A Study on Ikaite Growth in the Presence of Phosphate

Phosphate is a common component in natural growth solutions of ikaite. Although phosphate often occurs as a minor constituent, its presence may promote the formation of ikaite as it significantly inhibits the precipitation of calcite. The interactions of phosphate with ikaite and the role of a potential uptake of phosphate by ikaite, however, are poorly understood. In this study, the influence of phosphate on ikaite growth at 1 °C was investigated. Ikaite- and calcite-seeded growth experiments were conducted in cryo-mixed-flow reactors at saturation ratios 1.5 ≤ Ω_ikaite ≤ 2.9 (Ω = ionic activity product/solubility product). From these growth experiments, the rate constant k = 0.10 ± 0.03 µmol/m²/s and the reaction order n = 0.8 ± 0.3 were derived for ikaite. The reaction order implies a transport- or adsorption-controlled growth mechanism which supports a low energy pathway of ikaite growth via an attachment of hydrous CaCO₃⁰ complexes without any extensive dehydration of aqueous species as, for instance, required for calcite growth. A potential depletion of aqueous phosphate due to an uptake by ikaite growth was not detectable. Furthermore, growth retardation by phosphate, as known for calcite growth, was not evident. Thus, a significant incorporation of phosphate into growing ikaite could be precluded for the conditions applied in this study. The observed lack of incorporation of phosphate agrees with the previously suggested growth mechanism via the attachment of hydrous CaCO₃⁰ complexes which likely does not facilitate substantial substitution of carbonate by phosphate ions.