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Anion-induced metal binding in amorphous aluminium hydroxide
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Colloida and Surfaces, I 1 ( 1 9 8 4 ) 109--117
" Elsevier Science Publishers B . V . , A m s t e r d a m - - Printed in T h e Netherlands
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ANION-INDUCED METAL BINDING IN AMORPHOUS ALUMINIUM HYDROXIDE• , .~
C. O E S S A , M.L. D E CHERCHI, P. MEL|S l # t i t u t o d i C h l m i e a Agror[o, Uniuersit~, d i S o s ~ r t , Via E. d e N k o I a 10, 0 7 1 0 0 8 a m r [ (Italy) . . . . .
O. MICERA* and 1,. b'TRINNA ERRI$ l s t i t u t o d i C h i m i c a Generate ¢ lnorganica, Universitd d i 8aseart, Via Vienna 2, 0 7 1 0 0 S a m r i (Italy)
(Received 28 April 1983; accepted in final form 6 January 1984) ABSTRACT The adsorption o f VO*" and Cu r÷ ions on freshly-prepared aluminium hydroxide gels was studied using analytical and spectroscopic technlq,-es. The cation adsorption Is shown to involve hydrolytic c o m p l e x e s o f metal ions. In addi¢lon, a correlation is found between the adsorption capacity of the gels and the nature o f the anions present in the exchange solution, sulphate enhancing the copper uptake with respect to chloride. These findings are explained in terms o f negative charges arising o n the surfaces upon selective binding of sulphate, which promote the metal adsorption. Desorption experiments reveal that the adsorbed ions are quantitatively removed only by ~trong chelating agents. The role of such i,tteracttons in d e t e r m i n i n g the sou nutrient status is discussed. INTRODUCTION M e t a l b o n d i n g in n a t u r a l l y o c c u r r i n g a m o r p h o u s F e a n d AI o x i d e s is o n e of the main factors controlling the distribution and availability of trace m e t a l s in t h e s.~fl e n v i r o n m e n t . I n f a c t , c a t i o n i c s p e c i e s , a l t h o u g h n o t n e c e s s a r i l y g e n e t i c a l l y a s s o c i a t e d w i t h ~Lmorphous o x i d e s , c o n i n t e r a c t w i t h t h e m u p o n s u r f a c e c o n t a c t . S u c h p r o c e s s e s a p p e a r t o b e p ~ t i c u l a r l y r e l e v a n t in o x i d i c soils, w h e r e c o l l o i d s o f t h e v a r i a b l e - c h a r g e t y p e p~evafl [ l j . The adsorption phenomena occurring on the surfaces of hydrous oxides h a v e b e e n w i d e l y d e s c r i b e d [ 2 - - 7 ] . H o w e v e r , o n l y ~ e c e n t s t u d i e s h a v e att e m p t e d a s p e c t r o s c o p i c i d e n t i f i c a t i o n o f t h e bin0.jrg m e c h a n i s m s f o r h e a v y m e t a l i o n s [ 8 - - 1 0 ] .. I t is n o w a c c e p t e d t h a t , u n l i k e c l a y f r a c t i o n s , w h i c h u s u a l l y b e h a v e as n o n s p e c i f i c c a t i o n i c e x c h a n g e r s , a m o r p h o u s h y d r o u s Oxides a r e r e s p o n s i b l e f o r s p e c i f i c a n d s e l e c t i v e b o n d s w i t h m e t a l ions. A s a con~-.,. q u e n c e , t h e m e c h a n i s m s o f d e s o r p t i o n s h o u l d also b e s i g n i f i c a n t l y d i f f e r e n t from those resulting from nonspecific ion exchange, ~ *To whom correspondence should be addressed.
0166-66221841503.00
© 1 9 8 4 Elsevier S c i e n c e Publishers B.V.
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T h e use o f a VO 2~ p a r a m a g n e t i c p r o b e in a m o r p h o u s a l u m i n i u m h y d r o x . ide has p r o v i d e d i n f o r m a t i o n o n t h e s t r u c t u r ~ a n d m o b i l i t y o f m e t a l c o m plexes f o r m e d u p o n s o r p t l o n [ I 0 l . I n d e e d , rigid-limit species l i n k e d t o surface o x y g e n atoms have b e e n revealed*by m e a n s o f e l e c t r o n s p i n r e s o n a n c e ( E S R ) [10]. E S R s t udi e s o f t h e Cu 2÷ i on in a l u m i n i n m h y d r o x i d e gels s h o w . e d results similar t o t h o s e o b t a i n e d f o r VO 2+ [8, 9 | . T o o b t a i n a b e t t e r u n d e r s t a n d i n g o f t h e m e c h a n i s m s involved in m e t a l - o x i d e i n t e r a c t i o n s , w e investigated t h e VO 2+ a n d Cu T* a d s o r p t i o n b y alum i n i u m h y d r o x i d e over an e x t e n d e d c o n c e n t r a t i o n range a n d in t h e p r e s e n c e o f d i f f e r e n t anions. E v i d e n c e is p r e s e n t e d t o s h o w t h a t t h e m e t a l s o r p t i o n requires c a t i o n h y d r o l y s i s , is a n i o n - d e p e n d e n t , a n d yields species w h i c h a r e readi l y d e s o r b e d o n l y b y l t g a n d - d i s p l a c e m e n t reactions. EXPERIMENTAl'-
PART
Materials A l u m i n h l m h y d r o x i d e was p r e p a r e d b y a d d i n g a m m o n i a b y d r o p s t o a q u e o u s s o l u t i o n s o f KAI(SO4)~ • 12H=O u n til p h i - 9 was r e a c h e d . T h e precio p i t a t e was c e n t r i f u g e - w a s h e d . N o significant d i f f e r e n c e was o b s e r v e d bet w e e n t h e a m o u n t o f s u l p h a t e initially a d d e d a n d t h a t in t h e s u p e r n a t a n t s o l u t i o n . W h e n d r i e d a t 110°C, t h e gel w e i g h e d - - 6 2 m g m m o l -I AI.
