PFAS Stabilisation in Soil – Current Status from the USA

Wednesday 14th August, 2024

PFAS Stabilisation in Soil – Current Status from the USA

Dr Richard Stewart, Managing Director, RemBind Pty Ltd

I had the pleasure of attending the 2024 Battelle Remediation Conference in Denver, Colorado from 2^nd to 6^th June 2024. This conference is very industry focused and is a great way to get up to speed on where the industry is at concerning different remediation topics. This article gives an update on the status of PFAS soil stabilisation in the USA based on what was presented at the conference. It will cover the performance of various sorbents, long term stability testing, field application and QA/QC, carbon emissions, reductions in bioavailability, co-contaminants and the general acceptance of the technology in the USA.

Performance of Sorbents

The main PFAS sorbents being used in the USA for soil stabilisation are based on commercially available carbon/clay mixtures or organoclays. Jeff Bamer from CDM Smith presented some excellent data using PFAS impacted soil from a commercial airport site. Both RemBind and FluoroSorb showed significant reductions in PFAS leachability (up to 99.9%) after repeated leaching cycles (see Figure 1). This shows the feasibility of using such commercial sorbents to reduce the amount of PFAS leaching into surface water and ground water. This project is moving to field pilot scale in 2024 based on the successful lab trial results.

Figure 1: PFAS Leachability Reductions in AFFF- Contaminated Soil using 1% to 10% Sorbent Dosage. From Jeff Bamer, CDM Smith, In Situ Soil Stabilization to Mitigate PFAS Transport via Stormwater at an AFFF Source Area, Battelle Remediation Conference, Denver Colorado, USA, 2nd to 6th June 2024.

Long Term Stability

Long term stability is a critical pre-requisite for PFAS stabilisation in soil, because the PFAS is immobilised but not removed or destroyed (i.e. it remains in the soil). By its nature, long term stability is difficult to prove. It has been previously tested at laboratory scale using rigorous US EPA LEAF methods that simulate a wide range of environmental conditions under repeated leaching scenarios (Kabiri and McLaughlin, 2021). Jurgen Buhl of Cornelsen presented at the conference showing the results of stress-testing RemBind under freeze/thaw, wet/dry, accelerated solvent extraction (boiling methanol), and enhanced biological stress conditions (using sewage sludge).

Results showed that the RemBind treatment was very stable under all stress conditions which is consistent with the previous work of Kabiri and McLaughlin (2021).

A project initiated at a US field site in 2021 involved the in-situ treatment of PFAS impacted soil with RemBind and FluoroSorb with Portland cement (as a solidifier), and ongoing monitoring of PFAS leaching (McDonough et al., 2021). Theresa Guillette of Arcadis presented the 5-year time point for this project at the conference, showing that leaching has not increased over the 5-year period under worst-case scenario test conditions (using the Total Oxidisable Precursor assay), and leaching levels for some sorbent treatments remain less than 0.1% compared to the untreated soil. This infield data validates previous lab-scale simulations showing that immobilised PFAS soil is expected to be robust for many decades.

Field Applications and QA/QC

Field application methods and their validation are critical factors in the soil stabilization process. At the conference, Matthew Askeland of ADE Consulting presented on the stabilization of 7,000 tonnes of PFAS contaminated soil at Melbourne International Airport. Two new innovations were discussed; firstly, the Sorbent Standardized Sorbent Quotient Measure (SSSQM) method for assessing the performance of different sorbents using site specific soil and giving each sorbent a ‘score’ indicating its sorptive capacity. Secondly, the Sorbent Application Uniformity Test (SAUT) method was presented, a dye-based infield kit for confirming that the sorbent has been evenly applied to the soil at the correct dosage rate. These are both important tools for future use in PFAS soil stabilisation.

Dr Richard Stewart from RemBind (author) presented a Learning Lab demonstration of a new granular product, which essentially allows dust-free application of a sorbent as a granule while retaining the performance of a powder. This is an important innovation because while powdered sorbents are preferred (better surface area contact), they are relatively dusty which can lead to environmental issues and can slow production rates significantly in the field. Matt Richardson from the University of Adelaide presented extensive data on this new product, showing similar sorptive performance against a number of different PFAS species (Figure 2).

Figure 2: PFOS Leached from Soil Treated with Granulated versus Powdered RemBind. From Matt Richardson, University of Adelaide, Formulation and Development of a Novel Dust-Free Carbon-Based Amendment for PFAS Immobilization in Soil, Battelle Remediation Conference, Denver Colorado, USA, 2nd to 6th June 2024.

Carbon Emissions

Carbon emissions are becoming increasingly important as the remediation industry drives towards a more sustainable Circular Economy approach. Paige Molzahn from Jacobs presented some elegant work showing that the carbon emissions from stabilization are less than those from soil washing, incineration and landfill disposal under two treatment scenarios; five 50 gallon drums of soil and 10,000 cubic yards of soil (Figure 3).

Figure 3: Solid Scenario 2, Greenhouse Gas Emissions. From Paige Malzahn, Jacobs, Assessment of Environmental Footprints for Per- and Polyfluoroalkyl Substances (PFAS) Treatment Technologies for Liquids and Solids Battelle Remediation Conference, Denver Colorado, USA, 2nd to 6th June 2024.

Reductions in Bioavailability

It is well proven that sorbents can reduce the leachability of PFAS from soils, but how does that correlate with reductions in the bioavailability of the PFAS to sensitive environmental receptors? Professor Albert Juhasz from the University of South Australia presented new data showing that RemBind treatment reduces the bioavailability of PFAS to mice and earthworms by up to 99%. Since the conference, this work has been published in the peer reviewed literature (Caceres et al., 2024). This is consistent with previous work published on earthworms and plants Braunig et al. (2021), Juhasz et al. (2021) and Biec et al. (2023).

Co-Contaminants

PFAS is often found in soil alongside other co-contaminants including hydrocarbons and heavy metals, so any stabilisation design needs to take this into account. Sonya Carr from RemBind presented at the conference on the Immobilisation and On-Site Reuse of Soils Contaminated with Arsenic and Chromium: A Circular Economy Approach which showed that different sorbent formulations can be amended with other reagents to stabilise more than one contaminant in a single treatment step. The project used an alum sludge alongside RemBind to stabilise chromium and arsenic at an industrial site, allowing onsite reuse of the treated soil with full regulatory signoff.

Acceptance of PFAS Stabilization in the USA

Overall, the message from the 2024 Battelle Remediation Conference was that stabilisation is now accepted by industry and government alike as an important part of the PFAS soil remediation toolkit. It will likely be used alongside other technologies such as soil washing, thermal treatment and landfill, with each technology having its own ‘sweet spot’. Chatter from Industry Leaders at the conference implied that the ideal scenario for stabilisation would be for targeting relatively high volumes of diffuse PFAS contamination outside of the source zone, where other destructive technologies are simply not cost-effective. The coffee break talk had transitioned from “does stabilisation work for PFAS?” and “what about longevity?” at previous conferences, to “how can we implement stabilisation in the field?” and “what is the availability of sorbents?”.

References