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Infrared spectroscopy of goethite dehydroxylation. II. Effect of aluminium substitution on the behaviour of hydroxyl units
Spectrochimica Acta Part A 58 (2002) 479– 491 www.elsevier.com/locate/saa
Infrared spectroscopy of goethite dehydroxylation. II. Effect of aluminium substitution on the behaviour of hydroxyl units H.D. Ruan *, R.L. Frost, J.T. Kloprogge, L. Duong Centre for Instrumental and De6elopmental Chemistry, Queensland Uni6ersity of Technology, GPO Box 2434, Brisbane, Qld 4001, Australia Received 30 May 2001; accepted 1 June 2001
Abstract Dehydroxylation of goethite as affected by aluminium substitution was investigated using Fourier transform infrared spectroscopy (FT–IR) in conjunction with X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTGA). The band intensities of hydroxyl vibrations were indicative of the degree of dehydroxylation and the changes in band parameters due to aluminium substitution were observed. The effect of aluminium substitution on band parameters of FT– IR spectra of goethite and its partially and fully dehydroxylated products, the mixture of goethite/hematite and hematite, were interpreted. The results of this study have confirmed that aluminium substituted goethite is thermally more stable than non-substituted goethite and is in harmony with the results of XRD and DTGA. A larger amount of non-stoichiometric hydroxyl units is associated with a higher aluminium substitution. A shift to a higher wavenumber of bending and hydroxyl stretching vibrations is attributed to the effects of aluminium substitution associated with non-stoichiometric hydroxyl units on the a–b plane relative to the b–c plane of goethite. The results provide information for the characterisation of activated bauxite containing hematite and goethite. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Aluminium substitution; Bauxite; Dehydroxylation; Goethite; Hematite; Hydroxyl units; Infrared spectroscopy
1. Introduction Most research on the goethite dehydroxylation process has focussed on non-substituted goethite * Corresponding author. Present address: Agronomy Department, 1150 Filly Hall, Purdue University, West Lafayette, IN 47907-1150, USA. Tel.: + 1-765-49488786; fax: +1-7654962926. E-mail address: [email protected] (H.D. Ruan).
whereas goethite in the natural environment often contains appreciable aluminium as an isomorphous substitution. It has been reported that the proportion of aluminium substitution to iron has resulted in the decrease in unit cell dimensions and the increase in hydroxyl units, particularly non-stoichiometric hydroxyl units in the goethite structure [1–5]. Dehydroxylation of goethite can be affected by aluminium substitution [2,6,7]. The retention of hydroxyl units within the porous
1386-1425/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 1 ) 0 0 5 5 6 - X
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structure of partially dehydroxylated goethite is an important control of the transformation to hematite [8,9]. Aluminium substitution in goethite is thus considered as an important factor in controlling the stability of the lattice, affecting the decomposition of the crystal structure including the libration of hydroxyl units, re-arrangement of oxygen packing and cation migration [7,8,10,11]. The structure of goethite is based on the hexagonal close packing of oxygen atoms with 6-fold coordinated Fe atoms occupying the octahedral position. The Fe atoms are arranged in a double row to form what can be described as double chains of octahedra, which run the length of the c-axis. Within the double chains in the b – c plane, all bonds are covalent with each octahedron sharing four of its edges with neighbouring octahedra. In contrast, bonding between double chains consists of relatively weak hydrogen bonds directed through apical oxygen ions that are directed along the a-axis [3,12,13]. In this case, stacking of double chains along the a-axis can be easily disrupted and this consequently induces structural defects, such as non-stoichiometric hydroxyl units that are incorporated into the goethite structure during crystal growth, and aluminium substitution in goethite is found to accelerate this induction [1,2,4,5,13,14]. According to the model proposed by Schulze [15], the amount of non-stoichiometric hydroxyl units incorporated into the goethite structure can be explained by one iron or aluminium ion being replaced by three hydrogen ions. The proportion of iron and aluminium ions replaced by three hydrogen ions is expressed by the formula: a −(Fe1 − y Aly )1 − x O1 − 3x (OH)1 + 3x
(1)
where x represents the fraction of iron and aluminium ions replaced by three hydrogen ions, and y is the mole fraction of aluminium substitution. Hydroxyl units are liberated when goethite alters to hematite during thermal dehydroxylation: 2[a − (Fe1 − y Aly )1 − x O1 − 3x (OH)1 + 3x ] (1−x)a-(Fe1 − y Aly )2O3 +(1 +3x)H2O
(2)
Goethites formed from the ferrous systems usually contain non-stoichiometric hydroxyl units, which increase as aluminium substitution increases
[1,3,13]. These goethites have lower dehydroxylation temperatures than those synthesised from the ferric systems [1,2,7]. Dehydroxylation of goethite results in the topological alteration to hematite. Substitution of aluminium in goethite not only influences the cation migration and oxygen packing arrangement but also favours the incorporation of non-stoichiometric hydroxyl units into the goethite structure. Those hydroxyl units remaining in the goethite structure during the process of dehydroxylation are found to transfer to the resultant hematite, and this type of hematite is called hydrohematite [16–18]. The behaviour of hydroxyl units of synthetic goethite and its resultant dehydroxylated product has been reported in previous work [1,2,10,18]. This paper deals with the effect of aluminium substitution on topological alteration of goethite to hematite based mainly on the Fourier transform infrared spectroscopic (FT–IR) study. It provides an understanding of how aluminium substitution influences the mineralogical and surface chemistry of the partially and fully dehydroxylated goethites. Furthermore, it provides information for the study of thermal activation of bauxite that contains hematite and goethite as major impurities.
2. Experimental The goethites with and without aluminium substitution were synthesised from a ferrous salt involving the oxidation of a FeCl2·4H2O solution buffered with bicarbonate. A detailed description of this synthetic goethite was published elsewhere [1,2,13]. Dehydroxylation of goethite was carried out by heating sub-samples of goethite in a muffle furnace for 1 h at temperatures between 180 and 270 °C. X-ray diffraction (XRD) analysis using Cu Ka radiation was carried out on a computer-controlled Sieray modification of a Philips 1050 diffractometer. Ten percent by weight of NaCl was added to sub-samples to provide an internal standard for spacing and line broadening measurements. Scan speed was 0.3° 2q per minute and the step size was 0.01° 2q. Patterns were recorded from 10 to 70° 2q.
