Search

The increasing demand for gold has encouraged the development of cost effective and environmentally friendly techniques to recycle and recover this precious metal from waste liquors. Earlier experiments have shown the appreciable ability of alfalfa shoot biomass to accumulate Au(III) from aqueous solutions. Likewise, alfalfa biomass is able to bind other metal ions in noticeable amounts. Thus, experiments with solutions containing multiple metal ions were performed in order to determine the effect of other ions on gold accumulation by alfalfa biomass. Batch laboratory experiments showed that Au(III) binds in a rapid and pH-independent-like manner in the presence of other metal ions, which agrees with experiments using solutions containing only Au(III). In addition, Au(III) was found to have one of the highest metal binding affinities to the alfalfa biomass compared to Pb(II), Cr(III), Cu(II), Zn(II), Ni(II), and Cd(II), even at the optimum binding pH for the other metal ions (pH 5.0). A similar behavior for Au(III) accumulation was observed under flow conditions. Nevertheless, it was determined that the selectivity of the immobilized alfalfa biomass could be shifted towards Au(III) by decreasing the pH of the multi-metal solution to 2.0 (similar to mining and industrial environments where aqueous gold is found). Alfalfa biomass was also found to accumulate Au(III) efficiently even when Ca and Mg ions were added to the multi-metal solutions at concentrations up to 10 000 times higher than Au(III). These results will aid the development of a novel method to recover gold ions from aqueous solutions utilizing alfalfa biomass.

Existing technologies for the extraction and recovery of gold include cyanidation which uses toxic cyanide. Because of the potential health threat created by many of these processes, an increasing need to develop new safer technologies to extract and recover gold from aqueous solution has arisen. Previous experiments have shown that gold(III) is adsorbed by alfalfa biomass at high concentrations in an almost pH-independent manner. However, little data exist for the bio-adsorption of trace levels of gold(III) by alfalfa biomass. Batch laboratory pH profile experiments were performed at trace levels with native alfalfa biomass using a 100 ppb gold(III) aqueous solution. The observed trend in the binding by the alfalfa biomass was highest at pH 2–3 (≈97%) and decreased to approximately 50% at pH 4.0 and 5.0. To explore the effects of hard cations on the trace level gold(III) binding, calcium, magnesium, and sodium cations were added to the gold(III) solutions and binding experiments were repeated to determine how these ions might interfere with the gold(III) bio-adsorption by the biomass. Experiments were also performed using all three cations combined in solution to further explore the effects of hard cations on the trace level gold(III) binding to the alfalfa biomass. All of the interference experiments showed that under optimal conditions the presence of the hard cations in concentrations as high as 3000 times the ppm concentration of gold(III) did not greatly effect binding by the alfalfa biomass.