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Clays and Clay Minerals, Vol. 30, No. 3,200-206, 1982.

H Y D R O L Y S I S A N D D E H Y D R A T I O N R E A C T I O N S O FE X C H A N G E A B L E C u 2+ O N H E C T O R I T E 1

M . B . McBRIDEDepartment of Agronomy, Cornell University, Ithaca, New York 14853

Abstract--Electron spin resonance (ESR) analysis of Cu2+-hectorite suspensions provides evidence for thesurface-induced hydrolysis of Cu(H20)62+ at low pH and surface-inhibited hydrolysis (or precipitation) athigh pH. Dehydration of the hectorite by heating to 110~ appears to promote hydrolysis in high pH claysfurther. Heating to even higher temperatures removes ligand water from Cu2+, allowing the metal ion tocoordinate with silicate oxygen atoms. The planar Cu(H20)42+ on predominates in the interlamellar regionsof hectorite that has been air dried or heated to temperatures of 110~ or lower, but more extreme thermaltreatment changes the apparent orientation of the Cu2+-ligandaxes as some or all of the four water ligandsare removed, A loss in ESR signal intensity upon heating Cu2+-hectorite above 110~ is evidence for low-ered symmetry of the dehydrated, surface-coordinated Cu2+ ion.Key Words---Copper, Dehydration, Electron spin resonance, Hectorite, Hydrolysis, Water.

INTRODUCTIONThe ability of clay mineral surfaces to promote the

hydrolysis of adsorbed metal ions has been suggestedby nume rous studies (Turner and Brydo n, 1965; Farrahand Picketing, 1976a, 1976b; Bloom e t a l . , 1977). Theevidence for surface hydrolysis is an observed releaseof protons into solution upon metal adsorption. How-ever, because numerous potential sources of protonsexist in an aqueous clay system (McBride, 1978), amore direct method of observing metal hydrolysiswould be desirable.Because Cu 2+ is a d 9 transi tion metal ion, it dis playsan electron spin resonance (ESR) spectrum that per-mits its behavior on clay surfaces to be investigated i ns i t u (Clementz e t a l . , 1973; McBride e t a l . , 1975). Thesensiti vity of the ESR spectr um of Cu 2+ to changes inligand field symmetry might allow a distinction to bemade betw een hydrol yzed and hydrat ed Cu 2+. Thepresent study uses the E SR technique to detect changesin the n atur e of sur face-ads orbed Cu 2+ as a f unct ion ofpH and dehydration.

MATERIALS AND METHODSA natural hectorite obtained from the Baroid Divi-

sion of NL Industri es (cation-exchange capacity = 77meq/100 g) with stru ctural formula:

M0.64+ Mg~.42Lio.6sAlo.02)(Sis.00)O20(F,OH)4was used. The hectorite was washed once with 0.1 NHC1 to acidify it and to remove carbonate impurities,the n sev eral times wit h 0.1 M CuCt2. Re pea ted wate rwashing of the clay with remova of supernatant by cen-trifuging betwe en washes was the n carried out. The pHand pCu 2+ of the supernat ant was dete rmined after each

1 Contribution No. 1408.Copyright 9 1982, The Clay MineralsSociety

wash using a glass and specific ion electrode. The wash-ing procedure allowed a range of pH values to be at-tained in the clay-water system as the pH increasedafter each wash. Wet hectorite samples were removedand placed in capillary tubes for ESR analysis after cer-tain pH values had been reached. Because the samplesize varied from one analysis to the next, ESR signalintensities could be compared only semiquantitatively.To attain pH values above 5.5, small quantities of 0.1N NaOH had to be added to the system. The pH wasadjusted as high as 8.6, at which point the clay (whichhad bee n essentially white at lower pH) showed a dis-tinct greenish color.

In a mor e quanti tative exper iment to e valuate Cu 2+ESR signal-loss as a functio n of pH, similarly prepa redCu2+-hectorite (pH = 4.0-4.5) was doped with smallamounts of a nona dsorb ing nitroxide spin probe(2,2,6,6-tetr amethyl-4-pipe ridone-l-oxy l) before pHadjustment, and then reacted with small amounts ofNaOH to increase the pH. ESR spectra of these pH-adjusted samples, as well as the centrifuged clays de-scribed above, were obtained on a Varian E-104 (X-band) spectrometer at room temperature after placingthe wet samples in capillary tubes. Relative ESR signalintensity of the isotropic Cuz+ resonance (attributableto Cu(H20)62+) was measured approximately for thecentrifuged Cu2+-hectorite as the amplitu de of the sig-nal. However , the r elative Cu 2+ signal intensit y as afunction of pH in the nitroxide-doped Cu2+-hectoriteswas determined more exactly from the ratio of Cu2+ tonitroxide spin-probe signal intensities. Oriented clayfilms, obtaine d by drying the suspensi ons on a flat sur-face, were also ana lyze d by ESR in the air-dry state andafter various heat treatments in quartz tubes. Rehydra-tion of heated samples was prevented by sealing thetubes with parafilm. The films were aligned perpendic-

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Vol . 30 , No . 3 , 1982 Dehy dra t ion reac t ions o f Cu2+ on hecto ri te 201

u l a r ( 2 ) a n d p a r a l l e l ( II ) t o t h e m a g n e t i c f i e l d , H , o f t h es p e c t r o m e t e r .

