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An XPS study of the adsorption of chromate on goethite (α-FeOOH)
Applied Surface Science 108 Ž1997. 371–377
An XPS study of the adsorption of chromate on goethite ž a-FeOOH/ Hesham Abdel-Samad, Philip R. Watson
)
Department of Chemistry, Oregon State UniÕersity, Gilbert Hall 153, CorÕallis, OR 97331-4003, USA Received 30 May 1996; revised 21 August 1996; accepted 26 August 1996
Abstract The adsorption behavior of inorganic oxyanions on soil minerals is an important factor in the transport of subsurface environmental pollutants. We have studied the adsorption of chromate ŽCrO42y . from aqueous solution on the surface of the mineral goethite Ž a-FeOOH. as a function of pH and adsorbate concentration in 0.05 M NaNO 3 solution. Results obtained from the dried surface by X-ray photoelectron spectroscopy ŽXPS. are in good agreement with data from spectrophotometric analysis of chromate remaining in the supernatant liquid. Chromate adsorption increases with decreasing pH of the solution and eventually reaches a maximum at pH 6.5. The chromium XPS signal indicates that initially a small amount of chromium adsorbs in the q3 oxidation state via a redox reaction, but that the large majority of chromium remains in the q6 oxidation state.
1. Introduction The study of the adsorptionrdesorption behavior of inorganic oxyanions on soil mineral surfaces has particular relevance to plant nutrition and the mobility of subsurface environmental pollutants. Adsorption from aqueous solution at the solid surface significantly alters anion mobility and can affect the rate of dissolution and mechanism of ageing of the solid w1,2x. Chromate ŽCrO42y . is representative of inorganic oxyanions with intermediate binding strength to metal oxides. It is also of particular interest as it is a common toxic contaminant that is a regulated substance in U.S. domestic water supplies w3x. Goethite, the hydrated iron oxide a-FeOOH, is commonly found in soils in most climatic regions )
Corresponding author. Tel.: q1-541-7376740; fax: q1-5417372062.
w4x. It has been extensively studied as a model for hydrated oxide soil minerals. The orthorhombic structure of goethite can be best understood as an hexagonally close-packed array of O 2y and OHy anions with FeŽIII. ions in some of the octahedral holes. The FeŽIII. ions are arranged in such a way that double chains of distorted edge-sharing FeŽO,OH. 6 octahedra run parallel to the w001x direction. As usually prepared by direct precipitation it has a surface area of 10–100 m2rg w5x. Chromate adsorption on goethite has previously been studied by conventional titration techniques that measure the residual chromium in solution after adsorption from aqueous solution w5–9x. The adsorption process has been modeled using surface complexation models ŽSCMs. which assume that adsorption involves both a coordination reaction at specific surface sites and an electrostatic interaction between adsorbing ions and the charged surface w2,9,10x. The
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H. Abdel-Samad, P.R. Watsonr Applied Surface Science 108 (1997) 371–377
most successful have used either a two-layer diffuse layer model ŽDLM., or a triple layer model ŽTLM., using a single set of equilibrium constants w5,6x. The aim of the present investigation work is to use a modern surface sensitive technique, namely X-ray photoelectron spectroscopy ŽXPS., to supplement these wet chemical investigations in a more direct manner. XPS measurements for goethite itself have been published by several authors w11–14x. In addition XPS data are available for the adsorption of several other oxyanions Žphosphate, sulfate and selenite. on goethite w15x, but not for chromate. We have successfully monitored the adsorption process by XPS over a wide range of pH values and adsorbate concentrations. We find excellent agreement with wet chemical data performed concurrently. The Cr XPS signal is indicative of some redox chemistry occurring during the early stages of adsorption, but the bulk of the Cr is adsorbed as CrŽVI. on the surface. The nature of the surface complex has not been resolved using classical titration methods, although other indirect evidence suggests that the bonding in the chromatergoethite surface complex is primarily electrostatic, or ‘outer-sphere’, rather than coordinative w16x. Future work in this laboratory will address this problem.
2. Experimental 2.1. Synthesis and characterization of goethite (aFeOOH) A single batch of goethite was prepared according to the method of Schwertmann and Cornell w17x. 180 ml of 5 M KOH solution was rapidly added with continuous stirring to 100 ml of 1 M FeŽNO 3 . 3 P 9H 2 O solution in a 2 L polyethylene flask. The solutions were thoroughly purged with nitrogen before and during reaction to exclude carbon dioxide. Red brown ferrihydrite was precipitated at once. The suspension was then immediately diluted to 2 L with doubly-distilled water and held in a closed polyethylene flask at 708C for 60 h. During this period, the voluminous, red brown suspension of ferrihydrite became a compact, yellow brown precipitate of goethite. This was then centrifuged, washed to remove OHy and NOy 3 ions and dried at 408C for
