In situ AFM observations of Ca–Mg carbonate crystallization catalyzed by dissolved sulfide: Implications for sedimentary dolomite formation

It has been observed that the metabolism of sulfate-reducing bacteria can overcome the energy barrier to Mg2+ incorporation into growing Ca–Mg carbonates and enhance dolomite precipitation, although the role of SRB in dolomite formation is still under debate. In this study, we presented in situ AFM observations of Ca–Mg carbonate {1 0 4} surface growing from supersaturated solutions. Our data showed that not only can Mg2+ modify the morphology of the polygonal growth hillocks and impede step growth, but it can also inhibit 1-D step nucleation, resulting in the inability for spirals to continue their vertical growth. However, in the presence of dissolved sulfide, both the 1-D step nucleation and step growth which had been retarded by Mg2+ ions were significantly enhanced. For example, in the presence as low as 0.13 mM dissolved sulfide, the step velocity can be increased by more than 9 times compared to that in contact with solutions containing Mg2+ ions but no dissolved sulfide. The Ca–Mg carbonate growth hillock in contact with dissolved sulfide-bearing growth solutions eventually developed a micromosaic-like structure. Based on our observations, we propose that the overall catalytic effect of dissolved sulfide may be twofold, one to stabilize the critical nuclei during 1-D step nucleation by the adsorption of dissolved sulfide on Ca–Mg carbonate surfaces and two to facilitate the dehydration of surface Mg2+–water complexes during growth. We hypothesize that dissolved sulfide may adsorb on crystal faces through hydrogen bonding between the H in HS−/H2S and the O in calcite CO32- to weaken the rigid Mg2+ hydration shell, resulting in an elevated activation entropy for particle attachment and hence a larger kinetic coefficient for step growth. Together with previous studies on disorder dolomite precipitation induced by dissolved sulfide, we demonstrate the catalysis role of dissolved sulfide in sedimentary dolomite formation associate with SRB, which may shed new light on the long-standing “dolomite problem”.