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206 G.S .R. Krishnamurt i et al.prepared 0.1 M NaOH to pH 6.0 or 8.0 underambient conditions and were maintained at therespective pH value for 4 h on a Metrohmtitroprocessor using pH-stat mode. The NaOHconsumption during oxidation at a constant pHwas continuously recorded during the initial 2 hreaction period. The rate of Fe(II) oxidation wasdeduced from the rate of consumption of 0.1 raNaOH during the reaction period. The experimentswere also conducted in the presence of kaolinite(KGa-1, Kaolin, Georgia), an aluminosilicatemineral of low surface reactivity. The externaland total surface area of kaolinite (

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Influence of montmorillonite on Fe H) oxidation products 2 0 7t t3

K K G I K K~ ] K ~ K K K K ' d ',_: F

M t M t F hM I| , 1 ~ o Foh Fh Fh F h c )J ~ , e 4 ~ o o

OM t Lo (b)c4 c~M t ~ oM t ~ 1 L ~ - . _ . ~ ~ o

NNM h ~ t N G G L , Mh G MhL ~ L ~ ~ ~ ~ ,.Mh L ~ Mh L o o a ~/ ~ ~ ' k N [ ' ~ ~ ~ ~ ' - ; ' - ~

9 , , , , , , , , | , , , , . , , , , ~ , , ~ , , , , , , , . . . . . . . . . . .10 30 50 70 90~F IG . 1. X - ray d i f f r ac t o g ram s o f p rec i p i t a t i o n p ro d u c t s o f F e ( I I ) o x i d a t i o n fo rm ed a t 2 5 ~ i n 0 .02 MF e(C 1 O a)z .6 H z O -N a O H s y s t em a t p H 6 .0 a f t e r r e ac t i o n t im e o f 4 h . I n th e ab s en ce o f m o n t m o r i l l o n i te o rkao l in i te (a ), in the p resen ce o f m ontmo ri l lon i te a t an in i t ia l montm ori l lon i te /Fe(I I ) ra t io (w /w) o f 1 .4 (b) and 3 .4(c) , and in the p resence o f kao l in i te a t an in i t i a l kao l in i te /Fe(I I ) ra t io (w /w) o f 3 .4 (d ) . The d -spacings are in A .G = goeth i te , L = lep idocroci te , Mh = magh emi te , Fh = fer r ihydr i te , Mt = montm ori l lon i te , and K = kao l in i te ;

Fe-K~ rad ia t ion .

t o w a r d s m a g h e m i t e ( A n n e r s t e n H a f n e r , 1 9 7 3) ,m o s t p e d o g e n i c m a g n e t i c o x i d e s f o r m e d f r o mo x i d a t i o n o f F e ( I I ) s o l u ti o n a t p H > 7 . 0 w o u l d f a l l

i n t o t h e m ag h em i t e r an g e . I t i s a l s o p o s s i b l e t h a t t h ep r o d u c t f o r m e d is m a g n e t i t e s u r r o u n d e d b ys e c o n d a r y m a g h e m i t e , w h i c h p r o d u c e s o v e r l a p p i n g

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208 G . S . R . K r i s h n a m u r t i et al.peaks and cell dimensions intermediate betweenboth phases. The VI'IR spectrum showed character-istic strong absorption bands of maghemite at1088 cm -1, due to -OH deformation vibration,and at 627, 565, and 393 cm -1, due to -O-Fe-Odeformation vibrations. The TEM photographs ofthe sample showed isometric particles characteristicof maghemite (Fig. 2c).

The presence of montmori lloni te promoted theformation of highly crystal line lepidocrocite at anMt/Fe ratio of 1.4 and 3.4 as indicated by thecharacteristic XRD peaks (Fig. 3b), and by thewell-formed laths in TEM micrographs (Fig. 2d) oflepidocrocite.

The presence of kaolinite, even at a high K1/Feratio of 3.4, however, did not exert any significantinfluence on the formation of Fe oxides at either pH(Figs. ld and 3d).

