Imidazolium ionic liquids as potential persistent pollutants promote the conversion of waste activated sludge to volatile fatty acids in anaerobic fermentation without promoting hydrogen production

Abstract

In this study, [C8mim]PF6, or 3-methyl-1-octylimidazolium hexafluorophosphate was used to fill the gap in the knowledge regarding the effect of ionic liquids (ILs) on anaerobic hydrogen production of waste activated sludge (WAS). The 800 mg/L [C8mim]PF6 solution reduced hydrogen production by 66.26% and 45.78% but increased volatile fatty acids (VFAs) production by 140.15% and 133.75% at mesophilic and thermophilic conditions, respectively. Dynamics of the dissolved organic matter indicated that [C8mim]PF6 enhanced WAS solubilization by inducing cell rupture and enhancing complexation. In situ monitoring of the liquid aliquots revealed that the altered hydrogenase activity shifted the fermentation pathway shifted in a direction unfavorable to hydrogen production. Thermophilic fermentation mitigated the toxicity of [C8mim]PF6 via a more stable hydrogenase activity and fermentation pathway. Amplicon sequence variant analysis illustrated the evolution of microbial alliances and resistant strains that shared ecological niches. ILs leaking into WAS are not conducive to biohydrogen production. With the development of recovery technology, ionic IL may serve as potential substances facilitating the conversion of WAS to VFAs. Compared with thermophilic fermentation, mesophilic fermentation offers the advantages of higher VFA yield and lower cost, reflecting the cleanliness of the process. Our observations can serve as a reference for IL-rich wastewater treatment.