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Ponthieu M.,University of Reims Champagne Ardenne | Pourret O.,HydrISE | Marin B.,University of Reims Champagne Ardenne | Schneider A.R.,University of Reims Champagne Ardenne | And 3 more authors.
Journal of Geochemical Exploration | Year: 2016

To assess the influence of the composition of dissolved organic matter (DOM) on metal speciation, two models have been used: Model VI and NICA-Donnan. The speciation of Cu, Ni, Pb and Zn in the soil solution of 36 samples characterized by alkaline pH and different contamination levels has been determined. Four assumptions about DOM composition have been tested: (i) 100% of humic substances (HS) are fulvic acids (FA), (ii) 100% are humic acids (HA), (iii) 35% are HA and 65% FA and (iv) 84% are HA and 16% FA. The results obtained with the two distinct models are of the same order of magnitude; however, few differences have been highlighted regarding the description of the non-specific interactions, the Zn-FA interactions and the lowest free ion concentrations. The main result of this study is that the free ion concentrations calculated with the two models may differ by one order of magnitude depending on whether HA and/or FA are used to represent DOM. This work demonstrates the impact of the assumptions made about the DOM nature for metal speciation modelling. © 2016 Elsevier B.V.


Decree S.,Royal Belgian Institute Of Natural Sciences | Decree S.,Royal Museum for Central Africa | Pourret O.,HydrISE | Baele J.-M.,University of Mons
Journal of Geochemical Exploration | Year: 2015

Heterogenite (CoOOH) is the most abundant cobalt oxide mineral in the Katanga Copperbelt, which hosts around half of the world's known reserves of mineable cobalt. Heterogenite formed by the oxidation of Co-sulfides and accumulated as residual deposits during a Pliocene weathering event. Bulk analysis samples of oxidized cobalt ore samples containing with variable heterogenite concentration display two rare earth element (REE) patterns: (i) Type 1 is enriched in middle REE, with a negative cerium anomaly and a relatively low REE content; (ii) Type 2 is variably enriched in light REE (LREE), without a cerium anomaly and with higher REE content. However, in situ LA-ICP-MS reveals that the Type 2 patterns are due to the mixing of heterogenite with a LREE-rich mineral.Weathering processes leading to heterogenite formation mainly consist of water-rock interactions at high Co activity, in the near-surface environment. These result in the formation of a lateritic deposit. Heterogenite precipitates at near-neutral pH conditions together with manganese oxides. REE are mainly fractionated between these Co- and Mn-oxide minerals. In the deeper part of the oxidized profile, cobalt activity decreases and the heterogenite stability field shifts to alkaline pH conditions due to the dissolution of dolostone in the bedrock. In such an alkaline environment, REE speciation is mainly driven by carbonate complexation/precipitation. This environment would be favorable for the formation of REE-rich carbonate which is intimately associated with heterogenite (LREE-rich Type 2 patterns).The combined whole-rock and in situ geochemical analyses presented here clearly help (1) to distinguish the REE signature of the Co oxidized ore (mineral paragenesis comprising heterogenite) and of heterogenite itself, and subsequently (2) to highlight two different chemical environments for the formation of heterogenite in supergene ores. This study therefore improves the understanding of REE behavior and metal mobility in near-surface environments, and more particularly in environments where the supergene ores form. In the future, the approach developed here can be applied to other Co-Ni-Mn lateritic deposits such as those in New Caledonia and Cameroon. © 2015 Elsevier B.V.


Pourret O.,HydrISE | Lange B.,HydrISE | Lange B.,Free University of Colombia | Bonhoure J.,HydrISE | And 6 more authors.
Applied Geochemistry | Year: 2015

Metal and metalloid (As, Cd, Co, Cu, Pb and Zn) distribution in soils from the Katanga Copperbelt (Democratic Republic of Congo) is investigated in order to characterize the environmental impacts of mining and smelting activities in that area. The concentrations of Cu, Co, As, Zn, Pb and Cd in soils from mining sites are higher than in non-metalliferous sites and above permissible metal and metalloid concentrations in soils. Moreover, the fractionation and mobility of Co, and Cu in such environment is assessed using the application of both ammonium acetate-EDTA extraction and speciation modeling (WHAM 6). The resulting data set covers wide range of environmental conditions (pH, trace metals concentration, natural soils and soils affected by mining and ore processing). These extractions show that only a small fraction of Cu and Co is mobile, with variation depending on sites: mobility is higher in soils affected by mining and ore processing. The strong affinity of Mn-oxides for Co may explain lower Co mobility in Mn-rich soils. The high Mn and Fe contents of Cu-Co soils from Katanga may actually exert a protective effect against the toxic effects of Co. Finally, Cu-Co speciation modeling of contaminated sites emphasizes that organic matter strongly sorb Cu whereas Co speciation is mostly by Mn content. This type of study leads to a better understanding of metal fractionation and can guide to define different practices of phytoremediation. © 2015 Elsevier Ltd.

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