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Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTGA) were carried out using a Perkin– Elmer TGS-2 instrument. Approximately 10 mg of goethite sample was heated in flowing air to 620 °C at a heating rate of 10 °C min − 1. The goethite sample was preheated at 110 °C until weight was constant to remove adsorbed water. FT –IR absorption spectra of goethite and dehydroxylated goethite were obtained using a Digilab FTS-20/80 FT –IR spectrometer, of samples dispersed in KBr disks. Spectra of 4 cm − 1 resolution were acquired by coaddition of 256 scans. The ratio of sample to KBr was 1:300. Spectral manipulation such as baseline adjustment, smoothing and normalisation was performed using the Spectracalc software package (Galactic Industries Corporation, NH, USA). Band component analysis was undertaken with the Jandel ‘‘Peakfit’’ software package, which enables the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was carried out using a Lorentz–Gauss cross-product function with the minimum number of component bands used for the fitting process. The Gauss– Lorentz ratio was maintained at values greater than 0.7, and fitting was undertaken until reproducible results were obtained with squared correlations of r 2 greater than 0.995. 3. Results and discussion
3.1. Changes in the hematite/(goethite + hematite) ratio and unit cell dimensions during dehydroxylation The ratio of hematite/(goethite + hematite) was
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calculated from the intensities of XRD diffraction lines (Table 1). The 110 line for goethite and the 102 line for hematite were used for this calculation. Table 1 shows that the extent of transformation of goethite to hematite increases as dehydroxylation temperature increases. Aluminium substitution in goethite has been reported to increase the dehydroxylation temperature [1,2,7,10,19]. XRD lines of hematite in the dehydroxylated goethite samples were observed from 200 °C for non-substituted, 210 °C for 10 and 20 mol%, and 220 °C for 30 mol% aluminium substituted goethites. No goethite diffraction line was detected at temperatures higher than 230 °C for non-substituted goethite, 250 °C for 10 and 20 mol%, and 260 °C for 30 mol% aluminium substituted goethites [1,13]. Thus the transformation of goethite to hematite was affected by aluminium substitution (Table 1). The unit cell parameters derived from the diffraction lines in Tables 2 and 3 show a systematic decrease in unit cell dimensions of goethite and the thermally resultant hematite with increasing aluminium substitution. The linear relationships are significant at either P\ 0.05 or P \ 0.01 level. This finding is consistent with those reported in the literature [1,3,4,19–21]. However, the effect of thermal dehydroxylation on the decrease in unit cell dimensions is more complicated together with the effect of aluminium substitution. The correlation matrix for goethite and hematite in Table 4 shows that the r values of the relationships between unit cell dimensions and unit cell volume, and dehydroxylation temperature vary greatly from 0.089 to 0.939. The effect of dehydroxylation temperature on the change in unit dimensions shows a trend a b c for the non-substituted goethite (Table 4). However, the c-dimension of goethite becomes more sensitive to dehydroxylation temperature
Table 1 The effect of aluminium substitution on the ratio of hematite/(goethite+hematite) derived from XRD lines Mol% Al
0 10 20 30
Dehydroxylation temperature (°C) 200
210
220
230
240
250
260
B0.01 – – –
0.05 B0.01 B0.01 –
0.4 0.12 0.07 0.01
1 0.65 0.5 0.2
1 0.9 0.85 0.6
1 1 1 0.8
1 1 1 1
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Table 2 Unit cell dimensions and unit cell volume of goethitea Deh. temp. (°C)
Mol% Al
a (A, )
b (A, )
c (A, )
UCV (A, 3)
110
0 10 20 30 r2 0 10 20 30 r2 0 10 20 30 r2 0 10 20 30 r2 0 10 20 30 r2
4.6359 7 4.6309 9 4.6249 12 4.6199 12 0.999** 4.633 9 7 4.6289 12 4.6249 13 4.6159 9 0.967* 4.6329 7 4.6229 9 4.6199 12 4.6129 8 0.960* 4.630 9 7 4.6229 6 4.6169 12 4.6109 10 0.995** 4.6279 7 4.6209 8 4.6139 10 4.6089 11 0.994**
9.944 94 9.914 95 9.867 9 7 9.862 9 7 0.929* 9.936 94 9.906 97 9.870 9 7 9.851 95 0.987** 9.936 9 4 9.908 9 5 9.865 9 7 9.854 9 4 0.929* 9.939 94 9.916 94 9.868 97 9.849 9 6 0.971* 9.939 94 9.909 9 4 9.870 96 9.841 96 0.997**
3.030 9 3 3.017 93 3.001 95 2.992 9 4 0.990** 3.029 93 3.016 95 2.999 9 5 2.986 94 0.997** 3.030 9 3 3.016 94 2.997 9 5 2.985 9 3 0.993** 3.028 93 3.011 9 2 2.996 95 2.982 9 4 0.998** 3.029 93 3.009 93 2.995 9 4 2.981 94 0.991**
139.654 138.486 136.921 136.293 0.977* 139.435 138.268 136.871 135.751 0.998** 139.451 138.117 136.563 135.658 0.990** 139.341 137.999 136.470 135.394 0.996** 139.296 137.751 136.363 135.180 0.997**
180
190
200
210
a Linear relationships between unit cell dimensions a, b, c and unit cell volume of goethite, and aluminium substitution are obtained. Significant at P= 0.05 or 0.01 level for a r 2 value is 0.903* and 0.980**, respectively.
with the effect of aluminium substitution (column 4 of Table 4). The effect of dehydroxylation temperature on both the a- and c-dimensions of non-substituted hematite is not significant (Table 4). As aluminium substitution increases from 10 to 30 mol%, more hydroxyl units are incorporated into the goethite structure and some hydroxyl units still remain in hematite up to a dehydroxylation temperature of 270 °C [1]. Thus, the c-dimension of hematite, which is the crystallographic equivalent to the a- dimension of goethite [1,18,22], becomes significant in relation to the dehydroxylation temperature (Table 4).
3.2. Effect of aluminium substitution on weight loss and temperature of dehydroxylation maximum It has been reported that non-stoichiometric hydroxyl units are incorporated into the structure of goethite and hematite [2,7,23–25], and this amount of non-stoichiometric hydroxyl units increases with increasing aluminium substitution in these minerals [1,4,5]. TGA results in Fig. 1a show that weight loss increased linearly as aluminium substitution increased during the thermal decomposition of goethite and the formation of
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hematite at the dehydroxylation maximum stage. Aluminium substitution has been found to favour the incorporated hydroxyl units into the goethite structure during the process of synthesis [7,14,20]. This is why greater amounts of non-stoichiometric hydroxyl units are associated with synthetic aluminium substituted goethite than with non-substituted goethite [1,3,4]. This is because the Al3 + ion retains coordinated hydroxyl units more strongly than Fe3 + , due to the higher ionic potential of Al3 + in terms of its smaller ionic radius [1]. Consequently the temperature at dehydroxylation maximum of the aluminium substituted goethite is higher than that of the non-substituted one (Fig. 1b) [2,7,9]. It has been reported that aluminium ions, which replaced Fe3 + ions in the lattice of goethite, accelerate the protonation of more than the stoichiometric amount of oxygens in the structure, retaining the basic hexagonal close-packed
483
oxygen lattice, but replacing some of the hydrogen bonds by opposing hydroxyl units [15].
3.3. Effect of aluminium substitution on FT–IR spectra of goethite and hematite The effect of aluminium substitution on the FT –IR spectra of goethite, partially dehydroxylated goethite and hematite are compared in Fig. 2. Attention is particularly drawn to the vibrations at the regions of hydroxyl stretching, and hydroxyl deformation and water bending regions. To accurately manipulate the overlapped bands, the Peakfit software was employed. The resolved band positions are illustrated in Fig. 3. One band at 3216 cm − 1 of the hydroxyl stretching region and two bands at 897 and 801 cm − 1 of the hydroxyl deformation and water bending region are indicative of the liberation of hydroxyl units
Table 3 Unit cell dimensions and unit cell volume of hematitea Deh. temp. (°C)
Mol% Al
a (A, )
c (A, )
UCV (A, 3)
230
0 10 20 30 r2 0 10 20 30 r2 0 10 20 30 r2 0 10 20 30 r2 0 10 20 30 r2
5.040 95 5.027 95 5.0159 6 5.00296 0.999*** 5.03997 5.026 97 5.0119 5 5.00196 0.994** 5.037 96 5.023 96 5.012 96 5.0019 8 0.996** 5.03697 5.02196 5.010 96 5.00096 0.991** 5.0359 5 5.021 95 5.009 97 4.999 9 6 0.995**
13.787 98 13.782 98 13.773 912 13.762 911 0.975* 13.786 913 13.768 917 13.747 96 13.722 9 11 0.995** 13.777 9 11 13.753 9 10 13.707 9 9 13.682 917 0.983** 13.775 913 13.749 99 13.703 9 9 13.673 99 0.989** 13.761 9 5 13.740 97 13.679 914 13.664 910 0.942*
303.283 301.612 299.977 298.186 0.999*** 303.142 301.185 298.934 297.200 0.998** 302.703 300.498 298.183 296.334 0.998** 302.529 300.171 297.858 296.020 0.997** 302.111 299.834 297.217 295.707 0.990**
240
250
260
270
a Linear relationships between unit cell dimensions a, c and unit cell volume of hematite, and aluminium substitution are obtained. Significant at P = 0.05 or 0.01 level for a r 2 value is 0.903* and 0.980**, respectively.
