Research On The Release Performance And Mechanism Of Decomposing Pollution Of Common Aquatic Plants In Constructed Wetlands

The aquatic plants in constructed wetland can absorb nitrogen,phosphorus and other nutrients in the water,and further improve the water quality of the overlying water.However,the release of pollutants in the process of plant decay is easy to cause secondary pollution to the overlying water.In this thesis,reed,cattail,alternanthera philoxeroides and calamus were selected as experimental materials,and the density of aquatic plants was 3 g·L^-1 to simulate the decomposition and pollution release process of aquatic plants in different growth stages.The effects of decomposition and release on nutrients,organic matter and other water quality indexes of overlying water under different reoxygenation conditions were studied,The dissolved organic matter(DOM)in the overlying water was analyzed by three-dimensional fluorescence analysis,and the change of plant residue composition and decomposition rate was analyzed to investigate the release mechanism of aquatic plant decomposition pollution.The main results are as follows:(1)The water quality of overlying water changed significantly during the process of aquatic plant decomposition.The results showed that different aquatic plants had different pollution release intensity in the process of decomposition.The effects of plant decomposition on N,P components and organic matter concentration in overlying water were as follows:alternanthera philoxeroides>calamus>reed>cattail.The effect of natural reoxygenation of aquatic plants on the overlying water was higher than that of low reoxygenation.The TN concentration of reed under natural reoxygenation and low reoxygenation was 3.42 mg·L^-1 and 1.89 mg·L^-1,respectively.Except for alternanthera philoxeroides,the effect of plant decomposition at Seedling Stage on the quality of overlying water was higher than that at mature stage.The difference of TOC concentration between seedling stage and mature stage was 3.15 mg·L^-1.The results of principal component analysis(PCA)showed that the changes of p H and do had a strong inhibitory effect on the release of TN and TP during decomposition.(2)DOM was separated by three-dimensional fluorescence spectrum and parallel factor analysis to five kinds of fluorescence factor components:C1 and C3 components were Protein-like,C2 components were humus-like,C4 and C5 components were humic acid.In the early stage of decomposition,the intensity of Protein-like peaks B and T was higher,which indicated that there were a lot of Protein-like substances in the initial stage of plant decomposition;in the late stage of experiment,the concentration of Protein-like substances,humic acid substances and fulvic acid substances increased;in the end of experiment,fulvic acid substances C and humic acid substances F played a major role.DOM of aquatic plants in different growth stages had little difference in the process of decomposition.The results showed that the DOM spectra of natural reoxygenation and hypoxia were different:the fluorescence intensity of humic acid and fulvic acid was lower,and the release of refractory components was less.The biogenic index BIX>1,humification coefficient HIX<4 and fluorescence index FI>1.9,which indicated that most DOM in decaying water was derived from microbial activities,and the autotrophic characteristics were very significant.The plant growth stage has little effect on the humification degree of the plants.The influence of water reoxygenation conditions:natural reoxygenation>hypoxia.The results showed that calamus had the strongest autogeneity,Alternanthera philoxeroides had the highest degree of humification and the weakest autogeneity.(3)The contents of N and P in seedling stage were higher than that in mature stage,and the values of C/N,C/P and N/P in seedling stage were lower than those in mature stage.After decomposition,the content of C and N increased slightly,while the content of P decreased slightly.The rate of dry weight loss in the decomposition process was as follows:Alternanthera philoxeroides>calamus>cattail>Phragmites australis.The decomposition rate was as follows:Alternanthera philoxeroides>calamus>Phragmites australis>cattail.The decomposition rate was positively correlated with the initial N and P content in aquatic plants,and negatively correlated with the C content,C/P and N/P,which indicated that the higher the initial N and P content,the lower the C/P and N/P values,and the faster the decomposition of aquatic plants.