Effect of Adsorbed Carboxylates on the Dissolution of Boehmite Nanoplates in Highly Alkaline Solutions

Abstract

Understanding the dissolution of boehmite in highly alkaline solutions is important to processing complex nuclear waste stored at the Hanford (WA) and Savannah River (SC) sites in the United States. Here, we report the adsorption of model carboxylates on boehmite nanoplates in alkaline solutions and their effects on boehmite dissolution in 3 M NaOH at 80 °C. Although expectedly lower than at circumneutral pH, adsorption of oxalate occurred at pH 13, with adsorption decreasing linearly to 3 M NaOH. Classical molecular dynamics simulations suggest that the adsorption of oxalate dianions onto the boehmite surface under high pH can occur through either inner-or outer-sphere complexation mechanisms depending on adsorption sites. However, both adsorption models indicate relatively weak binding, with an energy preference of 1.26 to 2.10 kcal/mol. By preloading boehmite nanoplates with oxalate or acetate, we observed suppression of dissolution rates by 23 or 10%, respectively, compared to pure solids. Scanning electron microscopy and transmission electron microscopy characterizations revealed no detectable difference in the morphologic evolution of the dissolving boehmite materials. We conclude that preadsorbed carboxylates can persist on boehmite surfaces, decreasing the density of dissolution-active sites and thereby adding extrinsic controls on dissolution rates.

Effects of Ionizing Radiation on the Thermodynamic Stability of Boehmite and Gibbsite

“…For both boehmite and gibbsite, bands in the range of 500–900 cm –1 were assigned to the bending and stretching of Al–O bonds, and two bands in the range of 1050 to 1200 cm –1 (boehmite) and 950 to 1100 cm –1 (gibbsite), respectively, were attributed to the hydroxyl deformation. The OH stretch frequency region was from 3000 to 3400 cm –1 for boehmite and 3400–3600 cm –1 for gibbsite. ,− Compared to pristine boehmite, the intensity of peaks in the range of 1050–1200 cm –1 and 3000–3400 cm –1 on irradiated boehmite is lower (S1 and S2 from Figures S6a and S5c), which, not shown in the Raman data, indicates that irradiation removes a fraction of the hydroxyl groups on boehmite particles. But the OH stretch frequency region of 3400–3600 cm –1 for pristine and irradiated gibbsite does not show much difference (S3 and S4 from Figures S6b and S5d), while it shows a significant difference in the Raman data.…”