A Study Of Chemical Stabilization Of Heavy Metals

The demand of treatment and disposal solution of municipal solid waste incineration（MSWI）fly ash has been increasing,as inceneration treatment of MSW has been implemented more and more during last several years in China.At the same time,as treatment technology has been keeping updating,the standard of fly ash leachability detection before landfill is more strict than before.The current standard of MSWI fly ash leaching method in China is HJ/T 300-2007,which using acetic acid solution.Many agents used for fly ash stablization in the past studies can not reach the needs any longer,as the products after stablization is not stable enough under this acid environment.Thus reaching on new agents whether synthelized or multi-mixed is significant.Heavy metal containing waste water comes from metallurgy and electroplating industry,etc.Sediment of heavy metals with chemical agents added is the main method to purify heavy metal containing waste water.However,this method is not very efficient when heavy metal ions are at low concentration with the existence of complexing form.Very expensive technology such as reverse osimosis is often needed to solve this problem.But segment with chelating agents has also been studying in recent years,which is much cheaper.This study is mainly foucused on reducing leaching concentration of Pb,Zn and Cd in MSWI fly ash,which often been over standard limits,with chemical agents stablization method.We also tried to use the chelating agents in advanced treatment of heavy meatal containing waste water.The main conclusions are:（1）At first MSWI fly ash is collected from a MSW plant at different time in Zhongshan,Guangdong,China.The detecting of the samples showed that the leaching concentration is not stable at different time,which correspond with some related reports.especially Zn,which correspond with the record of the plant.Several agents have been tested,and the result showed that diethyldithiocarbamate has excellent ability on Pb and Cd stablization,but not Zn.Thiosemicarbazide showed well efficiency on Zn stablization but it is expensive and hard to dissolve in water.（2）Further study was carried on to get a multi mixed agents recipe in order to stablize znic in fly ash.The result showed that with a dosage lower than 2% of diethyldithiocarbamate and 4% of phosphoric can reduce the leaching concentration of Pb and Cd to a level much lower than the standard limits,and Zn was also reduced to be lower than the limits.（3）We also tried to use diethyldithiocarbamate and TMT-55 to remove heavy metals in waste wate.Firstly simulated water was prepared including an inorganic waste water sample and an organic waste water（with EDTA-2Na added）sample.And then tested how much these two agents could reduce the concentration of heavy metals.The results showed that the inorganic waste water is much easier to be purified than the organic one.Cr³⁺,Zn²⁺and Cr³⁺could be reduced a lot,and Cu²⁺could be reduced much lower than the standrad limits at 0.001 mg/L,but Ni was not apparently reduced.The electroplating waste water samples were gotten from an electroplating plant in Zhongshan,Guangdong,China,and heavy metals concentration was detected.The detection result showed that Cr³⁺,Cr³⁺and Zn²⁺have already been reduced low enough and do not need further treatment,but Cu and Ni were still over the level of standard limits.Not like that Cu²⁺in the simulated waste water that could be removed with diethyldithiocarbamate,Cu²⁺in electroplating waste water can not be reduced as much as expected.Because Cu²⁺in this sample could be Cu⁺and complexed with CN^-,whose complexed chemical with Cu⁺is much more stable.