Extracellular microbial products and sulfate reduction as potential tools for integrating methane production with nickel and cobalt recovery in anaerobic digestion technologies

Resumen

The rapidly growing production of waste materials containing relatively high nickel (Ni) and cobalt (Co) concentrations, from industries such as electronics and metallurgy, presents an acute environmental concern due to the ecological toxicity of these metals at high concentrations. Furthermore, given the extensive industrial applications and diminishing primary sources of these metals, the recovery of Ni and Co from secondary sources, such as mining waste, is now of paramount importance. However, due to the similar chemical characteristics of Ni and Co, selective recovery of these metals from secondary sources poses significant challenges in metallurgical practices. Microorganisms have shown promising results for application in selective metal recovery, offering an eco-friendly approach to address this challenge. In this context, microbial communities in anaerobic digestion (AD) systems present intriguing candidates for selective metal recovery, due to their innate metal-stress attenuation mechanisms, including sulfide production and extracellular microbial products. Notably, the capacity of AD microbiomes to reduce metal bioavailability presents the potential for integrating metal recovery with biomethane production. A crucial mechanism employed by microbial communities in AD to address metal stress, and enable metal recovery, is the production of extracellular microbial products, particularly Extracellular Polymeric Substances (EPS) and Soluble Microbial Products. These compounds encompass functional groups with metal-binding potential, that can selectively sequester metal ions and facilitate their selective recovery from mixed-metal wastes. The impacts of metal stress on extracellular microbial products and the potential for selective metal recovery has been extensively reviewed in this thesis. Furthermore, biomethane potential assays with different concentrations of Ni and Co were conducted using sewage sludge as the inoculum and glucose as the carbon source to study the relationships between extracellular microbial products, metal solubility, and methanogenic activity under Ni- and Co-stressed conditions. The protein content of EPS, and extracellular soluble protein fractions, decreased with increasing concentrations of Ni and Co. Decreasing protein content of EPS was accompanied by reduced methanogenic activity and an increase in the soluble metal fraction. The strong associations observed between these variables could be due to the critical role of EPS in protecting microbial cells against Ni and Co stress, possibly by sequestering metal cations through their functional groups, thus reducing metal availability to the methanogenic consortia underpinning the AD process. Sulfate reduction is another promising microbial process in anaerobic digesters for integrating metal recovery into methane production due to sulfide production which precipitate metal ions. However, sulfate-reducing bacteria (SRB) can compete with methanogens for substrates and potentially reduce the performance of anaerobic digesters when sulfate availability is high. Hence, investigating the complex interactions between SRB, methanogens and metals is required for using the potential of SRB for metal recovery in AD. Therefore, sulfate was introduced to a granular-sludge-based AD system to promote sulfate reduction and the interplay between SRB, methanogens and metal solubility in AD was investigated under no-metal, Ni- and/or Co-stressed conditions. The study showed that sulfate reduction activity in AD can enhance methanogenic activity of granular sludge both in the presence and absence of metal stress. This enhancement could be attributed to the production of sulfide, which not only precipitates metal and reduces their toxicity but also, as an alkaline agent, could neutralize the acidic pH resulting from VFA accumulation in AD. SRB also facilitated propionate anaerobic degradation by consuming H2, relieving thermodynamic constraints by H2 partial pressure. The novel discovery of enhanced methane production by SRB challenges conventional knowledge about the impact of SRB activity on AD methanogenesis, and points toward significant potential to leverage SRB activity to alleviate acidification and enhance the performance of acidified anaerobic digesters. Additionally, the metals in AD precipitated sulfide ions and reduced the emission of toxic and corrosive hydrogen sulfide. Furthermore, the activity of SRB in AD effectively precipitated Ni and Co and a significant disparity was observed in the solubility of Ni and Co in all experiments with added metals, indicating the potential for selective recovery of these metals from waste using AD. Considering these results collectively, coupling methane production with selective metal recovery through AD technology offers a promising approach for treating wastes containing high sulfate and metal concentrations.