Interactions of metal hydrous oxides with chelating agents

Interactions of Metal Hydrous Oxides with Chelating Agents IV. Dissolution of Hematite1 HAN-CHYEN CHANG 2 AND EGON MATIJEVIC Department o f Chemistry and Institute o f Colloid and Surface Science, Clarkson College, Potsdam, New York 13676 Received March 22, 1982; accepted July 20, 1982 The dissolution of hematite in the presence of EDTA and of related aminocarboxylic acids has been studied as a function of different parameters. It was found that the leaching of ferric species from this oxide depends greatly on the pH, temperature, and nature of the chelating agents. At lower temperatures (<64°C) EDTA enhances the dissolution of hematite in alkaline solution at pH ~ 10. As the temperature rises, the leaching of iron is the strongest in acidic solutions. HEDTA and DTPA exhibit behavior rather similar to that of the EDTA, while NTA differs from these complexing agents as it exerts the strongest effect in acidic solutions at all temperatures studied. The results are explained in terms of two different mechanisms: (A) ferric ions released from the bare surface form complexes with chelating ligands in solution, thus causing further leaching of metal ions from the solid, and (B) the chelating species are bound to the lattice metal ions and the complexes are then released into solution. Which of the two mechanisms prevails depends on the conditions of the system and the nature of the chelating agent. At high temperature (215 °C) hematite dissolves in acidic solutions with magnetite forming on extended aging. At higher pH values the leaching of iron is diminished and no change of phase is observed. INTRODUCTION

(6). Removal of these oxides by partial or complete dissolution on reaction with complexing organic species offers considerable advantages over other cleanup procedures. The most commonly tested chelating compounds for this purpose are oxalic and citric acids, and various homologues of EDTA (ethylenediaminetetraacetic acid). A vanadium(II) picolinate complex has also shown promise in this application (7). Processes involved in the dissolution of metal oxides by chelating agents are by no means simple; they are very sensitive to pH, temperature, and even to a relatively small change in the composition of the chelating agent. The purpose of this study is to evaluate some of these effects and to establish conditions for most efficient removal of a model corrosion product. Colloidal dispersion of uniform spherical hematite particles were interacted with a variety of chelating agents of m a t i t e , ot-Fe203

Dissolution of metal (hydrous) oxides by chelating agents has been used in the recovery of metals from ores (l), in boiler scale removal, and in other industrial applications. More recently these complexing species have been employed in the chemical decontamination of nuclear power plants in order to remove the radioactive "crud" deposited on out-of-core surfaces (2-5). Major products resulting from corrosion of steels and alloys used as structural materials in the primary and secondary heat transfer systems of water-cooled nuclear reactors are magnetite, Fe304, nickel and cobalt (or mixed) ferrites, Nix(CO)yFe3_x_yO4 (0 < x + y < 1), and he1Supported in part by the Electric Power Research Institute, Contract RP-966-2, and in part by NSF Grant CHE8013684. 2 Part of a Ph.D. thesis. 479

Journalof Colloidand InterfaceScience,Vol.92, No. 2, April 1983

0021-9797/83/040479-10503.00/0 Copyright© 1983by AcademicPress,inc. All rightsof reproductionin any formreserved.

480

CHANG

AND

MATLIEVI(~

the E D T A family in aqueous media o f different p H and over a temperature range between 25 and 215°C.

the solid adsorbent was separated by centrifugation at 15,000 rpm, the supernatant solution was filtered through a Nuclepore membrane of 0.08-#m pore size, and its p H EXPERIMENTAL was measured. At temperatures up to 100°C A. Materials. Colloidal dispersion o f and in the presence o f excess EDTA, the spherical hematite particles o f narrow size change in pH on aging was negligible. distribution (modal diameter 0.1 ~tm) was The concentration o f the iron released the same as used before (8, 9). The disso- from hematite was determined in the preslution o f this hematite was studied in the ence of an excess o f the chelating agent by presence o f the following chelating agents: a spectrophotometric method described earethylenediaminetetraacetic acid (EDTA), lier (8). Absorbances were measured at 263 hydroxyethylethylenediaminetriacetic acid and 325 nm. In all cases appropriate cali(HEDTA), cyclohexylenedinitrilotetraacetic bration curves were obtained. acid (CDTA), diethylenetriaminepentaacetic All p H data reported refer to measureacid (DTPA), ethylene glycol-bis(2-ami- ments at room temperature. noethyl)tetraacetic acid (EGTA), ethylenediamine-N,N'-diacetic acid (EDDA), and niRESULTS trilotriacetic acid (NTA), all Fluka chemicals Preliminary experiments have shown that (see Table I, Ref. (8)). B. Techniques. At lower temperatures the release o f iron from hematite particles (<105°C), the experiments were carried out used in this work was negligible in the abin Pyrex culture tubes with Teflon-lined sence o f the chelating agents. For example, screw caps. At temperatures > 105°C, a Tef- no iron could be detected in the supernatant lon vessel inserted in a stainless-steel pressure solution by the described analytical technique (<10 -6 M) after 1 week of aging at bomb was used. Mixtures containing known amounts o f 100°C of a sol at either p H 3 or 9. The adhematite, of chelating agents, and o f sodium dition of complexing organic species greatly hydroxide or nitric acid were agitated at a enhanced the dissolution o f hematite, which constant temperature for a desired period o f depended greatly on pH, temperature, and time. On quenching to room temperature, the natures o f chelating agents.

