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

Zinc?Zn?pollution is present in terrestrial and aquatic environments and is also one of the pollution that people are increasingly concerned about.Constructed wetland is an ecological system similar to the natural wetland that consists of plants,animals,microorganisms,substrates,and water through artificial intervention,and has the characteristics of good treatment effect and high economic efficiency.In this experiment,1/2 Hoagland nutrient solution of different concentrations(0,50,100,200 and 400 mg·L^-1)of Zn was used for hydroponic experiments.To study the mechanism of Zn accumulation in different concentrations of aquatic plants such as Oenanthe javanica,Juncus effusus,and Eichhornia crassipes.And select suitable Zn concentration and plant combinations.After that,using simulated artificial wetland technology and matching three different substrates,namely river sand?S?,fly ash?H?,river sand,and fly ash with a 1:1 volume ratio?S+H?,under the treatment of 100 mg·L^-1 Zn,simulated treatment effect of constructed wetland system on Zn-containing wastewater in order to select the appropriate substrates.And select the appropriate substrate.Finally,under different influent pH?3.0,4.0,5.0 and 6.0?conditions,the simulated wetland system was used to treat the Zn-containing wastewater and provide experience for the treatment of acidic heavy metal wastewater.Research indicates:?1?The 0¹00 mg·L^-1 Zn-containing wastewater was beneficial to the growth of three aquatic plants under hydroponic conditions,but applying 200⁴00 mg·L^-1 of Zn-containing wastewater was not conducive to its growth.The removal rate of Zn from three aquatic plants increased with the increase of culturing time.With the increase of Zn concentration,the removal rate reached the maximum at the end of the experiment?day 7?,reaching 57.02%,when different plants are combined to treat 100 mg·L^-1 Zn-containing wastewater,the combination of Oenanthe javanica + Juncus effusus + Eichhornia crassipes has the best effect.Malondialdehyde?MDA?and relative conductivity?REC?increase with the increase of Zn concentration,it shows that the membrane damage is getting more and more serious,which was not conducive to plant growth.With the increase of Zn concentration,the contents of Zn in three kinds of aquatic plants in the underground part?root?and the aboveground part?mixed stems and leaves?increased,in which the proportion of cell wall fraction?M1?and ribosomal soluble fraction?M4?accounted for the most.?2?Different substrates and influent pH had no adverse effect on the growth of three aquatic plants.The accumulation of Zn by the three plants in the same substrate is the Eichhornia crassipes>Juncus effususes > Oenanthe javanica.The plants suffered the least damage to the membrane in S+H substrate,followed by substrate S,so three plants were suitable for growth in S+H substrate.At different influent pH conditions,the change trends of REC and MDA were the same and all showed a gradual decrease with increasing pH,the higher the pH of the influent,the smaller the damage to the plant.?3?Different substrates and influent pH all achieved higher levels of Zn removal,which were all greater than 95%,and all met the secondary standards of Discharge standard of heavy metal wastewater.The substrate is S+H with plant wetland system and influent pH of 6.0 to reach the first level emission standard,with removal rates of up to 98.82% and 98.85%,respectively.?4?Under the influence of different substrates and influent pH,the correlation analysis showed that the DO in the simulated wetland system was positively correlated with pH.The total Zn content in all layers of the substrate gradually decreased from top to bottom regardless.The distribution of the total Zn content was upper?0⁵ cm?> middle?5¹2 cm?> lower?12²0 cm?,the change of effective Zn content was basically consistent with the change trend of total Zn content in all layers of the substrate,the correlation analysis showed that the total Zn content was positively correlated with the effective Zn content.In summary,under hydroponic conditions,combined with three plant growth and physiological indicators,it is suitable for treating wastewater containing 100 mg·L^-1 or less of Zn;for the substrate of the simulated constructed wetland system,a mixed substrate should be used,that is,S+H;Simulated the removal efficiency of Zn-containing wastewater from constructed wetlands and decreased with increasing pH.However,the treatment effect can reach the "Standard for the discharge of heavy metal wastewater",it shows that the treatment of acidic wastewater has a very good effect.However,lower acidity has a greater impact on plants and is not conducive to the plant's role in the system.In the treatment of acidic wastewater,it is advisable to use an alkaline substrate,which helps to adjust the pH of the effluent to reduce the damage to the system caused by excessive acidity,which is also beneficial to reduce the treatment cost.