Cu 2+ INTERACTION WITH MICROCRYSTALLINE GIBBSITE. EVIDENCE ... 32/32-1-12.pdf · Cu 2+ INTERACTION...

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

Cu 2+ I N T E R A C T I O N WITH 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 FOR O R I E N T E D C H E M I S O R B E D COPPER IONS

M. B. McBRIDE l, 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 chemi- sorbed small amounts of monomeric Cu 2+ (<0.5 mmole/100 g) which was oriented with its z-axis perpendicular to the (001) plane of the mineral. The proposed chemisorption sites are at gibbsite crystal "steps" observed by electron microscopy. Although Cu 2+ adsorption on gibbsite as a function of pH was largely reversible, exposure of the chemisorbed Cu E+ to NH3 did not result in desorption from the surface despite the displacement of several OH- or H20 ligands by NH3. The results indicate that at least one Cu-O-A1 bond is formed in the process of chemisorption.

At pH > 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 spectros- copy.

I N T R O D U C T I O N

Recent electron spin resonance (ESR) studies o fCu 2+- alumina systems have revealed that noncrystalline AI(OH)3 and microcrystalline A1OOH chemisorb Cu 2+ at isolated bonding sites, probably by the formation of one or two direct bonds between surface A1-O groups and Cu 2+ (McBride, 1982a). Because the reaction in- volves the release of 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 chem- isorb Cu 2+, except possibly in small quantities at crys- tallite edges. In fact, the study of a low surface-area gibbsite revealed very little interaction with Cu 2+ (McBride, 1982a). It is known, however, that phyllo- silicate surfaces can promote Cu 2+ hydrolysis at a given pH (McBride, 1982b; Farrah and Picketing, 1976), ap- parently by the preferential adsorption of hydrolyzed species of the metal. For this reason, a detailed study of Cu 2§ adsorption on high surface area gibbsite was carried out in an attempt to observe the influence of the planar AI(OH)3 surface on the behavior o f Cu 2+.

MATERIALS A N D METHODS

Gibbsite was prepared by the method of Gastuche and Herbillon (1962) and has surface properties as de- scribed 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 ofgibb- site suspension (26 mg/ml) with 10 ml o f 10 -3 or 10 -4

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 of the reaction was determined by readjusting the pH downward with HC104 and again measuring Cu 2+ activity. Reference titrations were car- tied out in the same way using 10 ml o f H20 in place of the gihbsite suspension. Adsorption isotherms after one day's reaction between 10 ml of gibbsite suspen- sion and 10 ml of 10 -3 or 10 -4 M Cu(NO3)2 were ob- tained by the initial adjustment of Cu(NO3)E-gibbsite mixtures over a range o f p H using NaOH, equilibration by shaking for one day, and measurement of the final pH and Cu 2+ activity using electrodes.

ESR spectra of self-supporting oriented gibbsite films were obtained on a Varian E-104 (x-band) spectrom- eter after immersing the films in l0 ml of C u ( N O 3 ) 2

solution and placing them in a tissue cell. This pro- cedure allowed spectra of the wet, unwashed Cu 2§ treated gibbsite to be obtained with the (001) faces oriented parallel (l[) and perpendicular ( l ) to the mag- netic field, H. Preliminary studies indicated that the Cu(NO3)z concentration over a range of 10 -2 M to 5 • 10 -4 M had little effect on the observed intensity of the Cu 2+ ESR spectrum, nor was there a significant effect of 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 of 10 -3 M Cu(NO3)2, and the pH was adjusted daily over a range from 2.7 to 6.2 using 1 M HC104 or NaOH. Immediately prior to each adjustment, the films were removed for ESR analysis,

Copyright �9 1984, The Clay Minerals Society 12

Vol. 32, No. 1, 1984 Chemisorbed CU 2+ on microcrystalline gibbsite 13

~ 1 o o Z o

80

o 60

o 40.

