Browsing by Subject "Transformation pathways"

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Thermal desorption transformation mechanisms and pathways promoted by metal oxides and organic matter in contaminated soils
(2020-05-08) Oden, Cameron Patrick; Katz, Lynn Ellen; Werth, Charles J.; Pope, Gary; Liljestrand, Howard; Saleh, Navid; Humphrey, Simon
Thermal desorption is a common soil remediation technology that can effectively treat soils contaminated with VOCs and SVOCs. However, due to the high remediation temperatures, contaminants can undergo transformation during treatment, but no studies have thoroughly investigated the transformation pathways and mechanisms. The goal of this work is to understand how soil components (minerals, organic matter, water) affect transformation under the conditions of thermal desorption, and whether those conditions are optimal for contaminant removal and destruction. First, pyrene was thermally treated in the presence of birnessite, magnetite, and hematite at temperatures lower than the typically employed 400 °C (150-250 °C for dry experiments and 75-150 °C for saturated experiments). Complete removal of pyrene was achieved by birnessite at 250 °C under dry conditions, and greater than 90% at 150 °C under saturated conditions. Pyrene was shown to degrade via reduction by hydrogenation under dry conditions and oxidative degradation caused by hydroxyl radicals in saturated experiments. Second, several substrates were employed to determine how soil organic matter affects removal: a humic substance, humic modified silica gel, magnetite in the presence of salicylic acid, and a natural soil. Greater than 90% removal was achieved by organic substrates at 400 °C. Hydroxyl radical formation was shown to be the dominant reaction mechanism at all temperatures, with and without water present. Unfortunately, several by-products were detected which were more toxic than pyrene, the most toxic produced at 500 °C. Lastly, thermal desorption of PFAS contaminated soil and hydrothermal regeneration of PFAS-laden GAC was investigated. Complete removal of PFOA was achieved at 400 °C by soil, birnessite, and magnetite, while greater than 90% removal was achieved for PFOS at the same temperature. Complete removal of both PFOA and PFOS was achieved by the GAC at 400 °C. Hydrothermal regeneration of GAC is a promising alternative, reducing treatment temperature compared to traditional methods from 700 to 400 °C. Extensive removal in both cases indicates that degradation of PFASs is occurring. The various components of this work have shown that transformation mechanisms are important to understand as they can lead to increased efficiency of thermal processes.