The reduction of plutonium(V) by aquatic sediments

J. Environ. Radioactivity 5 (1987) 169-184

The Reduction of Plutonium(V) by Aquatic Sediments

William R. Penrose, Donald N. M e t t a , J a m e s M. H y l k o a n d L o f t A. R i n c k e l Argonne National Laboratory, EnvironmentalResearchDivision,Argonne, IL 60439, USA

(Received22 September 1986;accepted 10November 1986)

ABSTRACT The reduction of plutonium (V) to the (III) or (IV) state proceeds very slowly in distilled water or filtered natural waters, but rapidly in the presence of natural sediments. The reaction rate is nearly first-order and is proportional to sediment concentration. Fresh or heat-dried (105°C) sediments mediated a rapid reduction reaction but ashed (500°C) sediments, silica, kaolin, alumina and goethite mediated much slower rates. Rare earths strongly inhibit the reduction; the affinity (inhibition) constant of neodymium for the reducing site is 4.5 x 107 M -1, whereas the affinity of neodymium for rare-earth binding sites is 5.0 x 105 M -1. Some hours of pre-equilibration of sediment and liquid phase are necessary to achieve constant reduction rates.

INTRODUCTION In natural waters, the fate of plutonium depends on the distribution of oxidation states. Plutonium in the 'reduced' forms (oxidation states (III) and (IV)) binds strongly to most natural surfaces but the 'oxidized' forms ((V) and (VI)) bind relatively weakly. In water bodies with long hydraulic residencies, binding and sedimentation with suspended particles is the most important mechanism for removal of plutonium; if a proportion of the element is present in the oxidized forms, the rate of natural removal will be slowed accordingly. In Lake Michigan and many ocean waters, plutonium removal by natural sedimentation processes has probably been retarded 169

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W. R. Penrose, D. N. Metta, J. H. Hylko, L. A. Rinckel

because 50%-90% is in the (V, VI) oxidation states (Wahlgren et al., 1977; Wahlgren & Orlandini, 1982). Other trace elements can exist in more than one oxidation state in natural waters, including nitrogen, sulfur, iodine, arsenic, manganese, and iron. In almost every case, it has been demonstrated that the oxidation state distributions are not under equilibrium control (Lindberg & RunneUs, 1984). Microorganisms have been shown to play a role in maintaining nonequilibrium distributions of arsenic (Johnson, 1972), iodine (Tsunogai & Sase, 1969), manganese and iron (Wetzel, 1983). A role for microorganisms has not been demonstrated in the speciation of plutonium. Plutonium speciation has been predicted from theoretical arguments more often than it has been measured directly. These predictions have been based on published solubility and equilibrium constants, many of which were determined at extremes of pH and concentration, and which do not take into account such reactive water constituents as humic substances (Aston, 1980, 1983; Silver, 1983). Silver (1983) has observed that small and reasonable errors in these constants are compounded in the calculations and lead to overall uncertainty as to the most stable oxidation state in any given situation, even assuming pure equilibrium control. Recently, Bondietti and Trabalka (1980) and Orlandini et al. (1986) have presented experimental evidence that the 'oxidized' plutonium in natural waters is in the (V)-state and not the (VI)-state. Depending on whether or not reduced and oxidized plutonium are in equilibrium with one another, two different questions might be asked. If they are in equilibrium, which physical and chemical factors influence the ratios of the oxidation states? Humic acids, for example, have been associated with greater proportions of the (IV) oxidation state (Wahlgren & Orlandini, 1982; Nelson et al., 1986). If they are not in equilibrium, which factors mediate interconversion of redox states? In certain natural waters containing low levels of dissolved organic matter, such as Lake Michigan and the mid-Pacific Ocean, Pu(V) exists in relatively high proportion (Wahlgren & Orlandini, 1982); in the laboratory, moreover, Pu(V) can be diluted into neutral distilled water and will remain indefinitely in that oxidation state. Yet without knowing whether distribution is at equilibrium or limited by kinetic processes, there is insufficient information to determine whether Pu(V) is the stable form under these conditions or merely lacks a catalyst to allow it to convert to the (IV) state. A potentially rewarding approach to resolving this problem is to investigate the observation (Wahlgren et al., 1977) that natural particles mediate a rapid conversion of Pu(V) to the reduced form. 'Mediate' in this context can mean either catalysis of a reaction that is thermodynamically favorable (i.e., Pu(III, IV) is most stable) or it can mean reaction, in which the sediment contains a reductant. In this paper, we have investigated the

The reduction of plutonium(V) by aquatic sediments

171

reduction reaction from the standpoint of the reactive sites on the particles which mediate it.

