Removal of Pollutants from Aquatic Plants and Its Application

The removal of contaminants by aquatic plants and their application to human activities can cause large amounts of industrial, agricultural, and domestic waste to be discharged into water, contaminating the water. Water pollution can be divided into three major categories: chemical pollution, physical pollution, and biological pollution, depending on the nature of the pollution. Basically, chemical pollution is the main cause. Specific pollution impurities are inorganic pollutants, inorganic toxic substances, organic toxic substances, and plant nutrient substances. For the removal of these pollutants, aquatic plants play a very important role. Aquatic plants refer to the group of plants that physiologically attach to the water environment and at least part of the reproductive cycle takes place in water or water. Aquatic plants can be roughly divided into four categories: emergent plants, submerged plants, floating leaf plants and floating plants. Large-scale aquatic plants are all groups of aquatic plants other than microalgae. Aquatic plants are an important part of the aquatic ecosystem and a major primary producer. They play a regulatory role in the circulation and transfer of ecosystem materials and energy. It also fixes suspended solids in water and can potentially detoxify it. The role of aquatic plants in the accumulation, metabolism and fate of environmental chemical substances cannot be ignored. Aquatic plants are used to monitor aquatic pollution, conduct eco-toxicological evaluation of pollutants, and bioaccumulate, modify, and translocate after entering the biological chain. This has important implications for the protection of plant ecology and human and animal health [1]. 1 Removal of Pollutants by Aquatic Plants 1.1 Removal of Nitrogen and Phosphorus by Aquatic Plants Lake eutrophication has become a worldwide environmental problem. The use of aquatic macrophytes to enrich nitrogen and phosphorus is one of the effective ways to control, regulate and inhibit lake eutrophication. The lake water environment includes two parts: the body of water and sediment. Nitrogen and phosphorus in the body of water can be migrated into the bottom soil by sedimentation of sediments, sediment adsorption, and deposition. Tracking past nutritional status suggests that aquatic plants can regulate nutrient concentrations in shallow water lakes at moderate temperatures [2]. Large-sized submerged plants absorb nitrogen and phosphorus from the substrate through the roots, and thus have a stronger ability to enrich nitrogen and phosphorus than floating plants. Submerged plants have enormous biomass, and undergo extensive exchanges of matter and energy with the environment, resulting in a very large environmental capacity and a strong self-purification capability. In the submerged plant distribution areas, the contents of COD, BOD, total phosphorus and ammonium nitrogen are generally much lower than the distribution areas of submerged plants outside [3]. The dense growth of floating plants makes the re-oxygenation of lake water obstructed, the dissolved oxygen in the water is greatly reduced, the self-purification capability of the water body is not improved, and secondary pollution is caused, which affects shipping. Emerging plants must grow on wetlands, shoals, shores, etc., that is, breeding sites with suitable depth, which have great limitations. Different submerged plants have significant removal of total nitrogen total phosphorus in the water. In the study on the removal rate of total nitrogen from common submerged macrophytes in Dianchi Lake water (including sediments), it was found that the order of species removal capacity was Elodea> Vallisneria> Foxtail> Potamogeton edulis> Goldfishes> Hemerocallis> Chara. With the prolongation of time, the concentration of total nitrogen in water declined in a negative exponential manner, and the removal rate of each submerged plant increased with the increase of total nitrogen concentration within the range of total nitrogen concentration (2.628-16.667 mg/L). [4]. In addition, Hydrilla verticillata (Lf) Royle has low phosphorus requirements and can use bicarbonate as a photosynthesis carbon source [5]. Phosphorus absorption is an active process [6]. In subtropical wetlands, phosphorus mainly flows in plants, and nitrogen mainly flows through sedimentation and denitrification. Phosphorus is a limiting factor for summer phytoplankton (mainly exotic blue algae). It has been speculated that the phosphorus cycle strongly depends on the regulation of large plants; the depletion of phosphorus in the sediment affects the reduction of the plant Typha domingensis, and subsequent increase in the availability of phosphorus recreates it [7]. In the enclosure experiment of Donghu Lake, the results showed that the submerged plants are in the key position of phosphorus retentate [8]. Submerged plants can remove labeled carbon in water from leaves and rhizomes (mainly leaves), thereby promoting carbon absorption, migration, and release in flowing water habitats [9]. The freshwater submerged plant system has a good effect on the removal of nutrients: nitrogen is mainly denitrified, and phosphorus is bioabsorbed and subsequently harvested [10]. 