Universal PFAS Sorbent Performance Comparison

By Dr. Matthew Askeland

To date, sorbent performance for PFAS immobilisation applications has been assessed using a variety of measures such as Iodine Number, Methylene Blue Number and Specific Surface Area (SSA). Because of the various binding mechanisms for PFAS to sorbents, these tests alone are not able to simulate PFAS binding efficiency for the numerous sorbent media on the market and across a variety of PFAS species (example image B). This has presented issues in the application of sorbents for immobilising PFAS, where the measures used in the specification either are unclear, do not represent performance, exclude key solutions, or potentially result in suboptimal outcomes.

This issue has been the driver behind the development of a methodology to unify and standardise language and analytical measures around how sorbents are characterised.

At present, a number of approaches are inconsistently used across the industry, these include:

Iodine Number measures such as ASTM, Standard Test Method for Determination of Iodine Number of Activated Carbon, ASTM D4607-14, 2021.
Specific Surface Area (SSA) akin to ISO 9277:2010
Determination of the specific surface area of solids by gas adsorption — BET method, 2010.
Methylene Blue Value such as European Council of Chemical Manufacturers’ Federations, Test Methods for Activated Carbon, 1986
Various Dye test for Capacity such as those described in Sörengård et al. (2020).
Sorption/desorption kinetics modelling as described by European Council of Chemical Manufacturers’ Federations, Test Methods for Activated Carbon, 1986.
Leachability testing such as that in Australian Standard Leaching Procedure (ASLP) tests (Australian Standard AS4439).

All the above-mentioned methods work well to describe the physical and chemical properties of powdered and granular activated carbon products. When these methods are applied to sorbents that are not wholly carbon based such as anion exchange resins, mixed mineral products, organoclays, and other novel sorbents, the iodine number, SSA and dye tests do not necessarily relate to the PFAS sorption of these other products or are not cross compatible. This then means that there is no way to make a comparison between carbon-based products (PAC, GAC) and the other sorbents to compare PFAS sorption efficacy. Further, this provides very little information regards behaviour for the variety of PFAS species usually present on contaminated sites.

ADE’s Standardised Sorbent Quality Measure(SSQM) method and Matrix Sorbent Qualities Measure (MSQM) method, provide results for PFAS stabilisation efficacy that are directly comparable between sorbent products, regardless of product composition. SSQM can be used alongside more detailed assessment methods such as kinetics and isotherm modelling. Kinetic and isotherm models offer a direct way to determine the sorption and desorption behaviours of PFAS to a sorbent product, which is then comparable to other products. The disadvantage of this method, compared to SSQM/MSQM is that kinetics and isotherm studies require multiple samples to be analysed over multiple time-points, rendering this approach neither cost nor time efficient. Furthermore, for the results to be comparable, studies need to be completed in the same concentration range and experimental conditions, which is often not the case.

By means of comparison, ASLP is a leachability test often used for sorbent performance, and measures contaminant leachability over a set period of time to simulate 100 years in a landfill. The method is only applied to soil or treated soil samples with no easily calculable kinetics information once testing is complete. The data collected from ASLP is useful but does not provide a cross comparable measure and only considers the desorption step once sorption is completed.

Overall, while several methods are available to directly or indirectly characterise sorbent performance, there is at present no harmonised approach, and comparing sorbents with different or incompatible measures to each other and specifications is a common challenge. This impacts decision making and at times drives negative immobilisation outcomes.

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

To resolve the above limitations, ADE developed the SSQM and MSQM method. These methods directly measure the sorption and desorption of PFAS to sorbent products using a sequential test to then calculate a sorption efficiency, sorption reversibility, net PFAS removal, performance quotient and SSQM/MSQM score (descriptions in Table 1). This one test is designed to be suitable across a range of different sorbent types, and has standardised testing conditions that do not bias the outcome but allow comparability between sorbents and PFAS sorption performance on a like-for-like basis.

