The ratio of Fe to A1 of the
Clays and Clay Minerals,
Vol.44, No. I, 113-120, 1996.
E F F E C T OF T I M E A N D T E M P E R A T U R E ON T HE C H E M I C A L C O M P O S I T I O N A N D C R Y S T A L L I Z A T I O N OF M I X E D IRON A N D A L U M I N U M SPECIES C. COLOMBO AND A. VIOLANTE Dipartimento di Scienze Chimico-Agrarie, Universith di Napoli "Federico II," 80055 Portici, Napoli, Italy Abstract We studied the influence of time ageing (up to 120 d at 50~ or 30d at 95~ on the mineralogical and chemical composition of hydrolytic species of mixed aluminum and iron samples formed at pH 5.0 and initial Fe/A1 molar ratio (Ri) ranging from 0.1 to 10. The partitioning distribution of Fe and A1 in soluble or solid phases of different sizes (0.2 ~m) depended on Ri and time. The ratio of Fe to A1 of the 1 aged 32 d at 50~ dissolved almost completely by acid ammoniumoxalate (82-93%), but the samples at Ri -< 0.5 were only partially solubilized (13--60%). After further 30 d at 95~ the percentages of Fe + A1 solubilized by oxalate from the samples at R -> 0.5 was still relatively high (22-39%). Key Words--Aluminum, Crystallization, Gibbsite, Iron.
INTRODUCTION M u c h w o r k has b e e n c o n d u c t e d on the effect o f ageing on the hydrolytic products o f A1 or Fe (III) at different p H values in the a b s e n c e or p r e s e n c e o f organic and inorganic ligands (Cornell and S c h w e r t m a n n 1979; H u a n g and Violante 1986; Cornell et al. 1989; Hsu 1989; S c h w e r t m a n n and Taylor 1989). The influe n c e o f A1 in the crystallization p r o c e s s o f F e - o x i d e s and in the f o r m a t i o n o f Al-substituted goethite and hematite has b e e n w o r k e d out in detail (Lewis and S c h w e r t m a n n 1979; S c h w e r t m a n n et al. 1979; Schulze and S c h w e r t m a n n 1984; Barrdn et al. 1984; Torrent et al. 1987). In m o s t o f the w o r k s o f S c h w e r t m a n n and his c o w o r k e r s (Taylor and S c h w e r t m a n n 1978; S c h w e r t m a n n et al. 1979; L e w i s and S c h w e r t m a n n 1979; S c h w e r t m a n n 1988), the s a m p l e s w e r e p r e p a r e d at Fe/A1 m o l a r ratio -->2 and/or at v e r y high p H values, and only the properties o f the final products were studied. It has b e e n s h o w n that A1 retards or inhibits the f o r m a t i o n o f s o m e F e - o x i d e s in favor o f others. There is little i n f o r m a t i o n available on the effect o f time on the c h e m i c a l c o m p o s i t i o n , nature and crystallization o f m i x e d Fe(III) and A1 precipitates obtained at different initial Fe/A1 m o l a r ratios (Gastuche et al. 1964; R e n g a s a m y and O a d e s 1979; G o h et al. 1987; Krishnamurti et al. 1995). R e n g a s a m y and O a d e s (1979) s h o w e d e v i d e n c e that p o l y m e r i z a t i o n o f A1 and Fe(III) in m i x e d solutions (OH/AI + F e --< 2.5) f a v o r e d the f o r m a t i o n o f AI-Fe c o p o l y - c a t i o n s rather than a mixture o f separate A1 and Copyright 9 1996, The Clay Minerals Society