1.2 Removal of Heavy Metals by Aquatic Plants Aquatic plants have a strong ability to absorb and accumulate heavy metals such as Zn, Cr, Pb, Cd, Co, Ni, and Cu. Numerous studies have shown that the content of heavy metals in the environment is positively correlated with the content of heavy metals in plant tissues. Therefore, it is possible to indicate the level of heavy metals in the environment by analyzing heavy metals in plants. Dai Quanyu monitored and evaluated Taihu Lake from the point of view of aquatic plants in the early 1980s, and believes that aquatic plants have the ability to monitor heavy metals in lakes. Aquatic macrophytes provide a cost-effective method for reducing the content of heavy metals in water by virtue of their rapid growth and absorption of large amounts of nutrients. For example, the concentration of Lemna minor can be controlled to minimize the content of organic and metal industrial wastes. [11]. In laboratory experiments, Lemna gibba can significantly reduce iron and zinc in wastewater, and the removal efficiency of manganese is 100% [12]. The concentration of heavy metals in duckweed exceeds that of algae and the angiosperm Azolla filliculoides. The enrichment coefficient of zinc, in particular, is very high. The concentration in plants is 2700 times higher than that in the medium outside [13]. The content of heavy metals in plants is very low and extremely uneven. In the same lake, the content of different kinds of aquatic plants varies greatly; in the same species in different lakes, the heavy metal content in aquatic plants also varies greatly. The order of the enrichment ability of aquatic plants is generally: submerged plants> floating plants> emergent plants. Plants are selective for the absorption of heavy metals. Cd can displace Zn when the essential elements Zn and Cd are combined with thiol groups in the sulfur protein. Therefore, the Zn/Cd value is a good indicator of the plant's ability to accumulate and it also indirectly indicates the degree of damage to the plant. Experiments have shown that although submerged plants and floating plants can absorb many heavy metals, especially Cd, this increase in absorption will lead to the loss of nutrients, if serious, it will lead to plant death. Therefore, submerged plants and floating plants are suitable as carriers for absorbing heavy metals in low-pollution areas, and at the same time they can monitor the metal content of water in heavy metals [14]. In addition, aquatic plants control the distribution of heavy metals in plants, allowing more heavy metals to accumulate in the roots. The heavy metal content in the roots of aquatic plants is generally much higher than that in the stems and leaves. But there are exceptions, which may be related to their different absorption pathways. The kinetics of the absorption of soluble metals by algae has been studied more clearly. The absorption of metals by algae is carried out in two steps: the first step is the passive adsorption process (ie physical adsorption or ion exchange on the cell surface), and the time of occurrence is very short, without any metabolic process and energy provision; It may be an active absorption process, which is related to metabolic activity. This absorption process is slow and is the main way for algal cells to absorb heavy metal ions. Algae enrich a large number of heavy metals and transfer them along the food chain to higher trophic levels, creating potential hazards, but on the other hand, it can use this feature to eliminate pollution in wastewater. Heavy metals enter natural water bodies in various ways, which are very harmful to water bodies. Therefore, it is of great significance to use algae to purify wastewater containing heavy metals [15]. Unlike metals, metals cannot be degraded by microorganisms and can only be removed from the environment by biological uptake. Plants have the advantages of large biomass and ease of post-treatment. Therefore, the use of plants to repair metal contamination sites is a very important choice to solve the problem of heavy metal pollution in the environment. There are three ways for plants to repair heavy metal contamination sites: plant fixation, plant volatilization, and plant uptake. Plants use these three methods to remove metal ions from the environment. There are also reports on the accumulation of radionuclides by aquatic plants. For example, Whicker et al. found that the aquatic macrophyte (Hydrocotyle spp.) accumulated 137Cs and 90Sr more strongly than other 15 aquatic plants [16]. Absorption of copper, lead, cadmium, nickel and other metals with Najas graminea Del. found that the absorption process is related to the Lagergren dynamic model at a constant rate of about 0.01 min-1, and the equilibrium results and the Langmuir absorption areotherm Line correlation [17]. 1.3 Removal of toxic organic contaminants by aquatic plants The presence of plants facilitates the degradation of organic pollutants. Aquatic plants may absorb and concentrate certain small-molecule organic contaminants, more often by purifying water bodies by promoting the precipitation of substances and promoting the decomposition of microorganisms. Agricultural pollution is a kind of "non-point source" pollution, and most agricultural pollutants include nitrogen and phosphorus and pesticides from crop fertilization or animal husbandry. For herbicide atrazine, it is abundant in the environment, generally 1 to 5 μg/L in the stream, 20 μg/L in the high level, and 500 μg/L in the area close to the farmland, or even 1 mg/L [18]. Aquatic macrophytes often grow near the point of application. The concentration of pesticides is high and the exposure time is long. Aquatic macrophytes and phytoplankton are more sensitive to atrazine than invertebrates, zooplankton and fish. Although higher plants cannot mineralize atrazine, they can be modified in different ways. Zablotowics et al. [19] found that algae can degrade the atrazine by studying the degradation of albus to alveolar. Clothing and green algae can also degrade atrazine [20]. Changes in the ratio of algae in a highly tolerant lichen (Parmelia sulcata Taylor) can show changes in local air pollution [21]. The distribution of chlorpyrifos in Elodea densa and water bodies shows that aquatic plants can absorb organic components and have the ability to remove them from the aquatic environment [22]. In the study