Abstract
The purpose of this PhD thesis was to explore the influence of magnesium sulphate (MgSO4 (aq))
on calcium carbonate (CaCO3) minerals and what role the MgSO40 ion pair had. CaCO3 minerals
are abundant and widespread on Earth, particularly in marine environments, and have been so
throughout Earth’s history. CaCO3 are formed by many ocean living organisms resulting in a wide
variety of biominerals. In biomineralisation the growth behaviour of CaCO3 is influenced by organic
molecules and inorganic ions. The inorganic ions, magnesium (Mg2+) and sulphate (SO42-),
are common seawater ions and both are known to influence the formation and growth of CaCO3.
Recent work reports a synergistic effect when the two ions are present at the same time. To understand
abiotic and biotic CaCO3 formation in the past and the present, it is important to develop our
understanding of the mechanisms and processes behind the observed influences of Mg2+ and SO42-
in combination. I addressed this through three different studies, where classical wet chemistry
methods and surface sensitive techniques were combined with computer modelling to explore
what the growth inhibition mechanisms of SO42-, Mg2+ and the two combined was.
In Study 1, the growth inhibition of MgSO4 (aq) on calcite precipitation was evaluated at near
equilibrium conditions. The effect of MgSO4 (aq) was compared with the effect of the single ions
Mg2+ and SO42-. I used a constant composition set up and the resulting calcite was characterised
with surface spectroscopy, crystallographic analysis and microscopic techniques. The study
showed that MgSO4 (aq) was the most effective growth inhibitor of the three and gave rise to large
surface disruptions in the form of changed morphology. The results suggest that MgSO4 (aq) leads to
increased Mg uptake in calcite precipitated at ambient pressure and temperature.
In Study 2, the newly published microkinetic growth model by Andersson et al. (2016b) was
extended and applied to the data obtained in Study 1. This gave insights to the underlying growth
inhibition mechanisms. The model assumes that calcite growth is inhibited by foreign ions that
adsorb on steps, hindering the attachment of CaCO30 ion pairs, which is considered the rate determining
step for calcite growth. The study showed that SO42- alone acts by step blocking, Mg2+
blocks steps and also binds CO32-, which decreases the activity of CaCO30 ion pairs. The use of
MgSO4 (aq) as an inhibitor led to the strongest growth rate reductions but only by individual effects
of Mg2+ and SO42-, thus revealing that the MgSO40 ion pair is without influence on growth kinetics.
In Study 3, I explored the influence of Mg2+, SO42- and MgSO4 (aq) on direct formation of crystalline
CaCO3 observed with UV-vis spectrometry. The results showed that the presence of SO42- in
the experiments led to delayed vaterite formation and inhibition of vaterite growth. The presence
of Mg2+ and MgSO4 (aq) led to formation of aragonite. Aragonite nucleation was delayed and aragonite
growth reduced by of Mg2+ and MgSO4 (aq). However there was no increase in aragonite growth
inhibition by MgSO4 (aq) compared with Mg2+.
on calcium carbonate (CaCO3) minerals and what role the MgSO40 ion pair had. CaCO3 minerals
are abundant and widespread on Earth, particularly in marine environments, and have been so
throughout Earth’s history. CaCO3 are formed by many ocean living organisms resulting in a wide
variety of biominerals. In biomineralisation the growth behaviour of CaCO3 is influenced by organic
molecules and inorganic ions. The inorganic ions, magnesium (Mg2+) and sulphate (SO42-),
are common seawater ions and both are known to influence the formation and growth of CaCO3.
Recent work reports a synergistic effect when the two ions are present at the same time. To understand
abiotic and biotic CaCO3 formation in the past and the present, it is important to develop our
understanding of the mechanisms and processes behind the observed influences of Mg2+ and SO42-
in combination. I addressed this through three different studies, where classical wet chemistry
methods and surface sensitive techniques were combined with computer modelling to explore
what the growth inhibition mechanisms of SO42-, Mg2+ and the two combined was.
In Study 1, the growth inhibition of MgSO4 (aq) on calcite precipitation was evaluated at near
equilibrium conditions. The effect of MgSO4 (aq) was compared with the effect of the single ions
Mg2+ and SO42-. I used a constant composition set up and the resulting calcite was characterised
with surface spectroscopy, crystallographic analysis and microscopic techniques. The study
showed that MgSO4 (aq) was the most effective growth inhibitor of the three and gave rise to large
surface disruptions in the form of changed morphology. The results suggest that MgSO4 (aq) leads to
increased Mg uptake in calcite precipitated at ambient pressure and temperature.
In Study 2, the newly published microkinetic growth model by Andersson et al. (2016b) was
extended and applied to the data obtained in Study 1. This gave insights to the underlying growth
inhibition mechanisms. The model assumes that calcite growth is inhibited by foreign ions that
adsorb on steps, hindering the attachment of CaCO30 ion pairs, which is considered the rate determining
step for calcite growth. The study showed that SO42- alone acts by step blocking, Mg2+
blocks steps and also binds CO32-, which decreases the activity of CaCO30 ion pairs. The use of
MgSO4 (aq) as an inhibitor led to the strongest growth rate reductions but only by individual effects
of Mg2+ and SO42-, thus revealing that the MgSO40 ion pair is without influence on growth kinetics.
In Study 3, I explored the influence of Mg2+, SO42- and MgSO4 (aq) on direct formation of crystalline
CaCO3 observed with UV-vis spectrometry. The results showed that the presence of SO42- in
the experiments led to delayed vaterite formation and inhibition of vaterite growth. The presence
of Mg2+ and MgSO4 (aq) led to formation of aragonite. Aragonite nucleation was delayed and aragonite
growth reduced by of Mg2+ and MgSO4 (aq). However there was no increase in aragonite growth
inhibition by MgSO4 (aq) compared with Mg2+.
| Originalsprog | Engelsk |
|---|
| Forlag | Department of Chemistry, Faculty of Science, University of Copenhagen |
|---|---|
| Antal sider | 115 |
| Status | Udgivet - 2017 |