Contaminated Sediment Remedial Designs are Potentially Adversely Impacted by Improper Application of an ASTM Standard Method for Measuring Activated Carbon Performance

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AquaBlok Review Indicates Orders-of-Magnitude Negative Field Performance as Compared to Modeled Remedy Performance May Result.

Powdered Performance GAC vs PAC

This review should focus on ensuring that the data collected provides a reliable indication of the expected performance of the activated carbon amendment in the full-scale remediation of these important contaminated sediment sites.

AquaBlok Ltd has a strong focus on improving materials and performance of remedies for contaminated sediments. Activated carbon is a proven amendment that is being increasingly deployed for remediating sediments contaminated with hydrophobic organic compounds (HOCs). The effectiveness of activated carbon relies on the carbon’s affinity for the target contaminants (e.g. PAHs, PCBs, and PFAS compounds). This affinity is often measured in terms of both the adsorption capacity (i.e. the equilibrium partition coefficient) as well as the speed of adsorption (i.e. the kinetics) for the selected HOCs. However, the generally accepted ASTM standard method for determining the equilibrium partition coefficient is not suited to a contaminated sediment setting and therefore introduces a bias that has the potential to have substantial adverse impact on remedy performance when granular activated carbon (GAC) is applied.

ASTM Bench-Scale Method Bias: Legacy Influence of Testing for Water Treatment

The approach commonly utilized for determining the partition coefficients for GAC introduces a bias that is often overlooked despite its significance. The ASTM Method D5919-96 is a standard often used for determining partition coefficients of activated carbon on a lab or bench-scale basis for water treatment applications. For water treatment, small scale column tests (i.e. RSSCT) are utilized to rapidly determine the relative performance of activated carbon. To accurately perform these tests, it is important that the lab columns retain the proportionality between the size of the vessels and the activated carbon particle size (ASTM D6586-03). This requires that the activated carbon used for testing has a very small particle size. Therefore, to determine the partition coefficient the standard recommends GAC is pulverized such that >90% of the activated carbon is 45-µm or smaller (i.e. 325-mesh or smaller). This step of the method effectively transforms the GAC into powder activated carbon (PAC). Although this method has been proven to be effective for application of activated carbon in water/wastewater treatment, it is not generally suitable for evaluating GAC when applied to active/amended capping of contaminated sediments.

Impacts of ASTM Method Bias on Expected Sediment Remedy Performance

A common practice of the remedial design engineer is to incorporate a GAC partition coefficient obtained from this ASTM method into a model of remedy performance, even though it is generally known and understood that a powdered form of activated carbon will provide significantly greater adsorption capacity than a granular form. Therefore, the use of partition coefficients determined based on powdered rather than granular sized materials over-estimates the performance of GAC in modeling and design for both kinetics and capacity. A second equally significant issue is that the speed of adsorption is typically not factored into the design and modeling of amended caps in contaminated sediments applications. The partition coefficient is an equilibrium measure of capacity and does not incorporate the rate or speed of adsorption. Therefore, contaminant transport mechanisms (such as groundwater up-welling) become critical to understanding whether the selected activated carbon remedy (GAC vs. PAC) has the capability to adsorb the contaminants at a rate equal to or exceeding the speed at which they may be moving through the amended cap.

It has been well documented by several academic and industry experts that PAC provides orders-of-magnitude better performance than GAC with respect to both total adsorptive capacity and kinetics. Yet a review of recent remedial designs reveals that there is a common assumption that a GAC-based approach will possess equal performance to a PAC-based approach. In addition, this assumption also appears in design inputs for modeling with respect to GAC performance in the design of amended/active caps. The potential effect is that the quantity of GAC applied may be far too low to provide the expected performance of the remedial design.

A number of high-profile contaminated sediment sites are planning to incorporate the use of activated carbon to improve the performance of the remedy. Where GAC is under consideration at these sites, it is recommended that any existing data or test/pilot programs planned should be carefully reviewed. This review should focus on ensuring that the data collected provides a reliable indication of the expected performance of the activated carbon amendment in the full-scale remediation of these important contaminated sediment sites.

About AquaBlok
AquaBlok, Ltd. provides a range of materials that address a variety of sealing and contaminated sediment remediation applications. Technical advantages include uniform delivery of high-value amendment materials at low concentrations and a higher rate of adsorption than granular materials. Both in-situ treatment and low-permeability products are supplied under the AquaGate® and AquaBlok® names, respectively. More information on this topic can be found in the News section at http://www.aquablok.com.

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Pooja Patel

John Collins
AquaBlok LLC
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