of the ability of Ceratophyllum demersum to chloriminate, the active branchlets were absorbed five times as much as the old branch. Membrane structure and its integrity seem to be important determinants [23]. In the absorption and accumulation of RHC, DDT, and PCBs in aquatic plants, fruits are stored more than plants and leaves than roots [24]. Some plants can also degrade TNT. According to Best et al., the screening and applied research on aquatic plants and wetland phytoremediation of surface water contaminated by explosives from the Iowa Army Ammunition Plant in the US found that the effect of Myriophyllum aquaticum Vell verdc good. Roxanne et al. studied the phytoremediation technology of surface water contaminated with TNT. Under the conditions of soil concentrations of 1, 5, and 10 mg/kg, compared with the control, the degradation of the plants can be achieved with 100% removal. William et al. studied the gasification and metabolic effects of plants on trichloroethylene (TCE)-contaminated shallow groundwater systems. It was found that all collected plant samples in contaminated sites can detect the vaporization of TCE and three intermediate products. Aitchison et al. found that the stems and leaves of hybrid poplars can be quickly removed from the contaminated 1,4-dioxane compound in water culture conditions, with an average removal of 54% within 8 days [25]. Polycyclic aromatic compounds (PAHs) are a large class of organic toxic substances. In duckweed, purple duckweed, water hyacinth, water peanut, and A. cinnamomum, five kinds of aquatic plants were all harmed by naphthalene. With the increase of the concentration of naphthalene, the degree of injury was deepened, and water hyacinth was the least affected, so it was polluted by naphthalene. The purification can be the preferred object. The duckweed has the greatest sensitivity and can be used as a test for the toxicity of naphthalene to aquatic plants [26]. In addition, aquatic plants can also effectively eliminate the toxicity of bisphenol, phthalate esters and other environmental hormones and rocket engine fuel heptyl. Lemna gibba metabolizes 90% of phenols into less toxic products within 8 days [27]. The removal efficiency of COD increased from 52% to 60% in the control group to 74% to 78% [28]. The presence of metals such as chromium, copper, and aluminum can also affect the removal efficiency of duckweed on COD to varying degrees [29]. 1.4 The synergistic effect of aquatic plants and other organisms on the removal of pollutants Root microorganisms and Eichhornia crassipes and other plants have significant synergistic purification. Some aquatic plants can also transport oxygen from the leaves to the roots through aeration tissue and then diffuse into the surrounding water. Microorganisms in the water supply, especially the rhizosphere microorganisms, can be used to respire and decompose pollutants. In the roots of plants such as Eichhornia crassipes and water lilies, a large amount of microorganisms and plankton are adsorbed, which greatly increases the diversity of organisms and allows different kinds of pollutants to be purified one by one. Immobilized Nitrogen Cycling Bacteria (INCB) allows nitrogen circulating bacteria to continuously release from the carrier into the water and spread in the water, affecting the number of bacteria in the roots of aquatic plants, and thus through nitrification-denitrification. The role is to further strengthen the natural water body's ability to remove nitrogen and strengthen the self-purification capability of the entire aquatic ecosystem. This is of great significance for the further study of the mechanism of the degradation of healthy aquatic ecosystems and their restoration [30]. Aquatic macrophytes can inhibit the growth of phytoplankton, thereby reducing the existing amount of algae. In the aquatic environment, the inhibitory effects of higher aquatic plants on algae are more obvious. The main manifestations are two aspects: First, the number of algae drastically decreased; second, the algae community structure changes. Aquatic plants and algae compete in nutrition, light, and living space. In addition to artificial control and low temperature conditions, the growth of aquatic plants is generally dominant. The interaction between aquatic plants and algae (exogenetic phenomena) has important application potentials in sewage purification and ecological optimization of water bodies. Gu Linxi et al31 found that Vallisneria secretes biochemical inhibitors, and the inhibitory effect is positively correlated with the planting water concentration. Planting higher plants such as Vallisneria in shallow lakes and stocking appropriate amounts of fish will not only protect water quality, but also develop fishery production and increase economic efficiency. Not only that, field experiments and laboratory studies have also shown that aquatic plants such as Eichhornia crassipes also secrete a series of organic chemicals into the water through the root system. These substances can affect the morphology, physiological and biochemical processes, and growth and reproduction of algae when the water content is extremely small, and the amount of algae is significantly reduced. Typha spp. often covers wetlands and other freshwater environments, resulting in single species. An important mechanism for the intrusion of this cattail is the release of phase-to-phase substances, plant toxins, into the surrounding environment [32]. The use of plant secretions and the symbiotic relationship between the microorganisms surrounding the plants and the algae remove algae. This makes sense for the prevention and treatment of eutrophic water pollution and the restoration and reconstruction of water ecosystems [33]. 