Formatted Title:

Activated Carbon Cap Amendments: Example of a Site-Specific Sorption Study to Reduce Uncertainty in Cap Design

Background/Objectives:

Design activities were conducted to evaluate a capping remedy to address contaminated sediments in a river adjacent to a former manufactured gas plant located in the northeastern United States. Initially, the main contaminants of concern were polycyclic aromatic hydrocarbons (PAHs). Upon review of groundwater data from monitoring wells along the shore of the river, elevated concentrations of benzene, toluene, ethylbenzene, and xylene (BTEX) were also identified. Initial modeling evaluations indicated the BTEX compounds became the cap design drivers, and a sorptive amendment would be needed in the cap to mitigate the flux of BTEX from the contaminated sediments into surface water.

Along with contaminant concentrations and groundwater seepage rates, information on the sorptive capacity of cap materials, including amendments, is key to designing an effective sediment cap. Adsorption of organic compounds on heterogeneous adsorbents, such as activated carbon, is often described using a Freundlich isotherm due to the nonlinearity of the relationship with concentration. For some contaminants, such as PAHs, Freundlich isotherm parameters reported in literature are fairly consistent and can often be reliably used for cap design. For other compounds, Freundlich isotherm parameters reported in the literature may be limited (or nonexistent) and can span a large range, making it difficult to rely on them; this was the case for BTEX. This limitation can add a significant source of uncertainty to amended sediment cap designs. As a result, cap designs based on such limited literature information can be jeopardized by: 1) use of conservative values that may result in an overly robust (or even infeasible) design; or 2) use of average values that may result in an ineffective cap design. For cases in which published data on amendment sorption properties for a given contaminant are lacking or highly variable, site-specific studies can greatly improve and optimize the cap design.

Approach/Activities:

Because published Freundlich isotherm data for BTEX were found to vary greatly, initial cap design modeling for the site was conducted for the full range of Freundlich isotherm parameters identified from the literature. This resulted in a large range of initial cap design configuration, ranging from a very thick cap with a large quantity of activated carbon (which would not be feasible to construct at the site) for the most conservative assumption to a much more modest cap design at the other end of the literature range.

Based on the cap’s chemical isolation layer design configuration being highly sensitive to the assumed BTEX sorption parameters, a site-specific adsorption isotherm study was conducted. Groundwater collected from the site was used in laboratory tests to identify an amendment that would effectively adsorb BTEX compounds. Freundlich isotherm parameters for BTEX in site groundwater were measured for three candidate amendment materials as part of this study, including two granular activated carbon and one powdered activated carbon. The resulting site-specific sorption parameters were then used to develop a refined cap design configuration for the site.

Results/Lessons Learned:

With the use of site-specific adsorption parameters derived from the laboratory isotherm tests in the cap modeling, the amendment content was significantly reduced relative to the initial conservative modeling based on literature parameters. In this presentation, the cap design based on the site-specific isotherm parameters will be compared to the initial designs derived from the range of literature-reported Freundlich isotherm parameters, including the most conservative, least conservative, and mid-range, to illustrate the importance of site-specific data when literature values are highly variable.

Masakazu Kanematsu

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Activated Carbon Cap Amendments: Example of a Site-Specific Sorption Study to Reduce Uncertainty in Cap Design