Spent-medium bioleaching of V and Ni from a power plant residual ash (PPR ash) was conducted using organic acids produced by Aspergillus niger. The production of organic acids in a bubble column bioreactor was optimized through selecting three most influencing factors. Under optimum condition of aeration rate of 762.5 (ml/min), sucrose concentration of 101.9 (g/l) and inoculum size of 40 (ml/l), respectively 17,185, 4539, 1042 and 502 (ppm) of oxalic, gluconic, citric and malic acids were produced. Leaching experiments were carried out using biogenic produced organic acids under leaching environment temperature of 60 degrees C and rotary shaking speed of 135 rpm, with various pulp densities of 1, 2, 3, 5, 7 and 9 (% w/v). The results showed that biogenic produced organic acids leached V much more efficiently than Ni so that even at high pulp density of 9 (% w/v), 83% of V was recovered while Ni recovery yield was 30%. (C) 2016 Elsevier Ltd. All rights reserved.