another 60 h. Powder X-ray diffraction pattern ˚ which showed main peaks at 4.177, 2.69 and 2.45 A, agreed well with the literature values w4x. The N2-BET surface area was 22 m2rg. 2.2. Adsorption experiments We performed two types of adsorption experiments. Adsorption pH edge experiments involved a series of measurements as a function of pH Ž4–11., using a constant adsorbate concentration Ž C s 1.4 mM. at a constant ionic strength Ž I s 0.05 M. set with sodium nitrate. Goethite is not very soluble in water in the pH range 3–12, although some anions, such as oxalate, can increase its solubility at pH - 5. Chromate is not known to have this effect w5x. All experiments were performed at 258C. Appropriate amounts of 1.0 M NaNO 3 stock solution and 25 ml of 0.10 M Na 2 CrO4 P 4H 2 O stock solution were mixed with N2-purged ultra-pure water. After mixing and N2-purging the solution, the pH was raised to 11 using 0.1 M NaOH solution. The total experimental volume was fixed at 180 ml, the addition of 0.36 g of goethite producing an adsorbent concentration of G s 2.0 grL. The solution was then divided into eight samples, placed in tightly capped, 250 ml polypropylene cups under an N2 atmosphere and the pH readjusted to a value in the range 4–11. The samples were then sealed and shaken. Trial experiments showed that the time required for achieving adsorption equilibrium was about 10 h, consistent with other studies w5x. After 12 h equilibration, two 1 ml aliquots were transferred to microcentrifuge tubes and centrifuged for 10 min at 13,000 rpm and the supernatant solution removed. The drift in pH over this period was less than 0.5 pH units. The solid residues were freeze-dried prior to XPS analysis. These samples were not washed prior to freeze-drying to lessen the possibility of removing adsorbed chromate. Calculations show that the maximum amount of chromate that might have precipitated out from residual solution on to the surface during drying is less than 5% of that measured on the surface, so the lack of washing does not lead to misleadingly high surface chromium levels. Adsorption isotherm experiments were performed in a similar manner at 258C with adsorbate concentrations between C s 0.055 mM and 1.7 mM using
H. Abdel-Samad, P.R. Watsonr Applied Surface Science 108 (1997) 371–377
appropriate volumes of 0.10 M Na 2 CrO4 P 4H 2 O stock solution as titrant at a constant pH of 6.5. 2.3. Residual aqueous Cr analysis Residual CrŽVI. ion concentrations in the liquid phase were quantified by a colorimetric method using diphenyl carbazide at pH f 1 w18x. Absorbances were measured at 540 nm on a diode-array spectrometer. Surface adsorbate concentrations were calculated as the difference between original and residual solution concentration. The maximum amount of Cr adsorbed was about 1r3 of that available in solution. 2.4. XPS analysis
373
plate using double-sided adhesive graphite tape. The instrumental work function was set to give a value of 83.9 eV for the binding energy of the Au 4f 7r2 line for metallic gold w19x, and an electron gun set at 0.7 eV and 0.02 mA was used to reduce sample charging. The XPS photoelectron binding energy of the adventitious carbon species, i.e., the C 1s line at 284.8 eV was used to correct the observed binding energies for surface charging w20x. Integrated peak intensities were evaluated by measuring the peak area above the background and correcting the measured intensity for the photoionization cross-section w21x. 3. Results and discussion
The XPS studies were carried out with a modified Hewlett-Packard 5950A ESCA spectrometer, using an aluminum anode ŽAl K a s 1486.6 eV. operating at 600 W. Samples were prepared by adhering a very thin layer of the powdered sample to an aluminum
3.1. XPS data from goethite and reference compounds Table 1 compares our XPS binding energies for goethite and reference iron compounds with those
Table 1 XPS binding energies Žin eV. for the major core lines of goethite and Fe and Cr reference compounds used in this study compared with literature values Material
Binding energy ŽeV. Fe 2p 3r 2
Goethite ŽFeOOH.
Fe 2 O 3
FeO Na 2 CrO4 P 4H 2 O Cr2 O 3
Reference
O 1s
Cr 2p 3r2
O 2y
OHy
711.0 708.2 Žsh. 711.2 711.7
528.4
531.0
533.0
529.8 530.1
531.1 531.2
711.1 710.9 710.4 711.1 710.7 711.3 710.0 709.2 — — — —
530.0 530.0 529.7 530.6 529.6 530.1 529.5 530.0 — — — —
531.0 531.7 531.4 — — — — — — — — —
— a 532.3 533.4 — a — a — a — — — — — — — — —
H 2O
b
—
This work
— —
w11x w12x
b
— — — — — — — — 579.4 579.4 576.2 576.8
w13x w14x w15x This work w12x w14x This work w12x This work w22x This work w22x
All binding energies are located to better than "0.2 eV. The data from Ref. w11–16x have been adjusted to use the same C 1s and Au 4f reference binding energies as used here. a Cannot be reliably estimated from data in reference. b Estimated from spectra in reference.
374
H. Abdel-Samad, P.R. Watsonr Applied Surface Science 108 (1997) 371–377
Fig. 1. XPS spectrum from a freeze-dried goethite sample in the Fe 2p region.
measured by other workers w11–15x. The literature data in Table 1 have been corrected to be referenced to the same C 1s or Au 4f binding energies as we found. Our values for the main Fe 2p 3r2 peak from goethite fall within the range of values reported for goethite and other FeŽIII. species such as Fe 2 O 3 . The spread of Fe 2p 3r2 peak energy values about 711 eV obtained by different authors presumably reflect to some degree variations in background subtraction methods, but it is possible that they also reflect differing amounts of associated water. Our goethite spectrum ŽFig. 1. also shows a shoulder to the low binding energy side of the main FeŽIII. 2p 3r2 peak. The position of this peak Ž708.2 eV. is consistent with a reduced iron species, although it lies to lower binding energy than the corresponding peak for FeO Ž; 709.6 eV.. A careful examination of the goethite spectrum of Allen et al. w14x shows such a low energy shoulder that is absent in their Fe 2 O 3 spectrum. The FeOOH spectrum of McIntyre and Zetaruk w12x may also have a low binding energy shoulder though it is less pronounced. It is our belief that synthesized goethite can contain some fraction of FeŽII. at least at the surface. This point becomes of some importance later in the context of redox reactions during Cr adsorption. The goethite O 1s region clearly contains several components. We have attempted fits to just oxide and hydroxide components, with various restrictions on widths etc. and also into three peaks from oxide, hydroxide and water. As our sample came from an aqueous synthesis with freeze-drying, but no heating,
Fig. 2. XPS spectrum from a freeze-dried goethite sample in the O 1s region, showing the best-fit deconvolution into signals from oxide, hydroxide and water Žsee text..
