Langmuir 1990, 6, 644-649
644
Table 11. Slope ( a ) ,Intersect at the Origin ( b ) ,and Correlation Coefficient ( p ) of the Different Linear Relationships Obtained between Absorbance and Surface Coverage pH 9.45, 0 < 8 < 2 pH 10.5, 0 < 8 < 1.66 wavenumber, cm-' a b a b P P 1465 0.0166 0.0004 0.8647 0.0179 0.0013 0.8696 0.9721 0.0341 -0.003 0.0326 0.0030 0.9154 1543 1561 0.9388 0.0313 0.0650 0.0038 0.8950 0.0328 1571 0.9907 0.0425 +0.0064 0.0387 0.0046 0.9057 1700 0.9858 0.0156 -0.0057 2854 0.9703 0.0318 0.0123 0.0329 0.0056 0.9455 2925 0.9709 0.0472 0.0190 0.0484 0.0106 0.9483 2962 0.9645 0.0213 0.0066 0.0170 0.0052 0.9598 0.8706 0.0038 0.0024 0.0057 -0.0001 3007 0.9463
neutral oleic acid. In this range of concentrations, { potential, turbidity, and absorbance are sensitive techniques that can be used to follow the coating of fluorite particles by oleate. Absorbances of all the bands investigated are more particularly linearly correlated with surface coverage. In this range, the suspension is stable (high value of the turbidity due to the bilayer formation). (c) In the micellar concentration range, a sharp (at pH 9.45) or monotonic (at pH 10.5) change in the absor-
bance relative to all the bands investigated and related either to hydrocarbon chains or carboxylate groups indicates a change in the state of the adsorbed layer. This change seems to be related to an increase in the sodium oleate concentration in the adsorbed layers due to a release of the calcium ions that adsorb on micelles in the bulk. Registry No. (C,,H,,COONa), 143-19-1; CaF,, 14542-23-5; calcium oleate, 142-17-6; oleic acid, 112-80-1.
Mossbauer Study of Ferric Ions Adsorbed at a-Ferric Oxide/Aqueous Solution Interface Shizuko Ambe* and Fumitoshi Ambe The Institute of Physical and Chemical Research (RIKEN), Wako-shi, Saitama 351 -01, Japan Received July 11, 1989. I n Final Form: October 27, 1989 In situ and ex situ Mossbauer observations were made on ',Fe3+ ions adsorbed on a-S6Fe,0, from a
0.1 mol dm-, NaCl solution of pH 2.5, 4.0, and 6. The in situ spectrum at pH 2.5 consisted of a sextet
with a reduced ma netic splitting compared with bulk a-Fe,O,. The result is interpreted to show that the adsorbed 57Fef+ ions form a new surface layer as an extension of the corundum structure of the substrate. Filtration and drying of the in situ sample brought about no appreciable change in the spectrum, while heating at and above 200 "C resulted in an increase of the splitting due to rearrangement of surface fine structures and diffusion of the surface ',Fe3+ ions into the bulk. The relative absorption areas of dried and heated samples were found to be the same as that of the in situ sample within the experimental uncertainty of about 10%. A t pH 6, the in situ sample gave no observable absorption, while an intense paramagnetic doublet and a weak sextet were observed in the frozen and dried states. On the basis of these results, physisorption of ferric hydroxide gel is concluded to be prevailing at pH 6. Measurement at pH 4.0 gave spectra indicating that both the mechanisms observed at 2.5 and 6 operate in parallel.
Introduction Adsorption of ferric ions on solid surfaces is a familiar and important phenomenon not only in chemical processes but also in circulation of iron in our natural environment. Radiotracer studies on the pH dependence of the adsorption were performed by several groups, and the chemical structures of the adsorbed ferric species were inferred from the data obtained.'-4 However, as far as (1)Davydov, Yu.P.; Jefremenkov, V. M.; Gratchok, M. A.; Bondar, Yu.J. J . Radioanal. Chem. 1976,30, 173.
