Cu 2+ INTERACTION WITH MICROCRYSTALLINE ... - CiteSeerX

('lay~ and (Tay Minerals. V01. 32, No. 1, 12-18, 1984.

Cu 2+ I N T E R A C T I O N W I T H M I C R O C R Y S T A L L I N E GIBBSITE. E V I D E N C E F O R O R I E N T E D C H E M I S O R B E D C O P P E R IONS M. B. McBRIDEl, A. R. FRASER, AND W. J. MCHARDY The Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen AB9 2QJ, United Kingdom and Department of Agronomy, Cornell University, Ithaca, New York 14853 Abstract--The ability of a high surface-area gibbsite to adsorb Cu 2. was studied using a Cu 2+ion-selective electrode, electron spin resonance, infrared spectroscopy, and electron microscopy. The gibbsite chemisorbed small amounts of monomeric Cu 2+ ( 5, the gibbsite appeared to promote the hydrolysis and polymerization of Cu 2§ with further adsorption at the surfaces. Infrared spectroscopy revealed no effect of the adsorption on the (001) surface hydroxyl groups, although the anisotropic diffusion of protons in the gibbsite structure was verified from deuteration studies. Key Words--Adsorption, Chemisorption, Copper, Electron spin resonance, Gibbsite, Infrared spectroscopy.

INTRODUCTION

M C u ( N O 3 ) 2 and adjusting the pH with N a O H while monitoring Cu 2+ activity with a specific ion electrode. The reversibility o f the reaction was determined by readjusting the pH downward with HC104 and again measuring Cu 2+ activity. Reference titrations were cartied out in the same way using 10 ml o f H 2 0 in place o f the gihbsite suspension. Adsorption isotherms after one day's reaction between 10 ml of gibbsite suspension and 10 ml o f 10 -3 or 10 -4 M Cu(NO3)2 were obtained by the initial adjustment o f Cu(NO3)E-gibbsite mixtures over a range o f p H using N a O H , equilibration by shaking for one day, and measurement o f the final pH and Cu 2+ activity using electrodes. ESR spectra o f self-supporting oriented gibbsite films were obtained on a Varian E-104 (x-band) spectrometer after immersing the films in l0 ml of C u ( N O 3 ) 2 solution and placing them in a tissue cell. This procedure allowed spectra o f the wet, unwashed Cu 2§ treated gibbsite to be obtained with the (001) faces oriented parallel (l[) and perpendicular ( l ) to the magnetic field, H. Preliminary studies indicated that the Cu(NO3)z concentration over a range o f 10 -2 M to 5 • 10 -4 M had little effect on the observed intensity o f the Cu 2+ ESR spectrum, nor was there a significant effect o f equilibration time beyond several hours. The most significant factor influencing the Cu 2+ ESR spectrum was pH. Thus, films weighing approximately 14 mg were immersed in 10 ml o f 10 -3 M C u ( N O 3 ) 2 , and the pH was adjusted daily over a range from 2.7 to 6.2 using 1 M HC104 or N aO H . Immediately prior to each adjustment, the films were r em o v ed for ESR analysis,

Recent electron spin resonance (ESR) studies o f C u 2+alumina systems have revealed that noncrystalline AI(OH)3 and microcrystalline A1OOH chemisorb Cu 2+ at isolated bonding sites, probably by the formation o f one or two direct bonds between surface A1-O groups and Cu 2+ (McBride, 1982a). Because the reaction involves the release o f nearly two protons per Cu E+ ion adsorbed, it is favored at high pH. It is likely that chemisorption occurs only at those surface hydroxyl groups which are coordinated to a single AP + ion. Thus, the ideal gibbsite structure should be unable to chemisorb Cu 2+, except possibly in small quantities at crystallite edges. In fact, the study o f a low surface-area gibbsite revealed very little interaction with Cu 2+ (McBride, 1982a). It is known, however, that phyllosilicate surfaces can promote Cu 2+ hydrolysis at a given pH (McBride, 1982b; Farrah and Picketing, 1976), apparently by the preferential adsorption o f hydrolyzed species of the metal. For this reason, a detailed study o f Cu 2§ adsorption on high surface area gibbsite was carried out in an attempt to observe the influence o f the planar AI(OH)3 surface on the behavior o f Cu 2+. MATERIALS AND METHODS Gibbsite was prepared by the method of Gastuche and Herbillon (1962) and has surface properties as described by Russell et al. (1974). The surface area of the (00 l) faces is estimated to be about 96 m2/g. Rapid Cu titrations were carried out by combining 10 ml ofgibbsite suspension (26 mg/ml) with 10 ml o f 10 -3 or 10 -4 Copyright 9 1984, The Clay Minerals Society