Adsorption measurements A l i q u o t s o f fresh gel c o n t a i n i n g 2.6 m m o l AI w e r e c o n t a c t e d w i t h VOSO4 a q u e o u s sol ut i on s o b t a i n e d b y dissolving w e i g h e d a m o u n t s o f t h e m e t a l sal~. Preli mi na ry e x p e r i m e n t s s h o w e d t h a t e q u i l i b r i u m was a t t a i n e d in <~24 h. NaCI w a s a d d e d to achieve a t o t a l ionic s t r e n g t h o f 0.1 M. T h e pH was adj u s t e d b y a d d i n g N a O H o r HCI a n d m e a s u r e d w i t h a n O r i o n m o d e l 901 pH m e t e r . T h e final v o l u m e was ca. 75 c m 3 . A f t e r s h a k i n g in p o l y p r o p y l e n e c e n t r i f u g e t u b e s at 2 0 . 0 ± 0.1°C, t h e samples w e r e c e n t r i f u g e d , a n d v a n a d i u m was d e t e r m i n e d in s o l u t i o n using a S p e c t m s p a n I V e m i s s i o n s p e c t r o m e t e r . Residual s u l p h a t e in s o l u t i o n was d e t e r m i n e d w i t h a Wesean m o d e l 2 6 0 io n a n a l y s e r using a Wescan 269-001 a n i o n c o l u m n a n d a 4 m ~ l p o t a s s i u m hyd r o g e n p h t h a ] a t e s o l u t i o n (pH 4.5) as e l u e n t . Similar e x p e r i m e n t a l t e c h n i q u e s w e r e e m p l o y e d t o m e a s u r e t h e Cu ~* ads o r p t i o n f r o m CuCI 2 o r CuSO4 s o l u t i o n s , w i t h t h e e x c e p t i o n t h a t t h e c h l o r i d e i on was n o t m o n i t o r e d . . f
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Desorptfon experiments Equa l aliquots o f At h y d r o x i d e , p r e v i o u s l y l o a d e d w i t h VO 2~ a n d centrifuge-washed, w e r e i m m e d i a t e l y s u s p e n d e d in CuCI= o r Na=EDTA solutions. A f t e r s e p a r a t i o n o f t h e solid phase, m e t a l ions in s o l u t i o n w e r e d e t e r m i n e d as d e s c r i b e d above.
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Spectroscopic analysis ESR spectra w e r e r e c o r d e d at r o o m t e m p e r a t u r e o n a Varian E-9 spectrom e t e r o p e r a t i n g at ca. 9.15 GHz. Equal a m o u n t s o f p o w d e r e d air,dried samples w e r e used f o r t h e q u a n t i t a t i v e analysis. C o n s t a n t values o f m o d u l a t i o n a m p l i t u d e a n d p o w e r level w e r e e m p l o y e d such t h a t n e i t h e r bro~de~ning n o r s a t u r a t i o n o f t h e signals was observed. O t h e r i n s t r u m e n t a l s e t t i n g s w e r e m a i n t a i n e d at p r e s e t values, w i t h t h e e x c e p t i o n o f t h e receiver gain (RG), w h i c h was varied t o give c o m p a r a b l e signals. T h e relative intensities were m e a s u r e d b y integrating t h e area o f t h e signals. I n f r a r e d (IR) spectra w e r e o b t a i n e d o n p o w d e r e d s a m p l e s spread over N a C l plates using a P e r k i n - - E l m e r 6 8 3 s p e c t r o p h o t o m e t e r . RESULTS T h e a d s o r p t i o n isotherms o f VO=* f r o m s u l p h a t e solutions at pH 3.0 and 3 . 5 are s h o w n in Fig. 1. A strong increase in a d s o r p t i o n was observed as t h e pH was raised, indicating t h e p H - d e p e n d e n c y o f t h e reaction. • 30
=2o i I 0
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10 15 20 ConceNtration-103,M
Fig. 1. Sorption of VO:* (tool V/tool AI) on amorphous alumi.lum hydroxide gels at pH 3.0 and 3.5.
ESR m e a s u r e m e n t s p e r f o r m e d o n t h e solid phase after t h e e x c h a n g e s h o w e d VO 2. s p e c i e s rigidly b o u n d t o f u n c t i o n a l groups o f t h e a m o r p h o u s h y d r o x i d e even in fully h y d r a t e d gels o b t a i n e d o n s u s p e n d i n g t h e samples in water. With t h e e x c e p t i o n o f t h e signal a m p l i t u d e , t h e parameters did n o t c h a n g e w i t h d i f f e r e n t a d s o r p t i o n levels (Fig. 2), ruling o u t variations o f t h e ~v a n a d i u m e n v i r o n m e n t . Incleed., t h e signal i n t e n s i t y w a s d i r e c t l y prop o r t i o n a l to t h e a m o u n t o f s o r b e d vanadium, indicating t h a t h o m o g e n e o u s
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Fig. 2. Comparative E S R spectra obtained on equal amounts o f VO~*-exehanged aluminium hydroxide gels with differing loading tevels: VIA1 (molar ratio): 6.04.10"3(a). 2 . 9 9 . 1 0 -~ (b) and 9.52- 10 "~(c). With the exception o f the receiver gain (RG). the instrumental settings were kept constant for all the samples.
filling o f t h e surfaces and c o m p l e x a t i o n o f t h e iron c o n t r o l t h e m e ( ~ l u p t a k e . The simultaneous adsorption o f sulphate o n t o t h e gels was measured by c h r o m a t o g r a p h z c analysis, as s h o w n in Fig. 3. S u l p h a t e b o u n d t o t h e s o l i d p h a s e is also revealed b y IR ( F i g . 4 ) t h r o u g h S - - O s t r e c h barLds similar t o those observed on goethite [11]. T h e a d s o r p t i o n o f c o p p e r ( I I ) b y a l u m i n i u m h y d r o x i d e w a s similarly inv e s t i g a t e d . H e r e t o o t h e r e a c t i o n p r o d u c e s i m m o b i l i s a t i o n o f Cu 2. o n t h e Fig. 4. IR spectra o f aluminium hydroxide (a) and alumtnlum hydroxide with differing VIAl molar ratios: 9.52- 10 "=(b), 2.62, 10 - i (e and d). The spectrum d is obtained after dehydration at I|CPC. 8: ~ - O stretching hands;M: V--O stretching band.
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Fig. 3. Uptake o f sulphate (tool sulphata/mol AI) from VOSO+ solutions (I = 0.1, HaCI) by amorphous alumlnlum hydroxide at pH 3.6.
