A three-dimensional (3D) nitrogen-doped carbon nanotubes/graphene (NCNTs/G) composite was prepared by pyrolysis of pyridine over a graphene-sheet-supported Ni catalyst. The morphology and structure of the NCNTs/G composite was investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. Tangled NCNTs with lengths of several hundred nanometers are sparsely, but tighly, distributed on graphene sheets, forming quasi-aligned NCNT arrays. The N content in the NCNTs/G measured by X-ray photoelectron spectroscopy is about 6.6 at. %. The NCNTs/G shows a higher activity and selectivity to the oxygen reduction reaction in alkaline electrolyte compared with undoped CNTs/G, as demonstrated by cyclic voltammetry, rotating disk electrode, and rotating ring-disk electrode measurements. The results indicate that the 3D NCNTs/G composite has potential application in fuel cells.
As one of the major sources of pollutions in the environments, effluents from municipal wastewater recently became a hot topic. This study quantified monthly county-level releases of five heavy metals, i.e., lead (Pb), cadmium (Cd), chromium (Cr), arsenic (As), and mercury (Hg), from municipal wastewater into the environment in the Heilongjiang Province of China, based on sampling, measurement, and modeling tools. Wastewater samples were collected from 27 municipal wastewater treatment plants (MWTPs) in 15 county-level cities of Heilongjiang every month from 2015 to 2017. The concentrations of five heavy metals were analyzed in both influents (Pb: 160 ± 100 μg/L; Cd: 15 ± 9.0 μg/L; Cr: 170 ± 64 μg/L; Hg: 0.67 ± 1.5 μg/L; As: 6.2 ± 4.8 μg/L) and effluents (Pb: 45 ± 15 μg/L; Cd: 5.2 ± 5.1 μg/L; Cr: 57 ± 13 μg/L; Hg: 0.28 ± 0.12 μg/L; As: 2.6 ± 1.4 μg/L). The removal ratios of the five heavy metals ranged from 50% to 67%. Inflow fluxes of Pb, Cr, and Cd displayed increasing trends first then decreased after reaching a maximum value, whereas those of Hg and Pb remained stable. Material flow analysis reveals that constructions of MWTPs are conducive to significantly reduce the releases of heavy metals from urban areas into the aquatic environment in the study area. Additionally, municipal wastewater sludge (used as fertilizer or spread on the land) could be a significant source of heavy metals in the land.
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