Formation of Covellite (CuS) Under Biological Sulfate-Reducing Conditions

Abstract

Sulfate-reducing bacteria (SRB) play a major role in the precipitation of metal sulfides in the environment. In this work, biogenic copper sulfide formation was examined in cultures of SRB and compared to chemically initiated Cu sulfide precipitation as a reference system. Mixed cultures of SRB were incubated at 22, 45, and 60 • C in nutrient solutions that contained copper sulfate. Abiotic reference samples were produced by reacting uninoculated liquid media with Na 2 S solutions under otherwise identical conditions. Precipitates were collected anaerobically by centrifugation, frozen in liquid N 2 , and freeze-dried, followed by analysis using X-ray diffraction (XRD), X-ray fluorescence, and scanning electron microscopy. Covellite (CuS) was the only mineral found in the precipitates. Covellite was less crystalline in the biogenic precipitates than in the abiotic samples based on XRD peak widths and peak to background ratios. Poor crystallinity may be the result of slower precipitation rates in bacterial cultures as compared to the abiotic reference systems. Furthermore, bacterial cells may inhibit the nucleation steps that lead to crystal formation. Incubation at elevated temperatures improved the crystallinity of the biotic specimens.

Formation of Fe-sulfides in cultures of sulfate-reducing bacteria

“…Copper, nickel, and zinc precipitation as biogenic sulfides involves the formation of covellite (CuS), heazlewoodite (Ni 3 S 2 ), and sphalerite (ZnS), respectively [10][11][12][13] as well as poorly crystalline sulfide phases. While the parameters for metal sulfide precipitation have been worked out for many treatment systems, properties and pathways of biogenic sulfide precipitates are, in general, poorly known.…”

Influence of electron donors and copper concentration on geochemical and mineralogical processes under conditions of biological sulphate reduction

“…In experiments described in previous reports, the concentration of copper was much lower than this. For example, Gramp et al (2006) conducted an experiment in which 8.95 mmol/L Cu (II) and sodium sulphate (VI) were added to a medium containing sodium citrate, yeast extract and sodium lactate as sources of carbon. In this case, only covelline was noted in the post-culture sediments as the product of copper biotransformation.…”

“…According to Gramp et al (2006), covelline of biogenic origin and sulphides of other metals do not have distinct crystalline structures like that of covelline of chemical origin. Moreover, poorly crystalline biogenic covelline is more susceptible to biological oxidation compared to the more resistant abiotic covelline.…”

“…Two minerals, covelline (CuS) or chalcocite (Cu 2 S), are formed from a solution that contains sulphide and copper ions. According to Gramp et al (2006), covelline is formed when there is a larger amount of sulphide ions than copper ions in the environment; a higher concentration of copper than sulphide ions results in the formation of chalcocite.…”

“…According to the commonly accepted model, the most important processes of sulphide formation in hypergenic settings, leading to the formation of such important metal deposits as those of iron, copper, zinc or lead, for example, are abiotic chemical reactions (Gramp et al 2006(Gramp et al , 2010Sarradin et al 2007). However, these are not the only processes by which metal sulphides of low solubility may be formed.…”