77.
(2) CvjetiCanin, N. M.; CvjetiCanin, D. N. J. Chromatogr. 1977, 140,
0743-7463/90/2406-0644$02.50/0
we know no spectroscopic methods have been successfully applied to characterize the chemical structure of the ferric ions a t solid/solution interfaces. As Mossbauer spectroscopy is sensitive only to ions bound in or on a solid and not to species dissolved in a solution, it can be a powerful tool for the study of the ions at interfaces. In our previous papers, it was demonstrated that in situ and ex situ emission Mossbauer analysis of magnetic interactions of adsorbed metal ions with metal oxide (3) Cvjetitanin, N.;CvjetiCanin, D. J.Radioanal. Nucl. Chem. 1984, 81, 49. (4) Davydov, Yu.P.; Bondar, Yu.I.; Efremenkov, V. M.; Voronik, N.
I. J . Radioanal. Nucl. Chem. 1984, 82, 247.
0 1990 American Chemical Society
Langmuir, Vol. 6, No. 3, 1990 645
Mossbauer Study of Adsorbed Ferric Ions surfaces provides valuable information on the chemical structures of the adsorbed metal and the kinetics of the adsorption was discussed in relation to the adsorption structure^.^ In the present paper, we report a study on ferric ions adsorbed on a-Fe203 (hematite) by in situ and ex situ absorption Mossbauer spectroscopy."' In order to obtain specific information on the adsorbed ions, ferric ions enriched in the Mossbauer-sensitive 57Fewere employed as the adsorbate, while the a-Fe O3 adsorbent used was depleted in 57Fe (enriched in 58Fe). The radioisotope 59Fe3+was mixed in the 57Fe3+adsorbate as a tracer for adsorption. The chemical states of adsorbed 57Fe3+ions at pH 2.5, 4.0, and 6 were determined on the basis of the in situ and ex situ Mossbauer spectra obtained. The effect of heating the dried C Y - ~ F ~ sample , O , with adsorbed 57Fe3+ was also studied. A relative recoilless fraction of the in situ, dried, and heated samples is reported.
i
Experimental Section Preparation of Adsorbent. The a-Fe203 powder used in the present work as the adsorbent was prepared according to the rescription of van der Kraan" from ferric oxide depleted in .pFe (56Fe,99.87 atom %; 57Fe,0.07 atom %) obtained from ORNL. The oxide was dissolved in concentrated HCl, and the solution was evaporated. The residue was dissolved in nitric acid, and the pH of the solution WBS adjusted with a dilute ammonia solution to pH 2.5. Fine precipitate was obtained on aging the solution a t 95 "C for 6 h. After thorough washing with distilled water, precipitates were calcined in air at 400 and 650 OC for 2 h. (The former was employed in adsorption a t pH 2.5 and the latter in that a t pH 2.5, 4.0, and 6.) The oxide samples were identified by measuring powder X-ray diffraction patterns and Mossbauer absorption spectra. Their shape and size were observed by a scanning electron microscope. The oxide samples are designated hereafter as ~ i - ~ ~ F e , othough ,, they contain 0.07 atom % of 57Fe and other iron isotopes besides 56Fe. Adsorption of 57Fe3+ on cu-58Fe,03-Preparation of Mossbauer Samples. Thirty milligrams of a-56Fe,0, was added to 5 cm3 of a 0.1 mol dm-3 NaCl solution a t pH 1.6 containing 300 pg of ferric ions (1.05 mmol Fe3+ dm-3) enriched in 57Fe (90.24 atom %, for Moossbauer study) and a small amount of radioactive "Fe3+ (for tracer study) in a Teflon vessel having a 0.5mm-thick Teflon window a t the bottom. Thus, the ratio of the amount of 57Fe3+used as the adsorbate to that in the substrate was 18 to 1. The pH of the solution was adjusted to 2.5, 4.0, and 6 by adding a dilute NaOH solution. The suspension was shaken a t room temperature. After settling of the powder a t the bottom of the vessel, the pH and the radioactivity of 59Fe in the supernatant solution were measured. The pH was in the ranges 2.5 i 0.1, 4.0 i 0.1, and 6.2 i 0.5, respectively. Then, about 4 cm3 of the supernatant solution was withdrawn to reduce the photoelectric absorption of Mossbauer y-rays in the aqueous layer. These samples were subjected to in situ Mossbauer measurement a t 297 and 78 K. In situ measurement was also performed on a sample of pH 6 after heating a t 100 OC for 2-6 h. After the in situ measurement, the a-56Fe,0, samples with adsorbed 57Fe3+were filtered out on a glass fiber filter, washed with distilled water, and dried. Mossbauer spectra of the dried (5) Okada, T.; Ambe, S.;Ambe, F.; Sekizawa, H. J . Phys. Chem. 1982,86,4726. (6)Ambe, F.; Okada, T.; Ambe, S.;Sekizawa, H. J . Phys. Chem. 1984,88,3015.