we feel it is likely that some adsorbed water remains on the surface of the goethite. The O 1s region of FeOOH in the spectrum of McIntyre and Zetaruk w12x also contains high binding energy components that they assign to adsorbed water. Fig. 2 shows the best fit deconvolution which uses oxide, hydroxide and water components, although the fit is not unique and a range of almost equally good fits with three components are possible. Fits omitting the water component give significantly poorer agreement. Table 2 provides peak intensity ratios for the O 1s peaks from our work compared with those that can be garnered from the literature. The derived oxide and hydroxide peaks energies shown in Table 1 are consistent with previous literature values. The fit produces an oxide peak that is considerable wider than the hydroxide component which complicates reliably quantifying the contributors to the O 1s signal. The ratio of our signal intensities from hydroxyl and oxide oxygen is close to 1 Žbut with a large error., in good agreement with the bulk FeOOH stoichiometry and can be compared with other values of 0.90 w12x and 1.04 w11x. Table 2 Estimates of the contributions of different oxygen species to the O 1s XPS peak of goethite from this study compared with literature values w11,12x Relative XPS OHyrO 2y
Peak area H 2 Ortotal O Reference
0.9–1.1 1.03 0.9
0.08–0.10 - 0.05 a 0.17 a
a
Estimated from spectra in reference.
this work w11x w12x
H. Abdel-Samad, P.R. Watsonr Applied Surface Science 108 (1997) 371–377
Fig. 3. XPS Cr 2p spectra after adsorption of 1.4 mM chromate solution at an ionic strength of 0.05 M onto goethite at various pH values. All spectra are on the same ordinate scale but shifted for clarity.
Our Cr 2p 3r2 spectra for the reference compounds Cr2 O 3 and Na 2 CrO4 P 4H 2 O ŽTable 1. compare very well with those from the large compilation by Allen and Tucker w22x. In general CrŽVI. compounds exhibit sharper peaks at higher binding energies Ž; 579.4 eV, D1r2 ; 2.1 eV. than those from CrŽIII. compounds Ž; 576.6 eV, D1r2 ; 3.3 eV..
375
Fig. 4. Comparison of the amount of Cr adsorbed from XPS Žcircles. and spectrophotometry data Žtriangles. for chromate adsorbed on 0.36 g goethite as a function of pH. The experiments used wNa 2 CrO4 x s1.4 mM at a constant ionic strength of 0.05 M and an adsorbent concentration of 2.0 grl.
amount of Cr removed from solution determined spectrophotometrically, mirrors closely the area of the Cr 2p 3r2 signal from the surface and show the breakthrough pattern at about pH 8 reported by other workers using wet chemical methods w5,6x. The max-
3.2. pH edge experiments For adsorption of chromate at pH values of 11 or higher any Cr 2p signal from the surface was below the level of detectability; spectrophotometry also revealed no surface adsorption ŽFigs. 3 and 4.. This is a result of the goethite surface being negatively charged in this pH range which is unfavorable for the adsorption of anionic chromate species. Adsorption of chromate became significant Žsee Figs. 3 and 4. between pH 9 and 10, reflecting the high point of zero charge value of the goethite substrate w5x. Between pH 10.5 and 6 the XPS signal from surface Cr increases dramatically ŽFig. 3.. These changes were accompanied by a corresponding decrease in the residual CrŽVI. measured in solution ŽFig. 4.. The
Fig. 5. Variations in the Fe 2p 3r 2 Žcircles. and Cr 2p 3r2 Žtriangles. binding energies for chromate adsorbed on 0.36 g goethite as a function of pH. The experiments used wNa 2 CrO4 x s1.4 mM at a constant ionic strength of 0.05 M and an adsorbent concentration of 2.0 grl.
376
H. Abdel-Samad, P.R. Watsonr Applied Surface Science 108 (1997) 371–377
Fig. 6. XPS Fe 2p spectra from goethite Ža. before reaction, Žb. after equilibration at pH 8, and Žc. after equilibration at pH 6 with 1.4 mM chromate solution at an ionic strength of 0.05 M. All spectra are on the same ordinate scale but shifted for clarity.
imum adsorption is reached by pH 6.5 in both sets of data. On metal oxides maximum adsorption of anions of diprotic acids typically occurs at a pH about equal to the second acid dissociation constant; for chromate, log pK a2 s 6.51. Fig. 5 shows that the binding energy of the main Fe 2p 3r2 peak from the goethite substrate after these experiments showed little change from the values before adsorption. In contrast the Cr 2p 3r2 binding energy increases from a value of about 576.2 eV at high pH to a value in excess of 579.5 eV at saturation. While there is a significant difficulty in accurately determining the binding energy of the low intensity peaks from the high pH experiments, the binding energy shift seems to be real Žcompare the spectra in Fig. 3.. These binding energies are consistent with adsorption of the first amounts of Cr as CrŽIII. and the later Cr as CrŽVI. at about pH 6.5 and below. Adsorption as CrŽIII. would necessitate some redox chemistry at the surface, presumably involving FeŽII.. This possibility is supported by the Fe 2p spectra taken after equilibration at pH 8 and pH 6 shown in Fig. 6. The obvious low binding energy shoulder that we have tentatively assigned to FeŽII.
