Bioleaching of metal polluted mine tailings aided by ultrasound irradiation pretreatment

Bioleaching of metal mine tailings is a well-known technology capable of recovering metals from these wastes in a sustainable way. Its major drawbacks are the relatively slow extraction rate and low efficiency, which usually imply long operating times. In this work it was assessed if the application of a pretreatment consisting in ultrasounds (US) irradiation could improve the rate and efficiency of the subsequent bioleaching process; additionally, it was also checked if US affects the activity of the acidophilic autochthonous microbial population present in the tailings. Bench-scale slurry phase batch experiments were performed in two steps: (1) US pretreatment and (2) bioleaching using a biostimulation strategy. Different frequencies (37 and 80 kHz) and energy dosages (0–18 kJ g−1) were used. The results obtained showed that the application of US had a positive impact in the process by increasing the leaching rate of all the metals studied (Fe, Al, Zn, Mn, and Cu). The effect of the US was more remarkable for a frequency of 80 than 37 kHz; in addition, a maximum of the bioleaching rate was obtained for an energy dosage in the range 10–15 kJ g−1. Specifically, bioleaching rates increased by 150, 95, 48, 38 and 28% for Zn, Al, Mn, Cu and Fe, respectively, due to the enhancement of the particle fragmentation and mass transfer rates caused by the US irradiation. However, it must be taken into account that high energy dosages could have a detrimental effect over the microorganism’s population, slowing down the overall process of metal leaching.