Treatment Efficiency Of Wastewater Contained Pb And Cd By Constructed Wetland Constituted With Different Substrates

Heavy metals presented a serious threat to our environment as hazardous pollutants because of their toxicity,persistence,and bioaccumulation problems.Among the differentheavy metals,lead and cadmium was considered to be one of the most harmful HMs to the ecological system and human life.The presence of lead in drinking water even at low concen-trations might cause diseases such as anemia,encephalopathy,hepatitis and nephritic Syndrome.Lead poisoning caused damage to liver,kidney and reduction in hemoglobin formation,mental retardation,infertility and abnormalities in pregnant women.Cd could cause chronic health problems,such as bone disease,lung edema,renal dysfunction,liver damage,anemia and hypertension.Their presences in aquatic systems posed,therefore,serious risks to humans,and also to living organisms in water and to other consumers of it.Therefore,more and more attention had been paid to the treatment of heavy metal wastewater.It was of great significance to study the treatment methods for the national production and social development.In this experiment sludge,fly ash,soil,fine sand,gravel and fine cinder were selected as raw materials,and five of them were selected to prepare five different mixed substrates of constructed wetland,named substrate SSFGF、substrate FSSGF、substrate FSSFF、substrate FSSFG and substrate FSFGF.The simulated constructed wetland systems were used to purify wastewater containing different concentrations of Pb and Cd,and the removal effect of heavy metals and their chemical speciation changes of two heavy metals in the substrates were studied.A more reasonable substrates were selected to achieve the ideal removal effect of heavy metals in order to provide the corresponding basis for the design,operation and maintenance of vertical flow constructed wetland,and the fraction changes of Pb and Cd in the substrates were studied to provide the corresponding basis for understanding the migration and transformation of Pb and Cd in the substrates,and they provided a theoretical basis for the selection of plant types and planting.The experiment results showed that:（1）The removal efficiencies of Pb and Cd were higher in the early stage,but they were lower in the later stage.Under the lower influent concentration,the highest removal efficiency of Pb in wastewater was appeared substrate SSFGF.The removal efficiency of Pb in substrate SSFGF was higher than that in the other four substrates.The results showed that the removal efficiencies of Pb in the five substrates with the medium initial concentration were higher than80%,while the removal efficiencies for Pb with the low initial concentrations were higher than 40%and the removal efficiencies for Pb with relatively high concentrations were the lowest.When the influent initial concentration of Cd was relatively low,the removal efficiency of substrate FSSGF was lower before 8h,followed by substrate SSFGF;after 8h,the removal efficiencies of substrate FSSFG was a bit lower.When the influent initial concentrations were a bit higher,the removal efficiency of substrate FSFGF was lower,and that of substrate SSFGF was the highest.The effluent concentration of substrate SSFGF was the highest when the influent concentration of Cd was low.The effluent concentration of substrate FSSFF was the lowest when the influent concentration of Cd was high.The removal efficiencies of Cd in the substrates increased with the increase of the influent initial concentrations of Cd.The removal efficiencies of Cd in substrate FSSFF and substrate FSFGF were relatively stable.The fluctuation range of substrate FSSGF was larger.The removal efficiencies of relatively high initial concentrations Cd in substrates were more than90%,while the removal efficiencies of relatively low initial concentrations Cd were generally lower,and the fluctuation range was 27.8%～90%.（2）The contents of exchangeable and carbonate-bound Pb increased in varying degrees after 12h.When the influent concentration was lower,the content of carbonate-bound form in substrate SSFGF was lower,while the content of carbonate-bound form in substrate FSSFF was lower at other influent initial concentrations.While the influent initial concentration was relatively high,the residual content of Pb in substrate SSFGF was the highest.While the residual content of Pb in substrates FSSFG and FSFGF were the highest at the other influent initial concentrations.When the influent initial concentration was lower,the residual form in the five substrates was the main form,followed by the iron manganese oxidation form.At medium influent concentration,the Fe-Mn oxide-binding form in five substrates was the the main form,especially in substrate FSSFF,which was significantly higher than that in other substrates.The main forms of Pb were Fe-Mn oxide-binding and residual form.The general trend of the content of the exchangeable form of Cd in different substrates was as follows:when the influent initial concentrations were lower,substrate FSFGF>substrate SSFGF>substrate FSSFF>substrate FSSFG>substrate FSSGF;while the influent initial concentrations were higher,substrate SSFGF>substrate FSFGF>substrate FSSGF>substrate FSSFF>substrate FSSFG.The general trend of the content of carbonate-bound of Cd was as follows:when the influent initial concentrations were lower,substrate FSSFF>substrate FSFGF>substrate SSFGF>substrate FSSFG>substrate FSSGF;while the influent initial concentrations were higher,substrate SSFGF>substrate FSSFG>substrate FSFGF>substrate FSSFF>substrate FSSGF.The main forms of Cd were exchangeable,carbonate-bound and Fe-Mn oxide-binding form.