Colloidal arsenic characterization at two abandoned gold mine sites in Nova Scotia, Canada

Nova Scotia was mined for gold in the late 1800's and many abandoned mine sites are contaminated with arsenic. Colloids may play an important role in providing a mechanism for contaminant transport and removal in natural waters. To assess the risk and toxicity of arsenic at two abandoned gold mine sites, the role of colloids needs to be assessed. Asymmetric-Flow Field-Flow Fractionation (AsFl-FFF) coupled to an inductively coupled plasma mass spectrometer (ICP-MS) was used to determine whether colloidal arsenic exists at Montague Gold Mine and Goldenville Gold Mine. Using this technique, the chemical composition and the diameter of colloids was determined. Ground water, pore water and tailings extracts were analyzed. A total of 64 samples were analyzed and seven of these samples contained colloidal arsenic at detectable concentrations. In each sample containing colloids, arsenic was associated with iron, suggesting that these elements are the main colloidal constituents in the tailings. The molar ratios indicate that both weathered mineral phases and iron oxyhydroxides were present as colloids. The colloids had a mean diameter between 6.2 nm and 8.5 nm. The colloidal fraction was determined to account for up to 20% of the total dissolved arsenic present in these samples. Although colloidal arsenic does exist at both locations, it was not a significant factor in the transport and risk associated with arsenic at either site. Colloidal arsenic was found in 10% of the samples analyzed and was only found in surface tailing samples. The primary form of arsenic present in natural waters at these mine sites is the truly dissolved analyte. It is important to distinguish between truly dissolved and colloidal arsenic as the form can affect the toxicity and mobility of arsenic at these locations.;Keywords: Arsenic, Colloid, Gold Mine Tailings, Asymmetric Flow-Field Flow Fractionation, ICP-MS, Nova Scotia...