The methodology consists of two parts:
1. SSQM – a methodology for providing specifications or information on sorbent performance under standardised conditions.
2. MSQM – a methodology that explores performance of sorbents within a given matrix.

The two methods are used in conjunction to specify, select, or demonstrate suitability of a sorbent for a suite of PFAS contaminants via SSQM, then demonstrate suitability of the selected sorbents for a given application using MSQM to measure and compare performance.

The SSQM method considers performance from the perspective of sorption and desorption, where a known mass of sorbent is introduced to a system containing a known mass of PFAS, agitated for 24 hours, and then the solution tested. Following this, the liquid fraction is decanted and replaced with clean ultrapure water to measure desorption after a 24-hour agitation period. Based on the known PFAS masses, sorbent and water additions, the extent of sorption, desorption, net removal, and SSQM score are calculated for each tested PFAS species and for each sorbent.

The MSQM method follows the same approach as SSQM, with the change that sufficient matrix is added to the system to attain a 1% w/w equivalent sorbent dose. The same standardised testing conditions are adhered to in MSQM as in SSQM. The impact of the matrix can then be measured by subtracting the MSQM scores (matrix) from the SSQM scores (standardised).

Overall, this provides a robust approach to first select sorbents for treatability studies, then assessing their suitability for a given matrix before progressing to isothermal, kinetics or dosing bench trials. The test is designed to be quick, to provide the evidence needed to clearly inform suitability of a sorbent for a selected application without holding up further testing by excessive laboratory analysis or data processing activities. SSQM unifies the language, providing a single score for each PFAS contaminant that can be used to describe the sorbents performance.

Application and Benefits

SSQM/MSQM addresses a gap in how we communicate about sorbents for use in PFAS immobilisation by offering a methodology that can be used to:

Provide a high level of clarity in treatment specifications.
Be used to inform and justify treatability trial decisions and sorbent selection.
Be used by manufacturers to communicate suitable applications and performance of their product.
Can be used to QA/QC batches of sorbent products to assess quality variation and conformity.
Assess the suitability of a product for a specific application based on matrix interference.

The key benefits of the method across these applications include:

Makes available a simple and unified language to cross compare sorbents and support other testing methodologies.
Provides a systematic process for sorbent selection.
Provides a direct measure of the PFAS sorption / desorption performance in a simple, cost effective and time efficient manner.
Enables benchmarking of sorbent products of any composition for comparison between product types.
Reduces the cost of testing and benchmarking PFAS sorption efficacy significantly overusing kinetics/isothermal modelling approaches.
Matrix component considered in MSQM to allow for site-specific performance testing.
Can be applied to other contaminants i.e. heavy metals, pesticides, explosive residues, other emerging contaminants.
Standardisation provides a high level of certainty in results.
Readily scalable to run large volumes of samples to cater for large testing factorials.

Case Study

SSQM and MSQM have recently been used for a project in Victoria to assess the viability of sorbents for immobilising large volumes of heavy clay soils that are diffusely impacted with PFAS (mean ASLP result ~30 µg/L PFOS).

The methodology was applied to screen a variety of sorbents for SSQM score for target PFAS: PFOS, PFHxS, and PFHxA. A total of 16 candidate sorbents were assessed in initial works utilising the SSQM process).

SSQM identified up to five sorbents that performed well, with the candidate sorbent list narrowed further based on availability. Hereafter two sorbents were selected based on performance and tested using MSQM to assess matrix impacts (see Table 2). Both sorbents performed similarly and therefore one was selected based on its cost, and merit (literature data supporting its viability and stability). The work then progressed on to bench scale tests to determine sorbent dosing rate and a full-scale trial was performed, treating over 8,000 tonnes of soil.