0 W > 2o o W

..... i [

gibbsile: 5X10 -5 M= 5X 10-4 M �9

4 5 6 pH

Figure 1. Adsorption/precipitation as a function of pH for 5 • 10 -5 M and 5 • 10-4 M Cu(NOa) 2 with and without gibb- site. Reaction time was 1 day with gibbsite, several minutes without gibbsite, for each pH adjustment.

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 gl resonance.

An infrared (IR) spectroscopic study of the surface properties ofa gibbsite film before and after equilibra- tion for one day in 10 -3 M Cu(NO3)2 adjusted to pH 6.0 was conducted on a Perkin-Elmer 580B spectrom- eter. 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 sys- tem, and by a Siemens Elmiskop 102 transmission electron microscope.

RESiJLTS AND 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 de- creased 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 rec ip i t a t ion /d i s so lu t ion react ion was invo lved . Chemisorption is likely to be less reversible than pre-

lOO

~ 80

on (rapid)

6o n- O

40

~ 20

0

A

�9 desorption (rapid) ---equilibration (t day)

1004

80-

v 60- s

LU m ~ 4 0 - 69 0 < 20- 0

0- 4

B

~ i d ) ~ r l ~ / ~o �9 desorption (rapid)

--~e-~_ / / / / " ---equilibration (1 day)

/

o ic ̀~

pH

Figure 2. Adsorption and desorption of Cu 2+ 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 prod- ucts of Cu(OH2) and CuO. The 5 • 10 -5 M Cu2+-gibb - site 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% ad- sorption, the gibbsite in the 5 • 10 5 M Cu(NO3)2 could have adsorbed a maximum 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 sur- face 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 McBride, Fraser, and McHardy Clays and Clay Minerals

5 X 1

i

5 X 1

8 e~

4 5 6 7 pH

Figure 3. Solubility diagram for the Cu2§ system with initial Cu 2+ concentrations of 5 • 10 -4 M and 5 • 10 -5 M. Data are shown for the rapid adjustment of pH upward and downward, and for a one-day equilibration.

the gibbsite. These results, like those obtained with the Cu 2§ electrode, indicate an ability o f the gibbsite sur- face to lower the solubility of copper, possibly by pro- moting 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 pH was raised, the isotropic signal weakened while the rigid-limit spectrum strengthened. The iso- tropic signal arises from free CH(H20)62+ in the aqueous phase of the wet f lms, while the rigid-limit spectrum is attributed to surface-bound Cu 2§ The latter spec- trum is strongly anisotropic, as is clearly shown by the dependence of the g• signal intensity on the orienta- tion of 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 maximum intensity near pH 5, decreased

.L

pH 2.7 II

.L

pH4.4 It

pH 4 . ~ ~

pH 5.4

\

1 lOO gauss i

Figure 4. ESR spectra of wet (unwashed) gibbsite films soaked for a day in Cu(NO3)2 at 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 of the rigid-limit Cu 2+ spec- trum occurs at lower pH than the onset o f copper hy- droxide precipitation.

More detailed investigation of the rigid-limit spec- trum revealed that short equilibration of the gibbsite with Cu 2§ solutions allowed some detail of the hyper- fine splitting to he observed. This effect may he attrib- utable to the slow adsorption reactions which even- tually result in higher loading levels of Cu 2+ on the surface and generate spectral broadening. With short reaction times, the four A• hyperfine lines were evi- dent, although the A, hyperfine lines could not be re- solved at room temperature (Figure 6a). At low tem- perature, the low field g, hyperfine lines (Figure 6b) permitted estimates o f A, and gt~ to be made. The ap- proximate 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 of gll and A~l are in- termediate 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 hy- droxyl or structural oxygen atoms. After exposure to NH 3 vapor, essentially all of the adsorbed Cu 2§ formed

Vol. 32, No. 1, 1984 Chemisorbed CU z+ on microcrystalline gibbsite 15

uJ r z

z 0

LU n"

l a .