1.5 Other Water Purification of Aquatic Plants (Improvement of Water Quality) Functional Aquatic plants have mechanisms for maintaining water cleanliness and their own advantages and stability at different trophic levels: Aquatic plants have the characteristic of excessive absorption of nutrients, which can reduce the nutrient levels of water bodies; The turbidity is reduced due to the resuspension of sediments caused by fish fed on benthic organisms. The functions of aquatic plants to improve water quality, such as stabilizing the sediment, inhibiting algae and inhibiting bacteria, also have important practical significance. Oxygen is a very important substance. Algae bloom caused by eutrophication of water causes the transparency of the water body to decrease, and the quality of drinking water decreases. Tissue hypoxia degrades large plants and reduces the diversity of aquatic plants. The lack of oxygen in the ocean's underlying continental shelf has caused a large number of deaths of submarine life, which has brought serious threats to the local economy and human survival. Submerged plants are closely related to sediments and water flow. In the ecosystem, it can play a role in improving water quality, stabilizing sediment and reducing turbidity [34]. 2 Application of aquatic plants in pollution control 2.1 Constructed wetland mediums, aquatic plants and microorganisms are the main components of constructed wetlands. Among them, aquatic plants not only directly absorb and use nutrients in sewage, but also absorb and enrich some toxic and harmful substances, as well as transport oxygen to the root zone and maintain the hydraulic transmission. Moreover, the presence of aquatic plants facilitates the expansion of microorganisms in the constructed wetlands. Part of the nitrogen in the sewage is absorbed by the plant and the available phosphorus can also be directly absorbed and utilized by the plant. Through the continuous harvest of aquatic economic crops, nitrogen, phosphorus and other pollutants are removed. At the same time, the roots of well-developed aquatic plants provide a good micro-ecological environment for micro-organisms and micro flora and fauna. Their large-scale reproduction ensures the efficient degradation, migration and transformation of contaminated organic matter. The organic combination of medium, aquatic plants and microorganisms, mutual relations and mutual causal relationships have formed a unified body of constructed wetlands, strengthening the function of wetland purification of sewage [35]. The use of constructed wetlands and large aquatic plants to purify water bodies has attracted increasing attention as a purification technology. It can create a rich ecosystem and minimal environmental output. Can protect the environment, with low operating costs and satisfactory purification efficiency. An aquatic plant system requires a large number of areas, design specifications, and maintenance methods to achieve the most optimal optimization effect per unit area. This has been done for three years in Japan's Lake Kasumigaura [36]. In Hungary, there are mainly three types of constructed wetlands: blank surface systems, subsurface systems, and artificial drift meadow systems. In the Nyirbogdny sewage treatment system, the removal rate of COD is about 60% on average, and the water quality meets the natural water standard [37]. 2.2 Bioremediation Bioremediation is a newly developed emerging technology that has low investment, high efficiency, convenient application, and large potential for development. It uses specific organisms (plants, microorganisms or protozoa) to absorb, transform, remove or degrade environmental pollutants, achieve environmental purification, and biological measures for the restoration of ecological effects. The bioremediation of inorganic (mainly heavy metal) pollution is mainly through the plant pathway, also known as phytoremediation, while the bioremediation of organic pollution relies mainly on microbial degradation, absorption and transformation. Although emphasis is placed on restricting emissions and strengthening waste management, with the continued growth of the population, the rapid development of industry and agriculture, and the continuous expansion of urbanization, the organic pollution of water bodies is still showing a significant increase. In particular, Xenobiotics have been extensively used in recent years because of their high resistance to microbial decomposition, making it more difficult to recover the polluted environment [38]. 2.3 Stabilization pond Stabilization pond method is also called biological pond and oxidation pond. It is a process for sewage treatment through manual control of biological oxidation process. It has the characteristics of low capital investment, simple treatment process, and easy management, and has the characteristics of small and medium-sized conventional sewage treatment. Wide application prospects. It mainly uses the combined action of bacteria and algae to treat organic pollutants in wastewater. The stable pond can be used for the treatment of domestic sewage, pesticide wastewater, food industry wastewater and papermaking wastewater, and the effect is significantly stable. Wu Zhenbin et al [39,40] used an integrated biological pond system to treat urban sewage. The results showed that COD, BOD, TSS, N, P and other pollutants were highly efficiently removed, and bacterial, viral, and mutagenic activities were significantly reduced. At the same time as sewage purification, a large number of aquatic plants and fish, fish and other aquatic products are harvested. The small-scale integrated enhanced oxidation ponds used slag adsorption and aquatic plant water hyacinths to treat dyeing wastewater in oxidation ponds by using a combination of physico-chemical and biological methods. Good results have been achieved with a COD removal rate of 76.5% and a high chroma decolorization