SJ

-7E o m O

E v O

<

I,&l J ! W

~~HEMATITE

0

5

=0.18 9

EDTA: 3.OxlO-SM pH:

0 3.0 [] 7.4 26"C

f

Z 0a,, m 0

B

I0

15

20

TIME (days) I~G. 1. Specificamount of iron released from 0.18 g hematite suspended in 8 c m 3 of a 3 × EDTA solution as a function of time at 26°C at two different pH values. Journal of Colloid and Interface Science, Vol. 92, No. 2, April 1983

10 -3

M

DISSOLUTION

Figure 1 gives the amounts o f iron (in pmole/m 2) released from a sol containing a total o f 0.18 g o f hematite in 8 cm 3 o f each sample at an initial E D T A concentration of 3 × 10 -3 M a s a function o f time at 26°C and at p H 3.0 and 7.4, respectively. The dissolution rate is strongly p H dependent and the reaction ceases once the entire a m o u n t of E D T A in solution has been complexed; at the plateau values, the concentration o f the free chelating agent in solution was found to be rather small (< 10-5 M). The difference in the final a m o u n t of iron released at the two pH values is accounted for by the adsorption o f E D T A solute species on hematite; the uptake is higher at the lower pH. Figure 2 gives a log-log plot of the a m o u n t of dissolved iron from hematite particles in the presence o f E D T A against time for an early reaction period at two different temperatures (26 and 100°C). Linear plots resuited in all cases. The respective slopes (n)

/

2.0

IE

0 --0

/

J

1.0

E

:1.

0 ; pH 4.5, 100"C O: 6.61

5.3 " : 33

A:

It o

26°C

0

0 --I -,o

0.5

~.o

1.5

2.o

LOG.(TIME/min)

FIG. 2. A log-log plot of the amount of iron dissolved (#mole/mE) vs time. Conditions: (3, l0 mg hematite in 8 cm3 ofa 0.1 M EDTA solution at 100°C at pH 4.5; (~, A, I-1 180 mg hematite in 8 cm3 of 0.1 M EDTA solution at 26° at three different pH values.

OF HEMATITE

481

show that at room temperature the rate decreases with time (n < 1), while at the high temperature accelerated dissolution (n > l) is observed. The time dependences o f hematite dissolution on the presence of excess EDTA, H E D T A , and D T P A at 100°C are presented in Figs. 3-5, respectively; the amounts of iron released per unit area of the solid are given for three different pH values (3.5, 6.2, and 9.0). In the studied system the leaching at p H 9.0 is initially faster than in acidic media but reaches a plateau value at a later time. However, the iron release continues in acidic solutions; thus, the amounts o f hematite eventually dissolved are higher at low p H values. Although the quantities or iron in solution at a given time differ for the three chelating agents used, the overall trends are the same. The influence o f p H over the range 3 - l 1 on the a m o u n t of hematite dissolved after 3 hr o f equilibration at different temperatures in the presence of E D T A is illustrated in Figs. 6-8. The results after 1 hr of equilibration are also given at 25 and 100°C for comparison purposes. In all cases the leaching o f iron is dramatically enhanced with rising temperature (note the difference in the ordinate scale in these figures). At lower temperatures (<65°C) a m a x i m u m in the a m o u n t o f iron released is observed in basic systems (at p H 10), whereas at high t e m p e r a t u r e s (> 100°C) the process is more efficient at the lowest pH values studied. Figure 9 shows that the a m o u n t o f iron leached after 0.5 hr o f aging is independent of the initial concentration o f E D T A under varying conditions indicated in the diagrams. The effect of several chelating agents on the dissolution o f hematite after 1 hr o f equilibration at three different temperatures as a function of p H is given in Figs. 10-12. For comparison purposes the results with E D T A are also included. An increase in temperature considerably enhances the dissolution o f this solid by some of the additives, but it has little effect on the others. N T A is consistently most efficient in dissolving hematite in acidic meJournal of Colloid and Interface Science. Vol. 92, No. 2, April 1983

