NanoCArB

marie_curieNanoCArB is an FP7 EU-funded Marie Curie project: “The role of boron in the crystallisation and surface properties of CaCO3 at the nanoscale”. The goal of NanoCArB is to carry out an in depth study of the effects of boron in the chemistry and crystallisation of carbonates. How surface properties of calcite in contact with borate-rich solutions are affected at the atomic- and nanoscale, is a subject that only has been addressed in the laboratory from a basic viewpoint. This study will enable me to provide key information on how to improve water cleaning treatments, scaling inhibition methods, develop better strategies to extract oil from chalk, and also to advance in the understanding of processes that occur at the global scale (e.g. ocean acidification, climate change).

This project was funded by the EU FP7 with a Marie Curie Intra-European fellowship for career development during 2015-2016.

BACKGROUND
Data on the measurements of boron isotopes in natural carbonates are abundant in the scientific literature, but more experimental work is required to understand the role of borate ions in the crystallization of CaCO3. We need to identify and understand the mechanisms by which boron is taken up during the formation of biominerals to validate the isotopic methods we use for palaeo-pH reconstructions.

I have combined laboratory experiments to study adsorbed and structurally incorporated borate in CaCO3 with synchrotron-based scattering techniques to follow the crystallization reactions in situ and real-time. This information will be crucial in the next years to understand the impact of climate change on ocean acidification and its biological systems.

FUNDAMENTAL QUESTIONS AND OBJECTIVES
NanoCArB aimed to answer three fundamental questions about borate-carbonate mineral chemistry:

1) What is the role of borate in CaCO3 crystallization and how does it affect the kinetics and mechanisms of calcite growth?
2) What is the effect of borate on the surface properties of calcite?
3) How do the different mechanisms of CaCO3 crystallization affect surface attachment, incorporation and isotopic fractionation of boron (B10/B11 ratio) in calcite and aragonite?

Work strategy, techniques involved and expected outcomes

Schematic summary of the main work strategy, techniques involved and expected outcomes of the NanoCArB project.

NanoCArB fulfilled two major research objectives:

1. To provide new insights into the factors that control CaCO3 crystallization in the presence of borate. This consisted of carrying out homogeneous crystallization experiments from solution in the presence of borate at a range of temperatures (5-95 °C) and Ca/B ratios and supersaturations to explore the effects of borate in CaCO3 crystallization mechanisms and pathways. The fate of borate (adsorption, incorporation) in amorphous and crystalline CaCO3 was also be in-depth studied.
2. To define the effect of borate on calcite surfaces. This entailed evaluation of the effect of borate on the growth rate of calcite seeds at a range of solution pH, temperature, Ca/B ratios and saturation states. It included an in-depth study of the surface properties of calcite (chemistry, surface wetting) in borate-rich solutions.

WORK PERFORMED
– Crystallization from solution experiments were carried out and excellent data on the effect of borate on polymorph selection, crystallization kinetics and pathways of CaCO3 minerals.
– Structural characterization of poorly-ordered precursor phases (amorphous CaCO3) grown in the presence of borate ions was carried out.
– Calcite was also grown from solution and data on the uptake on borate has been successfully obtained.
– Furthermore, as both boron and carbon speciation are strongly affected by pH, some collaboration studies were carried out in order to understand the effect of pH in ACC formation and also in calcite surface speciation in collaboration with members of the NanoGeoScience group.

My results show that the morphology, stability and polymorph distribution of CaCO3 and the amount of borate uptaken by CaCO3 are different as function of temperature and initial B/Ca ratios in the aqueous solutions. These data unambiguously suggest that amorphous CaCO3, a poorly-ordered and metastable precursor phase that forms prior to crystalline CaCO3 formation, has a very important role in the evolution of the reaction.

More updates on results will be posted in this page when they become available as papers in peer-reviewed journals.


FIRST PAPERS PUBLISHED:

Is bicarbonate stable in and on the calcite surface?
At neutral to slightly basic pH, where CaCO3 forms, B exists as boric acid and borate. These are present in all sea water so marine organisms automatically incorporate B into their shells. However, in order to understand how boric acid / borate are adsorbed on the surface of calcite, it is necessary to know what is the carbon speciation on the surface this mineral in solution. Does bicarbonate in calcite exist as an intact species? Does deprotonation follow immediately after adsorption?

Our results demonstrate that the pH where adsorbed bicarbonate converts to carbonate is 3 pH units lower than for bicarbonate deprotonation to carbonate in solution.

Andersson, M., Rodriguez-Blanco, J.D., Stipp, S.L.S. (2016) Is bicarbonate stable in and on the calcite surface? Geochimica et Cosmochimica Acta, 176, 198-205. doi:10.1016/j.gca.2015.12.016 [PDF]

The effect of pH on Amorphous Calcium Carbonate Structure and Transformation
Amorphous calcium carbonate (ACC) frequently forms in highly supersaturated solutions, as a precursor to crystalline CaCO3 minerals during the course of biomineralization processes. We do not know how boron (borate and boric acid) affects the structure at the atomic scale of ACC at different pH. However, when it comes to understanding the structure of ACC, we do not have information about the pH affects its structure. In this study we focused on the formation and crystallization of rapidly formed ACC in the pH range from 10.6 to 12.7. For this, we monitored the composition, local structure, and lifetime of rapidly formed ACC as a function of the concentration of the added base, NaOH, at ambient conditions and determined the crystallization pathway following ACC breakdown.

Tobler, D.J., Rodriguez-Blanco, J.D., Sørensen, H.O., Stipp, S.L.S., Dideriksen, K. (2016) Effect of pH on Amorphous Calcium Carbonate Structure and Transformation. Crystal Growth & Design, 16, 4500–4508.  DOI: 10.1021/acs.cgd.6b00630 [PDF] [Supl. Info]

 

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