The United States Patent and Trademark Office has awarded TRS Group, Inc. (TRS) a patent for the thermal treatment of per‐and polyfluoroalkyl substances (PFAS) in soil (Oberle, D and E. Crownover, 2020). The patented process uses heaters to raise soil temperatures to desorb and volatilize PFAS. Importantly, the target temperatures are high enough to degrade soil organic matter but still low enough to mitigate the risk of forming potentially dangerous PFAS by‐products. As the soil organic matter is destroyed, a vacuum is applied to the soil to remove PFAS by volatilization. The vaporized PFAS is routed to vapor treatment for further processing.
How the TRS Patent Differs from Other Patents
The TRS patent is the only soil heating patent that focuses specifically on thermal treatment of PFAS. Other thermal vendors may claim to have a “patented PFAS treatment;” however, the reference is to patented heating equipment or processes, not the treatment of PFAS.
PFAS Adsorb Strongly to Soil
PFAS include a wide class of anthropogenic fluorinated chemicals. Due to their special surface tension properties, they have been used by multiple industries for many products. Depending on the structure, PFAS can be ionic, non‐ionic or zwitterionic (Backe et al., 2013). Aqueous film‐forming foams (AFFFs) are a common source of PFAS contamination around the world (Houtz, 2013). The AFFFs typically consist of perfluoroalkyl acids (PFAAs), which have a hydrophobic fluorocarbon tail and a hydrophilic head, most often comprised of a carboxylate or sulfonate functional group. The carbon backbone allows PFAS to adhere to soil by van der Waals forces, due to the organics in the soil (Ferrey et al., 2012). Also, PFAS may interact with soil by ionic charge (Lixia et al., 2014). Adsorption of PFAS can be quite strong. For example, tests have shown that solvent flushing with alcohol was unable to break some of the adsorptive bonds of PFAS to carbon (Ziwen et al., 2014).
TRS’s Patented Process Breaks Adsorptive Bonds and Volatilizes PFAS from Soil
It has been demonstrated that low temperature thermal desorption can remove greater than 99.99 percent of PFAS from soil when performed at a temperature that degrades the organic material in the soil (Crownover et al., 2019). Soil organic matter is efficiently degraded at temperatures in the range of 360⁰C to 450⁰C (Nelson & Sommers, 1996). Greater than 99.99 percent removal of PFAS was achieved by heating and holding soil at a temperature of 400⁰C. The removal of organics from the soil is quite apparent when viewing photographs of the soil before and after treatment (See Figure 1). Figure 1: Soil Before and After Organic Material is Removed by Thermal Treatment
The TRS Process Minimizes the Potential for PFAS Thermal Breakdown Products
There has been some concern that heating PFAS might create toxic breakdown products (USEPA, 2020). The carbon to fluoride chemical bond is the strongest bond in organic chemistry and requires very high temperatures to break (Hurst, 2019). Studies have shown that temperatures in excess of 400⁰C are typically required to cause breakdown of perfluorinated substances (Madorsky et al., 1953; Marhevka, 1982; Muhammad, 2017). The patented TRS process generally operates in the temperature range of 350 to 400⁰C, which is below the temperatures where breakdown products would be anticipated. Therefore, TRS’s patented PFAS process is much safer than other thermal processes, such as rotary kilns or oven‐like chambers, that operate at higher temperatures.
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