Study On The Feasibility Of Three Solutions For Removing Cd, Pb, Zn, Cu From A Soil And The Optimization Of The Technical Conditions

The effects of three washing solutions, HCl, citric acid and Na2-EDTA, for removing Cd, Pb Zn and Cu from a Pb-Zn-Cd polluted soil colleted from a field near a smelter were studied in order to select a best washing solution. The extractibility of the washing solutions at each washing was compared to set a suitable concentration and washing number for each washing solution. The soil was successively washed for 5 times with each solution of which the concentrations were 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16 mol.L-1, respectively. The optimal conditions for the selected washing solution and the treatment of the leachate solution were also studied. The main results were as follows:The suitable concentrations, washing number and removal efficiencies for removing Cd, Pb and Zn from this soil by the solutions were: HCl, 0.1mol.L-1, twice; citric acid, 0.08mol.L-1, twice; Na2-EDTA, 0.02mol.L-1. Na2-EDTA solution was more effective than the other two solutions in removing Pb but it is the most expensive and its residues in soils are difficult to be degraded so it is not the most suitable washing solution. As compared with HCl, citric acid solution was more effective in removing Cd while less effective in removing Pb and Zn. For the more, Citric acid is more expensive than HCl. HCl had higher extractibility in removing Zn and lower cost than citric acid. Although washing with HCl solution will seriously destroy soil structures and remove other necessary nutrients, the adverse effects can be reclaimed by common soil improvement practices. Therefore, 0.1mol.L-1 HCl with two washings was the best for removing Cd, Pb, Zn and Cu from the soil.The suitable technical conditions for washing Cd, Pb, Zn and Cu with 0.1 mol.L-1 HCl from the soil were as follows: solution/soil ratio=2.5:1; shake speed=210r.min-1; reaction time=2h; washing number=twice.With the optimal washing conditons, the removal rate of Pb, Zn, Cd and Cu by 0.1 mol.L-1 HCl were significantly raised to 27.23%, 19.61%, 59.61% and 19.73%, respectively.In comparison with Na2S, CaO was found to be more economical and easy to operation in removing metal ions from the acidic leachate solution. When Na2S was added to the acidic solution, H2S gas which is difficult to protected produced. When CaO solution is used to treat the acidic leachate solution, it is enough to adjust the solution to pH 7-9. After the treatment with CaO, the leachate solution can be discharged directly or reused after acidification.The clean soil can be reused as agricultural soil while the precipitates including heavy metals and some fine particles can be landfilled.