XPS and XAES analysis of copper, arsenic and sulfur chemical state in enargites

Abstract

Enargite, a copper arsenic sulfide with the formula Cu3 AsS4 is of environmental concern due to its potential to release toxic arsenic species. The oxidation and dissolution of enargite are governed by the composition and chemical state of the outermost surface layer. Qualitative and quantitative analysis of the enargite surface can be initially obtained on the basis of X-ray photoelectron spectroscopy (XPS) binding energy and intensity data. However, a more precise determination of the chemical state of the principal elements of enargite (copper, arsenic and sulfur) in the altered surface layer and in the bulk of the mineral requires a combined analysis based on XPS photoelectron lines and the corresponding X-ray excited Auger lines.\ud On the basis of results obtained on natural and synthetic enargite samples and on standards of sulfides and oxides, the Auger parameter a′ of different compounds was calculated and the Wagner chemical state plots were drawn for arsenic, copper and sulfur. Arsenic in enargite is found to be in a chemical environment similar to that of arsenides or elemental arsenic, whereas copper in enargite is in a chemical state that corresponds to copper sulfide, Cu2S, for all samples irrespective of surface treatment (natural or freshly cleaved). Only sulfur changed from a chemical state similar to that of copper or iron sulfide in freshly cleaved samples to another state in natural enargite in the as-received state. Thus, it is the sulfur atom at the surface of enargite that is most susceptible to changes in the enargite surface state and composition. A more detailed interpretation of this behavior, based on differences in the initial and final state effects, is proposed here. The concept of Auger parameter and chemical state plot, used here for the first time for investigating enargite, has proved to be a method to unambiguously assign the chemical state of the principal elements copper, arsenic and sulfur in these minerals

The surface of enargite after exposure to acidic ferric solutions: an XPS/XAES study

“…It has been found 10 that arsenic shows a chemical environment similar to that of elemental arsenic or arsenides and copper is in a chemical environment similar to that of Cu 2 S. 10 Comparing the XPS and XAES line positions before and after exposure to FeCl 3 and Fe 2 SO 4 3 solutions, we did not observe any changes in the shape of the photoelectron lines or shifts for each of the enargite surface constituents. In order to determine their chemical state, the modified Auger parameter˛0 D BE (photoelectron) C KE (Auger) was calculated.…”

“…The arsenic component with a binding energy of 43.5 eV was assigned to atoms from the bulk mineral matrix. 7,10 As the kinetic energy of the As LMM signal did not change either upon exposure to the solutions, the Auger parameter˛0 and the chemical state of arsenic in enargite remained unchanged. This indicates that no solid oxidation products such as arsenic oxides 9 -11 formed during exposure to the solutions.…”

“…In order to determine their chemical state, the modified Auger parameter˛0 D BE (photoelectron) C KE (Auger) was calculated. 9,10 Arsenic As 3d spectra (Fig. 2(a)) showed only one component at 43.5 š 0.2 eV; no other components were detected.…”

“…The two-dimensional Wagner plot 28 clearly shows (Fig. 4) that the principal sulfur component of cleaved and immersed enargite lies on a diagonal line with˛0 D 2277.8 eV, close to sulfur in Cu 2 S. 10 Elemental sulfur shows an Auger parameter˛0 of 2277.2 š 0.2 eV. 29 The point group of the high-binding-energy component of sulfur at 163.5 š 0.2 eV does not match the Auger parameter for elemental sulfur (experimental S KLL peak calculated˛0 D 2279.2 š 0.2 eV, Table 1)…”

“…The calculated Auger parameter 0 is found at 1849.4, and therefore copper has a chemical state similar to that of Cu 2 S both before and after exposure. 10 No solid oxidation products of copper formed during immersion of enargite in the solutions.…”