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Boric acid adsorption on magnetite and zirconium dioxide
Boric Acid Adsorption on Magnetite and Zirconium Dioxide M I G U E L A. BLESA, ALBERTO J. G. MAROTO, AND ALBERTO E. R E G A Z Z O N I Departamento Qu[mica de Reactores, Comisidn Nacional de Energ[a Atdmica, Avenida del Libertador 8250, 1429 Buenos Aires, Argentina
Received May 25, 1983; acceptedOctober 4, 1983 The adsorption of boric acid on magnetite and zirconium dioxide in aqueous suspensionshas been characterizedthrough electrokineticmeasurementsand potentiometrictitrations. Both the pzc and the iep shift to lower pH values upon adsorption, remainingequal at all boric acid concentrationsstudied. From the shiftsin pzc, valuesfor the adsorptionequilibriumconstants are derivedusingthe site-binding model and assuming that boron is placed in the zero plane rather than in the r-plane. The predictions of the model for the pH-dependenceof adsorption are compared with available literature values. INTRODUCTION
niques, and must therefore be supplemented with reactions
The interaction o f metal oxides with weak acids dissolved in water is strongly influenced by the solution pH, because this parameter is involved in the charge generation reactions of the oxide interface, and furthermore determines the extent of the dissociative reactions of the weak acid. The usual trends indicate that for polybasic inorganic (1, 2) and organic (3, 4) acids, the affinity of the oxide for the adsorbing species decreases with increasing pH, showing maxima or humps at p H values which are close to the pKa value of the acid in bulk solutions. These profiles have been interpreted qualitatively on the basis of the expected changes in the chemical, electrical, and solvational changes of the Gibbs adsorption energy (4). Adsorption in these cases involves a chemical process in the interface, whereby surface hydroxyl groups are replaced by the adsorbing anion; this can be most readily written as in reaction
M - O H ~ + LH~- --~ M - O H m - - - LH~- --~ M-LH(qn-l)- + H 2 0 M-OH
M-O~
M_LH(qn-1)-
[2]
~
+ H +.
[31
M_LH(qn- 1)-
Furthermore, proteolytic equilibria of LH~may be shifted giving rise to a rather complex relationship between the measured acid consumption in the titration experiment and the desired magnitude ao. Within this frame, the adsorption of boric acid on soils has been studied by several authors; the data in general have been interpreted in terms of a differing affinity of the surface groups for uncharged B(OH)3 and its anion B(OH)~ giving rise to a maximum adsorptivity at around p H 9 (5-12). As boric acid is not a Lewis acid, in the sense that it does not release protons, but rather combines with hydroxide ions, its adsorption seems in principle of interest to probe the actual nature of the surface and the type of equilibria involved, in particular, up to what point are the current site-binding models (13-17) able to describe
M - O H + LH~- --o M_LH(q,-I)- + OH-. [11 Equation [1] by itself however does not account for the changes in surface charge density, ao measured by potenfiometric titration tech32 0021-9797/84 $3.00 Copyright © 1984 by Academic Press, Inc. All fights of reproduction in any form reserved.
Journal of Colloid and InterfaceScience, Vol. 99, No. 1, May 1984
BORIC ACID ADSORPTION ON METAL OXIDES
33
this adsorption. It has been previously pointed which in turn may change by the presence of out by Alwitt (18) that the effect of boric acid impurities or by changes in the surface layer on the dielectric properties of hydrous alumina Fe(III)/Fe(II) ratio (24, 25). films could be interpreted in terms of a penZirconium dioxide was the same material etration of borate anion into the solid phase, used in previous work (26), which is also a and the discussion of the better description of high purity nuclear grade ZrO2 (baddeleyte the location of adsorbed boron is one of the being the only phase present within X-ray difpurposes of the present paper. We present here fraction accuracy). Both materials were charthe results of a study of boric acid adsorption acterized chemically, structurally (X-ray difon magnetite and zirconium dioxide carried fraction and M/Sssbauer spectroscopy), and out through mobilities and surface charge de- morphologically (scanning electron microsterminations (potentiometric titrations). copy) as described in previous work (26). AvAdsorption of boric acid on those oxides is erage particle sizes were 0.18 (ZrO2) and 0.26 important per se in the context of our work. (Fe304) /zm. BET surface areas were 5.72 Boric acid is used as a soluble poison in order (ZrO2) and 5.00 (Fe304) m 2 g-l. to control the reactivity of nuclear reactors Prior to use, both oxides were repeatedly cooled and moderated by water, because of rinsed with bidistilled water until pH conthe high cross section of the nuclear reaction stancy was achieved in 10 successive rinses. l°B(n, a)7Li. Earlier studies showed that, at Analytical grade reagents were used in all least for reactors using enriched uranium fuels, cases. Water was bidistilled, conductivity less adsorption was not a possible nuisance in the than 0.1 uS cm -1. Nitrogen blanket atmocontrol of the stability of the nucleus (19, 20). sphere was kept in all cases, and deaereated A similar study, using the more powerful ex- solutions were used. perimental tools indicated was however beElectroldnetic measurements. Each oxide lieved to be useful because in the case of re- was suspended in 0.001 mole dm -3 KNO3 actors using natural uranium fuels neutron solution containing appropriate amounts of economy is much more strict, and very low BO3H3 to yield concentrations in the range amounts of boric acid adsorbed on fuel ele- 0.1-0.001 mole dm -3. Suspension pH was adments may affect nuclear reactivity by mea- justed by adding K O H or HNO3, and the syssurable amounts. tem was left standing overnight. Mobilities were determined in the thermostated cell of a Karl Zeiss cytopherometer. Thirty pairs of MATERIALS AND METHODS readings were averaged in each case. Potential Materials. Magnetite was prepared as de- gradient was 5 V c m -1, and the temperature scribed earlier (21, 22), by oxidation of a fer- was 25 °C. I soelectric points as measured were rous hydrous oxide gel with nitrate in the reproducible to within +0.1 pH units; error presence of hydrazine. The material thus ob- in calculated zeta potentials is 15%. tain was highly crystalline, practically stoiPotentiometric titrations. The technique has chiometric (x in Fe3_xO 4 w a s less than 0.005, been described previously (26), and is based i.e., within experimental error and undetect- in the fast-titration method developed by able by the analytical procedures employed) Breewsma (27). Titrations were performed in (23). Necessity to ensure purity and stoichi- a thermostated cell using a Mettler DK-10, ometry is particularly taxing in the case of DV-11 titrator. In every case, blank titrations magnetite because (a) magnetite easily devel- were performed in the oxide-free solution. ops an iron poor surface layer by oxidation Points of zero charge were highly reproducible, to a 3,-FeaOa-like material (24), and (b) double duplicate runs differing in less than 0.05 pH layer may, at least in principle, be influenced units. Apparent surface charge densities are by the semiconductor space charge region, correct to within +0.2 uC cm -2. Journal of Colloid and Interface Science, Vol. 99, No. 1, May 1984
34
BLESA, MAROTO, AND REGAZZONI RESULTS I01' , , ~ . . / m :1 V "1 S-1
The results of the electrokinetic measurements are shown in Figs. 1 and 2, in the form of mobility-pH profiles. Zeta potential values were calculated using the MOBLTY program of White and O'Brien (28) based on Wiersema's procedure (29) from the data in the plots. A shift in the isoelectric point toward the acid side upon boric acid addition is apparent, and is similar to the observations of Alwitt for the system pseudobohemite/boric acid (30). From the results of the potentiometric titrations, we have calculated the apparent surface charge density, ~PP, using ~8~" = F ( r ° + - r ° . -)
3
2
1
0
---t-~----4 5
7
8
pH 4--9
-1
-2
[41 -3
(r°+ - I?OH-) = AVN.onNN~orl/A.