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114 surfaces, as s h o w n b y rigid-limit E S I t s p e c t r a q u i t e similar to t h o s e r e p o r t e d b y McBride [ 8 } . T h e e x t e n t o f t h e a d s o r p t i o n is p H - d e p e n d e n t , being negligible at p H < 3 a n d increasing w i t h increasing p H values (Fig. 5). Again, t h e copper(II) uptake from sulphate-containing solutions, with sodium chloride as s u p p o r t i n g e l e c t r o l y t e , was a c c o m p a n i e d b y c h e m i s o r p t i o n o f s u l p h a t e , as s u b s t a n t i a t e d b y analytical a n d IR m e a s u r e m e n t s . T h e CU ~* a d s o r p t i o n f r o m s o l u t i o n s c o n t a i n i n g o n l y c h l o r i d e as a n i o n w a s m u c h l o w e r t h a n w h e n s u l p h a t e was also p r e s e n t (Fig. 5), p r o v i n g t h a t t h e c a t i o n a d s o r p t i o n o n t o a l u m i n i u m h y d r o x i d e is a n i o n - d e p e n d e n t . E n h a n c e m e n t o f m e t a l a d s o r p t i o n b y s u l p h a t e is also ~.ound in t h e case o f o x o v a n a d i u m ( I V ) b y varying t h e [ S O l - ] t o t : [VO=*Jto: ratio. F o r i n s t a n c e , at p H 3.0 an a d s o r p t i o n level ~ 2 . 5 * 10 -2 VIA1 is o b t a i n e d a t [ S O 4 2 - ] t o t : [ V O l t ] t o t = 1 , while a s t e p w i s e increase u p to "-3.0 • 10 -2 V/AI is o b s e r v e d at [SO42-]tot : | V O 2 + ] t o t = 2.5.
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Equ ilibrium Concentration . 1 0 3, M Fig, 6. :~orptton o f Cu :+ (reel Cufmol AI ! on amorphous a|uminium gels from CuSO4 (a) and CuCI 2 (b) solutions ( / - 0 . l , NaCI) at p H 4.3.
T o illustrate t h e c o m p e t i t i o n b e t w e e n t h e m e t a l ions, d e s o r p t i o n experi4 m e n t s w e r e carried o u t b y l oa di ng AI gels w i t h VO 2. ( s u l p h a t e ) a t varying levels ( p H - - 3 . 5 ) a n d testing t o replace v a n a d i u m w i t h Cu2* ( c h l o r i d e ) a t p H ~- 4.3. In all cases n o significant r e m o v a l o f v a n a d i u m o c c u r r e d (~ee, e.g. t h e results o f T a b l e 1), w h e r e a s t h e c o p p e r , p t a k e w a s c o m p a r a b l e t o t h a t s h o w n in Fig. 5. Again, ES R spectra o f t h e e x c h a n g e d gels e x c l u d e d clustering o f t h e o x o v a n a d i u m ( I V ) ions o r f o r m a t i o n o f h y d r o x i d e species. O n t h e o t h e r h a n d , a l m o s t c o m p l e t e r e m o v a l o f VO 2+ was achieved u p o n i n t e r a c t i o n o f t he h y d r o x i d e gels w i t h s t r o n g c h e l a t o r s (e.g., 0 . 0 1 M N a2 E DT A solutions, final p H ~ 6--7).
116 TABLE 1 -
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Analytical data concerning representative desozption experiments performed on vo z~a l u m [ n i u r n h y d r o x i d e g e l s ( V I A l m o l a r r a t i o : 5 . 7 6 . 1 0 "* ) w i t h c o p p e r ( H ) c h l o r i d e s o l u ' tionb at pH = 4.3 a [C~ Jeq.~f
C'u/AI molar ratio after exchange
2 . 7 * 1 0 "* 5 . 8 * 1 0 -~ 1 . 1 6 - 1 0 -s
4 . 1 3 - 1 0 "~ 5 . 2 3 - 1 0 -~ 1 . 1 5 , 1 0 -2
aResults obtained by triplicate experiments. DISCUSSION AI~D CONCLUSIONS
Metal ions• a r e s t r o n g l y a d s o r b e d o n t o A l ( O H ) 3 s t r u c t u r e even o n t h e acidic side o f t h e p o i n t o f z e r o c h a r g e ( p H i 9 . 4 , e.ccording t o Ref. [6] ), w h e n positive c ha r ge s p r e d o m i n a t e o n t h e surfaces. T h e a n a l y t i c a l d a t a are c o n s i s t e n t ~vith c h e m i s o r p t i o n m e c h a n i s m s a n d h o m o g e n e o u s filling o f t h e surfaces, r a t h e r t h a n p r e c i p i t a t i o n , w h i c h s h o u l d p r e c l u d e t h e a t t a i n m e n t o f a limit value for t h e a d s o r p t i o n level. O n t h e o t h e r b a n d , t h e E S R measurem e n t s i n d i c a t e t h a t n e i t h e r clustering n o r close i n t e r a c t i o n o f t h e a d s o r b e d ions is s i g n i f i c a n t . O u r results p o i n t o u t t h a t t h e r o l e o f t h e a n i o n s m u s t be c a r e f u l l y t a k e n i n t o a c c o u n t in evaluating t h e m e c h a n i s m s o f c a t i o n r e t e n t i o n o n h y d r o u s o x ide s. T h e r e is a c o n s i d e r a b l e b o d y o f e v i d e n c e t o s h o w t h a t a n i o n s app r o a c h i n g p o s i t i v e l y . c h a r g e d sites o n h y d r o u s o x i d e s are n o n s p e c l f i c a l l y (or electrostatica~.ly) a d s o r b e d as c o u n t e r i o n s in t h e o u t e r H e h n h o l t z p l a n e [ 1 2 - - 1 4 ] . O t h e r anions, s u c h as SO4a-, are in s tead selectively a d s o r b e d i n t o t h e i n n e r H e l m h o l t z l a ye r t h r o u g h ligand e x c h a n g e . T h e m e c h a n i s m s suggested for s u l p h a t e a d s o r p t i o n [ 1 5 ] involve t h e d i s p l a c e m e n t o f w a t e r f r o m p r o t o n a t e d e d g e sites a n d h y d r o x y l s f r o m n e u t r a l sites a c c o r d i n g to t h e scheme: -~AI--OH~ + SO~- ~
AI--(SO4)- + H20
--AI--OH
AI--(SO4)7
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+ OH-
I t has b e e n suggested t h a t n e u t r a l i z a t i o n o f t h e negative charges arising o n t h e surfa c e u p o n s u l p h a t e c h e m i s o r p t i o n m a y involve t h e release o f h y d r o x yls [ 1 5 l . In t h e p r e s e n c e o f suitable m e t a l ions, it is c o n c e i v a b l e t h a t t h e negative charge s c o u l d also be n e u t r a l i s e d via c a t i o n i c a d s o r p t i o n . T h u s , u n l i k e CI-, w h i c h d o e s n o t m o d i f y t h e s u r f a c e charge, s u l p h a t e , being a d s o r b e d b y t h e h y d r o x i d e t h r o u g h ligand e x c h a n g e , p r o d u c e s negative charges t h a t e n h a n c e t h e c a t i o n i c a d s o r p t i o n c a p a c i t y o f t h e gels.
