Clay Minerals (1986) 21, 93-100

I N F R A R E D A B S O R P T I O N OF S U R F A C E H Y D R O X Y L G R O U P S A N D L A T T I C E

V I B R A T I O N S IN L E P I D O C R O C I T E (3,-FEOOH) A N D B O E H M I T E (~ -ALOOH)

D. G. L E W I S AND V. C. F A R M E R

Department of Soil Science, Waite Agricultural Research Institute, Glen Osmond 5064, South Australia

(Received 4 November 1985)

A BS'I RAU 1 : l~oenmite and iepidocrocite, in th~ fu~m u~ ira,, VldL~.~ ~,t,l ~r . . . . . . . . . ]-~u •v .u, faces parallel to the constituent layers, show one predominant absorption band due to OH stretching of two-coordinated surface hydroxyl groups, which are shown to be unreactive towards phosphate. In very fine grained preparations of lepidocrocite, two other species of surface hydroxyl are detected, presumably associated with (100) and (001) faces. Marked differences in the frequencies of bulk vibrations between platy and rod shaped lepidocrocite preparations can be explained partly by their crystal morphology, and partly by degree of crystalline order.

Although the surface hydroxyl groups and surface reactions of gibbsite and goethite have been shown to be accessible to infrared study (Russell et al., 1974, 1975; Parfitt et al., 1976, 1977; Parfitt & Russell, 1977), no report of IR absorption associated with the surfaces of lepidocrocite and boehmite has appeared. These minerals can crystallize in the form of thin plates with well-developed (010) faces, on which a single species of two- coordinated OH group should predominate, and we have examined suitable crystal preparations to identify this OH vibration and to determine its reactivity towards phosphate.

In the course of this work, some striking differences between the IR spectra of platy and rod-shaped lepidocrocite crystals were seen in the region of bulk Fe-O vibrations. A possible explanation is proposed.

S A M P L E P R E P A R A T I O N A N D C H A R A C T E R I S T I C S

Boehmite

A solution of A1C13 (0.1 M) was neutralized to pH 6 with N a H C O 3 (0.4 N) and the suspension aged for one hour. The pH was then increased to pH 8 by addition of N a O H (0.15 ~) and the suspension stored for 60 h at 160~ in a sealed container. After cooling, the solid product was washed salt free by centrifugation and kept as a suspension (~20 mg m1-1) in a sealed polythene container.

L epidocroeite

Sample A. Pure solid FeC12.4H20 was added to 150 ml of a nitrogen-saturated sodium chloride solution (0.2 N) to give an Fe(II) concentration of 0.02 M. With continued stirring

(~ Waite Agricultural Research Institute, 1986

94 D. G. Lewis and V. C. Farmer

Y

J FIG. I. Structure of boehmite and lepidocrocite (Ewing, 1935). The double lines represent

hydrogen bonds between layers.

and nitrogen bubbling the solution was adjusted to pH 6.0 by addition of M NaOH from a Radiometer Autoburette. After the desired pH had stabilized, the nitrogen source was removed and air bubbled in at approximately 15 ml min -1, maintaining the pH constant at

6.0 throughout the oxidation period (2.5 h). The oxidized product was then washed salt free.

Sample B. Preparation similar to A but initial concentrations were 0-05 M Fe(II) and 0.75 M NaCI, and oxidation using air at 2 ml rain -1 was accomplished at pH 7.0 (Taylor 1984).

Sample C. Solid FeSO 4. 7H20 was added to 10 litres of 5 x 10 4 M NaHCO3 solution to give an Fe(II) concentration of 2.5 • 10 -4 M. Initially the stirred solution was at pH 6.2,

TABLE 1. Crystal dimensions (nm) of boehmite and lepidocrocite samples esti- mated from X-ray diffraction and electron microscopy.

Lepidocrocite

XRD p e a k Dimension Boehmite A B C

051/200 a* 10 33 52 14 020 b 8 10 44 4 002 c 60 electron microscopy width a 25 60 50 20-25 thickness b 7 15 50 5-10 length c 100 300 500 50-100

* Minimum value since doublet could not be resolved for the fine grained samples.