[5]
As discussed below, the magnitude thus calculated may differ from the actual value of a0 (they may even differ in sign), and consequently we have labeled it with the superscript. In Eqs. [4] and [5], F is the Faraday constant, r are the excess surface concentrations, AVis the difference in volume oftitrant required to bring blank and suspension to a
)O~/~C/ mZ v'~s-;
I
I
tpH
-1
-2
given pH value, Nis the titrant concentration, and A the total area of the oxide used in the experiment. The dependence of a~PPwith pH is shown in Fig. 3 for the case of zirconium dioxide. These curves shift upon boric acid addition in the same sense as the ~'-potential curves; furthermore, there is a close agreement of the pH of the apparent point of zero charge and the isoelectric point at every concentration of boric acid. For magnetite, similar trends are observed, although in this case the shifts in a~PP are much smaller, indicating a lesser affinity of the surface for boric acid; this is discussed further below in more quantitative terms. The values of pzc and iep for both oxides are plotted in Fig. 4 as a function of boric acid concentration. DISCUSSION
-3
F/G. 1. Mobility of magnetite in KNO3 0.001 mole dm -3 as a function of pH at 25°C: (llL in the presence of 0.1 mole dm -3 BO3H3; (0), without boric acid. Journal of Colloid and Interface Science.
FIG. 2. Mobility of zirconium dioxide in KNO3 0.001 mole dm -3 as a function o f p H at 25°C, at various boric acid concentrations: (0) 0.1; (A) 0.01; (I) 0.001 mole dm-3; (T) without boric acid.
Vol.99, No. 1, May 1984
Adsorption of boric acid on both magnetite and zirconium dioxide is dearly apparent through the changes in both a~PP and ~-. As opposed to the usual case for the specific ad-
BORIC ACID ADSORPTION ON METAL OXIDES
(~o app///). C cm "1
10
t 4
I 5
-~,~. \.
I
pH I
I
35
vious work, in which only adsorption isotherms (5-12) or mobilities only (30) were measured. According to the current site-binding models (13-17), any adsorbed borate ion should be placed in the inner Helmholtz layer (or/3-plane, see Fig. 5), and surface charge density should increase upon boric acid addition. The equation representing the adsorption of a weak polyprotic acid (which is in its simplest form equivalent to [2] plus the equation representing the protonation of surface sites) is:
-5
M - O H + LH~- + (1 + x ) H + M-OH~
• • • LH~+]
)-
[6]
-10
which takes into account the possible proton-t5
FIG. 3. Apparent surface charge density of zirconium dioxide in KNO3 0.1 mole dm-3 as a function of pH at 30°C, at various boric acid concentrations: (O) 0.01; (i) 0.001; (A) 0.0004 mole dm-3.
sorption of charged species, however, both ~r8pp and ~" decrease at any given p H value upon boric acid adsorption. This rather particular feature has not been observed in pre7.0
pzc(O) or iepll] .
.
OXIDE
.
8.o Zr 0 z - ' ~
0,,
'\
tog [ B]
5.0 ~
•
i
i
i
-5
-3
-1
FIG. 4. Point of zero charge (©) and isoelectric point (O) of magnetite and zirconium dioxide as a function of the logarithm of equilibrium boron concentration. Solid line calculated according to Eq. [ 19].
FIG. 5. Schematic representation of the oxide/solution interface. Journal of Colloid and Interface Science, VoL 99, No. 1, May 1984
36
BLESA, MAROTO, AND REGAZZONI
ation of the adsorbing anion, and x may be zero or one (16). We have attempted to reconcile our observations with the site-binding models by assuming that the equations representing adsorption must include the following features: (a) there must be a supraequivalent consumption of O H - ions in the adsorption (i.e., positive charge may be created in the zero plane, but the total equation must involve hydroxide ion consumption); (b) boric acid is restricted to the B-plane. In principle, such a mechanism can be envisaged as M - O H + B(OH)3 + O H - ~M - O H . • .HOB(OH)3
[7]
M - 0 H . • • HOB(OH)~ -.o M - O H ~ - - - O B ( O H ) 2-.
[8]
Equations [7] and [8] intend to reflect the stoichiometry of adsorption, the possibility of creation of positive charge in the zero plane, and the location of boron in the B-plane. Of course, and in view of the inorganic chemistry of boric acid, it is reasonable to supplement Eqs. [7] and [8] with M-OHm. • • OB(OH)32- ---* M+-OB(OH)~ - + H20.
[9]
Equation [9] has been written as to imply that a covalent borate-type bond is formed between the metal ion and borate, but that there is still a charge separation between the planes zero and BUnder these assumptions, the values of a8°p must be considered to differ from the real a0 calculated through a0 = F([M-OH~'] + [ M - O H f . • • NOr] -
[M-O-]
-
[M-O-.
• • K +]
+ [M-OHm. • .OB(OH)]-]).
[10]
On the other hand, ~PP is given by agpp = F([M-OH~] + [M-OHm. • • NO~] -
[M-O-] -
-
[M-O--
• • K +]
[M-OHm. • -OB(OH)2-]).
[ll]
This equation differs from [10] in the sign of the contribution of the sites of adsorbed borate ions, and this reflects the participation of O H in the stoichiometry of Eq. [7]. This adsorption scheme in principle describes the parallel shifts of the apparent pzc and of the iep, and predicts that in fact the sign of ao may be reversed in relation to agpp. The necessity to distinguish between a0 and agpp is the major drawback of this analysis, and gives rise to rather cumbersome algebra to model the adsorption phenomenon, included the observed moderately low --ad (=a0 + aa) values calculated from g'-potentials; this implies that charge creation in the r-plane (Eqs. [7]-[9]) must be neutralized to a large extent by other charge-creating processes. The origin of the difficulties in this model is the arbitrary attempt to restrict the location of boron to the B-plane. If this restriction is removed, reactions [7]-[9] are viewed as a simple surface reaction: M - O H + B(OH)3 + O H M-O-B(OH)~ + H20
[12]
which is complemented by the usual adsorption of counterions in the B-plane: M-O-B(OH)3 + K + M - O - B ( O H ) f . • • K +.