T o d e t e r m i n e t h e C u 2 c o n t e n t o f t h e C u 2 + - s a t u r a t e dh e c t o r i t e ( a f t e r s e v e r a l w a s h i n g s w i t h d i s t il l e d w a t e rh a d i n c r e a s e d t h e s u p e r n a t a n t p H t o 4 .6 ) , t h e c l a y w a st i tr a t ed w i th N a O H , a n d t h e e n d p o i n t w a s d e t e r m i n e db y a r a p i d p H i n c r e a s e . T h e r e s u l t s s h o w e d t h a t a t a p Ho f 7 . 7 e s s e n t i a l l y a ll o f t h e C u 2 h a d b e e n h y d r o l y z e da n d t h a t 1 0 3 m e q C u 2 + / 1 0 0 g w a s p r e s e n t i n t h e c l a y .T h i s h ig h v a l u e m a y b e d u e t o t h e r e m o v a l o f i m p u r i t i e ss u c h a s c a l c i t e d u r in g t h e a c i d w a s h o f h e c t o r i t e a s w e l la s t h e t e n d e n c y o f d i v a l e n t io n s t o g i v e h i g h e r e s t i m a t e so f th e C E C t h a n m o n o v a l e n t io n s .

T h e q u a n t i t y o f a d s o r b e d w a t e r o n C u 2 + - h e c t o r it e a ts e v e r a l t e m p e r a t u r e s w a s d e t e r m i n e d b y h e a t in g a i r-d r y c l a y t o 1 1 0 ~ 1 70 ~ a n d 3 5 0 ~ f o r 1 h r a n d r e c o r d i n gt h e w e i g h t l o s s . T h e n u m b e r o f w a t e r m o l e c u l e s p e rC u 2+ i o n w a s d e t e r m i n e d b y a s s u m i n g t h a t a l l a d s o r b e dw a t e r h a d b e e n d r i v e n o f f at 3 50 ~

R E S U L T S A N D D I S C U S S I O NE i g h t s a m p l e s o f t h e r e p e a t e d l y w a s h e d h e c t o r i t e

w e r e t a k e n f o r s t u d y , b a s e d u p o n t h e p H v a l u e s o f t h e i re q u i li b r iu m s u p e r n a t a n t s . T h e s e w e r e n u m b e r e d 1t h r o u g h 8 f o r r e f e r e n c e p u r p o s e s , a n d t h e p r o p e r t i e s o ft h e i r s u p e r n a t a n t s a r e s h o w n i n T a b l e 1 . T h e p C u 2+ +2 p O H v a l u e o f 1 8 .5 6 f o r s a m p l e 8 i n d i c a t e s t h a t t h es o l u b i li t y p r o d u c t o f C u ( O H ) 2 h a d b e e n e x c e e d e d , a c -c o u n t i n g f o r t h e g r e e n i s h c o l o r o f t h i s s a m p l e . H o w -e v e r , a l l o t h e r s y s t e m s w e r e u n d e r s a t u r a t e d w i t h re -s p e c t t o C u ( O H ) z , w h i c h h a s a p K s o = 1 9.3 6 ( B a e s a n dM e s m e r , 1 9 7 6 ) .

T h e E S R s p e c t r a o f t h e w e t h e c t o r i te s a m p l e s (F i g u r e1 ) w e r e e s s e n t i a l l y s y m m e t r i c a l s in g l e r e s o n a n c e s w i t hg = 2 . 1 9 a n d p e a k t o p e a k l in e w i d t h s o f 1 3 0 -1 4 0 g a u s s ,e v i d e n c e o f C u ( H 2 0 ) 6 z+ u n d e r g o i n g r a p i d t u m b l i n g o rJ a h n - T e l l e r d i s t o r t io n ( C l e m e n t z e t a l . , 1 9 7 3 ) . U p w a r da d j u s t m e n t o f t h e p H p r o d u c e d a d e fi n i te d e c r e a s e i ns i g n a l i n t e n s i t y f o r s i m i l a r q u a n t i t i e s o f s a m p l e , a ss h o w n b y t h e s p e c t r a o f F i g u r e 1 . T h e r e l a t i v e p e a kh e i g h t s ( d e t e r m i n e d w i t h a n a c c u r a c y o f a b o u t _ +2 5% )a r e p l o t t e d i n F i g u r e 2 , a n d c o m p a r e d w i t h t h e f r a c ti o no f s o l u b l e c o p p e r ( i n t h e a b s e n c e o f c l a y ) t h a t r e m a i n sa s C u (H 2 0) 62 w i t h c h a n g i n g p H a s c a l c u l a t e d f r o mk n o w n s t a b i l i t y c o n s t a n t s o f C u O H + , C u 2( OH )2 2 a n dC u C O 3 ~ ( B a e s a n d M e s m e r , 1 97 6) . T h e d e c r e a s e i n s i g -n a l i n t e n s i t y i s a l m o s t c e r t a i n l y a s s o c i a t e d w i t h a n h y -d r o l y s i s o r c o m p l e x a t i o n r e a c t i o n o f C u 2 o n t h e c l a ys u r f a c e , r e d u c i n g t h e c o n c e n t r a t i o n o f C u( H 2 0 )6 2 + . T h ec o m p l e x m u s t h a v e e i t h e r n o E S R s i g n al , o r o n e t h a ti s b r o a d e n e d b e y o n d d e t e c ti o n , b e c a u s e n o n e w s ig n a la p p e a r e d a t t h e e x p e n s e o f th e i s o t r o p ic r e s o n a n c e . T h ed o m i n a n t h y d r o l y s i s p r o d u c t o f C u 2+ i n c o n c e n t r a t e ds o l u t i o n i s C u 2 (O H ) 2 z + ( B a e s a n d M e s m e r , 1 9 76 ). A l -t h o u g h t h i s is n o t n e c e s s a r i l y t h e d o m i n a n t s p e c i e sf o r m e d i n t h e i n t e r l a y e r p o s i t io n s o f c l a y s , th e c o n c e n -

