ACID DISSOLUTION OF HEMATITES OF DIFFERENT MORPHOLOGIES

Transcription

1 Clay Minerals (1993) 28, ACID DISSOLUTION OF HEMATITES OF DIFFERENT MORPHOLOGIES R. M. CORNELL AND R. GIOVANOLI University of Berne, Laboratory for Electron Microscopy, Freiestrasse 3, 3000 Berne 9, Switzerland (Received 1 July 1992; revised 26 August 1992) ABSTRACT: Dissolution in HCl of hematite particles with different morphologies led to sigmoidal dissolution vs. time curves. The rate of dissolution was directly proportional to the sample surface area and independent of crystal morphology. Hematites produced by heating goethite at 600 or 800~ dissolved more rapidly per unit area than did hematites grown from solution. TEM showed that some platy crystals developed central holes after prolonged acid attack; the hole formation was attributed to enhanced dissolution at screw dislocations present on the (001) faces of the crystals. Except where particularly susceptible regions involving strained areas or dislocations were present, there appeared to be no preferential acid attack at any particular crystal face. The original morphology was usually maintained during the reaction. Goethite (c~-feooh) and hematite (o~-fe203) are two of the most widespread iron oxides in soils and sediments. Their dissolution behaviour is important because it can influence the availability of nutrients for plant growth and also helps regulate migration of pollutant ions in soils. An understanding of dissolution behaviour of these minerals may also assist in elucidation of weathering mechanisms. The dissolution behaviour of goethite and hematite has been investigated intensively (Cornell et al., 1975; Sidhu et al., 1981; Schwertmann, 1984; Cornell & Schindler, 1987; Torrent et al., 1987, LaKind & Stone, 1989). Considerable attention had been given to dissolution by proton attack. Dissolution in HC1 involves protonation of the oxide surface together with formation of a chloride-fe surface complex, followed by the rate determining step, the breaking of the Fe-O bond (Cornell et al., 1976; Sidhu et al., 1981). The rate of acid dissolution of goethite depends markedly on crystal morphology as well as on sample surface area (Cornell et al., 1974). Information concerning the effect of crystal morphology on the dissolution of hematite is lacking. Crystal morphology, however, does influence the number and position of bands in the infrared spectrum of hematite (Rendon & Serna, 1981) and the extent of phosphate adsorption (Barron et al., 1988), so it might be expected that this parameter would also influence dissolution behaviour. In the present study, the complete dissolution in HCI of a number of hematite samples with different crystal morphologies has been followed. Seven samples of synthetic hematite with distinct, well defined morphologies were examined. This work had four specific aims (1) to determine what rate law was followed; (2) to monitor changes in crystal morphology as dissolution progressed and to identify the sites of attack; (3) to determine whether crystals with different morphologies dissolve at different rates per unit area; (4) to determine whether method of preparation, i.e. dehydration of goethite at temperatures >600~ versus precipitation in solution, affected dissolution rate. A further point of interest was to compare the overall rate of dissolution of hematite with The Mineralogical Society

