The objective of this proof-of-concept project was to gain an understanding of processes influencing the effectiveness of particulate amendments for in situ treatment of chemicals of concern in groundwater. Specifically, the project assessed whether (i) chemicals sorbed to commercially-available particulate activated carbon (PAC) were bioavailable for biodegradation and (ii) whether bacteria sorbed to PAC influenced sorptive capacity.
14C-Phenanthrene was used as a model compound to study biodegradation following sorption to PAC, while 14C-perfluorooctanoic acid (PFOA) was used to determine the influence of PAC-bacterial adhesion on chemical sorption due to its limited biodegradation potential. A continuous flow method (flow cell) was adopted to simulate chemical transport and reactivity in model aquifers (loam-sand) to which PAC was amended. 14C-Labeled compounds were introduced into PAC-amended model aquifers with the effluent monitored for the determination of breakthrough and sorption capacity calculations. Bacterial influences (biodegradation of sorbed chemicals or decrease in chemical sorption capacity resulting from bacteria adhesion) was determined by introducing microorganisms into sterile PAC-amended aquifers pre- or post 14C-compound addition. In addition, matrix-PAC-bacterial interactions were visualized using environmental scanning electron microscopy (ESEM) and/or time-of-flight secondary ion mass spectrometry to provide complementary data regarding the distribution of PAC within the model aquifer and where/how bacteria were interacting with PAC, aquifer matrix, or chemicals of concern.
14C-Phenanthrene sorbed to the model aquifer material was readily mineralized by phenanthrene-degrading microorganisms. Although the model aquifer material had low capacity to sorb phenanthrene, the desorbable nature of phenanthrene on this material rendered it available for biodegradation. PAC had a significantly greater capacity to sorb phenanthrene; however, PAC-sorbed phenanthrene had limited potential to desorb under flow cell conditions when the influent solution was modified. Introduction of phenanthrene-degrading microorganisms into flow cells containing model aquifer material and phenanthrene-sorbed PAC resulted in limited phenanthrene mineralization, irrespective of phenanthrene loading onto the PAC. Assessment of PAC using ESEM identified that phenanthrene-degrading microorganisms were prevalent on PAC, illustrating that PAC-bacterial contact was not a limiting factor for the lack of phenanthrene mineralization. Although bacterial adhesion to PAC resulted in PFOA-PAC sorption variability (decreasing sorption with increasing biomass), there was no significant difference (p > 0.05) in maximum PFOA sorption when 104 -109 cells per ml were introduced into the flow cell.
Particulate amendments are increasingly being used to mitigate impacted groundwater impacts; however, a complete understanding of processes influencing their remediation efficacy is lacking. Understanding parameters influencing biodegradation and sorption will provide critical information regarding factors influencing the attenuation efficacy of particulate amendments. Research outlined in this project provided an insight into bioavailability constraints that may limit contaminant biodegradation following sorption to PAC. While bacterial sorption to PAC did not result in a reduction in sorption capacity, the timeframes utilized were too short for biofilm formation and growth which may, conceivably, influence chemical retention on PAC. These dynamic processes may be investigated in future endeavors. (Project Completion - 2023)