TY - JOUR

T1 - The mobility of palladium and platinum in the presence of humic acids

T2 - An experimental study

AU - Kotzé, Emmylou

AU - Schuth, Stephan

AU - Goldmann, Simon

AU - Winkler, Benjamin

AU - Botcharnikov, Roman E.

AU - Holtz, François

N1 - Funding Information: The scientific funding for this project was provided by the German Federal Ministry of Education and Research (grant number 033R119E as part of the AMREP project). The support of the DAAD under the Graduate School Scholarship Program of 2014 (program number 57034101 ) for personal funding is also gratefully acknowledged. The authors wish to thank the following collaborators for their leadership and assistance in sharing research ideas, data and discussion: Robert Trumbull, Deshentree Chetty, Malte Junge, Linda Oppermann, and the rest of the AMREP research team. Very constructive suggestions and comments by John Bowles and A. R. Cabral helped to improve the manuscript, and editorial handling by Donald Dingwell is gratefully acknowledged.

PY - 2019/6/5

Y1 - 2019/6/5

N2 - 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 (PdTe 2 ), palladium sulfide (PdS) and platinum disulfide (PtS 2 ). 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 PtS 2 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.

AB - 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 (PdTe 2 ), palladium sulfide (PdS) and platinum disulfide (PtS 2 ). 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 PtS 2 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.

KW - Humic acid

KW - Metal mobility

KW - Oxidized PGE deposits

KW - Platinum‑palladium geochemistry

KW - Solution experiments

UR - http://www.scopus.com/inward/record.url?scp=85063697089&partnerID=8YFLogxK

U2 - 10.1016/j.chemgeo.2019.03.028

DO - 10.1016/j.chemgeo.2019.03.028

M3 - Article

AN - SCOPUS:85063697089

VL - 514

SP - 65

EP - 78

JO - Chemical Geology

JF - Chemical Geology

SN - 0009-2541

ER -