Abstract

This report summarizes the findings of an extensive treatability study of stabilization technologies for mercury immobilization on materials collected from the Sulfur Bank Mercury Mine (SBMM), located north of San Francisco, in Lake County, California. The SBMM site is believed to be contaminating the adjacent Clear Lake environment with mercury derived from historic mining practices at the site. The study was conducted as a joint effort between EPA's Superfund Innovative Technology Evaluation Program and the Mine Waste Technology Program. Two mercury-contaminated materials were selected for treatment by three stabilization technologies.

The purpose of the study was to determine the effectiveness of the three stabilization technologies for immobilizing mercury in the waste rock materials and, therefore, reducing leachable mobile mercury in the effluent. Several mercury-bearing materials from the site were considered for testing. A material with high levels of leachable mercury was selected as the primary target of the study and is referred to as "mercury ore." As a secondary objective, treatment effectiveness was evaluated on material present in large quantities but whose mercury concentration was lower; this material is referred to as "waste rock."

The three stabilization technologies evaluated as part of this study were:

• A silica micro encapsulation process developed by Klean Earth Environmental Co. (KEECO)
• An inorganic sulfide stabilization technology (ENTHRALL) developed by E&C Williams
• A generic phosphate treatment

To determine the effectiveness of the stabilization technologies in reducing the quantity of leachable mercury from SBMM material, waste material was evaluated. The waste material consisted of mercury ore from the south white gate pile and waste rock from the north yellow pile. The mercury ore was the primary test material because of its demonstrated ability to produce consistent and detectable levels of leachable mercury. The waste rock was included because it is a common material at the site, even though it yields lower levels of leachable mercury.

In order to evaluate the performance of the three technologies, the leachable and mobile mercury (defined as the mercury in the less-than-25-micrometers [µm] filtered leachate fraction) from control columns receiving no treatment was compared to the leachable and mobile mercury in the treatment columns. Specifically, the objective was to achieve a 90 percent reduction in the total mass of mercury leached from each treatment relative to the control over a 12-week continuous column leaching study.

Leachability results from the no-treatment control columns revealed that approximately 96 percent of leachable mercury was found in the particulate fraction. The phosphate treatment dramatically increased the levels of both the particulate and dissolved fractions (less than 0.45 µm) during the 12-week study. The dramatic rise in leachable mercury brought about by the phosphate treatment invalidates its utility as a remedial alternative for materials at the SBMM site.

The E&C William's ENTHRALL technology did not appear to be effective in reducing the levels of mobile mercury in the mercury ore column tests. The total mass of mercury in both the particulate and dissolved fractions is statistically similar to the control. KEECO's silica micro encapsulation technology, applied both in situ and ex situ, was effective in reducing mobile mercury (less than 25 µm) at very close to the 90 percent reduction goal of the study. However, there was a significant increase in the mass mercury levels in the dissolved fraction (less than 0.45 µm). The in situ applications exhibited a 198 percent increase relative to the control, and the ex situ exhibited a 238 percent increase.

Contact

Ed Bates

See Also

Superfund Innovative Technology Evaluation

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