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Fe species. Furthermore, p o l y c a t i o n s o b t a i n e d by adding N a O H to A1 and Fe(III) m i x e d solutions up to O H / A1 + F e m o l a r ratio o f 2.5 had Fe/A1 m o l a r ratios close to the original solutions. M o r e recently, G o h et al. (1987) f o u n d that after p r o l o n g e d ageing o f m i x e d A1Fe s y s t e m s (Fe/A1 m o l a r ratio --< 1), crystalline AI(OH) 3 and m i x e d n o n c r y s t a l l i n e Fe-A1 oxide or highly A l - s u b s t i t u t e d n o n c r y s t a l l i n e F e - o x i d e s f o r m e d at p H 6.0-7.0. In this work, w e report on the effect o f time on the c h e m i c a l c o m p o s i t i o n , the nature and the stability o f hydrolytic species o f m i x e d a l u m i n u m and iron solutions (Fe/A1 molar ratio ranging f r o m 0.1 to 10) f o r m e d at p H 5.0 and aged up 4 m o n t h s at 50 ~ or for 32d at 50~ and 1 m o n t h at 95~ MATERIALS AND METHODS F r e s h stock solutions o f suitable a m o u n t s o f 0.01 M AI(NO3) 3 and 0.01 M Fe(NO3) 3 were m i x e d in order to have s a m p l e s at initial Fe/A1 m o l a r ratio o f 0, 0.1, 0.25, 0.5, 1.0, 4.0, 10.0 or c~ ( h e n c e f o r t h r e f e r r e d as R0, R0.1, R0.25, R0.5, R1, R4, R I 0 and Rc~). The solutions w e r e p o t e n t i o m e t r i c a l l y titrated to p H 5.0 b y adding CO2-free standard 0.25 M N a O H at a f e e d rate o f 0.5 ml/min. A M e t r o h m Herisau E 536 automatic titrator in c o n j u n c t i o n with an automatic syringe burette 655 D o s i m a t was used. The OH/A1 + F e m o l a r ratio o f the titrated solutions r a n g e d b e t w e e n 2.65 to 3.03 by increasing the initial Fe/AI m o l a r ratio. The final v o l u m e o f all s a m p l e s was adjusted to one liter.
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Clays and Clay Minerals
Colombo and Violante
Table 1. Chemical composition of the initial solution and amounts of iron and aluminum (mmol L 1) in the filtrates < 0.2 ixm of the samples after 7, 20, 32 and 120 d at 50~ Initial chemical composition
7 d
20 d
32 d
120 d
Samples
Fe
AI
Ri i
Fe
AI
Rs:
Fe
AI
Rs
Fe
A1
Rs
Fe
AI
Rs
R0 R0.1 R0.25 R0.5 R1 R4 R10 R~
0 0.45 1.00 1.66 2.50 4.00 4.55 5.00
5.00 4.55 4.00 3.34 2.50 1.00 0.45 0
0 0.10 0.25 0.50 1.00 4.00 10.00 ~
0 0.34 0.82 1.49 1.62 2.95 tr tr
3.22 3.22 3.11 2.59 2.15 0.95 tr tr
0 0.11 0.26 0.58 0.75 3.10 n.d. n.d.
0 0.30 0.63 1.32 1.76 3.33 tr tr
1.34 1.27 1.06 0.91 0.87 0.65 tr tr
0 0.24 0.59 1.45 2.70 5.12 n.d. n.d.
0 0.30 0.62 1.32 2.08 3.58 3.80 2.72
1.15 1.13 0.97 0.89 0.76 0.69 0.43 tr
0 0.27 0.64 1.48 2.73 5.18 8.83 n.d.
0 0.32 0.84 1.50 2.43 3.31 3.47 0.40
0.80 0.69 0.65 0.42 0.35 0.27 0.13 tr
0 0.46 1.29 3.57 7.51 12.3 26.7 n.d.
Ri = Initial Fe/A1 molar ratio. 2 Rs = Fe/A1 molar ratio in the filtrates < 0.2 txm.