(7) Ambe, F.; Ambe, S.;Okada, T.;Sekizawa, H. In GeochernicalProat Mineral Surfaces; Davis, J. A., Hayes, K. F., Eds.;ACS Symposium Series 323; American Chemical Society: Washington, DC, 1986; p 403. (8) Okada, T.;Ambe, S.;Ambe, F.; Sekizawa, H. Hyperfine Interact. 1988, 41, 685. (9) Ambe, S.Langmuir 1987,3, 489. (10) A part of the experimental results and preliminary discussion was presented at the 6th International Conference on Surface and Colloid Science, Hakone, Japan, June 1988. (11) Van der Kraan, A. M. Phys. Status Solidi A 1973, 18, 215. cesses
I 8 -16
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B J L K oC-FE2O3 I
,
,
-8 0 8 RELATTVE VELOClTY ( M M / S VS METALLIC ! ? O N 1
I6
Figure 1. Mossbauer spectra of 57Fe3+ ions adsorbed on a56Fe,03 from a 0.1 mol dm-3 NaCl solution of p H 2.5: (A) in situ measurement, (B) after drying, (C)after heating at 350 "C for 2 h, and (D) after heating at 600 OC for 6 h (all the Mossbauer measurements at room temperature). samples were recorded as they stand or after heating at various temperatures in air (ex situ measurement). Measurement of Mossbauer Spectra. Mossbauer measurement of the samples was performed by means of conventional Mossbauer spectrometers (Austin S-600 and Ranger 700 series) employing a 57C0 (740 MBq)/Rh (12 Mm) source and a Kr (+3% carbon dioxide)-filled proportional counter. In the in situ measurement, the source was driven vertically over the Teflon vessel, and the y-rays passing down through the sample submerged at the bottom of the aqueous layer were detected by the counter installed under the vessel. The data obtained were analyzed with FACOM M380 and M780 computers at our institute. Distribution of hyperfine magnetic field a t 57Fenuclei was calculated by the Hesse-Rubartsch analysis12 of the spectra.
Results The ferric oxide powders prepared as the adsorbent were identified to be a-Fe203by the X-ray diffraction analysis. Scanning electron microscopy showed that the particles have a spindle shape with a major axis of 0.1-0.3 pm. The Mossbauer spectra of the adsorbent gave a sextet assignable to a-Fe,O , although the intensity was weak due to depletion in "Fe. The radioactivity measurement on 59Fe3+showed that the adsorption of ferric ions was slow at pH 2.5. Only about 60% was adsorbed on ~ x - ~ ~ F e ,calcined 0, at 400 "C from the solution after 1 h of shaking. In case of the adsorbent calcined at 650 "C,the adsorption was much slower. The in situ Mossbauer spectra of 57Fe3+on the C Y - ~ ~ Fadsorbent ~ ~ O , thus prepared consist of six lines at room temperature. Figure 1A shows the spectrum of 57Fe3+on the Q - ~ ~ F ~ ,adsorbent O, calcined at 650 O C . The sample calcined at 400 O C gave a similar s ectrum with a smaller splitting. (The contribution of Fe3+ in the C Y - ~ ~ substrate F ~ , ~ , is estimated to be less than onetenth of the total absorption area of the spectrum.) Neither a paramagnetic nor a superparamagnetic doublet was observed in the spectrum within the experimental uncertainties. This observation demonstrates that all the
B
(12) Hesse, J.; Rubartsch, A. J. Phys. E 1974, 7, 526.