482

C H A N G A N D MATIJEVI(2

.-. 2 0 0 N

HEMATITE : 90 mg EDTA : O.1M

IE

E 1so =L

f

Q m.m (/) < 100 I¢1 .,I ill fP Z O

50

D

TIME (hr.) FIG. 3. Specific a m o u n t o f iron released from hematite (90 mg) suspended in 8 c m 3 o f a 0.1 M E D T A solution as a function o f time at 100°C at three different p H values.

extended aging a new phase was formed consisting of black magnetic material, indicating that partial reduction of iron took place. In basic systems the dissolution of hematite was incomplete and no magnetic solid was detected.

dia. The influence of EDTA was described earlier and it can be seen that HEDTA and DTPA, and to a lesser extent CDTA, follow the same trends, particularly at 55 and 100°C. The leaching of iron by other chelating agents studied is much less efficient and it is little affected by a change in the pH. The results of a series of experiments on the hematite-EDTA system, carried out at 215 °C, are summarized in Table I. Complete dissolution was noted in acidic media. On

a"

DISCUSSION

1. Kinetics of Dissolution Present data have shown that the rate of dissolution of hematite as well as the amounts

80

'E

HEMATITE: 9Omg HEOTA : OAM I

i" so

[

'°°*c

/

I

g

J

/ 6.2

4

TIME Ihr.I ~O. 4. The same plot as in Fig. 3, in the presence of H E D T A . Journal of Colloid and lnter~e Science, VoL 92, No. 2, April 1983

483

DISSOLUTION OF HEMATITE

E200

pH~

n.,

0 ~

HEMATITE : 90 rn9 DTPA:O.1M 1000C

150

,oo / 1 0

0

2

3

4

5

TIME (hr.) FIG. 5. The

same plot as in Fig. 3, in the presence of DTPA.

leached after a given period of time depend strongly on the temperature, pH, and nature of the chelating agent. Obviously, the mechanisms involved will be affected by the composition and the charge of the particle surface, by the state of the complexing solutes, and by the strength of the bonds formed between the constituent metal ions and the adsorbates. It was demonstrated that all these parameters influence the adsorption of the

2O

E o

HE!ATITE ! 90 mg

a

(/)1C <[ 1.1.1 .-I 1.1.1 re

G 5

7

9

pH FIG. 6. Solid lines show specific a m o u n t of iron released from hematite (90 mg) suspended in 8 cm 3 of a 0.1 M EDTA solution after 3 hr of equilibration as a function of pH at three different temperatures. The dashed line is for the same system aged for 1 hr at 25°C.

chelating agents on hematite particles (8) which, in turn, must affect the dissolution process. In considering the rates of iron leaching from hematite by the complexing species, it should be recognized that the adsorption process is much faster than dissolution. Indeed, the former is completed within minutes, while the latter continues for hours or weeks. It is also necessary to distinguish between the results at lower and higher temperatures. Figures 1 and 2 show that in the presence of EDTA at 26°C rates become higher as the pH rises. Since the adsorption density of this chelating agent on hematite decreases when the systems become less acidic, it appears that the dissolution process is primarily taking place on the bare surface. In all cases the leaching slows down as the process progresses. At higher temperature (100°C) the dissolution of hematite is considerably faster and the dependence of the rate on pH is much more involved. In the presence of EDTA and DTPA an accelerated leaching of iron is observed during the initial stages of the process (Figs. 2-4), followed by a decrease in the rate at later times. This pattern is strongly dependent on pH; at sufficiently long times the amount of iron released is higher at low pH Journal of Colloid and Interface Science, Vol. 92, No. 2, April 1983

484

CHANG AND MATIJEVIC 40

E 30

0

E

=. e~ h i 20

--I ILl

n1 Z

\

HEMATITE: 90 mg EDTA= 0.1 M #- 9 0 % V 80°C 3 hr

I

"-..2 J

0

ee

9

7

11

pH FIG. 7. The same plot as in Fig. 6 for systems equilibrated at 80 and 90°C for 3 hr.

than at high pH. Since the adsorption of these chelating agents changes little with temperature (10), it would appear that the dissolution of hematite in heated systems proceeds, at least in acidic media, through removal of

i ~E

---

.EMAT,90m. TE LOlOO°C

EDTA: O.1M ~0120"C 3hr OlO0°C

ferric chelate complexes from the particle surface. Indeed, the latter was indicated by adsorption measurements. It was shown that at low pH the amount of EDTA adsorbed on hematite first increases and after a relatively short reaction time it decreases until an equilibrium is established (10).

