A Kinetic Study of the Reduction of Plutonium with Humic Acid

The rate equation of the reduction of plutonium by humic acid has been derived. Preliminary results on the effect of the constituents of granite groundwater on the reduction rate of plutonium by humic acid are presented. Introduction Humic acid is one of the reductive organic electrolyte in the groundwater. The studies of the reaction of radioactive elements with humic acid have great significance for the understanding of adsorption and migration of radionuclides in geomedia and for safety assessment of geological disposal of radwaste. Similar studies on the reduction of Np(VI) by hydroxybenzoic acid [1] and by methylsalicylic acid [2] have been reported. In this paper, results of the reduction of plutonium by humic acid are reported. Experimental Reagen t s All solutions are prepared with water redistilled in presence of permanganate. The stock solution of Pu(VI) is prepared by electrolytic oxidation of Pu in 1 mol/1 HC104. Humic acid (Fluka Chimie AG, Sweden) is titrated by pH metry [3] and values of 8.7 mmol carboxy and phenolic group in 1 g of humic acid are determined. P rocedu re The stoichiometry of reduction of Pu is determined by coprecipitation of Pu(III, IV) with LaF3 [4] in the presence of humic acid added in great excess with respect to Pu (5.0 χ 10 7 mol/1). The ionic strength of the solution is kept constant with 0.10 mol/1 NaC104 and pH is adjusted to the desirable value with a solution of NaOH. Unless noted otherwise, all the experiments are performed at 25±0.5°C. Results and discussion Kine t i c s tudy of r e d u c t i o n of p l u t o n i u m The change of the Pu(VI, V) concentration as a function of time has been measured at constant concentration of the humic acid HA and at constant pH, typical curves are shown in Fig. 1. The rate equation of the reduction can be expressed as follows : -d[Pu(VI,V)]/dr = /qPu(VI,V)][HA][H] where Κ is the rate constant. Because the reduction of Pu(VI) to Pu(V) is very fast and the reduction of Pu(V) to Pu(IV) is much slower, the remaining is mainly Pu(V). All results show that the reaction of Pu(V) is of first order and the reduction rate is proportional to the concentration of Pu(V). Therefore the rate equation of reduction may be written: — d[Pu(V)]/di = /qPu(V)][HA][H]"