646 Langmuir, Vol. 6, No. 3, 1990
Ambe and Ambe About 90% of the ferric ions was adsorbed from a solution a t pH 4.0 after 1 day of shaking. The in situ spectrum consists of a sextet (Figure 3A) similar to the in situ spectrum a t pH 2.5. When the sample was dried, a doublet appeared in the spectrum besides the sextet (Figure 3B). The spectrum corresponds to a superposition of the ex situ spectra obtained a t pH 2.5 and 6.
6
-8
9
6
SEL'TIVE VELSEI-Y f'Y/S VS YE-AL-IC I9ONI
Figure 2. Mossbauer spectra of 57Fe3+ions adsorbed on a56Fe,03from a 0.1 mol dm-3 NaCl solution of pH 6: (A) in situ measurement and (B) after drying (Mossbauer measurements at room temperature).
- 6
-I
QE_:'-\/E VEL?: P Y / S " 5 YE-P__:C
a
6
-" P311
Figure 3. Mossbauer spectra of 57Fe3+ions adsorbed on a56Fe,03from a 0.1 mol dmW3NaCl solution of pH 4.0: (A) in situ measurement and (B) after drying (Mossbauer measurements at room temperature). adsorbed 57Fe3+ions are interacting magnetically with each other or with the ferric ions of the substrate. The isomer shift amounts to 0.33 f 0.03 mm s-l relative to metallic iron. The magnetic splitting is somewhat smaller than that of the ordinary bulk a-Fe,03 and the line width much larger. The ex situ spectrum of the sample after filtration and drying (Figure 1B) is essentially the same as the in situ spectrum. The increase in absorption intensity is ascribed mainly to improvement of S / N ratio by removal of the aqueous layer. Typical spectra recorded at room temperature after heating the sample at various temperatures are shown in C and D of Figure 1. At and above 200 O C , a gradual increase in the splitting and a decrease in the line width were observed. Virtually all the ferric ions were adsorbed by the a56Fe,03 sample from a solution a t pH 6 within 1h. However, the 67Fe3+species at the interface gave no appreciable Mossbauer absorption in the in situ measurement a t room temperature (Figure 2A). The weak but sharp sextet barely recognizable in the spectrum is attributable to 57Fe3+contained from the beginning in the bulk of the substrate. When the sample was dried after filtration, an intense quadrupole doublet with an isomer shift of 0.37 f 0.03 mm s-l and a quadrupole splitting of 0.75 f 0.05 mm s-l appeared along with weak magnetic components in the room temperature spectrum (Figure 2B). The paramagnetic doublet is still dominant over the magnetically split component at 78 K. Similar spectrum was obtained at 78 K for a sample frozen with the aqueous phase.
