Mechanisms of CuO Nanoparticles at an Environmentally Relevant Level Enhancing Production of Hydrogen from Anaerobic Fermentation of Waste-Activated Sludge

Abstract

To date, almost all previous studies reported the inhibitory effects of CuO nanoparticles on the production of methane from waste-activated sludge (WAS), whereas their potential effects on hydrogen production remained unknown. This study investigated how CuO nanoparticles affected the production of hydrogen from WAS to fill this knowledge gap. Experimental results surprisingly showed CuO nanoparticles at an environmentally relevant level (5 mg/g of TS) significantly enhanced hydrogen production by 45.2 ± 0.1% compared to the control, while CuO nanoparticles at 50–100 mg/g of TS had little impact on hydrogen production. A mechanistic study revealed that although CuO nanoparticles at 5 mg/g of TS weakly inhibited acetogenesis, they enhanced WAS solubilization significantly, leading to improved hydrogen production. In contrast, although CuO nanoparticles at 50 and 100 mg/g of TS promoted WAS solubilization, the acidogenesis and acetogenesis processes were both suppressed, with the improvement offsetting inhibition. Microbial community analysis confirmed that CuO nanoparticles at high levels reduced the populations of key bacteria (e.g., Rhodobacter sp.) involved in hydrogen generation. Reactive oxygen species-mediated oxidative stress was revealed to be a key factor of CuO nanoparticle toxicity. This work revealed that anaerobic fermentation for hydrogen production rather than for methane production would be a better option for anaerobic treatment of sludge containing CuO nanoparticles.