IR of lepidocrocite and boehmite 95

I #m I I

FIG. 2. Electron micrographs of (a) boehmite (b) lepidocrocite A (c) lepidocrocite B (d) lepidocrocite C (microscopists: A. Waters and J. M. Tait).

but as oxidation occurred (at the liquid-atmosphere interface) without air bubbling, the pH decreased slowly during the first hour and was eventually stabilized by the buffering effect of the bicarbonate solution at pH 5.9 +_ 0.1. After 18 h, when oxidation was complete, the pH was increased to 8.0 using M N a O H to facilitate flocculation and collection of the product which was then washed salt free,

96 D. G. Lewis and V. C. Farmer

In all cases, a portion of the washed product was kept as a suspension (approximately 20 mg m1-1) for surface hydroxyl examination but the major part was oven-dried at 55~ prior to characterization by IR, XRD and electron optical techniques. All samples were established as being mono-mineralic with properties as shown in Table 1 and Fig. 2.

S U R F A C E H Y D R O X Y L A B S O R P T I O N

Thin films (1 mg cm -2) of boehmite, when examined in an evacuated cell, showed a strong, sharp absorption band at 3665 cm -1 together with a much weaker, broader band near 3570 cm -1 on the high-frequency side of the broad band due to absorption by bulk hydroxyl groups (Fig. 3a--full line). The corresponding peaks for lepidocrocite sample A occurred at 3620 and near 3525 cm -1 (Fig. 3b--full line). The hydroxyls associated with these high-frequency bands exchanged their protons rapidly with D20 when exposed to its vapour, and then appeared as OD vibrations at 2705 and 2640 cm -1 (boehmite) or 2675 and 2612 cm -~ (lepidocrocite) following evacuation (Fig. 3a and b--broken lines). The bulk hydroxyl of boehmite and lepidocrocite exchanged slowly and progressively on exposure to D20 vapour by proton-deuterium transfer along the hydrogen-bonded chains (Fripat et al., 1967). The dominant OH absorption at 3665 or 3620 cm -~ is ascribed to absorption by hydroxyl groups on the (010) face; from Fig. 1 it can be seen that a single type of doubly-coordinated hydroxyl group, shared by two cations, is exposed on this (010) surface.

Exposing the platy boehmite and lepidocrocite preparations to dilute H3PO 4, followed by water washing, did not eliminate or weaken these surface hydroxyl adsorptions, indicating that the doubly-coordinated OH groups did not react with phosphate. This finding is consistent with the observation that only singly-coordinated OH groups on the (100) face of goethite are reactive, whereas 2- and 3-coordinated OH on goethite (Parfitt et al., 1976) and 2-coordinated OH groups on the (001) face of gibbsite are unreactive (Parfitt et al., 1977).

The rod-shaped preparation of lepidocrocite (sample B) gave a relatively weak absorption at 3620 cm -1 consistent with the more limited development of its (010) face and also its smaller specific surface area.

The second type of surface OH detected at 3570 (or 3525) is tentatively assigned to the (100) face. Inspection of the crystal structure (Fig. I) indicates that a section parallel to (100) will expose singly-coordinated oxide ions carrying an excess negative charge o f - 3 / 2 and 3-coordinated oxide ions carrying a charge o f - 1 / 2 . Addition of protons can compensate this negative charge and will convert the oxide ions to a mixture of hydroxyl groups and coordinated water molecules. The exact nature of the hydroxyl group that absorbs at 3570 (or 3525 cm -~) is uncertain. However, since this peak is also unaffected by treatment with HaPO 4 and, in the very small particles of lepidocrocite (sample C), occurs as a well defined peak at 3529 cm -~ rather than the shoulder given for sample A (Fig. 3b) it is probably derived from the 3-coordinated oxide ions.

