Associate Prof Koyanaka and his research team have developed a nanostructured MnO2-based adsorbent and a low-cost method that is capable of extracting gold, in the form of metallic nano-to-micrometer-size particles [Figure 1], 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 [Figure 2]. 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.
Figure. 1 TEM image of Au (dark) on MnO2 particles (bright): The gold deposited on the surface of adsorbent could be recovered as an enriched AuCl4– solution into added a small volume of diluted HCl solution by an effect of gold dissolution with HClO. After the enrichment process, the adsorbent can be reused with sustainable efficiency for collection of gold such as gold chloride and the sub-sulfuric acid gold complexes.
Figure. 2: 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.