3 research outputs found

    Acute and Chronic Responses of Activated Sludge Viability and Performance to Silica Nanoparticles

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    Recently, the potential health and environmental risks of silica nanoparticles (SiO<sub>2</sub> NPs) are attracting great interest. However, little is known about their possible impacts on wastewater biological nitrogen and phosphorus removal. In this study, the acute and chronic effects of SiO<sub>2</sub> NPs on activated sludge viability and biological nutrient removal performance were investigated. It was found that the presence of environmentally relevant concentration (1 mg/L) of SiO<sub>2</sub> NPs caused no adverse acute and chronic effects on sludge viability and wastewater nitrogen and phosphorus removal. However, chronic exposure to 50 mg/L SiO<sub>2</sub> NPs induced the increase of effluent nitrate concentration, and thus depressed the total nitrogen (TN) removal efficiency from 79.6% to 51.6% after 70 days of exposure, which was due to the declined activities of denitrifying enzymes, nitrate reductase and nitrite reductase. Wastewater phosphorus removal was insensitive to 1 and 50 mg/L SiO<sub>2</sub> NPs after either the acute or chronic exposure, because the critical factors closely related to biological phosphorus removal were not significantly changed, such as the activities of exopolyphosphatase and polyphosphate kinase and the intracellular transformations of polyhydroxyalkanoates and glycogen. Denaturing gradient gel electrophoresis (DGGE) analysis revealed that the bacterial community structure was changed after long-term exposure to 50 mg/L SiO<sub>2</sub> NPs, and the quantitative PCR assays indicated that the abundance of denitrifying bacteria was decreased, which was consistent with the declined wastewater nitrogen removal

    Effect of CO<sub>2</sub> on Microbial Denitrification via Inhibiting Electron Transport and Consumption

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    Increasing anthropogenic CO<sub>2</sub> emissions have been reported to influence global biogeochemical processes; however, in the literature the effects of CO<sub>2</sub> on denitrification have mainly been attributed to the changes it causes in environmental factors, while the direct effects of CO<sub>2</sub> on denitrification remain unknown. In this study, increasing CO<sub>2</sub> from 0 to 30 000 ppm under constant environmental conditions decreased total nitrogen removal efficiency from 97% to 54%, but increased N<sub>2</sub>O generation by 240 fold. A subsequent mechanistic study revealed that CO<sub>2</sub> damaged the bacterial membrane and directly inhibited the transport and consumption of intracellular electrons by causing intracellular reactive nitrogen species (RNS) accumulation, suppressing the expression of key electron transfer proteins (flavoprotein, succinate dehydrogenase, and cytochrome c) and the synthesis and activity of key denitrifying enzymes. Further study indicated that the inhibitory effects of CO<sub>2</sub> on the transport and consumption of electrons were caused by the decrease of intracellular iron due to key iron transporters (AfuA, FhuC, and FhuD) being down-regulated. Overall, this study suggests that the direct effect of CO<sub>2</sub> on denitrifying microbes via inhibition of intracellular electron transport and consumption is an important reason for its negative influence on denitrification

    Zinc Oxide Nanoparticles Cause Inhibition of Microbial Denitrification by Affecting Transcriptional Regulation and Enzyme Activity

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    Over the past few decades, human activities have accelerated the rates and extents of water eutrophication and global warming through increasing delivery of biologically available nitrogen such as nitrate and large emissions of anthropogenic greenhouse gases. In particular, nitrous oxide (N<sub>2</sub>O) is one of the most important greenhouse gases, because it has a 300-fold higher global warming potential than carbon dioxide. Microbial denitrification is a major pathway responsible for nitrate removal, and also a dominant source of N<sub>2</sub>O emissions from terrestrial or aquatic environments. However, whether the release of zinc oxide nanoparticles (ZnO NPs) into the environment affects microbial denitrification is largely unknown. Here we show that the presence of ZnO NPs lead to great increases in nitrate delivery (9.8-fold higher) and N<sub>2</sub>O emissions (350- and 174-fold higher in the gas and liquid phases, respectively). Our data further reveal that ZnO NPs significantly change the transcriptional regulations of glycolysis and polyhydroxybutyrate synthesis, which causes the decrease in reducing powers available for the reduction of nitrate and N<sub>2</sub>O. Moreover, ZnO NPs substantially inhibit the gene expressions and catalytic activities of key denitrifying enzymes. These negative effects of ZnO NPs on microbial denitrification finally cause lower nitrate removal and higher N<sub>2</sub>O emissions, which is likely to exacerbate water eutrophication and global warming
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