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Evaluation of Metal Partitioning and Mobility in a Sulfidic Mine Tailing Pile under Oxic and Anoxic Conditions
Citation:
Pinto, P. X., S. R. Al-Abed, C. Holder, AND D. J. Reisman. Evaluation of Metal Partitioning and Mobility in a Sulfidic Mine Tailing Pile under Oxic and Anoxic Conditions. A.L. Gill (ed.), JOURNAL OF ENVIRONMENTAL MANAGEMENT. Elsevier Science Ltd, New York, NY, 140:135-144, (2014).
Impact/Purpose:
Mine tailing piles are usually sampled while the sample surface is in contact with ambient air, which tends to oxidize iron and other metals. After the mine tailing piles are sampled, the traditional leaching tests are performed also under ambient air. However, it is unclear how much of the sample actually comes in contact with oxygen and how oxygen affects the mobility of the metals from these samples. In this study we evaluated metal mobility of the mine tailings and water from the Callahan Mine site when leaching under anoxic and oxic conditions. For this reason, the regulatory tests Synthetic Precipitation Leaching Protocol (SPLP) and Toxicity Characterization Leaching Procedure (TCLP) were conducted under oxic and anoxic conditions, while the Constant Acidity Leaching Procedure (CALP) was conducted at two different oxidative reduction potential (ORP) conditions at constant pH. Metal mobilization in the batch tests was also compared to the metal content of the overlying water collected with the mine tailings. A sequential extraction was used to determine metal associations and their potential mobility in the operationally defined fractions. This is one of the few studies that explore this topic with actual mine tailing samples. The presented conclusions may impact the sampling collection and leaching methods applied to evaluate metal mobility from solids found under anoxic conditions like mine tailings and sediments.
Description:
Mining-influenced water emanating from mine tailings and potentially contaminating surface water and groundwater is one of the most important environmental issues linked to the mining industry. In this study, two subsets of Callahan Mine tailings (mainly comprised of silicates, sulfides, and carbonates) were collected using sealed containers, which allowed keeping the samples under anoxic conditions during transportation and storage. Among the potential contaminants, in spite of high concentrations of Cu, Mn, Pb, and Zn present in the solid mine tailings, only small amounts of Mn and Zn were found in the overlying pore water. The samples were subjected to leaching tests at different reduction-oxidation (redox) conditions to compare metal and S mobilization under oxic and anoxic conditions. It was observed that Cd, Cu, Mn, Pb, S, and Zn were mobilized at higher rates under oxic conditions, while Fe was mobilized at a higher rate under anoxic conditions in comparable constant pH experiments. These results suggest that metal mobilization is significantly impacted by redox conditions. When anoxic metal mobilization assessment is required, it is recommended to always maintain anoxic conditions because oxygen exposure may affect metal mobilization. A sequential extraction performed under oxic conditions revealed that most of the metals in the samples were associated with the sulfidic fraction and that the labile fraction was associated with Mn and moderate amounts of Pb and Zn.
