Soil samples contaminated with heavy metals such as lead (Pb), copper (Cu) and zinc (Zn) were leached with citric acid solution. A five step-sequential extraction method involving "exchangeable", "bound to carbonate", "bound to FeMn oxide", "bound to organic matter" and "residue" fractions was also carried out to investigate the leaching behavior of Pb, Cu and Zn with citric acid.The leaching efficiencies of Pb, Cu and Zn increased with increasing citric acid concentration. About 86.5, 88.9 and 83.3% leaching efficiencies were obtained for Cu, Zn and Pb, respectively, under the following leaching condition: 2 kmol·m ¹3 in citric acid concentration, 120 min in leaching time, 10% in pulp density, 50°C in temperature, and 80 rpm in shaking speed. In the sequential extraction test, the heavy metal contaminants were extracted more in "bound to carbonate" and "bound to FeMn oxide" fractions than in "bound to organic matter" and "residue" fractions. When the leaching efficiencies of Pb, Cu and Zn with 2 kmol·m ¹3 citric acid solution were compared with the results of the sequential extraction, a good agreement with the summation of amount leached in "exchangeable", "bound to carbonate" and "bound to FeMn oxide" fractions was noted.

The effect of pH on the citrate leaching behavior of heavy metal ion was investigated to develop an eco-friendly process for removing heavy metals from soil contaminated with copper, zinc, and lead. The leaching tests were performed using citrate solution with pH adjusted by mixing citric acid and sodium citrate under the following leaching conditions: particle size, under 75 µm; temperature, 50 o C; citrate concentration, 1 kmol/m 3 ; pulp density, 5%; shaking speed, 100 rpm; leaching time, 1 hour. The difference of pH before and after the leaching test was not observed, and this result indicates the direct effect of hydrogen ion concentration on the leaching of metals was insignificant. The removal ratios of copper, zinc, and lead from the contaminated soil decreased with increasing pH. The thermodynamic calculation suggests that the leaching behaviors of metal ions were determined by two reactions; one is the reaction to form complex ions between heavy metal ions and citrate ion species, and the other is the reaction to form metal hydroxide between heavy metal ions and hydroxide ion.

No abstract