Preliminary results on PFAS Destruction in Soil

Our specialists have been working alongside peers from public and private entities to find the right solution to the challenges presented by Per- and Polyfluoroalkyl Substances (PFAS).

Since 2018, our team, a team from Queen’s University in Kingston, Ontario and a team from the Royal Military College of Canada (RMCC) in Kingston, in collaboration with a global oil and gas corporation, have worked together to develop an innovative approach to destroying PFAS using ball milling. PFAS were first developed more than 50 years ago and became popular due to their resistance to oil, heat and water. Unfortunately, they were eventually found to have the potential for unintended environmental and human health impacts.

Treatment of PFAS is challenging primarily because of the recalcitrant nature of many of these compounds and their precursors, and the large variability in their physical and chemical properties. Despite significant efforts by the environmental industry in the last decade, sustainable and reliable remediation solutions for removal of PFAS from the environment remain inadequate.

PFAS treatment challenges

In particular, the absence of proven cleanup technologies for PFAS in soil and sediment limits treatment of source areas. This allows for continued migration of PFAS from solids to groundwater and surface water, which are particularly sensitive exposure pathways for these contaminants.

Exposure controls are often implemented to limit the risk to people and ecological receptors from PFAS-impacted media. These typically require limiting site use and instituting health and safety procedures (e.g., using personal protective equipment), but do not control PFAS leaching from soil into groundwater or surface water.

The engineering approach most often used to deal with PFAS source areas consists of excavation followed by either landfill disposal or storage in engineered cells on site. While still unregulated in many jurisdictions, disposal of PFAS-impacted soil by landfill has inherent risks because many facilities licensed to accept solid waste are often not equipped for proper management and treatment of PFAS in landfill leachate, landfill gas or gas condensate.

Other technologies, such as high temperature thermal treatment, are often cost prohibitive. Solidification and stabilization options may not adequately address leaching, requiring long-term maintenance or monitoring and, in some cases, can limit a site’s redevelopment potential. Options such as soil washing and thermal desorption rely on the transfer of PFAS from soil to another medium that will need further treatment.

A new approach

Recent results from lab testing demonstrate that ball milling can be an effective PFAS destruction technology for impacted soils and, best of all, avoids many of the limitations associated with current remedial approaches. The ball milling trials using aqueous film-forming foam (AFFF) impacted sand and clay soils from a firefighting training area demonstrated the viability of this approach. Fluoride was recovered using a fluoride ion selective electrode (ISE), providing evidence that defluorination is occurring. Greater quantities of fluoride were recovered when potassium hydroxide (KOH) was added as a co-milling reagent.

These studies also demonstrated that co-milling with KOH enhances destruction kinetics. For example, while milling without KOH requires approximately one hour to achieve a 94% level of destruction, when co-milling with KOH, these levels are reached within the first 15 minutes of testing.

A patent application for the use of ball milling for PFAS remediation has been filed by Queen’s University and RMCC, with our team having royalty-free access to the technology. The team is now working on further optimization, scaling and future field implementation. This work is being undertaken as part of a Natural Sciences and Engineering Research Council (NSERC) of Canada Collaborative Research and Development Grant.