rate. 96.9%. The treated wastewater reaches the national level of an integrated emission standard. The unit throughput and operating costs are only 1/10 of the activated sludge process, so the investment in this way is low, the operating cost is low, the treatment effect is good, the management is convenient, and the environmental and economic benefits are significant [41]. In addition, from a small-scale production experiment, it can be concluded that the application of aerobic contact oxidation, a new biological treatment process in which the combination of a combination of a combination of algae-feeding bio-bed and aquatic plants removes COD, ammonia nitrogen, and other substances such as phosphorus and potassium in chicken manure anaerobic fermentation broth. Manganese, zinc, magnesium elements and pigments have a very good effect, so that the treated wastewater can meet the comprehensive wastewater discharge standard of GB 8978-88. The denitrification effect of the algae-attached biological bed is best, and it can be recovered as a good animal feed. Aquatic plant ponds, due to the huge fibrous root system of floating plants, extremely high growth rates and huge biomass, are beneficial to the absorption and absorption of pollutants in water, and have a strong removal effect on COD, averaging 71.7%.[42] ]. 2.4 Water Purification Water purification technology has become a bottleneck and a bargaining chip for the sustainable development of the fish farming industry. Since the 1980s, there have been reports on the use of phytoplankton to purify aquaculture wastewater. However, due to the difficulty in the separation of algae water, the application of this microalgae water purification model in the circulating water fish culture system is limited. Large plants have the combined effect of purifying water, saving energy, and harvesting bait [43]. Higher aquatic plants have a stronger absorption of pollutants in the water environment, and their effectiveness varies depending on the type of plant and the combination of treatments. The level of water purification effect of higher aquatic plants depends on the enhancement of their respective physiological activities (mainly reflected in the increase of enzyme activity). Eichhornia crassipes, water lettuce, and purple spruce grow and reproduce very quickly in the warm season, and they can quickly cover the water surface and have a good purification effect. Water peanuts, reeds and other strong resistance, population density, purification effect is good, and has resistance to wind waves and the separation of water and other functions. The growth of submerged plants such as Elodea and Valeriana under water does not affect the transmission of water, but also provides a large amount of oxygen to the water through photosynthesis, and it can also grow well in the cold season. Water peanuts, loquat leaves, duckweed and other plants have strong cold resistance. Lotus root itself has a certain economic value [44]. 2.5 Lake governance and vegetation restoration Submerged plants can significantly improve the physical and chemical properties of water bodies. Its presence effectively reduces the content of particulate matter, improves underwater lighting conditions, maintains transparency at a relatively high level, and the water conductivity is relatively low. Aquatic plants also enhance sediment stability and anchorage. It has been found that in the tropics, treatment systems that combine aquatic plants with bio-immobilized membranes are suitable for use in suitable areas [45]. In the eekhoven reservoir in Frederiks, Belgium, aquatic plants have also been used to pre-filter bio-regulation of stagnant reservoirs [46]. In dry climates, both higher aquatic plants, Typha latifolia and Juncus subulatus, exhibit high purification efficiency, and their porosity also contributes to the filtration of wastewater [47]. For shallow lakes, reconstructing aquatic vegetation is an important measure for eutrophication and lake ecological restoration. About 65% of the lakes in China have become eutrophic, and about 29% are turning to eutrophication. For its governance, we must consider the use of the self-pollution characteristics of aquatic plants. Aquatic plants can significantly improve the water quality of eutrophic water bodies, and also have a significant purifying effect on toxic organic pollution. The restoration of aquatic vegetation dominated by submerged plants is an important measure for rational and effective water purification and restoration of ecosystems. Many efforts have been made in this area [48]. The establishment of Submersed Aquatic Vegetation (SAV) is mainly limited to the presence or absence of shoots, and the transparency of the water body and the level of nutrients in the sediment (especially N) are the key to the establishment of plant communities [49]. Ma Jianmin et al. [50] conducted a preliminary study on vegetation restoration, structural optimization, and water quality in a cloth-surrounding and network-controlled ecosystem in Donghu Lake, Wuhan, from 1993-1995. It was found that controlling the scale of cultivation is the prerequisite for the restoration of aquatic vegetation; in the controlled ecosystem, the biomass of aquatic vascular plants increases, and well-developed aquatic vascular plants can significantly reduce the concentrations of N and P in the water; when aquatic vegetation is restored, Submerged plants should be the mainstay. Lotus, Phragmites australis, Vallisneria, Foxtail algae and Ceratophyllum albus have strong adaptability and can be used as species for reconstructing aquatic vegetation. Turbidity is one of the factors affecting recovery, and photosynthetic effective levels are most important for stem growth [51]. Kahl used a regression model to determine whether the light attenuation coefficient is different from the expected 5% light transmission area, thereby serving as an important reference for submerged plant management and restoration [52]. Studies on Bosten Lake have shown that when aquatic plants grow on the water, their evapotranspiration is lower than that of the free water surface, and they also reduce the salinity of the water body and purify the water body, and can provide a lot of quality feed for the breeding industry. The use of vegetation to improve its ecological environment has resulted in low investment and significant and lasting benefits [53]. Studies have also shown that aquatic plant beds play an important role in maintaining and short-term storage of Particulate Organic Matter (POM) in low transparency rivers at different spatial levels. Its importance varies with the density of grass bed, surface coverage and leaf fall time [54]. 