[13]
As these interactions take place in addition to the usual acid-base equilibria, the resulting magnitudes ~ko, ff~, ffa; a0, a~, and ad should be described in terms of six equilibrium constants. Accepting that the values for the equilibrium constants:
M-OH~ ~ M-OH + H +
(Ka,)
[14]
M - O H ~- M - O - + H +
(Ka2)
[15]
M - O H ~ + NO~s ~ M-OHm- • • NO3-
(K~b~)
M - O - + K~+ ~- M - O - - • • K +
(K~-~)
[ 161 [171
Journal of Colloid and Interface Science, Vol. 99, No. 1, May 1984
37
BORIC ACID ADSORPTION ON METAL OXIDES
are not changed upon boric acid adsorption, it is possible to calculate from the shifts in pzc with boron concentration the value of the equilibrium constant for boron adsorption (Eq. [12]). In order to do this, the surface charge expression
11/,
,
-6
2
2
7.5
5.0
ao = F ( [ M - O H ~ ] + [M-OHm-. • • NO~] -
[M-O-]
-
[M-O-.
-
[M-O-B(OH)~]
• • K +1
- [ M - O - B ( O H ) ~ . • • K+])
2.5
[18]
must be equated to zero, and the contribution from each type of site expressed in terms of the relevant constants, acidity and ionic strength. I f it is further assumed that K~)~, Ki~t, and K~3 are equal, a very simple expression is derived for the hydrogen ion concentration at the pzc: [H+]2zc =
10 t , [BJ/rnol dm-3 I
I
I
I
1
2
3
4
FIG. 6. Plot of [H+]2~o for magnetite as a function of the equilibrium boron concentration at 25°C.
BO3H3 -4- H 2 0 ~ B(OH)4 + H + O H - + H + ~ 1-120.
[21] [22]
K~lKa2 + K~IKwK~2[B]. [19] The ratio of borated sites, M - O - B ( O H ) ~
The assumed identity of all ion-pair formation constants m a y be a rough approximation, but even so a reasonable agreement is obtained with experimental data. This is shown in Figs. 6 and 7 in the form of plots of [H+]2z~ vs the total boron concentration [B] (which is assumed equal to equilibrium concentration). From the slope of the straight line, and using our previously determined values for Kal and K~2 (26), KI2 can be calculated to be 3.5 × 106 mole d m -6 for magnetite, and for zirconium dioxide 5.5 X 108 mole -2 d m 6. It must be emphasized that, although the calculation becomes independent of the value of K13, values of 63 and 10 mole -1 d m 3 have been implicitly assumed, and this in turn means that most of the borated sites are present as M - O - B ( O H ) 3 . . . K ÷ rather than as M - O B(OH)t. We are now in a position to calculate the p H dependence of boron adsorption. In order to d o this, Eq. [ 12] must be viewed as the sum of equations M - O H + B(OH)~ ~ M - O - B ( O H ) j + H 2 0
[20]
+ M - O - B ( O H ) ~ • • • K + to free (hydroxilated) sites M - O H is then given by [M-O-B]-r _ K2oK2~K~I [B] [M-OH] [H+] + KzlK~I X {1 + K13[K +]
exp(e~o/kT)
exp(-e~MkT) }.
[23]
In order to calculate the boron adsorption/ p H profile from Eq. [23] it is necessary to calculate 4o and ff~ at each p H value. This can be done by solving the whole set o f equations of the site-binding model, Eqs. [ 18] and a~ = F ( I M - O - . • • K +] + [ M - O - B ( O H ) 3 • • • K +] - [M-OHm. • .NO;l)
as = -ll.74IU2sinh[e~a~ \2kT]
[24] [25]
Ns = ( [ M - O H ] + [ M - O H f ] + [ M - O H f - • • NO3] + [ M - O - ] + [ M - O - . • • K +] + [ M - O - B ( O H ) 5 ] + [ M - O - B ( O H ) 5 - • • K+]) ~o - ~
= ao/C,
[26] [27]
Journal of Colloid and Interface Science, Vol. 99, No. 1, May 1984
38
BLESA, M A R O T O , A N D R E G A Z Z O N I
0.4
1011= [ H * ] 2 / r n o l 2 dm -~' pzc
0.3
0.2
0.1 102=[B]/ I
I
[
I
0.1
I
I
tool dm-3
I
l
0.5
I
I
1,0
FIG. 7. Plot o f [H+]2pz~ for zirconium dioxide as a function o f the equilibrium boron concentration
at 25°C.
6a - 6d = - a j C 2 ao + aa + ~a = 0.
[28] [29]
The procedure used to solve the set of equations was similar to that employed by other authors (14). By introducing Xb0and Se in Eq. [23] the adsorption profile on magnetite and zirconium dioxide were obtained. The results are shown in Fig. 8, and should be compared with the data reported in the literature. Thus, the maxima in adsorption at pH ca. 9 found by Keren (5, 6), Bingham (7, 8, 11), Chen (10), and Hingston (12) are naturally explained. It is also worth of mention that the experimental ao-pH profiles are reproduced well by the results of the calculations using the above equations. It is interesting to note that the pH dependency arises from the pH dependency of equilibrium [ 12] and from the pH dependency of the electrostatic interaction energy; in other words, not only the AGelectrostatic component to the total Gibbs energy of adsorption is sensitive to pH, but also the chemical component corresponding to the Gibbs energy change of reaction [12]. Separating the OH- activity contribution to AGehemicalit is obvious that this magnitude can be written as AGchem = AG0hem+ 2.3RT pOH, which changes with pH and is responsible for the decrease in adsorptivity in the acidic side of Fig. 8. Journal of Colloid and Interface Science, Vol. 99, No. l, May 1984
It is interesting to note that according to the ideas presented here, adsorption of boron gives rise to changes in the adsorption of all the ions defining the charge in the interface. The thermodynamics of adsorption in such mixed electrolyte systems have been hardly worked out, but the provisional discussion by Lyklema (34) regarding the adsorption of monovalent and divalent cations on haematite is directed in this sense, and shows that experimental quantities are related to EsinMarkov coefficients (Oao/O In COpH,q,i. Using these ideas, it is interesting to compare adsorption of boric acid and adsorption of other weak organic acids. In this latter case, the adsorption equation, which should be compared to [12], is M-OH+A-+H
+~M-A+H/O.
[30]
It is obvious that in a titration experiment this looks as a proton uptake by the surface, as compared to the hydroxide ion uptake of Eq. [12]. It should also be obvious that while Eq. [12] really represents a net decrease of the surface charge, no charge is created on the surface by the reaction of Eq. [30], i.e., only agppis increased. Using the Esin-Markov coefficient,
(a~o/a I n
[B]) =
+ F(0[Fn+
-
-F(orB/a In [B])
FOH-]/0FB)(0InB/0
In [B]) [31]
39
BORIC ACID ADSORPTION ON METAL OXIDES
,ot
/ mo, m2
0.8
Zr 0 2
0.6 <..--.
