Se(IV)/Se(VI) sorption on illite du Puy Breynaert, E. ; Bruggeman, C. ; Vancluysen, C. ; Dom, D. ; Maes, A. Katholieke Universiteit Leuven, Center for Surface Chemistry and Catalysis, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium Illite du Puy Purification procedure • • Se(IV) sorption on illite du Puy Short term (24 u) adsorption isotherm at p. H 4 grinding (ball mill) of the clay < 200 µm transformation to homo-ionic Na-form oxidation of organic matter (Kunze and Dixon, 1986) removal of free iron oxide (Kunze and Dixon, 1986) removal of carbonate (Van Reeuwijk, 1986) washing Size fractionation: < 2 µm, 2 -50 µm, 50 -200 µm Characterisation • CEC 108 µEq/g 22 Na-CEC at neutral p. H (7. 05 0. 07) 0. 01 N Na. Cl, ~ 8 g/L, overnight • specific surface BET surface area (N 2) : 132. 8 0. 6 m²/g Micropore area (< 2 nm): 11. 9 m²/g Solubility • Solubility data agrees with data from Bradbury and Baeyens (2005). • Ca concentration after purification may determine number of available sites • Ca concentrations too low for Ca. Se. O 3 precipitation • adsorption (p. H 4) ~ f([Se], I) • (almost) linear isotherm • independent of ionic strength Se(IV) sorption model Se(VI) [Se. O 42 -] sorption on illite du Puy • BASIS: 2 SPNE model ( Bradbury and Baeyens, 1997 ). illite du Puy surface description with 2 amphoteric surface hydroxyl sites ╟Swa, I 4. 5 x 10 -2 mol/kg ╟Swb, I 4. 5 x 10 -2 mol/kg ╟Swa, IOH + H+ ⇌ ╟Swa, IOH ⇌ ╟Swb, IOH + H+ ⇌ ╟Swb, IOH ⇌ ╟Swa, IOH 2+ log k = 5. 5 ╟Swa, IO- + H+ log k = -6. 2 ╟Swb, IOH 2+ log k = 9. 0 ╟Swb, IO- + H+ log k = -10. 5 • Selenite speciation (Olin et al. , 2005; Seby et al. , 2001) Se. O 3 -2 + H+ ⇌ HSe. O 3 - + H+ ⇌ No significant sorption of selenate between p. H 3 and 10 Se(IV) sorption on illite du Puy Short term (24 u) adsorption edge (0. 01 M Na. Cl) HSe. O 3 H 2 Se. O 3 log k = 8. 36 log k = 2. 64 • Selenite sorption model additions/changes - Small change in p. Ka value ╟Swb, IOH site ╟Swb, IOH + H+ ⇌ ╟Swb, IOH 2+ log k = 8. 2 - Addition of 2 extra minor surface sites to model the concentration dependent sorption behaviour ╟Swa, II 2 x 10 -6 mol/kg ╟Swb, II 4 x 10 -7 mol/kg ╟Swa, IIOH + H+ ⇌ ╟Swa, IIOH 2+ ╟Swa, IIOH ⇌ ╟Swb, IIOH + H+ ⇌ ╟Swb, IIOH ⇌ log k = 4. 5 ╟Swa, IIO- + H+ log k = -6. 2 ╟Swb, IOH 2+ log k = 9. 5 ╟Swb, IIO- + H+ log k = -10. 5 - 2 types of reactions with charged and uncharged Se(IV) ⇌ ╟Swa, IOH 2 HSe. O 3 log k = 2. 7 ╟Swa, IIOH 2+ + HSe. O 3 - ⇌ ╟Swa, IIOH 2 HSe. O 3 log k = 7. 7 ╟Swb, IOH 2+ + HSe. O 3 - ⇌ ╟Swb, IOH 2 HSe. O 3 log k = 2. 45 ╟Swb, IIOH 2+ + HSe. O 3 - ⇌ ╟Swb, IIOH 2 HSe. O 3 log k = 7. 65 ╟Swa, IOH 2+ + H 2 Se. O 3 ⇌ ╟Swa, IOH 2 H 2 Se. O 3+ log k = 3. 7 ╟Swa, IIOH 2+ + H 2 Se. O 3 ⇌ ╟Swa, IIOH 2 H 2 Se. O 3+ log k = 8. 0 ╟Swb, IOH 2+ + H 2 Se. O 3 ⇌ ╟Swb, IOH 2 H 2 Se. O 3+ log k = 3. 7 ╟Swb, IIOH 2+ + H 2 Se. O 3 ⇌ ╟Swb, IIOH 2 H 2 Se. O 3+ log k = 8. 0 ╟Swa, IOH 2+ + HSe. O 3 - Conclusions • Adsorption on short-term can be described based on 2 SPNE surface complexation model with ligand exchange • adsorption ~ f(p. H, [Se]) • classical anion sorption edge on oxidic surface • Se(IV) adsorbs onto illite clay forming surface complexes, most likely with aluminol sites at clay platelet edges Acknowledgements: ONDRAF/NIRAS – TRANCOM II – FUNMIG
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