Characterization of Colloids in Circumneutral Mine Waters Resulting from Pb-Zn Tailings
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Natural waters are typically filtered to 0.45 microns in order to discriminate between truly dissolved and non-aqueous phases. Acid is then added to the sample as a preservative, and all measured concentrations are considered dissolved. However, it is well-known that the <0.45 μm fraction contains a mixture of truly dissolved ions, colloids, and particulate matter. Drainage samples from tailings depositories at the New Calumet Mine in Quebec, Canada were collected and subsequently ultra-filtered to 0.01 μm, capturing all particles between 0.01 μm and 0.45 μm in size. SEM analysis of the filter membranes revealed 100-300 nm spherical iron oxyhydroxides, zinc carbonates and/or oxyhydroxides, and adsorbed Zn on Fe-oxyhydroxides. Zn was also identified on biogenic particles. Fe was found to be carried predominantly in colloidal form, while only a small fraction of the Zn was found to be transported this way. The majority of the Zn was found to be dissolved, and bioavailable. It is important to understand the fraction of metal carried in colloidal form as colloids play an integral role in the fate and mobility of trace contaminants. Failure to account for colloidal particles may cause the bioavailability of trace metals to be over- or under- estimated. This has important implications for mine permitting and environmental assessments.