Synthesis Of Dithiocarbamate Chelating Agents And Study On Stabilizing Effect Of Fly Ash

Domestic waste incineration fly ash contains a lot of heavy metals,improper handling will cause secondary pollution and endanger the environment and human health.Chemical agent stabilization treatment has attracted much attention due to its simple process,low energy consumption and small increase in capacity.Dithiocarbamates(DTCs)chelating agents have a strong coordination ability with heavy metals,and have great advantages in fly ash stabilization treatment.The most practical application is sodium dimethyl dithiocarbamate(SDD),The most practical application is sodium formaldehyde(SDD),but the problem of unstable chelating effect,heavy metals in the chelated products are re-dissolved or even exceed the standard.It is inferred that due to its small molecular weight,heavy metals are easy to migrate.Therefore,the synthesis of macromolecular heavy metal chelating agents with better chelating effect and stronger chelating product stability becomes the key to research.In this study,polyethyleneimine(PEI)with a molecular weight of 10000 was used as raw material to react with CS2 under alkaline conditions,through the single factor experiment and response surface analysis on synthesis conditions(reaction time,reaction temperature,CS2 drop rate,CS2 drop temperature,NaOH:PEI mole ratio,CS2:PEI mole ratio)were screened and optimized to obtain the best condition macromolecule chelating agent DTC.Compared the ability of SDD and DTC to remove heavy metals(Cd,Pb,Zn that are easily leached in fly ash),and further elaborated the chelating performance of DTC.The chelating agent was used to stabilize the fly ash,and the effects of chelating agent dosage,system water content,and stirring time on the chelating effect of fly ash heavy metals(Cu,Pb,Zn,Cd,Cr,Ni)were studied.Simulated landfill environment(ultraviolet radiation,exposure to air,35?,acid leaching agent)to analyze the stability of fly ash chelate products.Through heavy metal form classification researched and XRD,SEM characterization of fly ash before and after chelation to analyze the mechanism of fly ash stabilization.The results showed that the optimal synthesis conditions of DTC were reaction time 10 h,reaction temperature 70?,NaOH:PEI was 1.78(molar ratio),CS2:PEI was 0.8(molar ratio),and the molecular weight of DTC was more than 20000.Under the same dosage,DTC had a higher removal rate of heavy metals than SDD,and could remove heavy metals in a wide range of pH(3-12).The optimum conditions for fly ash chelation were 3%of the dosage,30%of the moisture content of the system,and the stirring time was no less than 2 min.Under different simulated environments,only Zn and Cr were leaching out of fly ash chelating products within 28 days.Ultraviolet irradiation and air contact were not conducive to the stability of Zn,and higher temperature was not conducive to the stability of Cr.But the leached concentrations of both met the GB16889-2008 standard,and the leaching concentration of heavy metals in DTC treated ash were lower.Under pH 2.0-2.7 leaching agent,the leaching concentration of Zn in the DTC-treated ash was reduced by 50%-60%compared with the raw ash,while the Zn leaching concentration in the SDD-treated ash was actually higher;a small amount of Cr was leached at pH 2.0-2.2 and the chelating treatment had no obvious effect on it.According to the research on the conversion mechanism of heavy metal forms,it was found that after SDD treatment,the stable form of Zn(oxidizable state and residue state)accounted for 11.2%of the raw ash,which is easier to be leached;Cr was mainly in the residue state,and the form of Cr changed little before and after chelation.After DTC treatment,the unstable forms of the six heavy metals decreased and the stable states increased,and the content of the stable states was higher than that of SDD.Therefore,DTC has a stronger ability to stabilize heavy metals.Compared with the commercial product sodium dimethyl dithiocarbamate,the DTC synthesized in this study has the advantages of stronger heavy metal chelating ability and more stable chelating products,so it has good application prospects and market value in the fly ash stabilization treatment.