116 It is also e v i d e n t t h a t h y d r o l y s i s o f t h e c a t i o n s is n e e d e d t o b r i n g a b o u t t h e s o r p t i o n process, in a g r e e m e n t w i t h previous suggestions [ 3 , 7 ] . In fact, t h e c a t i o n u p t a k e is n o t significant w h e n t h e c o n c e n t r a t i o n o f h y d r o l y s e d species is negligible in t h e e x c h a n g e s o l u t i o n a n d r a p i d l y i n c r e a s e s as p H is raised. More ove r , at a given p H t h e a f f i n i t y o r d e r o f t h e h y d r o x i d e t o w a r d s Cu a" a nd VO=* closely f ol l ow s t h e h y d r o l y s i s t e n d e n c y o f t h e ions, Using d a t a f r o m t h e l i t e r a t u r e [ 1 6 ] it is c a l c u l a t e d t h a t a t p H 3.0 a b o u t 0.1 2 % o f VO 2. is p r e s e n t as [ V O ( O H ) ] *, w h e r e a s t h e a m o u n t o f t h e din!iclear species [(VO)2 (OH)2 ! 2+ increases w i t h increasing v a n a d i u m concent~'at i o n (as high as ~-0.44% a t i V | t o t ffi 1 . 8 - 1 0 - = M ) . A t p H 3.5 [ V O ( O H ) ] * increases t o ~ 0 . 3 8 % a n d [ ( V O ) 2 ( O H ) 2 ] 2+ varies u p t o 4 . 0 5 % in t h e a b o v e c~nc e n t r a t i o n range. C o m p a r e d t o VO 2. a t p H 3.0 a n d 3.5, c o p p e r ( I i ) , d u e to th e h i g h e r p K * va l ue ( ' - 8 ) [ 1 6 ] , y i e l d s a l o w e r c o n c e n t r a t i o n o f h y d r o l y s e d species even a t p H 4.3 ( [ C I I ( O H ) | ÷ = ~ 0 . 0 2 % ) a n d , a c c o r d i n g l y , is less ads o r b e d o n t o t h e h y d r o x i d e . In t h e a b s e n c e o f h y d r o l y s i s r e q u i r e m e n t s , a n o p p o s i t e t r e n d s h o u l d have b e e n o b s e r v e d , as t h e s u r f a c e p o t e n t i a l is m o r e positive at p H 3.5 t h a n at p H 4.3. A c c o r d i n g l y , p r e l i m i n a r y results s h o w t h a t less h y d r o l y s a b i e ions such as Zn 2÷ are a d s o r b e d a t p H values s u b s t a n t i a l l y h i g h e r t h a n for C~t=+ a nd VO2% M u t u a l e ffe c t s b e t w e e n t h e s o r h e d tons a n d t h e h o s t s t r u c t u r e will o c c u r in t h e e x c h a n g e d gels. T h e p r e s e n c e o f ions t i g h t l y b o u n d o n t h e surfaces h i n d e r s ~h6 p o l y m e r i s a t i o n processes leading to et~ystallisation o f t h e h y d r o x ide [ 8 ] . O n t h e o t h e r h a n d , a c c o r d i n g to previous o b s e r v a t i o n s [ 6 , 8 , 1 7 ] , it a p p e a r s t h a t t h e s o r b e d ions a r e in a n o n e x c h a n g e a b l e f o r m an~! are c o m p l e t e l y r e m o v e d •only b y s t r o n g c h e l a t o r s t h a t c a n give c o m p l e x e s m o r e stable t h a n t h o s e o n tile surfaces. All these o b s e r v a l i o n s m a y p r o v i d e an e x p l a n a t i o n o f c e r t a i n processes o c c u r r i n g in soils. M l c r o e l e m e n t s in c a t i o n i c f o r m lose t h e m o b i l i t y t y p i c a l o f h y d r a t e d ions u p o n i n t e r a c t i o n w i t h a m o r h p h o u s h y d r o x i d e s clue to m e c h a n i s m s also involving t h e p a r t i c i p a t i o n o f suitab,*e anions. T h e e f f ~ t o f t h e a d s o r p t i o n is t h a t metals a r e in a n o n e x c h a n g e a b l e f o r m unless chelat i o n b y s t r o n g ligands oc c ur s . A l t h o u g h t h e relative i m p o r t a n c e o f th e i n t e r a c t i o n s o f t r a c e m e t a l ions w i t h o r g a e i c a n d inorganic soft c o m p o n e n t s is n o t well es tab lish ed [ 1 8 ] , t h e m o s t likely h y p o t h e s i s is t h a t b o t h c o n t r i b u t e to t h e k e e p i n g o f l o w b u t c o n s t a n t levels o t h y d r a t e d ions t h a t o n l y satisfy n u t r i e n t r e q u i r e m e n t s o f plants [ 1 9 ] . ACKNOWLEDGEMENTS T h a n k s are d u e to Consiglic N a z i o n a l e delle R i c e r c h e (C.N.R., Italy) fo r financial s u p p o r t a n d t o Mr A. Dess| f or t e c h n i c a l assistance.