In all samples, an extremely weak high-frequency peak occurred at 3695 cm -1 for boehmite and 3655 cm -1 for lepidocrocite. This peak could also be transformed by contact with D/O vapour to give an OD peak at 2730 cm -a (boehmite) or 2700 cm -~ (lepidocrocite). However, unlike the stronger peaks mentioned previously, treatment with H3PO 4 caused these high-frequency peaks to disappear. By analogy to the goethite interpretation, these may be due to singly-coordinated OH groups, probably associated

1R of lepidocrocite and boehmite 97

C o

m ~

E m C m

F-

,/17"o5 i / / 243!' 1

v AIOOH a

3695 ~ \\\

3b'~O\~ + F;",~ \ ~

"" \\ t 3665 ' ~

~ ~ - - ' ' , \ V F eOOH~/~..

)':k \ '---" / k ; - '~ \ ! ",,/

I I I

40 30

/ / /

J

20 Wavenumber ~/lOOcrn -1

FIG. 3. IR spectra of (a) boehmite (y-A100H) and (b) lepidocrocite sample A (FFeOOH) in the range of OH and OD stretching vibrations. Full line: untreated film; dotted line (+ P): after

treatment with H3PO4; dashed line: after treatment with D20.

with the crystal ends, i.e. (001) faces which normally comprise only a very small fraction of the total surface of these preparations. Since lepidocrocites found in soils generally show a platy morphology with (010) dominant, it would seem that this mineral probably plays little role in phosphate sorption.

B U L K V I B R A T I O N S

An IR study of the lattice vibrations of boehmite has shown that the stronger absorption bands could be assigned, at least approximately, on the basis of the average D~z~ (Amain) symmetry (Table 2) although the presence of some additional weak absorption bands

98 D. G. Lewis and V. C. Farmer

TABLE 2. Frequencies (cm -~) of the principal absorption bands of lepidocrocite and boehmite in KI disks, assigned to OH stretching (v), OH in-plane and out-of-plane bending (6 and y), and to displacements (r) of O 2- and OH-. The symmetry species and direction of the transition moments are based on the approximate D~Th (Amam) symmetry. Absorption bands showing orientation effects in oriented films are indicated (2_).

Frequencies with ambiguous assignments are bracketted.

Vibration Lepidocrocite symmetry species Boehmite

and transition moment rods(B) plates(A) plates(C) plates

vOH B2u (y) 3060 3160• 3160 3295A_ B3u (x) 2850 2850 sh 3088

fiOH B2u (y) 1150 ll60A_ 1150 1155A_ B3u (x) 1018 1018 1019 1067

yOH Blu (z) 752 742 742 736 rO B2u (y) 510 540A_ 530br 765A_

Blu (z) 478 480 470 ~622 B3u (x) 610 510 ~498

rOH B2u (y) (357 (355 ( 3 5 6 411A_ Blu (z) ~270 4270 ~260 f368 B3u (x) (223 (220 (225 1.323

sh = shoulder br = broad

indicated a locally ordered structure of lower symmetry, probably C~h (P2~/el 1), in which the chains of hydrogen-bonded OH groups alternate in direction in successive rows (Farmer, 1980).

Only the stronger absorption bands could be detected for lepidocrocite, and, as for boehmite, these could be grouped into proton vibrations, 02- displacements, and OH- displacements (Table 2). However, marked shifts in frequency, affecting principally the OH stretching and 02- displacements, were evident between well-crystallized preparations of platy (curve A, Fig. 4) and rod-shaped morphology (curve B, Fig. 4). These differences can be ascribed to the effects of surface charges developed on the very small crystals. In such crystals, a single vibration can vary in frequency between its longitudinal frequency (v0 as an upper limit, and its transverse frequency (vt) as a lower limit, where the difference, v I - v t, increases with increasing intensity of absorption (transition moment: Hadni, 1974). In thin platy crystals, vibrations perpendicular to the plates (parallel to y axis of lepidocrocite) occur at their v~ frequencies and those parallel to the plates (x-z plane) at their 1~ t frequencies. The rod-shaped form of lepidocrocite approximates to a cylinder, in which vibrations along the rod (z axis) will lie at their 1~ t frequencies and those perpendicular to the rod (x, y axes) will occur at frequencies intermediate between v 1 and vt, say v r Thus, for the two forms of lepidocrocite, vibrations in the y direction will decrease in frequency on passing from the platy (v0 to the cylindrical (vi) form; those in the x direction will rise in frequency in passing from the platy (vt) to the cylindrical (vi) form, while those in the z direction should change little in frequency (vt). It will be seen (Table 2) that the differences in spectra for OH stretching and O z- displacements might be explained on this basis for preparations B (rod-shaped) and A (platy). The assignments of the B2, vibrations for the platy crystals was confirmed by the marked increase in intensity of these absorption bands, when the angle of incidence of the IR beam on an oriented deposit of the plates was changed from perpendicular incidence (i.e. parallel to [010]) to 45 ~

