The Effect of the binding sites of humic acid for the complexation with Np,4

Tobitsuka, Sachiko; Iijima, Kazuki  ; Kohara, Yukitoshi*

The influence of humic acid for the solubility of tetravalent neptunium was investigated under reducing condition, by comparing at various pH. All of experiments were carried in an argon glove-box except analyses. At pH5, 8 and 9, at 20-24 degree C, Np solution in 1E-5 or 1E-3 mol/l was mixed with 0, 5-1,000 mg/l of purified Aldrich humic acid (PAHA). The ionic strength was 0.25 which was brought by 0.1 mol/l NaClOand 0.05 mol/l NaSOadded as a reductant to keep Np in tetravalent. The concentration of Np was quantified periodically by alpha spectrometry or ICP-MS. With the acid-base titration, analyzed by NICA-Donnan model which assumes the carboxyl group and the phenolic ydroxyl group as two main functional groups, the charge density of PAHA was 2.84, 4.36, 4.78 meq/g at pH5, 8, 9, respectively. In all cases, the total Np concentration ([Np]t) increases with increasing PAHA concentration. In the case of 1E-5 mol/l of initial Np concentration ([Np] especially, [Np]t increases almost up to [Np]in 50 mg/l PAHA at pH8, 9, or 25 mg/l at pH5. Even in the case of PAHA 5 mg/l, [Np]t becomes one order or more higher than that in the absence of PAHA. The apparent stability constant