| Summary: | We conducted a series of column experiments on tailing heap samples from the Picacho mine in California to determine the ability of the native Gram-negative bacteria, <i>Alcaligenes faecalis</i>, to leach gold. To mimic heap leaching using our technique, unprocessed and unsorted tailings of mixed grain sizes were placed into columns and leached for three weeks with four treatments: (1) deionized water, (2) bacteria, (3) NaCN by bacteria and (4) NaCN. In all of the experimental runs, the total Au (mg) recovered from the columns treated with <i>A. faecalis</i> and NaCN followed by <i>A. faecalis</i> yielded gold amounts that were higher than those from the deionized water control, and lower than the columns treated with cyanide. However, the total yields were variable across runs, which we attribute to the inherent heterogeneity of gold distribution in the samples. Statistical tests show that the yields from the treatments employing bacteria and/or cyanide were different from those that employed deionized water alone. Our results support previous studies that showed that exudates of <i>A faecalis</i> promoted reduction of Au<sup>3+</sup>, catalyzing extracellular Au<sup>0</sup> particles under alkaline conditions. We propose that <i>A. faecalis</i> is a possible novel alternative to cyanide treatment for recovering Au from mine tailings, and recommend optimization of the method.
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