Strategies for nickel and cobalt recovery from mine-impacted water using sulfate-reducing bacteria

With the increasing world demand and price of valuable metals such as nickel (Ni) and cobalt (Co), it has become more attractive to recover these metals from secondary streams such as mine-impacted water. Biogenic sulfide produced through a biological sulfate reduction process is a promising approach compared with the traditional hydroxide or chemical sulfide precipitation for metal recovery, as it can be produced on site and utilize the substances that already exist in the waste streams. Moreover, it is effective even with low metal concentrations and low pH solutions due to the low solubility products of the metal sulfides.However, the generation of fine particles in the sulfide precipitation process is a challenge and usually results in poor settling, which deteriorates the metal recovery efficiency. Despite this, the compositions of biogenic sulfide solutions from different sources are various depending on the electron donors and the influent characteristics, which may affect the metal precipitation reactions and particle settling properties. Therefore, the effect of using chemical sulfide and four biogenic sulfide-containing solutions for Ni and Co precipitation and settling was evaluated in this thesis. Results showed that phosphate was the main component enhancing particle settling whereas acetate and sulfate exhibited a smaller influence. The Ni and Co recovery were also assessed in a novel single-stage up-flow fixed-bed sulfidogenic bioreactor treating metal and sulfate-containing wastewater (0-200 mg/L Ni and 0-10 mg/L Co). Over 99% of Ni and Co precipitated and settled in the bioreactor regardless of the initial metal concentration. Bacterial communities attached on the carrier materials and metal precipitates collected from the bioreactor bottom were also characterized. These findings helped describe the simultaneous treatment of sulfate- and metal-containing wastewater in a sulfidogenic bioreactor and provided information on how to scale up the reactor and to optimize the metal recovery efficiency.Ni and Co often co-exist in ores and the associated mine waste; however, their similar chemical behaviors have made the selective recovery of both metals a challenge. The addition of chemicals and energy in hydrometallurgy are common practices for Ni-Co concentrates that allows selective separation through precipitation, ion exchange and solvent extraction. Although these options are economically feasible for concentrates, the same technologies may not necessarily be attractive for secondary sources with lower, but still significant metal concentrations. Therefore, a new approach to separate Ni and Co using sulfate-reducing bacteria (SRB) based on the different microbial responses to metal stress was proposed in this thesis. Results show that Ni displayed higher solubility (52-99%) than Co (0.6-15.5%) in the presence of SRB and sulfide. This could be partially explained by the higher Ni stress to SRB that induced the production of extracellular proteins that can selectively complex Ni and stabilize it in the aqueous phase. Metagenomic and metaproteomic analyses were also conducted in each metal system to identify the proteins that could be responsible for Ni complexation. The Ni inhibition to SRB with different substrates and pH values was also evaluated. The lactate-fed SRB showed the best Ni resistance capability and fastest Ni precipitation rate, while the SRB fed with formate were the most vulnerable to Ni. These results helped to better understand the metal-microbial interactions, providing insights for the optimized design of biological sulfate reduction process, which will contribute to a broader application in the treatment of mine-impacted waters