| With the rapid development of industry and urbanization, soil contaminated with heavy metals has become increasingly serious. It is urgent to deal with and remedy the heavy metal contaminated soils. In Europe and America, soil washing remediation technology is becoming matures, while it is just on beginning in China. Calcisols contaminated with Cd and Pb was taken from Gansu Province and the Na2EDTA washing remediation technique was investigated. The main results are as follows:(1) The heavy metal contaminated soil was washed with0.1mol/L inorganic acid (H2SO4, HNO3, HCl),0.1mol/L organic acids (oxalic acid, acetic acid, citric acid) and0.02mol/L chelators (Na2EDTA, Na3NTA, Na5DTPA), respectively. The results showed that organic and inorganic acids had a better Cd removal effectiveness than Pb. Among the three inorganic acid, H2SO4removed more Cd than HNO3and HCl.81.9%and71.0%of DTPA-extractable Cd and total Cd were removed by H2SO4washing. Among the three organic acids,0.1mol/L oxalic removed more Cd and Pb than citrate acid and acetic acid. The removal percentages of DTPA-extractable Cd, total Cd, DTPA-extractable Pb and total Pb reached up to76.8%,45.2%,9.2%,56.6%, respectively. The removal effectiveness of Cd and Pb by chelators (Na2EDTA, Na3NTA, Na5DTPA) washing were higher than that by organic and inorganic acids washing. Among the three chelators, Na2EDTA removed more Cd and Pb than Na3NTA and Na5DTPA. The removal of DTPA-extractable Cd, total Cd, DTPA-extractable Pb and total Pb were73.8%,60.7%,72.8%,55.8%, respectively. Therefore, Na2EDTA was the best washing agent.(2) The optimal conditions for washing Cd and Pb from the soil were as follows:Na2EDTA used as washing agent; the concentration and solution to soil ratio were0.04mol/L and8:1, respectively; the pH value of washing solution was4-5.(3) Na2S+Ca(OH)2and FeCl3+NaOH were employed to remove Cd and Pb in leachate and then recover Na2EDTA solution. The results showed that more than99%of Cd and Pb were removed by Na2S+Ca(OH)2in the leacheate, while FeCl3+NaOH was only effective to Pb and the removal of Cd in the leacheate was only56.7%.(4) The recovered Na2EDTA was used to washing three batches of soils for four cycles. The removal of DTPA-extractable Cd and Pb were80.1%and69.3%in the first batch washing. The removal efficiency had a slightly decrease comparing to washing with fresh Na2EDTA (the removal of DTPA-extractable Cd and Pb by washing with fresh Na2EDTA were81.3%and79.0%, respectively). However, in the second and third batch washing process, the removal of DTPA-extractable Cd decreased by26.2%and20.2%, while the removal of DTPA-extractable Pb decreased by33%and41%, respectively. Although a slightly decrease for DTPA-extractable Cd and Pb was observed, the total cost for recycled Na2EDTA washing decreased by70.6%as compared with fresh Na2EDTA washing.(5) Six ameliorants including gypsum, ammonium sulfate, monocalcium phosphate, diammonium hydrogen phosphate, ammonium acetate and H2O were chosen to remove the water soluble and exchangeable sodium in the washed soils. The results showed that ammonium salts were more efficient to decrease Na+concentration than calcium salt.(NH4)2HPO4obtained the highest Na+removal, but its price was high. Therefore,(NH4)2SO4was the most suitable amendment reagent and the optimal concentration of (NH4)2SO4was0.08mol/L at soil to solution ratio of1:3. The removals of water soluble and exchangeable sodium in the optimal condition were92.4%and91.1%, respectively.(6) When the contamined soil was washed with Na2EDTA followed by (NH4)2SO4amendment, soil pH was restored to7.8. The contents of available potassium decreased. The content of organic matter was1.6mg/kg and it still maintained a high level. The content of available P increased from low to medium lever. The contents of available nitrogen and total nitrogen increased significantly and the values were13.8g/kg and1069mg/kg, respectively. Therefore, the treated soil is benefits for plant growth and soil basic function restoration. |
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