t r a t e d i n t e r l a m e l l a r " s o l u t i o n " o f C u z+ m i g h t f a v o r i t sf o r m a t i o n o v e r m o n o m e r s s u c h a s C u O H S p i n p a i r in gi n C u2 (O H )2 2 c o u l d p o s s i b l y c a u s e a l o s s o f p a r a -m a g n e t i sm , a p h e n o m e n o n t h at m a y a l s o r e d u c e E S Rs i g n a l in t e n s i t y o f C u 2+ i n Y z e o l i t e s ( C o n e s a a n d S o f i a ,1 97 8a ). T h e o b s e r v a t i o n t h a t i o n s s u c h a s N i 2+ a n dC o 2+ d o n o t f o r m s i m i l a r d i m e r s u p o n h y d r o l y s i s ( B a e sa n d M e s m e r , 1 97 6) s u g g e s t s t h a t e l e c t r o n p a i r i n g s t a -b i l i z e s t h e C u 2( OH )2 2 s p e c i e s b e c a u s e o f t h e d 9 e l e c -t r o n c o n f i g u r a t i o n o f C u e . I t i s a l s o p o s s i b l e t h a tC u O H w i t h a r e d u c e d l i g a n d s y m m e t r y r e l a ti v e t o t h ea x i a l l y s y m m e t r i c a l C u (H 2 0) 62 + h a s a v e r y b r o a d r e s -o n a n c e n o t d e t e c t a b l e b y E S R . S i m i l a r l y , C uC O 3 ~ i ns o l u t i o n a p p e a r s t o g i v e n o E S R s i gn a l b e c a u s e o f it sl o w s y m m e t r y o r l o w c o n c e n t r a ti o n , w h i l e th e p l a n a rCu(OH)4 2- i o n w h i c h f o r m s i n a q u e o u s s o l u t i o n s a t v e r yh i g h p H p r o d u c e s a n i s o t r o p i c f o u r - li n e r e s o n a n c e .

R e g a r d l e s s o f th e p r o d u c t f o r m e d o n t h e c l a y , s u r f a c eh y d r o l y s i s ( o r c o m p l e x a t i o n ) i s e v i d e n t a t h ig h e r p H a si n d i c a t e d b y l o s s i n s i g n a l i n t e n s i t y ( F i g u r e 2 ) . C a l -c u l a t i o n s b a s e d u p o n t h e f o r m a t i o n c o n s t a n t s o fC u2 (O H )2 2+ a n d C u O H ( B a e s a n d M e s m e r , 1 97 6) i n -d i c a t e t h a t l e s s t h a n 3 % o f s o l u t i o n c o p p e r i n e q u i li b -r i u m w i t h t h e C u 2 + - h e c t o r i t e ( T a b l e 1 ) w a s i n t h eC u O H f o r m a t a p H o f 6 . 5 a n d t h a t C u 2( OH )2 2+ w a si n s i g n i f ic a n t . A t p H 7 , l e s s t h a n 1 0% o f t h e c o p p e r i ns o l u t io n c o u l d h a v e b e e n C u O H a n d n o s i g n if i c an ta m o u n t o f C u2 (O H )2 2 w a s p r e s e n t . H o w e v e r , a t p H7 .7 5 a n d h i g h e r , m o s t o f t h e s o l u b l e c o p p e r i n e q u i li b -r i u m w i t h t h e c l a y s h o u l d h a v e b e e n i n t h e f o r m o fC u C O z ~ T h e r e f o r e , t h e d e c r e a s e i n E S R s i g n al in t e n -s i ty o f th e h e c t o r i t e s u s p e n s i o n s a t l o w p H v a l u e s w h i c hd o n o t a l l o w s i g n i f i c a n t s o l u t i o n h y d r o l y s i s o r c o m -p l e x a t i o n o f c o p p e r i s p r o b a b l y a s s o c i a t e d w i t h s u r f a c e -e n h a n c e d h y d r o l y s i s . I t is p o s s i b l e t h a t n u c l e a t i o n o fC u ( O H ) 2 o n t h e s i l i c a t e s u r f a c e o c c u r s i n m u c h t h es a m e w a y t h a t a lu m i n u m h y d r o x i d e is a d s o r b e d o nc l a y s ( T u r n e r a n d B r y d o n , 1 96 5) w i t h a n a p p a r e n t s o l -u b i l i ty p r o d u c t c o n s i d e r a b l y l o w e r t h a n t h a t o f t h e h y -d r o x i d e p r e c i p i t a t e d i n s o lu t i o n . T h e d a t a o f T a b l e 1i n d i c a t e th a t t h e a p p a r e n t s o l u b i l it y p r o d u c t r e m a i n sb e l o w t h e l e v e l o f th e l e a s t s o l u b l e C u p r e c i p i t a t e s , t e n -o r i t e a n d m a l a c h i t e , f o r t h e f ir s t 7 s a m p l e s .