~

lhr

II zo

0 pH:4.5, IO0*C

I

v

I

HEMATITE 0:2m9 O,O :90mg

0

0.5 hr

,.., ,oo

'oli

-*: zoo ~

= ~1! ,¢.pH :9.5 --

2 3

5

7

9

pH

FIG. 8. The same plot as in Fig. 6 for systems equilibrated for 1 and 3 hr at 100°C and for 3 hr at 120°C. Journal of Colloid and Interface Science. Vol. 92, No. 2, April 1983

5o°c.~

fzs"co_

4

MOLAR CONC.OF EDTAxlO3 FIG. 9. Specific amount of iron released from hematite after aging for 0.5 hr in the presence of different initial EDTA concentrations.

DISSOLUTION OF HEMATITE

A

a,2,

HEMATITE:'90mg

OE0.rA 1 [] HEDTA 0.r A /

I

-

I I

HEMATITE :90mg 0 EDTA -] o HEDTA|

,,.,~ ....

a D'rPA ~O.IM

/ (~ / /

--

o CDTA /

.,,,.rA J 25"c, 1.,

~t

/ /

.: o...

~

/ .

/ /Y

OcO.rA ? 01u

11¢ --

485

1

~

0 N'rA | I EDDA I

/

E

& EGTA J

/

l/

?

// II ~/

5

/I

°

/? j C,.'--r

+ I 2

-

4

~

.

.

6

.

++.2

.

8

",4

10

12

pH

FIG. 10. Specific amount of iron released from hematite (90 mg) suspended in 8 cm 3 of a 0.1 M solution of different chelating agents as a function of pH equilibrated for 1 hr at 25°C.

The change of dissolution rate shown in Figs. 2-4 may be due to the progressive development of a surface insoluble film, which depends on conditions. It would seem that higher pH (e.g., 9) and the presence of the iron(III)-EDTA complex favors the formation of such a film. Such an explanation is supported by the observation that the addition of iron(III)-EDTA complexes to a hematite dispersion significantly reduced the amount of iron released from the particles; the effect was more pronounced at higher pH values. A similar explanation was offered for the dissolution of ferric oxides in mineral acids (11). Examples of accelerated and retarded dissolution processes of hematite under different conditions were reported in the literature. A decrease in the rate was attributed to a change in the surface of the particles: it was suggested that the concentration of defects

3

,5

7

pH

9

11

FIG. 11. The sameplot as in Fig. 10 for systemsequilibrated at 55°C. in the outermost layer was much higher than in the sublayers (12, 13). The increase in the rate of dissolution was explained in terms of deaggregation of the solid (14). It is noteworthy that colloidal spherical particles of hematite used in this work do consist of much smaller globular units (15).

2. Mechanisms of Dissolution Several mechanisms were suggested in order to explain the dissolution phenomena of metal (hydrous) oxides in the presence of chelating agents. Some investigators (16) stressed the reactions in solution, i.e., the processes which lead to complexation of the metal ion once it had been liberated from the surface. Interactions of the chelate-forming species with the constituent metal ions at the particle surface were considered by others (17-20) to govern the dissolution. The great sensitivity of hematite dissolution to pH and temperature established in this study suggests a change in the mechaJournal of Colloid and Interface Science. Vol. 92, No. 2, April 1983

486

CHANG AND MATIJEVIC I

i

I

i

Mechanism (B) assumes that following the surface complexation of the metal ion with the chelating species, the lattice bonds between the ferric ion and oxygen are sufficiently weakened for the entire complex to be released into solution. The overall process can be written as

I

HE MATITE : 9 0 rn9 O EDTA " n HEDTA DTPA ' OCDTA ,O.IM • NTA r=EDDA

8O

-7E Q

&EGTA lO0*C, 1h;

E

t Fe-0H + H+ + HnLn_i $ . . I 4O ILl n-

~FeHnLn+l-i

z

S

0 o: 2O

3

5

7

9

11

pH FIG. 12. The same plot as in Fig. 11 for systems equilibrated at 100°C.

