Surface Layer Alteration of Multi-Oxide Silicate Glasses at a Near-Neutral pH in the Presence of Citric and Tartaric Acid

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.