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Table 4 Correlation matrix (r 2 values) for unit cell parameters of goethite and hematite as affected by dehydroxylation temperature Mol% Al
0 10 20 30
Goethite
Hematite
a
b
c
UCV
a
c
UCV
0.706 0.752 0.581 0.877*
0.542 0.093 0.089 0.791*
0.346 0.507 0.860* 0.950**
0.939* 0.794* 0.676 0.921**
0.757 0.714 0.899* 0.726
0.526 0.826* 0.818* 0.898*
0.668 0.791* 0.888* 0.901*
* Significant at P50.05 level. ** Significant at P50.01 level.
from the goethite structure. The intensities of these bands decreased as dehydroxylation temperature increased (Fig. 2). For the 0 and 10 mol% aluminium substituted goethites, the 897 and 801 cm − 1 bands completely disappeared at a dehydroxylation temperature of 230 °C. These bands disappeared at 240 °C for the 20 mol% and at 250 °C for the 30 mol% aluminium substituted goethite, respectively (Fig. 2). However, the residual band at 3216 cm − 1 for goethite, which shifted to 3227 cm − 1 after goethite altered to hematite, was observed upon heating at 270 °C (Fig. 3d) although the intensity of this band decreased with increasing dehydroxylation temperature (Fig. 2). The trend of decrease in intensity of this band is non-substituted \10 mol%\20 mol% \30 mole% aluminium substituted hematite (Table 5). The change in band intensity at the hydroxyl stretching region is more complicated, thus reflecting the behaviour of stoichiometric and non-stoichiometric hydroxyl units as affected by both aluminium substitution and thermal dehydroxylation. The spectrum of unheated goethite shows bands of stoichiometric and non-stoichiometric hydroxyl units at 3233– 3206 and 3476– 3450 cm − 1, respectively (Fig. 4). It is noted that the band intensity at 3476–3450 cm − 1 increased relatively as aluminium substitution increased (Fig. 4), indicating that the increase in the amount of non-stoichiometric hydroxyl units was due to aluminium substitution. This is consistent with the results of XRD and thermal analysis as discussed earlier. By comparing spectra of the dehydroxylated samples with the non-dehydroxylated sample (110 °C), intensity of the non-stoichiometric hydroxyl band
at 3476–3450 cm − 1 decreased quicker than the stoichiometric hydroxyl band at 3233–3206 cm − 1 at a dehydroxylation temperature range of 180– 190 °C for the 0 and 10 mol% aluminium substituted goethite (Fig. 2a and b), whereas this change was less clearly shown by the higher levels of aluminium substituted goethite. Hydroxyl units released at such low temperatures (180– 190 °C) mostly represent the ‘‘loosely bound’’ hydroxyl
Fig. 1. Weight loss at TGA dehydroxylation maximum (a), and dehydroxylation maximum temperature of DTGA (b) as a function of aluminium substitution.
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Fig. 2. FTIR spectra of goethite, (a) 0 mol%, (b) 10 mol%, (c) 20 mol%, and (d) 30 mol% aluminium substitution, respectively.
units in the goethite structure [1,2,16,17]. However, this amount of hydroxyl units is bound more tightly in the higher aluminium (20– 30 mole%) substituted goethites than in the lower (10 mol%) or zero (0 mol%) aluminium substituted goethites [2,10,17]. Further decrease in band intensity and shift in band centre from 3233– 3206 cm − 1 to 3240– 3232 cm − 1 is indicative of the transformation of structure from goethite to hematite while the remaining 3434 cm − 1 band in the newly formed hematite (Fig. 3d) is the vibration of non-stoichiometric hydroxyl units that transferred directly from goethite to hematite. This structural alteration indicated by hydroxyl units is obviously observed only in the FT– IR spectra compared with the XRD technique. The decomposition of goethite and the formation of hematite are clearly illustrated in Fig. 2, indicating that some goethite bands disappeared
and hematite bands started to form eventually. The newly formed hematite bands were observed at 210 °C for the 0 mol%, 220 °C for the 10 and 20 mol%, and 230 °C for the 30 mol% aluminium substituted goethite, respectively. Some goethite and hematite bands overlapped at these dehydroxylation temperatures. The well resolved bands for hematite were recorded at 220 °C for the 0 mol%, 230 °C for the 10 and 20 mol%, and 240 °C for the 30 mol% aluminium substituted goethite, respectively (Fig. 2). Two typical bands at 540 and 452 cm − 1 (Fig. 3f) were observed and the intensities of these bands increased as dehydroxylation temperature increased (Fig. 2). This type of hematite has been reported to contain about 3–5% non-stoichiometric hydroxyl units [1,2,19]. The band at 3462 cm − 1 (Fig. 3a) in the hydroxyl stretching region remained (3434 cm − 1) after the completion of goethite–hematite
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transformation, followed by heating the samples up to 270 °C (Fig. 3d). This indicates that goethite altered to hematite without intermediate phases and the newly formed hematite inherited the remaining non-stoichiometric hydroxyl units from
the precursor goethite. A decreasing trend in intensity of this band indicated that the liberation of non-stoichiometric hydroxyl units from the hematite was continuing towards the progress of thermal dehydroxylation.
Fig. 3. Band component analysis of 10 mol% aluminium substituted goethite (110 °C, a– c) and its resultant hematite(270 °C, d– f).
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Table 5 Selected band component analysis of the infrared absorption spectra of hematite (270 °C) as affected by aluminium substitution Mol% Al w(HOH) 0 10 20 30 w(OH) 0 10 20 30 w[Fe(Al)O] 0 10 20 30
Band centre (cm−1)
Band intensity (% absorbance)
Band width (cm−1)
3435 3434 3448 3453
0.169 0.212 0.314 0.324
225 227 232 234
3232 3227 3239 3240
0.099 0.141 0.212 0.222
345 361 363 373
533 540 543 545
0.235 0.187 0.169 0.170
It has been reported that the dehydroxylation temperatures of aluminium oxy-hydroxides are higher than those of corresponding isostructural iron oxy-hydroxides [1,2,10,13]. For example, the dehydroxylation temperature of boehmite (gAlOOH) is 450–500 °C, and for isostructural lepidocrocite (g-FeOOH) is 230– 280 °C. Similarly, diaspore (a-AlOOH) dehydroxylates at 470– 500 °C compared with isostructural goethite (a-FeOOH) that dehydroxylates at 230– 280 °C [26–28]. One of the significant changes in goethite spectra affected by aluminium substitution is the shift of band positions. Fig. 5 shows the goethite band centres at 3450, 3206, 1413, 1269, 888, 798, and 461 cm − 1 shifted to higher positions as aluminium substitution increased. It is known that the goethite octahedra occupied by Al3 + are smaller than those occupied by Fe3 + so that average size of the unit cell and unit cell volume decreased (Tables 2 and 3) [1,3,18]. Schulze [15] has found that the shortening of the b- and c-dimensions caused a shortening and strengthening of the hydrogen bond, leading to a shift in the hydroxyl deformation and water bending vibrations to larger wavenumbers and this shift is observed in this work (Figs. 5 and 6). On the other hand, the influence of non-stoichiometric hydroxyl units was in many ways opposite to the influence of aluminium substitution
55.5 64.1 81.3 92.4
[1]. As the number of non-stoichiometric hydroxyl units increased, some of the hydrogen bonds were replaced by opposing hydroxyl units [15,18]. The opposing hydroxyl units tend to cause dilatation of the a- and b-dimensions because the main component of the hydrogen bond and of the opposing hydroxyl units is in the a– b plane [15].
