Deciphering colloidal transport of copper in partially remediated mine tailings by single particle analysis and transmission electron microscopy

Abandoned mine tailings are an environmental and financial liability worldwide and require often extensive reclamation efforts. A combination of single particle analysis and transmission electron microscopy, along with bulk chemical analysis, ultracentrifugation and ultrafiltration, are used to decipher the chemical, mineralogical and colloidal composition of an abandoned gold mine tailings at the Nopiming Provincial Park, Manitoba, Canada. The tailings are the result of the discharge of chalcopyrite-rich mine waste and were in recent years partially reclaimed through capping of two ponds with dolomite- and organic-rich soils. The surficial layers at the fully and partially affected sites have higher pH, inorganic and organic carbon content than the underlying tailings, whereas the surficial tailings at the unaffected site are more acidic and depleted in Cu relative to the reduced zone occurring at greater depth. The speciation of Cu in extracted acidic leachates is characterized by Cu²⁺ and CuSO₄ species, whereas Cu-organic complexes only play a role in weak acidic leachates extracted from the fully affected site and the reduced layer at the unaffected site. Studies of the colloidal fraction indicate that many colloids are coated by organic- or silica-rich layers, which in combination with the pH control their zeta potential and aggregation. The number of particles per time interval and the relative standard deviation of the Cu concentration per particle increase at a higher rate with increasing concentration of Cu in the colloidal fraction. These correlations are explained with a greater increase in the proportions of colloids containing trace (e.g. biomass-based colloids) and minor (clay mineral-based colloids) amounts of Cu relative to colloids where Cu is a major element (Cu-sulfate bearing colloids). A comparison of the mineralogical and chemical composition of the tailings and their colloidal fractions indicates that the formation of Cu-bearing colloids is favored in partly neutralized tailings of weak acidic pH and in the absence of efflorescent salts.