Conclusion

SSQM presents industries engaging in the treatment of soils for contamination using immobilisation technologies not only with a simple, easy to interpret, quick and cost-effective tool to select the best sorbent for a given application, but also a manner to demonstrate to stakeholders how that decision was made. Furthermore, the methodology unifies language and communication related to sorbent performance, giving clients, regulators, and sorbent providers an opportunity to provide and compare SSQM scores to justify immobilisation approaches. This also gives sorbent providers a clear way to verify the performance of their product and communicate with the market (including use of SSQM on a batch-to-batch basis for QA/QC).

Overall, SSQM brings uniformity to how we approach immobilisation agents and make decisions, and when paired with other more conventional tools such as sorption isotherms, a multiple lines of evidence approach can be undertaken to demonstrate the suitability of a given immobilisation approach and that the science behind its application is robust. This brings certainty to regulators and landowners regarding the quality and reliability of immobilisation tools in addressing contamination.

Table 1 – Measure descriptors

Term	Description
Percentage Sorbed (%)	The percentage sorbed measures the proportion of PFAS mass removed from the SSQM solution by the tested product (1-100%). A high percentage sorbed value (i.e., >90 %) means that the tested product has performed well at removing selected PFAS under the standardized testing conditions.
Percentage Desorbed (%)	The percentage desorbed measures the proportion of the PFAS remobilized (sorbed fraction desorbed) into a PFAS-free ultrapure water solution based on the initial PFAS concentration in solution. A low percentage desorbed value means that PFAS sorption to the tested product is not easily reversible.
Performance Quotient (dimensionless)	The performance quotient is a measure of the irreversible sorption/desorption and is heavily weighted towards desorption. A higher performance quotient is desirable, however, if a small amount of PFAS is remobilized during the desorption testing this will greatly decrease the performance quotient calculated. Therefore, the performance quotient needs to be considered in the context of the percentage sorbed/desorbed.
Net Removal (%)	The net removal is the total percentage mass that has been removed from the system irreversibly after the sorption/desorption testing under standardized SSQM conditions. As with Percentage sorbed, higher numbers show better performance of the tested product, with values ranging from 0 to 100%
SSQM Score (dimensionless)	The SSQM score is based on the performance quotient and net removal score with both weighted evenly. Higher performing sorbents will exhibit a higher SSQM score.
MSQM Score (dimensionless)	The MSQM score is much like the SSQM score, except that it presents the outcomes for the experiment including matrix, where the control is used to estimate the total PFAS sorbed by the matrix and the difference between SSQM and MSQM (once corrected for the control – just matrix, no sorbent) is the impact of the matrix on sorption/desorption.

Table 2 – Example PFHxA outputs for SSQM (16 candidate sorbents) and MSQM (2 Candidate sorbents) results for Clayey Melbourne Soils

Sorbent	Percentage Sorbed (%)	Percentage Desorbed (%)	Performance Quotient	Net Removal (%)	SSQM Score
SSQM
A	48	2.0	24	46	35
B	44	2.0	22	42	32
C	74	5.0	15	69	42
D	51	5.0	10	46	28
E	46	3.0	15	43	29
F	18	8.0	2	10	6
G	98	1.0	98	97	98
H	77	2.0	39	75	57
I	99	1.0	99	98	99
J	99	2.0	50	97	73
K	99	1.0	99	98	99
L	99	1.0	99	98	99
M	1	1.0	1	0	1
N	2	2.0	1	0	1
O	6	1.0	6	5	6
P	17	1.0	17	16	17
MSQM
G	98	1.0	98	97	98
I	100	1	100	99	100

dr. Matthew Askeland

State Environmental Lead
Victorian Environment Practice

Universal PFAS Sorbent Performance Comparison

Our Solution & Method

Table 1 – Measure descriptors

Table 2 – Example PFHxA outputs for SSQM (16 candidate sorbents) and MSQM (2 Candidate sorbents) results for Clayey Melbourne Soils

dr. Matthew Askeland

0452 377 551

matthew.askeland@ade.group

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