117 REFERENCES 1 W.A. Stoop, Geoderma, 26 (1981) 107. 2 d.W. Bowden, M.D.A. Bolland, A.M. P o m e r and J.P. Quirk, Nature ( L o n d o n ) Phys. Scl., 245 (1973) 81. 3 R.O. James and T.W. Hcaly, J. Colloid interface Sol., 40 (1972) 42, 53, 65. 4 C.P. Huang and W. S t u m m , J. Colloid Interface Sci., 43 (1973) 409. 5 D.(]. Kinnihurg, J.K. Syers and M.L. Jackson, Soft Sci. Soc. Am. Proc., 30 (1975) 464. 6 D.G. Kinnlburg, M.L. Jac]cson and J.K. Syers, Soil Scl. Soc. Am. J., 40 (1976) 796. 7 J.A. Davis and J.O. Leckie, J. Colloid Interface Sci., 67 (1976) 90. 8 M.B. McBride, Soil Sol. So¢. Am. J., 42 (1978) 27. 9 M.B. McBride, Clays Clay Miner., 30 (1982) 21. 10 C. Oessa, S. Deiana, L. Erre, G. Mieera and P. Piu, Colloids Surfaces, 2 (1981) 293. 11 R.L. Parfitt and R. St. C. Smart, Soll ScI. Soc. Am. J., 40- (1978) 48. 12 F.J. Hingston, R.J., Atkinson, A.M., Posner and J.P. Quirk, Nature ( L o n d o n ) Phys. Sol., 215 (1967) 1459. 13 F.J. Uingston, A.M. Posner and J.P. quLrk, J. Soil 8c|., 23 (1972) 17"/. 14 F.J. |[InEston, A.M. Posner and J,P. q u i r k , J. Soll 3el., 25 (1974) 16. 15 S.S.S. Rajan, Soil Sci. Soc. Am, J., 42 (1978) 39. 16 C.F. Ba~s and J.J. Mesmer, The Hydrolysis o f Catlons, Wiley, New York, NY, 1970, pp. 200, 269. 17 E.A. Forbes, A.M. Posner and J.P. Quirk, J. Soil S©i., 27 (1976) 154. 18 L~M. Shuman, SoIl Sci., 127 (1979) 10. 19 B.A. Goodman and D.J. Linehan, In J.L. Harley and 11. Scott Russell (Hds), The Soil--Root Interface, Academic, New York, NY, 1979, pp. 67--82.
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ANION-INDUCED METAL BINDING IN AMORPHOUS ALUMINIUM HYDROXIDE• , .~
C. O E S S A , M.L. D E CHERCHI, P. MEL|S l # t i t u t o d i C h l m i e a Agror[o, Uniuersit~, d i S o s ~ r t , Via E. d e N k o I a 10, 0 7 1 0 0 8 a m r [ (Italy) . . . . .
O. MICERA* and 1,. b'TRINNA ERRI$ l s t i t u t o d i C h i m i c a Generate ¢ lnorganica, Universitd d i 8aseart, Via Vienna 2, 0 7 1 0 0 S a m r i (Italy)
(Received 28 April 1983; accepted in final form 6 January 1984) ABSTRACT The adsorption o f VO*" and Cu r÷ ions on freshly-prepared aluminium hydroxide gels was studied using analytical and spectroscopic technlq,-es. The cation adsorption Is shown to involve hydrolytic c o m p l e x e s o f metal ions. In addi¢lon, a correlation is found between the adsorption capacity of the gels and the nature o f the anions present in the exchange solution, sulphate enhancing the copper uptake with respect to chloride. These findings are explained in terms o f negative charges arising o n the surfaces upon selective binding of sulphate, which promote the metal adsorption. Desorption experiments reveal that the adsorbed ions are quantitatively removed only by ~trong chelating agents. The role of such i,tteracttons in d e t e r m i n i n g the sou nutrient status is discussed. INTRODUCTION M e t a l b o n d i n g in n a t u r a l l y o c c u r r i n g a m o r p h o u s F e a n d AI o x i d e s is o n e of the main factors controlling the distribution and availability of trace m e t a l s in t h e s.~fl e n v i r o n m e n t . I n f a c t , c a t i o n i c s p e c i e s , a l t h o u g h n o t n e c e s s a r i l y g e n e t i c a l l y a s s o c i a t e d w i t h ~Lmorphous o x i d e s , c o n i n t e r a c t w i t h t h e m u p o n s u r f a c e c o n t a c t . S u c h p r o c e s s e s a p p e a r t o b e p ~ t i c u l a r l y r e l e v a n t in o x i d i c soils, w h e r e c o l l o i d s o f t h e v a r i a b l e - c h a r g e t y p e p~evafl [ l j . The adsorption phenomena occurring on the surfaces of hydrous oxides h a v e b e e n w i d e l y d e s c r i b e d [ 2 - - 7 ] . H o w e v e r , o n l y ~ e c e n t s t u d i e s h a v e att e m p t e d a s p e c t r o s c o p i c i d e n t i f i c a t i o n o f t h e bin0.jrg m e c h a n i s m s f o r h e a v y m e t a l i o n s [ 8 - - 1 0 ] .. I t is n o w a c c e p t e d t h a t , u n l i k e c l a y f r a c t i o n s , w h i c h u s u a l l y b e h a v e as n o n s p e c i f i c c a t i o n i c e x c h a n g e r s , a m o r p h o u s h y d r o u s Oxides a r e r e s p o n s i b l e f o r s p e c i f i c a n d s e l e c t i v e b o n d s w i t h m e t a l ions. A s a con~-.,. q u e n c e , t h e m e c h a n i s m s o f d e s o r p t i o n s h o u l d also b e s i g n i f i c a n t l y d i f f e r e n t from those resulting from nonspecific ion exchange, ~ *To whom correspondence should be addressed.
0166-66221841503.00
© 1 9 8 4 Elsevier S c i e n c e Publishers B.V.
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T h e use o f a VO 2~ p a r a m a g n e t i c p r o b e in a m o r p h o u s a l u m i n i u m h y d r o x . ide has p r o v i d e d i n f o r m a t i o n o n t h e s t r u c t u r ~ a n d m o b i l i t y o f m e t a l c o m plexes f o r m e d u p o n s o r p t l o n [ I 0 l . I n d e e d , rigid-limit species l i n k e d t o surface o x y g e n atoms have b e e n revealed*by m e a n s o f e l e c t r o n s p i n r e s o n a n c e ( E S R ) [10]. E S R s t udi e s o f t h e Cu 2÷ i on in a l u m i n i n m h y d r o x i d e gels s h o w . e d results similar t o t h o s e o b t a i n e d f o r VO 2+ [8, 9 | . T o o b t a i n a b e t t e r u n d e r s t a n d i n g o f t h e m e c h a n i s m s involved in m e t a l - o x i d e i n t e r a c t i o n s , w e investigated t h e VO 2+ a n d Cu T* a d s o r p t i o n b y alum i n i u m h y d r o x i d e over an e x t e n d e d c o n c e n t r a t i o n range a n d in t h e p r e s e n c e o f d i f f e r e n t anions. E v i d e n c e is p r e s e n t e d t o s h o w t h a t t h e m e t a l s o r p t i o n requires c a t i o n h y d r o l y s i s , is a n i o n - d e p e n d e n t , a n d yields species w h i c h a r e readi l y d e s o r b e d o n l y b y l t g a n d - d i s p l a c e m e n t reactions. EXPERIMENTAl'-
PART
Materials A l u m i n h l m h y d r o x i d e was p r e p a r e d b y a d d i n g a m m o n i a b y d r o p s t o a q u e o u s s o l u t i o n s o f KAI(SO4)~ • 12H=O u n til p h i - 9 was r e a c h e d . T h e precio p i t a t e was c e n t r i f u g e - w a s h e d . N o significant d i f f e r e n c e was o b s e r v e d bet w e e n t h e a m o u n t o f s u l p h a t e initially a d d e d a n d t h a t in t h e s u p e r n a t a n t s o l u t i o n . W h e n d r i e d a t 110°C, t h e gel w e i g h e d - - 6 2 m g m m o l -I AI.
