Solubility of Se in saline solutions – towards a consistent polythermal Pitzer dataset


Solubility of Se in saline solutions – towards a consistent polythermal Pitzer dataset

Bok, F.; Moog, H. C.

Selenium (with the isotope Se-79 being an important fission product) can occur in oxidation states varying between +VI and –II. Most often negatively charged species are formed rendering them extraordinarily mobile in groundwater systems. For a correct calculation of the solubilities of Se(VI) and Se(IV) phases, the Pitzer ion-ion interaction model is essential for solutions with high ionic strengths.
Beside solubility calculations – mostly relevant for low soluble earth alkali selenites – reliable thermodynamic data sets for selenium are also of importance for chemotoxicity estimations or as boundary system in S-Se-solid solutions.
The state-of-the-art thermodynamic data for Selenium are given in the OECD/NEA Chemical Thermodynamics. This compilation does not address the Pitzer ion-ion interaction model. A polythermal set of Pitzer interaction parameters was compiled by GRS. However, both compilations hold solubility data for T = 25 °C only.
Here, to enable the calculation of selenium solubility at various temperatures in high saline solutions, temperature functions for the solubility products of alkaline and earth alkaline selenium phases are presented.

The experimental solubility data of various alkaline and earth alkaline selenates and selenites have been collected. The temperature function’s parameters of the solubility products were fitted to these solubility data using the geochemical speciation code PHREEQC in combination with the parameter estimation software Ucode2014.
Beside the solubility data, also temperature function’s parameters for the relevant redox reactions have been determined. The THEREDA database was used for all auxiliary reactions. Thus, the obtained data are consistent with their actual Pitzer model.

Temperature function’s parameters for the solubility products of seven selenate and four selenite mineral phases as well as for the relevant redox reactions could be obtained. In all cases, simplified functions with only two parameters were sufficient to fit the data within the range of experimental uncertainty.
With this new temperature-dependent parameter set, the solubility of selenium(IV) and (VI) an be calculated over the temperature range (T = 0 – 100 °C) relevant for a nuclear waste disposal.
The literature on directly measured selenide solubility data is very limited and not sufficient to derive solubility products or Pitzer interaction parameters – even at 25 °C. Thus, for the Se(−II) system the solubility data for alkaline and earth alkaline solid phases were taken from thermochemical measurements. For the Pitzer interaction coefficients data from chemical analogs have been used (Br or H2S). For the selenide system, this dataset is valid for T = 25 °C only.
The new temperature functions provide a redox-enabled and self-consistent dataset for the solubility calculations of selenium within the oceanic salt system including carbonates from the binary up to ternary and some quaternary systems.
For oxidizing to light reducing conditions, this dataset is valid between T = 0 – 100 °C, for stronger reducing conditions, the dataset is valid at T = 25 °C only.

Keywords: THEREDA; Selenium; Solubility; Löslichkeit; Pitzer

  • Lecture (Conference)
    Workshop on Actinide-Brine-Chemistry and Workshop on High Temperature Aqueous Chemistry, 25.-27.06.2019, Karlsruhe, Deutschland

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Publ.-Id: 29193