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Soil characterisation using X-ray diffraction, photoacoustic spectroscopy and electron paramagnetic resonance
Applied Clay Science 21 (2002) 303 – 311 www.elsevier.com/locate/clay
Soil characterisation using X-ray diffraction, photoacoustic spectroscopy and electron paramagnetic resonance R.S.T. Manha˜es a, L.T. Auler a, M.S. Sthel a, J. Alexandre b, M.S.O. Massunaga a, J.G. Carrio´ a, D.R. dos Santos a, E.C. da Silva c, A. Garcia-Quiroz c, H. Vargas a,* a
Laborato´rio de Cieˆncias Fı´sicas, Centro de Ciencia e Tecnologia, Universidade Estadual do Norte Fluminense, Av. Alberto Lamego 2000, Campos dos Goytacazes, RJ, 28015-620, Brazil b Laborato´rio de Engenharia Civil, Universidade Estadual do Norte Fluminense, Av. Alberto Lamego 2000, Campos dos Goytacazes, RJ, 28015-620, Brazil c Instituto de Fı´sica Gleb Wataghin, Universidade Estadual de Campinas, Caixa Postal 6165, Campinas, SP, 13083-970, Brazil Received 10 May 2001; accepted 1 February 2002
Abstract The optical absorption spectra and chemical composition of soil samples were characterised using photoacoustic spectroscopy (PAS), electron paramagnetic resonance (EPR), X-ray diffraction and X-ray fluorescence. From fluorescence results, the chemical components were identified and an Fe mass concentration varying between 4% and 10% was determined. Besides that, the observed photoacoustic technique (PA) spectra showed transition bands associated with Fe3+ ions. From the phase behaviour, both the nonradiative relaxation time s and the characteristic thermal diffusion time sb were determined. The X-ray diffraction analysis showed that kaolinite is a major crystalline phase (86% in mass) followed by minor quantities of anatase, gibbsite and quartz. Rietveld refinements showed that the Fe3+ cations partially substitute for Al3+ cations in the octahedral sites of the kaolinite structure. EPR measurements were performed in order to determine the crystalline environment of Fe ions; the observed profiles indicate that Fe sites are embedded in a distorted cubic crystalline field. D 2002 Elsevier Science B.V. All rights reserved. Keywords: XRD; XRF; PAS; EPR; Soil; Thermal properties
1. Introduction The quaternary sedimentary basin of Campos dos Goytacazes, RJ, Brazil, is a low altitude plain which extends 50 km away from the Atlantic Ocean. Soil from this region has a high content of clay minerals. The sediments of this basin were deposited by the
*
Corresponding author. Fax: +55-22-2726-1532. E-mail address: [email protected] (H. Vargas).
Paraı´ba do Sul River meandering over its flood plain. Almost a hundred small ceramic industries explore these large deposits as a raw material for red bricks and tiles, a growing economic activity in the region. However, as the final products are in general of a low quality, there are several ongoing studies aiming at production of ceramic materials with better properties and a higher economic value. In a tropical country, building walls are often exposed to high temperatures and therefore, thermal properties of the raw material play an important role
0169-1317/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 ( 0 2 ) 0 0 0 9 2 - 3
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in the indoor climate of the houses. In a previous work, photoacoustic technique (PA) has been applied to determine some important properties, such as thermal diffusivity, heat capacity and thermal conductivity of the soil clay fraction (Alexandre et al., 1999). It was shown that thermal properties are very sensitive to the amorphous – crystalline phase transition occurring when clay samples are heated above 950 jC, which are typical temperatures applied in the manufacturing process to obtain ceramic materials with a desirable performance. The economic value of the ceramic product is also affected by its red pigmentation, which is mainly due to the presence of oxides, hydroxides and hydrated oxides of ferric iron. These iron oxides that occur in soil either as coatings on individual clay particles, or as discrete, fine particles throughout the clay mass, can be removed by organic acids (Ambikadevi and Lalithambika, 2000) or by chemical reduction of the iron to the ferrous form (Kunze, 1965; Mehra and Jackson, 1960). A small fraction of iron atoms can also be present as substitutional cations in the tetrahedral or octahedral units that form the clay lamellae (Moore and Reynolds, 1997). Depending on its content and location, Fe may substantially affect the suitability of natural clays for industrial applications, such as paper coating or ceramics.
In this work, a systematic study on selected raw materials used by ceramic industries is presented. The X-ray fluorescence was used to determine Fe concentration and PAS was applied to study the optical absorption spectra, also related to the Fe content and its valence state. EPR measurements were performed in order to study the average crystalline symmetry around Fe ions. In addition, a quantitative phase analysis was performed using the Rietveld method (Young et al., 1995) and major mineral phases were identified.
2. Experimental procedures The samples used in this work originated from three different locations, termed here simply as A, B and C. Representative quantity of the stratified material was obtained from each place of A, B and C; the number of stratification layers varied between two and five layers above the free ground water, which is about 3 m below ground. The number of layers on the chosen sites was 4, 2 and 5, respectively, resulting in 11 different samples. The samples were dried, grounded and passed through a sieve with nominal aperture of 74 Am (mesh 200). The obtained powder, comprising about 95% (in mass) of the original sample, was very representative for the soil studied.
Fig. 1. Scheme of the PA cell. The sample is placed in an open cavity of an attachable sample holder.
