Jun Ma (jxm140@po.cwru.edu)

and

Aaron A. Jennings, Professor (aaj2@po.cwru.edu)

Department of Civil Engineering

Case Western Reserve University

Cleveland, OH 44106-7201

Lead is a very common heavy metal contaminant often encountered in Superfund sites and Brownfields. When lead is sorbed be soil, most of the lead remains on the aggregate surface. However, some fraction of the lead continues to migrate into the aggregate and forms what is known as "sequestered lead". There are several alternative mechanisms by which lead can migrate within the soil aggregate, but all of these are dominated by diffusion limitations. Most remediation methods can remove lead from the surface of contaminated soil very effectively, but fail to remove sequestered lead due to slow diffusive mass transfer rates. Previous aggregate remediation modeling has indicated that diffusion-driven mass removal rates are proportional to the square of average aggregate size. However, these rates can be accelerated by the use of Electrokinetic remediation.

The impact of sequestered lead is illustrated using a lead-contaminated residential soil from the greater Cleveland area. The average grain size for this soil is approximately 0.3 mm. Based on simulations using a homogeneous distribution of sequestered lead it would take about 3 days to remove 90% of the lead from this average grain size. When these same calculations are repeated for a grain size of 0.7 mm, the numbers increase to 16.7 days for 90% removal and 48.5 % removal after 2 days. This demonstrates the need for a remediation method that will accelerate the rate of aggregate remediation.

Electrokinetic technology has recently emerged as an innovative in-situ remediation method. It involves using low-level direct current to accelerate remediation processes by mechanisms such as electromobility. However, in order to analyze the potential benefits of this process, one must be able to quantify the fraction of lead that is actually sequestered, and to quantify the required electromobility constants.

In an effort to develop an improved method of quantifying sequestered contamination, a series of Hydrochloric acid extractions were conducted on samples of a residential soil. Extractions were performed on unaltered samples, and on samples that had been pulverized to reduce the average size by a factor of 10. If the grain size is small enough, it is believed that diffusion is adequate to remove almost all of sequestered lead in two days. The difference between extraction result for pulverized and un-pulverized soils may then be used to determine the amount of sequestered lead. The mass of sequestered lead in this soil was about 12% of the total lead burden. Remediation methods targeting surface adsorbed lead would accomplish less than 90% treatment. Given time, the sequestered lead could diffuse from inside the aggregate and again contaminate the surface.