Fig. 4. Changes in hydroxyl stretching bands of unheated goethite (110 °C) as affected by aluminium substitution.
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Fig. 5. Shift of the goethite band centre as a function of aluminium substitution.
The largest dilatation was found in the a-dimension for the present samples due to the influence of opposing hydroxyl units. The bands shifted to higher wavenumbers (Fig. 5) in terms of the increase in the a-dimension resulting in the hydrogen bond to become longer. It has been reported that the energy required to break down an AlOH bond is higher than for an
FeOH bond [2,10]. This explains why Al ions may specifically retain hydroxyl units more steadily than Fe ions in the structure of goethite and thus the aluminium substituted goethite is thermally more stable than the non-substituted goethite. It has been confirmed by previous studies and this study that the dehydroxylation temperature of aluminium substituted goethite is
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higher than non-substituted goethite [2,5,7,13,15,26]. Since a high energy Al-bond may combine more hydroxyl units than a low energy Fe-bond, hydroxyl units in solution can be favoured by aluminium ions to be incorporated into goethite structure during the process of synthesis [5,14]. However, a decreasing trend in the goethite band centre at 619 cm − 1 with increasing aluminium substitution was observed (Fig. 5g). This trend is attributed to the rearrangement of cation and oxygen packing in the goethite structure after aluminium ion replaced the iron ion position elongating the b–c plane, resulting in a decrease in the AlO bonding distance. Some Al-bonds may become shorter than Fe-bonds due to this rearrangement. Other goethite band centres
Fig. 6. Shift of the hematite band centre as a function of aluminium substitution.
489
shown in Fig. 3 did not change systematically and significantly as affected by aluminium substitution, possibly due to the opposite effect of nonstoichiometric hydroxyl units. Fig. 6 shows that the hematite band centre increases as aluminium substitution increases. The band at 545– 533 cm − 1 in the low frequency region is indicative of the newly formed hematite and is reported as the ‘‘fingerprint’’ of hematite [15,18,29–31]. The band at 1340–1324 cm − 1 observed here was not reported in most of the previous work. This is due to the present samples containing non-stoichiometric hydroxyl units, which were synthesized from the ferrous system compared with those from the ferric system or natural samples. There was a 16, 19 and 12 cm − 1 increase in band centre due to aluminium substitution for the vibrations at 1324, 589 and 533 cm − 1, respectively (Fig. 6). These linear relationships again indicated that the effect of aluminium substitution on hematite is similar to its precursor goethite. No systematic change in band centre regarding aluminium substitution was observed for the hematite band at 3434, 3227, 1633, 1526, and 452 cm − 1 (Fig. 3). The bands at 3434 and 1633 cm − 1 were assigned as vibrations of the non-stoichiometric hydroxyl units [16,18], and this type of hematite is classified as hydrohematite [16–18,32]. Two bands at 3453–3434 and 3240–3227 cm − 1 of the hydroxyl stretching region were well characterized for hematite formed under the low dehydroxylation temperatures between 230–270°C. The intensities of these two bands decreased systematically as aluminium substitution decreased, indicating that the retention of hydroxyl units in the hematite structure was strongly related to the influence of AlOH bonding. In contrast, there is no systematic change in band intensity at the low frequency region regarding Fe(Al)O bonding (Table 5). It has been reported that aluminium substitution in goethite and hematite has resulted in the decrease in crystal size [1,4,7,19]. This function has been well characterised by the linear increase in band width (r 2 = 0.966, 0.918, and 0.985 for the bands at 3453–3434, 3240–3227, and 545–533 cm − 1, respectively) as shown in the last column of Table 5.
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4. Conclusions
Acknowledgements
The effect of aluminium substitution on the changes in band centre, width and intensity of goethite and hematite are well characterised using FT –IR spectroscopy. The shift of goethite band centre to a higher wavenumber in the hydroxyl stretching region is attributed to the dilatation of hydrogen bonds in the a – b plane, and in the hydroxyl deformation and water bending region is attributed to the shortening and strength of the hydrogen bonds in terms of the shortening of the b- and c-dimensions. A shift of the goethite band centre from 619 to 584 cm − 1 as aluminium substitution increased from 0 to 30 mol% reflects the shortening of AlO bonds relative to FeO bonds elongating the b –c plane. The linear relationships between aluminium substitution, and band centre and width are observed for both goethite and hematite. The decrease in band intensity resulted from the increase of dehydroxylation temperature is due to the liberation of stoichiometric and non-stoichiometric hydroxyl units. The effect of aluminium substitution has been found not only to favour hydroxyl units to incorporate into, but also to remain hydroxyl units in the goethite structure at a higher temperature. Thus, the aluminium substituted goethite is thermally more stable than the non-substituted goethite. The hematite formed from thermal dehydroxylation of goethite at low temperatures contains non-stoichiometric hydroxyl units. The vibrations of these non-stoichiometric hydroxyl units are the ‘‘fingerprints’’ for the characterisation of ‘‘hydrohematite’’. Additional work will be deserved for in situ study of goethite dehydroxylation in order to avoid the readsorption of hydroxyl units onto heated goethite samples and the study of goethite dehydroxylation at higher (300–1000 °C) temperatures to investigate the behaviour of non-stoichiometric hydroxyl units in hydrohematite. Since bauxite contains about 15– 20% hydrohematite and goethite, this study may provide information to distinguish the phase transformation in activated bauxite.
The financial support of Comalco Pty Ltd and Australian Research Council, and the infrastructure of the Centre for Instrumental and Developmental Chemistry, Queensland University of Technology are gratefully acknowledged.
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H.D. Ruan, R.J. Gilkes, Clays Clay Miner. 43 (1995) 196. H.D. Ruan, R.J. Gilkes, J. Therm. Anal. 46 (1996) 1223. D.G. Schulze, Clays Clay Miner. 32 (1984) 36. D.G. Schulze, U. Schwertmann, Clay Miner. 19 (1984) 521. D.G. Schulze, U. Schwertmann, Clay Miner. 22 (1987) 83. K. Jonas, K. Solynar, Acta Chim. Acad. Sci. Hung. 66 (1970) 383. M.B. Fey, J.B. Dixon, Clays Clay Miner. 23 (1981) 33. H. Naono, R. Fujuwara, J. Colloid Interface Sci. 73 (1980) 406. C.J. Goss, Miner. Mag. 51 (1987) 437. T. Grygar, H.D. Ruan, R.J. Gilkes, J. Therm. Anal. Calorimetry 55 (1999) 301. I. Rozenson, L. Heller-Kallai, Clays Clay Miner. 28 (1980) 391. M.H. Francombe, H.P. Rooksby, Clay Miner. Bull. 21 (1959) 1. H.D. Ruan, in: Department of Soil Science and Plant Nutrition, The University of Western Australia, Perth, 1995, p. 158. D.G. Lewis, U. Schwertmann, J. Colloid Interface Sci. 78 (1980) 543. D.G. Schulze, in: Lehrstuhl fur Bodenkunde der Technischen Universitat Munchen in Weihenstephan, Weihenstephan, 1982, p. 167. E. Wolska, W. Szajda, J. Mater. Sci. 20 (1985) 4407. E. Wolska, U. Schwertmann, Z. Kristallogr. 189 (1989) 223. H.D. Ruan, Frost, R.L., Kloprogge, J.T., Spectrochim. Acta Part A (2002) in press. H. Stanjek, U. Schwertmann, Clays Clay Miner. 40 (1992) 347. D.G. Lewis, U. Schwertmann, Clays Clay Miner. 27 (1979) 195. P.G. Frazey, B.H. O’Connor, L.C. Hammond, Clays Clay Miner. 39 (1991) 248. O. Ozdemir, D.J. Dunlop, Earth Planet. Sci. Lett. 177 (2000) 59. J.B. Pollack, D. Pitmen, B.N. Khare, C. Sagan, J. Geophys. Res. 75 (1970) 7841. R.R. Anand, R.J. Gilkes, Geoderma 34 (1984) 261.