~
0.4
e 3 0~,
0.2
4
6
8
10
FIG. 8. Adsorption isotherms of boric acid on magnetite and zirconium dioxide according to the present model (Eq. [23])at 0.001 mole din- 3boron equilibrium concentration and KNO3 0.1 m o l e d m ~-3. Temperature: 30°C.
where all partial derivatives must be understood at constant pH, temperature, and ionic strength. The analogous equation as applied to [30] would simply be (0~0/0 In [A-I) = 0.
[32]
Equation [32] is however hardly likely to hold valid, because of two reasons: (a) at sufficiently low pH values, a sizeable fraction of surface -OH groups are protonated, and as these can also be replaced by A-, a net decrease in the positive charge of the surface is expected; (b) usually data available refer to polyprotic weak acids, in which case the adsorbed ions may bear negative charges and these decrease the overall charge in the zero plane. Furthermore, shifts in the acidity constants upon complex-
ation may give rise to apparent contributions to ~o. It should be noted that Eq. [30] is just the first stage of the dissolution of the oxide by HA, and as such, it should be expected that adsorptivity should correlate with complex ion stability constants (4, 31). Going back to Eq. [31], it is obvious that the extent of adsorption of boron cannot be equated to the decrease in or0, as hydrogen ion co-adsorption neutralizes substantially the effect of boron adsorption. The idea that chemisorption of strongly interacting anions on hydrous oxides involves these species in the zero plane has already been forwarded by Stumm (31-33) for the case of inorganic acids such as HF, HaSiO4, and H3PO4. In these papers the close relaJournal of Colloid and Interface Science, Vol. 99, No. 1, May 1984
40
BLESA, MAROTO, AND REGAZZONI
tionship between the complexation chemistry of the metal ion and the anion with the surface chemistry of the oxide and the anion is also discussed. The present model complements Stumm's ideas with the site-binding model structure for the double layer, thus allowing for the existence of adsorbed ions in the Stern layer which partially neutralize the charge and explain the rather low ~'-potentials found in practice. In this sense, it may be considered as a mixture of both models, which accepts that strong chemisorption takes place in the zero plane, but uses the site-binding model to calculate the whole set of parameters describing the double layer. ACKNOWLEDGMENTS To SUBCYT for partial support. M.A.B. is a member of CONICET. REFERENCES 1. Hingston, F. J., Atkinson, R. J., Posner, A. M., and Quirk, J. P., Nature (London) 215, 1459 (1967). 2. Anderson, M. A., Ferguson, J. F., and Gavis, J., J. Colloid Interface Sci. 54, 391 (1976). 3. Chang, H.-C., Healy, T. W., and Matijevir, E., J. Colloid Interface Sci. 92, 469 (1983). 4. Blesa, M. A., Borghi, E. B., Maroto, A. J. G., and Regazzoni, A. E., J. Colloid Interface Sci., 98, 295 (1984). 5. Keren, R., Gast, R. G., and Bar-Yosef, B., Soil Sci. Soe. Amer. J. 45, 45 (1981). 6. Mezuman, U., and Keren, R., Soil Sci. Soc. Amen. J. 45, 722 (1981). 7. Bingham, F. T., Page, A. L., Coleman, N. T., and Flach, K., SoilSci. Soc. Amer. Proc. 35, 546 (1971). 8. McPhail, M., Page, A. L., and Bingham, F. T., Soil Sci. Soc. Amer. Proc. 36, 510 (1972). 9. Rhoades, J. D., Ingvalson, R. D., and Hatcher, J. T., Soil. Sci. Soc. Amen. Proc. 34, 938 (1970). 10. Choi, W-W., and Chen, K. Y., Environ. Sci. Technol. 13, 189 (1979). 11. Sims, J. R., and Bingham, F. T., Soil Sci. Soc. Amer. Proc. 32, 364 (1968).
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12. Hingston, F. J., Aust. J. Soil Res. 2, 83 (1964). 13. Yates, D. E., Levine, S., and Healy, T. W., J. Chem. Soc. Faraday Trans. L 70, 1807 (1978). 14. Davis, J. A., James, R. O., and Leckie, J. O.,J. Colloid Interface Sci. 63, 480 (1978). 15. Davis, J. A., and Leckie, J. O., J. Colloid Interface Sci. 67, 90 (1978). 16. Davis, J. A., and Leckie, J. O., J. Colloid Interface Sci. 74, 32 (1980). 17. James, R. O., and Parks, G. A., in "Surface and Colloid Science" (E. Matijevir, Ed.), Vol. 12, Chap. 2, p. 119. Plenum, New York, 1982. 18. Alwitt, R. S., J. Electrochem. Soc. 118, 1734 (1971). 19. Fletcher, W. D., Krieg, A., and Cohen, P., Report WCAP-1689, Westinghouse Electrical Corp. (1961). 20. Fletcher, W. D., Report WCAP-3730, Westinghouse Electrical Corp. (1964). 21. Regazzoni, A. E., Urrutia, G. A., Blesa, M. A., and Maroto, A. J. G., J. Inorg. Nucl. Chem. 43, 1489 (1981). 22. Maroto, A. J. G., Blesa, M. A., Regazzoni, A. E., and Urrutia, G. A., "Water Chemistry of Nuclear Reactor Systems," Vol. 2, p. 241. Ed. Brit. Nuclear Energy Soc., 1980. 23. Blesa, M. A., Maroto, A. J. G., Passaggio, S. L, Labenski, F., and Saragovi-Badler, C., Radiat. Phys. Chem. 11, 321~(1978). 24. Diggle, J. W., in "Oxides and Oxide Layers" (J. W. Diggle, Ed.), Vol. 2, p. 281. Dekker, New York, 1973. 25. Chazalviel, J.-N., J. Electrochem. Soc. 129, 964 (1982). 26. Regazzoni, A. E., Blesa, M. A., and Maroto, A. J. G., J. Colloid Interface Sci. 91, 560 (1983). 27. Breeuwsma, A., Ph.D. dissertation, Wageningen, Netherlands, 1973. 28. O'Brien, R. W., and White, L. R., J. Chem. Soc. Faraday Trans. H 74, 1607 (1978). 29. Wiersema, P. H., Loeb, A. L., and Overbeek, J. Th. G., J. Colloid Interface Sci. 22, 78 (1966). 30. Alwitt, R. S., J. Colloid Interface Sci. 40, 195 (1972). 31. Sigg, L., and Stumm, W., Colloids Surf. 2, 101 (1981). 32. Hohl, H., Sigg, L., and Stumm, W., Advan. Chem. Ser. 189, 1 (1980). 33. Stumm, W., Kummert, R., and Sigg, L., Croat, Chem. Acta 53, 291 (1980). 34. Ardizzone, S., Formaro, L., and Lyklema, J., J. Electroana[. Chem. 133, 147 (1982).