Adsorption measurements A l i q u o t s o f fresh gel c o n t a i n i n g 2.6 m m o l AI w e r e c o n t a c t e d w i t h VOSO4 a q u e o u s sol ut i on s o b t a i n e d b y dissolving w e i g h e d a m o u n t s o f t h e m e t a l sal~. Preli mi na ry e x p e r i m e n t s s h o w e d t h a t e q u i l i b r i u m was a t t a i n e d in <~24 h. NaCI w a s a d d e d to achieve a t o t a l ionic s t r e n g t h o f 0.1 M. T h e pH was adj u s t e d b y a d d i n g N a O H o r HCI a n d m e a s u r e d w i t h a n O r i o n m o d e l 901 pH m e t e r . T h e final v o l u m e was ca. 75 c m 3 . A f t e r s h a k i n g in p o l y p r o p y l e n e c e n t r i f u g e t u b e s at 2 0 . 0 ± 0.1°C, t h e samples w e r e c e n t r i f u g e d , a n d v a n a d i u m was d e t e r m i n e d in s o l u t i o n using a S p e c t m s p a n I V e m i s s i o n s p e c t r o m e t e r . Residual s u l p h a t e in s o l u t i o n was d e t e r m i n e d w i t h a Wesean m o d e l 2 6 0 io n a n a l y s e r using a Wescan 269-001 a n i o n c o l u m n a n d a 4 m ~ l p o t a s s i u m hyd r o g e n p h t h a ] a t e s o l u t i o n (pH 4.5) as e l u e n t . Similar e x p e r i m e n t a l t e c h n i q u e s w e r e e m p l o y e d t o m e a s u r e t h e Cu ~* ads o r p t i o n f r o m CuCI 2 o r CuSO4 s o l u t i o n s , w i t h t h e e x c e p t i o n t h a t t h e c h l o r i d e i on was n o t m o n i t o r e d . . f
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Desorptfon experiments Equa l aliquots o f At h y d r o x i d e , p r e v i o u s l y l o a d e d w i t h VO 2~ a n d centrifuge-washed, w e r e i m m e d i a t e l y s u s p e n d e d in CuCI= o r Na=EDTA solutions. A f t e r s e p a r a t i o n o f t h e solid phase, m e t a l ions in s o l u t i o n w e r e d e t e r m i n e d as d e s c r i b e d above.
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Spectroscopic analysis ESR spectra w e r e r e c o r d e d at r o o m t e m p e r a t u r e o n a Varian E-9 spectrom e t e r o p e r a t i n g at ca. 9.15 GHz. Equal a m o u n t s o f p o w d e r e d air,dried samples w e r e used f o r t h e q u a n t i t a t i v e analysis. C o n s t a n t values o f m o d u l a t i o n a m p l i t u d e a n d p o w e r level w e r e e m p l o y e d such t h a t n e i t h e r bro~de~ning n o r s a t u r a t i o n o f t h e signals was observed. O t h e r i n s t r u m e n t a l s e t t i n g s w e r e m a i n t a i n e d at p r e s e t values, w i t h t h e e x c e p t i o n o f t h e receiver gain (RG), w h i c h was varied t o give c o m p a r a b l e signals. T h e relative intensities were m e a s u r e d b y integrating t h e area o f t h e signals. I n f r a r e d (IR) spectra w e r e o b t a i n e d o n p o w d e r e d s a m p l e s spread over N a C l plates using a P e r k i n - - E l m e r 6 8 3 s p e c t r o p h o t o m e t e r . RESULTS T h e a d s o r p t i o n isotherms o f VO=* f r o m s u l p h a t e solutions at pH 3.0 and 3 . 5 are s h o w n in Fig. 1. A strong increase in a d s o r p t i o n was observed as t h e pH was raised, indicating t h e p H - d e p e n d e n c y o f t h e reaction. • 30
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Fig. 1. Sorption of VO:* (tool V/tool AI) on amorphous alumi.lum hydroxide gels at pH 3.0 and 3.5.
ESR m e a s u r e m e n t s p e r f o r m e d o n t h e solid phase after t h e e x c h a n g e s h o w e d VO 2. s p e c i e s rigidly b o u n d t o f u n c t i o n a l groups o f t h e a m o r p h o u s h y d r o x i d e even in fully h y d r a t e d gels o b t a i n e d o n s u s p e n d i n g t h e samples in water. With t h e e x c e p t i o n o f t h e signal a m p l i t u d e , t h e parameters did n o t c h a n g e w i t h d i f f e r e n t a d s o r p t i o n levels (Fig. 2), ruling o u t variations o f t h e ~v a n a d i u m e n v i r o n m e n t . Incleed., t h e signal i n t e n s i t y w a s d i r e c t l y prop o r t i o n a l to t h e a m o u n t o f s o r b e d vanadium, indicating t h a t h o m o g e n e o u s
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Fig. 2. Comparative E S R spectra obtained on equal amounts o f VO~*-exehanged aluminium hydroxide gels with differing loading tevels: VIA1 (molar ratio): 6.04.10"3(a). 2 . 9 9 . 1 0 -~ (b) and 9.52- 10 "~(c). With the exception o f the receiver gain (RG). the instrumental settings were kept constant for all the samples.
filling o f t h e surfaces and c o m p l e x a t i o n o f t h e iron c o n t r o l t h e m e ( ~ l u p t a k e . The simultaneous adsorption o f sulphate o n t o t h e gels was measured by c h r o m a t o g r a p h z c analysis, as s h o w n in Fig. 3. S u l p h a t e b o u n d t o t h e s o l i d p h a s e is also revealed b y IR ( F i g . 4 ) t h r o u g h S - - O s t r e c h barLds similar t o those observed on goethite [11]. T h e a d s o r p t i o n o f c o p p e r ( I I ) b y a l u m i n i u m h y d r o x i d e w a s similarly inv e s t i g a t e d . H e r e t o o t h e r e a c t i o n p r o d u c e s i m m o b i l i s a t i o n o f Cu 2. o n t h e Fig. 4. IR spectra o f aluminium hydroxide (a) and alumtnlum hydroxide with differing VIAl molar ratios: 9.52- 10 "=(b), 2.62, 10 - i (e and d). The spectrum d is obtained after dehydration at I|CPC. 8: ~ - O stretching hands;M: V--O stretching band.
