The mobility of palladium and platinum in the presence of humic acids: An experimental study

The economically important platinum-group elements (PGE) are known to be mobile in the surface weathering environment, but their mobility in the presence of soil organic matter under surficial conditions has been poorly constrained so far. In this paper, the behavior of synthetic platinum-group minerals (PGM) placed in solutions containing humic acids is described. In order to approximate the natural weathering environment in the area of the Bushveld Igneous Complex, South Africa, humic acid solutions were prepared at concentrations of 0.1, 1, and 10 mg/l, and allowed to react with synthetic palladium monotelluride (PdTe), palladium ditelluride (PdTe2), palladium sulfide (PdS) and platinum disulfide (PtS2). The solutions remained stationary for a period of up to 306–308 days, with aliquots being taken at various intervals. The solutions were then analyzed by inductively coupled plasma mass spectrometry (ICP-MS). With both telluride minerals, Pd was not mobilized above concentrations of about 4 μg/l, but tellurium proved to be more mobile, reaching concentrations of several thousand μg/l. In these experiments, tellurium oxide crystals formed on the substrate of the humic-acid leached palladium tellurides. The palladium content of the initial palladium tellurides increased relative to tellurium, possibly approximating the structure of extant naturally forming palladium telluride minerals. The PdS and PtS2 solutions appear to have undergone hydrolysis of sulfur, producing sulfurous/sulfuric acid, which lowered the pH of these solutions from 7 to 8 to about 3. Due to this lowering of the pH, Pt was observed to precipitate out of solution between days 50–150. Meanwhile, Pd steadily increased in the 10 mg/l humic acid solution, reaching a final value of 162 μg/l. The increase in Pd in this solution matches a linear equation of y = 0.5× + 4.9, where x = time in days and y = concentration of Pd in μg/l, corresponding to a rate of mobilization for Pd of 0.5 μg/l per day.