nism by which iron is leached from hematite as the conditions are altered. The observed phenomena can be interpreted in terms of either (A) a solution coordination mechanism, or (B) a surface complexation mechanism. In mechanism (A) the ferric ions are released from the bare surface and react with either hydroxide or chelating ions in solution, causing a further release of the metal ion from the solids. The processes involved can be schematically written as a-Fe203 + 3H20. ~-~

~2 Fe3+(aq) + 60H" . .3+n-I , ~Fe(0H)mL3"m-i ~FeNnL

Fe3+(aq) ~

Fex(OH)y(3x'y)+ Obviously, the rate of leaching by this mechanism will depend on the tendency of the metal ion to be released from the solid oxide. The latter is closely related to the surface properties (i.e., crystallinity, surface defects, etc.), to the pH, and to the chelating power of the complexing agent in solution. Journal of Colloid and Interface Science, Vol. 92, No. 2, April 1983

+ H20

FeHnLn+]'i (aq)

In this case the relative bond strength of the ferric ion for the lattice oxygen and for the chelating molecule will be of utmost importance. Furthermore, the conformation of the chelating agent in its adsorbed form will play an essential role. In reviewing the EDTA effects on dissolution of hematite (Figs. 6-8) a maximum appears in alkaline solutions at lower temperatures (<90°C) as observed earlier in a study of dissolution of/3-FeOOH in EDTA (21). Under these conditions the adsorption density of the chelating agent is the smallest. Thus, mechanisms (A) should prevail until the pH is too high. As the temperature is increased further (>~I00°C) dissolution at higher pH is somewhat enhanced, but a dramatic rise in iron leaching is observed in acidic systems. Since at low pH the adsorption is the highest, mechanism (B) should dominate. Apparently, the high temperature is needed to break the lattice bonds in order to release into solution the complexes formed on the hematite particle surface. A comparison of the effects of different chelating agents shows that at room temperature the behavior of EDTA is rather unique when compared to that of the other complexing species (Fig. 10); at higher temperatures (e.g., 100°C) EDTA, HEDTA, and DTPA (Fig. 12) exhibit similar behavior with respect to hematite dissolution as a function of pH. EDDA and EGTA are insensitive to pH and their ability to leach iron is little affected by temperature. CDTA dissolves some

487

DISSOLUTION OF HEMATITE TABLE I Dissolution of Hematite (0.01 g) in an EDTA Solution (4 × 10-2 M) on Aging at 215 °C pH

Appearance and characteristics

Aging

Initial

Final

period (hr)

3.0 3.0 3.0 3.0 3.0 4.2 4.2 4.2 6.1 6.1 7.5 7.5 9.5

7.0 8.2 8.6 9.8 9.8 8.0 8.8 8.8 9.1 9.4 9.0 9.7 10.5

1 2 3 10 30 1 20 30 1 30 1 30 30

Supematant solution

Light amber Dark amber Dark amber Dark amber Dark amber Light amber Light amber Light amber Faint brown Light amber Yellow Yellow Faint yellow

hematite at low and high pH at elevated temperatures only. Since no correlation can be found between dissolution of hematite in the presence of different (but related) chelating agents and their thermodynamic properties, it would seem that relatively small changes in molecular structure play a decisive role. EDTA may assume a particularly favorable interfacial configuration, especially at lower temperatures. NTA is exceptional as it exerts the strongest effect in acidic solutions and remains inactive in alkaline media at all temperatures studied. Dissolution of hematite by NTA at low pH can be explained in terms of mechanism (B). The finding that this chelating agent is more efficient than EDTA or other similar species under these conditions at all temperatures can be understood if the interfacial conformation of the complexing ligands is taken into consideration. The large EDTA molecule and the related chelating agents occupy several sites, requiring considerable energy to break a number of lattice bonds before the complex can be leached. In contrast the much more compact NTA molecule will probably bind one lattice ferric ion only which would necessitate less energy to

Precipitate

No solid left A small amount of black magnetic particles Black granular magnetic particles

No solid left Miniscule amount of red nonmagnetic particles No solid left Brown nonmagnetic particles A small amount of red nonmagnetic particles A large amount of red nonmagnetic particles remained

remove. The failure of NTA to dissolve hematite at high pH may be attributed to relatively weak complexing power for ferric ions in solution (22). No information on adsorption of EGTA and EDDA on hematite is available and neither are the necessary complex constants with Fe(III)-ion given. Thus, it is not possible at this time to offer an explanation for the inertness of these chelating species with regard to hematite dissolution.