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Vol. 32, No. 1, 1984

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69 0 and then returned to the solution. Semi-quantitative estimates of signal intensity for the rigid-limit Cu signal were based upon peak heights of the g l resonance. An infrared (IR) spectroscopic study of the surface properties o f a gibbsite film before and after equilibration for one day in 10-3 M Cu(NO3)2 adjusted to pH 6.0 was conducted on a Perkin-Elmer 580B spectrometer. The films were evacuated and exposed to D20 in order to shift the surface hydroxyl absorption bands away from the intense bulk hydroxyl absorption region between 3000 and 4000 cm-h The same untreated and Cu-treated films were analyzed by a Cambridge $4 stereoscan scanning electron microscope equipped with a Link Systems energy dispersive X-ray analysis system, and by a Siemens Elmiskop 102 transmission electron microscope. RESiJLTS A N D DISCUSSION

Adsorption data The gibbsite had a significant effect upon the quantity of Cu 2+ in solution at a given pH, as Figure 1 clearly shows. At both high (5 • 10 -4 M) and low (5 X 10 -5 M) levels of Cu(NO3)2, the presence of gibbsite decreased the pH at which the concentration ofCu(H20)62+ began to decrease. A portion of the adsorption and/or precipitation of Cu 2+ in the gibbsite suspension was not rapidly reversible, as demonstrated by the inability of part of the Cu 2+ to be desorbed or dissolved upon adjusting the pH downward (Figure 2). In addition, less Cu z+ was adsorbed (or precipitated) at a given pH for a quick reaction time than for a reaction time of one day. The adsorption and desorption curves were coincident at the high Cu 2+ level (5 • 10-4 M) when the pH was 6 or greater (Figure 2), suggesting that a p r e c i p i t a t i o n / d i s s o l u t i o n r e a c t i o n was i n v o l v e d . Chemisorption is likely to be less reversible than pre-

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Figure 2. Adsorption and desorption of Cu2+ as a function ofpH for 0.26 g gibbsite in (A) 5 • 10-5 M and (B) 5 • 10-4 Cu(NO3)2. Equilibration time for adsorption and desorption is several minutes for each pH adjustment. cipitation; however, most of the Cu 2+ was removed from solution by a rapidly reversible reaction. The solubility data, plotted on a pCu-pH diagram (Figure 3) reveal that the 5 • 10-4 M Cu2+-gibbsite approached an ion activity product (pCu + 2pOH) of 19.6 above pH 6, between the known solubility products of Cu(OH2) and CuO. The 5 • 10-5 M Cu2+-gibbsite system was undersaturated with respect to CuO, reaching an ion activity product of 21.7 after a reaction time of one clay (Figure 3). Therefore, although the reaction in the 5 X 10-4 M system may have involved precipitation, the gibbsite surface clearly lowered the solubility of Cu 2+ in the 5 • 10 -5 M system below that of any likely precipitated phase. Even at 100% adsorption, the gibbsite in the 5 • 10 5 M Cu(NO3)2 could have adsorbed a m a x i m u m of only 0.38 mmole/100 g. The effect of the gibbsite on Cu 1+ solubility, although easily measured, is very limited considering the high surface area of the gibbsite. Compared to boehmite and noncrystalline alumina, materials with similar surface areas (McBride, 1982a), the gibbsite is relatively nonreactive. The pH buffer curve of Cu(NO3)2 was significantly affected by the presence of gibbsite, as revealed by the titration of Cu(NO3)2 in the presence and absence of gibhsite (data not shown). The buffer range of the Cu(NO3)2 was shifted to lower pH by the presence of

14

Clays and Clay Minerals

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the gibbsite. These results, like those obtained with the Cu 2§ electrode, indicate an ability o f the gibbsite surface to lower the solubility o f copper, possibly by promoting Cu 2§ hydrolysis.

