Conditions affecting the release of thorium and uranium from the tailings of a niobium mine

Determinations of the mobility of metals from tailings is a critical part of any assessment of the environmental impacts of mining activities. The leaching of thorium and uranium from the tailings of different processing stages of a niobium mine was investigated for several pH, ionic strengths and concentrations of natural organic matter (NOM). The pH of the leaching solution did not have a noticeable impact on the extraction of Th, however, for pH values below 4, increased U mobilization was observed. Similarly, only a small fraction of Th (0.05%, <= 15 mu g kg(-1)) and U (1.22%, <= 6 mu g kg(-1)) were mobilized from the tailings in the presence of environmentally relevant concentrations of Ca, Mg or Na. However, in the presence of 10 mg L-1 of fulvic acid, much higher concentrations of ca. 700 mu g kg(-1) of Th and 35 mu g kg(-1) of U could be extracted from the tailings. Generally, colloidal forms of Th and dissolved forms of U were mobilized from the tailings, however, in the presence of the fulvic acid, both dissolved and colloidal forms of the two actinides were observed. Single Particle ICP-MS was used to confirm the presence of Th (and U) containing colloids where significant numbers (up to 10(7) mL(-1)) of Th and U containing colloids were found, even in 0.2 mu m filtered extracts. Although mass equivalent diameters in the range of 6-13 nm Th and 6-9 nm for U could be estimated (based upon the presence of an oxyhydroxide), most of the colloidal mass was attributed to larger (>200 nm) heterocomposite particles. (C) 2019 Elsevier Ltd. All rights reserved.