A t p H v a l u e s a b o v e 7 , t h e e x p e c t e d f r a c t i o n o f so -l u t i o n C u i n t h e C u (H 2 0) 62 + f o r m d e c r e a s e d r a p i d l y a sa r e s u l t o f t h e f o r m a t i o n o f C u O H + a n d C u C O 3 ~ A t p H8 a n d a b o v e m o s t o f th e s o l u b l e C u i s i n th e n e u t r a lC uC O 30 f o r m a s c a l c u l a t e d f r o m k n o w n s t a b i li t y c o n -s t a n t s a n d a s s u m i n g e q u i l i b r iu m o f t h e c l a y s u s p e n s i o nw i t h a t m o s p h e r i c C O z . T h u s , a c o m p a r i s o n o f t h e fr a c -t i o n o f C u a s C u( H2 0) 62 + i n t h e s o l u t i o n p h a s e a n d c l a yp h a s e s u g g e s t s t h a t , a t l o w e r p H s u r f a c e a d s o r p t i o ne n h a n c e s C u 2+ h y d r o l y s i s , w h i l e a t h i g h e r p H t h e r e -v e r s e e f f e c t o c c u r s ( F i g u r e 2 ). S u c h a n o b s e r v a t i o nw o u l d b e e x p e c t e d i f t h e s u r f a c e p r o m o t e d t h e f o r m a -t i o n o f a p o l y m e r w i t h t h e f o r m u l a C u ( O H ) x (2 -x ) w h e r e

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202 McBride Clays and Clay Mineral s

pH4.8 /

1200HGAUSSFigure I. ESR spectra of wet suspensions of Cu2+-hectoriteafter adjustment to pH values of 4.8 (sample 2), 6.5 (sample5), and 8.6 (sample 8). The instrument gain used for sample 8was four times greater than for samples 2 and 5. (The verticalmarker indicates the g = 2.0027 field position.)

x < 2. The pH-pCu solubility line for this surface-nu-cleated species would have a slope of x. Compared toCu(OH)2 with slope 2, this adsorbed species could thenhave a lower solubi lity than Cu(OH)2 at low pH a nd ahigher solubility than Cu(OH)2 at high pH, accou ntingfor the behavior of the ESR signal intensity.

A more quantitative ESR study of this phen omen onusing nitroxide as an internal standard shows the effectof the clay surface on Cu2+ complexation even moreclea rly. While the ESR signal of 0.01 M CuC12 de-creases abruptly above pH 5 as Cu(OH) 2 precipitates,the signal of exchangea ble Cu2+ on he ctorite decreasesmore gradu ally (Figure 3). The resul t again points to theability of the surface to p romot e Cu 2+ hydrolysi s at lowpH a nd inhibit (or delay) hydrolysis at high pH. T helatter effect may be largely a kinetic phenomen on, be-cause a stu dy of Cu2+-hectorites initially adjusted to p Hvalues greater th an 5 showed a slow downward drift inpH for several days until they equilibrated near theCu(OH)2 titration cur ve (Figure 3). It is likely that theadsorbe d Cu 2+ was partially protect ed f rom hy drolys isin interlamellar positions, but was slowly released tohydrolyze and possibly nucleate on surfaces.

After the clay suspensions were air-dried to formorient ed films, the ESR spectra (Figure 4) indica ted thealign ment of Cu(H20)a z+ ions in the inter lamel lar re-gions of the hectorite (Clementz e t a l . , 1973). The pa-rameters obtai ned from these spectra are g = 2.073,

9 rdt O 1.0~ - - ~ - - o - - - Q \ \O ~ o " ~oo Supernatant< . 7 5n.- C I ' ~ , , ~IL l II

Q_~ 0 \ \,,

0Z00r~

i I I0 5 6 7 8 - 9SUSPENSION pHFigure 2. The effect of suspension pH upon the fraction ofadsorbed Cu present as Cu(H20)~2+, as determined from theintensity of the isotropic ESR signal in Cu2+-hectorite. Forcomparison, the fraction of soluble Cu2+ present in the super-natant as Cu(H20)~~+ is calculated from the data of Table 1 andthe known stability constants of Cu-hydroxy and carbonatecomplexes.

gll = 2.347, and a hyper fine splitti ng (A/c) o f 0.0163cm 1. A signal attributab le to tumb ling or dist ortingCu(H20)62 was also observed when the clays were atambient conditions, with an averaged g-value of 2.164(position denoted by arrow in Figure 4). This signal waslargely obscured by the highest field hyperfine com-pon ent of gH when the clay film was ori ented pe rpen-dicular to the magnetic field, but was quite visible forthe parallel orientati on. The in tensity of the gx and gHspectral components was much reduced at high pH(Figure 4), indic ating tha t a po rtion of the Cu(H20)42ions converted to a hydrolyzed form at higher pH. Theupward adjus tment of pH had no significant effect onthe ESR parameters, suggesting that none of the hy-d[olysis products of Cu2+ (e.g. , CuOH +, Cu2(OH)22+)were observable by ESR.