R.S.T. Manha˜es et al. / Applied Clay Science 21 (2002) 303–311
Standard sedimentation methods disclosed narrow particle size distributions, with a large content (50%) of particles smaller than 2 Am. This powder was used in the photoacoustic, X-ray diffraction and EPR studies. For X-ray fluorescence measurements, samples were prepared from 4 g of powder that was pressed (at 15 t during 1 min) to form disks (32-mm diameter and 3 mm thick). The PA experiments were performed using a spectrometer consisting of a 1000-W xenon arc lamp, the radiation of which was modulated by a variable speed chopper (SRS, model SR540). A monochromator, in combination with the appropriate absorption filters, was used for wavelength selection and to eliminate higher order effects. The beam leaving the monochromator was directed into a conventional PA cell, a small gastight enclosure with a condenser microphone (Bruel and Kjaer, model 4165, 1/2 in. diameter) mounted in one wall as shown schematically in Fig. 1. Changes in temperature cause pressure alterations in the gas; these are subsequently converted to an electrical signal by the microphone. The signal was preamplified and fed to a lock-in amplifier (SRS, model SR830), connected to a microcomputer. Loading of the cell was accomplished via the attachable sample holder, with the powder placed in an open cavity. The PA signal was ratioed by signal obtained
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from a carbon black, in order to eliminate the spectral variation of the illumination source. The EPR measurements were performed using a Varian E-12 spectrometer operating at a microwave Xband (9.5 GHz). The X-ray powder diffraction data were acquired in Bragg – Brentano geometry using CuKa radiation at a conventional diffractometer (Seifert URD65, Germany), equipped with a graphite diffracted beam monochromator. The X-ray fluorescence spectra were obtained using a wavelength dispersive spectrometer (Seifert VRA35, Germany). These measurements were performed in a vacuum using a tungsten sealed tube, a LiF(220) analysing crystal and a flow detector; all studies were carried out at room temperature.
3. Results and discussion 3.1. X-ray fluorescence spectrometry A typical fluorescence spectrum is shown in Fig. 2. As expected, Fe, Ti, K, Ca, Si and Al were the chemical components detected from all the soil samples. The lighter elements, Si and Al, are found in another wavelength range, not shown in Fig. 2. The Fe content of one of the samples was determined by
Fig. 2. Typical X-ray fluorescence spectrum of soil samples. The Fe content was determined through the internal standard addition method as explained in the text.
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the standard addition method of X-ray fluorescence analysis (Bertin, 1975) and the concentration of the others calculated using this sample as a reference. Such a procedure is allowed as long as the matrix does not vary too much from sample to sample. In the present work, this requirement is met as the samples originated from soils not differing much from each other. The standard addition implied adding 0.4 g of Fe2O3 to 4 g of soil powder (prepared as described above) which after being well mixed and homogenised, was pressed in a disk following the same procedure as for a normal sample. Considering Ix and Ixs as the fluorescence intensities of the normal sample and of the sample plus additive, respectively, it can be shown (Bertin, 1975) that the concentration Cx of the element x is given by Cx ¼
Ix Ixs
1þ
wx ws
Cs 1 IIxsx
ð1Þ
where wx and ws are the weights of the sample and the standard, respectively, and Cs is the concentration of the element in the standard, 0.7 of Fe in Fe2O3. The results summarised in Table 1 show that Fe concentration ranges between 4% and 10%, although this variation does not show a trend relating the concentration to the site location or to depth. 3.2. Photoacoustic spectroscopy Fig. 3 shows a typical PA spectrum of the soil samples recorded at a chopping frequency of 20 Hz. It
Table 1 Fe weight concentration obtained from the fluorescence intensities for different soil samples Site location
Layer
Fe concentration (%)
A
1 2 3 4 1 2 1 2 3 4 5
9.5 8.3 9.4 4.3 6.8 4.9 7.5 10.3 6.6 6.3 7.7
B C
Fig. 3. Typical photoacoustic spectrum of soil samples recorded at a modulation frequency of 20 Hz. The arrows indicate the position of the absorption bands and dotted lines correspond to the deconvolution of the experimental curve.
was experimentally observed that this frequency is suitable for observing all transitions. Furthermore, PA spectra were very reproducible, although samples with higher Fe content displayed larger amplitudes of the PA signal. The arrows identified from 1 to 5 in Fig. 3 indicate the position of spectral bands associated with the electronic transitions of the Fe3+ from 6A1(6S) level, to respectively the levels 4T1(4G); 4T2(4G); 4E, 4 A1(4G); 4T2(4D); and 4E(4D). According to the ligand field theory, these transitions are expected when Fe3+ ions are in an octahedral or tetrahedral symmetry (Sugano et al., 1970). The same behaviour was observed in all samples. Some band positions could not be easily identified. In Fig. 3, for example, the spectrum displays a very small shoulder situated at 540 nm that upon Gaussian deconvolution discloses an absorption band. In Table 2, the mean position of the transition bands are listed. The results are in agreement with the theoretical prediction reported previously (Abritta and de Souza Barros, 1988; Abritta et al., 1989; Sosman et al., 1998).
R.S.T. Manha˜es et al. / Applied Clay Science 21 (2002) 303–311 Table 2 Positions of the absorption bands, nonradiative relaxation time s and the characteristic diffusion time sb for each band Band
1
2
3
4
5
Position (nm) s (ms) sb (As)
370 5.9 0.13
430 6.2 0.13
490 6.5 0.12
540 6.7 0.16
650 7.2 0.20
Having obtained the PA spectra, the procedure described in the paper of Lima et al. (1987) was used to obtain the nonradiative relaxation time s and the characteristic diffusion time sb for each absorption band. Fig. 4 shows the signal’s dependence on the modulation frequency for the 490-nm band. The signal exhibits f 1 dependence, which was also observed for the other absorption bands. This result contrasts the f 1.5 frequency dependence predicted by the model of Rosencwaig and Gersho (1976) for a thermally thick, optically transparent (unsaturated) sample, but agrees with that expected when the thermal expansion of the sample is a dominant mechanism responsible for PA signal. In this case, the acoustic signal is proportional to the average temperature in the sample. In fact, using the modified Rosencwaig and Gersho theory that includes the effect of a finite nonradiative relaxation time, the pressure fluctuation yP in the PA cell is given by:
yP ¼
cPo at Io r2 ½1 expðbls Þ lg ks ðb2 r2s Þð1 þ ixsÞ ðr bÞexpðbls Þ½expðls rs Þ expðls rs Þ þ 2br r ð1 þ bÞexpðls rs Þ ð1 bÞexpðls rs Þ expðixtÞ
307
Here, the subscript i denotes the sample (s), gas (g) and backing (b), respectively. For a thermally thick and optically transparent sample, namely lsrs 1 and bls 1, Eq. (2) reduces to yP ¼
cPo ls at bIo expðixtÞ lg ks r2s ð1 b=rs Þð1 þ ixsÞ
ð3Þ
The information about s and sb is obtained from the measurements of the thermal wave phase angle / as a function of the modulation frequency. In the thermally thick region of our experiment, these parameters are calculated from Eq. (3), by writing it as yP=AyPAexp[i(xt+/)], where AyPA ¼
h
cPo ls at bIo as
lg ks x ð1 bls =2Þ2 þ ðbls =2Þ2
i1=2 h i1=2 1 þ ðxsÞ2 ð4Þ
and p 1 / ¼ tan1 ðxsÞ tan1 2 ð2xsb Þ1=2 1
!
