Microbial hydrogen consumption leads to a significant pH increase under high-saline-conditions: implications for...

Salt caverns have been successfully used for natural gas storage globally since the 1940s and are now under consideration for hydrogen (H₂) storage, which is needed in large quantities to decarbonize the economy to finally reach a net zero by 2050. Salt caverns are not sterile and H₂ is a ubiquitous electron donor for microorganisms. This could entail that the injected H₂ will be microbially consumed, leading to a volumetric loss and potential production of toxic H₂S. However, the extent and rates of this microbial H₂ consumption under high-saline cavern conditions are not yet understood. To investigate microbial consumption rates, we cultured the halophilic sulphate-reducing bacteria Desulfohalobium retbaense and the halophilic methanogen Methanocalculus halotolerans under different H₂ partial pressures. Both strains consumed H₂, but consumption rates slowed down significantly over time. The activity loss correlated with a significant pH increase (up to pH 9) in the media due to intense proton- and bicarbonate consumption. In the case of sulphate reduction, this pH increase led to dissolution of all produced H₂S in the liquid phase. We compared these observations to a brine retrieved from a salt cavern located in Northern Germany, which was then incubated with 100% H₂ over several months. We again observed a H₂ loss (up to 12%) with a concurrent increase in pH of up to 8.5 especially when additional nutrients were added to the brine. Our results clearly show that sulphate-reducing microbes present in salt caverns consume H₂, which will be accompanied by a significant pH increase, resulting in reduced activity over time. This potentially self-limiting process of pH increase during sulphate-reduction will be advantageous for H₂ storage in low-buffering environments like salt caverns.