Retention of selenate at the water-mineral interface in the context of salt dome repositories


Retention of selenate at the water-mineral interface in the context of salt dome repositories

Franzen, C.; Hering, D.; Jordan, N.

One major process controlling the mobility and bioavailability of selenium, a long-lived fission product found in nuclear waste, is the adsorption onto mineral surfaces of both the engineered and geological barrier. In this context, it is important to understand to what extent this sorption is influenced particularly by characteristic parameters as expected in deep underground repositories for high level and long-lived radioactive waste. These parameters include inter alia the presence of different background salts which are important with regard to salt domes as potential repositories.
In the present study, a combination of macroscopic sorption experiments, electrophoretic mobility and in-situ ATR FT-IR spectroscopy measurements was used to study the interaction of selenate with aged γ-Al2O3 in the presence of NaCl and MgCl2. From in-situ ATR FT-IR spectra, a change in the symmetry of the aqueous tetrahedral selenate anion can be derived evidencing the formation of a surface complex on γ-Al2O3. From batch experiments, we observe a dependence of selenate sorption on the ionic strength and composition of the electrolyte. Additionally, the sorption generally decreases with increasing pH. However, in the presence of 0.1 M MgCl2, the sorption increased again at a pH above 9.5.
The isoelectric point (pHIEP) of γ-Al2O3 is located at pH 9.6 for low NaCl background electrolyte concentration (I = 0.1 M). The increase of ionic strength (up to I = 1 M) results in a decrease of the zeta potential for both the acidic and alkaline pH range. However, in the alkaline range the decrease of the zeta potential is more pronounced. Additionally, we observe that the pHIEP is shifted to more alkaline values and finally no charge reversal is observed. In the presence 0.1 M MgCl2, the surface charge of γ-Al2O3 is positive throughout the studied pH range (3-11). Above pH 10, a sharp potential decrease occurs due to Mg(OH)2 precipitation. The impact of the varied parameters on the sorption of selenate in the alkaline pH range will be verified in detail.

Keywords: sorption; selenate; gamma-al2O3

  • Poster
    Goldschmidt 2013, 25.-30.08.2013, Florence, Italy
  • Mineralogical Magazine 77(2013)5, 1107-1107
    DOI: 10.1180/minmag.2013.077.5.6

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Publ.-Id: 18687