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114 surfaces, as s h o w n b y rigid-limit E S I t s p e c t r a q u i t e similar to t h o s e r e p o r t e d b y McBride [ 8 } . T h e e x t e n t o f t h e a d s o r p t i o n is p H - d e p e n d e n t , being negligible at p H < 3 a n d increasing w i t h increasing p H values (Fig. 5). Again, t h e copper(II) uptake from sulphate-containing solutions, with sodium chloride as s u p p o r t i n g e l e c t r o l y t e , was a c c o m p a n i e d b y c h e m i s o r p t i o n o f s u l p h a t e , as s u b s t a n t i a t e d b y analytical a n d IR m e a s u r e m e n t s . T h e CU ~* a d s o r p t i o n f r o m s o l u t i o n s c o n t a i n i n g o n l y c h l o r i d e as a n i o n w a s m u c h l o w e r t h a n w h e n s u l p h a t e was also p r e s e n t (Fig. 5), p r o v i n g t h a t t h e c a t i o n a d s o r p t i o n o n t o a l u m i n i u m h y d r o x i d e is a n i o n - d e p e n d e n t . E n h a n c e m e n t o f m e t a l a d s o r p t i o n b y s u l p h a t e is also ~.ound in t h e case o f o x o v a n a d i u m ( I V ) b y varying t h e [ S O l - ] t o t : [VO=*Jto: ratio. F o r i n s t a n c e , at p H 3.0 an a d s o r p t i o n level ~ 2 . 5 * 10 -2 VIA1 is o b t a i n e d a t [ S O 4 2 - ] t o t : [ V O l t ] t o t = 1 , while a s t e p w i s e increase u p to "-3.0 • 10 -2 V/AI is o b s e r v e d at [SO42-]tot : | V O 2 + ] t o t = 2.5.
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I 15
Equ ilibrium Concentration . 1 0 3, M Fig, 6. :~orptton o f Cu :+ (reel Cufmol AI ! on amorphous a|uminium gels from CuSO4 (a) and CuCI 2 (b) solutions ( / - 0 . l , NaCI) at p H 4.3.
T o illustrate t h e c o m p e t i t i o n b e t w e e n t h e m e t a l ions, d e s o r p t i o n experi4 m e n t s w e r e carried o u t b y l oa di ng AI gels w i t h VO 2. ( s u l p h a t e ) a t varying levels ( p H - - 3 . 5 ) a n d testing t o replace v a n a d i u m w i t h Cu2* ( c h l o r i d e ) a t p H ~- 4.3. In all cases n o significant r e m o v a l o f v a n a d i u m o c c u r r e d (~ee, e.g. t h e results o f T a b l e 1), w h e r e a s t h e c o p p e r , p t a k e w a s c o m p a r a b l e t o t h a t s h o w n in Fig. 5. Again, ES R spectra o f t h e e x c h a n g e d gels e x c l u d e d clustering o f t h e o x o v a n a d i u m ( I V ) ions o r f o r m a t i o n o f h y d r o x i d e species. O n t h e o t h e r h a n d , a l m o s t c o m p l e t e r e m o v a l o f VO 2+ was achieved u p o n i n t e r a c t i o n o f t he h y d r o x i d e gels w i t h s t r o n g c h e l a t o r s (e.g., 0 . 0 1 M N a2 E DT A solutions, final p H ~ 6--7).
116 TABLE 1 -
. .
..
-~
Analytical data concerning representative desozption experiments performed on vo z~a l u m [ n i u r n h y d r o x i d e g e l s ( V I A l m o l a r r a t i o : 5 . 7 6 . 1 0 "* ) w i t h c o p p e r ( H ) c h l o r i d e s o l u ' tionb at pH = 4.3 a [C~ Jeq.~f
C'u/AI molar ratio after exchange
2 . 7 * 1 0 "* 5 . 8 * 1 0 -~ 1 . 1 6 - 1 0 -s
4 . 1 3 - 1 0 "~ 5 . 2 3 - 1 0 -~ 1 . 1 5 , 1 0 -2
aResults obtained by triplicate experiments. DISCUSSION AI~D CONCLUSIONS
Metal ions• a r e s t r o n g l y a d s o r b e d o n t o A l ( O H ) 3 s t r u c t u r e even o n t h e acidic side o f t h e p o i n t o f z e r o c h a r g e ( p H i 9 . 4 , e.ccording t o Ref. [6] ), w h e n positive c ha r ge s p r e d o m i n a t e o n t h e surfaces. T h e a n a l y t i c a l d a t a are c o n s i s t e n t ~vith c h e m i s o r p t i o n m e c h a n i s m s a n d h o m o g e n e o u s filling o f t h e surfaces, r a t h e r t h a n p r e c i p i t a t i o n , w h i c h s h o u l d p r e c l u d e t h e a t t a i n m e n t o f a limit value for t h e a d s o r p t i o n level. O n t h e o t h e r b a n d , t h e E S R measurem e n t s i n d i c a t e t h a t n e i t h e r clustering n o r close i n t e r a c t i o n o f t h e a d s o r b e d ions is s i g n i f i c a n t . O u r results p o i n t o u t t h a t t h e r o l e o f t h e a n i o n s m u s t be c a r e f u l l y t a k e n i n t o a c c o u n t in evaluating t h e m e c h a n i s m s o f c a t i o n r e t e n t i o n o n h y d r o u s o x ide s. T h e r e is a c o n s i d e r a b l e b o d y o f e v i d e n c e t o s h o w t h a t a n i o n s app r o a c h i n g p o s i t i v e l y . c h a r g e d sites o n h y d r o u s o x i d e s are n o n s p e c l f i c a l l y (or electrostatica~.ly) a d s o r b e d as c o u n t e r i o n s in t h e o u t e r H e h n h o l t z p l a n e [ 1 2 - - 1 4 ] . O t h e r anions, s u c h as SO4a-, are in s tead selectively a d s o r b e d i n t o t h e i n n e r H e l m h o l t z l a ye r t h r o u g h ligand e x c h a n g e . T h e m e c h a n i s m s suggested for s u l p h a t e a d s o r p t i o n [ 1 5 ] involve t h e d i s p l a c e m e n t o f w a t e r f r o m p r o t o n a t e d e d g e sites a n d h y d r o x y l s f r o m n e u t r a l sites a c c o r d i n g to t h e scheme: -~AI--OH~ + SO~- ~
AI--(SO4)- + H20
--AI--OH
AI--(SO4)7
+ SO~- ¢
+ OH-
I t has b e e n suggested t h a t n e u t r a l i z a t i o n o f t h e negative charges arising o n t h e surfa c e u p o n s u l p h a t e c h e m i s o r p t i o n m a y involve t h e release o f h y d r o x yls [ 1 5 l . In t h e p r e s e n c e o f suitable m e t a l ions, it is c o n c e i v a b l e t h a t t h e negative charge s c o u l d also be n e u t r a l i s e d via c a t i o n i c a d s o r p t i o n . T h u s , u n l i k e CI-, w h i c h d o e s n o t m o d i f y t h e s u r f a c e charge, s u l p h a t e , being a d s o r b e d b y t h e h y d r o x i d e t h r o u g h ligand e x c h a n g e , p r o d u c e s negative charges t h a t e n h a n c e t h e c a t i o n i c a d s o r p t i o n c a p a c i t y o f t h e gels.
