Nat Hazards (2007) 40:647–655 DOI 10.1007/s11069-006-9015-z ORIGINAL PAPER
Environmental mineralogy: spectroscopic studies on ferrous saponite and the reduction of hexavalent chromium G. Parthasarathy Æ B. M. Choudary Æ B. Sreedhar Æ A. C. Kunwar
Received: 27 June 2005 / Accepted: 17 November 2005 / Published online: 15 November 2006 Springer Science+Business Media B.V. 2006
Abstract A green-coloured phyllosilicate occurring on the walls of amygdaloidal cavities and along fractures in the Deccan Flood basalts at Killari, Maharashtra, India, has been identified as iron-rich saponite with a chemical composition [Na0.60 K0.40 Ca0.47] {Mg2.05Fe3.95} (Si6.45Al1.55) O20(OH)4. In order to explore the possible application of this phyllosilicate for environmental management, we have carried out X-ray photon spectroscopic (XPS) and diffuse reflectance spectroscopic (DRS) measurements on the dichromate solutions, in both the untreated and treated form. The dichromate solution treated with the saponite samples show a remarkable capability not only to adsorb hexavalent chromium but also effect a reduction of hexavalent to trivalent chromium at an efficiency of 75%. These valence states of chromium were characterised unambiguously by XPS and DRS spectra collected at room temperature. Our studies show that Killari saponite is capable of reducing Cr (VI) to Cr (III). The ferrous saponite in Deccan Flood basalts could therefore be a useful mineral in environmental management in areas affected by Cr(VI) effluents. Keywords
Chromium Æ Environmental hazards Æ Killari Æ Saponite
1 Introduction Environmental mineralogy is the study of minerals and related phases with the purpose of understanding and assessing their influence in the movement and fixing of organic and inorganic contaminants at and near the Earth’s surface. Chromium (Cr) exists at various valences, including elemental, trivalent and hexavalent chromium, G. Parthasarathy (&) National Geophysical Research Institute, Uppal Road, Hyderabad 500007, India e-mail:
[email protected] B. M. Choudary Æ B. Sreedhar Æ A. C. Kunwar Indian Institute of Chemical Technology, Hyderabad 500 007, India
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and undergoes reduction-oxidation reactions in the environment. This element is used in various industrial processes and has been introduced into the environment as by-products of these processes, among which are electroplating, leather tanning and ore and petroleum refining. The toxicity and mobility of chromium in the earthsurface environment are dependent on its oxidation state (James, 2003). As hexavalent chromium is a known human carcinogen, it is most important to find a simple method for converting hexavalent chromium to its reduced trivalent state. In addition, hexavalent chromium in ground waters is known to be highly toxic and environmentally hazardous (Blowes et al. 1997; Blowes 2002; Elderfield 1970). The treatment of hexavalent chromium-bearing waters aims at reducing Cr(VI) to Cr(III), the latter being non-toxic and immobile (Blowes 2002; Kim et al. 2002). Based on the results of their recent study, Taylor et al. (2000) suggested that dithionate clays can reduce hexavalent chromium. Smectites which contain Fe(II) are also considered to be very useful for immobilising and reducing hexavalent chromium. Iron-rich smectites are common as hydrothermal alteration products of basalts and andesites and could serve as a source for such clays. One example of this is the green clay mineral found in the Deccan Flood basalts of India, which is an iron-rich saponite. In this paper we present the results of X-ray spectroscopic (XPS) and diffuse reflectance spectroscopic (DRS) studies on the dichromate solution treated with ferrous saponite and demonstrate the application of this phyllosilicate in the adsorption and reduction of hexavalent chromium.