2 224 R. M. Cornell and R. Giovanoli that of goethite. During the first few percent reaction, hematite dissolved more rapidly per unit area than did goethite (Sidhu et al., 1981). There is qualitative evidence that this difference in rates persists over the entire dissolution reaction; transmission electron microscopy (TEM) showed that the hematite centre dissolved from epitaxial goethite twins more rapidly than did the goethite outgrowths (Cornell et al., 1974). Similarly, in mixtures of goethite and hematite, all the hematite dissolves before goethite, even when the former compound is in excess. In these examples, however, the surface area of each compound was unknown. In the present work, therefore, a brief comparison of the rate of dissolution over the entire reaction was made using samples of goethite and hematite of known surface area. MATERIALS AND METHODS Hematite samples 1-4 were grown from ferrihydrite suspensions (lg/1) at ph ranging from 6-15 and temperatures from 70-90~ Full descriptions of the preparation of samples 1,3-5 are given in Schwertmann & Cornell (1991). Sample 1 was obtained by transformation of ferrihydrite in 0.05 M NaHCO3 buffer (ph 8) at 90~ for 48 h. Sample 2 was obtained by transformation of ferrihydrite in 6 M KOH at 70~ for 500 h (Cornell & Giovanoli, 1985). Sample 3 was formed by conversion of ferrihydrite in the presence of oxalate (oxalate: Fe = ) at ph 6.5 and 70~ for 36 h. Sample 4 was obtained by holding a Cu/Fe (Cu/(Cu + Fe) = 0.1) co-precipitate at ph 12 and 90~ for 60 h. Sample 5 was prepared by heating a 0.02 M ferric chloride solution at 98~ for 24 h. Samples 6 and 7 were obtained by dehydrating goethite in air for 30 min at 600~ and 800~ respectively. The sample of goethite was grown from ferrihydrite at ph 13 and 70~ (Cornell & Giovanoli, 1985, Fig. 6b). X-ray powder diffraction patterns were obtained using a Guinier-Enraf camera (Mk IV) with Fe-Koq radiation. For each kinetic experiment, a suspension was prepared by adding 0.1 g oxide to 100 ml preheated M HC1. Some additional experiments were carried out using 0.1 M HC1 (to provide more information about the initial stage for sample 3) or 2 M HC1. The suspension was maintained at 65~ in a stoppered flask and stirred vigorously with a magnetic stirrer. Heating was by means of a hot plate. At intervals, samples were withdrawn, filtered through a 0-22/~m millipore filter and analysed for Fe by atomic absorption spectroscopy (AAS). The partly dissolved residues were examined by TEM and scanning electron microscopy (SEM). Transmission electron micrographs were obtained using a Hitachi H (100 kv) electron microscope. For TEM examination, the samples were dispersed in doubly distilled water using ultrasonic treatment and a drop of suspension was dried on a carbon-coated bronze grid. Some samples were shadowed with Cr at an angle of 19~ Replicas of partly dissolved crystals of sample 2 were obtained by drying a drop of suspension on a glass slide and coating it with evaporated carbon followed by Pt. The replica that resulted was floated off the glass slide onto a HF/HCI (40% HF/2 M HC1) solution and left for 36 h after which the hematite in the replica shells had dissolved. The replicas were then washed with doubly distilled water.

3 Acid dissolution of hematites 225 Scanning electron micrographs were obtained on gold sputtered samples using a JEOL JSM 840 scanning electron microscope. Sample surface areas were measured using the BET method with N 2 as the absorbent. Description of the samples RESULTS X-ray diffraction (XRD) showed that all samples consisted of 100% hematite except for samples 1 and 2 which also contained a few percent goethite. From TEM, sample 1 consisted of irregular, platy crystals nm in diameter, some diamond-shaped particles and -1-2% goethite needles (Fig. la). Such a sample is typical of hematite grown from ferrihydrite in weakly alkaline media. The sample surface area was 23 m2/g. Sample 2 also consisted of platy crystals, in this case, thick, approximately hexagonal plates 1-3/~m in diameter (Fig. lc). As the sample contained -5% goethite crystals as well, the surface area was not measured; calculations based on the dimensions of the hematite crystals indicated that the area of the hematite component was <5 m2/g. Sample 3 consisted of ellipsoidal, granular particles with lengths ranging from nm and a sample surface area of 83 m2/g (Fig. 2c). Both TEM and SEM indicated that sample 4 consisted of rhombohedra nm across (Fig. 3a, 3c). A proportion of these crystals displayed interpenetrant twinning on the rhombohedral faces (Fig. 3c, arrow). This hematite contained 8 mole% Cu and had a sample surface area of 5.5 m2/g. Sample 5 consisted of subrounded crystals with a diameter of -100 nm, a narrow size distribution and a surface area of 30 mz/g (Fig. 2e). This sample served as a reference model system for determination of the dissolution rate law. Sample 6 consisted of rod-like crystals 450 nm in length and 50 nm in width. As this sample was prepared by dehydration of goethite, the surfaces of the resulting hematite rods were covered with pores through which water had escaped (Fig. 2a). The sample surface area was 17 mz/g. Sample 7 consisted of similar rod-like crystals which were, however, rather distorted and also completely without pores; the surface area was 4.5 mz/g. The goethite sample had a sample surface area of 36 m2/g and consisted of acicular, multidomainic crystals 500 nm in length. Kinetics of dissolution Plots of the extent of dissolution, a, vs. time (Fig. 4) were sigmoidal for all the hematite samples investigated. Various rate laws including the cube root law and the Kabai equation were tested; neither of these laws applied to the initial slow stage of the reaction. It was found that the dissolution data from the start to 80-90% of the reaction could be fitted to the Avrami-Erofe'ev equation (cf. Brown et al., 1980), i.e. (-ln(1-c~)) ~= kt This rate law applies to reactions that are surface controlled and where nucleation of reaction sites is random. Despite its higher surface area, goethite dissolved far more slowly than the hematite

