Study On The Effect Of Several Aquatic Plants On The Purification Of Eutrophic Water

Eutrophication will affect man’s living environment negatively, which even impede the socio-economy development in some countries. Under such circumstances, purifying the eutrophic water, reducing the nutrient content and restoring its integrated function have become a hot topic in global environment research. Phytoremediation is much more widely used than physical or chemistry and other repairing methods for its advantage as less investment consumption, good treatment effect and easy to operate. Scholars have conducted the research of water body restoration via emergent, submerged and floating plants. However, researches above were mainly focus on the effects that the choosing plant could bring to the eutrophic water, few reaches studied on the purifying effect of eutrophic water by aquatic plants and the effect of plants root by eutrophic water simultaneously.In this study, four common aquatic plants of local water were selected as materials（Eichhornia crassipes, Myriophyllum elatinoides Gaudich, Ipomoea aquatica Forsk, Lemna minor L） and set low, middle and high eutrophication level in this pot experiment separately, to compare the purification of different plants toward the N and P of the eutrophic water and its activity and exudates in different eutrophication level. By conducting the static cell experiment at the same time, the comparison of purification of different plants toward N, P, COD of eutrophic water were attained, so to provide scientific theory to the selection and application of plant for aquatic plants repair techniques. Results are as follow:（1） In the pot experiment, all the plants have good removal effect to water total nitrogen（TN） and the difference was significant（P<0.05） comparing with the control check（CK）. The Eichhornia crassipes was the best one at removing TN in all three eutrophication level, the removal effect of other plants descending as: Ipomoea aquatica Forsk> Myriophyllum elatinoides Gaudich> Lemna minor L. The Eichhornia crassipes was best at removing the water ammonium（NH₄⁺-N）, whose removal ratio in low, middle and high eutrophication level were 87.49%, 95.68% and 92.41%, the rest three plants are Ipomoea aquatica Forsk（80.32%⁹3.74%） > Myriophyllum elatinoides Gaudich（76.61%⁹0.61%）> Lemna minor L（54.77%⁸2.86%）. While the Ipomoea aquatica Forsk was the best at removing water nitrate（NO₃^--N）, the concentration of NO₃^-N descending from initial 0.62¹9.38 mg/L to 0.07⁷.46 mg/L in the end. As the eutrophication level ascended, all the choosing plant removed the nitrogen element（TN、NH₄⁺-N、NO₃^-N） better in middle level than high level, which meaned that suitable concentration could promote the removal ability of the choosing plants while too high toncentration would impede this capacity, thereby reduced the purification effect. All the four choosing plants were significantly higher in removing TP than CK group in all eutrophication level（P<0.05）, the removal ration of them were descending as: Eichhornia crassipes> Ipomoea aquatica Forsk> Myriophyllum elatinoides Gaudich> Lemna minor L. Except for the CK group, the TP removal ration of all choosing plants were increased with the eutrophication concentration level.（2） During the experiment proceed, the secretion of dissolved organic carbon（DOC） of all the four plants were increased. In the same eutrophication level, the DOC secretion of different plants were very different（P<0.05）, Eichhornia crassipes and Ipomoea aquatica Forsk were better at secreting DOC, whose highest secretion at the low level were were 7.46 and 6.85 mg/（g·d） separately. The level of Myriophyllum elatinoides Gaudich and Lemna minor L were the least. In different eutrophication level, the same plant root secreted DOC decreased as eutrophication level increased, the secretion of plant root was significantly negative correlated with eutrophication level, which indicated that the reducing of nutrient content could promote the secretion of root DOC;For the experiment, Eichhornia crassipes had the best vitality in the same eutrophication level, which were 69.56 μg/（g·h）, 85.63 μg/（g·h） and 61.78 μg/（g·h） in low, middle and high eutrophication level, followed by Ipomoea aquatica Forsk and Myriophyllum elatinoides Gaudich, the Lemna minor L was the least one, which indicate that for the root absorption and purification capacity, the ability were Eichhornia crassipes> Ipomoea aquatica Forsk> Myriophyllum elatinoides Gaudich> Lemna minor L. In different eutrophication level, the root vitality decreased as eutrophication level increased;As time passed, the nutrition content continuously decreased, and the organic acids secretion of each plant were all increased. The organic acid of each plant were all decreased while the eutrophication level increased, which indicated that a low level nutrition content could promote the secretion of plant root. In different eutrophication level, the quantity and types of secreted organic acids were different. ]The Eichhornia crassipes secreted the most organic quantity, followed by Ipomoea aquatica Forsk and Myriophyllum elatinoides Gaudich and Lemna minor L. For the types of organic acids, Ipomoea aquatica Forsk and Myriophyllum elatinoides Gaudich mainly secreted oxalic acid and malic acid, Ipomoea aquatica Forsk secreted oxalic acid and citric acid while Lemna minor L secreted more types of organic acids than others including oxalic acid, citric acid, lactic acid and acetic acid. Among all these organic acids, oxalic acid was the basic one and its quantity was the largest.（3） In the whole static cell experiment, all plants could effectively reduce the water total suspended substance（TSS） and significantly different with CK group（P<0.05）. Among them, Myriophyllum elatinoides Gaudich and Eichhornia crassipes achieved the best results. In the experiment process, the TSS in all the test groups showed a trend of rapid reduction at first but slowly declined and stabilized finally, while CK group showed a trend of rapid reduction at first but gradually increased finally. Regarding the removal of water chemical oxygen demand(COD_cr), Myriophyllum elatinoides Gaudich worked best and the removal ratio was 86.11%, followed by Eichhornia crassipes and its removal ratio was 82.35%. the removal ratio of Ipomoea aquatica Forsk and Lemna minor L were 71.88% and 50.75%, thereby, all the choosing plants are effective in removing water COD_cr;In the static cell experiment, all the four plants had some effect on removing N and P in eutrophic water. Eichhornia crassipes and Myriophyllum elatinoides Gaudich showed the best results while Lemna minor L was the least. For the different tastes of the Eichhornia crassipes and Myriophyllum elatinoides Gaudich, the removal of inorganic nitrogen（NH₄⁺-N、NO₃^--N）behavied different. Eichhornia crassipes was better at removing NH₄⁺-N than Myriophyllum elatinoides Gaudich, while the removal of NO₃^-N was on the contrary. For TP and SRP, the removal ratios of all the plants were descending as: Eichhornia crassipes> Myriophyllum elatinoides Gaudich> Ipomoea aquatica Forsk> Lemna minor L.（4） In the static cell experiment, all the four plants could greatly promote the vitality of water extracellular enzymes（Urease, alkaline phosphatase）, among which Eichhornia crassipes was the top producer, its peak and poor value of urease activity（UA） were 8.13 and 5.91 ug/m L respectively, and alkaline phosphatase activity（APA） were 8.13 and 5.91 ug/m L, followed the Myriophyllum elatinoides Gaudich. The UA and APA in different treated water were negatively correlated with the concentration of NH₄⁺-N and SRP and the correlation coefficient r was among 0.955^-0.990 and-0.973^-0.995.To sum up, the removal capacity and adaptability toward different eutrophication level of Eichhornia crassipes and Myriophyllum elatinoides Gaudich are stronger than the other two plants, which can restore the eutrophic water better.