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
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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.