Speciation Analysis And Stabilizing Treatment Process Of Lead In Lead-Contaminated Soils

The research on phosphate remediation of heavy-metal-polluted soils was begun about 20 years ago, and a lot of great progress has been achieved. However, the cost is too high to stabilize heavy metals in soil, therefore, this method could not be widely spread in the practical application of soil remediation. In addition, there are only a few species of heavy metal stabilizers for soil remediation at the present, and the remediation effect needs to be improved. Hencethen, it is particularly important to select a economical and efficient stabilizer.In this paper, phosphate(sodium dihydrogen phosphate, disodium hydrogen phosphate and trisodium phosphate) and organic acids(humic acid, humic acid potassium and phytic acid) are applied to stabilize lead-contaminated soils, BCR sequential extraction method is used for morphological analysis of Pb in soils. The influences of the aforementioned treatment agent on the speciation distribution, leaching concentration of lead in soils, and the pH value of soils are explored. Meanwhile, X-ray diffraction(XRD) and infrared spectroscopy are applied for the exploration of the interaction between stabilizers and lead in soils. The stabilization mechanism on the heavy metal ions of different stabilizers is illustrated. The most efficient agents in phosphate and organic acids, namely sodium dihydrogen phosphate and phytic acid, are used to seek the optimum process ratio between the two agents by conduct orthogonal experiment. The results are as below:(1)The leaching concentration of Pb in soils is significantly decreased with addition of sodium dihydrogen phosphate, disodium hydrogen phosphate and trisodium phosphate respectively, the leaching concentration tends to decrease gradually with the treatment time increasing. According to BCR speciation analysis, the use of these three phosphates is promoted the transformation of Pb speciation in soils from effective state to residual state, and significantly reduced the Pb bio-availability in contaminated soils. Not only lead phosphate precipitate, but also a chloride phosphate minerals is generated according to XRD analysis.(2)The leaching concentration of Pb in soils is significantly decreased with addition of humic acid, humic acid potassium and phytic acid, and gradually decreased with the treatment time increasing. According to BCR speciation analysis, in contaminated soils, Pb content in acid-extractable state reduces greatly, and Pb in residual state increases significantly. The infrared spectroscopy shows that carboxy of humic acid and humic acid potassium, while P-O-CH3 phytic acid affects Pb in soils.(3)through the comparison of the stabilizing efficiency among phosphates and organic acid, it is concluded that the most efficient phosphate is Sodium dihydrogen phosphate, and for organic acid, is phytic acid.(4)Orthogonal experiment is used to study the effect of inorganic-organic combined chemicals to stabilize Pb-contaminated soils, and the optimum ratio of inorganic-organic combined chemicals as follows: the amount of sodium dihydrogen phosphate is 5%, the amount of phytic acid is 2%; and treatment time is 9 days. Under the above optimum ratio, the Pb leaching concentration is 0.0596mg/L,which meets the maximum concentration limit requirement(0.25 mg/L) of Landfill Pollution Control Standard.