Dissolved Mn2+ promotes microbially-catalyzed protodolomite precipitation in brackish oxidized water

Abstract

Manganese (Mn) is a common trace element in dolomite, yet its precise role in dolomite precipitation, if any, has not been explored. This study aims to investigate carbonate mineral formation in the presence of extracellular polymeric substances produced by Bacillus licheniformis Y1 in 14 cross-over experiments within the MgCl2-CaCl2-MnCl2 system. The study involved the analysis and comparison of water chemistry, mineralogy and bacterial metabolites. The findings indicate that B. licheniformis Y1 induces the formation of Mg-free calcite and low-Mg calcite in Mn-free solutions (Mg/Ca < 2). The addition of 0.001 mol/L Mn2+ results in the precipitation of Mg-rich calcite. In the presence of a relatively high Mg2+ content in the solution (Mg/Ca > 6), B. licheniformis Y1 in a Mn-free solution induces the formation of aragonite and dypingite, whereas the addition of 0.001 mol/L Mn2+ leads to the formation of Mn-bearing dolomite in the precipitate. Notably, at Mg/Ca = 6, an increase in Mn2+ content of the fluid to 0.002 and 0.003 mol/L yields Mn-rich dolomite. The Mn-bearing dolomite exhibits a regular spherical aggregate morphology with diameters ranging from 15 to 60 μm. The 2θ position of the (104) crystal face is close to that of dolomite, and the thermal decomposition temperature is lower than that of calcite. XPS results show that MnO bonds exist on the mineral surface. Acidic amino acids such as glutamic acid as well as humic acid in EPS, especially fulvic acid analogues, actually rise with increasing Mn2+ concentration and contain a large number of functional groups including carboxyl groups. Molecular dynamics simulations show that fulvic acid can effectively promote the dehydration of Mg-water complexes, thus promoting the formation of dolomite. These experiments indicate that the presence of Mn ions in the solution, along with bacteria and EPS, promote the precipitation of protodolomite.