0

I.-

z m I-- z

1 g

b 'j

O I

1 3 . 1

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

3 4 5 6 used to increase signal intensity, orientation relative to the E Q U I L I B R I U M pH magnetic field is imperfect. The vertical line denotes the g =

2.0027 field position. 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 complex with NH3 while remain ing rigidly bound at the surface. This process was detected by significant changes in the Cu ESR spectrum, showing large de- creases in gE~ and g• and an increase in A, (Figure 7). The values o f A,t (181 • 10 -4 c m - ' ) and g~t (2,265) for the surface Cu-NH3 complex are somewha t different f rom those o f Cu(NH3)42§ (Mart ini and Burlamacchi , 1979) (g~ = 2.245, A, = 192 X 10 -4 cm-]) , suggesting that Cu 2§ retains at least a single bond to the oxide surface after exposure to N H 3. The or ien ta t ion-depen- dence o f the spec t rum suggests that the symmet ry axis o f the apparent ly axially symmet r i c C u - N H 3 surface complex is not al igned perpendicular to the plane o f the gibbsite films, unl ike the C u - H 2 0 surface complex. In addit ion, the observa t ion that exposure to NH3 did not generate an isotropic spec t rum at t r ibutable to sol- uble C u - a m m o n i a complexes indicates that the Cu 2§ remained surface-bonded. This behavior contrasts with that o f Cu 2+ re ta ined at low p H on low surface-area gibbsite, because exposure to NH3 solubi l ized m u c h o f the Cu 2§ as a C u - a m m o n i a complex (McBride, 1982a). Litt le o f the Cu 2§ in the low surface-area system could have been chemisorbed at gibbsite surface sites; there- fore, the Cu 2+ (possibly in a Cu-hydroxy fo rm nucleated at surfaces) was readily solubi l ized as the a m m o n i a complex.

After equi l ibra t ion ofg ibbs i te films in Cu(NO3)2, the broad resonance near g = 2 .15-2 .20 becomes espe- cially ev iden t upon washing the films in dist i l led water to r e m o v e excess Cu(NO3) 2. After air-drying the films, this broad resonance is clearly anisotropic and orien- t a t ion-dependent (Figure 8). Washing in CaC12 did not appear to displace the surface-bound Cu 2+, because the b road resonance as well as the r igid- l imit spec t rum

remained after this t reatment. Acidification o f the Cu 2+- treated gibbsite films that had been equi l ibra ted at p H 6.15 (see Figure 4), however , regenerated a strong iso- tropic resonance due to free Cu(H20)62+ as well as the or ien ta t ion-dependent , r igid- l imit spect rum o f bound Cu 2+.

The ESR results suggest the existence o f two forms of Cu 2§ or iented on gibbsite planar surfaces. At low pH, m o n o m e r i c Cu E+ is adsorbed on the surface with the symmet ry axis al igned perpendicular t ~ the gibbsite (001) faces. The a m o u n t o f Cu 2+ bound in m o n o m e r i c form(s) must be very small, because adsorp t ion data obta ined at constant p H (4.9) indicate the re tent ion o f only about 0 .2-0 .4 m m o l e Cu2+/100 g. Thus, the max- i m u m rigid- l imit ESR spectrum, obta ined near p H 5, is at tr ibutable to a concentrat ion o f less than 0.5 mmoles CuZ+/100 g.

A different form o f Cu 2+ with a broad featureless resonance is ev iden t above p H 4.5, becoming more p redominan t at higher p H as the r igid- l imit spec t rum diminishes . The mos t reasonable in terpre ta t ion 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 po- sition. The unobscured g~ hyperfine resonances are shown at increased gain.

