Laboratory Experiments to Evaluate the Potential Mobilization of Contaminants from Hard Rock Lithium Mining

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Vengosh, Avner

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Saltman, Sam

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2023-04-26T16:41:11Z

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2023-04-26

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Nicholas School of the Environment

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Hard rock mining of lithium is projected to increase significantly over the next decade due to the demand for lithium in renewable energy. The potential impacts of lithium mine tailings and waste rock from a pegmatite source on water quality are not fully documented. Chemical interactions of lithium mine tailings and waste rock with meteoric water can promote leaching of contaminants from the residual rocks and potentially contaminate associated water. This study utilizes the Leaching Environmental Assessment Framework (LEAF) methods used by the US Environmental Protection Agency (EPA) to assess the potential of contaminants leaching from lithium tailings and possible impacts on water quality. Groundwater, surface water and waste rock samples from an undisclosed hard rock lithium mine site in North Carolina were collected. The results were compared to ecological and drinking water standards set by the EPA, US Geological Survey (USGS) and World Health Organization (WHO) as well as to the quality of effluents extracted from coal ash through the LEAF methodology. Results of the leaching experiments indicate that interactions between water and lithium tailings can cause the mobilization of metals and impact water quality. In all leaching scenarios, low liquid-solid (L/S) ratios caused higher mobilization of trace elements. The experimental acidity conditions affected mobilization of trace elements as well; under acidic conditions (lower than pH 4), the mobilization of beryllium, manganese, nickel, and cobalt are promoted, whereas under both acidic (lower than pH 4) and alkaline (greater than pH 10.5) conditions, zinc, lead, iron, aluminum, chromium, lithium, and uranium are mobilized into the effluents. Uranium in one sample leached 40.62% in pH 2, below the detection limit in pH 7 and 60.9% in pH 13 in comparison to the concentration of the bulk solid. Lead showed a similar pattern with one sample leaching 6.77% in initial pH of 2, below the detection limit in pH 7 and 3.54% in pH 12. These results highlight the dependence of ambient acidity conditions on the mobilization of contaminants from lithium tailings solids. The LEAF tests indicated that the lithium waste rock samples all had a buffer capacity between pH 6-10 in testing conditions between pH 4-10.5 and produced slightly to moderately alkaline effluent between a pH of 7-10 in deionized water. Both the alkaline effluent and high buffering capacity observed in the LEAF tests suggests that the formation of acidic effluent and the potential for acid rock drainage is low. Alkaline water observed in the two samples taken on-site, i.e., one surface water sample and one groundwater sample, supports this finding. A comparison to the effluent chemistry generated from a LEAF experiment of coal ash reveals that industrial effluent from a lithium mine will have less of an impact on water quality than coal ash residuals. Under expected moderately alkaline conditions (pH 9) of both coal ash and lithium residual rock effluent, results showed that concentrations of coal ash effluent exceeded lithium rock effluent concentrations by a factor of 19 for antimony, 130 for chromium and 440 for selenium. This study highlights the need to monitor and manage the conditions at tailings and waste rock storage areas. There were L/S and acidity conditions in the lab setting where contaminants of concern including chromium, lead, uranium, beryllium, cadmium, antimony and thallium mobilized. Although the results of this study cannot be directly applied to field conditions, environmental managers should be aware of the conditions in which contaminants of concern mobilized in the lab setting. The main conclusions are that acid rock drainage is unlikely to occur given that the lithium residual rock samples all produced alkaline effluent, the buffer capacity of the lithium residual rock can reduce the risk of contamination associated with highly alkaline conditions, and lithium residual rock effluent in expected moderately alkaline conditions will have less of an impact than coal ash effluent on water quality.

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https://hdl.handle.net/10161/27132

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lithium

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mining

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leach

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pegmatite

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tailings

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waste rock

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Laboratory Experiments to Evaluate the Potential Mobilization of Contaminants from Hard Rock Lithium Mining

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Master's project

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12

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2024-04-26

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