Gold recovery from parts-per-trillion-level aqueous solutions by a nanostructured Mn2O3 adsorbent

We introduce a nanostructured Mn2O3-based adsorbent and a low-cost method that is capable of extracting gold, in the form of metallic nano-to-micrometer-size particles, from down to sub-ppm-level aqueous solutions including seawaters with high yield, good selectivity and recyclability, and environmental benignity. The Mn2O3 adsorbent, prepared by low-temperature synthesis followed by acid treatments, exhibits a mass-fractal-like morphology of agglomerated nanometer-size crystallite grains covered with active protonated sites and a surface area of about 120 m2/g. For aqueous solutions containing ∼100 ppm gold, a yield of 70 mg of gold/g of adsorbent was achieved. A Langmuir-type adsorption isotherm was observed, showing a rapid uptake of gold. The method is well suited to gold recovery from very dilute solutions. For example, over 95% of the gold was recovered from seawater samples containing 0.1 and 1 ppm of added gold. A pilot study of seawater with 1 ppt added gold showed similar results. The protonation-suspension-filtration-washing process can be recycled without noticeable degradation in yield.