PFAS Immobilisation Dye-Based Field Quality Control

By Dr. Matthew Askeland

Syringe and vials

Until recently, there was no analytical methodology commercially available for use in the quality control of the soil remediation or blending process, when considering the addition of sorbents for the immobilisation of contaminants such as PFAS. Typically, sorbents are blended into soils at a known percentage weight by weight (w/w) to immobilise contaminants, with the application rate often listed in a treatment specification as determined by a bench scale treatability study. 

To date there has not been a direct measure to assess if the dosing rate stipulated in a specification was achieved, and how variably it was applied throughout the treated material. This causes quality issues, often resulting in undesirable performance and potential release of contaminants to the environment. Under these circumstances, the immobilisation process can appear not to have worked, but the cause is in fact the poor distribution of the sorbent through the treated material, as opposed to the performance efficiency of the sorbent itself.

Prior to the conceptualisation of ADE’s Sorbent Application Uniformity Test (SAUT) the approach to quantifying sorbent application for quality control has typically been one that measures efficiency (effective immobilisation) of sorbent by contaminant leachability reduction. The method used for this is the Australian Standard Leaching Procedure (ASLP). These values are compared pre- and post- treatment and only inform contaminant leachability reductions based on sorbent application/efficiency, and do not grant any details regarding sorbent dose accuracy or variation through material.  

This use of ASLP is limited by variables such as unpaired pre/post contaminant concentration samples, matrix impacts, sorbent efficiency, sorbent and soil and uniformity. As such this method cannot be used to assess dosing accuracy and or variability, but is a useful measurement for sorbent performance. 

Dye methods have been used in the past to characterise sorbents and assess their capacity (via a dye proxy measure) to sorb target contaminants (Sorengard et al., 2020). These do not however assess the dosing of sorbents in the soil or variability thereof, and are instead designed to estimate the sorption capability of sorbent or matrix.

Considering the above, there was a significant need for the development of a robust tool that could measure the mass of sorbent in a soil quickly and cost effectively to allow the measurement of immobilisation blending process quality.

Reference – Mattias Sörengård, Erik Östblom, Stephan Köhler, Lutz Ahrens, Adsorption behavior of per- and polyfluoralkyl substances (PFASs) to 44 inorganic and organic sorbents and use of dyes as proxies for PFAS sorption, Journal of Environmental Chemical Engineering, Volume 8, Issue 3, 2020, 103744, ISSN 2213-3437.

Our Solution & Method

ADE developed a dye-based analytical method for the quantification of sorbent mass, application (dose) and variability in treated soils, towards confirmatory quality control for immobilisation projects. 

Analysis is matrix corrected, meaning that the process is:

  • Suitable and robust enough for use with a variety of soils 
  • Takes into account the matrix interactions presented by non-homogenous soil types/ geochemistry across the globe

Analytical method was designed to be simple, affordable and quick to execute, supporting high throughput

Methodology is field deployable and easily understoodhelping training and upskilling of supervising staff 

Calculations and statistical analysis required to arrive on results are easy to undertake and interpret

Application and Benefits

The SAUT method presents a universal approach for measuring, specifying, communicating, and assessing sorbent blending quality on immobilisation projects. The method can be employed as a quality control method in near real-time during the blending process, post treatment or as a validation tool.

The key benefits of SAUT include:

  • Ensures that the soil treatment and immobilisation process is robust and has been properly executed
  • Offers better capability to prove that sorbent applications required for immobilisation have been achieved
  • Can alleviate the need for frequency of testing with more onerous proxy measures (i.e. ASLP)
  • Provides quick method that can be conducted on site and almost real-time QA/QC that allows for correction of non-conformities
  • Allows for less conservative use of sorbents and saves money by ensuring soils are not under or over treated, minimising the cost and risk
  • Delivers good value for money measure and provides more tangible measurement of success criteria

Case Study

ADE has used SAUT to quality control and assess the viability of different sorbent blending technologies (excavators, pug mills and trommel screens), as well as methodologies (single pass or double pass) in relation to a PFAS-impacted material stockpiling facility at a major Australian airport. An example output from this trial is demonstrated below, noting that this is only reflective of the particular soil conditions and sorbent type at that site. This data was used to demonstrate the scalability and feasibility of a variety of blending approaches for use at the site.

A treatment specification of 1% and 2% w/w was determined for the site based on the treatment curve, which was calculated using bench tests> the does were considered acceptable to conservatively achieve the site leachability criteria, which would enable reuse. 

Furthermore, samples were collected from the treated stockpile to enable the development of a simplified ‘heatmap’ to understand the distribution of sorbent across the very large, combined stockpile (6 x 2,000tonnes). This allowed further treatment of portions of stockpiles that did not comply with the specification, where required.

It was a valuable approach that could be used in the future to avoid underdosing, allow re-blending where sorbent content is non-uniform, or prevent the need to re-blend the entire material by instead only redressing the area of stockpile with the non-conformity.

The data collected in this study was used to confirm that treatment specifications had been met, and when combined with ASLP data presented strong evidence that immobilisation works were of a high quality and had been successful in achieving goals to achieve reuse of the PFAS-impacted material. The works provided significant evidence that while blending technologies impact immobilisation quality, specification could be achieved on this site using a variety of technologies.

A treatment specification of 1% and 2% w/w was determined for the site based on the treatment curve, which was calculated using bench tests> the does were considered acceptable to conservatively achieve the site leachability criteria, which would enable reuse. 

Furthermore, samples were collected from the treated stockpile to enable the development of a simplified ‘heatmap’ to understand the distribution of sorbent across the very large, combined stockpile (6 x 2,000tonnes). This allowed further treatment of portions of stockpiles that did not comply with the specification, where required.

It was a valuable approach that could be used in the future to avoid underdosing, allow re-blending where sorbent content is non-uniform, or prevent the need to re-blend the entire material by instead only redressing the area of stockpile with the non-conformity.

Mixing Equipment

Mean Actual RemBind (%)

Mean Accuracy (%)

Precision (%)

Blend Quality

1% RemBind Specified Application

Excavator (E)

0.35

35.05

67.13

Unacceptable*

Pugmill (P)

0.86

86.43

69.52

Acceptable

Trommel (T)

1.23

122.68

51.09

Good

2% RemBind Specified Application

Excavator (E)

1.21

60.50

85.31

Acceptable

Pugmill (P)

0.67

33.70

100.08

Unacceptable*

Trommel (T)

2.58

128.76

30.63

Good

The data collected in this study was used to confirm that treatment specifications had been met, and when combined with ASLP data presented strong evidence that immobilisation works were of a high quality and had been successful in achieving goals to achieve reuse of the PFAS-impacted material. The works provided significant evidence that while blending technologies impact immobilisation quality, specification could be achieved on this site using a variety of technologies.

Ultimately SAUT presents the industry with an additional tool to improve the quality and robusticity of soil treatment programs utilising immobilisation. The approach brings greater certainty by assuring the quality of the works meet that of the specification, but also makes additional options available to identify and rectify issues that would not have been noted via leachability testing alone. Combined with ASLP testing, SAUT provides strong lines of evidence that immobilisation can be used as a tool for managing contamination in soils with a high degree of certainty and control.

Matthew-Askeland-opt

dr. Matthew Askeland

State Environmental Lead
Victorian Environment Practice

0452 377 551