We have investigated the effect of changes in solution chemistry on the nature of uranyl sorption complexes on montmorillonite (SAz-1) at different surface coverages (1.43-53.6 µmol/g). Uranyl uptake onto SAz-1 between pH 3 and 7 was determined in both titration and batch-mode experiments. These pH values result in solutions that contain a range of monomeric and oligomeric aqueous uranyl species. Continuous-wave and time-resolved emission spectroscopies were used to investigate the nature of U(VI) sorbed to SAz-1. A discrete set of uranyl surface complexes has been identified over a wide range of pH values at these low to moderate coverages. For all samples, two surface complexes are detected with spectral characteristics commensurate with an inner-sphere complex and an exchange-site complex; the relative abundance of these two species is similar over these pH values at low coverage (1.43-2.00 µmol/g). In addition, surface species having spectra consistent with polymeric hydroxide-like sorption complexes form at the moderate coverages ( approximately 34-54 µmol/g), increasing in abundance as the capacity of the amphoteric surface sites is exceeded. Furthermore, a species with spectral characteristics anticipated for an outer-sphere surface complex is observed for wet paste samples at low pH (3.7-4.4) and both low ( approximately 2 µmol/g) and moderate ( approximately 40 µmol/g) coverage. There are only subtle differences in the nature of sorption complexes formed at different pH values but similar coverages, despite markedly different uranyl speciation in solution. These results indicate that the speciation in the solution has minimal influence on the nature of the sorption complex under these experimental conditions. The primary control on the nature and abundance of the different uranyl sorption complexes appears to be the relative abundance and reactivity of the different sorption sites. Copyright 2001 Academic Press.

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Raman spectroscopy is used to study the effect of pH on the solute/sorbent interactions and the chemical nature of the adsorbed species at the aqueous/silica interface for the aza-arenes acridine, quinoline, and pyridine. The neutral aza-arene is hydrogen-bonded to surface sites in aqueous media in which the pH is above the p Ka of the compound. A protonated aza-arene/ClO4− ion pair interacts with surface sites in aqueous solutions in which the pH is below the pKa of the compound. These results may be useful in predicting the fate of these environmentally significant compounds in groundwater.

The effects of the aqueous solution pH, temperature, initial solute concentration and non-equilibrium processes on the transport of acridine in saturated porous media (silica) were investigated in a series of continuous-flow column experiments. The enthalpy of the adsorption reaction was more exothermic when the solution pH was above acridine's pK a (5.6) than when it was below. The extent of adsorption was greater when the solution pH was below the pK a of acridine than when it was above. Non-equilibrium effects on the adsorption reaction were found to be unimportant at groundwater velocities. The results from this study suggest that the mobility of acridine in aquifers, in which adsorption to silica edge sites is significant, would increase as the temperature of the groundwater increased and the acridine mobility would be greatest when the pH of the groundwater is above the pK a of acridine. The transport of acridine in such aquifers can be effectively modeled using the local equilibrium assumption.

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