A novel bioleaching approach for metal(loid) mobilisation from mine waste by halophilic sulfur-oxidising bacteria

Mining industries in the European Union (EU) have been disposing of waste from mining activities for over a century, accounting for 29% of the EU-28's current waste output. These mine wastes usually contain elevated amounts of valuable and hazardous metal(loid)s, which may pose environmental risks but can also provide opportunities for resource recovery. Reprocessing of mine waste can benefit the economy by meeting some of the increasing global demand for raw materials, while also providing environmental benefits by mitigating the environmental risks associated with mine waste. Bioleaching is considered a more sustainable and cost-effective technology for the extractive metallurgy of refractory and low-grade ores (including waste materials), in comparison to other methods such as pyrometallurgy. Despite its acceptance, bioleaching has remained limited in its use in the mining industry and has only found application as a niche technology. Bioleaching operations are hindered by the presence of chloride ions, which affect the growth and activities of conventional acidophilic bioleaching prokaryotes. This vulnerability restricts the application of bioleaching, especially in areas such as Chile and Western Australia, where soil and water sources have high chloride content and obtaining fresh water for mineral processing is scarce and becoming a financial burden. This has generated significant interest in discovering halotolerant microorganisms capable of bioleaching in seawater media. Furthermore, bioleaching with acidophilic organisms is performed at a pH of ≤ 2 and can therefore lead to the acidification of the environment. Therefore, it is worthwhile to investigate the bioleaching of mine waste at circumneutral pH, as it may be beneficial to the environment.
In order to reduce the environmental risks associated with mine wastes, as well as economically recover valuable metals while contributing to the ongoing search for halotolerant organisms for saline water bioleaching, this thesis aimed to develop an alternative bioleaching approach for the bioprocessing of mine wastes in the presence of chloride ions and/or at circumneutral pH. Initially, three mine waste samples originating from the active Neves Corvo mine in Portugal and the closed Freiberg mine in Germany were assessed for their potential environmental risks (Chapter 2). The metal(loid)s in the waste samples were partitioned into seven operationally defined geochemical fractions using the Zeien and Brummer sequential extraction scheme (Zeien and Brummer, 1989) along with chemical and mineralogical analysis. This study revealed that certain elements, particularly Pb and Zn, were highly mobile in the three mine waste samples and could therefore be easily released into the environment, potentially contaminating important human resources such as surface water, soil, and plants, and may be incorporated into the food chain. The possibility to simultaneously generate economic value and reduce environmental risks via the bioprocessing of mine waste was demonstrated in Chapter 3. In this study, a novel acidophilic consortium mainly dominated by the iron-oxidizing Leptospirillum genus and Acidiphilium sp. simultaneously recovered both valuable and hazardous metal(loid)s from the Neves Corvo mine’s waste rock (NC_01) and tailings (NC_02) samples. Over 70% of the total Zn, Co, In, As and Cd contents of the two waste samples were solubilised, as well as 55 - 65% of Mn. However, the recovery of Cu was refractory (21 – 33%) and Pb was not solubilised, as they were mainly co-precipitated with biogenic jarosite. Scanning electron microscope-based automated image analyses (SEM/MLA-GXMAP) and X-ray diffraction (XRD) detected a reduction in the pyrite and silicate contents of both NC_01 and NC_02 after bioleaching by the acidophilic consortium, as well as the formation of secondary minerals, mainly jarosite.
The screening of new organisms for bioleaching potential is presented in Chapters 4, 5 and 6. Four halophilic neutrophile sulfur-oxidising bacteria (Thiomicrospira cyclica, Thiohalobacter thiocyanaticus, Thioclava electrotropha, and Thioclava pacifica) (Chapter 4) and two heterotrophic bacteria (Alicyclobacillus acidiphilus and Brevundimonas sp.) (Chapter 6) were screened for bioleaching potential by evaluating their rates of metal(loid) mobilisation from NC_01 into solution. Bioleaching results revealed T. electrotropha and T. pacifica as the most promising for bioleaching as they both leached about 30% of the total Co content of NC_01, as well as between 8 – 17% of other metal(loid)s (Cu, Pb, Zn, K, Cd, and Mn). The study also showed that roasting the waste rock in a microwave at 400 and 500 °C improved the bioleaching efficiency of T. electrotropha for Pb (13.7% to 45.7%), Ag (5.3% to 36%), and In (0% to 27.4%). In Chapter 5, the two promising organisms were assessed for their capacity to also mobilise metal(loid)s from NC_02. The maximum recoveries for Cu, Pb, Zn, Co, As, Cd, K, Sb, Ag & Mn from NC_02 were between 2 – 24%, slightly lower than the recoveries from NC_01. SEM/MLA-GXMAP did not detect any difference in the mineralogy of both NC_01 and NC_02 before and after bioleaching by the two promising halophilic bacteria.