ð5Þ where sb=(b2as)1. Eq. (4) implies that, provided bls<1, the PA signal is proportional to both the optical absorption coefficient and to the thermal expansion coefficient, and that it varies as f 1 for
ð2Þ
where c is the ratio of specific heats for air, Po is the ambient pressure, Io is the incident light intensity at a given wavelength, at is the thermal expansion coefficient of the sample, b is the optical absorption coefficient of the sample, s is the nonradiative relaxation time for the sample, x=2pf, where f is the applied modulation frequency, li is the length of material i, ki is the thermal conductivity of material i, ai=ki/qici is the thermal diffusivity of material i, ri=(1+i)li1 is the complex thermal diffusion coefficient of material i, li=(2ai/x)1/2 is the thermal diffusion length of material i, b=kbrb/ksrs and r=b/rs.
Fig. 4. Amplitude of the photoacoustic signal versus modulation frequency at 490 nm for a soil sample. The solid line corresponds to the best linear fit to the data, which in logarithmic scale represents a f 1 power law.
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that fitted values of s remain very close for all bands suggests that this heat exchange time is dominating nonradiative relaxation process. Comparing the values of s and sb obtained from the modulation frequency phase data fitting Eq. (5), it can be seen that the values of sb exhibit a large discrepancy. The reason for this is the sensitivity of the phase angle / with respect to sb. As pointed by Baesso et al. (1989), the variation of / as given by Eq. (5) is quite insensitive to the value of sb in the low modulation frequency region, thereby leading to a low accuracy in the value of sb obtained from the phase resolved method. 3.3. Electron paramagnetic resonance Fig. 5. Phase of the photoacoustic signal plotted versus modulation frequency for a soil sample. Measurement was performed at 490 nm. The solid line corresponds to data fit (Eq. (5)).
xs 1. The latter condition is normally well fulfilled, whereas the first condition requires that the thermal diffusion length, ls, is much smaller than the optical penetration depth, b1, a condition that is well satisfied for the studied samples within the modulation frequency range of our experiment. The dependence of the PA signal phase on the modulation frequency, Eq. (5), has been used by several authors to study the behaviour of s and sb in collisional deactivation of vibrational excitation in gases, polymers and in nonradiative relaxation of dopant ions in crystal and glasses (Sosman et al., 1998; Rosencwaig and Gersho, 1976; Baesso et al., 1989). Fig. 5 shows the typical results for the phase of PA signal obtained at 490 nm and the corresponding data fitting. The results for each absorption band together with the obtained values of s and sb are summarised in Table 2. Several conclusions can be drawn from the above results. The values of s (about 6.5 ms) are in very close agreement for all absorption bands. Note that the relaxation time s measured photoacoustically is not necessarily that appropriate to the level initially excited in the absorption process, but rather to the average lifetime of a variety of states before energy is lost as heat. Furthermore, for powder samples (as is the case in the present work), s also contains a contribution from a heat exchange time between the powder particles and the transducing gas in the PA cell. This heat exchange time depends not only on the shape of the particles but also on their size. The fact
Fig. 6 shows the EPR spectrum for one sample localised at place A, which is similar to those obtained from the other measured samples. The EPR spectra are typical of Fe3+ ions (Mansanares et al., 1989;
Fig. 6. Typical EPR spectrum of samples from site A. The circles represent the experimental data, continuous line is the best fit and dashed lines are the Lorentzian components.
R.S.T. Manha˜es et al. / Applied Clay Science 21 (2002) 303–311 Table 3 EPR parameters for three main resonances for samples taken from site A: the deviations on DH are about 50 G, on g-value of 0.1 and on I about 30% Layer g1 1 2 3
DH1 (G)
4.2 586 4.2 584 4.2 567
I1 g2 (a.u.) 6.5 6.8 9.0
DH2 (G)
2.3 1895 2.3 2112 2.3 2387
I2 g3 (a.u.) 559 735 1349
DH3 (G)
I3 (a.u.)
2.0 2433 2.0 2639 2.0 2485
4289 6979 6398
Goldfarb et al., 1994). The results are very reasonably fitted to two symmetric lines in the magnetic field region for gi2, corresponding to nondistorted sites, superposed to a gi4.2 line, corresponding to at least one distorted cubic site. The line structure observed in the low-field side of the spectra indicates a distribution of sites with different distortion levels, which are represented by a set of powder spectra. However, for comparison purposes, the gi4.2 line is assumed to be
309
representative of distorted sites because of its prevailing intensity. In order to obtain the usual EPR spectral parameters (position, peak-to-peak amplitude and line width), the experimental data were fitted to three Lorentzian curves. In Fig. 6, the experimental data (o), the best fitting (continuous line) and the Lorentzian components (dashed lines) are shown. The EPR parameters obtained from the fitting for samples from place A are listed in Table 3. The I parameter is equal to A(DH)2, which is proportional to the line spectral intensity, A being the peak-to-peak amplitude and DH the line widths. It can be concluded from the EPR data that Fe 3+ ions in samples are located in cubic distorted sites, occupying silicon tetrahedral framework positions ( gi4.2 and other low field lines), as well as in nondistorted sites, occupying oxy or hydroxy species interstitial positions ( g=2.3) and octahedral coordinated cation-exchange sites ( g=2.0) (Mansanares et al., 1989; Goldfarb et al., 1994). From
Fig. 7. Powder diffractogram of a soil sample and the result of Rietveld refinement for quantitative phase analysis. Peaks corresponding to each crystalline phase are indicated as K (kaolinite), Q (quartz), G (gibbsite) and A (anatase).
