Experimental constraints on lithium behavior during leaching of micas: Insights from lithium-isotope fractionation

Over the past decade, the surging demand for critical minerals essential in energy storage technologies has driven a global increase in lithium (Li) mining and exploration. Demand for Li is associated with significant environmental challenges, notably the potential increase in concentrations of Li and other trace elements in natural waters adjacent to and downstream of mining sites. Formation of authigenic clays can remove Li from aqueous solutions, offering a potential natural remediation of Li pollution from mining, but its efficiency in under low-temperature, short-timescale conditions remains uncertain. In this experimental study, two different Li-rich micas (zinnwaldite and lepidolite) were exposed to acidic conditions at room temperature to assess the potential for Li leaching and secondary-phase formation relevant to natural environments. These micas were leached using dilute sulfuric, acetic, and oxalic acids (all 0.01 M), as well as ultrapure water, over an 86-day period. We monitored Li concentration and isotopic composition in the evolving fluids and applied thermodynamic modeling to calculate saturation indices of minerals that are likely precipitating or dissolving under the experimental conditions. A decline in δ 7 Li of the fluid was observed at the start of the incubations, indicating preferential leaching of 6 Li during initial mineral dissolution. Subsequent increases in δ 7 Li of the fluid, together with calculated saturation indices, point to precipitation of secondary phases such as kaolinite, montmorillonite, goethite, and quartz. However, measurable decreases in fluid Li concentration were only observed in three of the eight experiments, and only during parts of the experimental period. Overall, decreases in Li concentration occurred for zinnwaldite in ultrapure water (by 0.036 mmol/L or 55.6%), and for lepidolite in both acetic (by 0.344 mmol/L or 19.1%) and oxalic acids (by 0.524 mmol/L or 37.2%). These results demonstrate that, under short-timescale, low-temperature conditions typical of terrestrial waters, authigenic clay formation is not a highly effective mechanism for removing Li from highly contaminated solutions. The findings underscore the limitations of natural remediation and the critical need for engineered solutions to prevent and mitigate Li pollution from mine drainage.