Treatability Study for Heavy Metal Removal from Mine Water
Introduction: Historical mining practices in the Coeur d’Alene River Basin (Idaho) have resulted in heavy metal contamination of soil, sediment, surface water, and groundwater. Canyon Creek, located in the upper basin, has elevated levels of dissolved zinc (average concentration ~ 3,000 μg/L), dissolved cadmium (average concentration ~ 22 μg/ L), and total lead (average concentration ~ 174 μg/L). Heavy metal loading near the mouth of Canyon Creek is influenced by surface water/groundwater interactions. Dissolved zinc concentrations in the groundwater have been detected in the 100,000 μg/L range while dissolved cadmium and lead have been detected in the hundreds to thousands μg/L ranges, respectively.
EPA’s consultant, URS Corporation (URS), developed a multi-phase treatability study to obtain quantitative information on a treatment process to effectively remove metals from the water of Canyon Creek. The treatment process incorporated different combinations of pH adjustment, chemical coagulation and coprecipitation, polymer flocculent additions, and additions of ballasted micro-sand to improve sludge settling. The results of the study will be used to help evaluate potential treatment technologies for surface water and/or groundwater at Canyon Creek. These data will also be used to help develop the pilotscale treatability study for Phase II of the study.
URS contracted Columbia Analytical Services, Inc.’s (CAS) Redding Lab to perform an in-depth, bench-scale treatability study. CAS performed the study in accordance with the procedure described in Canyon Creek Treatability Study Plan, Coeur d’Alene Basin, RAC, EPA Region 10, April 2004 and its Appendix A, Laboratory Scope of Work (SOW).
Treatability Study: The study involved performing a seven step process (see Table 1) involving sixteen jar and nineteen settling rate tests. Varying combinations of lime stabilization (adjusting the pH up to 11) with a calcium hydroxide slurry and coagulation / coprecipitation with ferric chloride followed by polymer flocculation (anion, cationic, and nonionic forms) and micro-sand additions (used as a flocculation aid) were used. The jar and settling rate tests were performed using a 6-place paddle stirrer or jar tester.
The bulk surface water feedstock collected for the treatability study was slightly acidic with a pH of 5.0 and low in conductivity (80 umhos/cm) and hardness (20 mg/L as CaCO3). The bulk sample was received in a Teflon lined 55-gallon drum by over-night courier. To help maintain sample stability the bulk sample was stored in a large walk-in cooler at 4°C for the duration of the study. Sub-samples (each approximately 8-L in volume) were taken for each jar or settling test by mixing the contents of the bulk sample using an acid washed, plastic paddle and then filling a 20-L cubitainer with the sample using an allplastic siphon pump. In addition to the treatability study, CAS analyzed 9 surface water and 4 groundwater samples for ICPMS and ICP metals, general chemistry, physical analyses, and neutralization curve analyses to evaluate site characteristics at the time of feedstock water collection.
Over two hundred samples were generated by the treatment study. CAS immediately analyzed the samples for pH, conductivity, turbidity, and zinc by the Zincon colorimetric procedure. The Zincon method was used so CAS and URS could quickly determine which combination of reagents provided the best reduction in zinc in order to quickly select the optimum combination of reagents for the next jar test. Selected samples were also analyzed for cadmium, lead, and zinc by ICPMS, calcium and magnesium by ICP, and a variety of general chemistry and physical measurements. After Step 3, all zinc analyses were performed by ICPMS because the treatment process was reducing the zinc concentration to below the Zincon detection limit of 20 μg/L.
Study Results: Overall the project was very complicated as each jar test required different combinations of lime, ferric chloride, and different types of flocculating agents depending on the results from the previous jar test. Due to the unknown stability of the water matrix CAS personnel worked through the initial weekend. The testing was completed over a period of 14 days. The bulk sample proved to be stable over this period based on daily analyses of dissolved metals concentrations and conventional parameters.
URS and CAS personnel worked closely with daily communications to insure a smooth progression and successful project outcome. Frequent communications between URS and CAS were essential, as minor modifications to the work-plan were required as the study progressed.
The first phase of the Canyon Creek Treatability Study provided quantitative information on the effectiveness of a variety of treatment approaches for removing dissolved metals from surface water collected from the mouth of Canyon Creek. The treatment process, using the optimum combinations of reagents, resulted in significant reduction of metals plus the formation of a sludge that has very good settling rates and ease of filtering (see Table 2).
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