Thermal conduction heating (TCH), as employed during in situ thermal remediation (ISTR), has been shown to effectively remove pentachlorophenol (PCP) and other chlorinated semi-volatiles from contaminated soil. However, much is still unknown about the preferred removal pathways during PCP thermal treatment. Identifying predominant removal pathways and developing accurate kinetic models could help provide better performance prediction and optimization of ISTR systems, thereby reducing energy costs and expanding technology application. The current research simulated ISTR treatment at the bench-scale and examined PCP removal and transformation across temperatures (120-440°C), under oxygen-rich conditions, and for both sea-washed sand and alluvial soil samples. Results demonstrate that for a 75 minute residence time, the temperature required to remove 99.6% of PCP in sea washed sand was 240°C. In contrast, it required 400°C to get comparable removal (99.7%) in the alluvial soil. In addition, quantification of chlorinated phenols in post-treated soil and off-gas samples indicate volatilization of PCP was the primary mechanism of removal (86.3 ± 13.9 of chlorinated phenols in the off-gas was PCP). However, dechlorination during ISTR occurred even at moderate temperatures and residence times (240°C for 50 minutes). In the post-treated soil, dechlorinated by-products made up 28.3 ± 7.3% of the total chlorinated phenols, the vast majority (22.1 ± 6.8%) of byproducts being 2,3,4,5-tetrachlorophenol, 2,3,4,6-tetrachlorophenol, 2,3,5,6-tetrachlorophenol, and 3,4,5-trichlorophenol. The results of this study suggest that PCP removal by volatilization and dechlorination occur in parallel, even at moderate temperatures. But while dechlorination enhances PCP removal, it also suggests the potential for formation of byproducts such as dioxin during ISTR treatment.

Pentachlorophenol (PCP) removal and dechlorination patterns during oxygen-rich thermal treatment

253rd National Meeting (2017)

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