| Eutrophication is one of the major environmental problems faced by the world currently. How to purify eutrophic waters economically, effectively, and easily is much sought-after technology to solve the problem in the world. Phytoremediation technology is an important research topic in 1990s. Phytoremediation on restoration of eutrophic water bodies has inherited advantages over physical and chemical methods, which are often having side effects on environment, or requiring large investment and expensive operation cost. On large scale and low concentrations of nutrients, phytoremediation is exceptionally welcome due to its environmental friendly, low cost, sustainable and good quality on water purification. The core technology of a phytoremediation method is to use macrophytes to remove nitrogen and phosphorus from eutrophic waters.Different macrophytes have their own biological characteristics on adaptability, capability on growth conditions and nutrient removals, so that the effects of water purification and principles of nutrient cycle in the aquatic ecosystems are different. Many researches pay attentions on plant assimilation, but ignored the dynamic pathways of nutrient cycle under the impacts of macrophytes, especially on the dynamic pathways of nitrogen cycle, which is the core process in the cause of eutrophication. Nitrogen is a critical element in the living ecosystem and actively recycled between air and water. The main components exchanged between them are nitrogen gas, nitrous oxide and ammonia. Nitrogen gas and nitrous oxide are the terminal products of the process of denitrification, which is mainly driven by varieties of microorganisms. Literature reported that denitrification is important in the process of eutrophication and water purification. However, the scale of effects, the quantitative amount and the dynamic pathways during the phytoremediation on eutrophic waters under different macrophytes are not clear, which may inevitably cumber the design of ecosystem engineering to restore water bodies.In this project, we designed a comparative study of denitrification process impacted by different types of macrophytes. We used simulation method and employed floating macrophyte (Eichhornia crassipes), floating leaved plants (Trapa and Nymphaea tetragona), and submerged plant (Hydrilla verticillata), which are important models in the application of remediating eutrophication waters. The objectives and the results of this research project are described as follows.1. To evaluate the accuracy of innovation research method on ebullition collecting device for in-situ measurement of denitrification. The result showed that the recovery rates of standard pure N2 and N2O were 99.81% and 99% respectively. The recovery rates of simulation release of denitrification gas at known composition of gas mixture were 99.1±0.8% for N2, and 101±2.2% for N2O,100.4±0.5% for O2,100.4±0.5% for CH4. The device is suitable to be used in researches of denitrification.2. To use new device to study the gas released from the eutrophication water und er the effects of different types of macrophytes; and to quantify gas volume, gas co mposition and flux rate. High temperature can promote aquatic nitrification and denitri fication in water and accelerate the release of gas. The amount of gas released from different aquatic plants in the order of Hydrilla verticillata>Nymphaea tetragona?ck >Trapa>Eichhornia crassipes. Hydrilla verticillata enhanced release gases through pla nt photosynthesis in water, thus increasing the total amount(43.4±14.6?109.3±24.7 m L m-2 h-1) of the gas (N2?N2O?O2?CO2?CH4) flux released from water. Therefo re, the flux of N2, N2O from water plant with Hydrilla verticillata was the highest (32. 3-62.5 mg m-2 h-1). The average nitrogen concentration reduced by nitrification and d enitrification in the treatment of Hydrilla verticillata during the experimental period was 38.59 mg L-1 in August and 19.93 mg L-1 in October, higher than the Ck of 18. 67 mg L-1 and 2.64 mg L-1 in the August and October. The covering surface density of Eichhornia crassipes was the biggest, which inhibited the photosynthetic producti vity in the water, therefore gas content (0.8-2.1mLm-2h-1) released from the water was lower than other treatments significantly, resulting in the N2 flux declined to 0.6-1.3 mg m-2 h-1. The Eichhornia crassipes assimilated nitrogen very quickly and the total nitrogen concentration dropped to below 1 mg/L in 3 to 5 days, and we suspect t hat it was the main reason leading to a low volume of N2 release. The flux of N2, N2O released from the remaining two aquatic plants was lower than the ck.3. Comparative study on purification effect of different types of macrophytes on nitrogen removal in eutrophic water. In August the decline rate of TN content is higher than Oct. and Nov. month. During the test in two month period, the content of TN in Eichhornia crassipes treatment decreased most quickly, the nutrient absorption of Eichhornia crassipes was fast. It can make the nitrogen concentration below 1mg L-1 in 3 to 5 days, and the final TN content in the treatment with Eichhornia crassipes was the lowest, which were 0.98± 0.21 mg L-1 and 1.16± 0.11mg L-1. Apparent removal rate of water nitrogen by Eichhornia crassipes was 91.56% and 88.08% respectively. The decline rates of NO3- and TN in eutrophic water of all the treatment were:the fastest by Eichhornia crassipes and Trapa, followed by Nymphaea tetragona and the slowest by Hydrilla verlicillata.4. Monitoring the changes of physical and chemical environment indices in water. Eichhornia Crassipes could sustain a relatively stable temperature and pH conditions, which are beneficial to the physiological activity and the metabolism of nitrifying and denitrifying bacteria. Planting Eichhornia crassipes and floating leaved Nymphaea tetragona could stabilize water pH around 7 during the test, which is favorable to the survival of microorganisms. The influences on aquatic DO various from one aquatic plant to another; submerged plant (Hydrilla verticillata) could maintain higher levels of dissolved oxygen, which were expected due to underwater photosynthesis. The effects of other three macrophytes on lowing dissolved oxygen content in water were significant.To sum up, the gas recovery device and the measurement method used in this study indicated that it was feasible and accurate. Four kinds of macrophytes influenced differently on the indices of physical and chemical properties in water. Evaluating systematically on temperature, pH and dissolved oxygen as a whole, the ability and mechanism of the four kinds of macrophytes to impact physical and chemical environmental factors in eutrophication water varied, which eventually led to different biological nitrogen transformation prosesses. Four kinds of macrophytes could effectively remove nitrogen in eutrophic water; and the removal rates of floating Eichhornia crassipes were significant, above 91.56+3.88% and 88.08±2.24% respectively. In addition to plant uptake, various types of aquatic plants also modulated the denitrification processes in eutrophic water. Comprehensive analysis of various factors indicated that Eichhornia crassipes is the most suitable aquatic plants for the purification of eutrophic water. |
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