4 R. M. Cornell and R. Giovanoli 226 FIG. 1. Transmission electron micrographs of hematite: (a) small platy crystals, (sample 1); (b) after 60% dissolution, M HCI, 65~ (c) large platy crystals and some goethite needles (sample 2); (d) after 50% dissolution, M HC1, 65~ (e) replicas of partly (25%) dissolved crystals; (f) after 80% dissolution. TABLE 1. Rate constant, k, as a function of surface area, for the dissolution of different hematites in M HCI (65~ Sample k x 104 min 1 m-z

5 Acid dissolution of hematites 227 samples; the inset in Fig. 4 compares plots of extent of dissolution vs. time for goethite. In the time required for complete dissolution of the hematite samples, less than 10% dissolution of goethite took place. Over the entire course of the reaction, goethite dissolved more slowly per unit area than did hematite; the reason for this is not clear as yet. In Fig. 5, the dissolution rate constant, k, for the different hematite samples is plotted against sample surface area. These values are also shown in Table 1. For the four samples grown in solution, (surface area 6-83 m2/g) the rate was directly proportional to surface FIc. 2. Transmissionelectron micrographsof hematite: (a) rod-like crystals(sample 6); (b) after 40% dissolution MHCI, 65~ (c) ellipsoidal crystals (sample 3); (d) after 55% dissolution, MHCI, 65~ (e) spherical crystal, sample 5; (f) after 40% dissolution, u HCI, 65~

6 228 R. M. Cornell and R. Giovanoli area. The same trend appears to be followed by hematite formed by dehydration of goethite. An interesting result is that hematites formed by heating goethite at temperatures at or above 600~ dissolved more rapidly per unit area than did those grown in solution. The Cu-substituted hematite (sample 4) had one of the lowest sample surface areas; there is no evidence that the rate of dissolution was influenced by the presence of Cu in the crystal structure. Electron microscope observations The predominant crystal face displayed by the platy crystals was (001); this lies parallel to the electron microscope grid. The edges of the large plates (sample 2) are bounded by 110 planes, whereas the edges of the small plates (sample 1) are presumably bounded by a number of different prismatic planes. The most striking characteristic of the dissolution of these crystals is that after prolonged acid attack, a proportion of the plates developed central holes indicating preferential attack at sites on the (001) faces (Fig. lb,d,f). The TEM observations suggested that such hole formation led to eventual disintegration of the crystals. Where holes did not develop, the crystals decreased in size by edge and surface attack. In the early stages of dissolution the edges became lobed (sample 1) or rounded (sample 2). Replicas of partly dissolved large plates also indicated that edge attack proceeded in a uniform manner from the outset of dissolution (Fig. le). Fro. 3. Electron micrographsof rhombohedralhematitecrystalswith8% Cu substitution (sample4): (a) TEM of originalcrystals;(b) Cr-shadowed,partlydissolved(25%) crystals,n HC1,65~ (c) SEM of original crystals; (d) SEM of partly dissolved (40%) crystals,mhc1,65~

