Aspects of the calcium carbonate-water interface

Thesis



The channel flow method has established the net dissolution kinetics of calcite single crystals at high pH (7.7-9.7) and varying bulk Ca²⁺ concentrations (0-10 mM), using wide ranges of solution flow rates (10^-3-0.3 cm³ s^-1). Literature rate equations were in poor agreement with experiment. Modelling with the following mechanism, where Ca²⁺ and CO₃^2- undergo Langmuirian adsorption:

Ca²⁺_(aq) ⇄ Ca²⁺_(ads)

CO^2-

3 (aq) ⇄ CO^2-_{3 (ads)}

Ca(sup>2+

(ads) + CO^2-_{3 (ads)} ⇄ CaCO_{3 (ads)}

CaCo_{3 (ads)} → CaCo_{3 (lattice)}

and the consequent rate law

J_net/mol cm^-2

s^-1 = k_pK_CaK_CO3 {K_sp - [Ca²⁺]_o[CO^2-₃]_o _________________________________________________________________________________________________ (1 + K_Ca[Ca²⁺]_o)(1 + K_{CO3[CO^2-3]o)}

gave excellent agreement with experiment under all conditions studied. This mechanism is shown to explain all literature streaming potential, electrophoresis and kinetic salt effect data.

Dissolution of calcite under the above conditions was strongly inhibited by Mg²⁺ and fully deprotonated forms of succinic acid, 2-sulphobutanedioic acid, phthalic acid and maleic acid. Mechanisms were established; for the maleate dianion, the inhibition was due to the blocking of the dissolution sites at which CaCO₃ units are incorporated into the crystal lattice. For the other ions, inhibition arose from competitive Langmuirian adsorption either between CO₃^{2- and the anions, or between Ca2+ and Mg2+.}

A new method to quantify the inhibited dissolution of particulate CaCO₃ (=10 μm) via enhanced mass transport of solution to the rotating disc electrode, due to the rotation of the particles in the diffusion layer, has been established. Good agreement was found with that measured independently using the channel flow cell.

A.c. impedance spectroscopy has been used to characterise scaled (CaCO₃) steel tubes. Results provide scope for (i) monitoring scale growth, and (ii) use in safety control devices for alerting to the scaling of pipe-work.

Authors: Brown, C

• Publication date: 1992
• DOI: 10.5287/ora-vekw8ndkb
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Research; Chemical kinetics; Solid-liquid interfaces