G E N E R A L D I S C U S S I O NIn the absence of complexing ligands, as Fe(II) isoxidized it passes through intermediate green solutioncomplexes or green rusts. The green solutioncomplexes are composed of both Fe(II) and Fe(III)held together by ol- and oxo-bridges formed duringthe consumption of OH-. The configuration of greensolution complexes and green rusts are the same andtheir composition may be represented as[ F e I I ) 2 F e I I I ) O x O H ) y ] ~ 7 - 2 x - y ) a nd[Fe(II)2Fe(III)Ox(OH)y] 5-2x-y) (Feitknech tKeller, 1950; Misawa e t a l . , 1974). Lepidocrocite(7-FeOOH) or magnetite (Fe304) form by furtheroxidation of either green solution complexes or greenrusts in acidic or mildly alkaline conditions,respectively (Misawa e t a l . , 1974). Lepidocrocite isless stable than its polymorph goethite (a-FeOOH)and the transformation is through a solution phaserather than being topotactic (Schwertmann Taylor,1972). Magnetite that forms by oxidation of Fe(lI) inslightly alkaline conditions slowly converts intomaghemite (7-Fe203) (Taylor Schwertmann,1974a,b). Taylor Schwertmann (1974b) wereable to make synthetic maghemite under conditionssimilar to those expected in soils.

The precipitation products formed from theoxidation of Fe(II) solutions are influenced mainlyby the rate of oxidation and the ionic environmentin the system. It was shown earlier that the rate ofFe(II) oxidation decreases with increase in thestability constant of Fe(II)-ligand complex in thepresence of ligands, and the nature of the

precipitation products formed varied s i g n i f i c a n t l y(Krishnamurti Huang, 1990). Further, it was alsoshown that the presence of highly surface-reactivecolloidal size Mn oxide minerals resulted in theformation of a wide range of different compoundssuch as maghemite, akaganeite (13-FeOOH), andpoorly-ordered Fe-oxides (Krishnamurti Huang,1988).

The presence of montmorillonite in the systemaccelerated the oxidation of Fe(II) as reflected inthe increased rate of OH consumption during theoxidation in the presence of increasing amounts ofmontmorillonite (Table 1). In the mildly acidicconditions, such as pH 6.0, the rapid oxidation inthe presence of large amounts of montmorillonite(Mt/Fe = 3.4) might have blocked the crystal-lization of lepidocrocite and goethite and promotedthe formation of relatively well-crystallized ferrihy-drite. In the alkaline conditions, the presence ofmontmorillonite probably retarded the dehydrationof the green rusts, and coupled with increased rateof oxidation might have inhibited the formation ofmaghemite. Initial dehydration and subsequent slowoxidation favour the transformation of green ruststo magnetite (Fe304) (Bernal e t a l . , 1959; Misawae t a l . , 1974), which eventually converts tomaghemite (7-Fe203) (Taylor Schwertmann,1974a,b). However, the presence of montmorillonitewith high surface reactivity accelerated the oxida-tion rate of Fe(II) and the intermediate greensolution complexes and green rusts, therebyinhib iting the formation of maghemite. Rapidoxidation of the intermediate green solutioncomplexes favours the formation of lepidocrocite(Bernal e t a l . , 1959; Misawa e t a L , 1974). Theproportion of lepidocrocite relative to that ofgoethite increased with increase in the rate ofFe(II) oxidation (Carlson Schwertmann, 1990),whereas slow oxidation favours the thermodynami-cally more stable goethite over lepidocrocite(Schwertmann, 1959). Lepidocrocite was observedto be the only species of Fe oxyhydroxides formedin alkaline conditions (at pH 8.0) in the presence ofmontmorillonite (Fig. 3b,c).

Another important factor which could haveinfluenced the formation of Fe oxides during theoxidation of Fe(II) is the presence of an ionicenvironment. Aluminosilicate minerals presentduring the oxidation of Fe(lI) solutions mightrelease A1 and Si to solution which could affectthe nature of oxidation products formed in thesystem. It has already been shown that the presence

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Influence of montmorillonite on Fe ll) oxidation products 209

Fie. 2. Transmis sion e lectron micrograph s of precipitation products of Fe(II) oxidation fo rmed in 0.02 raFe(C 104 )2.6H zO-N aOH system aged in suspension for 100 d at 25~ at pH 6.0 (a) in the absence ofmontmorillonite, and (b) in the presence of montmorillonite at an initial montmorillonite/Fe(II) ratio (w/w) of 3.4;at pH 8.0 (c) in the absence of montmorillonite, and (d) in the presence of montmorillonite at an initialmontmorillonite/Fe(II) ratio (w/w) of 3.4.