Discussion Adsorption of Fe3+Ions and Mossbauer Spectroscopy. As is well-known, hydrolysis of ferric ions begins around pH 2. The process is deeply affected by the concentration of ferric ions, counterions, temperature, and aging time. The complex mechanism of hydrolysis has been extensively studied by various physicochemical techniques such as electrochemical methods, spectrophotometry, measurement of magnetic moment, density gradient ultracentrifugation, and electron micro~copy.~~-'~ Mossbauer spectroscopy has also been applied to the study on dried hydrolysis productsz4 and frozen ferric solution~.'~-~' The results on the frozen solutions show some discrepancies,possibly due to a difference in detailed experimental conditions. The hydrolysis is often accompanied by the so-called hydrolytic adsorption, that is, adsorption of hydrolyzable species at solid/solution interfaces. The adsorption of ferric ions onto iron oxides is especially important, because it is one of the elementary processes in the formation of iron minerals in the hydrosphere. On the basis of the adsorption behavior of trace amounts of Fe3+on ferric oxide, an ion-exchange process (below pH 3) and adsorption of polynuclear complexes or colloidal particles (above pH 3) have been proposed as the adsorption rne~hanism.~ Van der Kraan reported the first Mossbauer study on surface 57Fe3+ions coated on fine a-Fe,03 particles of natural isotopic composition.'' Later, Shinjo et al. studied surface magnetism of a-Fe203 by Mossbauer spectroscopy using ~ - ~ ~ F ecovered , o , with a 57Fe3+ion monolayer by aging a t 90 " C for 20 h.33 These measurements (13)Hedstrom, B. 0.A. Arkiu for Kemi 1952,6,1. (14)Werbel, B.; Dibeler, V. H.; Vosburgh, W. C. J . Am. Chem. SOC. 1943,65,2329. (16)Mulay, L. N.;Selwood, P. W. J. Am. Chem. SOC.1955,77,2693. (16)Spiro, T. G.; Allerton, S. E.; Renner, J.; Terzis, A.; Bils, R.; Saltman, P. J. Am. Chem. SOC.1966,88,2721. (17)Knight, R. J.; Sylva, R. N. J . Inog. Nucl. Chem. 1975,37,779. (18)Murphy, P. J.; Posner, A. M.; Quirk, J. P. J. Colloid Interface Sci. 1976.56.270. (19) Murphy, P. J.; Posner, A. M.; Quirk, J. P. J. Colloid Interface Sei. 1976,56, 284. (20)Murphy, P. J.; Posner, A. M.; Quirk, J. P. J. Colloid Interface Sei. 1976,56, 298. (21)Murphy, P. J.; Posner, A. M.; Quirk, J. P. J . Colloid Interface Sci. 1976,56, 312. (22)Kauffman, K.; Hazel, F. J. Inorg. Nucl. Chem. 1975,37, 1139. (23)Andrianov, V. A.; Kalyamin, A. V.; Tomilov, S. B. J . Gen. Chem. USSR (Engl. Transl.) 1981,51,460. (24)Bowen, L.H. Mossbauer Effect Reference and Data J . 1979,2, 76 . -.
(25)DBzsi, I.; Gorobchenko, V. D.; Lukashevich, I. I.; VBrtes, A. Chem. Phys. Lett. 1968,2,665. (26)DCzsi, I.; VBrtes, A,; Komor, M. Inorg. Nucl. Chem. Lett. 1968, 4 , 649. (27)VBrtes. A.;Suba, M.; Komor, M. Radiochem. Radioanal. Lett. 1970,3,365. (28)Ohya, T.; b o , K. J . Chem. Phys. 1972,57,3240. (29)Chaves. F. A. B.: Gare. -. V. K. J . Znorg. Nucl. Chem. 1975,37, 2283. (30)Pan, H. K.; Yarusso, D. J.; Knapp, G. S.; Pineri, M.; Meagher, A.; Coey, J. M. D.; Cooper, S. L. J . Chem. Phys. 1983,79,4736. (31)Pan, H.K.; Meagher, A.; Pineri, M.; Knapp, G. S.; Cooper, S. L. J . Chem. Phys. 1985,82,1529. (32)Meagher, A.;Rodmacq, B. New J. Chem. 1988,12,961. (33)Shinjo, T.;Kiyama, M.; Sugita, N.; Watanabe, K.; Takada, T. J. Magn. Magn. Materials 1983,35, 133. '
Langmuir, Vol. 6, No. 3, 1990 647
