The application of MnO x -CeO 2 as the low temperature selective catalytic reduction (SCR) catalyst to control NO x emission from coal-fired power plants is extremely restricted due to the unrecoverable deactivation by SO 2 . There is little SO 2 in the flue gas from biomass-fired power plants, and the concentration of alkali metals in the flue gas after the electrostatic precipitator is very low, so the application of MnO x -CeO 2 may be possible to control NO x emission from biomass-fired power plants. However, a very small amount of alkali metals showed a seriously negative effect on NO reduction over MnO x -CeO 2 such that both NO x conversion and N 2 selectivity obviously decreased. In this work, the mechanism of NO reduction over MnO x -CeO 2 and K-MnO x -CeO 2 was investigated by the transient reaction study and the kinetic parameters of NO reduction were obtained from the steady-state kinetic study. After comparison of the kinetic parameters, the mechanism of potassium deactivation on NO reduction over MnO x -CeO 2 was discovered. The decrease of the SCR activity of MnO x -CeO 2 after potassium deactivation was mainly attributed to the decrease of acid site and Mn 4+ concentration on the surface, and the increase of N 2 O selectivity was mainly related to the occurrence of N 2 O formation over K-MnO x -CeO 2 through the Langmuir−Hinshelwood mechanism.
Direct use of zero-valent iron (ZVI) in reductive removal of selenate (Se(VI)) is inefficient due to the intrinsic passive layer of ZVI. Here we observed that ZVI pretreated with H2O2 (P-ZVI-O) performs much better in Se(VI) removal from a mining effluent than other three modes of ZVI alone, acid washing ZVI (P-ZVI-A), and simultaneous addition of H2O2 and ZVI (ZVI-O) as well. The P-ZVI-O exhibits exceptionally high Se(VI) removal at a low dosage, wide pH range, with Se dropping down from 93.5 mg/L to <0.4 μg/L after 7-h reaction. Interestingly, the initial pH (2–6) of the mining effluent exerted little influence on the final Se(VI) removal. H2O2/HCl pretreatment results in the formation of various reducing corrosion products (e.g. Fe3O4, FeO and Fe2+), which greatly favors the efficient Se(VI) removal. In addition, surface-bound Fe2+ ions participated in the reduction of Se(VI). Combined with the influence of Se valence as well as pH and Fe2+ (whether dissolved or surface bound), it is deduced that the P-ZVI-O mode induced efficient Se(VI) removal via the adsorption-reduction and/or co-precipitation. This study demonstrates that H2O2/HCl pretreatment of ZVI is a very promising option to enhance the efficiency of reductive removal of Se(VI) from real effluents.
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