Study Of Nickel-iron Hydrotalcite To Alleviate The Inhibition Of Anaerobic Hydrogen Production Fermentation By Ionic Liquids

The development of clean energy plays an important role in addressing the energy crisis and environmental issues.Biofuels have a promising future as a clean alternative to fossil fuels.Dark fermentation in anaerobic digestion as a green and sustainable treatment method can effectively solve the environmental pollution problem of waste activated sludge（WAS）.This method not only enables sludge stabilization,but also generates biofuels and other value-added products such as biohydrogen（Bio H₂）,biomethane（Bio CH₄）and volatile fatty acids（VFAs）.Anaerobic production of Bio H₂is highly attractive in the context of renewable energy technologies.Compared to Bio CH₄,Bio H₂has higher energy density,milder use conditions,cleaner products,and the ability to integrate waste treatment,environmental protection,and clean energy production.In addition,VFAs,as anaerobic products with high economic value,can be used as an in situ carbon source for phosphorus removal and feedstock for biodegradable plastics production.Despite the remarkable potential of anaerobic fermentation in the treatment of WAS,these substances may adversely affect the biodigestion process due to the wide variety of harmful organic substances in the sludge.For example,sludge may contain a variety of difficult-to-degrade organic compounds,such as antibiotics,heavy metals,and polycyclic aromatic hydrocarbons,which may inhibit microbial activity and reduce fermentation efficiency.Meanwhile,some novel organic contaminants,such as ionic liquids（ILs）,may exert selection pressure on specific microbial communities,thus altering the microbial community structure and function in the fermentation process.In addition,the anaerobic fermentation process involves multiple complex biochemical reactions,such as hydrolysis,acidification,hydrogen production and methanogenesis steps,and the interactions between these reactions and the stability of the microbial community are critical to the final fermentation outcome.However,there are still many unknown factors,such as the dynamics of microbial populations,the influence of substrate type on the fermentation process and the optimization of operating parameters,which are key issues limiting the development of this technology.Therefore,it is important to conduct research on the inhibition of hydrogen-producing fermentation in anaerobic environments.In this study,the effect of[C₈mim]PF₆,an environmental pollutant imidazole-based IL,on the anaerobic hydrogen production process of WAS was first investigated.The results showed that the IL inhibited anaerobic fermentation,leading to a decrease in hydrogen（H₂）production.Indeed,the synthesis of metal compounds and materials with high specific surface area has been shown to improve the stability of anaerobic systems.Therefore,in this study,an attempt was made to mitigate the adverse effects of ILs on anaerobic hydrogen production fermentation by synthesizing nickel-iron hydrotalcite（NiFe-LDH）,a metal compound with high specific surface area.The effects of environmentally relevant concentrations and high concentrations of the[C₈mim]PF₆on anaerobic hydrogen production fermentation were first investigated.It was found that[C₈mim]PF₆inhibited the anaerobic fermentation process of WAS,leading to a decrease in H₂production,but an increase in the production of VFAs.Under mesophilic and thermophilic fermentation conditions,800 mg/L[C₈mim]PF₆solution reduced the hydrogen production by 66.26%and 45.78%,respectively,while the yield of VFAs increased by 140.15%and 133.75%,respectively.Parallel factorial analysis of dissolved organic matter showed that[C₈mim]PF₆increased the dissolution rate of WAS by inducing cell rupture and enhancing complexation.Thermophilic fermentation mitigated the toxicity of[C₈mim]PF₆through more stable hydrogenase activity and fermentation pathways.Compared with high-temperature fermentation,mesophilic fermentation had higher yield of VFAs and lower cost.In addition,the evolution of the microbial community and resistant strains sharing ecological niches was revealed by amplicon sequence variation analysis,which revealed that even a small amount of IL leakage into the WAS could adversely affect Bio H₂production.The results also suggest that ILs have the potential to become a potential substance to facilitate the conversion of WAS to VFAs with the development of recycling technologies.The preformed metal compound NiFe-LDH was then used to develop a dark fermentation system with mixed flora promoting Bio H₂production,providing theoretical support for the optimization of fermentation systems to mitigate contamination by ILs.The highest Bio H₂yields of 232.8±8.5 m L/g glucose and 150.3±4.8 m L/g glucose were observed in the NiFe-LDH fermentation system with 100mg/L mesophilic fermentation and 400 mg/L thermophilic fermentation,which were65.4%and 43.3%higher than the control without NiFe-LDH addition,respectively.Fermentation pathway analysis showed that NiFe-LDH enhanced the butyric acid-type metabolic pathway and the corresponding nicotinamide adenine dinucleotide（NADH and NAD⁺）ratios,as well as increased hydrogenase activity during mesophilic fermentation.The interaction of NiFe-LDH with mixed bacterial flora suggested that NiFe-LDH may contribute to electron transfer and its release of metal elements was responsible for the increased hydrogenase activity.NiFe-LDH also promoted the evolution of the dominant microbial population and altered the content of extracellular polymers,thus increasing the production of Bio H₂.This suggests that anaerobic hydrogen production systems containing NiFe-LDH have the potential to mitigate the contamination of WAS by ILs.Finally,based on the above study a NiFe-LDH-enhanced dark fermentation system（NE-DFS）was developed using mixed flora to alleviate the inhibition of hydrogen production from anaerobic fermentation by ILs.When the IL concentration was below100 mg/L,NE-DFS was able to rapidly enhance the start-up time of mesophilic and thermophilic fermentations,with a greatly reduced lag time compared to the system without NiFe-LDH.At IL concentrations below 400 mg/L,the H₂yields of both the mesophilic and thermophilic fermentation experimental groups of NE-DFS were higher than those of the experimental group without NiFe-LDH addition.The results of these experiments showed that NE-DFS could still increase H₂yield and reduce the inhibitory effect of[C₈mim]PF₆on the fermentation process when the IL concentration was lower than 400 mg.Therefore,the application of NE-DFS in the IL-contaminated wastewater treatment system can improve its efficiency and sustainability.In summary,this study investigated the effects of environmentally relevant concentrations and high concentrations of[C₈mim]PF₆on the anaerobic fermentation process and revealed its inhibition effect.The adverse effects of ILs on anaerobic fermentation were successfully mitigated by constructing a NiFe-LDH-enhanced dark fermentation system.This strategy provides new ideas and insights into the IL industrial leakage problem and the final discharge treatment link of ILs.