Electron spin resonance spectra After equilibration in Cu(NO3)2 solutions for one day, unwashed gibbsite films revealed an isotropic ESR resonance at g = 2.20 (indicated by go in Figure 4) and a rigid-limit ESR spectrum with a readily observed g~ = 2.07 signal and broadened gll resonance (Figure 4). As the p H was raised, the isotropic signal weakened while the rigid-limit spectrum strengthened. The isotropic signal arises from free CH(H20)62+ in the aqueous phase o f the wet f l m s , while the rigid-limit spectrum is attributed to surface-bound Cu 2§ The latter spectrum is strongly anisotropic, as is clearly shown by the dependence o f the g• signal intensity on the orientation o f the gibbsite films in the magnetic field (Figure 4). At pH >4.5, an additional broad resonance in the g = 2.15-2.20 range intensified, weakening again above pH 6 (see Figure 4). The rigid-limit spectrum, after reaching a m a x i m u m intensity near pH 5, decreased

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Figure 4. ESR spectra of wet (unwashed) gibbsite films soaked for a day in Cu(NO3)2at various pH values and oriented with the (001) surfaces perpendicular (_L) and parallel ([[) to the magnetic field.

in intensity at higher pH. This semi-quantitative result is displayed graphically in Figure 5, demonstrating that a reduction in intensity o f the rigid-limit Cu 2+ spectrum occurs at lower pH than the onset o f copper hydroxide precipitation. More detailed investigation o f the rigid-limit spectrum revealed that short equilibration of the gibbsite with Cu 2§ solutions allowed some detail of the hyperfine splitting to he observed. This effect may he attributable to the slow adsorption reactions which eventually result in higher loading levels o f Cu 2+ on the surface and generate spectral broadening. With short reaction times, the four A• hyperfine lines were evident, although the A, hyperfine lines could not be resolved at r o o m temperature (Figure 6a). At low temperature, the low field g, hyperfine lines (Figure 6b) permitted estimates o f A, and gt~ to be made. The approximate ESR parameters for the chemisorbed Cu 2§ are A H= 154 X 10 -4 cm -1, A l = 18 X 10 -4 cm -~, gtt = 2.35, and g• = 2.06. The values o f gll and A~l are intermediate between those of Cu(H20)62§ and Cu(OHL 2(McBride, 1982), suggesting that Cu 2§ is equatorially coordinated to water molecules (probably two) and hydroxyl or structural oxygen atoms. After exposure to N H 3 vapor, essentially all o f the adsorbed Cu 2§ formed

Vol. 32, No. 1, 1984

Chemisorbed CU z+ on microcrystalline gibbsite 1

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Figure 5. Relationship of intensity of the rigid-limit ESR spectrum of adsorbed Cu 2§ on gibbsite to pH. Gibbsite films were equilibrated for l day in 10-3 M Cu(NO3):. a c o m p l e x with NH3 while r e m a i n i n g rigidly b o u n d at the surface. This process was detected by significant changes in the Cu E S R spectrum, showing large decreases in gE~and g• and an increase in A, (Figure 7). T h e values o f A,t (181 • 10 -4 c m - ' ) and g~t (2,265) for the surface C u - N H 3 c o m p l e x are s o m e w h a t different f r o m those o f Cu(NH3)42§ (Martini and Burlamacchi, 1979) (g~ = 2.245, A, = 192 X 10 -4 cm-]), suggesting that Cu 2§ retains at least a single b o n d to the oxide surface after exposure to N H 3. T h e o r i e n t a t i o n - d e p e n dence o f the s p e c t r u m suggests that the s y m m e t r y axis o f the apparently axially s y m m e t r i c C u - N H 3 surface c o m p l e x is not aligned perpendicular to the plane o f the gibbsite films, unlike the C u - H 2 0 surface complex. In addition, the o b s e r v a t i o n that exposure to NH3 did not generate an isotropic s p e c t r u m attributable to soluble C u - a m m o n i a c o m p l e x e s indicates that the Cu 2§ r e m a i n e d surface-bonded. This b e h a v i o r contrasts with that o f Cu 2+ retained at low p H on low surface-area gibbsite, because exposure to NH3 solubilized m u c h o f the Cu 2§ as a C u - a m m o n i a c o m p l e x (McBride, 1982a). Little o f the Cu 2§ in the low surface-area system could h a v e been c h e m i s o r b e d at gibbsite surface sites; therefore, the Cu 2+ (possibly in a C u - h y d r o x y f o r m nucleated at surfaces) was readily solubilized as the a m m o n i a complex. After equilibration o f g i b b s i t e films in Cu(NO3)2, the b r o a d resonance near g = 2 . 1 5 - 2 . 2 0 b e c o m e s especially e v i d e n t u p o n washing the films in distilled water to r e m o v e excess Cu(NO3) 2. A f t e r air-drying the films, this b r o a d resonance is clearly anisotropic and orient a t i o n - d e p e n d e n t (Figure 8). Washing in CaC12 did not appear to displace the surface-bound Cu 2+, because the b r o a d resonance as well as the rigid-limit s p e c t r u m

