The surface processes of Se(IV) on γ-alumina in the presence of carbonate. Evaluating competitive effects on the sorption behavior


The surface processes of Se(IV) on γ-alumina in the presence of carbonate. Evaluating competitive effects on the sorption behavior

Foerstendorf, H.; Mayordomo, N.; Jordan, N.; Alonso, U.; Missana, T.; Schmeide, K.

The sorption and desorption processes of selenium(IV) onto γ-Al2O3 at different pH values and ionic strengths, as well as the impact of carbonate ions, have been studied by classical batch sorption experiments and in situ Attenuated Total Reflection Fourier-transform Infrared (ATR FT-IR) spectroscopy.

From recent X-ray absorption spectroscopic investigations the predominant surface complexes of Se(IV) on γ-alumina were identified as bidentate bridging complexes with AlO6 surface groups in the circumneutral pH range (pH 4 – 8) [1]. However, contributions of outer-sphere complexes were not completely ruled out. Thus, we performed batch sorption series and vibrational spectroscopic experiments at different ionic strengths to evaluate the fraction of prevailing outer-sphere complexation. It could be shown that Se(IV) sorption was independent of the ionic strength suggesting that outer-sphere complexation can be neglected under the prevailing conditions.

At more alkaline pH level, the sorption processes proceeded more reversibly than under acidic and neutral conditions, as it was derived from the IR spectroscopic experiments providing molecular information of the ongoing sorption and desorption processes in real time [2]. The enhanced release of the selenite from the alumina phase is due to modification of alumina surface properties at a higher pH level.

Carbonate ions were found to form outer-sphere monodentate complexes at the γ-Al2O3 surface, and hence, the respective sorption process was highly reversible. In the ternary sorption system, the Se(IV) adsorption onto alumina was found to be slightly reduced and an enhanced release of selenite subsequent to the sorption reaction was observed. A change of the speciation of selenite on the alumina surface in terms of structural alterations or different binding mechanism was not observed.

Our results strongly suggested that Se(IV) surface affinity towards γ-Al2O3 is higher than the one of carbonate ions. Nevertheless, the competing effect of carbonate ions might impact the migration of Se(IV) by reducing the number of available sorption sites on sorbing surfaces and by enhancing desorption processes. Consequently, this should be taken into account in predicting the environmental fate of Se(IV). Therefore, we performed Surface Complexation Model (SCM) calculations triggered by the spectroscopic findings and first results will be presented. The modelling of the surface speciation based on spectroscopic results has proven to be a strong tool to gain a sustainable description of the surface processes occurring in sorption systems relevant for the safety assessment of a future nuclear waste disposal site.

[1] E. J. Elzinga, et al., J. Colloid Interface Sci. 340, 153 (2009).
[2] H. Foerstendorf, et al., J. Colloid Interface Sci. 416, 133 (2014).
[3] M. J. Comarmond, et al., Environ. Sci. Technol. 50, 11610 (2016).

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