Extreme halophilic bacteria promote the surface dolomitization of calcite crystals in solutions with various magnesium concentrations

Abstract

In this study, we conducted experiments on the transformation from calcite to protodolomite using bacteria. The surface dolomitization of calcite seed crystals was initiated by Vibrio harveyi J4, a newly isolated extreme halophilic bacterium, under increasing magnesium concentrations of 0, 0.05, 0.10, 0.15, to 0.20 mol L–1. The mineralogy, surface morphology, elemental composition, and lattice structure of the calcite crystals before and after bacterial treatment were analyzed by XRD, SEM-EDS, AFM and TEM, respectively. Organic functional groups within newly-formed minerals were determined by FTIR and XPS. The weight loss of minerals in the experimental group and control groups was measured by TGA and DSC. The experiments resulted in numerous nano-spheroidal-hemispheroidal precipitates being formed on the surface of the calcite seeds, whereas dissolution took place on the calcite crystal surfaces in control experiments with no bacteria. The nano-spheroids-hemispheroids were confirmed to be disordered protodolomite. The number of protodolomite spheroids on the calcite crystal surfaces increases dramatically with increasing concentration of magnesium. Since no calcium ions were added to the bioreactor, the protodolomite nano-spheroids-hemispheroids are considered to have been formed through some dissolution of the calcite to provide the Ca2+ ions in a local precipitation process. The mechanism of protodolomite precipitation is thought to be related to the presence of acidic amino acids and polysaccharides in the EPS of the extreme halophilic bacteria. These results suggest that in addition to promoting the precipitation of dolomite, micro-organisms may also play a key role in the dolomitization of calcite.