Study Of Remediation Of DDTs Contaminated Soils By Thermal Desorption Technology

The environmental risk of contaminated sites caused by POPs which was the potential sources of environmental pollution was tremendous. However, our country hadn't attached importance to this problem. DDT is one of the first control POPs mentioned in Stockholm Convention. The large scale contaminated sites formed due to the production of DDT pesticides in the past. The high concentration DDTs should be removed as soon as possible, to prevent the hazards caused by them. Thermal desorption technology was adopted in this study. And this paper mainly researched on the factors, such as device parameter, soil property and pollutant property and so forth, which may had significant effects on the DDTs desorption efficiency.The main device parameters were temperature and residual time. Black and brown earths were used as soil samples in this study. They were both mixed with DDT pesticide known as DDTs, including p,p′-DDT, o,p′-DDT, p,p′-DDD and p,p′-DDE. Under six gradually increase levels of residue times, 5, 10, 20, 30, 40, and 50min, respectively. Thermal desorption technology was adopted to study the removal rates, residual concentrations of DDT (p,p′-DDT), and its homologues in soils at 300℃. The results showed that The removal rates of DDTs increased sharply When the temperature was above 300℃. The removal rate (more than 97%) of DDTs under temperature of 300℃and 40 min was close to those under temperature of 400℃and 500℃with the residue time of 10 min. As for the residual concentrations of DDTs in the soil, the main pollutant, p,p′-DDT decreased significantly when the temperature was above 200℃and mainly transferred to p,p′-DDE. The concentrations of p,p′-DDE increased sharply when the temperature was between 200℃and 300℃. Also, it was removed when the temperature was above 400℃.The optimum temperature and residue time were300℃40min or 400℃10min, respectively. The main soil properties included the organic matter content and particle sizes. Two different soils, black earth and brown earth, were chosen as test samples.They were both mixed with DDT pesticide known as DDTs, including p,p′-DDT, o,p′-DDT, p,p′-DDD and p,p′-DDE. Under six gradually increase levels of residue times, 5,10,20,30,40,and 50min, respectively, thermal desorption technology was adopted to study the removal rates, residual concentrations of DDT (p,p′-DDT), and its homologues in soils at 300℃. The results showed that the removal of p,p′-DDT can be significantly enhanced by organic matter(P<0.05). In addition, organic matter can remarkably influence the transformation and desorption of p,p′-DDE. By contrast, it has little impact on the removal of o,p′-DDT and p,p′-DDD. Moreover, there are significant differences in the desorption rates of DDTs in soils before the removal of organic matter.and after that. As to the particle sizes, the results suggested that the particle sizes had considerable impact on the DDTs removal rates, the larger soil particle sizes were, the more DDTs desorpted. And the results of One-way ANOVA test further indicated that desorption and transformation efficiency of p,p′-DDT and p,p′-DDE were largely hinged on soil particle sizes.DDTs concentration was studied as the pollutant property in this research. Two different soils, black earth and brown earth, were chosen as test samples. They were both mixed with DDT pesticides known as DDTs. Thermal desorption technology was adopted to study the removal rates and residual concentrations of DDTs (p,p′-DDT, o,p′-DDT, p,p′-DDD, p,p′-DDE) in the artificially contaminated soil, brown earth, with three pollutant levels , 300, 600, 800 mg/kg, respectively, and three kinds of DDTs contaminated soils, black earth, identified by different particle sizes. The results showed that there were no significant differences between the removal rates of DDTs in soils with different pollutant levels. Also, pollutant level influenced little on the desorption of the four DDT and its homologues.