Contaminated Sediment Remedial Designs are Potentially Adversely Impacted by the Improper Use and Application of the ASTM Standard Method for Activated Carbon Performance

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

AquaBlok Ltd, has a strong focus on improving materials and performance of remedies for contaminated sediment. 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.

Importance and Difference in Adsorption Capacity and Kinetics for PAC vs GAC

The conventional and industry-accepted approach for determining activated carbon’s capacity is by developing adsorption isotherms using site-specific contaminants at concentrations similar to site conditions. The adsorption isotherm provides the equilibrium partition coefficients which reveal the maximum amount of contaminant a given quantity/weight of the carbon will remove. Further, when performed correctly, this provides an estimate of the total amount of contamination that the activated carbon-amended remedy may remove over its lifetime. Models, such as CAPSIM, often use partition coefficients to determine the relative ‘life’ or time to ‘break-through’ of a cap or layer amended with activated carbon. Powder activated carbon (PAC) has been demonstrated to have a higher adsorption capacity than GAC because the small particle size of PAC overcomes mass-transport limitations imposed by the activated carbon particle size. Therefore, remedies utilizing PAC would require less quantity of carbon to remediate a given amount of contaminant than when GAC is utilized.

Adsorption kinetics refers to how quickly the activated carbon removes HOCs from solution. The kinetic rate is typically performed by jar tests with fixed amounts of activated carbon and a certain concentration of the contaminant. The amended jars are sampled at different time intervals to establish a kinetic adsorption profile. A comparison of the kinetic profiles reveals which source (of carbon) and particle size of activated carbon will capture the contaminant at a faster rate, which determines the relative required residence time for the amended cap to be effective. A cap amended with powdered activated carbon (PAC) will require a much lower thickness than a cap amended with granular activated carbon (GAC) for the same concentration and flow rate due to the higher kinetic rates observed for PAC. This reduction in thickness and material quantity can amount to sizeable cost savings, particularly when pre-dredging is required to accommodate the treatment media.

While kinetics are important the industry has been primarily focused on equilibrium partition coefficient as the measure of performance incorporated into remedial designs 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 Activated Carbon for Water Treatment

Despite its significance, the approach commonly utilized for determining the partition coefficients for GAC introduces a bias that is often overlooked. The ASTM Method D5919-96 is a standard often used for determining partition coefficients of activated carbon on a lab or bench-scale basis. However, because this method was created primarily for application of activated carbon in water/wastewater treatment, it is not generally suitable for appraising GAC when applied for contaminated sediment remediation. In the water treatment industry, small scale column tests are utilized to rapidly appraise the relative performance of activated carbon. For 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 is reduced in particle size. In determining the partition coefficients, 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 PAC.

Impacts of ASTM Method Bias on Expected Remedy Performance

It is common practice for the remedial design engineers to incorporate the partition coefficients obtained from this ASTM method into modeling 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 adsorption 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 being an equilibrium measure of capacity does not incorporate the rate 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 many remedial designs revealed that where GAC-based approaches are specified, they are typically expected to possess equal performance to a PAC-based approach.

Several high-profile contaminated sediment sites are planning to incorporate the use of activated carbon to improve the performance of the remedy. It is recommended that any test/pilot programs or existing data should be carefully reviewed where GAC is under consideration at these sites. 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.