Recovery of gold and silver from concentrated solutions using tannin-based hydrophobic biosorbents

Tannin is a water-soluble compound and so it must be converted into an insoluble form to be used as an adsorbent in an aqueous medium. Three different versions of tannin were prepared: tannin resin (gel), tannin foam with chemical cross-linking, and torrefied tannin with heat treatment. Through torrefaction at 300 °C and carbonization at 600 °C, tannin powder was converted from hydrophilic tannin into a hydrophobic biosorbent. These prepared materials were tested for their ability to recover gold and silver under different pH conditions. It was determined that the torrefied tannin showed excellent adsorptive performance for gold, reaching up to 99.1 % with adsorbed amounts up to 2753 mg/g under acidic conditions. The adsorption capacities of the chemically cross-linked derivatives could not compete with that of torrefied tannin. Torrefied tannin provided a silver adsorption performance of 98.9 % at pH 9, which is superior to the resin and foam forms of tannin. It is concluded that torrefaction, which is a very simple and easy method, can be applied to prepare highly efficient hydrophobic biosorbents instead of using chemically cross-linked derivatives that require expensive and time-consuming procedures involving harmful chemicals.