SERDP & ESTCP Corner: Headlines from the Environmental Restoration Program Area

Abstract

In Situ Powdered Activated Carbon Amendments May Limit Contaminant Availability for Biodegradation The use of adsorbent media for in situ sequestration of chemicals of concern in groundwater has increased in recent years. Many technology vendors promote the use of adsorbent amendments such as powdered activated carbon (PAC) or colloidal activated carbon (CAC) in situ to sequester chemicals of concern in groundwater and minimize downgradient transport. Sorption to PAC and CAC are well documented; however, many issues regarding the application of amendments in situ are not well understood. In particular, the interactions between sorbent media and subsurface microbiota are largely unknown. Some technology vendors claim that biodegradation is enhanced with the use of amendments, especially when slowrelease nutrients are included as part of the amendment media, but the validity of those claims has not yet been conclusively demonstrated. It is generally believed that chemicals in groundwater need to be in the aqueous phase to be available for biodegradation. Previous studies have reported decreased biodegradation due to decreased chemical bioavailability for sorbed constituents that are typically able to be biodegraded, including petroleum hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), and pesticides. Therefore, it is possible that readily biodegradable compounds may be less available for biodegradation when sorbed to PAC regardless of nutrient availability. In addition, bacterial growth and biofilm formation on granular activated carbon (GAC) is a known problem in the water treatment industry. Bacterial growth can decrease GAC efficacy for chemical adsorption because bacterial growth can reduce the sorption sites available for chemicals. Microbial growth on PAC similarly has the potential to reduce the effective surface area available for chemical sorption. For chemicals such as perfluorooctane sulfonate and perfluorooctanoic acid (PFOA), where sorption is the predominant mechanism for the efficacy of in situ PAC, competition from bacterial growth may be significant. A recently completed SERDP project titled, “Biological Factors Influencing Sorption and Biodegradation of Chemicals of Concern on Particulate/Colloidal Activated Carbon (ER211059),” investigated the interactions between microbiota and PAC. The project team, led by Principal Investigator Dr. Albert Juhasz at the University of South Australia, aimed to answer two main questions: one, are sorbed contaminants available for biodegradation? Two, do bacteria sorbed onto PAC have an impact on sorptive capacity? The team investigated these two questions by constructing flow cells containing model aquifer materials and PAC. The team first performed sorptiondesorptionbiodegradation experiments using phenanthrene, a low molecular weight PAH that is readily biodegraded. The project team added phenanthrene and a Clabeled phenanthrene surrogate to the flow cells until the sorption capacity of the PAC in the model aquifer material was reached. Then, ultrapure water was added to the flow cells to desorb any phenanthrene that could be readily desorbed from the system, leaving behind only irreversibly sorbed phenanthrene. Finally, a known phenanthrenedegrading microbial consortium was added to the flow cells. No biodegradation was observed in the flow cells, supporting the hypothesis that phenanthrene sorbed to PAC was unavailable for biodegradation. The project team used scanning electron microscopy analysis on the PAC surface to confirm that phenanthrenedegrading microbes were prevalent on the PAC surface. Bacterial content of the PAC was not a limiting factor for mineralization, providing further evidence that sorbed constituents are not readily bioavailable. To assess whether bacterial growth/ biofilm formation decreases PAC sorption capacity, similar experiments were conducted in flow cells using PACamended model aquifer materials and PFOA. The project team did not find that bacterial growth limited PAC sorption capacity for PFOA during the period of the study. However, they concluded that a longer incubation time may be necessary to fully investigate this phenomenon. Additional detail about the project’s methods, results, and conclusions can be found on the project webpage at serdp-estcp.org where the project’s Final Report was published on May 11, 2023.