Se(IV)/Se(VI) sorption on illite du Puy

1. Purification procedure Solubility • BASIS: 2 SPNE model ( Bradbury and Baeyens, 1997 ). illite du Puy surface description with 2 amphoteric surface hydroxyl sites • 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 • 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 CaSeO3 precipitation ╟Swa, I 4.5 x 10-2 mol/kg ╟Swb, I 4.5 x 10-2 mol/kg ╟Swa,IOH + H+ ⇌ ╟Swa,IOH2+log k = 5.5 ╟Swa,IOH ⇌ ╟Swa,IO- + H+ log k = -6.2 ╟Swb,IOH + H+⇌ ╟Swb,IOH2+log k = 9.0 ╟Swb,IOH ⇌ ╟Swb,IO- + H+ log k = -10.5 • Selenite speciation (Olin et al., 2005; Seby et al., 2001) SeO3-2+H+ ⇌ HSeO3- log k = 8.36 HSeO3-+H+⇌ H2SeO3 log k = 2.64 • Selenite sorption model additions/changes • - Small change in pKa value ╟Swb,IOH site • ╟Swb,IOH + H+⇌ ╟Swb,IOH2+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,IIOH2+log k = 4.5 ╟Swa,IIOH ⇌ ╟Swa,IIO- + H+ log k = -6.2 ╟Swb,IIOH + H+⇌ ╟Swb,IOH2+log k = 9.5 ╟Swb,IIOH ⇌ ╟Swb,IIO- + H+ log k = -10.5 - 2 types of reactions with charged and uncharged Se(IV) ╟Swa,IOH2+ + HSeO3- ⇌ ╟Swa,IOH2HSeO3log k = 2.7 ╟Swa,IIOH2+ + HSeO3-⇌ ╟Swa,IIOH2HSeO3 log k = 7.7 ╟Swb,IOH2+ + HSeO3-⇌ ╟Swb,IOH2HSeO3 log k = 2.45 ╟Swb,IIOH2+ + HSeO3-⇌ ╟Swb,IIOH2HSeO3 log k = 7.65 ╟Swa,IOH2+ + H2SeO3⇌ ╟Swa,IOH2H2SeO3+log k = 3.7 ╟Swa,IIOH2+ + H2SeO3⇌ ╟Swa,IIOH2H2SeO3+ log k = 8.0 ╟Swb,IOH2+ + H2SeO3⇌ ╟Swb,IOH2H2SeO3+ log k = 3.7 ╟Swb,IIOH2+ + H2SeO3⇌ ╟Swb,IIOH2H2SeO3+ log k = 8.0 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 Se(IV) sorption on illite du Puy Short term (24u) adsorption isotherm at pH 4 Characterisation • CEC • 108 µEq/g • 22Na-CEC at neutral pH (7.050.07) • 0.01 N NaCl, ~ 8 g/L, overnight • specific surface • BET surface area (N2) :132.8  0.6 m²/g • Micropore area (< 2 nm): 11.9 m²/g • adsorption (pH 4) ~ f([Se], I) • (almost) linear isotherm • independent of ionic strength Se(IV) sorption model Se(VI) [SeO42-] sorption on illite du Puy No significant sorption of selenate between pH 3 and 10 Se(IV) sorption on illite du Puy Short term (24u)adsorption edge (0.01M NaCl) • Conclusions • Adsorption on short-term can be described based on 2 SPNE surface complexation model with ligand exchange • Se(IV) adsorbs onto illite clay forming surface complexes, most likely with aluminol sites at clay platelet edges • adsorption ~ f(pH, [Se]) • classical anion sorption edge on oxidic surface Acknowledgements: ONDRAF/NIRAS – TRANCOM II – FUNMIG