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Sorption of PFOA and PFOS to Ground Water Sediment
Citation:
Ferres, M. L., C. J. ADAIR, AND J. T. WILSON. Sorption of PFOA and PFOS to Ground Water Sediment. Presented at The PFAA Days II Workshop, RTP, NC, June 03, 2008.
Impact/Purpose:
Presentation on PFOA and PFOS to Ground Water Sediments
Description:
During its years of operation, the Washington County Sanitary Landfill near St. Paul, Minnesota accepted both municipal and industrial solid waste. Several years of ground water monitoring performed by the MPCA indicates that, some of the waste disposed of at this landfill contained PFOA. The PFOA has leached into the ground water and moved with the ground-water flow. It has also moved deeper, affecting the bedrock aquifer where it was found at low levels. As part of a risk evaluation, a microcosm study was performed to predict transport and fate of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonate (PFOS) in leachate from the landfill. Realistic concentrations of PFOA and PFOS were added to microcosms constructed with sediment that was collected from beneath the water table at the Washington County Landfill. Microcosms were then sealed and incubated in the laboratory. Three microcosms of each treatment were sacrificed at quarterly intervals for analysis. Aqueous concentrations of PFOA and PFOS increased in the microcosms over the incubation period. Shortly after the addition of PFOS and PFOA, the adsorption constant, Kd, averaged 0.9748 L Kg-1 for PFOA and 1.1503 L Kg-1 for PFOS. At 574 days, the Kd averaged 0.0690 L Kg-1 and 0.1973 L Kg-1 for PFOA and PFOS, respectively. Linear regression of the data generated slopes of -0.002 L Kg-1day-1 for PFOA and -0.0014 L Kg-1day-1 for PFOS. Corresponding retardation constants were 2.3 and 10.2 for PFOA and PFOS at the beginning of the study, which decreased to 1.55 and 2.58, respectively, after 574 days. The fraction organic carbon in the sediments was 0.034%. The Koc after 574 days of incubation was 203 L Kg-1 and 580 L Kg-1 for PFOA and PFOS, respectively. Higgins and Luthy (ES&T 40: 7251-7256, 2006) determined values of Koc for PFOA and PFOS for freshwater sediments of 130 L Kg-1 and 480 L Kg-1 for PFOA and PFOS, respectively. After 574 days of incubation, there was good agreement between Koc for sediment and Koc for aquifer material. At the concentrations of organic material found in water supply aquifers, both PFOA and PFOS should be highly mobile. The change in the extent of sorption was not expected. The decrease in the adsorptive properties of PFOA and PFOS observed in this study may be due to changing redox conditions over time in the microcosms. It can be rationalized as follows: The sediment as collected was impacted with leachate, but had a red color, indicating the presence of iron(III) minerals. The PFOA and PFOS may have initially sorbed to the iron(III) minerals, and then were released back into pore water as the iron(III) minerals were consumed or modified by iron reducing bacteria. No evidence of degradation of PFOA or PFOS was observed.