3 Summary and outlook In summary, aquatic plants can remove nitrogen, phosphorus, heavy metals and organic pollutants in polluted water to varying degrees, and have been widely used in sewage treatment. By analyzing the absorption and decomposition of aquatic elements such as nitrogen, phosphorus and other nutrients and pollutants in water, different aquatic plants and their combinations can be selected to adapt to different contaminated water bodies. It is also possible to control the size of the purification capacity by controlling the amount of aquatic plants to repair contaminated water bodies and maintain water quality. Scientific management and transformational use are the key to governance. If the amount of water hyacinth growth is conducive to the purification of water quality, the water hyacinth needs to be salvaged at the right time, and it will be converted and used through the subsequent conversion technology such as fermentation transformation to prevent its decay, because of the growth of a large number of submerged plants. It will also have a negative impact. For excessive growth of large plants, mechanical harvesting, scouring, draining and other measures can be used. Human activities cause large amounts of industrial, agricultural, and domestic waste to be discharged into the water, contaminating the water. Water pollution can be divided into three major categories: chemical pollution, physical pollution, and biological pollution, depending on the nature of the pollution. Basically, chemical pollution is the main cause. Specific pollution impurities are inorganic pollutants, inorganic toxic substances, organic toxic substances, and plant nutrient substances. For the removal of these pollutants, aquatic plants play a very important role. Aquatic plants refer to the group of plants that physiologically attach to the water environment and at least part of the reproductive cycle takes place in water or water. Aquatic plants can be roughly divided into four categories: emergent plants, submerged plants, floating leaf plants and floating plants. Large-scale aquatic plants are all groups of aquatic plants other than microalgae. Aquatic plants are an important part of the aquatic ecosystem and a major primary producer. They play a regulatory role in the circulation and transfer of ecosystem materials and energy. It also fixes suspended solids in water and can potentially detoxify it. The role of aquatic plants in the accumulation, metabolism and fate of environmental chemical substances cannot be ignored. Aquatic plants are used to monitor aquatic pollution, conduct eco-toxicological evaluation of pollutants, and bioaccumulate, modify, and translocate after entering the biological chain. This has important implications for the protection of plant ecology and human and animal health [1]. 1 Removal of Pollutants by Aquatic Plants 1.1 Removal of Nitrogen and Phosphorus by Aquatic Plants Lake eutrophication has become a worldwide environmental problem. The use of aquatic macrophytes to enrich nitrogen and phosphorus is one of the effective ways to control, regulate and inhibit lake eutrophication. The lake water environment includes two parts: the body of water and sediment. Nitrogen and phosphorus in the body of water can be migrated into the bottom soil by sedimentation of sediments, sediment adsorption, and deposition. Tracking past nutritional status suggests that aquatic plants can regulate nutrient concentrations in shallow water lakes at moderate temperatures [2]. Large-sized submerged plants absorb nitrogen and phosphorus from the substrate through the roots, and thus have a stronger ability to enrich nitrogen and phosphorus than floating plants. Submerged plants have enormous biomass, and undergo extensive exchanges of matter and energy with the environment, resulting in a very large environmental capacity and a strong self-purification capability. In the submerged plant distribution areas, the contents of COD, BOD, total phosphorus and ammonium nitrogen are generally much lower than the distribution areas of submerged plants outside [3]. The dense growth of floating plants makes the re-oxygenation of lake water obstructed, the dissolved oxygen in the water is greatly reduced, the self-purification capability of the water body is not improved, and secondary pollution is caused, which affects shipping. Emerging plants must grow on wetlands, shoals, shores, etc., that is, breeding sites with suitable depth, which have great limitations. Different submerged plants have significant removal of total nitrogen total phosphorus in the water. In the study on the removal rate of total nitrogen from common submerged macrophytes in Dianchi Lake water (including sediments), it was found that the order of species removal capacity was Elodea> Vallisneria> Foxtail> Potamogeton edulis> Goldfishes> Hemerocallis> Chara. With the prolongation of time, the concentration of total nitrogen in water declined in a negative