Narrow Results

Liquid and solid particles in polar stratospheric clouds (PSCs) have been known to play a crucial role in the chemical loss of stratospheric ozone over the Antarctic and Arctic regions in late winter and early spring. The stratospheric aerosol and cloud particles provide the sites where fast heterogeneous chemical reactions convert inactive halogen reservoir species into potential ozone destroying radicals. The sedimentation of nitric acid-containing PSC particles irreversibly removes HNO₃ gas (denitrification) from the lower stratosphere, which slows the return of chlorine to its inactive forms, resulting in more severe stratospheric ozone destruction. Although these clouds have been investigated extensively during the past decade using in situ field observation, laboratory experiment and modeling studies, the detailed microphysics processes under cold stratospheric conditions are still uncertain. This paper reviews the recent advances in our understanding of PSCs.

Graphene–Bi₂WO₆ composite photocatalyst with excellent photocatalytic properties were synthetized by ultrasonic-microwave synergistic method. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier infrared (FT-IR) spectrum, specific surface area analyzer (BET) and UV-Vis diffuse reflectance spectra. The photocatalytic activities of as-prepared samples were evaluated by degradation of pyridine in light oil under visible light irradiation. The results show that graphene changed appearance structure of Bi₂WO₆, which prevented the Bi₂WO₆ nanosheets reunion arrangement, and increased specific surface area of the catalyst. It can also reduce the recombination probability of photogenerated carrier, so as to improve the photocatalytic activity of the composite catalyst. The optimal amount of graphene is 2%, with the denitrification rate as high as 89.28%, much higher than that of pure Bi₂WO₆.

Nitrous oxide, or N₂O, is a greenhouse gas that is largely considered to be the cause for depletion of the ozone layer. As nearly 70% of the global emissions seem to arise from bacterial and fungal denitrifications and nitrification processes in soils, there is a growing concern to identify mitigation techniques to improve the atmosphere in which we live. Fertilizers in agricultural soils are the primary source. This chapter will explain the causes of nitrous oxide emissions in the atmosphere as well as explore these mitigation methods that have been researched, tested and successfully implemented.

Rates of selected soil nitrogen transforming processes as well as kinetic parameters of denitrification enzymes were determined in a cattle overwintering area. Soils from three localities differently impacted by the cattle (severe, moderate, control with no impact) were examined. In cattle-influenced soils, total N, organic C and pH were significantly increased. Consequently rates of potential mineralization, nitrifying enzyme activity and denitrifying enzyme activity (DEA) were enhanced compared to control, while potential nitrogenase activity was lowered in both soils influenced by cattle. The soils differed substantially in DEA, which was about 60 ng N₂O-Ng dw^-1h^-1 in control soil but 3 times and 34 times higher in moderately and severely impacted soils, respectively. The soils also exhibited significantly different maximum reaction velocity (V) and Michaelis-Menten constant (K_m) of enzymes responsible for reduction either of nitrate to nitrous oxide or of nitrous oxide to di-nitrogen. Results suggest that the cattle-induced stress alters the functioning of soil microbial community.

The effect of anammox coupling with heterotrophic denitrification process was investigated in an anaerobic reactor with an influent at the temperature of 35 °C, pH of 7.0, hydraulic detention time of 30h and the influent dilution ratio at 75%, the effects of different concentrations of nitrite on total nitrogen and organic matter removal were studied by changing ${\text{NO}}_3^{-}$-N/${\text{NH}}_4^{+}$-N ratio in the anaerobic reactor with the ammonia concentration of 140±5mg/L and COD of 900±5mg/L. The removal rate of ammonia nitrogen, nitrite, TN, COD were 54.71%, 81.49%, 73.58, 81.61%, respectively, with a ${\text{NO}}_2^{-}$-N/${\text{NH}}_4^{+}$-N ratio of 1.6. The optimum stoichiometric ratio can enhance anaerobic ammonium oxidation and denitrification, improving the removal efficiency of nitrogen carbon simultaneously.

The development of new processes and technologies like aerobic granulation has increased the interest in recent years to cope with disadvantages observed in conventional activated sludge process for wastewater treatment. Due to the increased interest in this topic, the fundamentals and applications of aerobic granulation have been compiled. The main factors affecting granulation, the biological processes involved, and the application of aerobic granules for nutrient removal at laboratory and pilot scale were delineated.