IR of lepidocrocite and boehmite

c -

O .m

(h . n

E I r

I - "

3060

61( 510

4 7 8 . I [

3160

3400

540

480

470

40 30 1'2 8 '

C

W a v e n u m b e r v / lOOcm -I

FIG. 4. IR spectra of lepidocrocite preparations B, A and C in KI disks. Absorption near 3400 cm -1 is due to adsorbed water.

99

In contrast to OH stretching and 02- displacements, the O H - displacements in the 400-200 cm -1 region did not show any marked shift in frequency between the platy and rod-like forms. None of these bands exhibited a clear orientation effect for oriented films of the platy form. It appears, therefore, that these vibrations are not aligned parallel to the crystal axes.

A second platy form of lepidocrocite (C), of poorer crystallinity as indicated by broadening of all its X-ray reflections, showed further changes in absorption pattern, affecting principally the region of 02- displacements around 450-550 cm-L In this spectrum (curve C, Fig. 4), the in-plane displacements merged into a single band at 470 cm -1, which also intensified considerably relative to the other bands in the spectrum. This feature was found to be a characteristic of other poorly crystalline preparations of platy morphology, prepared by different methods. It may be an effect of the increasing number of defects in these crystals, but no detailed interpretation can be offered.

100

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

V. C. Farmer was supported by a Hannaford Research Fellowship. We thank Mrs A. Waters for practical assistance and Dr R. M. Taylor, CSIRO, for sample B.

R E F E R E N C E S

EWING F.J. (1935) Crystal structure of lepidocrocite. J. Chem. Phys. 3, 420~,24. FARMER V.C. (1980) Raman and i.r. spectra of boehmite (y-A100H) are consistent with D2h 16 or C2h 5

symmetry. Spectrochim. Aeta 36A, 585-586. FRIPIAT J.J., BOSMANS H. & ROUXHET P.G. (1967) Proton mobility in solids. I. Hydrogenic vibration modes

and proton delocalization in boehmite. J. Phys. Chem. 71, 1097-1112. HADNI A. (1974) The interaction of infrared radiation with crystals. Pp. 27-49 in: Thelnfrared Speetra of

Minerals (V. C. Farmer, editor). Mineralogical Society, London. PARFITT R.L. & RUSSELL J.D. (1977) Adsorption on hydrous oxides. IV. Mechanisms of adsorption on

various ions on goethite. J. Soil Sei. 28, 297-305. PARFITT R.L., RUSSELL J.D. & FARMER V.C. (1976) Confirmation of the surface structures of goethite

(a-FeOOH) and phosphated goethite by infrared spectroscopy: J. Chem. Soe., Faraday Trans. I 72, 1082-1087.

PARFIT'r R.L., FRASER A.R., RUSSELL J.D. & FARMER V.C. (1977) Adsorption on hydrous oxides. II. Oxalate, benzoate, and phosphate on gibbsite. J. Soil Sci. 28, 40-47.

RUSSELL J.D., PARFITT R.L., FRASER A.R. & FARMER V.C. (1974) Surface structures of gibbsite, goethite and phosphated goethite. Nature, 248, 220-221.

RUSSELL J.D., PATERSON E., FRASER A.R. & FARMER V.C. (1975) Adsorption of carbon dioxide on goethite (ct-FeOOH) surfaces, and its implications for anion adsorption. J. Chem. Soe., Faraday Trans. I, 71, 1623-1630.

TAYLOR R.M. (1984) Influence of chloride on the formation of iron oxides from Fe(II) chloride. I Effect of [CI] on the formation of lepidocrocite and crystallinity. Clays Clay Miner. 32, 175-180.