exponential manner, and the removal rate of each submerged plant increased with the increase of total nitrogen concentration within the range of total nitrogen concentration (2.628-16.667 mg/L). [4]. In addition, Hydrilla verticillata (Lf) Royle has low phosphorus requirements and can use bicarbonate as a photosynthesis carbon source [5]. Phosphorus absorption is an active process [6]. In subtropical wetlands, phosphorus mainly flows in plants, and nitrogen mainly flows through sedimentation and denitrification. Phosphorus is a limiting factor for summer phytoplankton (mainly exotic blue algae). It has been speculated that the phosphorus cycle strongly depends on the regulation of large plants; the depletion of phosphorus in the sediment affects the reduction of the plant Typha domingensis, and subsequent increase in the availability of phosphorus recreates it [7]. In the enclosure experiment of Donghu Lake, the results showed that the submerged plants are in the key position of phosphorus retentate [8]. Submerged plants can remove labeled carbon in water from leaves and rhizomes (mainly leaves), thereby promoting carbon absorption, migration, and release in flowing water habitats [9]. The freshwater submerged plant system has a good effect on the removal of nutrients: nitrogen is mainly denitrified, and phosphorus is bioabsorbed and subsequently harvested [10]. 1.2 Removal of Heavy Metals by Aquatic Plants Aquatic plants have a strong ability to absorb and accumulate heavy metals such as Zn, Cr, Pb, Cd, Co, Ni, and Cu. Numerous studies have shown that the content of heavy metals in the environment is positively correlated with the content of heavy metals in plant tissues. Therefore, it is possible to indicate the level of heavy metals in the environment by analyzing heavy metals in plants. Dai Quanyu monitored and evaluated Taihu Lake from the point of view of aquatic plants in the early 1980s, and believes that aquatic plants have the ability to monitor heavy metals in lakes. Aquatic macrophytes provide a cost-effective method for reducing the content of heavy metals in water by virtue of their rapid growth and absorption of large amounts of nutrients. For example, the concentration of Lemna minor can be controlled to minimize the content of organic and metal industrial wastes. [11]. In laboratory experiments, Lemna gibba can significantly reduce iron and zinc in wastewater, and the removal efficiency of manganese is 100% [12]. The concentration of heavy metals in duckweed exceeds that of algae and the angiosperm Azolla filliculoides. The enrichment coefficient of zinc, in particular, is very high. The concentration in plants is 2700 times higher than that in the medium outside [13]. The content of heavy metals in plants is very low and extremely uneven. In the same lake, the content of different kinds of aquatic plants varies greatly; in the same species in different lakes, the heavy metal content in aquatic plants also varies greatly. The order of the enrichment ability of aquatic plants is generally: submerged plants> floating plants> emergent plants. Plants are selective for the absorption of heavy metals. Cd can displace Zn when the essential elements Zn and Cd are combined with thiol groups in the sulfur protein. Therefore, the Zn/Cd value is a good indicator of the plant's ability to accumulate and it also indirectly indicates the degree of damage to the plant. Experiments have shown that although submerged plants and floating plants can absorb many heavy metals, especially Cd, this increase in absorption will lead to the loss of nutrients, if serious, it will lead to plant death. Therefore, submerged plants and floating plants are suitable as carriers for absorbing heavy metals in low-pollution areas, and at the same time they can monitor the metal content of water in heavy metals [14]. In addition, aquatic plants control the distribution of heavy metals in plants, allowing more heavy metals to accumulate in the roots. The heavy metal content in the roots of aquatic plants is generally much higher than that in the stems and leaves. But there are exceptions, which may be related to their different absorption pathways. The kinetics of the absorption of soluble metals by algae has been studied more clearly. The absorption of metals by algae is carried out in two steps: the first step is the passive adsorption process (ie physical adsorption or ion exchange on the cell surface), and the time of occurrence is very short, without any metabolic process and energy provision; It may be an active absorption process, which is related to metabolic activity. This absorption process is slow and is the main way for algal cells to absorb heavy metal ions. Algae enrich a large number of heavy metals and transfer them along the food chain to higher trophic levels, creating potential hazards, but on the other hand, it can use this feature to eliminate pollution in wastewater. Heavy metals enter natural water bodies in various ways, which are very harmful to water bodies. Therefore, it is of great significance to use algae to purify wastewater containing heavy metals [15]. Unlike metals, metals cannot be degraded by microorganisms and can only be removed from the environment by biological uptake. Plants have the advantages of large biomass and ease of post-treatment. Therefore, the use of plants to repair metal contamination sites is a very important choice to solve the problem of heavy metal pollution in the environment. There are three ways for plants to repair heavy metal contamination sites: plant fixation, plant volatilization, and plant uptake. Plants use these three methods to remove metal ions from the environment.有关水生植物对放射性核素的积累也有报道，如Whicker等发现水生大型植物石莲花（Hydrocotyle spp.）比其他15种水生植物积累137Cs和90Sr的能力强[16]。用拂尾藻（Najas graminea Del.）吸收铜、铅、镉、镍等金属发现，吸收过程在约0.01 min-1 恒定速率下与Lagergren动力模型相关，同时平衡结果和朗缪尔（Langmuir）吸收等温线相关[17] 。 