in the unreacted goethite is decreased in the pH 8 spectrum Žwhere adsorption is very weak., and is smaller again in the pH 6 spectrum Žwhere Cr adsorption has saturated.. We speculate while the majority of Cr adsorbs as CrŽVI., there is some FeŽII. present in our goethite sample and initial Cr adsorption occurs as CrŽIII. via a redox reaction with this reduced iron. The origin of this reduced iron is not clear. At high pH, where little adsorption occurs, the Cr XPS signal is indicative of CrŽIII. at the surface and the signal indicative of FeŽII. has decreased from that seen in the unreacted substrate. By pH 6 Cr adsorption has saturated. The Cr signal ŽFig. 3. shows predominantly CrŽVI. with some CrŽIII. component and the FeŽII. signal ŽFig. 6. has decreased very markedly. The shoulder in the Fe 2p spectrum that we are assigning to FeŽII. does not disappear completely even at the highest Cr loadings. This might reflect the presence of some FeŽII. in subsurface layers that still contributes to the XPS Fe 2p signal, but is not accessible to Cr. The possibility of chromate removal from aqueous wastes by reduction with ferrous ion is well known w23x. 3.3. Adsorption isotherm experiments Fig. 7 shows the results obtained from both of the spectrophotometric analysis of residual aqueous
Fig. 7. Adsorption isotherms for chromate adsorbed on 0.36 g goethite from XPS Žtriangles. and spectrophotometry data Žcircles.. The experiments were carried out at pH 6.5"0.2 with constant ionic strength of 0.05 M and an adsorbent concentration of 2.0 grl.
H. Abdel-Samad, P.R. Watsonr Applied Surface Science 108 (1997) 371–377
chromate and XPS analysis of surface chromium after adsorption experiments using increasing concentrations of chromate at a constant pH of 6.5. The adsorption isotherms obtained by the two methods are in good agreement with each other. The data at low Cr values fit the Langmuir isotherm quite well but the data fall well above the plot at saturation. 4. Conclusions The Fe 2p XPS spectrum of goethite shows a small low binding energy feature that we attribute to FeŽII.. This feature is also present, but unidentified, in some other literature spectra. The O 1s spectrum shows an ; 1:1 OHyrO 2y ratio as expected, but also shows a peak attributable to adsorbed water Žalso seen by other workers.. We have observed the adsorption of chromium from an aqueous chromate solution onto goethite as a function of pH and chromate concentration by spectrophotometry and XPS. Results from the two methods agree very well. At high pH adsorption is small, becomes significant by pH 8 and is saturated by pH 6.5. The initial adsorption process appears to involve a redox reaction with small amounts of FeŽII. present on the oxide surface, leading to the adsorption of the metal as CrŽIII., while subsequent adsorption occurs as CrŽVI.. References w1x N. Caraco, J. Cole and G.E. Likens, Biogeochemistry 9 Ž1991. 277. w2x J.A. Davis and D.B. Kent, in: Mineral-Water Interface Geochemistry, eds. M.F. Hochella and A.F. White ŽMineralogical Society of America, Washington, D.C., 1990. pp. 176–260. w3x P.J. Sheehan, D.M. Meyer, M.M. Sauer and D.J. Paustenbach, J. Toxicol. Environ. Health 32 Ž1991. 161.
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w4x U. Schwertmann and R.M. Taylor, in: Minerals in Soil Environments, eds. J.B. Dixon and S.B. Weed, Soil Science of America ŽMadison, WI, 1989. pp. 379–438. w5x K.L. Mesuere and W. Fish, Environ. Sci. Technol. 26 Ž1992. 2365. w6x C.C. Ainworth, D.C. Girvin, J.M. Zachara and S.C. Smith, Soil Sci. Soc. Am. J. 53 Ž1989. 411. w7x J.M. Zachara, D.C. Girvin, R.L. Schmidt and C.T. Resch, Environ. Sci. Technol. 21 Ž1987. 589. w8x S. Music, ´ M. Ristic´ and M. Tonkovic, ´ Z. Wasser-AbwasserForsch. 19 Ž1986. 186. w9x J.A. Davis and J.O. Leckie, J. Colloid Interface Sci. 74 Ž1980. 32. w10x D.A. Dzombak and F.M.M. Morel, Surface Complexation Modeling: Hydrous Ferric Oxide ŽWiley, New York, 1990. p. 59. w11x D.T. Harvey and R.W. Linton, Anal. Chem. 53 Ž1981. 1684. w12x N.S. McIntyre and D.C. Zetaruk, Anal. Chem. 49 Ž1977. 1521. w13x C.R. Brundle, T.J. Chuang and K. Wandelt, Surf. Sci. 68 Ž1977. 459. w14x G.C. Allen, M.T. Curtis, A.J. Hooper and P.M. Tucker, J. Chem. Soc. Dalton Trans. Ž1974. 1525. w15x R.R. Martin and R.St.C. Smart, Soil Sci. Soc. Am. J. 51 Ž1987. 54. w16x J.M. Zachara, C.E. Cowan, R.L. Schmidt and C.C. Ainworth, Clays Clay Miner. 36 Ž1988. 317. w17x U. Schwertmann and R.M. Cornell, Iron Oxides in the Laboratory: Preparation and Characterization ŽVCH, Wieheim, 1991., p. 64. w18x Standard Methods for the Examination of Water and WasteWater, 17th Ed. ŽAmerican Public Health Association, Washington, D.C., 1989. pp. 3–91. w19x R.D. Seals, R.W. Alexander, L.T. Taylor and J.G. Dillard, Inorg. Chem. 12 Ž1973. 2485. w20x D. Briggs and M.P. Seah, in: Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy ŽJohn Wiley and Sons, New York, 1983. p. 438. w21x J.H. Scofield, J. Electron Spectrosc. Relat. Phenom. 8 Ž1976. 129. w22x G.C. Allen and P.M. Tucker, Inorg. Chim. Acta 16 Ž1976. 41. w23x L.E. Eary and D. Rai, Environ. Sci. Technol. 22 Ž1988. 972.