2 Data and analysis The samples of ferrous saponite were obtained from the secondary fillings of the Killari borehole in the Deccan Flood basalts, which had been drilled after the Latur earthquake on September, 30, 1993 (Gupta et al. 1999). The chemical composition of the samples was determined by electron probe micro analyser (EPMA) and also by using a Hitachi S-520 scanning electron microscope (SEM) in EDAX mode with a filament current of 110 mA and an accelerating voltage of 20 kV. The ferrous iron content was also independently determined by the titration method following the procedure suggested by Wilson (1960). For the powder diffraction study, the ground sample was sieved through a 40-lm filter to obtain uniformly sized material. Fourier transform-infrared spectroscopic (FT-IR) studies on the samples were carried out under ambient conditions using a Bio-Rad 175C FT-IR spectrometer (Bio-Rad, Hercules, Calif.) and adopting the potassium bromide (KBr) pellet method. Samples were scanned in the frequency range of 4000–300 cm–1. A blank KBr pellet of approximately the same thickness was used to determine the background spectrum. Ten to twenty scans were run for each spectrum at a resolution of 1 cm–1. Each experimental run was made on three different set of samples. Typical uncertainty in the wave numbers is ±5 cm–1 in the frequency region 2000–4000 cm–1. A solid-state MAS-nuclear magnetic resonance (NMR) [29Si] spectrum was obtained on the samples using a Varian UNITY-400 NMR spectrometer (Varian, Palo Alto, Calif.) at room temperature. The sample was spun at the magic angle (54.7) with respect to the direction of the magnetic field in a silicon nitrite rotor at about 5.3 kHz. The experimental conditions were as follows: transmitter frequency was 79.45 MHz; pulse length = 15 ls; 90 pulse = 15 ls; the probe head was supplied from Varian Unity-400. The chemical shift is obtained with reference to tetra-methyl
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silane (TMS) as external standard (dTMS = 0 ppm). The [27Al]- NMR spectrum was collected using the same instrument of Varian Unity-400 under the following experimental conditions: transmitting frequency of 104.22 MHz; pulse length = 5 ls; 90 pulse = 10 ls. A 1 M solution of Al(NO3)6 Æ 6H2Oin water was used as reference. Since the [27Al] relaxes due to the quadrupole moment the relaxation delays of 2 s are sufficient to achieve complete relaxation. The spectra were obtained by accumulating 800–1000 free induction decays with a recycle delay of 10 s. For the chromium-adsorption measurements, we placed the clay sample in a 15-ml centrifuge tube and added a 0.05 M potassium dichromate solution. The contents were first agitated for 1 hand then heated to 150C. Suspended particles were dried, and the XPS method was used to analyse the dried particles in order to determine the valence state of the adsorbed chromium. XPS measurements were conducted with a Kratos XPS Axis 165 spectrometer (Kratos, Chestnut Ridge, N.Y.) equipped with a hemispherical energy analyser. The non-monochromatised Mg-Ka X-ray source (hm=1253.6 eV) was operated at 5 kV and 15 mA with a pass energy of 80 eV and an increment of 0.1 eV. The samples were gassed out for several hours in the XPS chamber to minimise air contamination to sample surface. In order to overcome the charging problem, a charge neutraliser of 2 eV was applied and the binding energy of C 1s core level (B.E. = 284.6 eV) of adventitious hydrocarbon was taken as the standard. The obtained XPS spectra were fitted using a non-linear square method with the convolution of Lorentzian and Gaussian functions after the polynomial background subtraction from the raw spectra. Analysis by UV-Vis diffuse reflectance was performed under ambient conditions on a Cintra 10e GBC spectrometer (GBC, Victoria, Australia) operating in the diffuse reflectance mode and using BaSO4 as the reference. Measurements were obtained by pressing into the pellet a mixture of the sample with KBr powder in the wavelength region 200–800 nm with a scan speed of 400 nm/min.