T h e final F e + AI c o n c e n t r a t i o n was 0.005 M in a l l o f the samples. Table 1 s h o w s the c h e m i c a l c o m p o s i t i o n a n d the initial m o l a r ratio Fe/A1 (Ri) of the starting solutions. T h e s u s p e n s i o n s were k e p t in p o l y p r o p y l e n e containers a n d aged at 20~ for o n e week. A p H 5.0 was m a i n t a i n e d b y a d d i n g a f e w drops of 0.25 M N a O H . S u b s e q u e n t l y the s u s p e n s i o n s were aged up to 120 d at 50~ w i t h o u t p H adjustment. A l i q u o t s o f the samples, p r e v i o u s l y aged for 32 d at 50~ were k e p t at 95~ for a n o t h e r 30 d. D u r i n g the a g e i n g process (7, 20, 32 or 120 d at 50~ s u b s a m p l e s were collected a n d filtered t h r o u g h N a l g e n e acetate m e m b r a n e s (pore size < 0.2 ixm) or ultrafiltered t h r o u g h S p e c t r a / P o r m o l e c u l a r p o r o u s fluorocarbon F100 membranes (Molecular Weight (M.W.) c u t o f f < 1 0 0 , 0 0 0 ; ~ 0 . 0 1 p~m). T h e filtrates were a n a l y z e d for Fe a n d A1 b y a t o m i c a b s o r p t i o n s p e c t r o s c o p y after d i s s o l u t i o n with 6 M HC1. O t h e r s u b s a m p l e s were d i a l y z e d (M.W. c u t o f f = 15,000) in d e i o n i z e d w a t e r until C1 free, freeze-dried and lightly g r o u n d to pass t h r o u g h a 1 0 0 - m e s h sieve. T h e freeze-dried samples were m o u n t e d into a holder to o b t a i n r a n d o m particle o r i e n t a t i o n and a n a l y z e d u s i n g a R i g a k u Geigerflex D / M a x IIIC X - r a y diffract o m e t e r ( X R D ) e q u i p p e d w i t h iron-filtered C o - K ~ radiation g e n e r a t e d at 4 0 k V and 30 m A a n d a scan speed
Table 2. Percentages of Fe + A1 present in the precipitation products (particle size > 0.2 Ixm) after 7, 20, 32 and 120 d of ageing at 50~ Samples
(Fe + A1) 7 d
(Fe + AI) 20 d
(Fe + AI) 32 d
(Fe + AI) 120 d
R0 R0.1 R0.25 R0.5 R1 R4 R10 Rc~
36 29 21 18 25 22 100 100
73 74 66 55 52 36 100 100
77 77 68 56 43 15 17 46
84 80 71 61 44 29 26 92
o f 1~ 20/rain. T h e X R D traces are the results o f 8 s u m m e d signal. Differential t h e r m a l ( D T A ) and therm o g r a v i m e t r i c ( D T G ) analyses o f selected s a m p l e s were o b t a i n e d u s i n g a N e t z s c h T h e r m a l A n a l y z e r S T A 409 p r o g r a m m e d f r o m 25~ to 900~ at a rate o f 10~ min, u s i n g a l u m i n a as the reference material. F o r t r a n s m i s s i o n electron m i c r o s c o p i c ( T E M ) e x a m i n a tion, one drop o f a s a m p l e suspension, p r e v i o u s l y dialyzed, was d e p o s i t e d onto a c a r b o n - c o a t e d F o r m v a r film Cu grid. T E M e l e c t r o n m i c r o g r a p h s were t a k e n with a Philips c m 10. Fe and A1 were also d e t e r m i n e d b y atomic absorption for the dialyzed s a m p l e s after dissolution w i t h 6 M HC1, p H 3.0 NH4 oxalate ( S c h w e r t m a n n 1964) or dithionite-citrate-bicarbonate (Mehra and Jackson 1960). RESULTS
AND
DISCUSSION
C h e m i c a l C o m p o s i t i o n o f the Fe-A1 S a m p l e s Table 1 s h o w s the a m o u n t (retool L -1) o f Fe a n d A1 p r e s e n t in the filtrates < 0 . 2 ixm o f the samples aged 7, 20, 32, a n d 120 d at 50~ T h e c o n t e n t o f A1 o f each s a m p l e usually d e c r e a s e d with the time. T h e F e / A1 m o l a r ratio (Rs) in the filtrates < 0 . 2 txm of the samples R 0 . 1 - R 1 0 usually i n c r e a s e d with ageing. However, after 32 d at 50~ the Fe-AI species o f R0.5 c h a r a c t e r i z e d b y an initial c o n t e n t o f 6 7 % A1, R1 b y 5 0 % A1 a n d R 4 b y 2 0 % A1, still s h o w e d a h i g h perc e n t a g e o f A1 (40% for R0.5, 2 7 % for R I a n d 16% for R4), i n d i c a t i n g that the < 0 . 