Received May 25, 1983; acceptedOctober 4, 1983 The adsorption of boric acid on magnetite and zirconium dioxide in aqueous suspensionshas been characterizedthrough electrokineticmeasurementsand potentiometrictitrations. Both the pzc and the iep shift to lower pH values upon adsorption, remainingequal at all boric acid concentrationsstudied. From the shiftsin pzc, valuesfor the adsorptionequilibriumconstants are derivedusingthe site-binding model and assuming that boron is placed in the zero plane rather than in the r-plane. The predictions of the model for the pH-dependenceof adsorption are compared with available literature values. INTRODUCTION
niques, and must therefore be supplemented with reactions
The interaction o f metal oxides with weak acids dissolved in water is strongly influenced by the solution pH, because this parameter is involved in the charge generation reactions of the oxide interface, and furthermore determines the extent of the dissociative reactions of the weak acid. The usual trends indicate that for polybasic inorganic (1, 2) and organic (3, 4) acids, the affinity of the oxide for the adsorbing species decreases with increasing pH, showing maxima or humps at p H values which are close to the pKa value of the acid in bulk solutions. These profiles have been interpreted qualitatively on the basis of the expected changes in the chemical, electrical, and solvational changes of the Gibbs adsorption energy (4). Adsorption in these cases involves a chemical process in the interface, whereby surface hydroxyl groups are replaced by the adsorbing anion; this can be most readily written as in reaction
M - O H ~ + LH~- --~ M - O H m - - - LH~- --~ M-LH(qn-l)- + H 2 0 M-OH
M-O~
M_LH(qn-1)-
[2]
~
+ H +.
[31
M_LH(qn- 1)-
Furthermore, proteolytic equilibria of LH~may be shifted giving rise to a rather complex relationship between the measured acid consumption in the titration experiment and the desired magnitude ao. Within this frame, the adsorption of boric acid on soils has been studied by several authors; the data in general have been interpreted in terms of a differing affinity of the surface groups for uncharged B(OH)3 and its anion B(OH)~ giving rise to a maximum adsorptivity at around p H 9 (5-12). As boric acid is not a Lewis acid, in the sense that it does not release protons, but rather combines with hydroxide ions, its adsorption seems in principle of interest to probe the actual nature of the surface and the type of equilibria involved, in particular, up to what point are the current site-binding models (13-17) able to describe
M - O H + LH~- --o M_LH(q,-I)- + OH-. [11 Equation [1] by itself however does not account for the changes in surface charge density, ao measured by potenfiometric titration tech32 0021-9797/84 $3.00 Copyright © 1984 by Academic Press, Inc. All fights of reproduction in any form reserved.
Journal of Colloid and InterfaceScience, Vol. 99, No. 1, May 1984
BORIC ACID ADSORPTION ON METAL OXIDES
33
this adsorption. It has been previously pointed which in turn may change by the presence of out by Alwitt (18) that the effect of boric acid impurities or by changes in the surface layer on the dielectric properties of hydrous alumina Fe(III)/Fe(II) ratio (24, 25). films could be interpreted in terms of a penZirconium dioxide was the same material etration of borate anion into the solid phase, used in previous work (26), which is also a and the discussion of the better description of high purity nuclear grade ZrO2 (baddeleyte the location of adsorbed boron is one of the being the only phase present within X-ray difpurposes of the present paper. We present here fraction accuracy). Both materials were charthe results of a study of boric acid adsorption acterized chemically, structurally (X-ray difon magnetite and zirconium dioxide carried fraction and M/Sssbauer spectroscopy), and out through mobilities and surface charge de- morphologically (scanning electron microsterminations (potentiometric titrations). copy) as described in previous work (26). AvAdsorption of boric acid on those oxides is erage particle sizes were 0.18 (ZrO2) and 0.26 important per se in the context of our work. (Fe304) /zm. BET surface areas were 5.72 Boric acid is used as a soluble poison in order (ZrO2) and 5.00 (Fe304) m 2 g-l. to control the reactivity of nuclear reactors Prior to use, both oxides were repeatedly cooled and moderated by water, because of rinsed with bidistilled water until pH conthe high cross section of the nuclear reaction stancy was achieved in 10 successive rinses. l°B(n, a)7Li. Earlier studies showed that, at Analytical grade reagents were used in all least for reactors using enriched uranium fuels, cases. Water was bidistilled, conductivity less adsorption was not a possible nuisance in the than 0.1 uS cm -1. Nitrogen blanket atmocontrol of the stability of the nucleus (19, 20). sphere was kept in all cases, and deaereated A similar study, using the more powerful ex- solutions were used. perimental tools indicated was however beElectroldnetic measurements. Each oxide lieved to be useful because in the case of re- was suspended in 0.001 mole dm -3 KNO3 actors using natural uranium fuels neutron solution containing appropriate amounts of economy is much more strict, and very low BO3H3 to yield concentrations in the range amounts of boric acid adsorbed on fuel ele- 0.1-0.001 mole dm -3. Suspension pH was adments may affect nuclear reactivity by mea- justed by adding K O H or HNO3, and the syssurable amounts. tem was left standing overnight. Mobilities were determined in the thermostated cell of a Karl Zeiss cytopherometer. Thirty pairs of MATERIALS AND METHODS readings were averaged in each case. Potential Materials. Magnetite was prepared as de- gradient was 5 V c m -1, and the temperature scribed earlier (21, 22), by oxidation of a fer- was 25 °C. I soelectric points as measured were rous hydrous oxide gel with nitrate in the reproducible to within +0.1 pH units; error presence of hydrazine. The material thus ob- in calculated zeta potentials is 15%. tain was highly crystalline, practically stoiPotentiometric titrations. The technique has chiometric (x in Fe3_xO 4 w a s less than 0.005, been described previously (26), and is based i.e., within experimental error and undetect- in the fast-titration method developed by able by the analytical procedures employed) Breewsma (27). Titrations were performed in (23). Necessity to ensure purity and stoichi- a thermostated cell using a Mettler DK-10, ometry is particularly taxing in the case of DV-11 titrator. In every case, blank titrations magnetite because (a) magnetite easily devel- were performed in the oxide-free solution. ops an iron poor surface layer by oxidation Points of zero charge were highly reproducible, to a 3,-FeaOa-like material (24), and (b) double duplicate runs differing in less than 0.05 pH layer may, at least in principle, be influenced units. Apparent surface charge densities are by the semiconductor space charge region, correct to within +0.2 uC cm -2. Journal of Colloid and Interface Science, Vol. 99, No. 1, May 1984
34
BLESA, MAROTO, AND REGAZZONI RESULTS I01' , , ~ . . / m :1 V "1 S-1
The results of the electrokinetic measurements are shown in Figs. 1 and 2, in the form of mobility-pH profiles. Zeta potential values were calculated using the MOBLTY program of White and O'Brien (28) based on Wiersema's procedure (29) from the data in the plots. A shift in the isoelectric point toward the acid side upon boric acid addition is apparent, and is similar to the observations of Alwitt for the system pseudobohemite/boric acid (30). From the results of the potentiometric titrations, we have calculated the apparent surface charge density, ~PP, using ~8~" = F ( r ° + - r ° . -)
3
2
1
0
---t-~----4 5
7
8
pH 4--9
-1
-2
[41 -3
(r°+ - I?OH-) = AVN.onNN~orl/A.