1.3 水生植物对有毒有机污染物的清除植物的存在有利于有机污染物质的降解。水生植物可能吸收和富集某些小分子有机污染物，更多的是通过促进物质的沉淀和促进微生物的分解作用来净化水体。农业污染是一种“非点状源”的污染，大多数农业污染物包括来自作物施肥或动物饲养地的氮磷以及农药等。对除草剂莠去津来说，它在环境中大量存在，小溪中一般为1～5 μg/L，含量较高时为20 μg/L，而靠近农田的区域达500 μg/L，甚至1 mg/L[18]。水生大型植物常生长在施用点附近，农药浓度很高，暴露时间很长，所以水生大型植物和浮游植物对于莠去津比无脊椎动物、浮游动物和鱼类更敏感。高等植物虽不能矿化莠去津，但可以用不同的途径来修饰。 Zablotowics等[19]在研究藻类对伏草隆的降解中发现，纤维藻和月芽藻能使阿特拉津去烃基。衣、绿藻属也能降解阿特拉津[20]。一种高忍耐性地衣(Parmelia sulcata Taylor)的藻层比率的变化可显示出当地空气污染的变化[21]。毒死蜱(chlorpyrifos)在伊乐藻(Elodea densa)和水体中的分布表明，水生植物可吸收有机成分并有将其从水生环境中去除的能力[22]。金鱼藻(Ceratophyllum demersum)对灭害威的吸着能力的研究中，生长活跃的小枝是老枝吸收的5倍。膜构造及其完整性好象是重要的决定因子[23]。水生植物对RHC,DDT，PCBs残留的吸收和积累中，果实比植株，叶比根贮存更多[24]。某些植物也可降解TNT。据Best等报道，对受美国依阿华陆军弹药厂爆炸物所污染的地表水进行水生植物和湿地植物修复的筛选与应用研究中发现，狐尾藻属植物（Myriophyllum aquaticum Vell verdc）的效果甚佳。 Roxanne等研究了受TNT污染地表水的植物修复技术，在所用浓度为1、5、10 mg/kg的土壤条件下，与对照相比，利用植物的降解，移除量可达100%。 William等研究了植物对三氯乙烯（TCE）污染浅层地下水系的气化、代谢效应，结果发现，污染场所中所有采集的植物样品都可检测出TCE的气化挥发以及3种中间产物。 Aitchison等发现，水培条件下杂交杨的茎、叶可快速去除污染物1，4-二氧六环化合物，8 d内平均清除量达54%[25]。多环芳香烃化合物(PAHs)是一大类有机毒性物质。在浮萍，紫萍，水葫芦，水花生，细叶满江红等5种水生植物中，均受到萘的伤害，随萘浓度的增加而伤害程度加深，其中水葫芦受害最轻，所以对萘污染的净化可作为首选对象。而浮萍的敏感性最大，可用作萘对水生植物的毒性检测[26]。此外水生植物也可有效消除双酚、酞酸酯等环境激素和火箭发动机的燃料庚基的毒性。浮萍(Lemna gibba)在8 d内把90%的酚代谢为毒性更小的产物[27]。 COD的去除效率由对照组的52%～60%上升为74%～78%[28]。铬，铜，铝等金属的存在也可不同程度地影响浮萍对COD的去除效率[29]。 1.4 水生植物与其他生物的协同作用对污染物的清除根系微生物与凤眼莲等植物有明显的协同净化作用。一些水生植物还可以通过通气组织把氧气自叶输送到根部，然后扩散到周围水中，供水中微生物，尤其是根际微生物呼吸和分解污染物之用。在凤眼莲、水浮莲等植物根部，吸附有大量的微生物和浮游生物，大大增加了生物的多样性，使不同种类污染物逐次得以净化。利用固定化氮循环细菌技术（Immobilized Nitrogen CyclingBacteria，INCB），可使氮循环细菌从载体中不断向水体释放，并在水域中扩散，影响了水生高等植物根部的菌数，从而通过硝化-反硝化作用，进一步加强自然水体除氮能力和强化整个水生生态系统自净能力。这对进一步研究健康水生生态系统退化的机理及其修复均具有重要意义[30]。水生大型植物能抑制浮游植物的生长，从而降低藻类的现存量。在水生态环境中，水生高等植物对藻类的抑制作用较为明显。主要表现在两个方面：一是藻类数量急剧下降；二是藻类群落结构改变。水生植物与藻类在营养、光照、生存空间等方面存在竞争。除人工控制和低温等条件下，一般是水生植物生长占优势。水生植物与藻类之间的相生相克（异株克生现象）作用在污水净化和水体生态优化方面有重要应用潜力。顾林娣等[31]发现苦草能分泌生化抑制物质，且抑制作用的大小和种植水浓度呈正相关。在浅水湖泊中种植苦草等高等植物，放养适量的鱼类，这样就既可以保护水质，又可以发展渔业生产，增加经济效益。不仅如此，野外实验和实验室研究还表明，凤眼莲等水生植物还通过根系向水中分泌一系列有机化学物质。这些物质在水中含量极微的情况下即可影响藻类的形态、生理生化过程和生长繁殖，使藻类数量明显减少。有害植物(Typha spp.)常覆盖湿地和其他淡水环境，造成物种单一。这种香蒲侵入的一个重要机制就是向周围环境中释放相生相克物质——植物毒素[32]。利用植物分泌物和植物周围的微生物与藻类间的相生相克关系，来去除藻类。这对于富营养化水体污染的防治和治理，水生态系的恢复和重建很有意义[33]。 1.5 水生植物的其他净水（改善水质）功能水生植物在不同的营养级水平上存在维持水体清洁和自身优势稳定状态的机制：水生植物有过量吸收营养物质的特性，可降低水体营养水平；减少因为摄食底栖生物的鱼类所引起沉积物重悬浮，降低浊度。水生植物的改善水质的功能，如稳定底泥、抑藻抑菌等，也具有重要的实践意义。氧气是一种非常重要的物质。水体富营养化引起的藻类水华造成水体透明度降低，饮用水质量下降。组织缺氧使大型植物退化，减少了水生植物多样性。海洋底层大陆架的缺氧，使海底生物大量死亡，给当地经济和人类生存带来了严重的威胁。沉水植物与沉积物、水体流动间有紧密联系。在生态系统中，它能起到提高水质，稳定底泥，减小浑浊的作用[34]。 2 水生植物在污染治理中的应用2.1 人工湿地介质、水生植物和微生物是人工湿地的主要组成部分。其中的水生植物除直接吸收利用污水中的营养物质及吸附、富集一些有毒有害物质外，还有输送氧气至根区和维持水力传输的作用。而且水生植物的存在有利于微生物在人工湿地纵深的扩展。污水中的氮一部分被植物吸收作用去除，同时可利用态磷也能被植物直接吸收和利用。通过对水生经济作物的不断收获，从而移出氮、磷等污染物。同时发达的水生植物根系为微生物和微型动植物提供了良好的微生态环境，它们的大量繁殖为污染有机物的高效降解、迁移和转化提供了保证。介质、水生植物和微生物的有机组合，相互联系和互为因果的关系形成了人工湿地的统一体，强化了湿地净化污水的功能[35]。利用人工湿地和水生大型植物来净化水体，作为一种净化技术，日益受到关注。它可以创立丰富的生态系统和最小的环境输出。可以保护环境，具有运行费用低和令人满意的净化效率等特点。一个水生植物系统需要大量区域、设计规格和维护方法，从而达到单位面积上的最适宜的优化效应。这在日本的琵琶湖(Lake Kasumigaura)已经进行了三年的实验[36]。在匈牙利，人工湿地主要有三种类型：空白水面系统、潜流系统和人工漂移草地系统。在Nyirbogdny的污水处理系统中，COD的去除速率平均约为60%，水质达自然水体标准[37]。 2.2 生物修复生物修复（Bioremediation）是新近发展起来的一项清洁环境的低投资、高效益、应用方便、发展潜力较大的新兴技术。它利用特定的生物（植物，微生物或原生动物）吸收，转化，清除或降解环境污染物，实现环境净化，生态效应恢复的生物措施。对无机（主要是重金属）污染的生物修复主要是通过植物途径，又称植物修复（Phytoremediation），而对有机污染的生物修复则主要靠微生物的降解，吸收与转化等途径。虽然强调限制性排放，加强废物管理，然而随着人口的持续增长，工农业的迅速发展以及都市化的不断扩大，对水体的有机污染仍呈大幅度增长趋势。特别是近年来大量使用生物异源物质（Xenobiotics），因抗性强，难以被微生物分解，使污染环境的恢复更加困难[38]。 2.3 稳定塘稳定塘法也叫生物塘、氧化塘，是通过人工控制生物氧化过程来进行污水处理的工艺，具有基建投资少、处理过程简单、易管理等特点，在中小型常规污水处理领域具有广泛的应用前景。它主要利用菌藻的共同作用处理废水中的有机污染物。稳定塘可用于生活污水、农药废水、食品工业废水和造纸废水等的处理，效果显著稳定。吴振斌等[39，40]用综合生物塘系统处理城镇污水，结果发现COD、BOD、TSS、N、P等污染组分去除效率较高，细菌、病毒及诱变活性明显下降。在污水净化的同时，收获大量的水生植物及鱼，蚌等水产品。小型综合强化氧化塘通过采用物理化学与生物相结合的方法，将炉渣吸附和水生植物水葫芦运用于氧化塘处理印染废水，取得了良好的效果，COD 去除率达76.5%，色度脱色率高达96.9%。经处理后的废水达到国家综合排放一级标准。而单位处理量投资和运行费用只有活性污泥法的1/10，因此采用这种方式投资省、运转费用低、处理效果好、管理方便、环境与经济效益显著.另外，从小规模生产实验可以得出，应用好氧接触氧化，颤藻附着生物床和水生植物联合的生物处理新工艺对去除鸡粪厌氧发酵液中的COD，氨氮和其他如磷、钾、锰、锌、镁元素及色素等有很好的效果，能使处理后的废水达GB 8978—88污水综合排放标准。其中颤藻附着生物床脱氮效果最好，且可回收作为良好的牲畜饲料。而水生植物塘由于漂浮植物体的庞大的须根系，极高的生长速率和巨大的生物量都有利于吸附、吸收水中的污染物，从而对COD的去除作用较强，平均达71.7%[42]。 2.4 水质净化水质净化技术已成为养鱼工业可持续发展的瓶颈与筹码。 20世纪80年代以来，已有利用浮游植物净化养殖污水的研究报道。但因藻水分离困难，使这种微藻净水模式在循环水养鱼系统中的应用受到限制。而大型植物则具有净化水质、节省能源和收获饵料的综合效果[43]。高等水生植物对水环境中的污染物具有较强的吸收作用，其效能因植物种类及处理组合方式不同而异。高等水生植物净水效果的高低依赖于各自生理活性的增强（主要体现在酶活性的提高）。凤眼莲、水浮莲、紫萍等植物在温暖季节生长繁殖极快，能迅速覆盖水面，净化效果好。水花生、芦苇等抗性较强，种群密度大，净化效果较好，并具有抵抗风浪和分隔水面等功能。伊乐藻，菹草等沉水植物在水下生长不影响水的透光，还通过光合作用向水中提供大量氧气，并且在低温季节也可很好生长。水花生、槐叶萍、浮萍等植物的抗寒性较强。莲藕等本身即具有一定的经济价值[44]。 2.5 湖泊治理与植被修复沉水植物可以明显改善水体的理化性质。它的存在有效降低了颗粒性物质的含量，可改善水下光照条件，使透明度保持在较高水平，水体电导率也相对较低。水生植物还可以增强底质的稳定和固着。有人发现在热带地区，把水生植物和生物固定膜结合起来的处理系统在适宜的地带非常地适用[45]。在比利时的佛来德斯的eekhoven水库，水生植物还被用于预过滤停滞水库的生物调节[46]。在干燥气候下，两种高等水生植物Typha latifolia 和Juncus subulatus 都表现出较高的净化效率，其多孔性也有助于污水的过滤[47]。对于浅水湖泊而言，重建水生植被是富营养化治理和湖泊生态恢复的重要措施。我国的湖泊已有约65%呈现富营养状态，还有约29%正在转向富营养状态。对其治理，必须考虑利用水生植物的自身治污特性。水生植物可以显著提高富营养水体的水质，对有毒的有机污染也有明显的净化作用。恢复以沉水植物为主的水生植被是合理有效的水质净化和生态系统恢复的重要措施，在这个方面已有人做了不少工作. 沉水植被(Submersed Aquatic Vegetation，SAV)的建立主要受限制于芽植体的有无，而水体的透明度和沉积物中的营养（尤其是N）的水平是植物群落建立的关键[49]。马剑敏等[50]在1993—1995年间对武汉东湖的布围和网围受控生态系统中的植被恢复、结构优化及水质进行了初步研究。结果发现：控制养殖规模是恢复水生植被的前提；在受控生态系统中，水生维管束植物生物量增加，生长良好的水生维管束植物能使水中N、P浓度明显降低；恢复水生植被时，应以沉水植物为主体，莲、芦苇、苦草、狐尾藻和金鱼藻适应性较强，可作为重建水生植被的物种。而浑浊是影响恢复的因素之一，光合有效水平对茎生长最重要[51]。 Kahl通过衰退模型来确定光衰减系数是否与预计的5%透光区相异，从而作为沉水植物治理和修复的重要参考[52]。通过对博斯腾湖的研究表明，水面上有水生植物生长时，其蒸发蒸腾量低于自由水面的蒸发量，而且降低了水体的矿化度并净化了水体，并且可为养殖业提供大量优质饲料。利用植被改善其生态环境，投资少，效益明显而持久[53]。研究还表明，水生植物床对于低透明度河流中颗粒性有机物质(Particulate Organic Matter，POM)的保持和短期贮存在不同空间层次上有重要作用。其重要性因草床密度、表面覆盖率及叶落时间的不同而有差异[54]。 3 小结与展望综上所述，水生植物能够不同程度地清除被污染水体的氮、磷，重金属及有机污染物，并在污水治理中得到了广泛的应用。通过分析水生植物对水中氮、磷等营养元素和污染物的吸收及分解作用，可选择不同的水生植物及其组合来适应不同的受污染水体。还可通过控制水生植物的数量来调控净化能力的大小，以修复受污染水体并保持水质。科学的管理和转化利用是治理的关键。如适量的水葫芦生长有利于水质的净化，在水葫芦长到适当的时候就需要适时打捞，并通过发酵转化等后续技术将之转化利用，防止其腐烂造成的二次污染。沉水植物的治理对湖泊生态系统有着重大影响，但如果缺乏反馈机制结果会更恶劣，因为大量的沉水植物的生长也会带来负面影响。对过多的大型植物生长可采用机械收割、冲刷、抽干等措施。的二次污染。沉水植物的治理对湖泊生态系统有着重大影响，但如果缺乏反馈机制结果会更恶劣

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