7 Acid dissolution of hematites 229 The rod-like hematite (sample 6) was formed by dehydration of acicular goethite, hence the predominant crystal face was (001) ((100) in the original goethite). This face was extremely porous. The sides of the rods were bounded by 110 planes and the ends by 100 planes. It was shown by TEM that dissolution took place at the crystal edges; holes and corrugation developed as the reaction proceeded (Fig. 2b). Other sites of attack were the pores on the (001) faces which gradually enlarged during the reaction. Sample 7 particles also maintained an acicular shape during the dissolution process; the crystals just became thinner and more distorted. The ellipsoidal crystals (sample 3) maintained their shapes to a late stage in the reaction (Fig. 2d). As dissolution proceeded, the crystals developed a fibrous appearance and then abruptly disintegrated after --80% reaction. The rounded crystals (sample 5) appeared to dissolve in an overall uniform manner (Fig. 2f). The rhombohedral crystals (sample 4) were bounded by six (102) faces. Examination by TEM of the partly dissolved crystal suggested that after initial attack at the corners and any surface irregularities the crystals decreased in size fairly uniformly (Fig. 3b). Examination by SEM confirmed that initial attack occurred at the corners; Fig. 3d shows that after acid._~ 1,0 0,6 O c.'0 x W Time (min) v o 06 "E x Time (rni n) FIG. 4. Extent of dissolution of solution grown hematites vs. time; M HC1, 65~ 1 = small platy crystals; 2 = large platy crystal; 3 = ellipsoidal hematite; 4 = rhombohedral crystals; 5 = spherical crystals. Inset: extent of dissolution of calcined hematites and goethite vs. time; M HC1, 65~

8 230 R. M. Cornel1 and R. Giovanoli attack, the crystals developed a rounded appearance and this shape was then maintained for most of the reaction. DISCUSSION The sample of spherical, monodisperse hematite displayed a sigmoidal dissolution curve and the dissolution data followed the Avrami-Erofe'ev law. Similar dissolution behaviour was observed for all other samples. It can be concluded, therefore, that the same rate law is followed regardless of crystal morphology and method of preparation. Why the calcined material dissolved more rapidly than that grown in solution is not clear. The XRD patterns of the calcined samples showed uneven broadening of the XRD lines with 110 and t02 being sharper than the other lines, which indicates incomplete ordering of the crystal structure; this may account for the faster dissolution of such hematite. The cause of the inital slow dissolution of hematite is also not clear, but is presumably associated with slow nucleation of dissolution sites. The major crystal faces displayed by the samples studied here were (001), (110) and (102). These are among the commonest faces exhibited by hematite crystals. The kinetic data suggested that the different planes dissolved at approximately the same rate per unit area. However, TEM indicated that some preferential attack took place at the (001) faces of the platy crystals as indicated by hole formation in some of the plates. In a study of natural hematite crystals, Sunagawa (1962a) established that the basal 001 planes of platy crystals usually grow by a two dimensional spreading mechanism and in some cases, growth is initiated by the presence of a screw dislocation. By means of etching experiments, he showed that such dislocation served as sites of preferential dissolution (Sunagawa, 1962b). The distribution of these dislocations appeared to be random; they were only observed in a proportion of the crystals examined. It seems probable that the hole formation observed in the synthetic hematite plates was the result of the presence of screw dislocation in some 5 6 A 3 r C ; 3 =6 c ~ B 5e O =2 "6 u~ 7 le u~ i:5 1 O Surface area m 2g-1 FIG. 5. Plots of dissolution rate constants vs. sample surface area; (A) calcined hematite, (B) solution grown hematite. The numbers by the points refer to the appropriate samples.

