Neptunium(V) sorption on quartz and albite in aqueous suspension; Annual progress report

The behavior of neptunium in the subsurface environment is of interest since neptunium isotopes are included in nuclear waste. Previous work investigated the sorption behavior of Np onto {alpha}-Fe{sub 2}O{sub 3} (hematite), an accessory mineral of the Yucca Mountain repository. The work reported herein involves the much more abundant silicate minerals quartz and albite, and is a logical continuation of the ongoing task. In previous work increased sorption was observed in systems containing hematite and EDTA, a ligand which acts as a surrogate for organic complexing agents. In addition, increased partial pressures of CO{sub 2} are common in many ground waters and the effects of carbonate on sorption of radionuclides have to be studied as well. At concentration levels of 10{sup {minus}7} M, Np(V) does not adsorb strongly on quartz and albite up to pH values of approximately 9 at solid/solution ratios of 30 to 40 g/l. Significant adsorption (> 20%) occurs on both minerals only at pH > 9. Pretreatment of albite affects the sorption behavior of this mineral at pH > 9, possibly due to the formation of secondary mineral phases at the albite surface. EDTA does not adsorb on quartz at concentrations of 10{sup {minus}6} M. In the presence of 50 {mu}M EDTA, Np(V) sorption seems to be restricted. EDTA at the 10{sup {minus}6} M level adsorbs onto albite to an appreciable degree at pH values < 7.5. One {mu}M EDTA has no effect on Np(V) adsorption onto albite. Carbonate species adsorb on quartz and albite, both cases showing a maximum in sorption at pH 6.5 to 7 where HCO{sub 3}{sup {minus}} is the predominant solution species