MATERIALS AND METHODS The sample of natural sediment used in these experiments was obtained in 1977 from a deep, high-sedimentation area of Lake Michigan. It has been used frequently for experiments similar to those reported here (e.g., Nelson et al., 1985). The sediment was graded by shaking with 20-30 volumes of water, settling for 30 seconds and decanting the suspension from the large particles. The suspension was allowed to continue settling overnight. The supernatant was discarded and the settled sediment mixed with distilled water. Concentration was determined by weighing an aliquot dried at 110°C. It was stored in the dark at 4°C and stirred vigorously for several minutes before use. The sediment had a low content of organic matter, as indicated by the 3% loss of dry weight upon ignition. Plutonium-237 was prepared in a cyclotron by Dr J. J. Hines of Argonne National Laboratory. Cleanup of the partially purified material was continued by adsorption on anion resin from 8 M HNO3, washing with 9 M HCI and elution with 0.1 M HCI-0.01 M HF. This latter medium was evaporated over low heat and the residue taken up in 8 M HNO3. Prepared in this way, the plutonium was tetravalent. Oxidized plutonium (V, VI) was prepared by removing the solvent over low heat and dissolving the residue in a small amount of 0.1 M HNO3. The solution was placed in a Teflon-lined Parr bomb and heated to 150°C for 36--48 h. Most of the Pu in the stock solution in 0.1 M HNO3 was found to be in the (VI) state, using lanthanum fluoride coprecipitation (see below) and silica gel adsorption (Orlandini et al., 1986). When added to the medium used in the following experiments, however, the plutonium was rapidly converted to the (V) state, and Pu(VI) could no longer be detected. The solution of Pu(V, VI) prepared was stable for the useful life of the isotope, at least six months. Plutonium(V) reduction rates

Measurements of Pu(V) reduction were made in 0.01 M dimethylarsenic (cacodylic) acid-0.1 M K N O 3 . This medium was found to prevent losses of Pu to the container walls and to effectively maintain pH (pKa = 6.1). Of several buffers tried, cacodylic acid had the least propensity to complex plutonium. Strong nitrate solutions have been routinely used in adsorption experiments (Sanchez et al., 1985 ). In a typical experiment, 425 ml of this medium was adjusted to pH 6-0. A known amount of sediment suspension

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W. R. Penrose, D. N. Metta, J. H. Hylko, L. A. Rinckel

was added. Since the properties of the sediment were shown to change after dilution, the mixture was pre-equilibrated with stirring for 6 to 24 hours before adding Pu(V). At the end of the pre-equilibration time, 20002500 cpm of 237pu(W) were added, yielding an estimated final Pu concentration of <10-~2M. After mixing for 15 seconds, a 100ml aliquot was removed for a zero-time measurement. Further samples were removed at appropriate times, typically 30 min, 60 min, and 24 or 48 hours. The samples from the assay were fractionated for the measurement of Pu(III, IV) and Pu(V) by a method similar to that of Lovett and Nelson (1981). The 100ml samples were added to 5 ml 'holding oxidant' (5 M H 2 S O r 0 " 0 1 M K2Cr2OT). This reagent eluted adsorbed plutonium from the sediments and prevented further reduction of Pu(V) without re-oxidizing Pu(III, IV). After mixing, 10 ml 8 M HNO3, 0.2 ml 0.25 M La(NO3)3 and 10 ml 0.7 M hydrofluoric acid were added separately, with mixing. A precipitate of LaF3 formed within 15 minutes and was filtered off with a 4 7 m m cellulose acetate filter (0-45/xm). This filter contained the Pu(III, IV) fraction. About 0.1-0-2 g ferrous sulfate was then dissolved in the filtrate to reduce the Pu(V) to Pu(III, IV). An additional 0.2 ml of La(NO3)3 solution was added to coprecipitate the fraction representing Pu(V). The Pu(III, IV) and Pu(V) fractions were counted using a NaI(TI)based gamma-ray spectrometer. 'Percent Pu(V)' was calculated by dividing Pu(V) counts by the sum of Pu(III, IV) and Pu(V) counts. Reduction rate constants were estimated from the slope of a semilogarithmic plot of percent Pu(V) divided by concentrations in (g liter -L) and expressed as liters g-1 h-l.