[5]
As discussed below, the magnitude thus calculated may differ from the actual value of a0 (they may even differ in sign), and consequently we have labeled it with the superscript. In Eqs. [4] and [5], F is the Faraday constant, r are the excess surface concentrations, AVis the difference in volume oftitrant required to bring blank and suspension to a
)O~/~C/ mZ v'~s-;
I
I
tpH
-1
-2
given pH value, Nis the titrant concentration, and A the total area of the oxide used in the experiment. The dependence of a~PPwith pH is shown in Fig. 3 for the case of zirconium dioxide. These curves shift upon boric acid addition in the same sense as the ~'-potential curves; furthermore, there is a close agreement of the pH of the apparent point of zero charge and the isoelectric point at every concentration of boric acid. For magnetite, similar trends are observed, although in this case the shifts in a~PP are much smaller, indicating a lesser affinity of the surface for boric acid; this is discussed further below in more quantitative terms. The values of pzc and iep for both oxides are plotted in Fig. 4 as a function of boric acid concentration. DISCUSSION
-3
F/G. 1. Mobility of magnetite in KNO3 0.001 mole dm -3 as a function of pH at 25°C: (llL in the presence of 0.1 mole dm -3 BO3H3; (0), without boric acid. Journal of Colloid and Interface Science.
FIG. 2. Mobility of zirconium dioxide in KNO3 0.001 mole dm -3 as a function o f p H at 25°C, at various boric acid concentrations: (0) 0.1; (A) 0.01; (I) 0.001 mole dm-3; (T) without boric acid.
Vol.99, No. 1, May 1984
Adsorption of boric acid on both magnetite and zirconium dioxide is dearly apparent through the changes in both a~PP and ~-. As opposed to the usual case for the specific ad-
BORIC ACID ADSORPTION ON METAL OXIDES
(~o app///). C cm "1
10
t 4
I 5
-~,~. \.
I
pH I
I
35
vious work, in which only adsorption isotherms (5-12) or mobilities only (30) were measured. According to the current site-binding models (13-17), any adsorbed borate ion should be placed in the inner Helmholtz layer (or/3-plane, see Fig. 5), and surface charge density should increase upon boric acid addition. The equation representing the adsorption of a weak polyprotic acid (which is in its simplest form equivalent to [2] plus the equation representing the protonation of surface sites) is:
-5
M - O H + LH~- + (1 + x ) H + M-OH~
• • • LH~+]
)-
[6]
-10
which takes into account the possible proton-t5
FIG. 3. Apparent surface charge density of zirconium dioxide in KNO3 0.1 mole dm-3 as a function of pH at 30°C, at various boric acid concentrations: (O) 0.01; (i) 0.001; (A) 0.0004 mole dm-3.
sorption of charged species, however, both ~r8pp and ~" decrease at any given p H value upon boric acid adsorption. This rather particular feature has not been observed in pre7.0
pzc(O) or iepll] .
.
OXIDE
.
8.o Zr 0 z - ' ~
0,,
'\
tog [ B]
5.0 ~
•
i
i
i
-5
-3
-1
FIG. 4. Point of zero charge (©) and isoelectric point (O) of magnetite and zirconium dioxide as a function of the logarithm of equilibrium boron concentration. Solid line calculated according to Eq. [ 19].
FIG. 5. Schematic representation of the oxide/solution interface. Journal of Colloid and Interface Science, VoL 99, No. 1, May 1984
36
BLESA, MAROTO, AND REGAZZONI
ation of the adsorbing anion, and x may be zero or one (16). We have attempted to reconcile our observations with the site-binding models by assuming that the equations representing adsorption must include the following features: (a) there must be a supraequivalent consumption of O H - ions in the adsorption (i.e., positive charge may be created in the zero plane, but the total equation must involve hydroxide ion consumption); (b) boric acid is restricted to the B-plane. In principle, such a mechanism can be envisaged as M - O H + B(OH)3 + O H - ~M - O H . • .HOB(OH)3
[7]
M - 0 H . • • HOB(OH)~ -.o M - O H ~ - - - O B ( O H ) 2-.
[8]
Equations [7] and [8] intend to reflect the stoichiometry of adsorption, the possibility of creation of positive charge in the zero plane, and the location of boron in the B-plane. Of course, and in view of the inorganic chemistry of boric acid, it is reasonable to supplement Eqs. [7] and [8] with M-OHm. • • OB(OH)32- ---* M+-OB(OH)~ - + H20.
[9]
Equation [9] has been written as to imply that a covalent borate-type bond is formed between the metal ion and borate, but that there is still a charge separation between the planes zero and BUnder these assumptions, the values of a8°p must be considered to differ from the real a0 calculated through a0 = F([M-OH~'] + [ M - O H f . • • NOr] -
[M-O-]
-
[M-O-.
• • K +]
+ [M-OHm. • .OB(OH)]-]).
[10]
On the other hand, ~PP is given by agpp = F([M-OH~] + [M-OHm. • • NO~] -
[M-O-] -
-
[M-O--
• • K +]
[M-OHm. • -OB(OH)2-]).
[ll]
This equation differs from [10] in the sign of the contribution of the sites of adsorbed borate ions, and this reflects the participation of O H in the stoichiometry of Eq. [7]. This adsorption scheme in principle describes the parallel shifts of the apparent pzc and of the iep, and predicts that in fact the sign of ao may be reversed in relation to agpp. The necessity to distinguish between a0 and agpp is the major drawback of this analysis, and gives rise to rather cumbersome algebra to model the adsorption phenomenon, included the observed moderately low --ad (=a0 + aa) values calculated from g'-potentials; this implies that charge creation in the r-plane (Eqs. [7]-[9]) must be neutralized to a large extent by other charge-creating processes. The origin of the difficulties in this model is the arbitrary attempt to restrict the location of boron to the B-plane. If this restriction is removed, reactions [7]-[9] are viewed as a simple surface reaction: M - O H + B(OH)3 + O H M-O-B(OH)~ + H20
[12]
which is complemented by the usual adsorption of counterions in the B-plane: M-O-B(OH)3 + K + M - O - B ( O H ) f . • • K +.
