Perchlorate (ClO4 -) has aroused wide concern as a global ecosystem pollutant, especially in the aquatic environment. A rapid and visible detection of ClO4in aqueous solution using alkali methylene blue (MB) and liquid-liquid extraction (LLE) was established with high selectivity and sensitivity. The alkali MB exclusively change from red into blue in organic solvent when exposed to aqueous ClO4 -. Organic solvent, amount of MB and alkalinity of solution were systematically optimized, while underlying mechanism was revealed using UV-vis spectroscopy and mass spectrometry.Under optimum conditions, the UV-vis absorbance at 600 nm showed a good linear relationship with the concentrations of ClO4in the range of 0.0025-0.025 mM (R 2 = 0.9945). The visual detection limit was 0.005 mM in environmental water and 0.02 mM in black tea infusion. This method provides an economical and simple visual detection of ClO4in aqueous solution.
BACKGROUND Pymetrozine is a widely used pesticide. It is challenging to analyze and difficult to manage due to the large gap in its global maximum residue limits (MRLs) in tea. The development of a high‐efficiency detection method for the evaluation of the transfer of residual pymetrozine from tea plantations to tea cups is therefore of prime significance. RESULTS An analytical method for the determination of pymetrozine residues in tea was established based on Cleanert PCX solid‐phase extraction. The average recoveries were 72.2–93.7%, with relative standard deviations (RSDs) of less than 12%. The limits of quantification (LOQs) were 0.005 mg·kg−1 in fresh tea leaves and dry tea, and 0.00025 mg·L−1 in tea brew. Pymetrozine degraded rapidly in tea plants with a half‐life (t1/2) of 1.9 days in open tea plantations, and decreased by 9.4–23.7% in the green tea‐processing procedure, which was concentration dependent. The residual pymetrozine levels in green tea collected at 6 and 21 days were below the MRLs in China and EU at a dosage of 30 g a.i. ha−1, respectively. The leaching rates of pymetrozine from dry tea to tea brew were 58.7–96.3%. Hazard quotient (HQ) values of pymetrozine were significantly <100% when tea shoots were plucked in 6 days, which indicated a negligible risk to humans. CONCLUSION This work allows the determination of residual pymetrozine in tea and illustrates a low intake risk with the use of pymetrozine in tea plantations. It could serve as reference for further regulation consideration for maximum residue limits (MRLs). © 2020 Society of Chemical Industry
With the prevalence of renewable energy sources such as wind power in the power system, analyzing the fault characteristics of systems composed of DFIGs is becoming increasingly important. Therefore, this article analyzes, at first theoretically, the fault characteristics of a doubly fed induction generator (DFIG) during fault periods. It was found that the fault current of the DFIG exhibited the frequency offset phenomenon, which is affected by the depth of voltage dips and can negatively impact traditional distance protection. Furthermore, a method using a dynamic voltage restorer (DVR) based on superconducting magnetic energy storage (SMES) was adopted to compensate for the fault voltage of DFIG, which can mitigate the voltage dips of the DFIG. This method can not only achieve the fault ride through for DFIG but also significantly improve the frequency offset of the fault current during fault periods. Finally, a model composed of a 2.5 MW DFIG-based wind turbine and a 2.5 MW DVR-based SMES was built using a real-time digital simulator (RTDS) platform, and the simulation results showed that the fault stator voltage of DFIG can be compensated at a rated value of 0.69 kV, and the frequency of fault current can be maintained at 50 Hz These results validate the excellent performance of the method in achieving the fault ride through of DFIG and improving the frequency offset of the fault current by comparing multiple type faults while employing different protection methods.
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