Per- and polyfluoroalkyl substances (PFAS) have become pollutants of global concern due to their widespread usage and presence in the environment. Investigation of PFAS-impacted sites results in investigation-derived waste (IDW). The IDW is a mixture of soil, purge water from groundwater sampling, and fluid from decontamination of drilling equipment, and likely to have PFAS and other co-occurring chemicals. PFAS are also becoming a subject of environmental regulations.
The broad objective of this project was to develop an innovative, low cost, simple to use technology for the treatment of groundwater and soil IDW at Department of Defense (DoD) sites. This project evaluated the use of a novel adsorbent and ultrasound for the treatment of groundwater and soil, and destruction of PFAS (perfluorooctanoic acid, perfluorooctanesulfonic acid [PFOS], perfluorobutanoic acid, perfluorononanoic acid, perfluorohexanoic acid, perflurobutane sulfonic acid [PFBS], perfluorohexane sulfonic acid, fluorotelomer sulfonic acid 6:2). The removal of chlorinated volatile organic compounds (CVOC) co-occurring chemicals also was investigated.
Overall, the technology employs a three-step approach to treat soil and groundwater and destroy PFAS.
- Remove PFAS and co-occurring chemicals from impacted groundwater using a novel, low cost adsorbent.
- Desorb chemicals from the soil and adsorbent with a chemical (desorption) solution in the presence of ultrasound.
- Destroy PFAS and other co-occurring chemicals in the desorption solution and those sorbed on soil using ultrasound in one reactor.
In batch system, the novel polymer adsorbent efficiently removed PFAS from the groundwater and soil mixture; final concentrations ranged from non-detect to less than 40 parts-per-trillion (ppt). The adsorbent also removed trichloroethene and tetrachloroethene from water in the presence of PFAS; PFAS removal was not adversely affected by the presence of CVOCs. The adsorption of PFAS from groundwater was very fast, and equilibrium was reached in approximately five minutes.
Site soil was treated using ultrasound and a desorption solution. The final PFAS concentrations on the soil were much less than the target level of 10 micrograms per kilogram (μg/kg) (except for PFOS, which was 13 μg/kg). The site soil had a very high level of PFOS (243 μg/kg); 95% of PFOS was removed from the soil in four hours of sonication. Higher removal of PFOS from the soil could be achieved by process optimization. The ultrasound process was able to destroy PFAS (10 parts per million each) in the desorption solution in six hours to concentrations <100 ppt for all PFAS except PFBS (final PFBS concentration was 3900 ppt). Higher destruction of PFAS could be achieved by process optimization. The sonication process with desorption solution was effective to clean the adsorbent in six hours, and the final concentrations of all PFAS were < 10 μg/kg. The initial PFOS loading on the adsorbent (from treatment of site groundwater) was 1725 μg/kg, and the final was seven μg/kg, representing a cleanup efficiency of 99.6%.
Overall, this proof-of-concept project showed that the groundwater and soil (including the spent adsorbent) can be treated for on-site disposal, and the PFAS are destroyed, offering an environmentally sustainable solution to manage IDW at DoD sites. The technology needs to be tested for different types of groundwater and soils, different soil-water slurry ratios, and optimized. A pilot-scale demonstration would allow the remedial project managers and stakeholders to evaluate the economic and technical suitability of the technology for IDW management at their sites. In addition, further optimization of this technology could provide a low cost and environmentally green solution for the management of PFAS-impacted soil as an alternative to soil incineration or landfilling.