The International Information Center for Geotechnical Engineers

# Permeable Reactive Barriers - GAC Laboratory and Modeling Case Study

GAC LABORATORY AND MODELING

This is a summary of a 2D model study done by F. Di Natale: (Di Natale et al., 2008)

Background and Site Description

A 2D numerical model was created to assess the applicability of using a granular activated carbon (GAC) PRB for the removal of cadmium (Cd(II)) from contaminated groundwater. Laboratory tests were first performed to determine the adsorption characteristics between GAC and Cd(II). This is because “the concentration of ionic species in solution is the driving force of adsorption.” (Di Natale et al., 2008) A series of lab tests were performed and led to an equation relating the adsorption capacity of GAC to concentrations of cadmium, pH, and sodium.

The 2D model used the equation from the laboratory tests and modeled a case study involving a topsoil layer, located near a riverbank, contaminated with Cd(II).  A geometric representation of the model of the site is shown in the figure below.

A figure demonstrating the cross-sectional flow through the barrier

Parameters necessary for the model were established, and can be found in the article. Some characteristics included aquifer bed depth, porosity, hydraulic conductivity, topsoil depth and average Cd concentration. The weather was modeled by assuming several intense rainfall periods, that greatly increase the cadmium concentrations in the aquifer to dangerous levels.

PRB Design and computer model

The barrier was designed as a continuous trench, shown in the above figure. The barrier width was determined using an inequality relating groundwater velocity, mass transfer coefficient, and external specific surface of the adsorbent. The width and adsorbent specific surface area must be wide enough for adsorption to occur. This equation can be found in the article.

A first order finite difference implicit scheme was integrated numerically by Fortran and using the SEEP™ code to model the movement of Cd due to advection–dispersion processes. The exact equation can be found in the article.

Results

The simulation results showed that the PRB completely remediated the Cd from the groundwater for the first three months. After the first three months the barrier began to saturate, but sustained an outflow concentration of Cd below 0.005 mg/L for over 7 months. The figure below shows the concentrations of Cd for a 7 month time period.

A figure demonstration the contaminant level over a 7 month period.

The simulation results also showed a peak Cd concentration at the center of the PRB, which the author concluded was due to Cd desorption and resorption when clean groundwater flowed through.

Discussion and Conclusion

The results of the computer model simulation provided insight to the usage of a GAC PRB for cadmium removal. The tendency for the Cd to desorb when exposed to clean incoming groundwater demonstrated that the PRB is self-cleaning. However outflowing Cd concentrations may be present due to the desorption process. The author suggests that the PRB should be utilized to dampen the concentration peaks of a contaminant, as opposed to a full removal technology. With a properly designed barrier the author believes that the PRB can be used to assure compliance with safety regulations and reduce hazardous pollution because the Cd at its highest remained well below the maximum contaminant level.