Formatted Title
Sorbed PFAS under Weather Conditions: Resilient Enough?
Background/Objectives
At a former fire training pit on an industrial site, soil had to be excavated that showed PFAS contamination. Landfilling was chosen as the main disposal route, but immobilization of the PFAS was planned for part of the soil. The intention of the client behind this was that this treated soil should be used to conduct tests in the laboratory. These tests should clarify whether weather conditions and/or certain ambient conditions can reduce or eliminate the sorption of PFAS over time.
Approach/Activities
The process of immobilization of the PFAS in that soil was first simulated in a bench test. That was done in a technical lab. The bench test should show what amounts of sorbent substance are required to bind the PFAS in a representative sample of the soil from this site. To identify this, different amounts of the immobilizing substance were added to sub-samples of that soil. Treated samples went to an external laboratory where they determined the corresponding eluate values of PFAS in an analytical laboratory. The immobilization proved to be effective.
The measure was implemented on site and after immobilization was completed, samples of the treated soil were again taken on site. These samples were sent to different institutions to test the elution behaviour during simulated ageing of the material. The investigations considered, for example: freeze-thaw stress, different water-solid ratios, biological stress (activated sludge), wet-dry changes and chemical stress.
Results/Lessons Learned
In the untreated soil, 2.3 µg/l PFAS could be determined in the eluate (65% PFOS and 21% PFHxS). The simulation of the immobilization process itself in the technical lab showed that already with an addition of only 0.5 wt.% of the immobilizing powder no PFAS above the determination limit of 0.01 µg/l (ppb) per individual parameter were present in the eluate (2:1) of that soil sample.
In the next step a sieve drum machine mixed the contaminated soil with the sorbent powder on site. This application confirmed the results of the test in the technical lab. No PFAS were detectable in the eluate of that soil after mixing.
The simulations of the treated samples under stressing conditions resulted in the following findings. Activated sludge as well as freeze/thaw cycles, wet-drying changes or different water-soil-ratios have no influence on the binding of PFAS. Only chemical stress carried out as accelerated solvent extraction (ASE) at 10 bar and 100°C was able to dissolve a few PFAS species to a small extent; this test represents very harsh conditions which are unlikely to occur in the environment.
As a general result of the investigations of the treated soil, it was found that only the harshest ageing test involving a solvent could achieve mobilization of the PFAS from the sorbent, albeit to a small extent.