Characterization of organic-rich colloids from surface and ground waters at the actinide-contaminated Rocky Flats Environmental Technology Site (RFETS), Colorado, USA

Colloids, i.e. nanoparticles and macromolecules, play an important role in the environmental dispersion of actinides. Thus, colloids (3 kDa–0.5 μm) were collected and purified from three different environments, i.e. surface water, pond water and near-surface ground water at the Rocky Flats Environmental Technology Site, Colorado, where elevated actinide concentrations had previously been documented. Their chemical composition was determined in order to better understand their role in actinide migration. All three types of colloid samples were found to be similar in chemical composition, with a higher percentage of organic carbon, OC (5–18%), than any other measured component, and only small amounts of Si, Mn, Al, and Fe (1.5% or below). Analytically determined components account for 40–56% of the colloidal matter, with water likely making up the difference. Transmission electron microscopy (TEM) images of colloidal material from all three sample types indicate the presence of cellulose or chitin, likely from plant (terrestrial and/or aquatic) material. Other major components include humic acid type particles, with only small amounts (<5%) of mineral particles. Our findings of colloids high in organic and low in inorganic matter content agree with previously reported results on Pu(IV) associated with an acidic natural macromolecular organic compound that also contains small amounts of Fe. Pu/OC and Fe/OC ratios both showed a steady decrease from surface water to pond water to groundwater, with a more marked decrease in the Fe/OC ratio, but no significant change in overall colloidal organic carbon (COC) concentrations.