Natural attenuation of heavy metals via secondary hydrozincite precipitation in an abandoned Pb-Zn mine

The Aran Valley in Catalonia (Spain) was the site of large-scale Zn, Pb, Cu, Fe, and Ag mining from the late 19th century until approximately 1950. Although mining activities ceased over 70 years ago, some of the abandoned mining relics (e.g. tunnels, processing facilities, and tailings dumps) exhibit elevated concentrations of metals in the associated water systems, thus posing a health risk in the neighboring ecosystem. In this study, the largest underground zinc mine in the Aran Valley (the Victoria Mine) was chosen as a field site to showcase the processes affecting metal mobility in the environment. Three sampling campaigns to the mine (2019, 2020 and 2021) provide a spaciotemporal dataset showing the evolution of solute concentrations through the system along a flow path. Rainwater from the upper catchment flows into the host rock above the mine, dissolving ore materials (primarily Zn (sphalerite) and Fe (pyrite) sulfides with Ni and Cd impurities) that cause elevated Zn, Ni, and Cd concentrations in the water that enters the mine. Precipitation of hydrozincite (Zn5(CO3)2(OH)6) along the gallery where water flows serves as a metal-removal mechanism resulting in significantly diminished metal concentrations (e.g. Zn from 155 to 10 ppm, Ni from 377 to 32 ppb, and Cd from 105 to 22 ppb). Characterization of the solid samples taken from the mine reveals several different morphologies, Zn zonations in hydrozincite (i.e. purity changes), and small amounts of smithsonite (ZnCO3) and calcite (CaCO3). In general, all solids show layering, which is a result of intermittent precipitation of distinct solid products. These precipitation patterns are likely a result of changing solute concentrations and precipitation rates. Variations in rain events change the water residence time in both the host rock and in the gallery, thereby altering the water composition. Solubility experiments and speciation calculations demonstrate that hydrozincite here does not have a constant Keq value, but rather a range of values (30.0 < log[Keq] < 37.68 at 7 degrees C). This behavior is presumed to be a result of precursor effects as amorphous solids with higher solubilities may form before the structured hydrozincite. Notably, all analyzed solids from the collected mine samples show significant amounts of amorphous material, demonstrated by broad XRD peaks along with significant impurities in the analyzed solids, leading to the hypothesis that there exists a variability in the solubility of hydrozincite beyond a pure end-member value.