Binding measurements Measurements of the binding of tracers to sediments were carried out in 100 mL volumes of 0-01 M cacodylic acid--0.1 M K N O 3 . Normally, all ingredients except sediment were mixed first. The pH was then adjusted and sediment suspension was added. The mixture was stirred for 3-5 days. Sediment accumulating above the meniscus of the solution was occasionally rinsed down by swirling the flask. At the end of the equilibration time, the sediment was filtered and 5 ml of holding oxidant was added to the filtrate. It was fractionated as indicated in the above section. If a rare earth was used as the tracer, only the first lanthanum fluoride fraction was taken.

Changes in properties of sediment upon dilution The reducing properties of the sediment were observed to change slowly after dilution of the stock suspension into the reaction medium. In order to

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173

observe pseudo-first-order kinetics, it was necessary to pre-equilibrate reaction mixtures with the sediment before adding the Pu(V). An experiment was designed to optimize the pre-equilibration time. Lake Michigan sediment suspension (200/xl of 20mgm1-1) was added to 100ml of cacodylic-nitrate buffer at varying intervals before 'zero time' to allow differing periods of pre-equilibration. At zero time, sediment was added to a control flask (no pre-equilibration) and Pu(V) was added to all flasks. The reaction was stopped after 3 hours and the relative amounts of Pu(V) and Pu(IV) were measured. It is possible that substances may have been leaching from the sediment into solution and subsequently interfering with the Pu(V) reduction. An experiment was devised in which the effects of pre-equilibration were measured separately on solid and liquid phases. 1.00 ml of sediment suspension (20 mg m1-1) was diluted into 500 ml of buffer and stirred for 24 hours. The suspension was centrifuged for 30 minutes and as much of the supernatant solution as possible was removed. Fresh buffer was added to restore the total volume to 500 ml and 425 ml of this suspension was measured into a flask for measurement of the reduction rate (Flask A--preequilibrated sediment). 425 ml of the pre-equilibrated solution removed after centrifugation was put into Flask B (pre-equilibrated solution). Two controls were prepared: Flask C (no pre-equilibration) contained 425 ml of fresh buffer and Flask D (both sediment and solution pre-equilibrated) contained 425 ml of buffer and 0-85 ml of sediment suspension that had been pre-equilibrated together for 24 hours. At time zero, 0.85 ml of sediment suspension was added to Flasks B and C, and Pu(V) was added to all four flasks to begin the reaction. Samples were measured at 0, 1, 3 and 19 hours.

RESULTS Reduction rate measurements

The reduction of Pu(V) with time exhibits first-order kinetics over at least three hours, provided that the sediment has been allowed to equilibrate with the buffer (Fig. 1). Only very slow reduction was observed in the absence of particles and the rate was seen to be proportional to the sediment concentration (Fig. 2). It was necessary to control pH in the rate measurements. In preliminary experiments without buffering, the pH often drifted one or two units during the reaction. Since the plutonium tracer was added in acid solution, this affected the pH of unbuffered reactions. Experiments involving higher

W. R. Penrose, D. N. Metta, J. H. Hylko, L. A. Rinckel

174

1,0

1.8,

1.6

1.5

~ 1

0

2

3

4

Time - hr Fig. 1. The reduction of Pu(V) by Lake Michigan sediments. Curve A: both sediment and Pu(V) were added at time zero; B: sediment was added 24 hours before time zero; and C: Pu(V) was added 24 hours before time zero. Percent Pu(V) was calculated as described in Methods. The final sediment concentration was 32 mg liter-~.