[13]
As these interactions take place in addition to the usual acid-base equilibria, the resulting magnitudes ~ko, ff~, ffa; a0, a~, and ad should be described in terms of six equilibrium constants. Accepting that the values for the equilibrium constants:
M-OH~ ~ M-OH + H +
(Ka,)
[14]
M - O H ~- M - O - + H +
(Ka2)
[15]
M - O H ~ + NO~s ~ M-OHm- • • NO3-
(K~b~)
M - O - + K~+ ~- M - O - - • • K +
(K~-~)
[ 161 [171
Journal of Colloid and Interface Science, Vol. 99, No. 1, May 1984
37
BORIC ACID ADSORPTION ON METAL OXIDES
are not changed upon boric acid adsorption, it is possible to calculate from the shifts in pzc with boron concentration the value of the equilibrium constant for boron adsorption (Eq. [12]). In order to do this, the surface charge expression
11/,
,
-6
2
2
7.5
5.0
ao = F ( [ M - O H ~ ] + [M-OHm-. • • NO~] -
[M-O-]
-
[M-O-.
-
[M-O-B(OH)~]
• • K +1
- [ M - O - B ( O H ) ~ . • • K+])
2.5
[18]
must be equated to zero, and the contribution from each type of site expressed in terms of the relevant constants, acidity and ionic strength. I f it is further assumed that K~)~, Ki~t, and K~3 are equal, a very simple expression is derived for the hydrogen ion concentration at the pzc: [H+]2zc =
10 t , [BJ/rnol dm-3 I
I
I
I
1
2
3
4
FIG. 6. Plot of [H+]2~o for magnetite as a function of the equilibrium boron concentration at 25°C.
BO3H3 -4- H 2 0 ~ B(OH)4 + H + O H - + H + ~ 1-120.
[21] [22]
K~lKa2 + K~IKwK~2[B]. [19] The ratio of borated sites, M - O - B ( O H ) ~
The assumed identity of all ion-pair formation constants m a y be a rough approximation, but even so a reasonable agreement is obtained with experimental data. This is shown in Figs. 6 and 7 in the form of plots of [H+]2z~ vs the total boron concentration [B] (which is assumed equal to equilibrium concentration). From the slope of the straight line, and using our previously determined values for Kal and K~2 (26), KI2 can be calculated to be 3.5 × 106 mole d m -6 for magnetite, and for zirconium dioxide 5.5 X 108 mole -2 d m 6. It must be emphasized that, although the calculation becomes independent of the value of K13, values of 63 and 10 mole -1 d m 3 have been implicitly assumed, and this in turn means that most of the borated sites are present as M - O - B ( O H ) 3 . . . K ÷ rather than as M - O B(OH)t. We are now in a position to calculate the p H dependence of boron adsorption. In order to d o this, Eq. [ 12] must be viewed as the sum of equations M - O H + B(OH)~ ~ M - O - B ( O H ) j + H 2 0
[20]
+ M - O - B ( O H ) ~ • • • K + to free (hydroxilated) sites M - O H is then given by [M-O-B]-r _ K2oK2~K~I [B] [M-OH] [H+] + KzlK~I X {1 + K13[K +]
exp(e~o/kT)
exp(-e~MkT) }.
[23]
In order to calculate the boron adsorption/ p H profile from Eq. [23] it is necessary to calculate 4o and ff~ at each p H value. This can be done by solving the whole set o f equations of the site-binding model, Eqs. [ 18] and a~ = F ( I M - O - . • • K +] + [ M - O - B ( O H ) 3 • • • K +] - [M-OHm. • .NO;l)
as = -ll.74IU2sinh[e~a~ \2kT]
[24] [25]
Ns = ( [ M - O H ] + [ M - O H f ] + [ M - O H f - • • NO3] + [ M - O - ] + [ M - O - . • • K +] + [ M - O - B ( O H ) 5 ] + [ M - O - B ( O H ) 5 - • • K+]) ~o - ~
= ao/C,
[26] [27]
Journal of Colloid and Interface Science, Vol. 99, No. 1, May 1984
38
BLESA, M A R O T O , A N D R E G A Z Z O N I
0.4
1011= [ H * ] 2 / r n o l 2 dm -~' pzc
0.3
0.2
0.1 102=[B]/ I
I
[
I
0.1
I
I
tool dm-3
I
l
0.5
I
I
1,0
FIG. 7. Plot o f [H+]2pz~ for zirconium dioxide as a function o f the equilibrium boron concentration
at 25°C.
6a - 6d = - a j C 2 ao + aa + ~a = 0.
[28] [29]
The procedure used to solve the set of equations was similar to that employed by other authors (14). By introducing Xb0and Se in Eq. [23] the adsorption profile on magnetite and zirconium dioxide were obtained. The results are shown in Fig. 8, and should be compared with the data reported in the literature. Thus, the maxima in adsorption at pH ca. 9 found by Keren (5, 6), Bingham (7, 8, 11), Chen (10), and Hingston (12) are naturally explained. It is also worth of mention that the experimental ao-pH profiles are reproduced well by the results of the calculations using the above equations. It is interesting to note that the pH dependency arises from the pH dependency of equilibrium [ 12] and from the pH dependency of the electrostatic interaction energy; in other words, not only the AGelectrostatic component to the total Gibbs energy of adsorption is sensitive to pH, but also the chemical component corresponding to the Gibbs energy change of reaction [12]. Separating the OH- activity contribution to AGehemicalit is obvious that this magnitude can be written as AGchem = AG0hem+ 2.3RT pOH, which changes with pH and is responsible for the decrease in adsorptivity in the acidic side of Fig. 8. Journal of Colloid and Interface Science, Vol. 99, No. l, May 1984
It is interesting to note that according to the ideas presented here, adsorption of boron gives rise to changes in the adsorption of all the ions defining the charge in the interface. The thermodynamics of adsorption in such mixed electrolyte systems have been hardly worked out, but the provisional discussion by Lyklema (34) regarding the adsorption of monovalent and divalent cations on haematite is directed in this sense, and shows that experimental quantities are related to EsinMarkov coefficients (Oao/O In COpH,q,i. Using these ideas, it is interesting to compare adsorption of boric acid and adsorption of other weak organic acids. In this latter case, the adsorption equation, which should be compared to [12], is M-OH+A-+H
+~M-A+H/O.
[30]
It is obvious that in a titration experiment this looks as a proton uptake by the surface, as compared to the hydroxide ion uptake of Eq. [12]. It should also be obvious that while Eq. [12] really represents a net decrease of the surface charge, no charge is created on the surface by the reaction of Eq. [30], i.e., only agppis increased. Using the Esin-Markov coefficient,
(a~o/a I n
[B]) =
+ F(0[Fn+
-
-F(orB/a In [B])
FOH-]/0FB)(0InB/0
In [B]) [31]
39
BORIC ACID ADSORPTION ON METAL OXIDES
,ot
/ mo, m2
0.8
Zr 0 2
0.6 <..--.