An XPS study of the adsorption of chromate on goethite ž a-FeOOH/ Hesham Abdel-Samad, Philip R. Watson
)
Department of Chemistry, Oregon State UniÕersity, Gilbert Hall 153, CorÕallis, OR 97331-4003, USA Received 30 May 1996; revised 21 August 1996; accepted 26 August 1996
Abstract The adsorption behavior of inorganic oxyanions on soil minerals is an important factor in the transport of subsurface environmental pollutants. We have studied the adsorption of chromate ŽCrO42y . from aqueous solution on the surface of the mineral goethite Ž a-FeOOH. as a function of pH and adsorbate concentration in 0.05 M NaNO 3 solution. Results obtained from the dried surface by X-ray photoelectron spectroscopy ŽXPS. are in good agreement with data from spectrophotometric analysis of chromate remaining in the supernatant liquid. Chromate adsorption increases with decreasing pH of the solution and eventually reaches a maximum at pH 6.5. The chromium XPS signal indicates that initially a small amount of chromium adsorbs in the q3 oxidation state via a redox reaction, but that the large majority of chromium remains in the q6 oxidation state.
1. Introduction The study of the adsorptionrdesorption behavior of inorganic oxyanions on soil mineral surfaces has particular relevance to plant nutrition and the mobility of subsurface environmental pollutants. Adsorption from aqueous solution at the solid surface significantly alters anion mobility and can affect the rate of dissolution and mechanism of ageing of the solid w1,2x. Chromate ŽCrO42y . is representative of inorganic oxyanions with intermediate binding strength to metal oxides. It is also of particular interest as it is a common toxic contaminant that is a regulated substance in U.S. domestic water supplies w3x. Goethite, the hydrated iron oxide a-FeOOH, is commonly found in soils in most climatic regions )
Corresponding author. Tel.: q1-541-7376740; fax: q1-5417372062.
w4x. It has been extensively studied as a model for hydrated oxide soil minerals. The orthorhombic structure of goethite can be best understood as an hexagonally close-packed array of O 2y and OHy anions with FeŽIII. ions in some of the octahedral holes. The FeŽIII. ions are arranged in such a way that double chains of distorted edge-sharing FeŽO,OH. 6 octahedra run parallel to the w001x direction. As usually prepared by direct precipitation it has a surface area of 10–100 m2rg w5x. Chromate adsorption on goethite has previously been studied by conventional titration techniques that measure the residual chromium in solution after adsorption from aqueous solution w5–9x. The adsorption process has been modeled using surface complexation models ŽSCMs. which assume that adsorption involves both a coordination reaction at specific surface sites and an electrostatic interaction between adsorbing ions and the charged surface w2,9,10x. The
0169-4332r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 6 . 0 0 6 0 9 - 5
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H. Abdel-Samad, P.R. Watsonr Applied Surface Science 108 (1997) 371–377
most successful have used either a two-layer diffuse layer model ŽDLM., or a triple layer model ŽTLM., using a single set of equilibrium constants w5,6x. The aim of the present investigation work is to use a modern surface sensitive technique, namely X-ray photoelectron spectroscopy ŽXPS., to supplement these wet chemical investigations in a more direct manner. XPS measurements for goethite itself have been published by several authors w11–14x. In addition XPS data are available for the adsorption of several other oxyanions Žphosphate, sulfate and selenite. on goethite w15x, but not for chromate. We have successfully monitored the adsorption process by XPS over a wide range of pH values and adsorbate concentrations. We find excellent agreement with wet chemical data performed concurrently. The Cr XPS signal is indicative of some redox chemistry occurring during the early stages of adsorption, but the bulk of the Cr is adsorbed as CrŽVI. on the surface. The nature of the surface complex has not been resolved using classical titration methods, although other indirect evidence suggests that the bonding in the chromatergoethite surface complex is primarily electrostatic, or ‘outer-sphere’, rather than coordinative w16x. Future work in this laboratory will address this problem.
2. Experimental 2.1. Synthesis and characterization of goethite (aFeOOH) A single batch of goethite was prepared according to the method of Schwertmann and Cornell w17x. 180 ml of 5 M KOH solution was rapidly added with continuous stirring to 100 ml of 1 M FeŽNO 3 . 3 P 9H 2 O solution in a 2 L polyethylene flask. The solutions were thoroughly purged with nitrogen before and during reaction to exclude carbon dioxide. Red brown ferrihydrite was precipitated at once. The suspension was then immediately diluted to 2 L with doubly-distilled water and held in a closed polyethylene flask at 708C for 60 h. During this period, the voluminous, red brown suspension of ferrihydrite became a compact, yellow brown precipitate of goethite. This was then centrifuged, washed to remove OHy and NOy 3 ions and dried at 408C for
