Abstract
This study aimed at determining the chemical alterations occurring at the surface of multi-oxide silicate glasses in the presence of organic ligands—citrate and tartrate—at a near-neutral pH. Batch surface titration experiments for basaltic glass and blast furnace slag (BFS) were conducted in the range of 6.4 < pH < 8 to investigate the element release, and speciation and solid phase saturation were modeled with PHREEQC software. Surface sensitive XPS and zeta potential measurements were used to characterize the alterations occurring on the surface. The results show that, while Al/Si and Fe/Si surface molar ratios of the raw materials increase at a near-neutral pH, the presence of organic ligands prevents the accumulation of Al and Fe on the surface and increases their concentration in the solution, particularly at pH 6.4. The Al- and Fe-complexing ligands decrease the effective concentration of these cations in the solution, consequently decreasing the surface cation/Si ratio, which destabilizes the silicate surface and increases the extent of dissolution by 300% within the 2 h experiment. Based on the thermodynamic modeling, 1:1 metal-to-ligand complexes are the most prevalent aqueous species under these experimental conditions. Moreover, changes in Ca/Si and Mg/Si surface ratios are observed in the presence of organic ligands; the direction of the change depends on the type of ligand and pH. The coordination of Al and Fe on the surface is different depending on the ligand and pH. This study provides a detailed description of the compositional changes occurring between the surface of multi-oxide silicate materials and the solution in the presence of citrate and tartrate. The surface layer composition is crucial not only for understanding and controlling the dissolution of these materials but also for determining the activated surface complexes and secondary minerals that they evolve into.
Highlights
Various industrial materials, such as glasses, cements, and ceramics, consist of multi-oxide silicates
The release of Si is only slightly higher at pH 8 than at pH 10, which shows that the extent of the dissolution of the basalt glass is similar at pH 10 after 5 min of mixing as it is at pH 8 after 124 min of mixing, demonstrating the low solubility of basalt glass at pH 8
The results demonstrated that the leached Al and Fe are adsorbed or precipitated as hydroxides or silicates on the surface of the multi-oxide raw materials based on the X-ray Photoelectron Spectroscopy (XPS) analysis and thermodynamic modeling
Summary
Various industrial materials, such as glasses, cements, and ceramics, consist of multi-oxide silicates. In contact with the solution, the solid silicate surface can undergo a number of different reactions, such as dissolution, leaching, adsorption, and precipitation. For research fields such as nuclear waste stabilization in glasses,[1] geosciences,[2] reactivity of cements and supplementary cementitious materials,[3] and biosolubility of glasses,[4] to mention just a few, it is central to understand the mechanisms involved. When in contact with water, a leaching of cations from the silicate surface occurs by proton−metal exchange reactions, the depth of which depends on the solid and solution properties and the experimental conditions.[2,5−7] At 2 < pH < 5, all cations are typically leached, while Si dissolves only at a very low rate. The release of Si can be taken as an estimation of the extent of the dissolution of the raw material in cases when re-precipitation of Si can be excluded
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