Utilization of A. niger strains isolated from pistachio husk and grape skin in the bioleaching of valuable elements from red mud

Due to the high heavy and rare element contents, bauxite residue rejects (red mud) are simultaneously an ecological hazard and economic virtue. The abilities of two strains of Aspergillus niger isolated from pistachio husk and grape skin were investigated in the bioleaching of metals from the red mud produced in the Iran Alumina Company. Citric acid secretion (~15 g/l) by both strains was ten times more than oxalic acid (~1.5 g/l) and kinetically obeyed the Luedeking-Piret model. Also, both Gompertz and logistic equations correlated with cell growth and substrate consumption. Considering the best results of the bioleaching experiments, the grape-skin strain extracted about 92% Al, 88% V, 67% Ti, 66% Sr, and 29% Sc from the activated residue; while, the metal recoveries by pistachio-husk strain were 53% Al, 31% V, 67% Ti, 44% Sr, and 38% Sc. Finally, according to the kinetic studies, the shrinking core model could perfectly simulate the process, and the product layer diffusion was determined as the rate-determining step.