0.25

-

0.20 -

T

.~

0.15

-

I

0,I0-

0.05 -

m

0.00 210

410

6E0

010

q 100

Sediment - mg/L Fig. 2. Dependence of the Pu(V) reduction rate on sediment concentration. Curve A: 30 minute rate measurement; and B: 60 minutes.

concentrations of lanthanides also needed buffering, since the solubilities of these elements were strongly dependent on pH. Concentrations of lanthanum and neodymium up to 10-3 M could be sustained at pH 6-0, so we selected this value for most experiments. Virtually all convenient buffer ions were found to interfere with the reaction, including tris(hydroxymethyl)aminomethane, orthophosphate and carboxylic acids. Cacodylic acid (pKa = 6.1) inhibited the reduction reaction about 60% at 0.01 M, relative to unbuffered reactions. This

The reduction of plutonium (V) by aquatic sediments

175

inhibition, although severe, was less than that caused by other buffers and was at least consistent from experiment to experiment. Relative rates of binding of Pu(IV) and Pu(V) to sediments

It is known that the plutonium bound to sediment particles is in the (III, IV) states (Lovett & Nelson, 1981; Nelson et al., 1986). The binding of Pu(V) to sediments in these experiments would be expected to involve at least two steps--reduction to Pu(IV) and subsequent binding----each with its own rate.

TABLE 1 Relative Rates of Binding of Plutonium to Lake Michigan Sediments When Added as Pu(IV) or Pu(V). (The Sediment Concentration was 56 mg liter -l)

Percent Pu bound Time 15 s 30 min 60 min 24 h

Pu(IV) 53.3, 70.8, 83.5, 81.6,

57.2 a 81.1 84.6 78.4

Pu(V) 6.9, 22.5, 32-1, 92.0,

6.1 a 21.5 33-3 92.8

a Replicate measurements.

The rates of binding of Pu(IV) and Pu(V) to Lake Michigan sediment were measured (Table 1). The initial binding of Pu(IV) (binding only) was much more rapid than that of Pu(V) (reduction and binding), indicating that the reduction step was rate-limiting. The kinetics of the reduction step could therefore be studied without needing to correct for rates of binding. Reduction of Pu(V) by other solids

Sediment from the 1977 Lake Michigan core mediated a high rate of Pu(V) reduction (Table 2). When this sediment was dried at 105°C and resuspended in water, it continued to reduce Pu(V) at a similar high rate. If, however, the dried sediment was ashed at 500°C to remove organic matter, the rate of reduction decreased by 98%. Other solids mediated the reduction at rates much lower than those observed with natural sediments. Kaolin, alumina, silica and goethite yielded specific rates less than 8% that of natural sediment.

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W. R. Penrose, D. N. Metta, J. H. Hylko, L. A. Rinckel

TABLE 2 Reduction of Pu(V) Mediated by Various Types of Solids. (Lake Michigan Sediments were used Wet, Dried 16 h at 105°C, or ashed 16 h at 500°C) Concentration (mg liter- ~)

Rate (h - 1)

Normalized rate (liters. g - t . h - I)

Wet sediment Dried sediment Ashed sediment

56 50 50

0-693 0.668 0.012

12.4 13.4 0.24

Kaolin Alumina Silica Goethite

50 50 50 50

0.043 0.053 0-018 0.023

0.86 1.06 0-36 0-47

Solid

pH sensitivity T h e rate o f the reduction reaction was measured at several p H values from 5.0 to 7.0, the effective range of the cacodylic acid buffer. T h e r e were no large variations in rate over this range (Table 3). T h e lack of suitable buffers p r e v e n t e d examination of a wider range; for example, 0.01 M acetate buffer at p H 5.0 inhibited the reaction a further 40% relative to cacodylic acid at the same p H. TABLE 3 pH Sensitivity of Pu(V) Reduction (Sediment Concentration was 11 mg liter-l)

pH

7.0 6.5 6"0 5"5 5-0

Normalized rate (liters. g - t. h - t)

8"0 9-8 10.2 11.5 10.9

Inhibition of Pu(V) reduction by rare earths A search was m a d e to find metal ions which might interfere with the availability o f reducing sites to plutonium. Lanthanide elements were shown to h av e such properties. L a n t h a n u m and n e o d y m i u m are both sufficiently soluble to sustain concentrations up to 10-3 M in the medium used for these

The reduction of plutonium(V) by aquatic sediments

177

TABLE 4 Inhibition of Pu(V) Reduction by Rare Earth Ions. (The Sediment Concentration was 56 mg liter -1)

