Neptunium(V) sorption on hematite (α-Fe2O3) in aqueous suspension: the effect 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 and quartz are also presented. The sorption of carbonate species on hematite was investigated and the data are compared to carbonate sorption by goethite as reported by van Geen et al. [1]. Finally, the sorption data are analyzed 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 constants 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 (pKa1 = -8.4; pKa2 = 10.4; pKNa+ = 8.55; pKCIO4- = -10.33; ns = 1.86 nm-2; C1 = 1.6 F/m2; C2 = 0.2 F/m2). A pHPZC of 9.4 ± 0.1 was determined. Neptunium sorption by hematite is independent of ionic strength (0.005 to 0.1 M NaClO4) for the experimental conditions of Np(V)T = 1.2 x 10-7 M and 4.46 x 10-5 M hematite surface sites. Under these conditions, fractional Np sorption was in the pH range of 6 to 8. FITEQL analysis of the adsorption data was achieved with an inner-sphere surface complexation reaction ≡FeOH + NpO2+ KNpO2+ <=> ≡ FeONpO2 + H- where KNpO2- = 10-2.09. This adsorption reaction is sufficient to describe Np sorption data under conditions of low surface site occupancy (i.e., less than 10%). FITEQL analysis of data for Np(V) sorption by goethite and quartz yielded surface complexation values of -1.57 and -6.93, respectively, using the same adsorption reaction as for hematite. Thus, logKNpO2+goethite > logKNpO2+hemapite > logKNpO2+quartz. Carbonate species sorbed stronger to hematite than to goethite. FITEQL analysis of the carbonate sorption data for both inner and outer sphere TLM configuration gave good fits to the observed data. In the ternary systems, Np(V)/carbonate/hematite or Np(V)/carbonate/goethite, Np sorption was strongly reduced relative to the CO2-free systems for pH values greater than 7.5 and PCO2 levels of 2% due to the formation of neptunyl carbonato solution complexes. TLM simulations using parameters estimated from the respective binary systems (e.g., carbonate/hematite and Np/hematite or Np/goethite) underestimate Np sorption due to competition of carbonate species with Np for surface sites. Invoking a ternary neptunyl-carbonato surface complex greatly improved the model fits for goethite and hematite in these systems for all partial pressures of CO2. (orig.)