Remediation of PFAS-Contaminated Soil and Granular Activated Carbon by Smoldering Combustion

Abstract

This study explored smoldering combustion for remediating polyfluoroalkyl substance (PFAS)-impacted granular activated carbon (GAC) and PFAS-contaminated soil. GAC, both fresh and PFAS-loaded, was employed as the supplemental fuel supporting smoldering in mixtures with sand (≈175 mg PFAS/kg GAC-sand), with PFAS-spiked, laboratory-constructed soil (≈4 mg PFAS/kg soil), and with a PFAS-impacted field soil (≈0.2 mg PFAS/kg soil). The fate of PFAS and fluorine was quantified with soil and emission analyses, including targeted PFAS and suspect screening as well as hydrogen fluoride and total fluorine. Results demonstrated that exceeding 35 g GAC/kg soil resulted in self-sustained smoldering with temperatures exceeding 900 °C. Post-treatment PFAS concentrations of the treated soil were near (2 experiments) or below (7 experiments) detection limits (0.0004 mg/kg). Further, 44% of the initial PFAS on GAC underwent full destruction, compared to 16% of the PFAS on soil. Less than 1% of the initial PFAS contamination on GAC or soil was emitted as PFAS in the quantifiable analytical suite. Results suggest that the rest were emitted as altered, shorter-chain PFAS and volatile fluorinated compounds, which were scrubbed effectively with GAC. Total organic fluorine analysis proved useful for PFAS-loaded GAC in sand; however, analyzing soils suffered from interference from non-PFAS. Overall, this study demonstrated that smoldering has significant potential as an effective remediation technique for PFAS-impacted soils and PFAS-laden GAC.

Heat losses in a smouldering system: The key role of non-uniform air flux

“…Smouldering Front Results' errors encompass a conservative estimate of experimental variability as they represent the normalized standard deviations from three DRUM and LAB repeat experiments from smouldering wastewater sewage sludge, a highly variable fuel [30]. These errors align well with similar experimental studies [30,54,65,80,82]. 1 At standard temperature and pressure (21.1°C at 1 atm).…”

“…1. Granular activated carbon (GAC, at least 90% particles larger than 80 mesh, DARCO® 12X40, Cabot Corp.) was used as a model fuel for this study because: 1) simple oxidation chemistry, 2) lack of pyrolysis reactions, 3) ease of mixing and experimental preparation, and 4) there is an emerging, practical interest in smouldering spent GAC from adsorption treatment [80]. GAC is similar to the charcoal used as a model fuel in [55], which reported the same benefits and simplified chemistry that was followed here.…”

“…Table 1 presents the experimental conditions, illustrating that the variables adjusted were column diameter, fuel concentration, and applied air flux. The experimental conditions reflect both common values used in applied smouldering research (e.g., [30,54,65,80,82]) and application (e.g., [3,28,45]). The GAC was mechanically mixed with coarse grain sand (1.180 ≤ mean grain diameter ≤ 2.000 mm, porosity (D) = 0.37, bulk density to desired fuel concentrations (example photo in the Supplementary Materials, Fig.…”

An Overview of the Formation of PFOA and PFOS in Drinking-Water and Wastewater Treatment Processes

“…Spent or exhausted GAC can be thermally reactivated or regenerated, where the carbon is heated with inert gases (e.g., N 2 ), CO 2 , or steam (Marsh and Rodríguez-Reinoso 2006). Heating PFAS-laden GAC at high temperatures (≥ 500 o C) is highly effective for decomposition of PFAS, including PFOA and PFOS (Duchesne et al 2020;Sasi et al 2021;Watanabe et al 2018;Xiao et al 2020). However, at low temperature conditions (< 400 o C), PFAS can transform to shorter-chained homologues or other PFAS species (Sasi et al 2021;Xiao et al 2021).…”

Challenges in the PFAS Remediation of Soil and Landfill Leachate: A Review

“…Alkaline soils may promote the leaching of PFAS from the soil since the sorption of PFAS to the soil has been shown to decrease with an increase in the pH of the leachate. This is due to a reduction in the protonation of the adsorbent surfaces and the associated reduction in the frequencies of positively-charged sites on the sorbent [54][55][56]. Thus, landfilling of PFAS-contaminated soil together with alkaline materials should be avoided.…”