More and more wastewater containing hexavalent chromium (Cr(VI)), which causes increasingly threatening environmental events including death of plants or organisms, soil inactivation and canceration of human organs, has been caused by rapid industrial growth. Various methods, such as photocatalytic reduction, physical adsorption, electrochemical and photoelectrochemical approaches have been proposed to detoxify/remove Cr(VI) contained in wastewater. Quite significantly, photocatalytic Cr(VI) reduction grabs increasing attention with many advantages, including environmental friendliness, no sludge, low secondary pollution risk, high utilization of solar energy and low dosage of chemical reagents. For the purpose of improving the Cr(VI) removal efficiency during the photocatalytic reduction process, various kinds of catalysts were developed. In this mini-review, the photocatalytic reduction of Cr(VI) by ion doping photocatalysts, faceted photocatalysts, and heterostructure photocatalysts are briefly introduced. Furthermore, some suggestions for modifying photocatalysts to enhance their photocatalytic performance on Cr(VI) reduction are put forward.
In this work, carbon nanotubes (CNTs) were synthesized by catalytic pyrolysis from waste polyethylene in Ar using an in-situ catalyst derived from ferric nitrate and nickel nitrate precursors. The influence factors (such as temperature, catalyst content and Fe/Ni molar ratio) on the formation of CNTs were investigated. The results showed that with the temperature increasing from 773 to 1073 K, the carbon yield gradually increased whereas the aspect (length-diameter) ratio of CNTs initially increased and then decreased. The optimal growth temperature of CNTs was 973 K. With increasing the Fe/Ni molar ratio in an FeNi bimetallic catalyst, the yield of CNTs gradually increased, whereas their aspect ratio first increased and then decreased. The optimal usage of the catalyst precursor (Fe/Ni molar ratio was 5:5) was 0.50 wt% with respect to the mass of polyethylene. In this case, the yield of CNTs reached as high as 20 wt%, and their diameter and length were respectively 20–30 nm, and a few tens of micrometers. The simple low-cost method developed in this work could be used to address the environmental concerns about plastic waste, and synthesize high value-added CNTs for a range of future applications.
Phase pure HfB 2 SiC powders were efficiently synthesized via microwave/molten salt assisted boro/ carbothermal reduction (MMS-BCTR) method by using HfO 2 , SiO 2 , activated carbon and B 4 C powders as raw materials, and NaClKCl as a molten salt medium. The effects of mass ratio of salt to reactant (m s /m r ), holding period, reaction temperature and B 4 C content on the phase composition and microstructure of the synthesized HfB 2 SiC powders were investigated. The results showed that the optimal molar ratio of HfO 2 / SiO 2 /B 4 C/C was 3.0:1.0:3.2:4.0, m s /m r was 2.0, and reaction temperature and holding period were respectively 1250°C and 20 min. Rod-shaped HfB 2 and irregular SiC particles were uniformly distributed in the products, the former with a length of 0.5¯m and a diameter of 300 nm exhibited a single-crystal structure with a preferred growth along the [100] direction, while the latter had a particle size of about 300 nm.
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