Figure 6. ESR spectra of wet (unwashed) gibbsite films after soaking for 1 day in 10 3 M Cu(NO3)2 (unadjusted pH) at (a) room temperature, and (b) -160~ Because thick films were used to increase signal intensity, orientation relative to the magnetic field is imperfect. The vertical line denotes the g = 2.0027 field position.

r e m a i n e d after this treatment. Acidification o f the Cu 2+treated gibbsite films that had been equilibrated at p H 6.15 (see Figure 4), however, regenerated a strong isotropic resonance due to free Cu(H20)62+ as well as the o r i e n t a t i o n - d e p e n d e n t , rigid-limit spectrum o f b o u n d Cu 2+. T h e E S R results suggest the existence o f two forms o f Cu 2§ oriented on gibbsite planar surfaces. A t low pH, m o n o m e r i c Cu E+ is adsorbed on the surface with the s y m m e t r y axis aligned perpendicular t ~ the gibbsite (001) faces. T h e a m o u n t o f Cu 2+ b o u n d in m o n o m e r i c form(s) m u s t be v e r y small, because a d s o r p t i o n data o b t a i n e d at constant p H (4.9) indicate the retention o f only about 0.2-0.4 m m o l e Cu2+/100 g. Thus, the m a x i m u m rigid-limit E S R spectrum, o b t a i n e d near p H 5, is attributable to a concentration o f less than 0.5 m m o l e s CuZ+/100 g. A different f o r m o f Cu 2+ with a b r o a d featureless resonance is e v i d e n t a b o v e p H 4.5, b e c o m i n g m o r e p r e d o m i n a n t at higher p H as the rigid-limit s p e c t r u m diminishes. T h e m o s t reasonable interpretation o f this

Figure 7. ESR spectra of wet (unwashed) oriented gibbsite films, after exposure to 10 -3 M Cu(NO3) 2 followed by NH 3 vapor. The films are oriented I and II to the magnetic field, with the vertical line at high field marking the g = 2.00 position. The unobscured g~ hyperfine resonances are shown at increased gain.

16

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McBride, Fraser, and McHardy

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Clays and Clay Minerals

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Figure 8. ESR spectra of gibbsite films after soaking in 5 • 10-3 M Cu(NO3)2, washing in water and air-drying. The films are oriented perpendicular (_1_)and parallel (11)to the magnetic field.

effect is that higher pH favors further hydrolysis and polymerization o f Cu 2+ at the initial chemisorption sites, thereby decreasing the surface population o f m o n o m e r s relative to hydroxy polymers. The gibbsite surface must enhance the reaction, because Cu2+-hy droxy polymers do not form in aqueous solution under the conditions o f pH and Cu 2+ concentration that generated the broad resonance. Once the pH was adjusted above 6.1, the loss in ESR signal intensity (Figure 4) probably arose from surface nucleation of Cu(OH)2. The Cu 2+ solubility and pH-titration data indicate that Cu(OH)2 precipitation began near pH 6.1.

