Investigation of platinum recovery from a spent refinery catalyst with a hybrid of oxalic acid produced by Aspergillus niger and mineral acids

•Three modes of direct (one step and two step) and indirect bioleaching were examined.•pH adjustment during the direct bioleaching improves the platinum recovery significantly.•Recovery increased by pH reduction and temperature increase in the indirect system.•37% of the catalyst platinum content was recovered at the optimum conditions.•Fungi produced oxalic acid showed a great potential for platinum recovery. The capability of oxalic acid produced by Aspergillus niger was investigated for bioleaching of platinum from a refinery reforming catalyst. The spent medium mode was selected for bioleaching because of its higher efficiency at favorable pH and temperature conditions. The effects of several important factors such as the pulp density, pH and temperature on platinum recovery were optimized using Box-Behnken design of response surface methodology. The results indicated that pH adjustment during the bioleaching process increases the final platinum recovery significantly. The obtained optimum conditions were 1% for the pulp density, 0.5 for the medium pH, and 70 °C for the temperature which led to 37% platinum recovery. The significance of oxalic acid as the leaching agent in platinum bioleaching was highlighted by investigating the recovery of a blank medium without oxalic acid at the optimum conditions which was just about 13%. The presented method can be utilized in an environmentally friendly process to recover platinum from industrial catalysts.