Study of the feasibility of in-situ stabilization of PFAS by injection of surface-modified clay

Intermediate results:

Objective

The objective of the project is to evaluate the feasibility of in‑situ PFAS stabilisation through injection of SMC (surface‑modified clay, marketed as Fluorosorb). SMC is available as a powder but is not soluble. Injecting particles into the subsurface is challenging and is therefore only possible in the most permeable soil pores or by opening fractures (‘fracturing’) in the soil. For remediation efficiency, it is desirable to distribute the SMC as homogeneously as possible in order to immobilise the maximum amount of PFAS.

Method

To achieve this, the innovative and patented SPIN injection technique from Injectis was used. This technique is suitable for effectively injecting particle‑based mixtures (slurries) into heterogeneous soils.

At both laboratory and field scale, several tests were carried out to examine whether the commercial product could be separated into different fractions. The conclusion was that injecting the original product provides the best cost‑benefit ratio. Additional pre‑treatment was therefore not performed.

Two sites belonging to project partners were selected to test SMC injection, both characterised by challenging and very different lithologies. One site is located at Brussels Airport in “Brussel Sands,” which also contains stone layers. The second site is in the Port of Antwerp and is characterised by highly incohesive soft layers of clay and peat‑bearing fine sand.

The injection mixtures at both sites were prepared using a mobile bentonite mixer. From the test injections and the preparation of these mixtures, it became clear that mixing and achieving sufficient homogeneity posed no issues. The bentonite mixer ensures proper blending through integrated high‑energy water jets. The required water volume is monitored using a flow meter.

During two testing phases at the respective sites, practical experience was gained regarding the most suitable injection tools for the prevailing lithological conditions. In addition, the necessary knowledge was gathered to determine the optimal injection and distribution parameters. The SPIN injection technique essentially involves injecting the product using a rotating movement of a specially designed drill head. A crucial aspect is that the operator can monitor all injection parameters (pressure, depth, volume, flow rate) in real time on a screen, allowing immediate intervention if injection conditions change.

Results

For the injection campaign with the SPIN technology at the POAB site:

For the injection campaign with the SPIN technology at the BAC site:

Conclusion

Overall, the results of the pilot projects using the SPIN injection technology at both locations are in line with the intended objectives. Specifically, the aim was to achieve the most homogeneous possible distribution of solid particles in a slurry within the subsurface, even under two very different lithological conditions.

The nature of the injection mixture—very fine solid SMC particles suspended in water—did not in itself lead to any execution issues. The mixture could be injected smoothly at both low and high concentrations. When problems occurred, they were related to the lithology rather than the product characteristics.

The applicability of the technique has been demonstrated at the pilot project sites. Given that SPIN technology has already been widely used at many other locations with various, often more challenging, injectable products, it can be concluded that the technology is suitable for use under a broad range of conditions.