（3）The bulk density of substrate was generally positivelycorrelated with the exchangeable form of Pb,and it was generally negatively correlated with Fe-Mn oxidized and residual forms of Pb.At the higher influent initial concentrations,bulk density and density were significantly positively correlated with exchangeable,carbonate-bound and organic bound forms of Cd,and it was generally significantly negatively correlated with residual form of Cd.The effective particle size D₁₀ was significantly positively correlated with exchangeable,carbonate-bound and residual form of Pb.D₁₀ was positively correlated with exchangeable and carbonate-bound form of Cd,significantly positively correlated with residual form of Cd,and extremely significantly positively correlated with Fe-Mn oxidized and organic bound forms of Cd at the higher concentration.The effective particle size D₈₀ was significantly negatively correlated with Fe-Mn oxidized form of Pb and generally positively correlated with exchangeable form of Cd,resulting in the decrease of Fe-Mn oxidized form of Pb in the substrate.The total porosity was generally negatively correlated with exchangeable,carbonate-bound,Fe-Mn oxide-binding and organic bound forms of Cd,but generally positively correlated with residual form of Cd,it indicated that the higher the porosity was,the lower the exchangeable and carbonate-bound forms of Cd contents were in the substrate.The heterogeneity coefficient K₈₀ was generally positively correlated with carbonate-bound and residual forms of Cd,and generally negatively correlated with organic bound form of Cd.Cation exchangeablecapacity（CEC）was generally positively correlated with five chemical forms of Pb and Cd in the substrate.Exchangeable Ca was positively correlated with five forms of Pb and Fe-Mn oxidized form of Cd,but it was generally negatively correlated with organic bound form of Cd.The total amount of Al had significantly and generally negatively correlation between the five forms of Pb and Cd in the substrate,but it was generally positively correlated with Fe-Mn oxidized form of Pb at the lower influent initial concentration,and significantly positively correlated with residual form of Cd.Available Fe was significantly and generally significant positive correlated with the five forms of Pb in the substrate.And there was a general significant positive correlation between the other four forms of Cd except carbonate-bound form of Cd,which indicated that Fe in the substrate was beneficial to the removal of Pb and Cd in wastewater.The total Mg was significantly negatively correlated with exchangeable form of Cd,and generally negatively correlated with carbonate-bound,Fe-Mn oxidized and residual forms of Cd.Organic matter,exchangeable Al,available Fe and Mn were generally positively correlated with residual form of Cd.Exchangeable Al was generally positively correlated with exchangeable form of Cd.Available Mn was generally negatively correlated with exchangeableand carbonate-bound forms of Cd.Exchangeable Mg was generally positively correlated with Fe-Mn oxidized,organic bound and residual forms of Cd.There was a generally significantly negative correlation between the total amount of Fe and organic bound and residual forms of Cd.p H was generally negatively correlated with residual form of Pb and exchangeable and Fe-Mn oxidized forms of Cd,and extremely positively correlated with Fe-Mn oxidized form of Pb and organic bound form of Cd.（4）The removal efficiency of Pb was negatively correlated with K₈₀ and positively correlated with bulk density.The removal efficiency of Cd in wastewater had significant negative correlation with K₈₀,and general significant positive correlation with bulk density and extremely significant positive correlation with specific gravity at higher influent concentration.Exchangeable Ca,CEC and total Al were generally significant positively correlated with the removal efficiency of Cd,which was,the removal efficiency of Cd would increase with the increase of CEC,which was conductive to the removal of Cd.However,the other regularly chemical properties of the substrate had a negative correlation with the removal efficiency of Cd in wastewater,which would result in inhibition of the removal of Cd in wastewater.Through this study,it could provide a theoretical basis for the selection and composition of the mixed substrate of constructed wetland,and provide a scientific basis for the process design of heavy metal wastewater treatment by constructed wetland in the future,and provide a new idea for the systematic and ecological treatment of heavy metal wastewater.