Formation and transport of PCDD/Fs in the packed bed of soil containing organic chloride during a thermal remediation process.

The authors previously proposed the concept of a new thermal remediation process for particulate/powder materials contaminated by polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and experimentally verified its validity on the basis of process efficiency. However, contaminees such as soils and fly ashes from waste incinerators often contain a considerable amount of other chlorides, which may act as a main source of chlorine in the formation of PCDD/Fs via thermal processes. The present study aims to examine the formation and transport of PCDD/Fs in the packed bed of soil containing a chloride during the process. Polyvinyl chloride (PVC) polymer was mixed with soil sample as an organic chloride model. A laboratory-scale apparatus was employed as a process simulator. Further, a technique to quench the process was applied to observe the concentration distribution of PCDD/Fs in the solid bed in the vertical direction. The result shows that the PCDFs tend to form dominantly in the high temperature (calcination and/or combustion) zone and are successively trapped in the low temperature (wet) zone. Especially, TeCDF is the most dominant homologue contained in the wet zone and outlet gas. Although PCDD/Fs are once trapped at the wet zone, the concentration of the remediated materials gives fairly low value (1.9 pg/g-dry, 0.04 pg-TEQ/g-dry). It signifies that organic chlorides mingled in the solid contaminee not affect the removal efficiency of PCDD/Fs in the process. Nevertheless, attention should be paid to the potential emission of PCDD/Fs in the outlet gas due to the presence of organic chloride in the soil.

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 13C-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.