3 Results and discussion The composition of the Killari saponite shows that it is a trioctahedral smectite and that its composition is similar to that of an iron-rich saponite that has been found in the rhyolitic glassy tuffs at Oya, Japan (Kohyama et al. 1973). An interesting observation is that the mineral has iron only in the ferrous state. Figure 1 shows the typical FT-IR spectra of the Killari clays in different frequency ranges. The FT-IR technique investigates OH vibrations, whose absorption bands appear at different frequencies depending on the cations directly linked to the hydroxyls. These vibrations enable the distribution of cations around the hydroxyls to be determined, thereby allowing the short-range cation ordering to be assessed (Cuadros et al., 1999; Wilson, 1994). The spectrum in the region of 2500–4000 cm–1 shown in trace A of Figure 1 exhibitsstrong absorption peaks at 3678, 3590 and 3450 cm–1. The bands between 3450 and 3670 cm–1 are attributed to the OH stretching mode. As almost all trioctahedral smectites show similar IR spectral bands in the hydroxyl region, it is very difficult to distinguish between hectorite, saponite and stevensite in FT-IR spectra obtained at room temperature. Russell and Fraser (1994) suggested that the spectra taken following heat treatment of the KBr-pressed samples would distinguish the different trioctahedral smectites. The FT-IR spectrum of the heattreated Killari clay is shown in trace B of Fig. 1. The appearance of an additional
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Fig. 1 Fourier Transform infrared (FT-IR) spectra of the ferrous saponite. Trace A is the spectra of the hydroxyl component of ferrous saponite, trace B shows the spectra collected from a heat-treated (at 450 K) sample and indicates the trioctahedral nature of the ferrous saponite. Trace C is the FT-IR spectrum of the sample in the higher frequency range. The peaks in the range of 600–700 cm–1 are typical Fe–O out-of-plane vibrations in trioctahedral smectites
high-frequency satellite band at 3722 cm–1 confirms that the Killari clay is saponite. The additional satellite band is due to the interlayer K+ ions (from KBr) electrostatically repelling the proton of the OH group (Russell and Fraser 1994). This occurs in saponite because of the highly localised negative charge on the Si–O surface, which is theresult of a Al for Si substitution. In ferric saponite hydroxyl peaks at 3610 and 3400 cm–1 are characteristic. Their absence suggests that the Killari sample is ferrous saponite. Trace C in Fig. 1 shows the IR bands in the frequency range 350–2000 cm–1. The bands between 600 and 690 cm–1 are considered to arise from the trioctahedral smectites (Farmer, 1958). In trioctahedral smectites, the bands near 1000 cm1 are assigned to OH-deformation (Al–Fe–Mg OH), a sharp band at 650–690 cm–1 to Fe– O out-of-plane vibration, and very broad peaks at lower frequencies of 390, 440 and 465 cm–1 to Si–O–Mg stretching modes (Farmer 1958). Note that the oxidised saponite (ferric saponite) shows IR absorption bands at 455, 514, 616, 670, 724, 798 and
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878 cm–1. The absence of these peaks in the iron-rich saponite of Killari shows that iron is not in the ferric state, thereby corroborating the inference based on hydroxyl vibrational modes. Figure 2a shows the [29Si]-MAS-NMR spectrum of the Killari sample spun at 5.3 kHz. The strong peak at the chemical shift of –102.19 ppm indicates the tetrahedral coordination of silicon. The other strong peak at around –47 ppm is due to the rotor. The [27Al]-MAS-NMR spectrum (Fig. 2b) exhibits a strong peak at
Fig. 2 (a) Magic angle spinning [29Si]-NMR spectrum of ferrous saponite. The peak at –102.2 ppm is due to the tetrahedral Si, and the peak R denotes the peak due to the silicon nitride rotor. (b) Magic angle spinning [27Al]-NMR spectrum of ferrous saponite. The peak at 57.7 ppm is due to the tetrahedral Al. The peaks marked by asterisks are spinning side bands