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line intensity results, one cannot report much concerning the concentration of Fe3+. Nevertheless, it is clear that the amount of the distorted sites is much less than that of nondistorted ones. On the other hand, the line-width indicates more concentrated Fe3+ clusters for the nondistorted sites, which is expected for oxides and other similar species. Free iron oxides were removed applying the sodium dithionite – citrate procedure (Mehra and Jackson, 1960), with a minimum of destructive action to the clay minerals. The PAS and EPR spectra for the chemically treated samples confirmed a decrease in Fe3+ content. The observed EPR profiles were very similar in shape to the spectra of nontreated samples. They were also fitted to three Lorentzian curves with positions fixed at the previously determined values ( g =2.0, 2.3 and 4.2). For the chemically treated samples, the peak-to-peak amplitude ratio of the resonance g =4.2 (distorted sites at the crystalline structure) to g =2.0 and 2.3 (nondistorted sites) was 60%, in contrast to the 30% ratio obtained for the natural soil. This result suggests that the chemical treatment removed preferentially Fe ions from the nondistorted sites. 3.4. X-ray diffraction X-ray powder diffraction data were collected in the angular range 3jv2hv70j with a step 0.02j and a 5-s counting time. All soil samples presented very similar results. A quantitative phase analysis was performed using the Rietveld method with the DBWS program (Young et al., 1995). Fig. 7 shows the Rietveld fitting for one powder diffractogram. The results indicate kaolinite as the main phase (85.8% in mass), followed by quartz (5.5%), anatase (5.2%) and gibbsite (3.6%). Iron oxides such as hematite, magnetite and ilmenite were not detected. Atomic positions, cell parameters, site occupation factors and texture were refined for kaolinite. Structural parameters of the minor phases were not refined. Taking into account the important presence of Fe ions detected by X-ray fluorescence, PAS and EPR, the possibility of Al3+ substitution by Fe3+ cations in the octahedral sites of the kaolinite structure was considered. A significant improvement of the quality of fitting (residuals RBragg=7.36%, RP=13.42% and Rwp=16.95%) was achieved by substituting 50% of the Al3+ ions by
Fe3+ ions. Meads and Malden (1975) who used Mo¨ssbauer spectroscopy also mentioned the possibility of this substitution. Finally, it was observed that the chemical treatments for removal of free iron oxides do not affect significantly the crystalline phases mass fractions obtained in the Rietveld refinements.
4. Conclusions The X-ray fluorescence spectra allowed the identification of the chemical components in soil samples as being Fe, Ti, K, Ca, Si and Al. The weight concentration of Fe atoms that was determined for all the samples varied between 4% and 10%. Results of Xray diffraction analysis showed that the main crystalline phase present in these samples is kaolinite (86% in mass) followed by minor quantities of anatase, gibbsite and quartz. Rietveld refinements showed that Fe3+ cations partially substitute for Al3+ cations in the octahedral sites of the kaolinite structure. The PAS showed transition bands associated with Fe3+ ions in tetrahedral or octahedral symmetry; samples with higher Fe content also presented larger amplitudes of the PA signal. The PA spectroscopy and EPR results are in close agreement, since EPR spectra presented a dominant component ( g =2.0) that corresponds to Fe3+ ions occupying octahedral coordinated cation-exchange sites. The results presented in this work, which refer to the characterisation of the soil in natura, are part of a larger project which comprises the analysis of the optical, thermal and rheological properties of extruded pieces as a function of the firing temperature, as well as other procedures adopted for the production of bricks and tiles. The Fe content affects the final properties of the solid pieces, for instance, the ideal firing temperature for recrystallization, which is responsible for a series of rheological effects such as hardening and shrinkage. This temperature could be lower for samples with higher Fe content, since Fe atoms act as an ‘‘internal source of heat’’, accelerating the involved reactions. Furthermore, we are concerned with the environmental impact related to the clay extraction and gases emitted during production, which are also studied and will be reported in another paper.
R.S.T. Manha˜es et al. / Applied Clay Science 21 (2002) 303–311
Acknowledgements This work was partially financed by CNPq and FAPERJ, the support of which is gratefully acknowledged.
References Abritta, T., de Souza Barros, F., 1988. Luminescence and photoacoustic measurements of LiAl5O8 – Fe3+. J. Lumin. 40, 187 – 188. Abritta, T., Cella, N., Vargas, H., 1989. A photoacoustic study of LiAl5O8 doped with several percent of iron(III). Chem. Phys. Lett. 161, 12 – 15. Alexandre, J., Saboya, F., Marques, B.C., Ribeiro, M.L.P., Salles, C., da Silva, M.G., Sthel, M.S., Auler, L.T., Vargas, H., 1999. Photoacoustic thermal characterization of kaolinite clays. Analyst 124, 1209 – 1214. Ambikadevi, V.R., Lalithambika, M., 2000. Effect of organic acids on ferric iron removal from iron-stained kaolinite. Appl. Clay Sci. 16, 133 – 145. Baesso, M.L., Mansanares, A.M., da Silva, E.C., Vargas, H., 1989. Phase resolved photoacoustic spectroscopy and EPR investigation of MnO2- and CoO-doped soda – lime glasses. Phys. Rev. B 40, 1880 – 1884. Bertin, E.P., 1975. Principles and Practice of X-ray Spectrometric Analysis. Plenum, New York. Goldfarb, D., Bernardo, M., Strohmaier, K.G., Vaughan, D.E.W., Thomann, H., 1994. Characterization of iron in zeolites by Xband and Q-band ESR, pulsed ESR, and UV – visible spectroscopies. J. Am. Chem. Soc. 116, 6344 – 6353.