another 60 h. Powder X-ray diffraction pattern ˚ which showed main peaks at 4.177, 2.69 and 2.45 A, agreed well with the literature values w4x. The N2-BET surface area was 22 m2rg. 2.2. Adsorption experiments We performed two types of adsorption experiments. Adsorption pH edge experiments involved a series of measurements as a function of pH Ž4–11., using a constant adsorbate concentration Ž C s 1.4 mM. at a constant ionic strength Ž I s 0.05 M. set with sodium nitrate. Goethite is not very soluble in water in the pH range 3–12, although some anions, such as oxalate, can increase its solubility at pH - 5. Chromate is not known to have this effect w5x. All experiments were performed at 258C. Appropriate amounts of 1.0 M NaNO 3 stock solution and 25 ml of 0.10 M Na 2 CrO4 P 4H 2 O stock solution were mixed with N2-purged ultra-pure water. After mixing and N2-purging the solution, the pH was raised to 11 using 0.1 M NaOH solution. The total experimental volume was fixed at 180 ml, the addition of 0.36 g of goethite producing an adsorbent concentration of G s 2.0 grL. The solution was then divided into eight samples, placed in tightly capped, 250 ml polypropylene cups under an N2 atmosphere and the pH readjusted to a value in the range 4–11. The samples were then sealed and shaken. Trial experiments showed that the time required for achieving adsorption equilibrium was about 10 h, consistent with other studies w5x. After 12 h equilibration, two 1 ml aliquots were transferred to microcentrifuge tubes and centrifuged for 10 min at 13,000 rpm and the supernatant solution removed. The drift in pH over this period was less than 0.5 pH units. The solid residues were freeze-dried prior to XPS analysis. These samples were not washed prior to freeze-drying to lessen the possibility of removing adsorbed chromate. Calculations show that the maximum amount of chromate that might have precipitated out from residual solution on to the surface during drying is less than 5% of that measured on the surface, so the lack of washing does not lead to misleadingly high surface chromium levels. Adsorption isotherm experiments were performed in a similar manner at 258C with adsorbate concentrations between C s 0.055 mM and 1.7 mM using
H. Abdel-Samad, P.R. Watsonr Applied Surface Science 108 (1997) 371–377
appropriate volumes of 0.10 M Na 2 CrO4 P 4H 2 O stock solution as titrant at a constant pH of 6.5. 2.3. Residual aqueous Cr analysis Residual CrŽVI. ion concentrations in the liquid phase were quantified by a colorimetric method using diphenyl carbazide at pH f 1 w18x. Absorbances were measured at 540 nm on a diode-array spectrometer. Surface adsorbate concentrations were calculated as the difference between original and residual solution concentration. The maximum amount of Cr adsorbed was about 1r3 of that available in solution. 2.4. XPS analysis
373
plate using double-sided adhesive graphite tape. The instrumental work function was set to give a value of 83.9 eV for the binding energy of the Au 4f 7r2 line for metallic gold w19x, and an electron gun set at 0.7 eV and 0.02 mA was used to reduce sample charging. The XPS photoelectron binding energy of the adventitious carbon species, i.e., the C 1s line at 284.8 eV was used to correct the observed binding energies for surface charging w20x. Integrated peak intensities were evaluated by measuring the peak area above the background and correcting the measured intensity for the photoionization cross-section w21x. 3. Results and discussion
The XPS studies were carried out with a modified Hewlett-Packard 5950A ESCA spectrometer, using an aluminum anode ŽAl K a s 1486.6 eV. operating at 600 W. Samples were prepared by adhering a very thin layer of the powdered sample to an aluminum
3.1. XPS data from goethite and reference compounds Table 1 compares our XPS binding energies for goethite and reference iron compounds with those
Table 1 XPS binding energies Žin eV. for the major core lines of goethite and Fe and Cr reference compounds used in this study compared with literature values Material
Binding energy ŽeV. Fe 2p 3r 2
Goethite ŽFeOOH.
Fe 2 O 3
FeO Na 2 CrO4 P 4H 2 O Cr2 O 3
Reference
O 1s
Cr 2p 3r2
O 2y
OHy
711.0 708.2 Žsh. 711.2 711.7
528.4
531.0
533.0
529.8 530.1
531.1 531.2
711.1 710.9 710.4 711.1 710.7 711.3 710.0 709.2 — — — —
530.0 530.0 529.7 530.6 529.6 530.1 529.5 530.0 — — — —
531.0 531.7 531.4 — — — — — — — — —
— a 532.3 533.4 — a — a — a — — — — — — — — —
H 2O
b
—
This work
— —
w11x w12x
b
— — — — — — — — 579.4 579.4 576.2 576.8
w13x w14x w15x This work w12x w14x This work w12x This work w22x This work w22x
All binding energies are located to better than "0.2 eV. The data from Ref. w11–16x have been adjusted to use the same C 1s and Au 4f reference binding energies as used here. a Cannot be reliably estimated from data in reference. b Estimated from spectra in reference.
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H. Abdel-Samad, P.R. Watsonr Applied Surface Science 108 (1997) 371–377
Fig. 1. XPS spectrum from a freeze-dried goethite sample in the Fe 2p region.
measured by other workers w11–15x. The literature data in Table 1 have been corrected to be referenced to the same C 1s or Au 4f binding energies as we found. Our values for the main Fe 2p 3r2 peak from goethite fall within the range of values reported for goethite and other FeŽIII. species such as Fe 2 O 3 . The spread of Fe 2p 3r2 peak energy values about 711 eV obtained by different authors presumably reflect to some degree variations in background subtraction methods, but it is possible that they also reflect differing amounts of associated water. Our goethite spectrum ŽFig. 1. also shows a shoulder to the low binding energy side of the main FeŽIII. 2p 3r2 peak. The position of this peak Ž708.2 eV. is consistent with a reduced iron species, although it lies to lower binding energy than the corresponding peak for FeO Ž; 709.6 eV.. A careful examination of the goethite spectrum of Allen et al. w14x shows such a low energy shoulder that is absent in their Fe 2 O 3 spectrum. The FeOOH spectrum of McIntyre and Zetaruk w12x may also have a low binding energy shoulder though it is less pronounced. It is our belief that synthesized goethite can contain some fraction of FeŽII. at least at the surface. This point becomes of some importance later in the context of redox reactions during Cr adsorption. The goethite O 1s region clearly contains several components. We have attempted fits to just oxide and hydroxide components, with various restrictions on widths etc. and also into three peaks from oxide, hydroxide and water. As our sample came from an aqueous synthesis with freeze-drying, but no heating,
Fig. 2. XPS spectrum from a freeze-dried goethite sample in the O 1s region, showing the best-fit deconvolution into signals from oxide, hydroxide and water Žsee text..