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57.69 ppm, indicating the tetrahedral coordination for aluminum. The peaks with an asterisk symbol are due to spinning side bands. The spinning side band centered around 2 ppm suggests the possible presence of traces of aluminum in octahedral coordination. It is well known that octahedral Al shows very strong peaks at +2 and –2 ppm and an intrinsic asymmetry in the more negative chemical shift range (Cuadros et al. 1999; Kinsey et al. 1985). The NMR results on Killari clays are found to be in good agreement with the NMR spectra obtained on trioctahedral smectites (Goodman and Stucki, 1984). The presence of a broad side band at 0 ppm could be attributed either to the Al–Al and Al–OH dipole-dipole interactions or to the possible presence of some amorphous phase. However, earlier NMR studies on montmorillonite revealed the presence of tetrahedral and octahedral aluminum (Fig 2b) and demonstrated that NMR can be a very powerful technique for determining the octahedral-to-tetrahedral Al ratio (Kinsey et al., 1985). In the present study, we have observed most of the aluminum to be tetrahedral with a chemical shift of 57.69 ppm, which is consistent with the nature of the trioctahedral character (or ferrous-saponite). 3.1 Adsorption and reduction of hexavalent chromium Adsorption of hazardous Cr(VI) by the Fe(II)-containing clay was a prerequisite for the coupled adsorption-reduction reaction. The capacity of clays to reduce Cr(VI) has been correlated with the ferrous iron content of the clays. As the clay mineral under study is found to be a ferrous saponite, we examined its effectiveness in reducing Cr(VI). In this study, we have adopted the XPS 7 technique to determine the valence state of chromium and iron after treating the clay with the 0.05 M potassium dichromate solution. It is well known that potassium dichromate solution contains Cr in the hexavalent state. The narrow scan of Cr 2p for the Killari saponite treated with dichromate solution is shown in Fig. 3. Two peaks at binding energies of 587.9 eV and 577.7 eV corresponding to Cr(VI) and two peaks at 585.8 eV and 576.0 eV corresponding to Cr(III) were observed. It is clear from the intensity of the peaks in Figure 3 that the clay sample not only adsorbs the Cr(VI) but also reduces it at an efficiency of more than 78% to Cr(III). Similarly, the two Cr 2p3/2 peaks observed were attributed to the Cr3+ and Cr6+ oxidation states following Chastain (1992). The lowest binding energy component at 576.0 is due to the chromium oxide phase-like Cr2O3 phase (Chung et al. 2002). We also observed that the ferrous iron in the clay sample was oxidised to ferric state following the treatment with dichromate solution. The Fig. 3 The XPS spectrum of saponite treated with dichromate solutions. The volume ratio of the sample to the dichromate solution is 2:1. The binding energy peaks are assigned to various valence states of chromium
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samples treated with dichromate solutions showed two peaks for Fe 2p3/2 — at 710.9 eV and 713.0 eV — which are ascribed to the Fe2+ and Fe3+ oxidation states, respectively. Figure 4 shows the diffuse reflectance spectra of the chromate samples treated with saponite recorded in the range of 200–800 nm at room temperature. As can be seen in trace C of Fig. 4, there are two absorption bands at k = 260–280 nm and 370– 380 nm in the charge transfer region which are characteristic of the presence of chromium in the Cr6+ oxidation state. Trace B shows that together with these two intense bands, there is a shoulder at about 470 nm and two additional bands at k = 450–465 and 580–600 nm due to the 4A1g fi 4T1g and 4A2g fi 4T2g transitions for Cr3+ ions in a strongly distorted octahedral crystal field symmetry. It has also be suggested that the band at 600 nm could be due to the presence of Cr6+ and Cr3+ ions which interact through dMd electron exchange (Chastain, 1992; Wong and Angell, 1976). These observations are in agreement with the ESCA (electron spectroscopy for chemical analysis) results, which also support the coexistence of both Cr3+ and Cr6+ species in saponite-treated chromate samples. Trace A of Fig. 4 shows the UV-Vis-diffuse reflectance spectrum of saponitetreated chromate samples at the ratio of 3:1 of saponite to dichromate solution. From the ESCA spectra it is clear that these samples contain mostly Fe ions in both the Fe2+ and Fe3+ oxidation states. The absorption spectrum has three bands centred at 750, 375 and 280 nm, respectively,that can be attributed to