Table 1. Properties of the supernatants in equilibrium withCu2+-hectorite gels.S a m p l e n o . p n p C u p C u + 2 p O H

1 4.13 2.55 22.292 4.78 4.49 22.933 5.28 5.44 22.884 5.83 5.87 22.215 6.46 6.63 21.716 6.98 6.80 20.847 7.74 7.49 20.018 8.58 7.72 18.56

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Vol. 30, No. 3, 1982 Dehydration reactions of Cuz+ on hectorite 203

1 . 0 . . . . . . . .~ ) - - , ~ - s u p e r s a t u ra e d,,~ .8 9 C u 2 ~ h e c t o ri te s h o r t t e r m )

i~ 9 A C u 2 ~ h e c t o r it e l o n g t e rm )n.- ~ 0 I0 -2 M C u C IbJ(2_ .6 . . . . . pC u + 2 p0H =20.210

~ .42I .-%1< .2 9LL

""" 5 9|4 5 6 7 8

pH

F i g u r e 3. T h e e f fe c t o f p H o n t h e f r a c t io n o f G u p re s e n t a sCu(H20)62+ in pure solution (0.01 M CuCI2)and Cu2+-hectoriteas determined by the intensity of the isotropic ESR signal andinternal spin standard. For comparison, a theoreticall ine (dot-ted) is plotted assuming the precipitation of a Cu(OH)2 phasewith solubility given by pCu + 2pOH = 20.21. The effect oftime on the Cu2+-hectorite data is denoted by open trianglesaccompanied by numbers indicating the number of days of re-action time.

Further removal of water from the clay by heatingproduced more changes in the ESR spectra. After heat-ing the clay at ll0 ~ for 1 hr, the g = 2.164 signal dis-appeared, signifying complete con versio n of anyCu(H20)62+ ions to Cu(H20)4 z+ (Figure 5a). In add ition,significant shifts in the spectral para meters occurr ed(g, = 2.37, g = 2.08, A/c = 0.0158 cm-1). The va lueof g, is consid ered the mo st sensitive indica tor of cov-alenc y, an d an increas e in gFtaccompanied by a decreasein A/c is evidence of a decrease in the covalent natureof the Cu2+-ligand bond s (Kive lson and Nei man , t961).However, calculation of a z , a para meter indicating thedegree of Cu -O ~r-bond covalency, produces values of0.87 for both the air-dried and heated clays. Thus, theo--bonds are quite ionic in nature and unaffected byheating. The effect of heating on ESR parameters mayarise from distortion of the square planar symmetryaccompanying partial collapse of interlayers. Previousstudies (McBride and Mortl and, 1974) showed thatCu2+-smectites collapse from 12.4 to 11.7 A upon heat-ing to 100~ indicati ng partial removal of the mono-layer of interlamellar water a nd possible distortion ofthe planar Cu(H20)42+ ion.

More extreme heating at 170~ and 205~ greatly re-duced the apparent intensity of the g, component,whereas the g resonan ce remained distinct (Figure 5b,5c). The loss of g, signal inten sity is probab ly associat edwith the further removal of H20 from Cu(H20)42thereby allowing direct silicate oxygen-Cu2+ contact.

Figure 4. ESR spectra of air-dry Cu2+-hectoritefilms orient-ed perpendicular ( and parallel ( I1 ) to the magnetic fielddirection. Films used were prepared from suspensions of hec-torite samples 2 (pH = 4.8), 7 (pH = 7.7), and 8 (pH = 8.6).

This result is consistent with the observed collapse ofCu2+-smectite basal spacings to 9.7/~ upon heating at150~ or highe r (McBride and Mortla nd, 1974). Simil arheating studies of Cu2 in zeolites have shown maxi-mum ESR signal loss in the 100~176 range, suggest-ing lowered symmetry as the Cu2+ ion coordinates tosurface oxygens (Conesa and Sofia, 1978b). In addi-tion, hydrolysis reactions of Cu(H20)42+ are likely tooccur in the heated clays as the remaining ligand wateris str ongly polarized by Cu 2+. The res ulting surface-co-ordinated and hydrolyzed species with lower symmetrythan square plana r, may hav e a range of g, and A/c val-ues which greatly broa dens the obser ved spectral com-pone nts of g,. Because g is much less sensitiv e tochanges in ligand symmetry or covalency, and the hy-perfine splitting of the g reson ance is small, much lessbroad enin g of the g signal should occur as verified bythe strong gz reso nanc e of Figure 5b and 5c.

After 170~ heating, the weak g, hyperfine lines wereobserved for b o t h orient ations of the clay films relativeto the magnetic field (Figure 5b). This is an indicationof the loss of orien tation of Cu2+ as a square p lan ar com-plex aligned with the clay surface, resulting from thecomplete interlamellar collapse at temperatures above150~ (McBride and Mortland, 1974). After 205~ heat-ing, there is evidence that the parallel orientation of theclay produced the most distinct g, components (Figure

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2 0 4 M c B r i d e Clays and Clay Minerals

b 1 7 o ~

~ o o ~ - ~ - ~ s s : J - ~ . ~

d ~ 500 ~

F i g u r e 5 . E S R s p e c t r a o f C u 2+ - h e ct o ri t e f il m s o r i e n t e d p e r -p e n d i c u l a r ( a n d p a r a l l e l ( }} ) t o t h e m a g n e t i c fi e l d d i r e c t i o na f t e r h e a t t r e a t m e n t f o r 1 h r a t 1 10 ~ 1 70~ 2 0 5 ~ a n d 5 0 0 ~

