Behavior of dioxin during thermal remediation in the zone combustion process

In the previous study, a new process concept for the thermal remediation of particulate/powder materials contaminated by polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) has been verified. It leads to removal efficiencies of more than 99.9% in the soil contaminated by PCDD/Fs in terms of toxicity equivalent quantity (TEQ). However, details of the reactions and phenomena during the process, i.e., decomposition, vaporization, reformation and trap of PCDD/Fs and their relating compounds, have not sufficiently been clarified yet. The present study aims to examine experimentally the transport and fate of PCDD/Fs in the process. In the experiment, a laboratory-scale process simulator and a soil sample preliminary mixed with octachlorinated dibenzo-p-dioxin spiked by carbon-13 isotope (¹³C-OCDD) were used. The distribution of ¹³C-OCDD in the soil bed during the process was measured by applying a quench technique that rapidly cools-down the bed. Further, the total amount of ¹³C-OCDD discharged with outlet gas was measured. Using the obtained data, mass balance of 13^C-OCDD in the process was estimated. The results show that about 99% of ¹³C-OCDD preliminary admixed with the soil was decomposed rather than released to the outlet gas. Only a trace amount of ¹³C-OCDD remained in the treated soil. In addition, a very small amount of other congeners having the ¹³C-cycles was detected in the treated soil and outlet gas although its TEQ values are not significant. These were probably formed by dechlorination reactions occurring in the process.