The influence of clay matrix and activity of sulphidogenic microbial communities on the formation of sulphide mineral phases
Description
The study aimed to determine the role of sulphidogenic microorganisms in the formation of iron sulphide minerals synthesized in laboratory conditions for 6 and 12 months with or without the addition of mineral matrix (in the form of kaolinite). The presence of clay minerals may promote the crystallization of sulphides. In all variants, Baar medium was used with the addition of FeCl2 as a source of iron and Na2SO4 as a source of sulphur for sulphate reducers or Na2S in abiotic variants. The obtained post-culture sediments were subjected to XRD analysis and SEM imaging. The test results confirmed the presence of iron sulphides, i.e. mackinawite, greigite and pyrite. It was found that the formation of pyrite and other existing mineral phases is influenced by the presence of sulphur(0) and iron(III) as an oxidizing agent, as well as the activity of microorganisms that modify the crystallization environment. Additionally, phyllosilicates constitute a potential crystallization matrix of sulphide phases. These nucleation sites are created due to the mineral packaged structure, the presence of acidic sites in the crystal structure and the lamellar shape. Moreover, in biotic variants, the products of bacterial metabolism (exopolymeric compounds) influenced the flocculation of the kaolin sediment, causing a decrease in microbiological activity and thus inhibiting the conversion of greigite into pyrite. The extension of the experiment time primarily influenced the degree of order in the structure of the resulting mineral phases.
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XRD data collection settings: powder diffraction patterns were collected acquired from 4° to 78° with a 2θ step of 0.0263°, at 40 kV and 30 mA with Co Kα radiation on a the PANalytical X’Pert Pro MPD diffractometer equipped with a PIXcel3D detector mounted in the Bragg-Brentano configuration. The total time of registration for one sample was 4 hours. SEM imaging settings: samples were imaged without sputter coating, current intensity 4 pA, excitation voltage 8-20 kV, and working distance 7.5 mm, EDS- 15 kV and the same working distance