Neptunium(V) sorption on hematite (alpha-Fe2O3) in aqueous suspension: Theeffect of CO2

A data set for the sorption of Np(V) on hematite is provided and the effect of carbonate species on neptunyl (NpO2+) sorption as a function of partial pressures of CO2 is investigated. Data for neptunyl sorption on goethite a nd quartz are also presented. The sorption of carbonate species on hematite was investigated and the data are compared to carbonate sorption by goethi te as reported by van Geen et al. [1]. Finally, the sorption data are analy zed with FITEQL (v 3.1; [2]) and the Triple Layer Model configuration (TLM: Davis et al. [3]) of the particle/water interface is invoked. Binding cons tants of postulated surface species are presented. Surface charge density data from potentiometric titrations of the hematite suspension at different ionic strengths (NaClO4) were analyzed with FITEQL and TLM parameters were estimated (pK(a1) = -8.4; pK(a2) = 10.4; pK(Na+) = 8.55; pK(ClO4)(-) = -10.33; n(s) = 1.86 nm(-2); C-1 = 1.6 F/m(2); C-2 = 0.2 F/m(2)). A pH(PZC) of 9.4 +/- 0.1 was determined. Neptunium sorption by he matite is independent of ionic strength (0.005 to 0.1 M NaClO4) for the exp erimental conditions of Np(V)(T) = 1.2 x 10(-7) M and 4.46 x 10(-5) M hemat ite surface sites. Under these conditions, fractional Np sorption was in th e pH range of 6 to 8. FITEQL analysis of the adsorption data was achieved w ith an inner-sphere surface complexation reaction [GRAPHICS] where KNpO2+ = 10(-2.09). This adsorption reaction is sufficient to describ e Np sorption data under conditions of low surface site occupancy (i.e., le ss than 10%). FITEQL analysis of data for Np(V) sorption by goethite and qu artz yielded surface complexation values of -1.57 and -6.93, respectively, using the same adsorption reaction as for hematite. Thus, logK(NpO2)(goethi te)> logK(NpO2)(hematite) > logK(NpO2)(quartz). Carbonate species sorbed st ronger to hematite than to goethite. FITEQL analysis of the carbonate sorpt ion data for both inner and outer sphere TLM configuration gave good fits t o the observed data. In the ternary systems, Np(V)/carbonate/hematite or Np(V)/ carbonate/goethi te, Np sorption was strongly reduced relative to the CO2-free systems for p H values greater than 7.5 and P-CO2 levels of 2% due to the formation of ne ptunyl carbonate solution complexes. TLM simulations using parameters estim ated from the respective binary systems (e.g., carbonate/hematite and Np/he matite or Np/goethite) underestimate Np sorption due to competition of carb onate species with Np for surface sites. Invoking a ternary neptunyl-carbon ato surface complex greatly improved the model fits for goethite and hemati te in these systems for all partial pressures of CO2.