5 c ), a r e v e r s a l o f t h e r e s u l t s f o r t h e a i r - d r y o r l l 0 ~h e a t e d c l a y . T h i s c h a n g e m a y m e a n t h a t t h e l a s t w a t e rm o l e c u l e s a s s o c i a t e d w i t h C u 2+ w e r e d r i v e n o f f , a n d t h eC u ~ c o o r d i n a t e d i n s u c h a w a y w i t h s u r f a c e o x y g e n st o c h a n g e t h e s y m m e t r y a x i s o r i e n t a ti o n r e l a t iv e to t h ec l a y p la t e l e t s. U p o n h e a t i n g to v e r y h i g h t e m p e r a t u r e s( 50 0 ~ t h e l o s s o f r e s o l u t i o n o f h y p e r f i n e s t r u c t u r e a sw e l l a s t h e l a c k o f o r i e n t a t i o n c o m p a r e d t o t h e l o w e rt e m p e r a t u r e s ( F i g u r e 5 d ) s u g g e s t s t h a t t h e C u 2+ e n t e r e ds i t es o f p o o r l y d e f i n e d s y m m e t r y .

T h e p r o c e s s o f w a t e r r e m o v a l a t l l 0 ~ d o e s n o tg r e a t l y r e d u c e t h e i n te n s i ty o f th e E S R s p e c t r u m o fC u ( H 2 0 ) 4 ~+ i n t h e a c i d i f i e d c l a y s . F o r e x a m p l e , t h es p e c t r u m o f s a m p l e 3 h e a t e d t o 1 10 ~ w a s a l m o s t a si n t e n s e a n d d i s t i n c t a s t h e s p e c t r u m o f t h e a i r - d r ie d

F i g u r e 6 . E S R s p e c t r a o f C u Z + - h ec t or i te f i lm s o r ie n t e d p e r -p e n d i c u l a r ( a n d p a r a l l e l ( II ) t o t h e m a g n e t i c f i e l d d i r e c t i o n .T h e s p e c t r a s h o w n a r e f o r s a m p l e 3 ( p H = 5 .3 ) a i r - d ri e d ( A )a n d h e a t e d t o 11 0~ ( B ) , a n d f o r s a m p l e 7 ( p H = 7 . 7) a i r - d r i e d( C ) a n d h e a t e d to l l 0 ~ ( D ) . I n s t r u m e n t g a i n s w e r e 2 . 5 x 1 0z(A ) , 4 x 102 (B) , 2 .5 x 10~ (C) , 6 .3 x 102 ( D ) .

A J J

I

B

C

F

1 2 0 0 G A U S S f

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Vol . 30 , No . 3 , 1982 De hyd ra t ion reac t ion s of Cu 2+ on hec tor i t e 205s a m p l e ( F i g u r e 6 a ). I n c o n t r a s t , t h e h i g h e r p H c l a y sd e m o n s t r a t e d v e r y i n d i s ti n c t s p e c tr a u p o n d r y i n g a t1 1 0 ~ a s s h o w n b y s a m p l e 7 ( F i g u r e 6 c ). I t i s l i k e l y t h a tt h e d r y i n g p r o c e s s p r o m o t e s h y d r o l y s i s o f C u ~+ t o h y -d r o x y - C u s p e c i e s , e s p e c i a l l y w h e n t h e p H i s i n it ia l lyh i g h . S t u d i e s o f t h e a c i d i t y o f c l a y s u r f a c e s a s a f u n c t i o no f h y d ra t io n h a v e s h o w n t h a t th e r e m o v a l o f w a t e r g e n -e r a t e s p r o t o n s b y m e t a l i on h y d r o l y s i s (M o r t l a n d a n dR a m a n , 1 9 6 8 ) .

I n s u m m a r y , t h e c l a y s u r f a c e a f f e c t s C u ~+ h y d r o l y s i sr e a c t i o n s . W h e r e a s a h e c t o r i t e s u s p e n s i o n e n h a n c e sC u ~ h y d r o l y s i s a t l o w p H b y t h e p r e f e r e n t i a l a d s o r p -t i o n o f h y d r o l y s i s p r o d u c t s , t h e c l a y i n h ib i t s o r a t l e a std e l a y s C u - h y d r o x y o r C u - c a r b o n a t e c o m p l e x f o rm a -t i o n a t h i g h p H .

R e m o v a l o f a d s o r b e d w a t e r b y h e a ti n g t h e a ir - d ryC u ~ + - h e c t o r i t e t o l l 0 ~ e l i m i n a t e s a l l r e m a i n i n gC u (H ~ O )6 ~+ s p e c i e s w h i c h m a y h a v e b e e n p r e s e n t o ne x t e r n a l s u r f a c e s a n d i n t e rn a l " m i c r o p o r e s " c r e a t e d b yt h e i r r e g u l a r s t a c k i n g o f c l a y p l a t e le t s . T h e p l a n a rC u ( H ~ O ) 4 ~+ i o n i s f o r m e d b y t h e r e m o v a l o f a x i a l w a t e ra n d b e c o m e s h i g h l y o r i e n t e d b e t w e e n c l a y p l a t e l e t s .D r y i n g a t 1 10 ~ a l s o a p p e a r s t o p r o m o t e h y d r o l y s i s o fC u ~ +, a t l e a s t i n c l a y s i n i t ia l l y a d j u s t e d t o h i g h e r p H .T e m p e r a t u r e s o f 1 70 ~ a n d h i gh e r r e m o v e s o m e o f t h el i g a n d w a t e r o f C u ( H ~ O ) 4 ~ +, t h e r e b y f o r c i n g C u ~ t oc o o r d i n a t e w i t h s i l i c a t e o x y g e n s a s t h e i n t e r l a m e l l a rs p a c i n g c o l l a p s e s . T h e s e c o n c l u s i o n s f r o m E S R d a t aa r e c o n s i s t e n t w i t h t h e r m o g r a v i m e t r i c d a t a w h i c h i n -d i c a t e t h a t 1 7 - 1 8 , 4 - 5 , a n d 2 - 3 w a t e r m o l e c u l e s a r e a s -s o c i a t e d w i t h e a c h a d s o r b e d C u ~+ i o n i n h e c t o r i t e t h a th a s b e e n a i r - d ri e d , h e a t e d to l l 0 ~ a n d h e a t e d to1 7 0 ~ r e s p e c t i v e l y .

