Testing PFAS-Immobilization

In the region Rastatt/Baden-Baden in the Upper Rhine Valley, Germany, approximately 1000 ha of predominantly agricultural land is contaminated with per- and polyfluoroalkyl substances (PFASs). About one decade ago, paper-fibre biosolids mixed with compost were applied as fertiliser. This affects various land uses and the underlying aquifer as the main drinking water resource for surrounding cities and municipalities. Besides perfluorinated carboxylic and sulfonic acids, the soil pollution is characterised by high contents of polyfluorinated precursors.

Remediation attempts have been limited to date, particularly due to the large spatial extent of the contamination and the related high costs. Currently, the possibility to immobilise the PFASs in the soil material is discussed. One strategy is an in-situ approach: substances with a high sorption capacity would be applied on the ground surface and mixed with the soil. The altered soil should still fulfil its original purpose (e.g., for agriculture). In this project, two soil mixtures treated with different active carbon-based products are used. Another strategy could be to remove the contaminated soil and use it for construction (e.g., noise protection embankment) after treatment with the immobilisation agents. This is tested with a liquid soil mixture and a concrete mixture.

The purpose of this research is to develop a test strategy to evaluate the long-term leaching characteristics of treated soils. Therefore, tests on three scales (batch experiments, column experiments, lysimeters) including different saturation conditions (saturated, variably saturated) are conducted. Effluent concentrations are monitored over time with different analytical methods (target analysis, determination of sum parameters (EOF/AOF), Total Oxidisable Precursor Assay (TOP)). In Hydrus-1D, mathematical models are employed to evaluate the appropriateness of various processes (e.g., equilibrium sorption) and the leaching behaviour for time scales larger than laboratory experiments can reproduce. The measured and modelled time-series of effluent concentrations serve as the basis for a simple and cost-effective method for the experimental testing of immobilisation measures for PFASs.

The current data illustrate significant reductions in PFAS desorption rates in the soils treated with active carbon-based additives. The immobilisation efficacy is chain-length dependent with less retention for short-chain carboxylic acids (PFBA, PFPeA); similar characteristics are observed in all experimental methods. In the variably saturated lysimeter experiments, delayed elution of short-chain PFAS in treated soils indicate additional processes (such us biotransformation).

The presentation focuses on the illustration and interpretation of PFAS desorption characteristics in the differently treated soils, on a data-based comparison of the experimental methods and challenges in the numerical simulations.