Immobilization Of Lead And Cadmium In Contaminated Soil Using Phosphate,Bentonite And Calcium Hydroxide

Soil heavy metal pollution treatment especially lead(Pb)and cadmium(Cd)had become an urgent and difficult task.In our study,Chemical immobilization experiment for Pb-Cd combined pollution was conducted by using phosphate,bentonite and calcium hydroxide.We changed the fixitives adding mode on the basis of previous studies,Design-Expert software was used in the experiment to maximize the materials performance.Firstly,in order to accurately characterize the immobilization efficiency of the fixitives,an optimal leaching method was selected commonly used leaching methods(0.1 M CaCl2?1 M NH4OAc?0.005 M DTPA?0.05 M EDTA-2Na and TCLP)of bio-available heavy metals through an experiment.The correlation and regression analysis between every leaching method extractable Pb,Cd concentration and uptake of Pb,Cd content by wheat grass were conducted.The results showed that DTPA extractable Pb,Cd concentration was the most appropriate bio-available fixative.So DTPA was selected as the characterization methods of soil heavy metals bio-availability in following studies.Secondly,doses of fixatives were studied.The ratio of phosphate,bentonite,calcium hydroxide was set in 3:1:1 according to their contribution during the immobilization process.Soil physical and chemical properties,XRD analysis,DTPA extractable Pb and Cd concentration,uptake of Pb and Cd content by wheat grass,sequential extraction procedure and soil enzyme activities were studied before and after adding fixatives in the proportion of 0,0.83%,1.66%,5.00% and 8.33%(w/w).The results implied that dose of fomula fixative at 2% could meet the experiment requirements.Lastly,method of mixture Design in Design-Expert software was used to explore the influence of different fixatives ratios on Pb,Cd immobilization rate and soil pH?Meanwhile,the optimal formula was obtained through the software.The software results indicated that the optimal formula was 75.6% phosphate and 24.4% bentonite,in which Pb,Cd immobilization rate were 72.98 and 61.31%.The verified experiment demonstrated that Pb,Cd immobilization rate were 65.34% and 54.73% under the optimal formula,respectively.