~
0.4
e 3 0~,
0.2
4
6
8
10
FIG. 8. Adsorption isotherms of boric acid on magnetite and zirconium dioxide according to the present model (Eq. [23])at 0.001 mole din- 3boron equilibrium concentration and KNO3 0.1 m o l e d m ~-3. Temperature: 30°C.
where all partial derivatives must be understood at constant pH, temperature, and ionic strength. The analogous equation as applied to [30] would simply be (0~0/0 In [A-I) = 0.
[32]
Equation [32] is however hardly likely to hold valid, because of two reasons: (a) at sufficiently low pH values, a sizeable fraction of surface -OH groups are protonated, and as these can also be replaced by A-, a net decrease in the positive charge of the surface is expected; (b) usually data available refer to polyprotic weak acids, in which case the adsorbed ions may bear negative charges and these decrease the overall charge in the zero plane. Furthermore, shifts in the acidity constants upon complex-
ation may give rise to apparent contributions to ~o. It should be noted that Eq. [30] is just the first stage of the dissolution of the oxide by HA, and as such, it should be expected that adsorptivity should correlate with complex ion stability constants (4, 31). Going back to Eq. [31], it is obvious that the extent of adsorption of boron cannot be equated to the decrease in or0, as hydrogen ion co-adsorption neutralizes substantially the effect of boron adsorption. The idea that chemisorption of strongly interacting anions on hydrous oxides involves these species in the zero plane has already been forwarded by Stumm (31-33) for the case of inorganic acids such as HF, HaSiO4, and H3PO4. In these papers the close relaJournal of Colloid and Interface Science, Vol. 99, No. 1, May 1984
40
BLESA, MAROTO, AND REGAZZONI
tionship between the complexation chemistry of the metal ion and the anion with the surface chemistry of the oxide and the anion is also discussed. The present model complements Stumm's ideas with the site-binding model structure for the double layer, thus allowing for the existence of adsorbed ions in the Stern layer which partially neutralize the charge and explain the rather low ~'-potentials found in practice. In this sense, it may be considered as a mixture of both models, which accepts that strong chemisorption takes place in the zero plane, but uses the site-binding model to calculate the whole set of parameters describing the double layer. ACKNOWLEDGMENTS To SUBCYT for partial support. M.A.B. is a member of CONICET. REFERENCES 1. Hingston, F. J., Atkinson, R. J., Posner, A. M., and Quirk, J. P., Nature (London) 215, 1459 (1967). 2. Anderson, M. A., Ferguson, J. F., and Gavis, J., J. Colloid Interface Sci. 54, 391 (1976). 3. Chang, H.-C., Healy, T. W., and Matijevir, E., J. Colloid Interface Sci. 92, 469 (1983). 4. Blesa, M. A., Borghi, E. B., Maroto, A. J. G., and Regazzoni, A. E., J. Colloid Interface Sci., 98, 295 (1984). 5. Keren, R., Gast, R. G., and Bar-Yosef, B., Soil Sci. Soe. Amer. J. 45, 45 (1981). 6. Mezuman, U., and Keren, R., Soil Sci. Soc. Amen. J. 45, 722 (1981). 7. Bingham, F. T., Page, A. L., Coleman, N. T., and Flach, K., SoilSci. Soc. Amer. Proc. 35, 546 (1971). 8. McPhail, M., Page, A. L., and Bingham, F. T., Soil Sci. Soc. Amer. Proc. 36, 510 (1972). 9. Rhoades, J. D., Ingvalson, R. D., and Hatcher, J. T., Soil. Sci. Soc. Amen. Proc. 34, 938 (1970). 10. Choi, W-W., and Chen, K. Y., Environ. Sci. Technol. 13, 189 (1979). 11. Sims, J. R., and Bingham, F. T., Soil Sci. Soc. Amer. Proc. 32, 364 (1968).
Journalof Colloidand InterfaceScience.Vol.99. No. 1, May 1984
12. Hingston, F. J., Aust. J. Soil Res. 2, 83 (1964). 13. Yates, D. E., Levine, S., and Healy, T. W., J. Chem. Soc. Faraday Trans. L 70, 1807 (1978). 14. Davis, J. A., James, R. O., and Leckie, J. O.,J. Colloid Interface Sci. 63, 480 (1978). 15. Davis, J. A., and Leckie, J. O., J. Colloid Interface Sci. 67, 90 (1978). 16. Davis, J. A., and Leckie, J. O., J. Colloid Interface Sci. 74, 32 (1980). 17. James, R. O., and Parks, G. A., in "Surface and Colloid Science" (E. Matijevir, Ed.), Vol. 12, Chap. 2, p. 119. Plenum, New York, 1982. 18. Alwitt, R. S., J. Electrochem. Soc. 118, 1734 (1971). 19. Fletcher, W. D., Krieg, A., and Cohen, P., Report WCAP-1689, Westinghouse Electrical Corp. (1961). 20. Fletcher, W. D., Report WCAP-3730, Westinghouse Electrical Corp. (1964). 21. Regazzoni, A. E., Urrutia, G. A., Blesa, M. A., and Maroto, A. J. G., J. Inorg. Nucl. Chem. 43, 1489 (1981). 22. Maroto, A. J. G., Blesa, M. A., Regazzoni, A. E., and Urrutia, G. A., "Water Chemistry of Nuclear Reactor Systems," Vol. 2, p. 241. Ed. Brit. Nuclear Energy Soc., 1980. 23. Blesa, M. A., Maroto, A. J. G., Passaggio, S. L, Labenski, F., and Saragovi-Badler, C., Radiat. Phys. Chem. 11, 321~(1978). 24. Diggle, J. W., in "Oxides and Oxide Layers" (J. W. Diggle, Ed.), Vol. 2, p. 281. Dekker, New York, 1973. 25. Chazalviel, J.-N., J. Electrochem. Soc. 129, 964 (1982). 26. Regazzoni, A. E., Blesa, M. A., and Maroto, A. J. G., J. Colloid Interface Sci. 91, 560 (1983). 27. Breeuwsma, A., Ph.D. dissertation, Wageningen, Netherlands, 1973. 28. O'Brien, R. W., and White, L. R., J. Chem. Soc. Faraday Trans. H 74, 1607 (1978). 29. Wiersema, P. H., Loeb, A. L., and Overbeek, J. Th. G., J. Colloid Interface Sci. 22, 78 (1966). 30. Alwitt, R. S., J. Colloid Interface Sci. 40, 195 (1972). 31. Sigg, L., and Stumm, W., Colloids Surf. 2, 101 (1981). 32. Hohl, H., Sigg, L., and Stumm, W., Advan. Chem. Ser. 189, 1 (1980). 33. Stumm, W., Kummert, R., and Sigg, L., Croat, Chem. Acta 53, 291 (1980). 34. Ardizzone, S., Formaro, L., and Lyklema, J., J. Electroana[. Chem. 133, 147 (1982).