       (3)

1. 1. 1. 1. 1. Overall reaction
               

Au3+ + 3/2OH- → Au + 3/4O2 + 3/2H+       (4)

REFERENCES

8. H. Koyanaka, K. Takeuchi, A. I. Kolesnikov, Charge/discharge cycle of electron, proton, and oxygen evolution with precious metal deposition based on a water-oxidation effect of a high purity ramsdellite type manganese dioxide, The Electrochemical Society of Japan, The Spring Annual Conference, (2013)
   

19. S.Iikubo, H. Koyanaka, S. Shamoto, K. Takeuchi, S. Kohara, K. Kodama, C-K. Loong, Local crystal structure of nano-manganese-oxide gold adsorbent, Journal of Physics and Chemistry of Solids, Vol. 71, pp. 1603-1608, (2010)
    

8. H.Koyanaka, K. Takeuchi, and C.-K. Loong, Gold recovery from parts-per-trillion-level aqueous solutions by a nanostructured adsorbent, Separation and Purification Technology, Vol. 43, pp.9-15, (2005)
   

C. -K. Loong and H. Koyanaka : Harvesting precious metals and removing contaminants from natural waters - Can neutrons benefit industrial researchers in Japan, J. Neutron Res. Vol. 13, (1-3), pp.15-19, Mar-Sep., (2005)