Mossbauer Study of Adsorbed Ferric Ions were performed only on dried samples. There seems to have been no Mossbauer study so far on adsorption structure of 57Fe3+ions on the a-Fe203surface in contact with an aqueous phase. In our in situ Mossbauer measurement, the observation was made on the adsorbed 57Fe3+ species a t the solid/aqueous solution interface. If the adsorbent employed (30 mg) is postulated to consist of smooth spheres of 0.2-pm diameter, it is calculated to have a surface area of 0.18 m2. On the other hand, 300 pg of 57Fe3+ions is estimated to form a monolayer of 0.34 m2 as an extension of the crystal structure of a-Fe,O,. Therefore, if the fine structures of the surfaces and presence of finer particles are taken into consideration, the amount of 57Fe3+used in each run is estimated to correspond roughly to monolayer adsorption on the C Y - ~ ~sample. F ~ , ~ In , the in situ measurement, exclusive information on adsorbed 57Fe3+ions is obtained even if a part of 57Fe3+remains dissolved in the aqueous phase, because Mossbauer absorption does not occur on species dissolved in the solution as was described above. Moreover, in situ measurements without the substrate yielded no Mossbauer absorption of 57Fe3+under the same experimental condition^.^^ Therefore, it is reasonable to conclude that the in gitu spectra reflect the 57Fe3+species associated with a-Fe203particles. Adsorption at pH 2.5. On the basis of the equilibrium constants reported by Hedstrom,13 the 1.05 mmol dm-, ferric ion solution is estimated to contain Fe3+, FeOH2+,Fe(OH),+, and Fe,(OH),4+ in concentrations of 0.70,0.20, 0.03, and 0.06 mmol dm-3, respectively, at pH 2.5 and 25 "C. Since the adsorption was carried out with a ferric solution freshly prepared from a 0.5 mol dm-, HC1 stock solution for each run of the experiment, polymerization due to aging is estimated to have been negligible. Consequently, the cationic ferric species above are considered to take part in the adsorption. The surface of C Y - ~ ~ isF positively ~ , ~ , charged at pH 2.5, since its zero point of charge is pH 6.5-8.6.35 As both the adsorbate and adsorbent are positively charged, the adsorption has to proceed against the Coulomb repulsion between them. This is compatible with the observation that the adsorption of ferric ions at pH 2.5 is slow. Figure 4 shows distribution of the hyperfine magnetic field at 57Fe3+obtained by the Hesse-Rubartsch analysis of the spectra in Figure 1. For the in situ measurement at pH 2.5, the distribution has a maximum approximately at 490 kOe (Figure 4A). The value is smaller than that of bulk cy-Fe203by about 5%. It can be seen from Figure 1A that the electric quadrupole interaction parameter 2e = (1/4)e2qQ(3cos2 0 - 1) of the adsorbed species amounts to about -0.1 mm/s. It is roughly onehalf the value for the bulk. As no doublet is observed in the in situ spectrum at pH 2.5, formation of P-FeOOH (NBel temperature: 295 K)36 at the a-Fe2O, surface is disproved in spite of the presence of 0.1 mol dm-, of chloride ions, which promote formation of P-Fe00H.19'37This indicates that reaction of ferric ions with the cy-Fe203surface prevails over the growth of P-FeOOH under our experimental conditions. The surface 57Fe3+species observed is assignable to neither cy- nor y-FeOOH, since the hyperfine magnetic field of the former is as low as 384 kOe at room temperature3' and the latter has none at room temperature (NBel temperature: 73 K).39 (34) Ambe, S.; Ambe, F. Unpublished data. (35) Parks, G. A.; de Bruyn, P. L. J. Phys. Chem. 1962,66,967. (36) DBzsi, I.; Keszthelyi, L.; Kulgawczuk, D.; Molnhr, B.; Eissa, N. A. Phys. Status Solidi 1967,22, 617. (37) Ohyabu, M.; Ujihira, Y. Bull. Chem. SOC.Jpn. 1982,55, 1651.
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