3. Phase transformation On aging at 215°C for 2 hr the hematite sol in the presence of EDTA at an initial pH < 4 yields a black solid which was identified by X-ray diffraction to be magnetite (Table I). During this process the pH of the system increased considerably. The experimental evidence indicates that magnetite was generated after the hematite particles dissolved. It was reported (23) that the Fe 3+ ion is much more difficult to reduce while incorporated in Fe(OH)3 than when in solution. Thus, it is reasonable to assume that the formation of Fe 2+ ion takes place once the ferric ions have been released from the solid. At high temperatures, EDTA decomposes Journal of Colloid and Interface Science, VoL 92. No. 2. April 1983

488

CHANG AND MATLIEVIC

stepwise through a sequential loss of acetate groups to form N-(2-hydroxyethyl)iminodiacetic acid and iminodiacetic acid (24) which have higher pKa values and lower complexing ability for the iron ions (22). This process is accompanied with an increase in pH and a decrease in the total binding ability of the ligands for the iron in solution. These conditions favor the formation of magnetite. REFERENCES 1. Bauer, D. J., and Lindstrom, R. E., J. Met. 23, 31 (1971). 2. Johnson, Jr., A. B., Griggs, B., and Kustas, F. M., "Water Chemistry II," BNES, p. 273 (1980). 3. Anstine, L. D., "BWR Decontamination and Corrosion Product Characterization." General Electric Report NEDE-12665, March 1977. 4. Anstine, L. D., "Dilute Chemical Decontamination Program." General Electric Report for the U. S. Dept. of Energy, DOE/ET/34203-43, NEDC12705, August 1981. 5. Marguiova, T. KI., Rassokhin, N. G., Tevlin, S. S., Milaev, A. I., Gruzdev, N. I., and Shchapov, G. A., Teploenergetika 24, 8 (1977). 6. Romeo, G., "Characterization of Corrosion Product on Recirculafion and Bypass Lines at Millstone1." EPRI Report NP-949, December 1978. 7. Wood, C., Communicated at Electric Power Research Institute Contractors Meeting. Schenectady, N. Y., 1980; Segal, M. G., and Sellers, R. M., J. Chem. Soc. Chem. Commun., 991 (1980). 8. Chang, H. C., Healy, T. W., and Matijevi6, E., J. Colloid Interface Sci. 92, 469 (1983).

Journal of Colloid and Interface Science, Vol. 92, No. 2, April 1983

9. Matijevi~, E., and Seheiner, P., J. Colloid Interface Sci, 63, 509 (1978). 10. Chang, H. C., Ph.D. thesis, Clarkson College, 1982. I 1. Azuma, K., and Kametani, H., Trans. Metall. Soc. A I M E 230, 853 (1964). 12. Pryor, M. J., and Evans, W. R., J. Chem. Soc. (London) 1949, 3330. 13. Simnad, M., and Smoluchowski, R., J. Chem. Phys. 23, 1961 (1955). 14. Warren, I. H., and Roach, G. I. D., Trans. Inst. Min. Metall. Sect. C 80, 152 (1971). 15. Eisenlauer, J., and Matijevi6, E., J. Colloid Interface Sci. 75, 199 (1980). 16. Green, J. B., and Manahan, S. E., J. Inorg. Nucl. Chem. 39, 1023 (1977). 17. Cornell, R. M., Posner, A. M., and Quirk, J. P., J. lnorg. Nucl. Chem. 38, 563 0976). 18. Surana, W. S., and Warren, I. H., Trans. Inst. Min. Metall. 78, C133 (1969). 19. Warren, I. H., Bath, M. D., Prosser, A. P., and Armstrong, J. T., Trans. Inst. Min. Metall. Sect. C 78, 21 (1969). 20. Shying, H. E., Florence, J. M., and Carwell, D. J., J. Inorg. Nucl. Chem. 34, 213 (1972). 21. Rubio, J., and Matijevi~, E., J. Colloid Interface Sci. 68, 408 (1979). 22. Martell, A. E., and Smith, R. M., "Critical Stability Constant," Vol. I. Plenum, New York/London, 1974. 23. Cobble, J. W., "Chemical Thermodynamic Studies of Aqueous Trace Components in Light Water Reactors at High Temperature and Pressure." EPRI, RP-311-2, 1976. 24. Martell, A. E., Motekaites, R. J., Fried, A. R., Wilson, J. S., and MacMillan, D. T., Canad. J. Chem. 53, 3471 (1975).