Enhancing Silicate Dissolution Kinetics in Hyperalkaline Environments

Abstract

The dissolution of silicate minerals and glasses in aqueous solutions is important in many natural and engineered contexts including mineral weathering, nuclear waste stabilization, cementation, and infrastructure degradation. The influences of electrolytes on dissolution rates have been extensively studied, but previous studies have used widely varying minerals and electrolytes, experimental conditions, and measurement techniques. Comparatively fewer studies have been conducted in hyperalkaline solutions that are encountered in concrete, geopolymers, and nuclear stabilization systems. This study seeks to control many of these variables to isolate the effects of electrolyte composition on altering the degree of dissolution of a soda lime silicate glass in hyperalkaline electrolyte solutions. A glass powder is dissolved in one of a series of static solutions having 10 mmol L–1 NaOH (pH 12) at (25 ± 0.2) °C and either an organic or an inorganic electrolyte at a concentration of 1 or 10 mmol L–1. The solution series was designed to reveal qualitatively how ion identity and concentration alter the glass’ degree of dissolution after prescribed exposure times. The results indicate that a relative dissolution enhancement of no greater than about 2.4 times can be induced at pH 12, with the greatest enhancements being observed for Na-benzoate, Na-citrate, and Na-malonate. The degree of dissolution is unaffected by most of the other salts examined. Broadly, the nucleophilic attack by OH– on Si–O bonds and the formation of O––Na+ surface complexes appear to be the most important factors influencing the dissolution rate at high pH. The adsorption and the influence on dissolution of other electrolyte ions are comparatively weak.

Electrochemically Enhanced Dissolution of Silica and Alumina in Alkaline Environments

“…Although inorganic oxide materials typically exhibit low dissolution rates, such rates have been observed to increase dramatically at confined asymmetric solid–liquid–solid interfaces. For example, the geological phenomenon known as “pressure solution” involves enhanced dissolution at mineral–mineral grain boundaries in the presence of water, such as at the interface between quartz and clays, including muscovite mica. − While pressure solution effects were previously attributed to high geologic pressures, , recent studies of dissolution in acidic and neutral solutions have shown that enhanced dissolution can occur for confined surfaces at ambient pressures and furthermore indicate that the rates of dissolution depend on the electrostatic potentials of the two surfaces. , The electrochemical environment also influences corrosion at metal-mineral boundaries, , and other studies have described the influence of solution conditions (e.g., pH, ionic strength) and surface potential on the dissolution kinetics of isolated surfaces. − Although the dissolution kinetics of inorganic materials, including silicates and aluminates, under technologically relevant conditions have been widely studied, fundamental aspects that account for dissolution of these materials remain poorly understood, − especially under alkaline conditions that are specifically relevant to CMP and structural materials . Furthermore, the impact of electrochemical considerations on the dissolution of oxide materials at asymmetric solid–liquid–solid interfaces in alkaline solutions has not been explored.…”

New insights into the mechanisms of carbon dioxide mineralization by portlandite

“…This is unsurprising, especially in Class F fly ashes, wherein a high degree of silicate‐polymerization/connectivity implies decreased reactivity. Such connectivity of the network forming elements is known to strongly affect the dissolution rates of minerals and glasses 33,35,36 …”

Fly Ash–Ca(OH)2 Reactivity in Hypersaline NaCl and CaCl2 Brines

“…Thermodynamic modeling based on the minimization of Gibbs free energy is a powerful tool for assessing stable phase equilibria and equilibrium compositions of the solid and liquid (and gas) phases as relevant to encapsulation applications. However, the current implementations of such modeling often rely on assumptions including congruent dissolution of the fly ash and, most often, an inability to incorporate the actual degree of fly ash reaction which could be particularly affected in hypersaline environments that feature a reduced water activity. − For example, the dissolution rates of crystalline and amorphous materials could either enhance or decrease in hypersaline environments in relation to their compositions and concentrations. − For example, seawater has been suggested to enhance fly ash’s pozzolanic reactions . Since changes in reactivity alter not only the kinetics of reactions but also the phase assemblage that forms, it is important to assess these aspects due to the obvious implications on contaminant retention by the solidified waste forms.…”