Plutonium desorption

The sorption and desorption of aqueous plutonium in the range of 10 7 to 10 8M was studied on quartz and other silica surfaces. Sorption continued typically for 12 to 15 days before apparent equilibrium was reached, and the distribution of plutonium particle sizes sorbed on the silica was different from that in solution. At pH 7, sorption increased with increasing ionic strength, but decreased when bicarbonate was added. The amount of sorption varied at pH 5 and 7, but differently at high and low ionic strengths, as well as with the age of the solution. Plutonium desorption indicated that there were two basically different sorbed species, and the rate and quantity of desorbed material increased at pH 5 compared with 7 and 9. 

These observations contrast with some of the results obtained in natural waters. In the experiments where contaminated sediments were equilibrated with Lake Michigan water for a number of days, the Pu(IV) that was on the sediments and was transferred to the water was oxidized to Pu(V), with the oxidation occurring either during or after desorption (15). The studies in the Irish Sea near Windscale show that although no more than 1 percent of the waste effluent stream is oxidized plutonium, approximately 5 percent of the plutonium released leaves the area in the currents of the Irish Sea as oxidized plutonium. Most of the plutonium, therefore, must be oxidized fairly rapidly in sea water. 

Erdal, B.R. Aguilar, R.D. Bayhurst, B.P. Daniels, W.R. Duffy, C.J. Lawrence, F.O. Maestas, S. Oliver, P.Q. Wolfsberg, K. "Sorption-Desorption Studies on Granite. I. Initial Studies of Strontium, Technetium, Cesium, Barium, Cerium, Europium, Uranium, Plutonium, and Americium", in "Proceedings of the Task 4 Waste Isolation Safety Assessment Program Second Contractor Information Meeting", Vol. II, Report PNL-SA-7352, Battelle Pacific Northwest Laboratory, 1978, pp. 7-67. 

The results of two investigations from two diverse locations provided evidence that this distribution ratio might be due to reversible sorption-desorption reactions. Scientists at Argonne National Laboratory (ANL) equilibrated filtered Lake Michigan water with sediments that had been contaminated with Pu ten years earlier. As many as nine extractions on the same portions of sediment produced essentially identical values(15). Noshkin has observed that the concentrations of plutonium in the waters of Enewetak remain relatively constant although there is a continual replacement of water at an estimated residence time of 144 days (16). The concentration corresponds well with that predicted from the experimental distribution ratio between lagoon sediment and water. 

There have been several laboratory and field studies concerned with the uptake of aqueous plutonium by plants, marine biota, soils, minerals, and glass. These have been discussed in the first paper of this series (I), which shows that several solution variables, as they influence the particle size distribution of the aqueous plutonium, greatly affect its interaction with silica surfaces. Studies were conducted to determine the rates, equilibria, and mechanism of the sorption and desorption of aqueous, colloidal plutonium-239 onto the surfaces of quartz silica. The orientation of these studies is the understanding of the likely behavior and fate of plutonium in environmental waters, particularly as related to its interaction with suspended and bottom sediments. 

Desorption of Plutonium from Silica. A study was made of the effect of pH on the rate of desorption by first sorbing Pu onto silica at pH 7, then desorbing into an "infinite volume solution at pH s 5, 7, and 9. The "infinite volume solution condition was approximated by removing the supernatant liquid at each sampling time and replenishing with fresh solution of the same pH. Thus, there was no significant buildup of the Pu concentration in the solution. The results, plotted in Figure 6, indi-... 

Figure 6. Effect of pH on the rate of desorption of plutonium from granular silica into an infinite solution at an ionic strength...

Curve 1, after sorption from fresh solution Curve 2, after sorption from aged solution Curves 3 and 4, desorption of loosely held plutonium from fresh and aged systems, respectively. See text for definitions of A0(d and Aow. 

A comparison of the desorption rates at pH 7, shown in Figure 7 for the plutonium sorbed from fresh and aged solutions, indicates that the total desorption curve may be interpreted in terms of two different sorbed species. This is expressed in Equations 2, 3, and 4 as two first order processes. For both the fresh and aged systems, the relative quantities of the Ao(d or loosely-held species were almost identical, as were their desorption rate constants. It is likely that the A0<2 or tightly-held species were colloidal in size, since irreversibility is a widely known characteristic of colloid sorption. This was found to apply, for example, in the case of the sorption of colloidal americium on quartz (27). 

The partitioning of plutonium from surface water to sediments in freshwater and marine environments depends on the equilibrium between plutonium(IV) and plutonium(V), and the interaction between plutonium(IV) in solution and plutonium sorbed onto sediment particle surfaces (NCRP 1984). Sorption onto marine clays was found to be largely irreversible (Higgo and Rees 1986). Higgo and Rees (1986) also found that the initial sorption of plutonium onto clays was effective in removing most of the plutonium species that would be able to sorb onto the clay. When sorption to carbonate marine sediments was investigated, it was found that some desorption from the surface would also occur. This behavior was due to the presence of plutonium carbonate complexes on the sediment surfaces which were sorbed less strongly than plutonium dioxide... 

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