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Kunze, G.W., 1965. Pretreatment for mineralogical analysis. In: Black, C.A., Evans, D.D., White, J.L., Ensminger, L.E., Clark, F.E., Dinauer, R.C. (Eds.), Methods of Soil Analysis. American Soc. of Agronomy, Madison, USA, pp. 568 – 577. Lima, G.A.R., Baesso, M.L., Arguello, Z.P., da Silva, E.C., Vargas, H., 1987. Phase-resolved photoacoustic spectroscopy: application to metallic-ion-doped glasses. Phys. Rev. B 36, 9812 – 9815. Mansanares, A.M., Baesso, M.L., da Silva, E.C., Gandra, F.C.G., Vargas, H., Miranda, L.C.M., 1989. Photoacoustic and ESR studies of iron-doped soda – lime glasses—thermal-diffusivity. Phys. Rev. B 40, 7912 – 7915. Meads, R.E., Malden, P.J., 1975. Electron spin resonance in natural kaolinites containing Fe3+ and other transition metal ions. Clay Miner. 10, 313 – 345. Mehra, O.P., Jackson, M.L., 1960. Iron oxide removal from soil and clays by a dithionite – citrate system buffered with sodium carbonate. Clays Clay Miner. 7, 317 – 327. Moore, D.M., Reynolds Jr., R.C., 1997. X-ray Diffraction and the Identification and Analysis of Clay Minerals Oxford Univ. Press, UK. Rosencwaig, A., Gersho, A., 1976. Theory of photoacoustic effect with solids. J. Appl. Phys. 47, 64 – 69. Sosman, L.P., Abritta, T., Amaral Jr., M.R., Cella, N., Vargas, H., 1998. Optical properties of LiGaTiO4:Fe3+. Solid State Commun. 105, 135 – 138. Sugano, S., Tunabe, Y., Kamimura, H., 1970. Multiplets of Transition-metal Ions in Crystals Academic Press, New York. Young, R.A., Sakthivel, A., Moss, T.S., Paiva-Santos, C.O., 1995. DBWS-9411, an upgrade of the DBWS programs for Rietveld refinement with PC and mainframe computers. J. Appl. Crystallogr. 28, 366 – 367.
Soil characterisation using X-ray diffraction, photoacoustic spectroscopy and electron paramagnetic resonance R.S.T. Manha˜es a, L.T. Auler a, M.S. Sthel a, J. Alexandre b, M.S.O. Massunaga a, J.G. Carrio´ a, D.R. dos Santos a, E.C. da Silva c, A. Garcia-Quiroz c, H. Vargas a,* a
Laborato´rio de Cieˆncias Fı´sicas, Centro de Ciencia e Tecnologia, Universidade Estadual do Norte Fluminense, Av. Alberto Lamego 2000, Campos dos Goytacazes, RJ, 28015-620, Brazil b Laborato´rio de Engenharia Civil, Universidade Estadual do Norte Fluminense, Av. Alberto Lamego 2000, Campos dos Goytacazes, RJ, 28015-620, Brazil c Instituto de Fı´sica Gleb Wataghin, Universidade Estadual de Campinas, Caixa Postal 6165, Campinas, SP, 13083-970, Brazil Received 10 May 2001; accepted 1 February 2002
Abstract The optical absorption spectra and chemical composition of soil samples were characterised using photoacoustic spectroscopy (PAS), electron paramagnetic resonance (EPR), X-ray diffraction and X-ray fluorescence. From fluorescence results, the chemical components were identified and an Fe mass concentration varying between 4% and 10% was determined. Besides that, the observed photoacoustic technique (PA) spectra showed transition bands associated with Fe3+ ions. From the phase behaviour, both the nonradiative relaxation time s and the characteristic thermal diffusion time sb were determined. The X-ray diffraction analysis showed that kaolinite is a major crystalline phase (86% in mass) followed by minor quantities of anatase, gibbsite and quartz. Rietveld refinements showed that the Fe3+ cations partially substitute for Al3+ cations in the octahedral sites of the kaolinite structure. EPR measurements were performed in order to determine the crystalline environment of Fe ions; the observed profiles indicate that Fe sites are embedded in a distorted cubic crystalline field. D 2002 Elsevier Science B.V. All rights reserved. Keywords: XRD; XRF; PAS; EPR; Soil; Thermal properties
1. Introduction The quaternary sedimentary basin of Campos dos Goytacazes, RJ, Brazil, is a low altitude plain which extends 50 km away from the Atlantic Ocean. Soil from this region has a high content of clay minerals. The sediments of this basin were deposited by the
*
Corresponding author. Fax: +55-22-2726-1532. E-mail address: [email protected] (H. Vargas).
Paraı´ba do Sul River meandering over its flood plain. Almost a hundred small ceramic industries explore these large deposits as a raw material for red bricks and tiles, a growing economic activity in the region. However, as the final products are in general of a low quality, there are several ongoing studies aiming at production of ceramic materials with better properties and a higher economic value. In a tropical country, building walls are often exposed to high temperatures and therefore, thermal properties of the raw material play an important role
0169-1317/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 ( 0 2 ) 0 0 0 9 2 - 3
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in the indoor climate of the houses. In a previous work, photoacoustic technique (PA) has been applied to determine some important properties, such as thermal diffusivity, heat capacity and thermal conductivity of the soil clay fraction (Alexandre et al., 1999). It was shown that thermal properties are very sensitive to the amorphous – crystalline phase transition occurring when clay samples are heated above 950 jC, which are typical temperatures applied in the manufacturing process to obtain ceramic materials with a desirable performance. The economic value of the ceramic product is also affected by its red pigmentation, which is mainly due to the presence of oxides, hydroxides and hydrated oxides of ferric iron. These iron oxides that occur in soil either as coatings on individual clay particles, or as discrete, fine particles throughout the clay mass, can be removed by organic acids (Ambikadevi and Lalithambika, 2000) or by chemical reduction of the iron to the ferrous form (Kunze, 1965; Mehra and Jackson, 1960). A small fraction of iron atoms can also be present as substitutional cations in the tetrahedral or octahedral units that form the clay lamellae (Moore and Reynolds, 1997). Depending on its content and location, Fe may substantially affect the suitability of natural clays for industrial applications, such as paper coating or ceramics.
