NANOSCALE EFFECTS OF NICKEL CATIONS ON CALCITE GROWTH OBSERVED BY IN SITU ATOMIC FORCE MICROSCOPE

Abstract

Biomineralization of calcite has been the focus of numerous studies, yet some intricate problems, especially those related to the mechanism of calcite-additive interactions remain poorly understood. Previous research has investigated how additives attach on the calcite growth steps and change step-edge properties of calcite hillocks. More recently, based upon both computational and experimental results, there is a growing interest in understanding what role the hydration layer on calcite plays in directing the interactions of additives. Nickel (II), an important micronutrient for organisms, has a solvation shell that is quite different from calcium. Such differences include the structure of the hydration shell, the enthalpy of solvation, and the hydrolysis kinetics. For these reasons, we chose to explore calcite growth in the presence of Ni cations to see how this cation disrupted the calcite hydration layer. Specifically, we used fluid cell AFM to monitor calcite growth in the presence of Ni2+. We find that nickel cations have concentration-dependent effects on calcite hillock growth, including modification of hillock geometry, decrease in step density on both acute and obtuse sites, and increase in step velocities. Morphology of the growth hillocks is modified largely only when nickel chloride concentration is high enough (Ni:Ca ≈ 3% mol). After nickel is added, the step density on both acute and obtuse steps, as well as the height of a growth hillock keeps decreasing with time regardless of nickel concentration, until the hillock disappears from the surface, or the hillock reaches a steady state. Step velocities increase in every tested nickel concentration and most of them do not reach a steady state after a running time of ~2400 seconds. The acute step velocities always increase greatly (~200%) after nickel is added, while the obtuse step velocities first decrease and then start to increase. The faster step velocities are believed to result from modification of surface kinetics, which could indicate that the Ni2+ disrupts the hydration layer on the calcite surface, making it easier for growth units to add to both acute and obtuse steps. These results provide insight into how small cations with large hydration shells can lead to unexpected changes to calcite growth kinetics, which may provide a route to modify the hydration layer on crystal surface and thus further influence the interaction between crystal and organic impurities.