we feel it is likely that some adsorbed water remains on the surface of the goethite. The O 1s region of FeOOH in the spectrum of McIntyre and Zetaruk w12x also contains high binding energy components that they assign to adsorbed water. Fig. 2 shows the best fit deconvolution which uses oxide, hydroxide and water components, although the fit is not unique and a range of almost equally good fits with three components are possible. Fits omitting the water component give significantly poorer agreement. Table 2 provides peak intensity ratios for the O 1s peaks from our work compared with those that can be garnered from the literature. The derived oxide and hydroxide peaks energies shown in Table 1 are consistent with previous literature values. The fit produces an oxide peak that is considerable wider than the hydroxide component which complicates reliably quantifying the contributors to the O 1s signal. The ratio of our signal intensities from hydroxyl and oxide oxygen is close to 1 Žbut with a large error., in good agreement with the bulk FeOOH stoichiometry and can be compared with other values of 0.90 w12x and 1.04 w11x. Table 2 Estimates of the contributions of different oxygen species to the O 1s XPS peak of goethite from this study compared with literature values w11,12x Relative XPS OHyrO 2y
Peak area H 2 Ortotal O Reference
0.9–1.1 1.03 0.9
0.08–0.10 - 0.05 a 0.17 a
a
Estimated from spectra in reference.
this work w11x w12x
H. Abdel-Samad, P.R. Watsonr Applied Surface Science 108 (1997) 371–377
Fig. 3. XPS Cr 2p spectra after adsorption of 1.4 mM chromate solution at an ionic strength of 0.05 M onto goethite at various pH values. All spectra are on the same ordinate scale but shifted for clarity.
Our Cr 2p 3r2 spectra for the reference compounds Cr2 O 3 and Na 2 CrO4 P 4H 2 O ŽTable 1. compare very well with those from the large compilation by Allen and Tucker w22x. In general CrŽVI. compounds exhibit sharper peaks at higher binding energies Ž; 579.4 eV, D1r2 ; 2.1 eV. than those from CrŽIII. compounds Ž; 576.6 eV, D1r2 ; 3.3 eV..
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Fig. 4. Comparison of the amount of Cr adsorbed from XPS Žcircles. and spectrophotometry data Žtriangles. for chromate adsorbed on 0.36 g goethite as a function of pH. The experiments used wNa 2 CrO4 x s1.4 mM at a constant ionic strength of 0.05 M and an adsorbent concentration of 2.0 grl.
amount of Cr removed from solution determined spectrophotometrically, mirrors closely the area of the Cr 2p 3r2 signal from the surface and show the breakthrough pattern at about pH 8 reported by other workers using wet chemical methods w5,6x. The max-
3.2. pH edge experiments For adsorption of chromate at pH values of 11 or higher any Cr 2p signal from the surface was below the level of detectability; spectrophotometry also revealed no surface adsorption ŽFigs. 3 and 4.. This is a result of the goethite surface being negatively charged in this pH range which is unfavorable for the adsorption of anionic chromate species. Adsorption of chromate became significant Žsee Figs. 3 and 4. between pH 9 and 10, reflecting the high point of zero charge value of the goethite substrate w5x. Between pH 10.5 and 6 the XPS signal from surface Cr increases dramatically ŽFig. 3.. These changes were accompanied by a corresponding decrease in the residual CrŽVI. measured in solution ŽFig. 4.. The
Fig. 5. Variations in the Fe 2p 3r 2 Žcircles. and Cr 2p 3r2 Žtriangles. binding energies for chromate adsorbed on 0.36 g goethite as a function of pH. The experiments used wNa 2 CrO4 x s1.4 mM at a constant ionic strength of 0.05 M and an adsorbent concentration of 2.0 grl.
376
H. Abdel-Samad, P.R. Watsonr Applied Surface Science 108 (1997) 371–377
Fig. 6. XPS Fe 2p spectra from goethite Ža. before reaction, Žb. after equilibration at pH 8, and Žc. after equilibration at pH 6 with 1.4 mM chromate solution at an ionic strength of 0.05 M. All spectra are on the same ordinate scale but shifted for clarity.
imum adsorption is reached by pH 6.5 in both sets of data. On metal oxides maximum adsorption of anions of diprotic acids typically occurs at a pH about equal to the second acid dissociation constant; for chromate, log pK a2 s 6.51. Fig. 5 shows that the binding energy of the main Fe 2p 3r2 peak from the goethite substrate after these experiments showed little change from the values before adsorption. In contrast the Cr 2p 3r2 binding energy increases from a value of about 576.2 eV at high pH to a value in excess of 579.5 eV at saturation. While there is a significant difficulty in accurately determining the binding energy of the low intensity peaks from the high pH experiments, the binding energy shift seems to be real Žcompare the spectra in Fig. 3.. These binding energies are consistent with adsorption of the first amounts of Cr as CrŽIII. and the later Cr as CrŽVI. at about pH 6.5 and below. Adsorption as CrŽIII. would necessitate some redox chemistry at the surface, presumably involving FeŽII.. This possibility is supported by the Fe 2p spectra taken after equilibration at pH 8 and pH 6 shown in Fig. 6. The obvious low binding energy shoulder that we have tentatively assigned to FeŽII.