In this work, a systematic study on selected raw materials used by ceramic industries is presented. The X-ray fluorescence was used to determine Fe concentration and PAS was applied to study the optical absorption spectra, also related to the Fe content and its valence state. EPR measurements were performed in order to study the average crystalline symmetry around Fe ions. In addition, a quantitative phase analysis was performed using the Rietveld method (Young et al., 1995) and major mineral phases were identified.
2. Experimental procedures The samples used in this work originated from three different locations, termed here simply as A, B and C. Representative quantity of the stratified material was obtained from each place of A, B and C; the number of stratification layers varied between two and five layers above the free ground water, which is about 3 m below ground. The number of layers on the chosen sites was 4, 2 and 5, respectively, resulting in 11 different samples. The samples were dried, grounded and passed through a sieve with nominal aperture of 74 Am (mesh 200). The obtained powder, comprising about 95% (in mass) of the original sample, was very representative for the soil studied.
Fig. 1. Scheme of the PA cell. The sample is placed in an open cavity of an attachable sample holder.
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Standard sedimentation methods disclosed narrow particle size distributions, with a large content (50%) of particles smaller than 2 Am. This powder was used in the photoacoustic, X-ray diffraction and EPR studies. For X-ray fluorescence measurements, samples were prepared from 4 g of powder that was pressed (at 15 t during 1 min) to form disks (32-mm diameter and 3 mm thick). The PA experiments were performed using a spectrometer consisting of a 1000-W xenon arc lamp, the radiation of which was modulated by a variable speed chopper (SRS, model SR540). A monochromator, in combination with the appropriate absorption filters, was used for wavelength selection and to eliminate higher order effects. The beam leaving the monochromator was directed into a conventional PA cell, a small gastight enclosure with a condenser microphone (Bruel and Kjaer, model 4165, 1/2 in. diameter) mounted in one wall as shown schematically in Fig. 1. Changes in temperature cause pressure alterations in the gas; these are subsequently converted to an electrical signal by the microphone. The signal was preamplified and fed to a lock-in amplifier (SRS, model SR830), connected to a microcomputer. Loading of the cell was accomplished via the attachable sample holder, with the powder placed in an open cavity. The PA signal was ratioed by signal obtained
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from a carbon black, in order to eliminate the spectral variation of the illumination source. The EPR measurements were performed using a Varian E-12 spectrometer operating at a microwave Xband (9.5 GHz). The X-ray powder diffraction data were acquired in Bragg – Brentano geometry using CuKa radiation at a conventional diffractometer (Seifert URD65, Germany), equipped with a graphite diffracted beam monochromator. The X-ray fluorescence spectra were obtained using a wavelength dispersive spectrometer (Seifert VRA35, Germany). These measurements were performed in a vacuum using a tungsten sealed tube, a LiF(220) analysing crystal and a flow detector; all studies were carried out at room temperature.
3. Results and discussion 3.1. X-ray fluorescence spectrometry A typical fluorescence spectrum is shown in Fig. 2. As expected, Fe, Ti, K, Ca, Si and Al were the chemical components detected from all the soil samples. The lighter elements, Si and Al, are found in another wavelength range, not shown in Fig. 2. The Fe content of one of the samples was determined by
Fig. 2. Typical X-ray fluorescence spectrum of soil samples. The Fe content was determined through the internal standard addition method as explained in the text.
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the standard addition method of X-ray fluorescence analysis (Bertin, 1975) and the concentration of the others calculated using this sample as a reference. Such a procedure is allowed as long as the matrix does not vary too much from sample to sample. In the present work, this requirement is met as the samples originated from soils not differing much from each other. The standard addition implied adding 0.4 g of Fe2O3 to 4 g of soil powder (prepared as described above) which after being well mixed and homogenised, was pressed in a disk following the same procedure as for a normal sample. Considering Ix and Ixs as the fluorescence intensities of the normal sample and of the sample plus additive, respectively, it can be shown (Bertin, 1975) that the concentration Cx of the element x is given by Cx ¼
Ix Ixs
1þ
wx ws
Cs 1 IIxsx
ð1Þ
where wx and ws are the weights of the sample and the standard, respectively, and Cs is the concentration of the element in the standard, 0.7 of Fe in Fe2O3. The results summarised in Table 1 show that Fe concentration ranges between 4% and 10%, although this variation does not show a trend relating the concentration to the site location or to depth. 3.2. Photoacoustic spectroscopy Fig. 3 shows a typical PA spectrum of the soil samples recorded at a chopping frequency of 20 Hz. It
Table 1 Fe weight concentration obtained from the fluorescence intensities for different soil samples Site location
Layer
Fe concentration (%)
A
1 2 3 4 1 2 1 2 3 4 5
9.5 8.3 9.4 4.3 6.8 4.9 7.5 10.3 6.6 6.3 7.7
B C
Fig. 3. Typical photoacoustic spectrum of soil samples recorded at a modulation frequency of 20 Hz. The arrows indicate the position of the absorption bands and dotted lines correspond to the deconvolution of the experimental curve.
was experimentally observed that this frequency is suitable for observing all transitions. Furthermore, PA spectra were very reproducible, although samples with higher Fe content displayed larger amplitudes of the PA signal. The arrows identified from 1 to 5 in Fig. 3 indicate the position of spectral bands associated with the electronic transitions of the Fe3+ from 6A1(6S) level, to respectively the levels 4T1(4G); 4T2(4G); 4E, 4 A1(4G); 4T2(4D); and 4E(4D). According to the ligand field theory, these transitions are expected when Fe3+ ions are in an octahedral or tetrahedral symmetry (Sugano et al., 1970). The same behaviour was observed in all samples. Some band positions could not be easily identified. In Fig. 3, for example, the spectrum displays a very small shoulder situated at 540 nm that upon Gaussian deconvolution discloses an absorption band. In Table 2, the mean position of the transition bands are listed. The results are in agreement with the theoretical prediction reported previously (Abritta and de Souza Barros, 1988; Abritta et al., 1989; Sosman et al., 1998).
