Comparative study of Se oxyanions retention on three argillaceous rocks: Upper Toarcian (Tournemire, France), Black Shales (Tournemire, France) and Opalinus Clay (Mont Terri, Switzerland)

Ex and In Situ Reactivity and Sorption of Selenium in Opalinus Clay in the Presence of a Selenium Reducing Microbial Community

“…Figure 4 compares the sorption distribution coefficients (Rd, Equation ( 1)) calculated from the aqueous Se(IV) measurements from the batch tests with clay only (Ab_Se(IV)-C_a) and with clay and filter (Ab_Se(IV)-CF_a), considering a total initial concentration of 9.3 µM, with Rd measurements presented by Frasca et al [18] for Se(IV) sorption onto Opalinus Clay (obtained from the Mont Terri Rock Laboratory). The data from Frasca et al [18] were undertaken with an initial Se(IV) concentration of 100 µM (approximately 70 µM at equilibrium), and so the higher Rd values calculated for the present abiotic experiments were expected, given the effect of the sorption isotherm [18]. However, the Rd values calculated using the 2-site Langmuir isotherm model of Frasca et al [18] for the range of aqueous Se(IV) concentrations (3.8 to 5.6 µM) measured in the present study were lower than (around half) the coefficients calculated from the concentration changes.…”

“…Modeling of the abiotic experiments was performed to examine the extent of sorption of Se(IV) onto Opalinus Clay using the 2-site Langmuir isotherm model parameterized by Frasca et al [18]. Considering an initial Se(IV) concentration of 9.3 µM (Figure 2), the PHREEQC model of the abiotic experiments calculated that after sorption onto Opalinus Clay, the aqueous Se(IV) concentration would be 7.8 µM.…”

“…The data from Frasca et al [18] were undertaken with an initial Se(IV) concentration of 100 µM (approximately 70 µM at equilibrium), and so the higher Rd values calculated for the present abiotic experiments were expected, given the effect of the sorption isotherm [18]. However, the Rd values calculated using the 2-site Langmuir isotherm model of Frasca et al [18] for the range of aqueous Se(IV) concentrations (3.8 to 5.6 µM) measured in the present study were lower than (around half) the coefficients calculated from the concentration changes. This suggests that sorption of Se(IV) was enhanced in the BN batch experiments compared to what was observed by Frasca et al [18].…”

“…where C 0 is the initial aqueous Se(IV) concentration (mol L −1 ), C is the remaining aqueous Se(IV) concentration (mol L −1 ), V is the solution volume (mL), and m is the mass of the clay (g). Frasca et al [18] compared the sorption of selenium oxyanions on three argillaceous rocks, including samples from a core of Opalinus Clay from a borehole at the Mont Terri Rock Laboratory. They found that while Se(VI) was non-sorbing, Se(IV) sorption behavior could be reproduced by 1-and 2-site Langmuir isotherms, with more accurate representation being obtained by the 2-site model:…”

“…where C s is the concentration of Se(IV) sorbed onto the solid phase (mol kg −1 ), C is the remaining aqueous concentration of Se(IV) (mol L −1 ), K L is the sorption potential (L mol −1 ), and S max is the sorption capacity (mol kg -1 ) of each site. Parameter values obtained by Frasca et al [18] for Opalinus Clay are shown in Table 3. No distinction in the sorption mechanism is made (i.e., direct sorption of Se(IV) vs. reduction followed by surface precipitation).…”