Theme: 01. Remediation Technology Innovations

Topic Area: 01j. Injectable Activated Carbon Amendments: Lessons Learned and Best Practices

Formatted Title:

Laboratory Assessment of Injectable Activated Carbon on Biological Reductive Dechlorination of Chlorinated Ethenes

Background/Objectives:

The project investigates the fundamental mechanisms and processes involved in an increasingly used in situ groundwater remediation approach for chlorinated solvents, which combines sorption by activated carbon with biodegradation. Currently, there is no independent evaluation that substantiates the claim that the presence of activated carbon can promote biodegradation. This SERDP project aims to address this fundamental question by investigating the extent and rate of biodegradation affected by amendment, long-term sorption capacity, and contaminant re-release potential. The primary focus will be on characterizing and quantifying the dynamic interactions between adsorption and biodegradation, which are the core processes inherent to the technology but have not been rigorously examined for assessing its long-term effectiveness for in situ groundwater remediation.

Approach/Activities:

The research was conducted at laboratory scale, using experimental systems that have been previously applied to study contaminant sorption and sorbent-based biofilms separately. The team acquired five different types of ACs, including commercial colloidal AC products and FiltraSorb 400 that has been grinded to powder size and milled to colloidal size. The AC products were characterized for surface area, porosity, contact angle, and elemental composition. TCE sorption to these ACs were investigated. The commercial CAC with the polymer stabilizer was incubated with sand and KB-1®, a dechlorinating culture, at four different doses: 0, 200, 2,000, and 4,000 mg CAC/kg sand. Parent and dechlorination products were monitored in both aqueous phase and by total extraction of the sacrificial reactors such that the sorbed CVOCs can also be monitored. Additionally, microbial dechlorination biomarkers were monitored during the incubation with less frequency. The batch test is currently being finished and the samples will be imaged by scanning electron microscopy to investigate biomass and potential biofilm formation. Additionally, column tests are being conducted to evaluate CVOC biodegradation under flow through conditions as well as the migration of CAC and microbes.

Results/Lessons Learned:

The research performed to date showed that AC affected microbial reductive dechlorination primarily by decreasing aqueous-phase CVOC bioavailability via sorption. The results showed a lack of dechlorination but enhanced methanogenesis in the presence GAC. Biomass appears to attach to the GAC surface and form biofilm, despite the lack of dechlorination. The sorption results showed the presence of polymer stabilizer decreased TCE sorption affinity to CAC but decreasing particle size from granular to colloidal range did not alter TCE sorption affinity. The batch microcosm results showed that increasing CAC loading in the microcosms decreased aqueous TCE concentrations, resulting in further limited reductive dechlorination of TCE. However, the reductive dechlorination biomarkers showed increasing abundance of one to two orders of magnitude over incubation in all biotreatment with CAC, which was not observed in the biotreatment without CAC. The contrast between reductive dechlorination and biomarker abundance among CAC treatments may have some physiological implications that remain to be further investigated. Additional results from the column test are expected to be available in Spring 2024.

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Birthe Kjellerup