Mechanochemical degradation of per- and polyfluoroalkyl substances in soil using an industrial-scale horizontal ball mill with comparisons of key operational metrics

Nicholas Battye; David Patch; Iris Koch; Ryan Monteith; Dylan Roberts; Natalia O'Connor; Bernard Kueper; Michael Hulley; Kela Weber

doi:10.1016/j.scitotenv.2024.172274

Mechanochemical degradation of per- and polyfluoroalkyl substances in soil using an industrial-scale horizontal ball mill with comparisons of key operational metrics

Sci Total Environ. 2024 Jun 10:928:172274. doi: 10.1016/j.scitotenv.2024.172274. Epub 2024 Apr 9.

Authors

Nicholas Battye¹, David Patch¹, Iris Koch¹, Ryan Monteith², Dylan Roberts¹, Natalia O'Connor¹, Bernard Kueper³, Michael Hulley⁴, Kela Weber⁵

Affiliations

¹ Environmental Sciences Group, Department of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, ON, Canada.
² SGS Canada Inc., Lakefield, ON, Canada.
³ Department of Civil Engineering, Queen's University, Kingston, ON, Canada.
⁴ Environmental Sciences Group, Department of Civil Engineering, Royal Military College of Canada, Kingston, ON, Canada.
⁵ Environmental Sciences Group, Department of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, ON, Canada. Electronic address: Kela.Weber@rmc.ca.

PMID: 38604365
DOI: 10.1016/j.scitotenv.2024.172274

Abstract

Horizontal ball mills (HBMs) have been proven capable of remediating per- and polyfluoroalkyl substances (PFAS) in soil. Industrial-sized HBMs, which could easily be transported to impacted locations for on-site, ex-situ remediation, are readily available. This study examined PFAS degradation using an industrial-scale, 267 L cylinder HBM. This is the typical scale used in the industry before field application. Near-complete destruction of 6:2 fluorotelomer sulfonate (6:2 FTS), as well as the non-target PFAS in a modern fluorotelomer-based aqueous film forming foam (AFFF), was achieved when spiked onto nepheline syenite sand (NSS) and using potassium hydroxide (KOH) as a co-milling reagent. Perfluorooctanesulfonate (PFOS) showed much better and more consistent results with scale-up regardless of KOH. Perfluorooctanoate (PFOA) was examined for the first time using a HBM and behaved similarly to PFOS. Highly challenging field soils from a former firefighting training area (FFTA) were purposefully used to test the limits of the HBM. To quantify the effectiveness, free fluoride analysis was used; changes between unmilled and milled soil were measured up to 7.8 mg/kg, which is the equivalent of 12 mg/kg PFOS. Notably, this does not factor in insoluble fluoride complexes that may form in milled soils, so the actual amount of PFAS destroyed may be higher. Soil health, evaluated through the assessment of key microbial and associated plant health parameters, was not significantly affected as a result of milling, although it was characterized as poor to begin with. Leachability reached 100 % in milled soil with KOH, but already ranged from 81 to 96 % in unmilled soil. A limited assessment of the hazards associated with the inhalation of PFAS-impacted dust from ball-milling, as well as the cross-contamination potential to the environment, showed that the risk was low in both cases; however, precautions should always be taken.

Keywords: Aqueous film forming foam; Firefighting training area; Horizontal ball mill; Mechanochemical destruction; Per- and polyfluoroalkyl substances.