A C K N O W L E D G M E N TR e s e a r c h w a s s u p p o r t e d b y t h e E a r t h S c i e n c e s S e c -

t i o n , N a t i o n a l S c i e n c e F o u n d a t i o n G r a n t E A R7 9 2 3 2 9 0 .

R E F E R E N C E SBa e s , C . F . a n d M e s me r , R . E . 0 9 7 6 ) T h e H y d r o l y s i s o f C a t -i o n s : W i l e y , Ne w Yo r k , 4 8 9 p p .B l o o m, P . R . , M c Br i d e , M . B . , a n d Ch a d b o u r n e , B . ( 1 9 7 7 )Ad s o r p t i o n o f a l u mi n u m b y a s me c t i t e : I . Su r f a c e h y d r o l y s i sd u r i n g Ca ~+ - AP + e x c h a n g e : S o i l S c i . S o c . A m e r . J . 41,1068-1073.C l e me n t z , D . M . , P i n n a v a i a , T . J . , a n d M o r t l a n d , M . M .( 19 7 3) S t e r e o c h e m i s t r y o f h y d r a t e d c o p p e r ( I I ) io n s o n t h ei n t e r la me l l a r s u r f a c e s o f l a y e r s i l ic a t e s . An e l e c t r o n s p i nr e s o n a n c e s t u d y : J . P h y s . C h e m . 77, 196--200.Co n e s a , J . C . a n d So f i a , J . ( 1 9 7 8 a ) E l e c t r o n s p i n r e s o n a n c eo f d i p o l e - c o u p l e d c o p p e r ( I I ) p a i r s i n Y z e o l i t e s: J . P h y s .C h e m . 82, 1575-1578.Co n e s a , J . C . a n d So f i a , J . ( 1 9 7 8 b ) E l e c t r o n s p i n r e s o n a n c eo f u n d e t e c t e d c o p p e r ( I I ) i o n s i n Y z e o l i t e s : J . P h y s . C h e m .82, 1847-1850.Fa r r a h , H . a n d P i c k e t i n g , W . F . ( 19 7 6a ) Th e s o r p t i o n o f c o p -p e r s p e c i e s b y c l a y s . I . Ka o l i n i t e : A u s t . J . C h e m . 29,1167-1176.Fa r r a h , H . a n d P i c k e t i n g , W . F . ( 19 7 6b ) Th e s o r p t i o n o f c o p -p e r s p e c i e s b y c l a y s : I I . I l l i te a n d mo n t mo r i l l o n i t e : A u s t . J .C h e m . 29, 1177-1184.Ki v e l s o n , D . a n d Ne i m a n , R . ( 19 6 1) ES R s t u d i e s o n t h e b o n d -i n g i n c o p p e r c o mp l e x e s : J . C h e m . P h y s . 35, 149-155.McBr ide , M. B. (1978) Copper ( I I ) in te rac t ions wi th kaol in i t e :f a c t o r s c o n t r o l l i n g a d s o r p t i o n . C l a y s & C l a y M i n e r a l s 26,101-106.M c B r i d e , M . B . a n d M o r t l a n d , M . M . ( 1 9 7 4 ) Co p p e r ( I I ) in -t e r a c t i o n s wi t h mo n t mo r i l l o n i t e : Ev i d e n c e f r o m p h y s i c a lme t h o d s : S o i l S c i . S o c . A m e r . P r o c . 38, 408-415.M c B r i d e , M . B . , P i n n a v a i a , T . J . , a n d M o r t l a n d , M . M . ( 19 7 5)E l e c t r o n s p i n r e s o n a n c e s t u d i e s o f c a t i o n o r i e n t a t i o n i n r e -s t r i c ted water l ayers on phyl los i l i ca te ( smect i t e ) sur faces :J . P h y s . C h e m . 79, 2430-2435.M o r t l a n d , M . M . a n d Ra m a n , K . V . 0 9 6 8 ) Su r f a c e a c i d i t y o fs me c t i t e s i n r e l a t io n t o h y d r a t i o n , e x c h a n g e a b l e c a t i o n , a n ds t ruc ture : C l a y s & C l a y M i n e r a l s 16, 393-398.Tu r n e r , R . C . a n d B r y d o n , J . E . ( 1 9 6 5) Fa c t o r s a f f e c t i n g t h es o l u b i li t y o f A l ( OH) a p r e c i p i ta t e d i n t h e p r e s e n c e o f mo n t -mot i l loni t e : S o i l S c i . 100, 176-181.