Added concentration of rare earth

10 -6 M 10-7 M

Reduction rate (% of control) La

Nd

Sm

Gd

5"9 59.7

21"9 81.6

7"8 74.9

6"7 77.5

experiments. This concentration was sufficiently high to saturate the binding sites on the sediment. Reduction rates were measured in the presence of two concentrations of lanthanum, neodymium, samarium and gadolinium. Even at 10 -7 M, the reduction reaction was significantly inhibited by each lanthanide (Table 4). Since at these low concentrations, a large proportion of the added lanthanide might be consumed in forming complexes with the binding sites, the effective (free) concentrations of lanthanide ions would be expected to be lower than these values. Because of its higher solubility, neodymium was selected for a more detailed study of inhibition. Initial neodymium concentrations from 10-7 M to 3 × 10-s M, in threefold steps, were added to Pu(V) reduction assays and used to derive pseudo-first-order rate constants. In order to estimate the free neodymium concentrations, a second series of mixtures was prepared with identical compositions, except that the 237pu tracer was replaced by a tracer quantity of gadolinium-153; ~53Gdhas an easily-measured gamma-ray and its binding behavior is not greatly different from that of neodymium. The ratio of unbound to total ~53Gdwas then multiplied by the total (added) neodymium to estimate the free Nd concentration. The variation in the Pu(V) reduction rate with free [Nd] is shown in Fig. 3. The maximum change in inhibition is observed over the 10 -9 t o 10-TM concentration range. The assumed binding equation is: g r -

[SNd] [S]-[Nd]

where Kr is the affinity constant of the reducing site for neodymium, [S] is the binding site concentration, [Nd] is the free neodymium concentration and [SNd] is the concentration of the complex of the two. At the point where the rate is one-half of the maximum, one-half of the binding sites are

W. R. Penrose, D. N. Metta, J. H. Hylko, L. A. Rinckel

178

r-

15-

-J !

10 0 (.3

tl: "0

5-

to 0

z

o -,o

-'9

-'8

-7

?5

-~

log [Nd]

Fig. 3. Effect of neodymium on the reduction of Pu(V) by Lake Michigan sediment. The points are observed data; the curve represents the expected behavior of a single type of binding site with an affinity constant of 4.5 x 107 M-I. The sediment concentration was 50 mg liter -l. The leftmost point actually represents [Nd] = 0 M, but was assigned a finite value in order to plot on the log scale.

assumed to be occupied by Nd and therefore unavailable to Pu(V). Then [SNd] = [S] and Kr = 1/[Nd]. For these data, Kr = 4.5 x 107 M -a. If this binding equation, which assumes only a single value for the affinity constant, is used to reconstruct the curve from the affinity constant and the maximum rate constant (12.41itersg -lh-1), the curve coincides almost exactly with the observed data. The binding experiment that had been used to calculate the proportion of n e o d y m i u m bound to the sediments was used to determine whether the lanthanide binding sites could also be described by a single affinity constant. In order to compare binding data to plutonium reduction rate data, both sets of data were expressed in terms of the proportion of sites occupied by neodymium. The proportion occupied, P, was calculated from: p _

Ka[Nd] 1 + Ka[Nd]

where K a is the affinity constant estimated from the binding plot (5.0 x 105) (Thakur et al., 1980). Kinetic data were converted by dividing each rate constant by the maximum rate (when [Nd] = 0). This assumed that the rate constant was proportional to the fraction of plutonium-reducing sites not blocked by neodymium. The data transformed in this way are plotted as points in Fig. 4. The curves

The reduction of plutonium(V) by aquatic sediments

179

1-

o

~" 0.6-

o

©

•

(.1 0

o

0.6-

'~

0.4-

_o 0 O. 0

0.2-

0 ,~-10

-9

-8

-7

log

-0

-fl

-4

[Nd]

Fig. 4. Effect of neodymium on the reduction of Pu(V) (open circles) and on the binding of 153Gdtracer (filledcircles). Both sets of data are expressedin terms of the proportion of reducing or binding sites, respectively, occupied by neodymium. As in Fig. 3, the circles represent actual data and the curves are calculated from the behavior of single binding sites with affinities of 4.5 x 10 7 M -1 and 5.0× 105 M - l , respectively. The leftmost point actually represents [Nd] = 0 M, but was assigned a finite value in order to plot on the log scale.

shown are reconstructed from estimates of the affinity constants made from the data points and by making the assumption that only a single type of binding site exists. A simple calculation based on the affinity constant for neodymium binding, the amount of neodymium bound at half-saturation and the sediment concentration estimated the equivalent weight of a neodymiumbinding site at 25 000 'daltons' of sediment. Effects of dilution on suspended sediment