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[29] H.W. van der Marel, H. Beutelspacher, Atlas of Infrared Spectroscopy of Clay Minerals and their Admixtures, Elsevier, Amsterdam, The Netherlands, 1976. [30] E. Schwerzmann, H. Sparr, Z. Naturforsch. 23b (1968) 767. [31] E. Schwerzmann, H. Sparr, Z. Naturforsch. 24b (1969) 8. [32] E. Wolska, Z. Kristallogr. 154 (1981) 69.
Infrared spectroscopy of goethite dehydroxylation. II. Effect of aluminium substitution on the behaviour of hydroxyl units H.D. Ruan *, R.L. Frost, J.T. Kloprogge, L. Duong Centre for Instrumental and De6elopmental Chemistry, Queensland Uni6ersity of Technology, GPO Box 2434, Brisbane, Qld 4001, Australia Received 30 May 2001; accepted 1 June 2001
Abstract Dehydroxylation of goethite as affected by aluminium substitution was investigated using Fourier transform infrared spectroscopy (FT–IR) in conjunction with X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTGA). The band intensities of hydroxyl vibrations were indicative of the degree of dehydroxylation and the changes in band parameters due to aluminium substitution were observed. The effect of aluminium substitution on band parameters of FT– IR spectra of goethite and its partially and fully dehydroxylated products, the mixture of goethite/hematite and hematite, were interpreted. The results of this study have confirmed that aluminium substituted goethite is thermally more stable than non-substituted goethite and is in harmony with the results of XRD and DTGA. A larger amount of non-stoichiometric hydroxyl units is associated with a higher aluminium substitution. A shift to a higher wavenumber of bending and hydroxyl stretching vibrations is attributed to the effects of aluminium substitution associated with non-stoichiometric hydroxyl units on the a–b plane relative to the b–c plane of goethite. The results provide information for the characterisation of activated bauxite containing hematite and goethite. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Aluminium substitution; Bauxite; Dehydroxylation; Goethite; Hematite; Hydroxyl units; Infrared spectroscopy
1. Introduction Most research on the goethite dehydroxylation process has focussed on non-substituted goethite * Corresponding author. Present address: Agronomy Department, 1150 Filly Hall, Purdue University, West Lafayette, IN 47907-1150, USA. Tel.: + 1-765-49488786; fax: +1-7654962926. E-mail address: [email protected] (H.D. Ruan).
whereas goethite in the natural environment often contains appreciable aluminium as an isomorphous substitution. It has been reported that the proportion of aluminium substitution to iron has resulted in the decrease in unit cell dimensions and the increase in hydroxyl units, particularly non-stoichiometric hydroxyl units in the goethite structure [1–5]. Dehydroxylation of goethite can be affected by aluminium substitution [2,6,7]. The retention of hydroxyl units within the porous
1386-1425/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 1 ) 0 0 5 5 6 - X
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structure of partially dehydroxylated goethite is an important control of the transformation to hematite [8,9]. Aluminium substitution in goethite is thus considered as an important factor in controlling the stability of the lattice, affecting the decomposition of the crystal structure including the libration of hydroxyl units, re-arrangement of oxygen packing and cation migration [7,8,10,11]. The structure of goethite is based on the hexagonal close packing of oxygen atoms with 6-fold coordinated Fe atoms occupying the octahedral position. The Fe atoms are arranged in a double row to form what can be described as double chains of octahedra, which run the length of the c-axis. Within the double chains in the b – c plane, all bonds are covalent with each octahedron sharing four of its edges with neighbouring octahedra. In contrast, bonding between double chains consists of relatively weak hydrogen bonds directed through apical oxygen ions that are directed along the a-axis [3,12,13]. In this case, stacking of double chains along the a-axis can be easily disrupted and this consequently induces structural defects, such as non-stoichiometric hydroxyl units that are incorporated into the goethite structure during crystal growth, and aluminium substitution in goethite is found to accelerate this induction [1,2,4,5,13,14]. According to the model proposed by Schulze [15], the amount of non-stoichiometric hydroxyl units incorporated into the goethite structure can be explained by one iron or aluminium ion being replaced by three hydrogen ions. The proportion of iron and aluminium ions replaced by three hydrogen ions is expressed by the formula: a −(Fe1 − y Aly )1 − x O1 − 3x (OH)1 + 3x
(1)
where x represents the fraction of iron and aluminium ions replaced by three hydrogen ions, and y is the mole fraction of aluminium substitution. Hydroxyl units are liberated when goethite alters to hematite during thermal dehydroxylation: 2[a − (Fe1 − y Aly )1 − x O1 − 3x (OH)1 + 3x ] (1−x)a-(Fe1 − y Aly )2O3 +(1 +3x)H2O
(2)
Goethites formed from the ferrous systems usually contain non-stoichiometric hydroxyl units, which increase as aluminium substitution increases
[1,3,13]. These goethites have lower dehydroxylation temperatures than those synthesised from the ferric systems [1,2,7]. Dehydroxylation of goethite results in the topological alteration to hematite. Substitution of aluminium in goethite not only influences the cation migration and oxygen packing arrangement but also favours the incorporation of non-stoichiometric hydroxyl units into the goethite structure. Those hydroxyl units remaining in the goethite structure during the process of dehydroxylation are found to transfer to the resultant hematite, and this type of hematite is called hydrohematite [16–18]. The behaviour of hydroxyl units of synthetic goethite and its resultant dehydroxylated product has been reported in previous work [1,2,10,18]. This paper deals with the effect of aluminium substitution on topological alteration of goethite to hematite based mainly on the Fourier transform infrared spectroscopic (FT–IR) study. It provides an understanding of how aluminium substitution influences the mineralogical and surface chemistry of the partially and fully dehydroxylated goethites. Furthermore, it provides information for the study of thermal activation of bauxite that contains hematite and goethite as major impurities.
2. Experimental The goethites with and without aluminium substitution were synthesised from a ferrous salt involving the oxidation of a FeCl2·4H2O solution buffered with bicarbonate. A detailed description of this synthetic goethite was published elsewhere [1,2,13]. Dehydroxylation of goethite was carried out by heating sub-samples of goethite in a muffle furnace for 1 h at temperatures between 180 and 270 °C. X-ray diffraction (XRD) analysis using Cu Ka radiation was carried out on a computer-controlled Sieray modification of a Philips 1050 diffractometer. Ten percent by weight of NaCl was added to sub-samples to provide an internal standard for spacing and line broadening measurements. Scan speed was 0.3° 2q per minute and the step size was 0.01° 2q. Patterns were recorded from 10 to 70° 2q.