( R e c e i v e d 7 A p r i l 1 9 8 1; a c c e p t e d 8 J u l y 1 9 8 1)

PeamMe---Hocpe~ICTBOMa~IeKTpOUl~OrOcnn noa oro p eao nan ca (~)CP) cycneu 3nf i CUZ+-reKTOpHT o~ yqe no~oKa3aTe~bCTBO ~ n Cyll~eCTBOaann~ noBepxnOCTHO-nn~IyrTnpOBaHnoro rn~ po ~n aa Cu(H~O)6~+ npnHUaKnX pH n n oaep xnoc Tno -nnru6 nTo pnoro rn~Ipo~naa (n~n oca~K~Ienn~) npn BUCOKnX pH . ~e rn-~paT aun~ re rTo pnT a npn HarpeBe ~Io 110~ npnao ~nT K arTnBaUnn rn~ poJIna a B r~n nax c Bb~COKnMpH , ~Ia_~bnefimnfi o6 orp en ~o BbICOKHX TeMnepaTyp y~la-~neT ~nra F~ BO~IbI n3 CU2 + , U O a B O ~ n o n yueTa Ji~a Koop~InnnpoaaTb C aTouaMn Knc~opo~a cn~nKaTa. H~OCKOCTHb~fi On Cu(H20)42+ npeo6 za~ aeTB Me ~aM e~hu b~x 3onax reKTopnTa, KOTOpbIfi cym nac n Ha Boa~yxe ~n6o o6orpeBan Ca ~Io l l0~I/l/ii, FII,3IIIe~ TeM IlepaTypbI. ]~OYlee npe~Ie~bnafl cBepxT epMnq ecKa~I o6pa6OT Ka 1,13Melq$1eT Ka~K y~ytocflopHeFITaiinio ocei~ 2ii, raH~[a C u 2+ no Mepe TOFO KaK HeKOTO pbIe 3II460 Bce H3 qeTepex JIHrarI~IOB BO,l~bly~Ia-qem,L HoT ep~ nnTencnnnOCTn cnrHa~a 3 C P n pn o 6or pea e C uZ+-reKTopnTa cab~me 11 0~ ~B~ eTC ~~OKaaaTe~bCTBOM 3~In o6 nu ~e nn a cnu ueT pnn ~ern~lpaTnpoaanuoro noaepxnocTHO-KOOp~InmlpOBannoroaorta Cu ~+. [E.C .]

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206 McBride Clays and Clay Minerals

Resfimee---Die Electr onensp innr esonan z-Anal yse ESR) von CuZ+-Hektorit-suspensionenzeigen, dab einedurc h die Oberfliiche gefiSrderte Hydrol yse yo n Cu(H~O)6z+ bei niedrigen pH-Wer ten und eine durch dieOberfliiche verhi nderte Hy droly se (oder Ausf'fillung) bei ho hen p H-Wert en stattfindet. Die Dehydr atationdes Hektorit durch Erhitzen auf 110~ scheint die Hydrol yse in Tonen mit hohem pH wei ter zu f6rdern.Beim Erhitzen auf noch h6h ere Temperatu ren wird das Ligand enwasse r yon Cu z entfern t, wodurch sichdas Metallion mit Silikatsauerstoffatomen koordinieren kann. Das pl anare Cu(H~O)4~+-Ion ist in deninterlamellaren Bereich en yon Hektorit, der Luft getrockn et oder bis maximal 110~ erhitzt wurde, vor-herrsc hend. Eine Erhitzung auf hShere Temperat uren verfindert edoch die Orientierung der Cu2+-Ligand -enachsen, da einige oder alle der vier Wasserliganden entfernt werden. Eine Verminderung der ESR-Signalitensit~it nach dem Erhi tzen yon Cu2+-Hektorit fiber 110~ zeigt die Symet rieverri ngerung derdehydra tisiert en Oberfliichen-koordinierten Cu2+-Ionen. [U.W.]R6sum6----L'analyse par resonance/~ spin d'61ectrons de suspen sions d'he ctori te-Cu2+ fournit l'6vi dencepour l 'hyd roly se induite h la surface de Cu(H20)6 z+ ~ un pH b as, et pour l'hy drol yse inhib6e/ t la surface(ou pour la pr6cipitation) hun pH 61ev6. La d6shydration de l'hecto rite par 6chauffement h 110~ sembled'avan tage promo uvoir l'hy droly se dans des argiles h pH 61ev6. L'6ch auffe ment h des temp6r atures encor eplus 61ev6es enlbve l'eau ligande de Cu 2+, permettan t une coordinat ion de l' ion m6tal avec des ato mesoxyg~nes silicates. L'i on Cu(HzO)4z+ plane domine dans les r6gions interfolaires de l'hectorite sech6el'air ou 6chauff6e/~ 110~ ou b. des temp6ratu res plus basses, mais un trait ement thermal plus extremechange l'orient ation appa rente des axes ligands Cu 2 puisque certai ns ou to us les quatre ligands aqueuxsont retir6s. Une per te d'intensi t6 du signal ESR lorsque l'hec torit e est chauff6e au del~ de i 10~ est unepreuve de la symm6trie amoindrie de l'ion Cu 2 d6shydrat6 co ordonn 6 h la surface. [D.J.]