9 Acid dissolution of hematites 231 crystals. As the dislocation passes right through the crystal, attack at such a site could lead to hole formation. It is of interest that similar hole formation after partial dissolution has been observed for platy soil hematites (Schwertmann, 1991). Formation of holes via preferential dissolution at dislocation sites has also been observed for goethite (Cornell et al., 1974). Enhanced attack leading to hole formation on the (001) faces might be expected to increase specific surface area and hence accelerate sample dissolution. This effect was not noticed, most probably because, despite their spectacular appearance, holes formed in relatively few platy crystals. In aqueous media the surface of hematite is hydroxylated. The surface hydroxyl groups are coordinated to underlying Fe atoms and are the sites at which dissolution starts. The OH groups are of three kinds; they may be coordinated to one, two or three underlying Fe atoms. Earlier work with goethite (Cornell et al., 1974) and lepidocrocite (7-FeOOH) (Cornell & Govanoli, 1988) suggested that only the singly coordinated OH groups took part in the dissolution reaction; those crystal faces on which the density of such groups was highest dissolved most readily. For hematite, however, the nature and distribution of surface OH groups does not appear to influence dissolution behaviour. Singly coordinated OH groups are present only on the (110) faces (in the samples studied), yet other crystal faces were also attacked. It can be concluded that except where susceptible regions such as dislocations or strains are present, all crystal faces of hematite dissolve equally well. This behaviour may be related to the internal crystal structure which is approximately equidirectional. Dissolution of hematite is dependent only on surface area, not crystal morphology. It follows that marked changes in crystal morphology during dissolution are unlikely to be observed except where definite area of strain are present. In fact, a rare example of a change in morphology following acid attack was noted in an earlier investigation in which rod-like hematite formed by growth from a central nucleus in both directions along the c axis (in the presence of maltose) was partly dissolved (Cornell, 1985). Such crystals are bounded by six (110) faces. Instead of uniformly decreasing in width as they dissolved, the crystals developed a marked hourglass form as a result of enhanced attack at the centre of the rod. Preferential dissolution occurred in the vicinity of the original hematite nucleus because this was surrounded by a strained region. ACKNOWLEDGMENTS We are indebted to Miss E. Ettinger for carrying out the electron microscopy and to Mrs B. Wild for the X-ray powder diffraction measurements. Thanks are due to Mr B. Trusch for the AAS measurements and for carrying out the surface area determinations. REFERENCES BARRON V., H~RRUZO M. & TORRENT J. (1988) Phosphate adsorption by aluminous hematites of different shapes. Soil Sci. Soc. Am. J. 52, BROWN W.E.B., DOLLIMORE D. & GALWAY A.K. (1980) Chapter 3, Heterogeneous Reactions Pp in: Comprehensive Chemical Kinetics (C.H. Bamford & C.F.H. Tipper, editors). Elsevier, Amsterdam. CORNELL R.M, (1985) Effect of simple sugars on the alkaline transformation of ferrihydrite into goethite and hematite. Clays Clay Miner. 33, CORNELL R.M. & GIOVANOLI R. (1985) Effect of solution conditions on the proportion and morphology of goethite formed from ferrihydrite. Clay Clay Miner. 33,

10 232 R. M. Cornell and R. Giovanoli CORNELL R.M. & GIOVANOLI R. (1988) Acid dissolution of akagan6ite and lepidocrocite: The effect on crystal morphology. Clays Clay Miner. 36, CORNELE R.M. & SCHINDLER P.W. (1987) Photochemical dissolution of goethite in acid/oxalate solution. Clays Clay Miner. 35, CORNEEL R.M., POSNER A.M. & QUIRK J.P. (1974) Crystal morphology and the dissolution of goethite. J. Inorg. Nucl. Chem. 36, CORNEEL R.M., POSNER A.M. & QUIRK J.P. (1975) The complete dissolution of goethite. J. Appl. Chem. Biotechnol. 25, CORNEEL R.M., POSNER A.M. QUIRK J.P. (1976) Kinetics and mechanisms of the acid dissolution of goethite. J. Inorg. Nud. Chem. 38, LAKINO J.S. & STONE A.T. (1989) Reductive dissolution of goethite by phenolic reductants. Geochim. Cosmochim, Acta 59, RENDON J.L. & SERNA C.J. (1981) IR spectra of powder hematite; effects of particle size and shape. Clay Miner. 16, SCHWERTMANN U. (1984) The influence of aluminium on iron oxides: IX. Dissolution of Al-goethites in 6 M HC1. Clay Miner. 19, SCHWERTMANN U. (1991) Solubility and dissolution of iron oxides. Plant Soil, 130, SCHWERTMANN U. & CORNELL R.M. (1991) Hematite. Pp in: Iron Oxides in the Laboratory. VCH, Weinheim. SIDHU P.S., G1LKES R.J., CORNELL R.M., POSNER A.M. & QUIRK J.P. (1981) Dissolution of iron oxides and oxyhydroxides in hydrochloric and perchloric acids. Clays Clay Miner. 29, SUNAGAWA ][. (1962a) Mechanism of growth of hematite. Am. Miner. 47, SUNAGAWA I. (1962b) Mechanism of natural etching of hematite crystals. Am. Miner. 47, TORRENT J., SCHWERTMANN U. & BARRON V. (1987) The reductive dissolution of synthetic goethite and haematite in dithionite. Clay Miner. 22,