16 McBride, Fraser, and McHardy Clays and Clay Minerals

/ ~ J " g/2"2

100 gauss i i

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.

cl b

)

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 Cu E+ ion is in a square planar arrangement, with one ligand position occupied by a surface oxygen ion coordinated to a single Al 3§ ion, and the other 3 ligand positions occupied by non-structural H20 and OH- (indicated by darker outline).

effect is that higher pH favors further hydrolysis and polymerization of Cu 2+ at the initial chemisorption sites, thereby decreasing the surface population of monomers 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 of pH and Cu 2+ concentration that gen- erated 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 of the structural OH vibrations at the surface (Russell et al., 1974). After the Cu 2+ treatment, however, the rate of deuter- ation of surface OH groups was greatly reduced, and the deuteration of bulk structural OH was prevented

Figure 9. Transmission electron micrograph ofgibbsite film.

almost completely. These results were shown to be an effect of pH adjustment and not of Cu 2§ addition, be- cause 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 dif- ferently. Although the surface hydroxyls again deuter- ated readily, bulk hydroxyl showed almost no tendency to deuterate. One implication of these results is that deuteration of bulk structural OH in gibbsite occurs by diffusion of protons inward from edges rather than across structural layers from the (001) surfaces. This highly anisotropic diffusion of deuterium in gibbsite can be explained by the rapid migration of H + ions in the plane of the hydrogen-bonded hydroxyl network.

Whereas no CuOH 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 un- stable under mild heating. No NO3- was detected in the water-washed film by IR, indicating that all of the Cu 2§ present in the gibbsite at pH 6 was charge-bal- anced by OH or 0 2- groups. In summary, the IR spectra were unable to reveal any interaction between the solid phase of Cu 2§ formed at pH 6 and the gibbsite surfaces.

Electron microscopy

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 differ- ence between the Cu-treated and untreated films, with

Vol. 32, No. 1, 1984 Chemisorbed CU ~+ on microcrystalline gibbsite 17

the ~ 0 . 5 - u m hexagona l g ibbs i te plates be ing the only o b s e r v e d features in bo th . Because the r e so lu t ion o f the T E M tech ique was o f the o rde r o f 100 ~ , any par t ic les o f Cu oxide or hyd rox ide p resen t m u s t be ve ry small . T E M also revea led t ha t m a n y o f the g ibb- site plates h a v e crystal " s t e p s " at in te rva l s o f a b o u t 200 A, obse rvab le o n the (001) faces (Figure 9). T h u s the g ibbs i te h a d " e d g e " surfaces a long the basa l p lanes wh ich m a y be ac t ive in Cu 2+ chem i s o r p t i on . Such a poss ib le surface is s h o w n in Figure 10, r evea l ing t ha t Cu ~+ could b o n d via one A 1 - O H or A1-OH~ group to fo rm a p l ana r C u - h y d r o x y species a l igned o n a " s t e p " surface. T h e " s t e p " sites m e e t the dua l r e q u i r e m e n t t h a t Cu 2+ b o n d s so t ha t i t is o r i en ted wi th i ts z-axis n o r m a l to the ab p lane o f the g ibbs i te crystal, a n d t ha t O H - or H 2 0 l igands c o o r d i n a t e d to on ly one A1 a t o m be ava i l ab le for Cu 2+ bond ing . A n es t ima te o f the q u a n - t i ty o f these c h e m i s o r p t i o n sites (based u p o n the ob- se rved dens i ty o f s teps on the crystal l i tes a n d the sur- face area o f the gibbsi te) was m a d e a s s u m i n g t ha t b o n d i n g could on ly occur at h a l f the su i tab le A 1 - O H groups exposed o n crystal s teps because o f s ter ic con- s t ra ints . T h e ca lcu la t ion ind ica t ed t ha t a m a x i m u m o f 1.6 m m o l e / 1 0 0 g cou ld be c h e m i s o r b e d , conf i rming t h a t the s tep sites cou ld accoun t for all o f the a d s o r p t i o n o f m o n o m e r i c Cu 2+ a t low pH.

