Comparison of nitrogen and phosphorus purification effects of different wetland plants on eutrophic water

Using hydroponic, 13 plants were selected from 2 type (aquatic plants, hygrophytes) of new varieties and common varieties of wetland plants in Tianjin to explore the differences of purification effect of nitrogen and phosphorus in water. The result shows that the plants on the removal rate of total nitrogen was significantly higher than control group, but the differences were small. Among them, yellow iris, Siberia iris, Cyperus alternifolius, aquatic Canna and Miscanthus sinensis had the best effect on TN, and the removal rate was above 99%; The removal rates of total phosphorus in plants were significantly higher than those in control group (except for M.sinensis cv. ‘Variegatus’), and the differences among the groups were relatively significant. Among them, Irispseudoacorus, Cyperus alternifolius, iris pseudacorus, Scirpus tabernaemontani, Iris siberian, C.glauca and M.sinensis cv. ‘Gracillimus’ had the best effect on TP, and the removal rate was more than 70%. Therefore, Irispseudoacorus, Cyperus alternifolius, iris pseudacorus, Scirpus tabernaemontani, Iris siberian, C.glauca and M.sinensis cv. ‘Gracillimus’ can be used as an alternative plant for eutrophic, wetlands or landscape water.


Introduction
Water eutrophication is one of the important causes of water pollution [1] , and it is also a hot and difficult problem in water pollution control [2] . Water eutrophication further deteriorates water quality, seriously affecting the ecological environment and human life, restricting the sustainable development of the national economy [3] . At present, there are many ways to manage eutrophic water at home and abroad, whether it is to reduce the source of exogenous nutrition or take various measures to eliminate algae can not achieve lasting and ideal results [4] . Through a lot of practice, ecological restoration has become the best way to manage water eutrophication. plants as the core of ecological restoration from the water can absorb nitrogen, phosphorus and other nutrients and provide a suitable environment for microbial degradation of pollutants [5] ，which not only had less investment, low energy consumption, no secondary pollution risk, but also can restore original function of the aquatic ecology system, to create a good landscape effect. Due to different plants between the nitrogen and phosphorus absorption capacity are different, and different types of plants between the growth characteristics of the differences. At present, although there are many studies on the management of eutrophic water bodies [6][7] , most of them focus on several indigenous and common plant studies, and few studies on the combination of large numbers of new varieties and local common plants for nitrogen and phosphorus adsorption to optimizate plants.
In this study, 13 wetland plants from 2 types (aquatic plants, hygrophytes) were selected. The effects of different wetland plants on the removal of nitrogen and phosphorus were studied by means of indoor hydroponics, and the best new varieties were selected. So as to provide a theoretical basis for the application of new varieties of aquatic plants and the application of common varieties in engineering and eutrophic water.

plant materials
Through the field collection and purchase of plant seedlings suitable for growth in wetlands, thirteen kinds of wetland plants with a certain landscape value were selected from six kinds of aquaticplants All plants were transferred to the configured nutrient solution for one week before the experiment began,and then one by one into the artificial allocation of sewage ,which TN concentration was 9.558mg/L, TP concentration was 1.005mg/L in the actual, to cultivate.

Experimental design
The experiment was conducted in the greenhouse of the Nankai University in May 2017. Experiments were carried out by using foam floating plate as carrier. Each plant was cultivated according to its growth characteristics and consistent growth. The diameter of the plastic bucket was 30cm and the height was 22cm, and the plastic bucket was into water 8L. Each plant was grouped with 3 replicates and 1 set of blank controls. Each group of plastic barrels according to the amount of water added to determine the standard water level, Water level is added to the tick mark before each water sample. The experimental period is 35 days and the water sample is taken once a week.

Experimental methods and data process
To avoid the sampling error, each sampling time is 9: 00-10: 00AM. The water samples were taken and measured within two days. Taking into account the latest monitoring methods of the State Environmental Protection Administration, the total nitrogen (TN) was determined by ammonium persulfate digestion ultraviolet spectrophotometry (GB11894-89) and the total phosphorus (TP) was determined by ammonium molybdate spectrophotometry (GB11893-89).
Excel 2007 is used for data processing and mapping. Spss20.0 is used to calculate the mean and standard deviation of each index.  Figure.1 shows the curve of TN removal rate in the aquatic plants groups with time. As shown in Figure 1, compared with control group, each group on the total nitrogen in water have a good removal effect. During the first week of experiment, the removal rate of total nitrogen was the highest, reaching more than 90%. The removal rate of total nitrogen was almost flat after the end of the second week. Figure. 2 shows the curve of TN removal rate in hygrophyte groups with time. It can be seen from Figure. 2 that the removal of total nitrogen in the water was significantly improved compared with control group. Similar to the emerged plant, the removal efficiency of total nitrogen was the highest in the first week of the experiment, with the exception of L.salicaria cv.white flower, the other groups reaching more than 90%. However, the removal rate of total nitrogen was more obvious after the end of the second week of the experiment, which may be related to the growth characteristics of hygrophyte, and the part of plant leaf abscission could easily fall into the water, leading to the recovery of total nitrogen concentration.  Removal of total phosphorus in water In addition to the plant's own absorption, there are plant roots pr ovide a good habitat for the growth and reproduction of polyphosphate bacteria, and promote the remo val of phosphorus in water [8] . There is a difference in the removal of total phosphorus between differe nt aquatic plants, which may be related to plant growth to phosphorus requirement and root oxygen ca pacity [9] .

TP removal rate changes over time
It can be seen from Figure. 3 that the total phosphorus removal rate of each group was significantly higher than that of control group. In addition to Pontederia cordata and Cortaderia selloana, the other groups of total phosphorus removal rate of more than 60%. And the effect of total phosphorus removal was significant in the first three weeks of the experiment, and then became gentle. It can be seen from Figure. 4 that compared with the control group, in addition to M.sinensis cv.'Variegatus', the other groups of plants on the removal of total phosphorus in water were significantly improved. Reaching the third week of the experiment, the removal rate of total phosphorus reached the maximum and then fluctuated slightly. The removal rate of total phosphorus between different plants was different. The highest removal rate was Iris siberian, which was 77.54%; the lowest removal rate was M.sinensis cv.'Variegatus', only 25.13%.
In all, the aquatic plants are similar to hygrophyte to phosphorus removal in water, and there is no significant difference. This indicates that under the existing culture conditions, the plant roots provide a good environment for microorganisms such as polyphosphate bacteria, and promote the transformation of phosphorus in water [10] .

Conclusions
(1) The removal rate of total nitrogen in different wetland plants was significantly higher than that in control group, but the difference among the groups was small. Removal effect of Irispseudoacorus, Iris siberian, Cyperus alternifolius, C.glauca and M.sinensis cv.'Gracillimus' are better, and the removal rate of them are more than 99%.
(2) The removal rate of total phosphorus in different wetland plants was significantly higher than that in control group, and the difference was significant. Removal of the better effect are Irispseudoacorus, Cyperus alternifolius, iris pseudacorus, Scirpus tabernaemontani, Iris Siberian, C.glauca and M.sinensis cv.'Gracillimus', and the removal rate of them are more than 70%.
(3) Considering the efficiency of Wetland Plant on TN and TP removal, Irispseudoacorus, Cyperus alternifolius, iris pseudacorus, Scirpus tabernaemontani, Iris Siberian, C.glauca and M.sinensis cv.'Gracillimus'can be used as an alternative to eutrophic water, wetlands or landscape water.