of S i excee ding an Si /Fe molar ra t io of 0 .05 Huang, 1990) , and the presence of A excee ding the(Schwer tm ann & T ha lm ann , 1976; Kr i shnam urt i & AI / (AI +F e) m o la r r a t io o f 0 , 30 (T a y lo r &

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Influence of montmoriUonite on Fe ll) oxidation products 2 1 1T A a L E 1 . F i n a l c o m p o s i t i o n , r a t e o f c o n s u m p t i o n o f N a O H , a n d t h e n a t u r e o f i r o n o x i d e s / o x y h y d r o x i d e s f o r m e d

i n t h e s y s t e m .

I n i t i a l C o m p o s i t i o n o f t h e R a t e o f c o n s u m p t i o n M i n e r a l o g y *c o m p o n e n t / F e ( I I ) s y s t e m a t t h e e n d o f o f N a O H d u r i n g t h era t i o 4 h r eac t i on pe r iod i n i t i a l 2 h r eac t i on

Si AIm g 1 -1

p.moles m in -1p H o f t h e s y s t e m : 6 . 0C o n t r o l 0 0 7 . 8 G , L , M hM ffFe = 1 .4 0 .31 0 .042 9 .8 L , G ( t r ) tM f f F e = 3 . 4 5 . 1 6 0 . 0 1 6 1 2 .9 F hKl /Fe = 3 .4 1 .65 0 9 .8 G, L ( t r )p H o f t h e s y s t e m : 8 . 0Co n t ro l 0 0 n .d . Mh , GM t /Fe = 1 .4 1 .21 0 .132 n .d . LMffFe = 3 .4 4 .37 0 .045 n .d . LK t / F e = 3 . 4 0 . 3 0 0 n . d . M h , G

t L - l ep idoc roc i t e , G - goe th i t e , Mh- m aghem i t e , Fh - f e r r i hydr i t e* t t r - t r a ce sn . d . - n o t d e t e r m i n e d

l e p i d o c r o c i t e a t t h e e x p e n s e o f m a g h e m i t e , t h e F eo x i d e f o r m e d i n t h e a b s e n c e o f m o n t m o r i l l o n i t e ,c a n o n l y b e a t t r i b u t e d t o t h e p r e s e n c e o f m o n t -m o r i l l o n i t e , a c o l l o i d a l a l u m i n o s i l i c a t e m i n e r a l w i t hh i g h s u r f a c e r e a c t i v i t y . T h e i n c r e a s e d i n t e n s i t y o ft h e 0 2 0 X R D p e a k i n th e o r i e n te d s a m p l e o f t h ep r e c i p i t a t i o n p r o d u c t s f o r m e d a t p H 8 . 0 ( d a t a n o ts h o w n ) i n d i c a t e d t h e p r e f e r e n t i a l d e v e l o p m e n t o ft h e c r y s t a l s o f l e p i d o c r o c i t e a l o n g t h e b - a x is . T h ep r o n o u n c e d g r o w t h o f l e p i d o c r o c it e a l o n g b isp r o b a b l y m o r e a n e f f e c t o f g r o w t h v e l o c i t y a n d / o rs u r f a c t a n t s , w h i c h m i g h t h a v e r e t a r d e d t h e d e v e l -o p m e n t o f o t h e r d i r e c t i o n s .

T h e i n f l u e n c e o f k a o l in i t e , a h ig h l y c r y s t a l l i n e1 : 1 l a y e r s i l i c a t e m i n e r a l w i t h l o w s u r f a c er e a c t i v i t y , o n t h e p r e c i p i t a t i o n p r o d u c t s o f F e ( I I )o x i d a t i o n w a s a l s o s t u d i e d a t K I / F e r a t i o s o f 0 to3 . 4 , a t t h e p H s o f b o t h 6 . 0 a n d 8 . 0 , T h e r a t e o fF e ( I I ) o x i d a t i o n i n t h e p r e s e n c e o f k a o l i n i t e ( K 1 / F e= 3 . 4 ) w a s o b s e r v e d t o b e t h e s a m e a s t h a t o b s e r v e di n th e p r e s e n c e o f m o n t m o r i l l o n i t e ( M t / F e = 1 .4 )( T a b l e 1 ), a n d a p p r e c i a b l e a m o u n t s ( 0 . 3 -1 . 6 5 m g 1 - 1 ) o f S i w e r e r e l e a s e d t o s o l u t i o n d u r i n gt h e r e a c t i o n ( T a b l e 1 ). H o w e v e r , t h e p r e s e n c e o fk a o l i n i t e ( e v e n a t K I / F e = 3 . 4 ) d i d n o t s h o w a n ye f f e c t o n t h e n a t u r e o f F e o x y h y d r o x i d e s f o r m e d

