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Electrochemical and XPS surface analytical studies on the reactivity of enargite

¹ Department of Inorganic and Analytical Chemistry, INSTM research unit, University of Cagliari, 09042 Monserrato (Ca), Italy
² ETH Zurich, Institute for Building Materials, ETH Hönggerberg, 8093 Zurich, Switzerland

^* Corresponding author, e-mail: rossi{at}unica.it

Enargite, a copper arsenic sulfide having the formula Cu₃AsS₄, is a source of arsenic and may cause environmental problems owing to the release of toxic elements upon oxidation, especially in acid mine effluents. In this work the oxidative dissolution of enargite has been studied on freshly cleaved samples exposed to distilled water, sulfuric acid solution at pH 4 and acidic FeCl₃ or Fe₂(SO₄)₃ solutions at pH ca. 2 with 0.025 M Fe³⁺ simulating abiotic acid mine drainage environments. The open circuit potential of the acidic solutions with ferric ions achieved stabilization at + 0.72 ± 0.02 V NHE thus, according to the mixed potential theory, the redox couple Fe³⁺/Fe²⁺ strongly polarizes the enargite surface towards positive potentials. Solution analyses showed that in these conditions about 10–14 µg copper are released every 24 h into solution from approximately 0.5 cm² enargite surface. Based on the amount of dissolved copper, the thickness of the dissolved enargite has been calculated to be about 60–130 nm. XPS analyses of the reacted enargite surfaces revealed no changes in the binding energy of copper Cu2p3/2 found at 932.4 ± 0.2 eV, arsenic As3d5/2 found at 43.3 ± 0.2 eV and of sulfur, S2p3/2 at 161.9 ± 0.2 eV compared to the pristine surface, whereas a prominent sulfur signal appeared at ca. 163.5 ±0.2 eV, assigned to sulfur in a copper-deficient layer. The XPS quantitative analysis performed by applying a three-layer model revealed the presence of a metal-deficient layer of ca. 0.7 nm thickness on the enargite surface. The interface beneath this layer (estimated thickness 5–10 nm) was slightly enriched in sulfur and depleted in copper. Based on these complementary data from solution analysis and XPS surface analysis, a model similar to the dissolution of binary metallic alloys is here proposed for enargite dissolution under oxidizing conditions.

Key-words: redox potential, mixed potential theory, XPS surface analysis, dissolution, sulfide minerals.