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Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTGA) were carried out using a Perkin– Elmer TGS-2 instrument. Approximately 10 mg of goethite sample was heated in flowing air to 620 °C at a heating rate of 10 °C min − 1. The goethite sample was preheated at 110 °C until weight was constant to remove adsorbed water. FT –IR absorption spectra of goethite and dehydroxylated goethite were obtained using a Digilab FTS-20/80 FT –IR spectrometer, of samples dispersed in KBr disks. Spectra of 4 cm − 1 resolution were acquired by coaddition of 256 scans. The ratio of sample to KBr was 1:300. Spectral manipulation such as baseline adjustment, smoothing and normalisation was performed using the Spectracalc software package (Galactic Industries Corporation, NH, USA). Band component analysis was undertaken with the Jandel ‘‘Peakfit’’ software package, which enables the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was carried out using a Lorentz–Gauss cross-product function with the minimum number of component bands used for the fitting process. The Gauss– Lorentz ratio was maintained at values greater than 0.7, and fitting was undertaken until reproducible results were obtained with squared correlations of r 2 greater than 0.995. 3. Results and discussion
3.1. Changes in the hematite/(goethite + hematite) ratio and unit cell dimensions during dehydroxylation The ratio of hematite/(goethite + hematite) was
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calculated from the intensities of XRD diffraction lines (Table 1). The 110 line for goethite and the 102 line for hematite were used for this calculation. Table 1 shows that the extent of transformation of goethite to hematite increases as dehydroxylation temperature increases. Aluminium substitution in goethite has been reported to increase the dehydroxylation temperature [1,2,7,10,19]. XRD lines of hematite in the dehydroxylated goethite samples were observed from 200 °C for non-substituted, 210 °C for 10 and 20 mol%, and 220 °C for 30 mol% aluminium substituted goethites. No goethite diffraction line was detected at temperatures higher than 230 °C for non-substituted goethite, 250 °C for 10 and 20 mol%, and 260 °C for 30 mol% aluminium substituted goethites [1,13]. Thus the transformation of goethite to hematite was affected by aluminium substitution (Table 1). The unit cell parameters derived from the diffraction lines in Tables 2 and 3 show a systematic decrease in unit cell dimensions of goethite and the thermally resultant hematite with increasing aluminium substitution. The linear relationships are significant at either P\ 0.05 or P \ 0.01 level. This finding is consistent with those reported in the literature [1,3,4,19–21]. However, the effect of thermal dehydroxylation on the decrease in unit cell dimensions is more complicated together with the effect of aluminium substitution. The correlation matrix for goethite and hematite in Table 4 shows that the r values of the relationships between unit cell dimensions and unit cell volume, and dehydroxylation temperature vary greatly from 0.089 to 0.939. The effect of dehydroxylation temperature on the change in unit dimensions shows a trend a b c for the non-substituted goethite (Table 4). However, the c-dimension of goethite becomes more sensitive to dehydroxylation temperature
Table 1 The effect of aluminium substitution on the ratio of hematite/(goethite+hematite) derived from XRD lines Mol% Al
0 10 20 30
Dehydroxylation temperature (°C) 200
210
220
230
240
250
260
B0.01 – – –
0.05 B0.01 B0.01 –
0.4 0.12 0.07 0.01
1 0.65 0.5 0.2
1 0.9 0.85 0.6
1 1 1 0.8
1 1 1 1
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482
Table 2 Unit cell dimensions and unit cell volume of goethitea Deh. temp. (°C)
Mol% Al
a (A, )
b (A, )
c (A, )
UCV (A, 3)
110
0 10 20 30 r2 0 10 20 30 r2 0 10 20 30 r2 0 10 20 30 r2 0 10 20 30 r2
4.6359 7 4.6309 9 4.6249 12 4.6199 12 0.999** 4.633 9 7 4.6289 12 4.6249 13 4.6159 9 0.967* 4.6329 7 4.6229 9 4.6199 12 4.6129 8 0.960* 4.630 9 7 4.6229 6 4.6169 12 4.6109 10 0.995** 4.6279 7 4.6209 8 4.6139 10 4.6089 11 0.994**
9.944 94 9.914 95 9.867 9 7 9.862 9 7 0.929* 9.936 94 9.906 97 9.870 9 7 9.851 95 0.987** 9.936 9 4 9.908 9 5 9.865 9 7 9.854 9 4 0.929* 9.939 94 9.916 94 9.868 97 9.849 9 6 0.971* 9.939 94 9.909 9 4 9.870 96 9.841 96 0.997**
3.030 9 3 3.017 93 3.001 95 2.992 9 4 0.990** 3.029 93 3.016 95 2.999 9 5 2.986 94 0.997** 3.030 9 3 3.016 94 2.997 9 5 2.985 9 3 0.993** 3.028 93 3.011 9 2 2.996 95 2.982 9 4 0.998** 3.029 93 3.009 93 2.995 9 4 2.981 94 0.991**
139.654 138.486 136.921 136.293 0.977* 139.435 138.268 136.871 135.751 0.998** 139.451 138.117 136.563 135.658 0.990** 139.341 137.999 136.470 135.394 0.996** 139.296 137.751 136.363 135.180 0.997**
180
190
200
210
a Linear relationships between unit cell dimensions a, b, c and unit cell volume of goethite, and aluminium substitution are obtained. Significant at P= 0.05 or 0.01 level for a r 2 value is 0.903* and 0.980**, respectively.
with the effect of aluminium substitution (column 4 of Table 4). The effect of dehydroxylation temperature on both the a- and c-dimensions of non-substituted hematite is not significant (Table 4). As aluminium substitution increases from 10 to 30 mol%, more hydroxyl units are incorporated into the goethite structure and some hydroxyl units still remain in hematite up to a dehydroxylation temperature of 270 °C [1]. Thus, the c-dimension of hematite, which is the crystallographic equivalent to the a- dimension of goethite [1,18,22], becomes significant in relation to the dehydroxylation temperature (Table 4).
3.2. Effect of aluminium substitution on weight loss and temperature of dehydroxylation maximum It has been reported that non-stoichiometric hydroxyl units are incorporated into the structure of goethite and hematite [2,7,23–25], and this amount of non-stoichiometric hydroxyl units increases with increasing aluminium substitution in these minerals [1,4,5]. TGA results in Fig. 1a show that weight loss increased linearly as aluminium substitution increased during the thermal decomposition of goethite and the formation of
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hematite at the dehydroxylation maximum stage. Aluminium substitution has been found to favour the incorporated hydroxyl units into the goethite structure during the process of synthesis [7,14,20]. This is why greater amounts of non-stoichiometric hydroxyl units are associated with synthetic aluminium substituted goethite than with non-substituted goethite [1,3,4]. This is because the Al3 + ion retains coordinated hydroxyl units more strongly than Fe3 + , due to the higher ionic potential of Al3 + in terms of its smaller ionic radius [1]. Consequently the temperature at dehydroxylation maximum of the aluminium substituted goethite is higher than that of the non-substituted one (Fig. 1b) [2,7,9]. It has been reported that aluminium ions, which replaced Fe3 + ions in the lattice of goethite, accelerate the protonation of more than the stoichiometric amount of oxygens in the structure, retaining the basic hexagonal close-packed
483
oxygen lattice, but replacing some of the hydrogen bonds by opposing hydroxyl units [15].