S U M M A R Y

G i b b s i t e ad so rbs smal l a m o u n t s o f m o n o m e r i c Cu 2+ at low p H wh ich are o r i en t ed re la t ive to the (001) surfaces. These m o n o m e r s are c o n v e r t e d to po lymer i c fo rms o f Cu 2+ as the p H is ra ised, a n d a d d i t i o n a l Cu 2+ is nuc lea t ed at surfaces. T he g ibbs i te surfaces p r o m o t e Cu ~+ hydrolys is , lower ing the a p p a r e n t solubi l i ty o f the meta l . Because the ev idence ind ica tes (1) n o i n t e r ac t i on be tween the p l a n a r surfaces a n d a d s o r b e d Cu ~+, (2) an e v e n d i s t r i bu t i on o f Cu 2+ on the g ibbs i te wi th no de-

tec table par t ic les o f sepa ra te -phase Cu h y d r o x i d e or oxide, a n d (3) a s ignif icant effect o f the g ibbs i te on the solubi l i ty o f Cu ~+, the m o s t l ikely loca t ion o f Cu ~§ b o n d i n g is the edges o f the crystal s teps on the (001) faces. C h e m i s o r p t i o n is m o s t l ikely here because A1- O H groups (with singly c o o r d i n a t e d hydroxyls ) a n d A1- OH2 groups are p resen t at these steps. F u r t h e r adso rp - t ion at the s ame pos i t ions would cause the po lymer i - za t ion o f Cu 2§ Thus , the crystal s teps cou ld act as sites o f Cu(OH)z nuc lea t ion , lower ing the a p p a r e n t solubi l - i ty o f the copper .

A C K N O W L E D G M E N T S

T h e research was s u p p o r t e d in pa r t by N a t i o n a l Sci- ence 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 inanc ia l as- s is tance f rom the U n d e r w o o d fund is grateful ly ac- knowledged by M. B. McBr ide . T h e ass i s tance o f Dr. J. D. Russel l in in t e rp re t ing the IR da ta is grateful ly acknowledged .

R E F E R E N C E S

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 cop- per-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 reac- tions of exchangeable Cu 2+ on hectorite: Clays & Clay Min- erals 30, 200-206.

Russell, J. D., Parfitt, R. L., Fraser, A. R., and Farmer, V. C. (1974) Surface structures of gibbsite, goethite, and phos- phated goethite: Nature 248, 220-221.

(Received 15 January 1983; accepted 27 April 1983)

Pe31oMe---CIJoCOrHOCTh rn66CriTa c 6o~IhtUOfi naottta~hro noBepxHocTrl K ajlcop6tma Cu 2+ aCC.lleJloBa.nacb lIpH I1OMOLLIH CU2+-I4OHOCe3IeKTBBHOFO 3aeKTpoAa, 3JIeKTpOHHOFO CrlHHOBOFO pe3oHaHca , HnqbpaKpaCHO,~ cneKTpocKonnr i n 3J1eKTpOHnOro MHKpOCKOna. F n 6 6 c n T xeMncop6r ipoBa~ Marb le KOJIHqeCTBa MOHO- MepHoro HOHa Cu z+ (<0,5 MMOa~,/100 r), ocb z KOTOpOFO 6bI~a HaapaB~eHa BepTriKa~brIO K n.IIOCKOCTI4 (001) MrlHepa~a. l-lpejiJlox<eHm,ie MecTa xeMrmop6mm HaXO~TCa 8a gpncTaJlanqecKrix "cTyneH~x" r1466C14Ta, Ha6aIo/laeMbiX nprt nOMOUm 3JIeKTpOHHOFO MHKpOCKOrla. XOTSI a j l c o p 6 i m a Cu 2+ Ha rI466C14Te gag qbynKima pH ~IB.rI~IeTC~I B 3aaqHTe~bno~ cTene rm o6paTrlMOfi, 3KCnOHrlpOBaHHe xeMncop6npoBaaHoro Cu z+ K NH3 He nprmo~rt~o K ztecop6r~rm c noHepxHOCTn HeCMOTp~I pa aaMe~eHne HeCKOa~,KnX OH- H.~H JII4raHj1OB H20 rpynnaMn NHa. Pe3yJlbTaTbI yKa3blBa~OT Ha TO, qTO II0 KpafiHefi Mepe o~8a caaab Cu-O-AI qbopMnpyeTc~ BO Bpeua npottecca xeMncop6mm.

