An XPS analytical approach for elucidating the microbially mediated enargite oxidative dissolution

In this work, the microbe-mediated oxidative dissolution of enargite surfaces (Cu₃AsS₄) was studied on powdered samples exposed to 9K nutrient solution (pH 2.3) inoculated by Acidithiobacillus ferrooxidans initially adapted to arsenopyrite. These conditions simulate the acid mine environment. The redox potential of the inoculated solutions increased up to +0.72 V vs normal hydrogen electrode (NHE), indicating the increase of the Fe³⁺ to Fe²⁺ ratio, and correspondingly the pH decreased to values as low as 1.9. In the sterile 9K control, the redox potential and pH remained constant at +0.52 V NHE and 2.34, respectively. Solution analyses showed that in inoculated medium Cu and As dissolved stoichiometrically with a dissolution rate of about three to five times higher compared to the sterile control. For the first time, X-ray photoelectron spectroscopy (XPS) was carried out on the bioleached enargite powder with the aim of clarifying the role of the microorganisms in the dissolution process. XPS results provide evidence of the formation of a thin oxidized layer on the mineral surface. Nitrogen was also detected on the bioleached surfaces and was attributed to the presence of an extracellular polymer substance layer supporting a mechanism of bacteria attachment via the formation of a biofilm a few nanometers thick, commonly known as nanobiofilm. Figure SEM image of enargite is in the background of the figure; in foreground the scheme of the dissolution mechanism in presence of microorganisms showing a sulphur enriched layer; the mechanism is supported by the presence of the high binding energy signal in the S2p photoelectron spectrum (upper-right).