in the unreacted goethite is decreased in the pH 8 spectrum Žwhere adsorption is very weak., and is smaller again in the pH 6 spectrum Žwhere Cr adsorption has saturated.. We speculate while the majority of Cr adsorbs as CrŽVI., there is some FeŽII. present in our goethite sample and initial Cr adsorption occurs as CrŽIII. via a redox reaction with this reduced iron. The origin of this reduced iron is not clear. At high pH, where little adsorption occurs, the Cr XPS signal is indicative of CrŽIII. at the surface and the signal indicative of FeŽII. has decreased from that seen in the unreacted substrate. By pH 6 Cr adsorption has saturated. The Cr signal ŽFig. 3. shows predominantly CrŽVI. with some CrŽIII. component and the FeŽII. signal ŽFig. 6. has decreased very markedly. The shoulder in the Fe 2p spectrum that we are assigning to FeŽII. does not disappear completely even at the highest Cr loadings. This might reflect the presence of some FeŽII. in subsurface layers that still contributes to the XPS Fe 2p signal, but is not accessible to Cr. The possibility of chromate removal from aqueous wastes by reduction with ferrous ion is well known w23x. 3.3. Adsorption isotherm experiments Fig. 7 shows the results obtained from both of the spectrophotometric analysis of residual aqueous
Fig. 7. Adsorption isotherms for chromate adsorbed on 0.36 g goethite from XPS Žtriangles. and spectrophotometry data Žcircles.. The experiments were carried out at pH 6.5"0.2 with constant ionic strength of 0.05 M and an adsorbent concentration of 2.0 grl.
H. Abdel-Samad, P.R. Watsonr Applied Surface Science 108 (1997) 371–377
chromate and XPS analysis of surface chromium after adsorption experiments using increasing concentrations of chromate at a constant pH of 6.5. The adsorption isotherms obtained by the two methods are in good agreement with each other. The data at low Cr values fit the Langmuir isotherm quite well but the data fall well above the plot at saturation. 4. Conclusions The Fe 2p XPS spectrum of goethite shows a small low binding energy feature that we attribute to FeŽII.. This feature is also present, but unidentified, in some other literature spectra. The O 1s spectrum shows an ; 1:1 OHyrO 2y ratio as expected, but also shows a peak attributable to adsorbed water Žalso seen by other workers.. We have observed the adsorption of chromium from an aqueous chromate solution onto goethite as a function of pH and chromate concentration by spectrophotometry and XPS. Results from the two methods agree very well. At high pH adsorption is small, becomes significant by pH 8 and is saturated by pH 6.5. The initial adsorption process appears to involve a redox reaction with small amounts of FeŽII. present on the oxide surface, leading to the adsorption of the metal as CrŽIII., while subsequent adsorption occurs as CrŽVI.. References w1x N. Caraco, J. Cole and G.E. Likens, Biogeochemistry 9 Ž1991. 277. w2x J.A. Davis and D.B. Kent, in: Mineral-Water Interface Geochemistry, eds. M.F. Hochella and A.F. White ŽMineralogical Society of America, Washington, D.C., 1990. pp. 176–260. w3x P.J. Sheehan, D.M. Meyer, M.M. Sauer and D.J. Paustenbach, J. Toxicol. Environ. Health 32 Ž1991. 161.
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w4x U. Schwertmann and R.M. Taylor, in: Minerals in Soil Environments, eds. J.B. Dixon and S.B. Weed, Soil Science of America ŽMadison, WI, 1989. pp. 379–438. w5x K.L. Mesuere and W. Fish, Environ. Sci. Technol. 26 Ž1992. 2365. w6x C.C. Ainworth, D.C. Girvin, J.M. Zachara and S.C. Smith, Soil Sci. Soc. Am. J. 53 Ž1989. 411. w7x J.M. Zachara, D.C. Girvin, R.L. Schmidt and C.T. Resch, Environ. Sci. Technol. 21 Ž1987. 589. w8x S. Music, ´ M. Ristic´ and M. Tonkovic, ´ Z. Wasser-AbwasserForsch. 19 Ž1986. 186. w9x J.A. Davis and J.O. Leckie, J. Colloid Interface Sci. 74 Ž1980. 32. w10x D.A. Dzombak and F.M.M. Morel, Surface Complexation Modeling: Hydrous Ferric Oxide ŽWiley, New York, 1990. p. 59. w11x D.T. Harvey and R.W. Linton, Anal. Chem. 53 Ž1981. 1684. w12x N.S. McIntyre and D.C. Zetaruk, Anal. Chem. 49 Ž1977. 1521. w13x C.R. Brundle, T.J. Chuang and K. Wandelt, Surf. Sci. 68 Ž1977. 459. w14x G.C. Allen, M.T. Curtis, A.J. Hooper and P.M. Tucker, J. Chem. Soc. Dalton Trans. Ž1974. 1525. w15x R.R. Martin and R.St.C. Smart, Soil Sci. Soc. Am. J. 51 Ž1987. 54. w16x J.M. Zachara, C.E. Cowan, R.L. Schmidt and C.C. Ainworth, Clays Clay Miner. 36 Ž1988. 317. w17x U. Schwertmann and R.M. Cornell, Iron Oxides in the Laboratory: Preparation and Characterization ŽVCH, Wieheim, 1991., p. 64. w18x Standard Methods for the Examination of Water and WasteWater, 17th Ed. ŽAmerican Public Health Association, Washington, D.C., 1989. pp. 3–91. w19x R.D. Seals, R.W. Alexander, L.T. Taylor and J.G. Dillard, Inorg. Chem. 12 Ž1973. 2485. w20x D. Briggs and M.P. Seah, in: Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy ŽJohn Wiley and Sons, New York, 1983. p. 438. w21x J.H. Scofield, J. Electron Spectrosc. Relat. Phenom. 8 Ž1976. 129. w22x G.C. Allen and P.M. Tucker, Inorg. Chim. Acta 16 Ž1976. 41. w23x L.E. Eary and D. Rai, Environ. Sci. Technol. 22 Ž1988. 972.