Calcination Remediation Of Soils Contaminated With Heavy Metals

With the urbanization process in China, the urban planning layout has greatly changed. In order to meet the needs of urban development, a large number of chemical plants, refineries and many other key polluted enterprises in the urban centers moved to the suburbs and other remote areas, remaining many compound polluted sites contaminated by heavy metals and many other pollutants. These sites will be reconstructed for residential, commercial and so on. However, heavy metals, polycyclic aromatic hydrocarbons and many other pollutants will stay for a long time, which may cause great harm to human health and the environment.As a physical remediation, heat treatment possesses simple operation technology and high process efficiency. Circle period of physical remediation is so short that secondary contamination can be avoided effectively. This kind of remediation is appropriate for pollution sites needing short remediation time and where environmental emergency happens. After heat treatment of heavy metals and PAHs in the sites, good effect can be obtained. Nowadays, as contamination sites are more and more, heat treatment is a site remediation technology which is deserved to study and promote.In this paper, we chose zinc and copper as the typical heavy metal pollutants, and naphthalene and fluoranthene as the typical polycyclic aromatic hydrocarbon pollutants. The treatment effect of calcination remediation to the soil contaminated by heavy metal pollution and heavy metal-PAHs combined pollution, and its impact factors were studied by experiments. And proper substance was searched as stabilizer on this basis, so that we can study the synergism to the effect of calcination remediation by adding stabilizer. Zinc, copper, naphthalene and fluoranthene are representative as they are common pollutants in the contaminated sites in China. The main research results are as follows:1. The law and impact factor of different calcination ways in dealing with heavy metal contaminated soil were studied by batch experiments. Results indicate that temperature and time have effects on calcination efficiency. Initial concentrations of bioavailable Zn and Cu in soil samples were 995.73 and 769.92 mg/kg, respectively. After calcinating at 300℃ for 4 h, concentration of bioavailable Zn decreased to 398.07 mg/kg, and concentration of bioavailable Cu decreased to 117.08 mg/kg, the corresponding fixed rates were just 60.02% and 84.79%, respectively. During the period of temperature raising to 600 ℃ concentrations of bioavailable Zn and Cu were decreasing gradually, till 8.75 mg/kg and 55.75 mg/kg respectively, far below the grade A standard limit (Zn 200 mg/kg and Cu 63 mg/kg) of soil environmental quality assessment standards (HJ 350-2007) for exhibition land. The treatment efficiency was not obvious if temperature increased to 700℃ continually. When the temperature was constant, concentration varitional rule of bioavailable Zn and Cu was uncertain. At low temperatures, residual concentrations of Zn and Cu in soils were relatively high after calcinated for 2 h, and low when calcinated for 4 h. If the calcinating temperature continued rising to 8 h, there was no obvious effect. Under the condition of high temperature, there were no significant changes to the concentrations of bioavailable Zn and Cu as the calcinating time prolonged.2. Some stabilizers which can stabilize heavy metals were chosen, substance which has the most synergistic effect when calcinating the Zn and Cu contaminated soil were seeking at the same temperature conditions. Calcination temperature was set at 400℃, different substances were added and sufficiently mixed for 24 h, calcinating 0.5,1,2,4 and 8 h respectively. The study found that when bone meal and quicklime were added into the soil to calcinate, fixed rate of bioavailable Zn and Cu in soils were higher than adding other substance, reaching 94.6%. When the soil was calcinated for 1 h, fixed rate was 99.9%. So bone meal and quicklime mixture was chosen as the best stabilizer. Under the condition of adding a certain amount bone meal and quicklime, contaminated soil was calcinated for 0.5, 1,2,4 and 8 h at different temperatures of 100℃,200℃,400℃,500℃ and 600℃. We found that when the soil was calcinated for 0.5 h at 100-200℃, the fixed rates of bioavailable Zn and Cu in soils reached 99.82% and 97.91%, respectively. When the temperature was set to 400℃, bone meal and quicklime mixtures with the proportions of 1%,3%,5%,10% and 15% of soil mass were added to the soils as stabilizer. Results indicated that within a certain range, the more the stabilizer doses were adding, the higher the fixed rates of Zn and Cu in the calcinated soils, and the fixed rate of Zn increased faster than that of Cu.3. In this section, processing discipline of heavy metals and PAHs contaminated soil by calcinating technology was studied. The results indicated that the calcinating effect was affected much by the temperature. Concentrations of bioavailable Zn and Cu in soils were decreased gradually when temperature was 300-600 ℃, and concentration of Zn decreased faster than that of Cu. When temperature was 600-700℃, concentrations of bioavailable Zn and Cu were constant, indicating that treatment effect was no markedly increasing. Calcinate temperatures had significant effect on the removal of naphthalene and fluoranthene from soil. The naphthalene and fluoranthene reduced to the minimum concentration at 500℃ and 700℃, respectively. Comparing heavy metal pollution and heavy metal-PAHs combined pollution, in the same calcinate codition, the fixed rates of zinc in heavy metal contaminated soils were greater than that in heavy metal-PAHs combined pollution contaminated soils, nevertheless, the fixed rate of copper in heavy metal contaminated soils were less than that in heavy metal-PAHs combined pollution contaminated soils. The results indicated that PAHs can promote the stabilization of zinc; however they inhibit the stabilization of copper in soil.