Flpa pH >5, ra66CnT cnoco6cTByeT rH~po~nsy n nOanMepn3aamt Cu 2+, c ~o6aBoqnofi a~cop6lmefi Ha noHepxHOCTaX. HHqbpaKpacnaa cneKTpocKonna He 'o6napyxnBaaa uHKaKOrO aqbqbe~Ta aAcop6uan na rn;tpoKcn~IOm,ix rpynnax nnOCKOCTH (001), XOTa arm3oTponHaa ~n~qby3n~ npoTo~o~ H cTpyKTypy rn66cnTa no~Tsepn~a~lac~, ncc~e~lonannnMn npn ncrlo~3oBannn ~lehTepna. [E.G.]

Resiimee--Es wurde die F~ihigkeit von Gibbsit mit groBer Oberfliiche Cu(II) zu adsorbieren untersucht, wobei eine Cu(II) Ionen-selektive Elektrode, Elektronenspinresonanz, Infrarotspektroskopie und Elektro- nenmikroskopie verwendent wurde. Der Gibbsit chemisorbierte kleine Mengen von monomerem Cu(II) (<0,5 • 10 3 Mol/100 g), wobei das Ion mit seiner z-Achse senkrecht zur (001) Ebene des Minerals orientiert wurde. Die vorgeschlagenen Chemisorptionsstellen sind beim Gibbsit Ktistall-"Stufen," die

18 McBride, Fraser, and McHardy Clays and Clay Minerals

miltels Elektronenmikroskopie beobachtet wurden. Obwohl die Cu(II)-Adsorption an Gibbsit in Abh~in- gigkeit vom pH-Wert weitgehend reversibel war, ergab die Behandlung des chemisorbierten Cu(II) rail NH3 keine Desorption yon der Oberfl~iche, obwohl einige OH-- oder H20-Liganden dutch NH3 ersetzt wurden. Diese Ergebnisse deuten darauf bin, dab mindestens eine Cu-O-A1-Bindung im Verlauf der Chemisorption gebildet wird.

Bei pH-Werten tiber 5 schien der Gibbsit die Hydrolyse und Polymerisation von Cu(II) zu f'6rdem, bei weiterer Adsorption an die Oberfl~ichen. Infrarotspektroskopische Untersuchungen zeigten keine Aus- wirkung aufdie Adsorption an die Hydroxylgruppen der (001) Oberfl~iche, obwohl die anisotrope Diffusion yon Protonen in der Gibbsitstruktur durch Deuterierungsuntersuchungen best~tigt wurde. [U.W.]

R6sum6--L'abilit6 d'une gibbsite ayant une large aire de surface pour adsorber Cu 2+ a 6t6 6tudi6e en utitisant une 61ectrode s61ectionnant l'ion Cu 2§ le spin ~t r6sonnance d'61ectrons, la spectroscopie infra- rouge, et la microscopie 61ectronique. La gibbsite a ch6misorb6 de petites quantit6s de Cu 2+ monom6rique (< 0,5 mmole/100 g) qui 6tait orient6 avec son axe-z perpendiculaire au plan (001) du min6ral. Les sites de chemisorption propos6s sont ~t des "6tapes" du cristal de gibbsite observ6es par microscopie 61ec- tronique; Quoique l'adsorption de Cu 2* sur la gibbsite en fonction du pH 6tait largement r6versible, l'exposition de Cu 2§ ch6misorb6 au NH3 n'a pas r6sult6 en une d6sorption de la surface malgr6 le d6pla- cement de plusieurs ligands OH- ou H20 par NH 3. Les r6sultats indiquent qu'au moins un lien CU--O- A1 est form6 pendant le proc6d6 de ch6misorption.

A un pH > 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.]