R.S.T. Manha˜es et al. / Applied Clay Science 21 (2002) 303–311 Table 2 Positions of the absorption bands, nonradiative relaxation time s and the characteristic diffusion time sb for each band Band
1
2
3
4
5
Position (nm) s (ms) sb (As)
370 5.9 0.13
430 6.2 0.13
490 6.5 0.12
540 6.7 0.16
650 7.2 0.20
Having obtained the PA spectra, the procedure described in the paper of Lima et al. (1987) was used to obtain the nonradiative relaxation time s and the characteristic diffusion time sb for each absorption band. Fig. 4 shows the signal’s dependence on the modulation frequency for the 490-nm band. The signal exhibits f 1 dependence, which was also observed for the other absorption bands. This result contrasts the f 1.5 frequency dependence predicted by the model of Rosencwaig and Gersho (1976) for a thermally thick, optically transparent (unsaturated) sample, but agrees with that expected when the thermal expansion of the sample is a dominant mechanism responsible for PA signal. In this case, the acoustic signal is proportional to the average temperature in the sample. In fact, using the modified Rosencwaig and Gersho theory that includes the effect of a finite nonradiative relaxation time, the pressure fluctuation yP in the PA cell is given by:
yP ¼
cPo at Io r2 ½1 expðbls Þ lg ks ðb2 r2s Þð1 þ ixsÞ ðr bÞexpðbls Þ½expðls rs Þ expðls rs Þ þ 2br r ð1 þ bÞexpðls rs Þ ð1 bÞexpðls rs Þ expðixtÞ
307
Here, the subscript i denotes the sample (s), gas (g) and backing (b), respectively. For a thermally thick and optically transparent sample, namely lsrs 1 and bls 1, Eq. (2) reduces to yP ¼
cPo ls at bIo expðixtÞ lg ks r2s ð1 b=rs Þð1 þ ixsÞ
ð3Þ
The information about s and sb is obtained from the measurements of the thermal wave phase angle / as a function of the modulation frequency. In the thermally thick region of our experiment, these parameters are calculated from Eq. (3), by writing it as yP=AyPAexp[i(xt+/)], where AyPA ¼
h
cPo ls at bIo as
lg ks x ð1 bls =2Þ2 þ ðbls =2Þ2
i1=2 h i1=2 1 þ ðxsÞ2 ð4Þ
and p 1 / ¼ tan1 ðxsÞ tan1 2 ð2xsb Þ1=2 1
!
ð5Þ where sb=(b2as)1. Eq. (4) implies that, provided bls<1, the PA signal is proportional to both the optical absorption coefficient and to the thermal expansion coefficient, and that it varies as f 1 for
ð2Þ
where c is the ratio of specific heats for air, Po is the ambient pressure, Io is the incident light intensity at a given wavelength, at is the thermal expansion coefficient of the sample, b is the optical absorption coefficient of the sample, s is the nonradiative relaxation time for the sample, x=2pf, where f is the applied modulation frequency, li is the length of material i, ki is the thermal conductivity of material i, ai=ki/qici is the thermal diffusivity of material i, ri=(1+i)li1 is the complex thermal diffusion coefficient of material i, li=(2ai/x)1/2 is the thermal diffusion length of material i, b=kbrb/ksrs and r=b/rs.
Fig. 4. Amplitude of the photoacoustic signal versus modulation frequency at 490 nm for a soil sample. The solid line corresponds to the best linear fit to the data, which in logarithmic scale represents a f 1 power law.
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that fitted values of s remain very close for all bands suggests that this heat exchange time is dominating nonradiative relaxation process. Comparing the values of s and sb obtained from the modulation frequency phase data fitting Eq. (5), it can be seen that the values of sb exhibit a large discrepancy. The reason for this is the sensitivity of the phase angle / with respect to sb. As pointed by Baesso et al. (1989), the variation of / as given by Eq. (5) is quite insensitive to the value of sb in the low modulation frequency region, thereby leading to a low accuracy in the value of sb obtained from the phase resolved method. 3.3. Electron paramagnetic resonance Fig. 5. Phase of the photoacoustic signal plotted versus modulation frequency for a soil sample. Measurement was performed at 490 nm. The solid line corresponds to data fit (Eq. (5)).
xs 1. The latter condition is normally well fulfilled, whereas the first condition requires that the thermal diffusion length, ls, is much smaller than the optical penetration depth, b1, a condition that is well satisfied for the studied samples within the modulation frequency range of our experiment. The dependence of the PA signal phase on the modulation frequency, Eq. (5), has been used by several authors to study the behaviour of s and sb in collisional deactivation of vibrational excitation in gases, polymers and in nonradiative relaxation of dopant ions in crystal and glasses (Sosman et al., 1998; Rosencwaig and Gersho, 1976; Baesso et al., 1989). Fig. 5 shows the typical results for the phase of PA signal obtained at 490 nm and the corresponding data fitting. The results for each absorption band together with the obtained values of s and sb are summarised in Table 2. Several conclusions can be drawn from the above results. The values of s (about 6.5 ms) are in very close agreement for all absorption bands. Note that the relaxation time s measured photoacoustically is not necessarily that appropriate to the level initially excited in the absorption process, but rather to the average lifetime of a variety of states before energy is lost as heat. Furthermore, for powder samples (as is the case in the present work), s also contains a contribution from a heat exchange time between the powder particles and the transducing gas in the PA cell. This heat exchange time depends not only on the shape of the particles but also on their size. The fact
Fig. 6 shows the EPR spectrum for one sample localised at place A, which is similar to those obtained from the other measured samples. The EPR spectra are typical of Fe3+ ions (Mansanares et al., 1989;
Fig. 6. Typical EPR spectrum of samples from site A. The circles represent the experimental data, continuous line is the best fit and dashed lines are the Lorentzian components.