Infrared spectroscopy Infrared spectroscopic investigations of the gibbsite films after equilibration in 10 -3 M Cu(NO3)2 at pH 6 produced no evidence of perturbation o f the structural O H vibrations at the surface (Russell et al., 1974). After the Cu 2+ treatment, however, the rate of deuteration o f surface O H groups was greatly reduced, and the deuteration o f bulk structural O H was prevented

Figure 10. View along the a-axis of a possible site of Cu 2+ chemisorption at the gibbsite crystal "step" on the (001) face. The CuE+ ion is in a square planar arrangement, with one ligand position occupied by a surface oxygen ion coordinated to a single Al3§ ion, and the other 3 ligand positions occupied by non-structural H20 and OH- (indicated by darker outline).

almost completely. These results were shown to be an effect o f pH adjustment and not o f Cu 2§ addition, because the untreated acidic gibbsite films on exposure to D20 were deuterated at the surface and in bulk positions, whereas the same films soaked overnight in a weakly basic solution (pH 7.5) behaved much differently. Although the surface hydroxyls again deuterated readily, bulk hydroxyl showed almost no tendency to deuterate. One implication o f these results is that deuteration o f bulk structural O H in gibbsite occurs by diffusion o f protons inward from edges rather than across structural layers from the (001) surfaces. This highly anisotropic diffusion o f deuterium in gibbsite can be explained by the rapid migration of H + ions in the plane o f the hydrogen-bonded hydroxyl network. Whereas no C u O H groups could be detected in the gibbsite by IR spectroscopy, it was observed that the thermal energy of the IR beam had changed the color of the Cu-treated film from faint blue to yellow. Thus, it is likely that a Cu(OH)2 phase had dehydrated to form CuO, inasmuch as Cu(OH)2 is known to be unstable under mild heating. N o NO3- was detected in the water-washed film by IR, indicating that all o f the Cu 2§ present in the gibbsite at p H 6 was charge-balanced by O H or 0 2- groups. In summary, the IR spectra were unable to reveal any interaction between the solid phase o f Cu 2§ formed at pH 6 and the gibbsite surfaces.

Electron microscopy

Figure 9. Transmission electron micrograph ofgibbsite film.

Electron microprobe analysis of the same Cu2+-treat ed gibbsite films studied by IR revealed that Cu 2§ was evenly distributed on the film surfaces (within the ~ 1#m resolution o f the technique). Transmission electron microscopy (TEM) of film replicas indicated no difference between the Cu-treated and untreated films, with

Vol. 32, No. 1, 1984

Chemisorbed CU ~+ on microcrystalline gibbsite

t h e ~ 0 . 5 - u m h e x a g o n a l g i b b s i t e plates b e i n g t h e only o b s e r v e d features in b o t h . Because t h e r e s o l u t i o n o f t h e T E M t e c h i q u e was o f t h e o r d e r o f 100 ~ , a n y particles o f C u o x i d e or h y d r o x i d e p r e s e n t m u s t b e v e r y small. T E M also r e v e a l e d t h a t m a n y o f t h e g i b b site plates h a v e crystal " s t e p s " at i n t e r v a l s o f a b o u t 200 A, o b s e r v a b l e o n the (001) faces (Figure 9). T h u s t h e g i b b s i t e h a d " e d g e " surfaces a l o n g t h e b a s a l p l a n e s w h i c h m a y b e a c t i v e i n C u 2+ c h e m i s o r p t i o n . Such a p o s s i b l e surface is s h o w n in Figure 10, r e v e a l i n g t h a t C u ~+ c o u l d b o n d via o n e A 1 - O H or A1-OH~ g r o u p to f o r m a p l a n a r C u - h y d r o x y species aligned o n a " s t e p " surface. T h e " s t e p " sites m e e t t h e d u a l r e q u i r e m e n t t h a t C u 2+ b o n d s so t h a t it is o r i e n t e d w i t h its z-axis n o r m a l to the ab p l a n e o f t h e g i b b s i t e crystal, a n d t h a t O H - or H 2 0 ligands c o o r d i n a t e d to o n l y o n e A1 a t o m b e a v a i l a b l e for C u 2+ b o n d i n g . A n e s t i m a t e o f t h e q u a n tity o f these c h e m i s o r p t i o n sites ( b a s e d u p o n t h e o b s e r v e d d e n s i t y o f steps o n t h e crystallites a n d t h e surface area o f t h e gibbsite) was m a d e a s s u m i n g t h a t b o n d i n g c o u l d o n l y o c c u r at h a l f t h e s u i t a b l e A 1 - O H g r o u p s e x p o s e d o n crystal steps b e c a u s e o f steric c o n straints. T h e c a l c u l a t i o n i n d i c a t e d t h a t a m a x i m u m o f 1.6 m m o l e / 1 0 0 g c o u l d b e c h e m i s o r b e d , c o n f i r m i n g t h a t t h e step sites c o u l d a c c o u n t for all o f t h e a d s o r p t i o n o f m o n o m e r i c C u 2+ a t low p H . SUMMARY G i b b s i t e a d s o r b s s m a l l a m o u n t s o f m o n o m e r i c C u 2+ at low p H w h i c h are o r i e n t e d relative to t h e (001) surfaces. T h e s e m o n o m e r s are c o n v e r t e d to p o l y m e r i c f o r m s o f C u 2+ as t h e p H is raised, a n d a d d i t i o n a l C u 2+ is n u c l e a t e d at surfaces. T h e g i b b s i t e surfaces p r o m o t e C u ~+ hydrolysis, l o w e r i n g t h e a p p a r e n t solubility o f the metal. Because t h e e v i d e n c e i n d i c a t e s (1) n o i n t e r a c t i o n b e t w e e n t h e p l a n a r surfaces a n d a d s o r b e d C u ~+, (2) a n e v e n d i s t r i b u t i o n o f C u 2+ o n t h e g i b b s i t e w i t h n o de-

