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PAH Biodegradation in Sediment Cap Environments: Evaluating how Performance Varies with Materials and Redox Zones
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Reactive caps containing sorptive materials have been shown to effectively sequester contaminants; however, the limitations of sorptive capacity must be considered in the design. At many sites, the majority of the contaminant mass sequestered via adsorption from dissolved phase pore water flux is expected to consist of more soluble compounds, which are likely to be biodegraded to some extent under the right conditions. Biological natural attenuation within sediment caps, oxidizing contaminants in situ, represents a design enhancement opportunity. Implementing the design of bioactive caps can support and stimulate the native biodegradation potential of benthic microbial communities, ultimately improving the effectiveness of the remedy.

This work investigated the performance of capping materials in systems mimicking bioactive sediment caps. We examined three conditions representative of a stratified sediment environment: i) the cap-water interface (aerobic), ii) the transition zone, characterized by interlayered electron acceptors (nitrate, iron, and sulfate), and iii) strictly anaerobic, sulfate-reducing conditions. Specifically, this work aimed to: i) compare biodegradation kinetics in the presence of different capping materials; ii) analyze changes in microbial community composition; iii) correlate capping amendment to biological activity; iv) provide recommendations to design bioactive caps.


Microbial communities were enriched from sediments collected from a river adjacent to a former manufactured gas plant. Microcosms were prepared with the sediment enrichments, common capping materials (sand, organoclay, and activated carbon), mineral media mimicking a freshwater system, and electron acceptors characterizing the sediment-cap environment. Separate experiments were performed, and different redox conditions were tested (oxygen, nitrate, sulfate, and iron). Experimental systems were periodically analyzed for aqueous concentrations of naphthalene, a model PAH, mineralization (under sulfate-reducing conditions), biomarker abundance (under aerobic conditions), and 16S RNA gene surveys.

Results/Lessons Learned. 

Results showed that selection of capping material significantly impacts microbial communities at the capping water interface (PERMANOVA p<0.05). In aerobic systems, activated carbon (AC) amendments are linked to high biological activity, enrichment of biodegraders, and early biofilm growth; in contrast, organoclay strongly affected the behavior of microbial communities, but did not enhance removal performance.
Multivariate analysis showed that electron acceptors and capping materials both affected microbial community shifts when transitioning to anaerobic conditions (PERMANOVA p<0.05). Naphthalene biodegraders were enriched, and positive correlations (Spearman's p<0.05) indicated the formation of biodegrading consortia that varied with capping material type and electron accepting conditions.

Under sulfate-reducing conditions, naphthalene biodegradation was significant in presence of AC (ANOVA, p<0.05). AC stimulated naphthalene biodegradation and mineralization, which significantly correlated (Spearman's, p<0.05) with the enrichment of naphthalene biodegraders (Geobacter and Desulfovirga).

This study demonstrates that sediment caps can enrich PAH-degrading microorganisms under different thermodynamic conditions, and appropriate selection of capping materials can improve biodegradation.

These results suggest that incorporation of elements favorable to in-situ biodegradation in reactive cap design have the potential to prolong cap life or reduce the amount of amendment required to achieve the design objectives.

Publication Summary

  • Geosyntec Authors: Giovanna Pagnozzi
  • All Authors: Giovanna Pagnozzi, (Geosyntec Consultants, Seattle, WA, USA), Kayleigh Millerick, and Danny Reible, (Texas Tech University, Lubbock, TX, USA), Sean Carroll, (Haley & Aldrich, Inc., Rocky Hill, CT, USA)
  • Title: 2023 Battelle Sediments Conference
  • Event or Publication: Event
  • Practice Areas: Sediment assessment and remediation, Contaminated site assessment and clean up
  • Citation: Battelle's International Conference on the Remediation and Management of Contaminated Sediments at the JW Marriott in Austin, Texas, on January 9 through 12, 2023
  • Date: January 9 through 12, 2023
  • Location: JW Marriott in Austin, Texas
  • Publication Type: Platform Presentation