R.S.T. Manha˜es et al. / Applied Clay Science 21 (2002) 303–311 Table 3 EPR parameters for three main resonances for samples taken from site A: the deviations on DH are about 50 G, on g-value of 0.1 and on I about 30% Layer g1 1 2 3
DH1 (G)
4.2 586 4.2 584 4.2 567
I1 g2 (a.u.) 6.5 6.8 9.0
DH2 (G)
2.3 1895 2.3 2112 2.3 2387
I2 g3 (a.u.) 559 735 1349
DH3 (G)
I3 (a.u.)
2.0 2433 2.0 2639 2.0 2485
4289 6979 6398
Goldfarb et al., 1994). The results are very reasonably fitted to two symmetric lines in the magnetic field region for gi2, corresponding to nondistorted sites, superposed to a gi4.2 line, corresponding to at least one distorted cubic site. The line structure observed in the low-field side of the spectra indicates a distribution of sites with different distortion levels, which are represented by a set of powder spectra. However, for comparison purposes, the gi4.2 line is assumed to be
309
representative of distorted sites because of its prevailing intensity. In order to obtain the usual EPR spectral parameters (position, peak-to-peak amplitude and line width), the experimental data were fitted to three Lorentzian curves. In Fig. 6, the experimental data (o), the best fitting (continuous line) and the Lorentzian components (dashed lines) are shown. The EPR parameters obtained from the fitting for samples from place A are listed in Table 3. The I parameter is equal to A(DH)2, which is proportional to the line spectral intensity, A being the peak-to-peak amplitude and DH the line widths. It can be concluded from the EPR data that Fe 3+ ions in samples are located in cubic distorted sites, occupying silicon tetrahedral framework positions ( gi4.2 and other low field lines), as well as in nondistorted sites, occupying oxy or hydroxy species interstitial positions ( g=2.3) and octahedral coordinated cation-exchange sites ( g=2.0) (Mansanares et al., 1989; Goldfarb et al., 1994). From
Fig. 7. Powder diffractogram of a soil sample and the result of Rietveld refinement for quantitative phase analysis. Peaks corresponding to each crystalline phase are indicated as K (kaolinite), Q (quartz), G (gibbsite) and A (anatase).
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line intensity results, one cannot report much concerning the concentration of Fe3+. Nevertheless, it is clear that the amount of the distorted sites is much less than that of nondistorted ones. On the other hand, the line-width indicates more concentrated Fe3+ clusters for the nondistorted sites, which is expected for oxides and other similar species. Free iron oxides were removed applying the sodium dithionite – citrate procedure (Mehra and Jackson, 1960), with a minimum of destructive action to the clay minerals. The PAS and EPR spectra for the chemically treated samples confirmed a decrease in Fe3+ content. The observed EPR profiles were very similar in shape to the spectra of nontreated samples. They were also fitted to three Lorentzian curves with positions fixed at the previously determined values ( g =2.0, 2.3 and 4.2). For the chemically treated samples, the peak-to-peak amplitude ratio of the resonance g =4.2 (distorted sites at the crystalline structure) to g =2.0 and 2.3 (nondistorted sites) was 60%, in contrast to the 30% ratio obtained for the natural soil. This result suggests that the chemical treatment removed preferentially Fe ions from the nondistorted sites. 3.4. X-ray diffraction X-ray powder diffraction data were collected in the angular range 3jv2hv70j with a step 0.02j and a 5-s counting time. All soil samples presented very similar results. A quantitative phase analysis was performed using the Rietveld method with the DBWS program (Young et al., 1995). Fig. 7 shows the Rietveld fitting for one powder diffractogram. The results indicate kaolinite as the main phase (85.8% in mass), followed by quartz (5.5%), anatase (5.2%) and gibbsite (3.6%). Iron oxides such as hematite, magnetite and ilmenite were not detected. Atomic positions, cell parameters, site occupation factors and texture were refined for kaolinite. Structural parameters of the minor phases were not refined. Taking into account the important presence of Fe ions detected by X-ray fluorescence, PAS and EPR, the possibility of Al3+ substitution by Fe3+ cations in the octahedral sites of the kaolinite structure was considered. A significant improvement of the quality of fitting (residuals RBragg=7.36%, RP=13.42% and Rwp=16.95%) was achieved by substituting 50% of the Al3+ ions by
Fe3+ ions. Meads and Malden (1975) who used Mo¨ssbauer spectroscopy also mentioned the possibility of this substitution. Finally, it was observed that the chemical treatments for removal of free iron oxides do not affect significantly the crystalline phases mass fractions obtained in the Rietveld refinements.
4. Conclusions The X-ray fluorescence spectra allowed the identification of the chemical components in soil samples as being Fe, Ti, K, Ca, Si and Al. The weight concentration of Fe atoms that was determined for all the samples varied between 4% and 10%. Results of Xray diffraction analysis showed that the main crystalline phase present in these samples is kaolinite (86% in mass) followed by minor quantities of anatase, gibbsite and quartz. Rietveld refinements showed that Fe3+ cations partially substitute for Al3+ cations in the octahedral sites of the kaolinite structure. The PAS showed transition bands associated with Fe3+ ions in tetrahedral or octahedral symmetry; samples with higher Fe content also presented larger amplitudes of the PA signal. The PA spectroscopy and EPR results are in close agreement, since EPR spectra presented a dominant component ( g =2.0) that corresponds to Fe3+ ions occupying octahedral coordinated cation-exchange sites. The results presented in this work, which refer to the characterisation of the soil in natura, are part of a larger project which comprises the analysis of the optical, thermal and rheological properties of extruded pieces as a function of the firing temperature, as well as other procedures adopted for the production of bricks and tiles. The Fe content affects the final properties of the solid pieces, for instance, the ideal firing temperature for recrystallization, which is responsible for a series of rheological effects such as hardening and shrinkage. This temperature could be lower for samples with higher Fe content, since Fe atoms act as an ‘‘internal source of heat’’, accelerating the involved reactions. Furthermore, we are concerned with the environmental impact related to the clay extraction and gases emitted during production, which are also studied and will be reported in another paper.
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Acknowledgements This work was partially financed by CNPq and FAPERJ, the support of which is gratefully acknowledged.
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