3.3. Effect of aluminium substitution on FT–IR spectra of goethite and hematite The effect of aluminium substitution on the FT –IR spectra of goethite, partially dehydroxylated goethite and hematite are compared in Fig. 2. Attention is particularly drawn to the vibrations at the regions of hydroxyl stretching, and hydroxyl deformation and water bending regions. To accurately manipulate the overlapped bands, the Peakfit software was employed. The resolved band positions are illustrated in Fig. 3. One band at 3216 cm − 1 of the hydroxyl stretching region and two bands at 897 and 801 cm − 1 of the hydroxyl deformation and water bending region are indicative of the liberation of hydroxyl units
Table 3 Unit cell dimensions and unit cell volume of hematitea Deh. temp. (°C)
Mol% Al
a (A, )
c (A, )
UCV (A, 3)
230
0 10 20 30 r2 0 10 20 30 r2 0 10 20 30 r2 0 10 20 30 r2 0 10 20 30 r2
5.040 95 5.027 95 5.0159 6 5.00296 0.999*** 5.03997 5.026 97 5.0119 5 5.00196 0.994** 5.037 96 5.023 96 5.012 96 5.0019 8 0.996** 5.03697 5.02196 5.010 96 5.00096 0.991** 5.0359 5 5.021 95 5.009 97 4.999 9 6 0.995**
13.787 98 13.782 98 13.773 912 13.762 911 0.975* 13.786 913 13.768 917 13.747 96 13.722 9 11 0.995** 13.777 9 11 13.753 9 10 13.707 9 9 13.682 917 0.983** 13.775 913 13.749 99 13.703 9 9 13.673 99 0.989** 13.761 9 5 13.740 97 13.679 914 13.664 910 0.942*
303.283 301.612 299.977 298.186 0.999*** 303.142 301.185 298.934 297.200 0.998** 302.703 300.498 298.183 296.334 0.998** 302.529 300.171 297.858 296.020 0.997** 302.111 299.834 297.217 295.707 0.990**
240
250
260
270
a Linear relationships between unit cell dimensions a, c and unit cell volume of hematite, and aluminium substitution are obtained. Significant at P = 0.05 or 0.01 level for a r 2 value is 0.903* and 0.980**, respectively.
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484
Table 4 Correlation matrix (r 2 values) for unit cell parameters of goethite and hematite as affected by dehydroxylation temperature Mol% Al
0 10 20 30
Goethite
Hematite
a
b
c
UCV
a
c
UCV
0.706 0.752 0.581 0.877*
0.542 0.093 0.089 0.791*
0.346 0.507 0.860* 0.950**
0.939* 0.794* 0.676 0.921**
0.757 0.714 0.899* 0.726
0.526 0.826* 0.818* 0.898*
0.668 0.791* 0.888* 0.901*
* Significant at P50.05 level. ** Significant at P50.01 level.
from the goethite structure. The intensities of these bands decreased as dehydroxylation temperature increased (Fig. 2). For the 0 and 10 mol% aluminium substituted goethites, the 897 and 801 cm − 1 bands completely disappeared at a dehydroxylation temperature of 230 °C. These bands disappeared at 240 °C for the 20 mol% and at 250 °C for the 30 mol% aluminium substituted goethite, respectively (Fig. 2). However, the residual band at 3216 cm − 1 for goethite, which shifted to 3227 cm − 1 after goethite altered to hematite, was observed upon heating at 270 °C (Fig. 3d) although the intensity of this band decreased with increasing dehydroxylation temperature (Fig. 2). The trend of decrease in intensity of this band is non-substituted \10 mol%\20 mol% \30 mole% aluminium substituted hematite (Table 5). The change in band intensity at the hydroxyl stretching region is more complicated, thus reflecting the behaviour of stoichiometric and non-stoichiometric hydroxyl units as affected by both aluminium substitution and thermal dehydroxylation. The spectrum of unheated goethite shows bands of stoichiometric and non-stoichiometric hydroxyl units at 3233– 3206 and 3476– 3450 cm − 1, respectively (Fig. 4). It is noted that the band intensity at 3476–3450 cm − 1 increased relatively as aluminium substitution increased (Fig. 4), indicating that the increase in the amount of non-stoichiometric hydroxyl units was due to aluminium substitution. This is consistent with the results of XRD and thermal analysis as discussed earlier. By comparing spectra of the dehydroxylated samples with the non-dehydroxylated sample (110 °C), intensity of the non-stoichiometric hydroxyl band
at 3476–3450 cm − 1 decreased quicker than the stoichiometric hydroxyl band at 3233–3206 cm − 1 at a dehydroxylation temperature range of 180– 190 °C for the 0 and 10 mol% aluminium substituted goethite (Fig. 2a and b), whereas this change was less clearly shown by the higher levels of aluminium substituted goethite. Hydroxyl units released at such low temperatures (180– 190 °C) mostly represent the ‘‘loosely bound’’ hydroxyl
Fig. 1. Weight loss at TGA dehydroxylation maximum (a), and dehydroxylation maximum temperature of DTGA (b) as a function of aluminium substitution.
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485
Fig. 2. FTIR spectra of goethite, (a) 0 mol%, (b) 10 mol%, (c) 20 mol%, and (d) 30 mol% aluminium substitution, respectively.
units in the goethite structure [1,2,16,17]. However, this amount of hydroxyl units is bound more tightly in the higher aluminium (20– 30 mole%) substituted goethites than in the lower (10 mol%) or zero (0 mol%) aluminium substituted goethites [2,10,17]. Further decrease in band intensity and shift in band centre from 3233– 3206 cm − 1 to 3240– 3232 cm − 1 is indicative of the transformation of structure from goethite to hematite while the remaining 3434 cm − 1 band in the newly formed hematite (Fig. 3d) is the vibration of non-stoichiometric hydroxyl units that transferred directly from goethite to hematite. This structural alteration indicated by hydroxyl units is obviously observed only in the FT– IR spectra compared with the XRD technique. The decomposition of goethite and the formation of hematite are clearly illustrated in Fig. 2, indicating that some goethite bands disappeared
and hematite bands started to form eventually. The newly formed hematite bands were observed at 210 °C for the 0 mol%, 220 °C for the 10 and 20 mol%, and 230 °C for the 30 mol% aluminium substituted goethite, respectively. Some goethite and hematite bands overlapped at these dehydroxylation temperatures. The well resolved bands for hematite were recorded at 220 °C for the 0 mol%, 230 °C for the 10 and 20 mol%, and 240 °C for the 30 mol% aluminium substituted goethite, respectively (Fig. 2). Two typical bands at 540 and 452 cm − 1 (Fig. 3f) were observed and the intensities of these bands increased as dehydroxylation temperature increased (Fig. 2). This type of hematite has been reported to contain about 3–5% non-stoichiometric hydroxyl units [1,2,19]. The band at 3462 cm − 1 (Fig. 3a) in the hydroxyl stretching region remained (3434 cm − 1) after the completion of goethite–hematite
486
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transformation, followed by heating the samples up to 270 °C (Fig. 3d). This indicates that goethite altered to hematite without intermediate phases and the newly formed hematite inherited the remaining non-stoichiometric hydroxyl units from
the precursor goethite. A decreasing trend in intensity of this band indicated that the liberation of non-stoichiometric hydroxyl units from the hematite was continuing towards the progress of thermal dehydroxylation.
Fig. 3. Band component analysis of 10 mol% aluminium substituted goethite (110 °C, a– c) and its resultant hematite(270 °C, d– f).