( F i g s . 1 , 3 ) ; m o n t m o r i l l o n i t e , o n t h e o t h e r h a n d ,w i t h h i g h s u r f a c e r e a c t i v i t y , s i g n i f i c a n t l y i n f l u e n c e dt h e c r y s t a l l i z a t i o n p r o c e s s e s r e s u l t i n g i n t h ef o r m a t i o n o f p o o r l y - o rd e r e d F e o x i d e s a t p H 6 . 0a n d o f l e p i d o c r o c i t e a t p H 8 .0 .

A C K N O W L E D G M E N T ST h i s s t u d y w a s s u p p o r t e d b y R e s e a r c h G r a n t G P 2 3 8 3 -H u a n g o f t h e N a t u r a l S c i e n c e s a n d E n g i n e e r i n gR e s e a r c h C o u n c i l o f C a n a d a . C o n t r i b u t i o n N o . R 8 3 1 ,S a s k a t c h e w a n C e n t r e f o r S o i l R e s e a r c h , U n i v e r s i t y o fS a s k a t c h e w a n , 5 1 , C a m p u s D r i v e , S a s k a t o o n , S K ,C a n a d a S 7 N 5 A 8 . T h e a u t h o r s t h a n k D r H e l g eS t a n j e k , a n d a n a n o n y m o u s r e v i e w e r , f o r t h e i r c r i t i c a lc o m m e n t s a n d m a n y h e l p f u l s u g g e st io n s .

R E F E R E N C E SA n n e r s t e n H . H a f n e r S .S . ( 1 9 7 3 ) V a l e n c y d i s t r ib u t i o n

in syn the t i c sp ine l s o f t he se r i e s Fe304-7 -Fe203 . Z .Kristallogr. 1 3 7 , 3 2 1 - 3 4 0 .B e r n a l J . D ., D a s g u p t a D . R . M a c k a y A .I . ( 1 9 5 9 ) T h eo x i d e s a n d h y d r o x i d e s o f i r o n a n d t h e i r s t r u c t u r a li n t e r r e l a t i onsh ips . Clay Miner. Bull. 4 , 1 5 - 3 0 .

C a r l s o n L . S c h w e r t m a n n U . ( 1 9 9 0 ) T h e e f f e c t o f C O za n d o x i d a t i o n r a t e o n t h e f o r m a t i o n o f g o e t h i t e

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2 1 2 G . S . R . K r i s h n a m u r t i et al.versus lep idocroci te f rom an Fe( l I ) sys tem a t pH 6and 7. C l a y M i n e r . 2 5 , 6 5 - 7 1 .

E l tan tawy I ,M. Arno ld P .W. (1973) Reappra i sa l o fe t h y l en e g l y co l m o n o e t h y l e th e r (E G M E ) m e t h o d fo rsu rface area es t imat ion o f c lays . J . So i l Sc i . 242 3 2 - 2 3 8 .

Fei tknech t W. (1959) Uber d ie Oxydet ion von fes tenh y d ro x y v e rb i n d u n g en d es e i s en s i n w as s r i n g enlosungen . Z E l e c t r o c h e m . 6 3 , 3 4 - 4 3 .

Fei tknech t W . Kel ler G . (1950) The dark -g reenhydroxy compounds o f i ron . Z. anorg a l l g . Chem ie2 6 2 , 6 1 - 6 8 .

