NEPTUNIUM ADSORPTION ON SYNTHETIC AMORPHOUS IRON OXYHYDROXIDE

Neptunium adsorption on synthetic amorphous iron oxyhydroxide (Fe2O3·H2O(a)) was investigated at 25°C for initial Np(V) concentrations between 4.5 × 10-11 and 4.5 × 10-13 M in 0.1 M NaNO3 electrolyte solutions undersaturated with atmospheric CO2 for pH > 7. The surface properties of the Fe2O3·H2O(a) were determined by CO2-free acid-base titration (pH(PZC) 8.0 ± 0.2), and surface acidity and electrolyte complexation constants were determined using the triple-layer model (TLM). The aqueous speciation of Np(V) is dominated by the neptunyl oxycation, NpO2+, in the region of the adsorption edges (5 < pH < 8). At the Np concentrations investigated, the adsorption data are consistent with the hypotheses that (1) Fe2O3·H2O(a) can be treated as an "ideal" single-site ligand, (2) no deprotonation of surface sites need be invoked to describe Np(V) adsorption, and (3) NpO2(OH)0 is the dominant Np surface species. A single surface-coordination reaction, XOH + (NpO2+)s + H2O = (XOH - NpO2(OH))0 + Hs+ (log K = -3.2), described all adsorption data and is independent of the surface-coordination model. Surface hydrolysis of NpO2+ occurs 2.4 pH units below NpO2+ hydrolysis in bulk solution (pKHY1 = 8.85). © 1991.