Longevity of colloidal activated carbon for in situ PFAS remediation at AFFF‐contaminated airport sites

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The treatment of per‐ and polyfluoroalkyl substances (PFAS) within groundwater is an emerging topic, with various technologies being researched and tested. Currently, PFAS‐impacted groundwater is typically treated ex situ using sorptive media such as activated carbon and ion exchange resin. Proven in situ remedial approaches for groundwater have been limited to colloidal activated carbon (CAC) injected into aquifers downgradient of the source zones. However, treatment of groundwater within the source zones has not been shown to be feasible to date. This study evaluated the use of CAC to treat dissolved PFAS at the air–water interface within the PFAS source zone. Studies have shown that PFAS tends to preferentially accumulate at the air–water interface due to the chemical properties of the various PFAS. This accumulation can act as a long‐term source for PFAS, thus making downgradient treatment of groundwater a long‐term requirement. A solution of CAC was injected at the air–water interface within the source zone at a site with PFAS contamination using direct push technology. A dense injection grid that targeted the interface between the air and groundwater was used to deliver the CAC. Concentrations of PFAS within the porewater and groundwater were collected using a series of nine lysimeters installed within the vadoze and saturated water columns. A total of six PFAS were detected in the porewater and groundwater including perfluorobutanoic acid (PFBA), perfluoropentanoic acid (PFPeA), perfluorohexanoic acid (PFHxA), perfluoroheptanoic acid (PFHpA), perfluorooctanoic acid (PFOA), and perfluorononanoic acid (PFNA). Detectable concentrations of PFAS within the pore and groundwater before treatment ranged from values greater than 300 µg/L for PFPeA to less than 3 µg/L for PFNA. Following the injection of the CAC, monitoring of the porewater and groundwater for PFAS was conducted approximately 3, 6, 9, 12, and 18 months postinjection. The results indicated that the PFAS within the porewater and groundwater at and near the air–water interface was effectively attenuated over the 1.5‐year monitoring program, with PFAS concentrations being below the method detection limits of approximately 10 ng/L, with the exception of PFPeA, which was detected within the porewater during the 18‐month sampling event at concentrations of up to 55 ng/L. PFPeA is a five carbon‐chained PFAS that has been shown to have a lower affinity for sorption onto activated carbon compared to the longer carbon‐chained PFAS such as PFOA. Examination of aquifer cores in the zone of injection indicated that the total organic carbon concentration of the aquifer increased by five orders of magnitude postinjection, with 97% of the samples collected within the target injection area containing activated carbon, indicating that the CAC was successfully delivered into the source zone.

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Pfas Treatmentmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The in situ treatment of PFAS within porewater at the air–water interface of a PFAS source zone

McGregor¹

2023

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“…Therefore, long‐term monitoring is essential to understand the effectiveness of stabilization treatment with CAC. The longevity of a CAC barrier is directly proportional to the CAC fraction applied (Carey et al, 2022), therefore reapplication of CAC might be considered necessary in future scenarios.…”

Section: Conclusion and Environmental Implicationsmentioning

confidence: 99%

In‐situ application of colloidal activated carbon for PFAS‐contaminated soil and groundwater: A Swedish case study

Niarchos

Ahrens

Kleja

et al. 2023

Soil and groundwater contamination by per-and polyfluoroalkyl substances (PFAS) has been a significant concern to human health and environmental quality. Remediation of contaminated sites is crucial to prevent plume expansion but can prove challenging due to the persistent nature of PFAS combined with their high aqueous mobility. In this case study, we investigated the potential of colloidal activated carbon (CAC) for soil stabilization at the pilot scale, aiming to entrap PFAS and prevent their leaching from soil into groundwater. Monitoring of the site revealed the presence of two potential sources of PFAS contamination at concentrations up to 23 μg L −1 for ∑ 11 PFAS in groundwater. After CAC application, initial results indicated a 76% reduction of ∑ 11 PFAS and high removal rates for long-chain PFAS, such as perfluorooctane sulfonic acid and perfluorooctanoic acid. A spike in concentrations was noticed 6 months after injection of CAC, showing a rebound of the plume and a reduction of treatment effectiveness. Based on long-term monitoring data, the treatment effectiveness for ∑ 11 PFAS dropped to 52%.The rebound of concentrations was attributed to the plume bypass of the barrier due to the presence of high conductivity zones, which likely occurred because of seasonal changes in groundwater flow directions or the CAC application at the site. This demonstrates the need for a detailed and accurate hydrogeological understanding of contaminated sites before designing and applying stabilization techniques, especially at sites with high geologic and hydrologic complexity. The results herein can serve as a guideline for treating similar sites and help avoid potential pitfalls of remedial efforts.

Analysis of colloidal activated carbon alternatives for in situ remediation of a large PFAS plume and source area

Carey,

Anderson,

Van Geel

et al. 2023

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This study evaluated optimal locations for in situ remediation of per‐ and poly‐fluoroalkyl substances (PFAS) in groundwater with colloidal activated carbon (CAC). New Freundlich isotherms for PFAS adsorption to CAC were estimated to illustrate the effect of competitive adsorption with dissolved organic carbon and other PFAS in a groundwater sample. A hypothetical model scenario was constructed based on source area characteristics similar to a site impacted by aqueous film forming foam in South Dakota. Modeling indicates that, even with high PFAS concentrations, CAC would still be capable of maintaining concentrations below proposed maximum contaminant levels in the adsorption zone for at least 30–40 years. Two‐dimensional areal modeling indicates that the future breakthrough of PFAS is likely to occur in the localized core of the plume, and that the corresponding future reinjection of CAC will only need to be conducted over a smaller portion of the original injection footprint. The benefits of implementing a phased remedial approach at PFAS sites are discussed. Source area and mid‐plume CAC treatments are shown to be ineffective at attenuating PFAS concentrations at the downgradient property boundary within a reasonable timeframe when PFAS travel time is relatively slow. Among the CAC alternatives evaluated here, a downgradient CAC permeable reactive barrier has the best performance with respect to protecting downgradient receptors.