Jackson M.L . (1958) So i l Chemica l Ana ly s i s . Pren t iceHal l Inc . , Eng lewood Cl i f fs , NJ . 498 pp ,

Jackson M.L . (1979) S o i l C h e m i c a l A n a l y s i s - A nA d v a n c e d C o u r s e 2nd ed i t ion , pub l i shed by theau thor, Depa r tmen t o f So i l Sc ience , Unive rs i ty o fW i s co n s i n , M ad i s o n , W I , U S A .

Krishnamu rt i G .S .R. Huang P ,M. (1987) The ca ta ly t icro le o f b i rness i te in the t rans fo rmat ion o f i ron . Can.J . Soi l Sc i . 6 7 , 5 3 3 - 5 4 3 .Krishnamu rt i G .S .R. Huang P .M. (1988) In f luence o fm an g an es e o x i d e m i n e ra l s o n t h e fo rm a t i o n o f i ro nox ides . C l a y s C l a y M i n e r . 3 6 , 4 6 7 - 4 7 5 .

Krishnamurt i G .S .R. Huang P .M. (1990) K inet ics o fFe(I I ) oxygenat ion and the na tu re o f hydro ly t icproducts as influenced by l igands. Sci. Geol. Mem.8 5 : 1 9 5 -2 0 4 .

Krishnamurt i G .S .R. , Sarm a V .A.K. Reng asamy P.(1974) Sp ect ropho tom et r ic de terminat ion o f a lumi-n i u m w i t h a l u m i n o n . I n d i a n J . T e c h n o l . 12,2 7 0 - 2 7 1 .Misa wa T . , Hash im oto K . Sh im odai ra S . (1974) Them ech an i s m o f fo rm a t i o n o f i ro n o x i d e and o x y h y d r -

ox ides in aqueous so lu t ions a t room tempera tu re .Corros. Sc i . 1 4 , 1 3 1 -1 3 9 .

Schwer tmann U . (1959) . Uber d ie Syn these def in ier te rE isenoxyde un ter versch iedenen Bed ingungen . Z .anorg a l l g . Chem ie 2 9 8 , 3 3 7 -3 4 8 .S ch w er t m an n U . (1 9 79 ) T h e i n f l u en ce o f a l u m i n u m o ni ron ox ides : 5 . Clay m inera l s as source o f a luminum .Soi l Sc i . 1 2 8 , 1 9 5 -2 0 0 .

Schwer tmann U . (1988) Goeth i te and hemat i te fo rma-t ion in the p resence o f c lay minera l s and g ibbs i te a t25~ Soi l Sc i . Soc . Am. J . 5 2 , 2 8 8 - 2 9 1 .

S ch w er t m an n U . T ay l o r R .M . (1 9 72 ) T h e i n f lu en ce o fs i l i ca te on the t rans fo rmat ion o f l ep idocroci te togoethi te. C l a ys C l a y M i n e r 2 0 , 1 5 9 -1 6 4 .

Schwe r tmann U . Tay lo r R .M. (1989) I ron ox ides . Pp .3 7 9 - 4 3 8 i n : M i n e r a l s i n S o i l E n v i r o n m e n t s . (J.B.Dixon S .B. W eed , ed i to rs) , 2nd ed i t ion , SS SABoo k Ser ies no . 1 , So i l Sc i . Soc . Am . , Madison , W I,U S A .Schwe rtmann U . Thalm ann H . (1976) The in f luenceof Fe(I I ) , S i and pH on the fo rmat ion o f l ep idocroci teand fer r ihydr i te dur ing ox ida t ion o f aqueous FeC12solutions. C l a y M i n e r 1 1 , 1 8 9 -2 0 0 .

Tay lo r R .M. Schwer tm ann U . (1974a) M aghem i te insoils and i ts origin . I . Propert ies and observations ofso i l maghemi tes . C l a y M i n e r . 10 , 289--298 .

Tay lo r R .M. Schw er tmann U . (1974b) M aghe mi te inso i l s and i t s o r ig in . I I . Maghemi te syn thes i s a tambien t t empera tu res and pH 7 . C l a y M i n e r . 10,2 9 9 - 3 1 0 .Tay lo r R .M. Schwe r tmann U . (1978) The in f luence o fAI on i ron ox ides . I. The in f luence o f A1 on F e ox idefo rmat ion f rom the Fe(I I ) sys tem. C l a y s C l a y M i n e r .2 6 , 3 7 3 - 3 8 3 .