Study On Synergism Passivation Of Fly Ash And Chelating Agent On Heavy Metals In Municipal Sludge And Its Bioavailability

Heavy metals in sewage sludge are one of the major factors restricting their land use.In this paper,The dehydrated sludge from the Ji’nan West Sewage Treatment Plant was studied.The heavy metals in the sludge were passivated by the synergistic action of fly ash and 2,4,6-mercapto-s-triazine trisodium salt(TMT),and the optimum passivation conditions were screened by orthogonal test.The fertilizer efficiency of immobilized sewage sludge was evaluated and its effects on the growth of Lolium perenne from ryegrass and the bioavailability were studied.The risk assessment of heavy metals in the passivated municipal sludge was performed using the improved BCR method,TCLP method and leaching experimenta,respectively.Finally,the passivation mechanism of the passivating agent was analyzed in order to provide basic data and scientific basis for heavy metals fixation and risk assessment of municipal sludge,and also to provide reference for the land use of municipal sludge and fly ash.The main research contents were as follows:(1)The sludge was rich in nutrients like organic matter,nitrogen,phosphorus and potassium.The contents of Cu,Ni,Pb and Zn in the sludge were higher than that in the backgroung soil.The fertilizer efficiency indicators and heavy metals contents were all accord with the standard for forestland and could used as forestland fertilizer.The sludge showed different inhibitory effect on different plants.When sludge ratio was 10%~20%,sludge could effectively promote the growth of L.perenne,but show severe inhibitory affect on Ch.speciosa and C.chinensis.(2)Fly ash and TMT showed differnet immobilizing effect on different heavy metals.The results of the orthogonal test showed that the addition amount of fly ash had the greatest impact on Ni,while the immobilization of Zn was most affected by contents of TMT and Cu was most most affected by the immobilizating time.The synergistic effect was found to decrease the bioavailability and the ecotoxicity of heavy metals in sludge.(3)The applied amount of passivated sludge reached 20%,the oil showed the improved physicochemical propertie.And the inhibitory effect on the growth of L.perenne was significantly reduced.The planting of L.perenne affected the existing forms of heavy metals and their order was Zn>Ni>Pb>Cu.(4)The passivated sludge showed slow leaching rates of N and P,while the leaching rate of K was increased.When the amount of passivated sludge was 20%,the concentration of TN in the leachate would reach 38.78mg/L,beyond the Class V limit of the “Standard for groundwater quality”(GB/T 14848-2017,China),which showed a risk of groundwater contamination,But the concentrations of Cu,Ni,Pb,Zn were not exceeded the Category V limits,the would not pose a risk of groundwater contamination.The migration potential of the four heavy metals was Zn>Ni>Cu>Pb,which was consistent with the conclusions drawn from the TCLP leaching test,which showed that Zn had the greatest risk of contamination.(5)The passivation mechanism analysis showed that the passivation effect of fly ash was mainly ion exchange and raising the pH to convert heavy metals into insoluble salts.The addition of TMT could make the adhesion of fly ash and sludge closer together,which facilitated the ion exchange.New substances were formed in the passivated sludge,increasing the bufferiong performance of the sludge.