In Fig. 1, it was shown that the Pu(V)-reducing properties of the sediment changed within a few hours of dilution into the cacodylic acid-potassium nitrate solution used for the experiments. This p h e n o m e n o n was investigated further as a possible additional means of probing the structure of the reducing site. T h e time course of the change in Pu(V)-reducing properties was measured. The reaction rate initially changed rapidly and converged on a constant rate at about three hours (Fig. 5). In all subsequent measurements of reduction rate, the reaction mixtures were pre-equilibrated for six to sixteen hours after dilution of the sediment, before addition of the Pu(V). The effects of pre-equilibration could take many forms, including the leaching from the sediment of substances which could inhibit the reduction

180

W. R. Penrose, D. N. Metta, J. H. Hylko, L. A. Rinckel 25-

.2 20rr~

15-

.~

10-

~

5-

0-

i 1

i 2

i 3

i 4

Pre-equilibration Time Fig.

i 5

-

hr

5 . T h e change in the initial rate of Pu(V) reduction during the first hours of preincubation of the diluted sediments. The sediment concentration was 40 mg liter-1.

100-

e~ e~

(D e~

e~

0~ 0~

311

i 1

i 2

Time

i 3

-

i 4

hr

Fig. 6. Effect on Pu(V) reduction rate of pre-incubationof sediments and medium. Curve A: sediment only pre-equilibrated; B: solution only pre-equitibrated; C: no pre-equilibration; and D: solution and sediment both pre-equilibrated.

reaction in the same way that rare earths do. On the other hand, changes might be taking place in the structure of the reactive site as a result of dilution into the buffer constituents. A n experiment was devised in which a pre-equilibrated dilution of Lake Michigan sediment was separated into solid and liquid phases. The centrifuged sediment was mixed with fresh buffer and the supernatant liquid was mixed with fresh sediment. Only the flask containing the pre-equilibrated sediment revealed a decrease in reduction rate similar to that of a control which was pre-equilibrated but not separated (Fig. 6). It therefore appears that the effects of pre-equilibration

181

The reduction o f plutonium(V) by aquatic sediments TABLE 5

The Effect of Stirring on Pre-equilibrationof Lake MichiganSediment Treatment

Reduction rate (liters. g - 1. h - ~)

No pre-equilibration No stirring, 24 h pre-equilibration Slow stirring (1 s -I) Rapid stirring (>5 s-l)

0"15 0"038 0-044 0"037

TABLE 6 Initial Reduction Rates After Pre-equilibration as a Function of the Composition of the Reaction Medium. (Sediment Concentration was 40 mg liter-l)

Rate (liters. g - t . h - 1) Medium composition

0.01 i cacodylic acid/ 0.1 MKNO3 0-05 u cacodylic acid/ 0-1 MKNO3 0.01 i cacodylic acid/ no KNO3

No pre-equil,

5-0 3.75 4-75

With pre-equil,

1.25 10-0 2.00

Percent change

-75 +267 -42

act directly on the sediment particles and do not involve leaching of materials into the liquid phase. Mechanical abrasion of the sediment particles by stirring was ruled out as a cause of the pre-equilibration effect. Mixtures that were pre-equilibrated with and without stirring showed the same degree of loss of reducing activity (Table 5). A n inhibitor in the cacodylic acid-potassium nitrate buffer was similarly ruled out by doing the pre-equilibrations in different compositions of buffer m e d i u m . A n increased concentration of cacodylic acid caused the reducing activity to increase, rather than decrease, and the deletion of potassium nitrate had no effect (Table 6).

DISCUSSION U n d e r the conditions of these experiments, natural Lake Michigan sediments bring about the reduction of Pu(V) to Pu(III, IV). A small

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W. R. Penrose, D. N. Metta, J. H. Hylko, L. A. Rinckel