17

t e c t a b l e particles o f s e p a r a t e - p h a s e C u h y d r o x i d e or oxide, a n d (3) a significant effect o f t h e g i b b s i t e o n t h e solubility o f Cu ~+, t h e m o s t likely l o c a t i o n o f C u ~§ b o n d i n g is t h e edges o f t h e crystal steps o n t h e (001) faces. C h e m i s o r p t i o n is m o s t likely here b e c a u s e A1O H g r o u p s (with singly c o o r d i n a t e d h y d r o x y l s ) a n d A1OH2 g r o u p s are p r e s e n t at t h e s e steps. F u r t h e r a d s o r p t i o n at t h e s a m e p o s i t i o n s w o u l d cause t h e p o l y m e r i z a t i o n o f Cu 2§ T h u s , t h e crystal steps c o u l d act as sites o f Cu(OH)z n u c l e a t i o n , l o w e r i n g t h e a p p a r e n t solubility o f t h e copper. ACKNOWLEDGMENTS T h e research was s u p p o r t e d in p a r t b y N a t i o n a l Science F o u n d a t i o n G r a n t E A R - 7 9 2 3 2 9 0 . F i n a n c i a l assistance f r o m t h e U n d e r w o o d f u n d is gratefully ack n o w l e d g e d b y M. B. M c B r i d e . T h e a s s i s t a n c e o f Dr. J. D. Russell in i n t e r p r e t i n g t h e I R d a t a is gratefully acknowledged. REFERENCES Farrah, H. and Picketing, W. F. (1976) The sorption of copper species by clays: II. Illite and montmorillonite: Aust. J. Chem. 29, 1177-1184, Gastuche, M. C. and Herbillon, A. (1962) Etude des gels d'alumine; cristallisation en milieu desionise: Bull. Soc. Chim. Fr. 5, 1404-1412. Martini, G. and Burlamacchi, L. (1979) ESR study of copper-ammonia complexes in solution adsorbed on silica gels. 1. Wide-pore silica gels: J. Phys. Chem. 83, 2505-2511. McBride, M. B. (1982a) Cu2+-adsorption characteristics of aluminum hydroxide and oxyhydroxides: Clays & Clay Minerals 30, 21-28. McBride, M. B. (1982b) Hydrolysis and dehydration reactions of exchangeable Cu 2+ on hectorite: Clays & Clay Minerals 30, 200-206. Russell, J. D., Parfitt, R. L., Fraser, A. R., and Farmer, V. C. (1974) Surface structures of gibbsite, goethite, and phosphated goethite: Nature 248, 220-221.

(Received 15 January 1983; accepted 27 April 1983)

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MepHoro HOHa Cu z+ ( 5, la gibbsite semblait promouvoir l'hydrolyse et la polymerisation de Cu e+, avec d'avantage d'adsorption aux surfaces. La spectroscopie infrarouge n'a r6v616 aucun effet de l'adsorption sur les groupes hydroxyles de surface (001), quoique la diffusion anisotropique de protons dans la structure de la gibbsite a 6t6 verifi6e par des 6tudes de deuteration. [D.J.]