2 txm Fe-A1 species initially f o r m e d were relatively stable. O n l y after 120 d at 50~ was the c o n t e n t o f A1 for the < 0.2 txm FeA1 species o f these s a m p l e s drastically r e d u c e d (21.8% for R0.5, 11.7% for R1 a n d 7.5% for R4). C o n v e r s e l y , large quantities o f Fe ( 7 1 - 9 8 % o f the Fe initially added) were p r e s e n t in the < 0 . 2 ~ m fraction of s a m p l e s R 0 . 1 - R 1 0 after 120 d. T h e p e r c e n t a g e s o f Fe + A1 p r e s e n t in the precipitation p r o d u c t s ( > 0 . 2 Ixm) d u r i n g the a g e i n g are rep o r t e d in Table 2. A f t e r 120 d, the precipitation products f r o m s a m p l e R 0 c o n t a i n e d 84% A1 and s a m p l e
Vol. 44, No. 1, 1996
Nature of mixed Al-fe species
Table 3. Amounts (mmol L -1) of the iron and aluminum present in the < 0.01 ixm fractions of the samples aged 7 and 120 d at 50~ 7 d Samples
R0 R0.1 R0.25 R0.5 R1 R4 RI0 R~
Fe
0 0.31 0.81 1.03 1.19 0.08 tr tr
AI
0.92 0.90 1.22 1.24 0.42 tr tr tr
115
Table 4. Percentages of Fe + AI solubilized by dithionitecitrate bicarbonate (DCB) o1" mmnonium oxalate solution (OXA) from the samples aged 32 d at 50~ or 32 d at 50~ and 30 d at 95~
120 d Rs I
0 0.34 0.75 0.83 2.83 n.d. n.d. n.d.
Fe
0 0.02 0.16 0.07 0.01 tr tr tr
A1
0.76 0.55 0.48 0.34 0.16 tr tr tr
After 32 d at 50~
Rs
0 0.04 0.33 0.20 0.06 n.d. n.d. n.d.
After 30 d at 95~
Samples
DCB
OXA
OXA
R0 R0.1 R0,25 R0.5 R1 R4 R10 Roo
2 25 36 69 98 98 92 100
9 13 36 60 82 93 92 74
tr 6 15 22 36 39 36 tr
Rs = Fe/AI molar ratio. Ro0 contained 92% Fe, of the amount initially added. In the coprecipitates o f Fe and A1, after 120 d, the higher the initial concentration of Fe the lower the percentage of Fe + A1 present in the solid phase. In fact, 80% o f the initial amount of Fe + At was precipitated for R0.1., 61% for R0.5, 29% for R4 and 26% for R10. Reported in Table 3 are the amounts (mmol L 1) of Fe and A1 present in the filtrates 1) appeared distorted (Figure 5d) and of extremely small size. In sample R10, the sparse crystals of hematite appeared to have been formed of fine-grained particles with spherical shape and an inner granular structure (Taylor and Schwertmann 1978). Finally, in sample R~, acicular goethite and few small hexagonal crystals of hematite were evident (Figure 513. Nature of fe-al precipitation products. Table 4 shows the percentages of Fe + A1 solubilized by DCB or oxalate solution at pH 3.0 from the samples aged 32 d at 50~ or 32 d at 50~ plus 30 d at 95~ as referred to the total amounts of Fe and A1 solubilized by 6 M HCI. Samples R I - R I 0 aged 32 d at 50~ which showed only low-crystalline felxihydrite by XRD (Figures 2 c e), were almost completely solubilized by DCB and
oxalate (82-98%). On the contrary, samples R0-R0.5, which showed gibbsite by XRD were only partially solubilized. The amounts of Fe + A1 removed from these samples by oxalate ranged from 9% (R0) to 60% (R0.5), confirming that increasing amounts of shortrange ordered and/or finely divided Fe-A1 materials were formed by increasing the initial Fe/A1 molar ratio. From the sample R~, which principally contained goethite and traces of hematite (Figures 2f-5f), 74% of the Fe was solubilized by oxalate. This indicates the presence of large quantities of ferrihydrite or very poorly crystalline Fe-oxides. After ageing at 95~ samples R0, R0.1, R0.25 and R ~ had negligible quantities of Fe + A1 solubilized by oxalate (--
