Study On Degradation Of NCL And Its Influence Factors In Wetland Within Poyang Lake Region

Poyang Lake is one of the most precious fresh water resources in China. More than 170 tons of 50% niclosamide ethanolamine salt wettable powder is used within Poyang Lake region in Jiangxi province every year. Serious impacts may be generated because of a repeatedly massive use of niclosaniede for a long time in Poyang lakeshore. Degradation of iclosamide is totally dependent on the natural environment. In order to ensure the safety of the public and the environment, the degradation mechanism of niclosamide and its influence factors were studied in wetland within Poyang Lake region in this work. This research was mainly carried out in three aspects: degradation dynamics and mechanism of niclosamide in different plant rhizosphere soil; the absorption laws of niclosamide in carex and artemisia selengensis; residual dynamics of niclosamide in soil percolate. Methanol was used in ultrasonic extraction for sample preparation, which was later tested by HPLC(High Performance Liquid Chromatography). At last, mathematical statistics method was used to analyze the data. The main results are as follows:(1) The degradation rate of niclosamide in carex rhizosphere soil is greater than that in artemisia selengensis. While in the same depth of control group without plant, the degradation rate is the lowest. Degradation process of niclosamide in rhizosphere soil follows first-order reaction kinetics. The half-life of these three type soil samples are 14.6d, 18.3d and 21.7d respectively, which shows that plant roots have a promoting effect on the degradation of niclosamide.(2) In the process degradation of niclosamide in rhizosphere soil, the overground and underground part of artemisia selengensis and carex all have the absorption effect on niclosamide in the rhizosphere soil to different extent. The niclosamide residues of plants gradually increase in the beginning and then decrease slowly. Before reaching degradation half-life, niclosamide in rhizosphere soil is mostly accumulated in the ground part of the plant after being absorbed by the root.(3) Niclosamide residues show a significant negative correlation(P < 0.05) with the amount of three kinds of microbes in carex and artemisia selengensis rhizosphere soil. Microbes have important contribution to the degradation of niclosamide in rhizosphere soil. Study shows that bacteria play a leading role in the microbial degradation of niclosamide in carex rhizosphere soil. While in Artemisia selengensis rhizosphere soil, the actinomycetes plays a leading role. In addition, the amount of fungi in these two rhizosphere soil is far less than the control group, it indicates that Root secretion has regulating effect on physical and chemical properties of soil.(4) Degradation process of niclosamide in water follows first-order reaction kinetics. Degradation rate of niclosamide in water(pH = 7) is much faster than that in the soil. Its half-life is only 13.3h. In wet season, the degradation rate of niclosamide in the soil leachate is faster than that in dry season. The half-life in wet season is 11.4 d while 16.0d in dry season. Sunlight, rainfall, humidity are some of the important factors which affect the degradation of niclosamide.(5) In soil percolate, there are two kinds of intermediates: 2-chloro-4-nitroaniline and 5-chloro salicylic acid. While the intermediates in the rhizosphere soil are 5-chloro salicylic acid and 2,5-dihydroxy benzoic acid. The concentration of 2-chloro 4-nitroaniline and 2,5-dihydroxy benzoic acid increases gradually in experiment. However, the change of 5-chloro salicylic acid is not significant.