The Investigation of Sulphides Composition Created by Bioprecipitation

Abstract

The objective of this work was to examine the structure and characteristics of biogenic iron sulphides, created by sulphate-reducing bacteria (SRB) cultivation under various conditions. SRB are anaerobic microorganisms characterized by the ability to perform dissimilatory sulphate reduction with the simultaneous ox - idation of the organic substrates. By oxidizing low molecular weight organic compounds (e.g. lactate, acetate) they obtain energy and nutrients. Bacteria reduce sulphate to hydrogen sulphide and this reacts with certain metals dissolved forming insoluble precipitates in process which is called bioprecipitation. The initial sulphide phase is very often amorphous with poor crystallinity. According to the conditions, solution composition and with longer time more stable and crystalline sulphides can be formed. The precipitates studied in this paper were synthesized in reagent bottles with SRB culture and modified growth medium at 30°C, in 2 modes, under anaerobic conditions during 10 months. The modification consisted of iron ions addition in form of sulphates and double organic substrate dose. During batch and semi-continuous modes were created 4 biogenic iron sulphides samples. They were examined by EDX, SEM and XRD. The analyses revealed how variety in bacteria cultivation (nutrient medium compound, cultivation process) reflects in crystallinity, structure, particle size and composition. EDX results confirmed the presence of iron and sulphur as a major part in all samples. XRD showed mainly amorphous or poorly crystalline precipitates (with partial mackinawite and gregite occurrence).