116 It is also e v i d e n t t h a t h y d r o l y s i s o f t h e c a t i o n s is n e e d e d t o b r i n g a b o u t t h e s o r p t i o n process, in a g r e e m e n t w i t h previous suggestions [ 3 , 7 ] . In fact, t h e c a t i o n u p t a k e is n o t significant w h e n t h e c o n c e n t r a t i o n o f h y d r o l y s e d species is negligible in t h e e x c h a n g e s o l u t i o n a n d r a p i d l y i n c r e a s e s as p H is raised. More ove r , at a given p H t h e a f f i n i t y o r d e r o f t h e h y d r o x i d e t o w a r d s Cu a" a nd VO=* closely f ol l ow s t h e h y d r o l y s i s t e n d e n c y o f t h e ions, Using d a t a f r o m t h e l i t e r a t u r e [ 1 6 ] it is c a l c u l a t e d t h a t a t p H 3.0 a b o u t 0.1 2 % o f VO 2. is p r e s e n t as [ V O ( O H ) ] *, w h e r e a s t h e a m o u n t o f t h e din!iclear species [(VO)2 (OH)2 ! 2+ increases w i t h increasing v a n a d i u m concent~'at i o n (as high as ~-0.44% a t i V | t o t ffi 1 . 8 - 1 0 - = M ) . A t p H 3.5 [ V O ( O H ) ] * increases t o ~ 0 . 3 8 % a n d [ ( V O ) 2 ( O H ) 2 ] 2+ varies u p t o 4 . 0 5 % in t h e a b o v e c~nc e n t r a t i o n range. C o m p a r e d t o VO 2. a t p H 3.0 a n d 3.5, c o p p e r ( I i ) , d u e to th e h i g h e r p K * va l ue ( ' - 8 ) [ 1 6 ] , y i e l d s a l o w e r c o n c e n t r a t i o n o f h y d r o l y s e d species even a t p H 4.3 ( [ C I I ( O H ) | ÷ = ~ 0 . 0 2 % ) a n d , a c c o r d i n g l y , is less ads o r b e d o n t o t h e h y d r o x i d e . In t h e a b s e n c e o f h y d r o l y s i s r e q u i r e m e n t s , a n o p p o s i t e t r e n d s h o u l d have b e e n o b s e r v e d , as t h e s u r f a c e p o t e n t i a l is m o r e positive at p H 3.5 t h a n at p H 4.3. A c c o r d i n g l y , p r e l i m i n a r y results s h o w t h a t less h y d r o l y s a b i e ions such as Zn 2÷ are a d s o r b e d a t p H values s u b s t a n t i a l l y h i g h e r t h a n for C~t=+ a nd VO2% M u t u a l e ffe c t s b e t w e e n t h e s o r h e d tons a n d t h e h o s t s t r u c t u r e will o c c u r in t h e e x c h a n g e d gels. T h e p r e s e n c e o f ions t i g h t l y b o u n d o n t h e surfaces h i n d e r s ~h6 p o l y m e r i s a t i o n processes leading to et~ystallisation o f t h e h y d r o x ide [ 8 ] . O n t h e o t h e r h a n d , a c c o r d i n g to previous o b s e r v a t i o n s [ 6 , 8 , 1 7 ] , it a p p e a r s t h a t t h e s o r b e d ions a r e in a n o n e x c h a n g e a b l e f o r m an~! are c o m p l e t e l y r e m o v e d •only b y s t r o n g c h e l a t o r s t h a t c a n give c o m p l e x e s m o r e stable t h a n t h o s e o n tile surfaces. All these o b s e r v a l i o n s m a y p r o v i d e an e x p l a n a t i o n o f c e r t a i n processes o c c u r r i n g in soils. M l c r o e l e m e n t s in c a t i o n i c f o r m lose t h e m o b i l i t y t y p i c a l o f h y d r a t e d ions u p o n i n t e r a c t i o n w i t h a m o r h p h o u s h y d r o x i d e s clue to m e c h a n i s m s also involving t h e p a r t i c i p a t i o n o f suitab,*e anions. T h e e f f ~ t o f t h e a d s o r p t i o n is t h a t metals a r e in a n o n e x c h a n g e a b l e f o r m unless chelat i o n b y s t r o n g ligands oc c ur s . A l t h o u g h t h e relative i m p o r t a n c e o f th e i n t e r a c t i o n s o f t r a c e m e t a l ions w i t h o r g a e i c a n d inorganic soft c o m p o n e n t s is n o t well es tab lish ed [ 1 8 ] , t h e m o s t likely h y p o t h e s i s is t h a t b o t h c o n t r i b u t e to t h e k e e p i n g o f l o w b u t c o n s t a n t levels o t h y d r a t e d ions t h a t o n l y satisfy n u t r i e n t r e q u i r e m e n t s o f plants [ 1 9 ] . ACKNOWLEDGEMENTS T h a n k s are d u e to Consiglic N a z i o n a l e delle R i c e r c h e (C.N.R., Italy) fo r financial s u p p o r t a n d t o Mr A. Dess| f or t e c h n i c a l assistance.
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