Effect of Biofilm Formation on the Sorption of Per- and Polyfluoroalkyl Substances to Colloidal Activated Carbon

Abstract

Per- and polyfluoroalkyl substances (PFAS) are a class of contaminants that have garnered increasing concern due to their widespread presence and harmful effects on humans and ecosystems. PFAS enter the environment via many different pathways, with the release of PFAS-containing aqueous firefighting foams being a major source of groundwater contamination. Because PFAS are highly resistant to most chemical and biological degradation processes, they are currently removed from groundwater mainly by ex-situ adsorption, which is expensive and energy intensive.

Recently, activated carbon (AC) permeable reactive barriers (PRBs) have been proposed and used in-situ to limit the downgradient migration of PFAS by groundwater. AC PRBs are created by injecting powdered activated carbon (PAC) or colloidal activated carbon (CAC) into the subsurface to generate a stationary zone that removes PFAS by adsorption. As with any adsorption technology, however, PFAS breakthrough will occur once adsorptive sites in the barrier are exhausted. To improve our understanding of the ability of AC PRBs to adsorb PFAS and their longevity, there is a need for research that evaluates the adsorption of PFAS on AC and the factors affecting this process. The research reported in this thesis focused on one potential influencing factor, namely biofilm. Specifically, the objectives of this study were first, to evaluate if a biofilm can form on small (<5 µm) CAC particles, and second, to examine the impact that biofilm may have on the adsorption of PFAS to CAC.

To address the first objective, the growth of Pseudomonas putida (P.putida), an aerobic bacterium, in the absence of particulate and in the presence of either CAC or fine silica was investigated. P.putida was selected because it has been shown to readily form a biofilm, is not infectious to humans, is commonly found in the environment, and has applications in the bioremediation of organic contaminants. Analyses of the bacterial samples by confocal laser scanning microscopy (CLSM) indicated that the bacteria remained planktonic when no particulate was present but formed a biofilm consisting of cells and CAC or sand particles held together by extracellular polymeric substances (EPS). Over seven days of growth, the biofilm formed on CAC increased in thickness and decreased in roughness as it developed and formed more cohesive structures. Results suggest that P.putida is capable of forming a biofilm on CAC particles. Rather than the classical depiction of a biofilm adhered to a single surface, the P.putida biofilm was formed on an aggregate of CAC particles, which were held together by EPS.

To address the second objective, the adsorption of perfluoroctane sulfonate (PFOS, a hydrophobic PFAS) and perfluoropentane carboxylate (PFPeA, a hydrophilic PFAS) on virgin and biofilm-coated CAC was investigated. P.putida was grown in the presence of CAC, and either PFOS or PFPeA was added to the microcosms once a biofilm was formed. Because the adsorption of PFAS to CAC is known to be impacted by the presence of dissolved organic carbon (DOC), experiments were also conducted to determine the impact of broth (used for culture growth) concentration on the extent of PFAS sorption to CAC and the development of the biofilm.

In the experiments without bacteria, the amount of PFOS adsorbed to CAC decreased as the concentration of broth was increased. The relationship between aqueous and sorbed PFOS could not be described by a linear, Freundlich, or Langmuir isotherm model, likely due to competitive sorption between the DOC present in the broth and PFOS. In the experiments with P.putida, it was observed that as the broth concentration increased, the biofilm became thicker and smoother, as the additional broth appeared to have aided biofilm development. Subsequent experiments, conducted with 3 mg/L broth and 80 mg/L broth (which represented high and low DOC concentrations, respectively), revealed that the majority of PFOS sorption on virgin and biofilm-coated CAC occurred during the first three days, and the biofilm resulted in a decrease in PFOS adsorption. This decreased adsorption is presumed to be due to biofilm blocking sorption sites. For PFPeA, limited sorption occurred, and no significant difference was observed between the amount adsorbed in the bacteria-free CAC and P.putida-containing CAC systems. The difference in sorption between PFOS and PFPeA was attributed to decreased hydrophobic interactions between CAC and the shorter fluorinated tail of PFPeA.

The results of this study improve our understanding of how biofilm may impact CAC PRBs implemented for the management of PFAS. Biofilm can form on cell-sized particles and, as a result, may reduce the adsorption of long-chain compounds, such as PFOS. The effect of biofilm on the adsorption of short-chain compounds, such as PFPeA, may be less prominent than for PFOS, as the extent of sorption is comparatively limited. Further investigation is required to evaluate the impacts of biofilm on CAC sorption of other PFAS, the interactions of biofilm with other groundwater parameters, and the extent to which biofilm plays a role in the longevity of CAC PRBs in column or field scale studies.