Fe3+ ions in octahedral symmetry; these observed bands are assigned to 6A1g(s) fi 4T1g(G), 6A1g(s) fi 4Eg(D) and 6A1g(s) fi 4T1g(P), respectively. On the other hand, Fe2+ has no optical bands in the measured range. Trace B of Figure 4 shows the UV-Vis-diffuse reflectance spectrum of saponite treated with chromate solution at the ratio of 2:1 of saponite to dichromate solution SAPGP2; this spectrum shows the presence of both Cr3+ and Cr6+ and Fe2+ and Fe3+ in the XPS spectrum. Bands centred at 300, 375, 400, 490, 555 and 640 nm are observed that can be attributed to both chromium and iron ions. These bands are overlapping and broad and have shoulders that result from the coexistence of these two ions in multivalent oxidation states. Our XPS and DRS studies on the Killari saponite revealed that this saponite is capable of reducing Cr(VI) to Cr(III). The ferrous saponite in the Deccan Flood basalts could therefore be useful a mineral in environmental management in areas affected by Cr(VI) effluents. Larger quantities of the minerals could be expected to occur along cooling cracks and in the intensely fractured and altered flow top breccias. As the uptake and release of the contaminant by the clay can change with Fig. 4 Diffuse reflectance spectra of saponite treated with dichromate solutions of varying concentrations. Spectra A, B and C represent those samples in which the ratio of saponite to dichromate solution is 1:3, 2:1 and 3:1, respectively
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eH and pH, further studies are in progress that aim at improving our understanding of the mechanism of chromium adsorption-reduction reactions. These studies would provide a basis for the utilisation of ferrous saponite in environmental management. Studies by White and Yee (1985), Eary and Rai (1988), Blowes et al. (1997), Blowes (2002) and Kim et al. (2002) have shown that ferrous iron-bearing minerals such as pyrite, biotite, augite, hornblende and siderite can reduce Cr(VI) to CrIII) as the minerals undergo dissolution to release ferrous iron into solution or as Cr(VI) adsorbs onto a surface to allow direct contact with Fe(II). The usage of pyrite is known to cause environmental problems, such as acidification of the soil and surface water. Moreover, a 50-lM chromate solution has been converted to Cr(III), even though both Cr(VI) and Cr(III) were present at higher concentrations of more than few millimoles (Doyle et al. 2004). However, the reduction of hexavalent chromium by FeS is favoured at a lower pH. Earlier studies by Kim et al. (2002) have found that the finely ground hydro, thermally altered pyrite-rich andesites was effective in reducing hexavalent chromium only in KCl and K2SO4 solutions. The reduction of Cr(VI) to Cr(III) by pyrite in the presence of dissolved oxygen involved both reactions at the solid-solution interface and in solution. There are other studies which have used the microbial reduction of hexavalent chromium by natural organic materials at low pH, suggesting that polycondensed aromatic humic materials may be useful in mediating the bioreduction and rapid immobilisation of chromium (Gu and Chen 2003). The present study provides yet another new material for the application of treatment of ground water contaminated with hexavalent chromium. Acknowledgments We are grateful to V.P. Dimri, Director, National Geophysical Research Institute, and Dr. J.S. Yadav, Director, Indian Institute of Chemical Technology, Hyderabad for their encouragement and support. We thank Dr. H.K. Gupta, Secretary, Department of Ocean Development, Government of India, New Delhi, for his enthusiastic support and also for his keen interest in the studies of the mineralogy of the Killari borehole. We thank Dr. R. Srinivasan for many useful discussions. GP is grateful to Dr. Tom Beer, Australia for his kind appreciation of this paper. We thank Dr. R. K. Chadha for inviting us to make this contribution to the special issue of Natural Hazards. We gratefully acknowledge the partial financial support by ISRO, Department of Space Government of India under the PLANEX programme. The manuscript has been improved by constructive comments from the reviewers and from Prof. W.A. Bassett, Cornell University, Ithaca, USA.
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