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487
Table 5 Selected band component analysis of the infrared absorption spectra of hematite (270 °C) as affected by aluminium substitution Mol% Al w(HOH) 0 10 20 30 w(OH) 0 10 20 30 w[Fe(Al)O] 0 10 20 30
Band centre (cm−1)
Band intensity (% absorbance)
Band width (cm−1)
3435 3434 3448 3453
0.169 0.212 0.314 0.324
225 227 232 234
3232 3227 3239 3240
0.099 0.141 0.212 0.222
345 361 363 373
533 540 543 545
0.235 0.187 0.169 0.170
It has been reported that the dehydroxylation temperatures of aluminium oxy-hydroxides are higher than those of corresponding isostructural iron oxy-hydroxides [1,2,10,13]. For example, the dehydroxylation temperature of boehmite (gAlOOH) is 450–500 °C, and for isostructural lepidocrocite (g-FeOOH) is 230– 280 °C. Similarly, diaspore (a-AlOOH) dehydroxylates at 470– 500 °C compared with isostructural goethite (a-FeOOH) that dehydroxylates at 230– 280 °C [26–28]. One of the significant changes in goethite spectra affected by aluminium substitution is the shift of band positions. Fig. 5 shows the goethite band centres at 3450, 3206, 1413, 1269, 888, 798, and 461 cm − 1 shifted to higher positions as aluminium substitution increased. It is known that the goethite octahedra occupied by Al3 + are smaller than those occupied by Fe3 + so that average size of the unit cell and unit cell volume decreased (Tables 2 and 3) [1,3,18]. Schulze [15] has found that the shortening of the b- and c-dimensions caused a shortening and strengthening of the hydrogen bond, leading to a shift in the hydroxyl deformation and water bending vibrations to larger wavenumbers and this shift is observed in this work (Figs. 5 and 6). On the other hand, the influence of non-stoichiometric hydroxyl units was in many ways opposite to the influence of aluminium substitution
55.5 64.1 81.3 92.4
[1]. As the number of non-stoichiometric hydroxyl units increased, some of the hydrogen bonds were replaced by opposing hydroxyl units [15,18]. The opposing hydroxyl units tend to cause dilatation of the a- and b-dimensions because the main component of the hydrogen bond and of the opposing hydroxyl units is in the a– b plane [15].
Fig. 4. Changes in hydroxyl stretching bands of unheated goethite (110 °C) as affected by aluminium substitution.
488
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Fig. 5. Shift of the goethite band centre as a function of aluminium substitution.
The largest dilatation was found in the a-dimension for the present samples due to the influence of opposing hydroxyl units. The bands shifted to higher wavenumbers (Fig. 5) in terms of the increase in the a-dimension resulting in the hydrogen bond to become longer. It has been reported that the energy required to break down an AlOH bond is higher than for an
FeOH bond [2,10]. This explains why Al ions may specifically retain hydroxyl units more steadily than Fe ions in the structure of goethite and thus the aluminium substituted goethite is thermally more stable than the non-substituted goethite. It has been confirmed by previous studies and this study that the dehydroxylation temperature of aluminium substituted goethite is
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higher than non-substituted goethite [2,5,7,13,15,26]. Since a high energy Al-bond may combine more hydroxyl units than a low energy Fe-bond, hydroxyl units in solution can be favoured by aluminium ions to be incorporated into goethite structure during the process of synthesis [5,14]. However, a decreasing trend in the goethite band centre at 619 cm − 1 with increasing aluminium substitution was observed (Fig. 5g). This trend is attributed to the rearrangement of cation and oxygen packing in the goethite structure after aluminium ion replaced the iron ion position elongating the b–c plane, resulting in a decrease in the AlO bonding distance. Some Al-bonds may become shorter than Fe-bonds due to this rearrangement. Other goethite band centres
Fig. 6. Shift of the hematite band centre as a function of aluminium substitution.
489
shown in Fig. 3 did not change systematically and significantly as affected by aluminium substitution, possibly due to the opposite effect of nonstoichiometric hydroxyl units. Fig. 6 shows that the hematite band centre increases as aluminium substitution increases. The band at 545– 533 cm − 1 in the low frequency region is indicative of the newly formed hematite and is reported as the ‘‘fingerprint’’ of hematite [15,18,29–31]. The band at 1340–1324 cm − 1 observed here was not reported in most of the previous work. This is due to the present samples containing non-stoichiometric hydroxyl units, which were synthesized from the ferrous system compared with those from the ferric system or natural samples. There was a 16, 19 and 12 cm − 1 increase in band centre due to aluminium substitution for the vibrations at 1324, 589 and 533 cm − 1, respectively (Fig. 6). These linear relationships again indicated that the effect of aluminium substitution on hematite is similar to its precursor goethite. No systematic change in band centre regarding aluminium substitution was observed for the hematite band at 3434, 3227, 1633, 1526, and 452 cm − 1 (Fig. 3). The bands at 3434 and 1633 cm − 1 were assigned as vibrations of the non-stoichiometric hydroxyl units [16,18], and this type of hematite is classified as hydrohematite [16–18,32]. Two bands at 3453–3434 and 3240–3227 cm − 1 of the hydroxyl stretching region were well characterized for hematite formed under the low dehydroxylation temperatures between 230–270°C. The intensities of these two bands decreased systematically as aluminium substitution decreased, indicating that the retention of hydroxyl units in the hematite structure was strongly related to the influence of AlOH bonding. In contrast, there is no systematic change in band intensity at the low frequency region regarding Fe(Al)O bonding (Table 5). It has been reported that aluminium substitution in goethite and hematite has resulted in the decrease in crystal size [1,4,7,19]. This function has been well characterised by the linear increase in band width (r 2 = 0.966, 0.918, and 0.985 for the bands at 3453–3434, 3240–3227, and 545–533 cm − 1, respectively) as shown in the last column of Table 5.
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4. Conclusions
Acknowledgements
The effect of aluminium substitution on the changes in band centre, width and intensity of goethite and hematite are well characterised using FT –IR spectroscopy. The shift of goethite band centre to a higher wavenumber in the hydroxyl stretching region is attributed to the dilatation of hydrogen bonds in the a – b plane, and in the hydroxyl deformation and water bending region is attributed to the shortening and strength of the hydrogen bonds in terms of the shortening of the b- and c-dimensions. A shift of the goethite band centre from 619 to 584 cm − 1 as aluminium substitution increased from 0 to 30 mol% reflects the shortening of AlO bonds relative to FeO bonds elongating the b –c plane. The linear relationships between aluminium substitution, and band centre and width are observed for both goethite and hematite. The decrease in band intensity resulted from the increase of dehydroxylation temperature is due to the liberation of stoichiometric and non-stoichiometric hydroxyl units. The effect of aluminium substitution has been found not only to favour hydroxyl units to incorporate into, but also to remain hydroxyl units in the goethite structure at a higher temperature. Thus, the aluminium substituted goethite is thermally more stable than the non-substituted goethite. The hematite formed from thermal dehydroxylation of goethite at low temperatures contains non-stoichiometric hydroxyl units. The vibrations of these non-stoichiometric hydroxyl units are the ‘‘fingerprints’’ for the characterisation of ‘‘hydrohematite’’. Additional work will be deserved for in situ study of goethite dehydroxylation in order to avoid the readsorption of hydroxyl units onto heated goethite samples and the study of goethite dehydroxylation at higher (300–1000 °C) temperatures to investigate the behaviour of non-stoichiometric hydroxyl units in hydrohematite. Since bauxite contains about 15– 20% hydrohematite and goethite, this study may provide information to distinguish the phase transformation in activated bauxite.
The financial support of Comalco Pty Ltd and Australian Research Council, and the infrastructure of the Centre for Instrumental and Developmental Chemistry, Queensland University of Technology are gratefully acknowledged.
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