number of sites with very homogeneous properties seems to be responsible for this interconversion. Due to the limited solubility of all reduced actinides at near-neutral pH, these sites cannot be saturated to determine their number or their affinity constants. The lighter lanthanides, however, can be maintained in solution at concentrations sufficient to occupy the Pu(V)reducing sites and completely inhibit the reaction. The binding of trace metals and organic compounds to sediment particles does not normally resemble the simple equilibria found in, for example, chelation with a single, discrete ligand. Typically, a distribution of binding affinities is observed: Low concentrations of adsorbate bind to the highestaffinity sites first. Increasing concentrations bind to sites of progressively lower affinity. The result is a binding curve of lower slope than those obtained with the single-site model, often exhibiting an irregular shape (Shuman et al., 1983). A method for extracting affinity constants from complex binding data has been given by Thakur et al. (1980). In the case of the binding of neodymium to particles, however, the expected heterogeneity is not observed. The inhibition by neodymium of both Pu(V) reduction and gadolinium binding behaves as though only single types of binding sites were involved for each; both data sets closely match the curves calculated from the single-site model. The displacement of the reduction curve to a much lower concentration range than the binding curve implies that the Pu(V)-reducing sites comprise only a very small fraction of the total number of neodymium-binding sites. The nature of the reducing activity has not been determined. The binding site may be catalytic in nature, facilitating the reaction of Pu(V) with trace reductants (by providing appropriately juxtaposed binding sites, for example). Alternatively, the sites themselves may contain reductants; salicylate groups, for instance, are common in humic substances and can potentially act as both ligand and reductant. The fact that incineration of sediments results in a loss of activity also suggests that adsorbed organic matter may act as either a reductant or catalyst. Direct biological mediation of the reduction reaction is unlikely, since the activity was unaffected by heating the sediments to 105°C. Other solids are capable of mediating the reduction of Pu(V), although at a slower rate. Sanchez et al. (1985) demonstrated that the 'adsorption edge' of Pu(V) on goethite shifted over a period of 20 days to resemble that of Pu(IV); they suggested that reduction was occurring. Keeney-Kennicutt and Morse (1985) made a similar observation for goethite but said that no such reduction was seen on calcium carbonate. Our experiments show a reduction reaction mediated by goethite but at a rate much slower than that for natural sediment. If the oxidation state distribution of plutonium is determined by an

The reduction of plutonium(V) by aquatic sediments

183

equilibrium mechanism and if the effect of the solids is catalytic rather than reactive, it will make little difference whether organic or metal oxide surfaces mediate the reaction in natural waters. The rates of either the sediment- or goethite-mediated reactions are faster than the time constants of most natural water systems and the equilibrium distribution of oxidation states w o u l d probably be unaffected. If the distribution is determined by the relative rates of opposing reactions, however, or if the solids act as reagents rather than catalysts, then the rate of the reduction reaction matters a great deal in determining the fate of plutonium in natural waters.

ACKNOWLEDGMENTS W e wish to thank D. M. Nelson and K. A. Orlandini for continuing discussions and for suggesting the simple method of making Pu(V, VI) from Pu(IV). J. M. H y l k o and L. A. Rinckel were Student Research Participants with A r g o n n e National Laboratory's Department of Educational Programs. This w o r k was supported by the US Department of Energy, Office of Health and Environmental Research, under Contract W-31-109-Eng-38.

REFERENCES Aston, S. R. (1980). Evaluation of the chemical forms of plutonium in seawater. Marine Chem., 8, 319-25. Aston, S. R. (1983). Evaluation of the chemical forms of plutonium in seawater (reply). Marine Chem., 12, 97. Bondietti, E. A. & Trabalka, J. R. (1980). Evidence for plutonium/V/ in an alkaline, freshwater pond. Radiochem. Radioanal. Letters, 42, 169-76. Johnson, D. L. (1972). Bacterial reduction of arsenate in sea water. Nature (London), 240, 44-5. Keeney-Kennicutt, W. L. & Morse, J. W. (1985). The redox chemistry of Pu(V)O2 interaction with common mineral surfaces in dilute solutions and seawater. Geochim. Cosmochim. Acta, 49, 2577-88. Lindberg, R. A. & Runnells, D. D. (1984). Ground water redox reactions: an analysis of equilibrium state applied to Eh measurements and geochemical modeling. Science, 225,925-7. Lovett, M. B. & Nelson, D. M. (1981). Determination of some oxidation states of plutonium in sea water and associated particulate matter. In Techniques for identifying transuranic speciation in aquatic environments, 27-35. International Atomic Energy Agency, Vienna. Nelson, D. M., Larsen, R. P. & Penrose, W. R. (1986). Chemical speciation of Pu in natural waters. In Symposium on environmental research for actinide elements, Hilton Head, November, 1983. (In press.) Nelson, D. M., Penrose, W. R., Karttunen, J. O. & Mehlhaff, P. (1985). Effects of

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