Study On The Effect And Mechanism Of Cd Removal From Wastewater By Constructed Wetlands

With the rapid development of modern industrial production,China's economy has made progress by leaps and bounds,but it also has a prominent negative impact on human society,that is,environmental pollution.In particular,heavy metal emission in heavy industry makes the pollution of heavy metals in the environment become more and more serious.The primary object of pollution is the natural body of water.For the artificial discharge of heavy metal waste water can not be recycled,will be part of the benthic sediment in rivers and lakes and rivers,some will transfer and accumulation in the food chain,and the greater part will cause more pollution through the water cycle,and ultimately endanger human health.At the same time,the acidic wastewater is also bring huge losses to industrial production that strong corrosion can damage all kinds of pipe and equipment,after entering the water body,it isl also damage the ecological environment and affect the production of agriculture and fishery.As a new technology of wastewater treatment,constructed wetland?CW?has many advantages,such as less investment,low energy consumption,simple operation and maintenance,high efficiency and so on.Now it has been gradually applied to treat various kinds of wastewater,including urban sewage,industrial wastewater and mine wastewater.This subject studies hydroponic plant experiment,the substrates experiment of wetland system and acid waste water experiment,which in order to simulate the removal effect and mechanism of CW for Cd in wastewater.First,in the hydroponics experiment with a concentration of 5mg·L^-1 under a single heavy metal Cd pollution,we studies the purification effect and adsorption mechanism of two combinations and three combinations of three kinds of wetland plants,including Oenanthe javanica,Juncaceae and Eichhornia crassipes,then selects the best purification combination,planting them in the small subsurface flow constructed wetland?SSFW?system system of next substrates experiment.This experiment study the purification effect and mechanism of different substrates?river sand,fly ash,fly ash-river sand mixture?wetland system and planted system,unplanted system on Cd wastewater,and evaluate the best constructed wetland system in this subject.Finally,based on the wetland system,the effect of different influent pH on the effect of heavy metal Cd on wetland was studied,and the suitable range of pH value for acid wastewater was analyzed.The results of the study show that:?1?Under the low concentration Cd pollution with initial concentration of 5mg·L^-1,the combination of water celery,rushes and water hyacinth has the best effect on the purification of wastewater.At this time,the removal rate of Cd in the environment is 68.14%,the DO in the solution is more sufficient than the other combinations.The three plants have a favorable coordination effect and have a good enrichment effect,which makes the whole micro system more effective.The removal efficiency of the combination of rushes and water hyacinth was significantly lower than that of the other combinations,which may be the result of the interspecific inhibition of interspecific competition when these two plants only exist.?2?Small SSFW with mixed planting?water celery+rushes+water hyacinth?and mixed substrates?fly ash-river sand mixture?is evaluated as a better CW system.The purification rate of Cd in the wastewater is 99.27%,the effluent pH is 9.62,the conductivity of the effluent is233?S·cm^-1,and the DO concentration of the effluent is 7.97 mg·L^-1,the three plants can grow normally The adsorption and precipitation effect of fly ash substrate on heavy metal Cd is the best,but the pH value and conductivity value of effluent index are too high,and inhibit the growth and development of plants.The effluent quality of traditional river sand substrate is well evaluated,but the removal efficiency is relatively low,the Cd removal rate of plant effluent is only 97.77%.?3?The removal efficiency of the experimental CW system for Cd is shown to be pH-6>pH-5>pH-4>pH-3,indicating that the system also has a stable purification operation for strong acid wastewater.When the influent p H is 3,the removal efficiency of heavy metal Cd is 98.18%,and the effluent pH is 7.38,reaching the neutral water quality,the conductivity of the effluent is 187?S·cm^-1.?4?Water celery,rushes and water hyacinth all have strong developed roots,and have good enrichment effect on heavy metal Cd.Different plant species had different responses to heavy metal availability,of which water celery has the best accumulation effect,but the resistance of rushes and water hyacinth to environment was better.The Cd subcellular distribution of the three plants was basically the same.The proportion of the cell wall part?M1?and the soluble part containing ribosome?M4?was the most.The proportion of Cd distribution in the cell wall components of the rushes was significantly higher than that of water celery and water hyacinth.Cd is dominated by free state in 3 plants,and less Cd content in combination with organic ligands,thus alleviated the harm of Cd to plants.In the alkaline substrate,three plants are subjected to more severe stress with the increase of substrate pH,both the electrolyte permeability?REC?and the content of malondialdehyde?MDA?increase.In the acid wastewater,the REC and MDA content of the three plants increase with the decrease of influent pH,the lower the acidity,the greater the effect on the persecution of the plants.The three plants all have high tolerance to Cd and can adapt to the wide range of pH.?5?The adsorption of Cd in each layer gradually decreases with the increase of the substrate depth,regardless of unplanted system or planted system,and there is a very significant positive correlation between the changes of organic carbon and total Cd and effective Cd in the same substrate layer.The adsorption capacity of Cd is significantly related to the substrate type in the medium and lower layers.When pH>7,the larger the substrate pH,the greater the adsorption of heavy metals.The total Cd content in the unplanted system was significantly higher than that of the planted system in the upper substrate with the most root distribution,and in the planted system,the higher the content of the active Cd in the upper substrate,the better the effect of plant absorption on Cd.In this experiment,the conclusion is embodied in the operation effect of fly ash-river sand mixture substrate system.