Clay Minerals (1987) 22, 329-337

THE REDUCTIVE D ISSOLUTION OF SYNTHETIC GOETHITE AND HEMATITE IN D ITHIONITE

J. TORRENT, U. SCHWERTMANN* AND V. BARRON

Departamento de Ciencias y Recursos Agricolas, Escuela T~cnica Superior de Ingenieros Agr6nomos, Apdo. 3048, 14080 C6rdoba, Spain, and *Institut J~r Bodenkunde, Technische Universitiit Mi2nchen, 8050 Freising-

VVeihenstephan, Federal Republic of Germany.

(Received 30 July 1987)

ABSTRACT: The reductive dissolution by Na-dithionite of 28 synthetic goethites and 26 hematites having widely different crystal morphologies, specific surfaces and aluminium substitution levels has been investigated. For both minerals the initial dissolution rate per unit of surface area decreased with aluminium substitution. At similar aluminium substitution and specific surface, goethites and hematites showed similar dissolution rates. These results suggest that preferential, reductive dissolution of hematite in some natural environments, such as soils or sediments, might be due to the generally lower aluminium substitution of this mineral compared to goethite.

Although thermodynamically extremely stable, goethite and hematite, the most frequent Fe(III)-oxides in soils, may be completely redissolved under an anoxic environment if the redox potential drops below about 0.15 V (at pH 7). This reductive dissolution is usually caused by the anaerobic respiration of microorganisms which transfer an electron from the metabolized organic compounds to the Fe(III)-oxides, as exemplified by the following reaction:

FeOOH + 2H + + CH20-~ Fe 2+ + CO2 + H20.

The dissolution of Fe(III)-oxides can be easily recognized in soils as the typical red or yellow colours give way to the grey colours of the matrix minerals.

The ease with which the reduction takes place under fixed conditions with respect to Eh and pH should depend on the type of Fe(III)-oxide, and its crystal properties such as crystal size, morphology, disorder and chemistry (e.g. Al-for-Fe substitution).

Observations from red tropical soils containing hematite and goethite have led to the conclusion that under anoxic conditions a yellowing, i.e. an apparent preferential dissolution of hematite as opposed to goethite, occurs. Fey (1983) predicted this from thermodynamic considerations based on an increasing stability of goethite if its AI substitution increases (see also Yapp, 1983; Tardy & Nahon, 1985; Trolard & Tardy, 1987). However, because of the variability of thermodynamic data for fine-grained minerals, which has resulted in a certain arbitrariness of the selected solubility products (Trolard & Tardy, 1987), an experimental investigation into the dissolution kinetics of fine-grained goethites and hematites appeared desirable. This paper reports the results of a study in which the dissolution in Na-dithionite of 28 synthetic goethites and 24 hematites of widely varying crystallinity and Al-for-Fe substitution was investigated. Dithionite was chosen because, under the conditions used, the

9 1987 The Mineralogical Society

330 J. Torrent et al.

rate of dissolution was high but still easy to follow analytically. In addition, dithionite is widely used to extract Fe(III)-oxides from soils (Mehra & Jackson, 1960).

MATERIALS AND METHODS

Several series of synthetic goethites and hematites were used. Series 34 and 35 were A1 goethites synthesised in 0.3 M KOH at 70 ~ and 25~ respectively (Schulze & Schwertmann, 1984, 1987). Series 39 were pure goethites synthesised in 0.3 M KOH between 4 ~ and 80~ (Schwertmann et al., 1985). Goethites GV1, GV2 and GV3 were produced by adding 2 u KOH to a solution of Fe(NO3)3 (60 mmol in 200 ml) until the final pHs were 11.5, 12 and 13, respectively, and storing the suspensions at 35~ for 30 days. Goethite GV4 was prepared as for GV1 but Al(NO3)3 (6 mmol) was added to the initial solution. For goethite GT2, 50 mmol of Fe and 25 mmol of A1 (as nitrates) were dissolved in 200 ml of water, and 100 ml of 3 M KOH were added (final OH concentration: 0.5 M); the resulting suspension was stored for 16 days at 28~ Goethite GT4 was similar to GT2 but no A1 was added.

All hematites of series A, B, C, D, E, M and MH were synthesised from Fe(NO3)3 solutions (60 mmol in 300 ml) having different amounts of AI(NO3)B and to which 2 M KOH was added. For series A the final pH was 8 and the suspensions were stored at 98~ for 25 days. For series B the same procedure was followed except that oxalate was added at a concentration of 10 -2 M. For series C different concentrations of oxalate were added: 2 x l0 -2 for C1, 2.5 x 10 -4 for C2, 5 x 10 -3 for C3 and C4 and 2 x 10 -2 for C5. In series D the final pH was 9 and citrate (10 -5 M) was added. In series E the final pH was 10 and citrate concentration was 10 -4 M. In series M, maltose (10 -4 M) was added and final pH was 10 (M3) or 9.5 (M4). Samples MH22 and MH27 were prepared as for M3 but storage temperature was 120~ Samples MH41 and MH47 were prepared by adding oxalic acid (10 -2 M) to the solutions, precipitating to a final pH of 5, and storing the suspensions at 98~ Hematites S1, $2 and $3 were prepared by hydrolysing Fe(C104)3 at different concentrations at 90-100~ All the hematites were washed once with acid NH4-oxalate to remove noncrystalline Fe- oxides, several times with 1 M (NH4)2CO3, once with water and then finally dried at 70~ Tables 1 and 2 summarize the properties of the goethites and hematites, respectively.

All samples were X-rayed using a Philips instrument with Co-Kct radiation. The crystal thickness perpendicular to a given hkl plane (MCDhkl) was calculated using the widths at half weight (WHH) of the most important lines and the Scherrer formula, after correcting for instrumental broadening as described by Schulze & Schwertmann (1984) for the goethites and by a folding procedure (H. Stanjek, unpublished) for the hematites. From MCDhk~ the thickness perpendicular to any plane can be calculated by multiplying MCDhk~ by the cosine of the angle between hkl and the plane. For the goethites MCD was calculated along the a and b axes (i.e. MCD, and MCDb). For the hematites MCD a and MCDc were calculated. These MCD values were used to obtain 'calculated' specific surfaces as follows. It was assumed that the goethite particles were endless prisms along the e direction. In this case, if A is the mole percent A1 substitution, it can be shown that, for a specific gravity of the pure goethite equal to 4.37 g cm -3 and that of diaspore equal to 3.44g cm -3, the specific surface (m 2 g- l ) is:

1

where MCDa and MCDb are given in nm.

Reduction of goethite and hematite

TABLE 1. General characteristics of the goethites.

331

Goethite no.

MCDa (nm)

Specific surface MCD b measured calculated A1 substitution hISS* (nm) (m2/g) (m2/g) (mol ~) Feo/F% (nmol Fem -2 min -1)

34/0A 34/1 34/2 34/3 34/4 34/5 34/6 34/7

39'4 39'10 39r15 39'25 39'30 39'40 39'50 39'60 3~70 3980

21 60 32 30 0'0 0'000 31 24 63 29 29 0.9 0.002 27 27 77 23 23 1'5 0"002 31 31 83 22 20 3.0 0.001 23 33 94 20 19 3.4 0.001 28 36 66 17 20 5.8 0-001 17 40 59 15 19 7"9 0.006 17 38 48 21 22 10.9 0-017 10

9 38 94 63 0'0 0'020 22 10 43 82 56 0-0 0-014 23 11 53 73 50 0"0 0.013 25 11 43 70 52 0.0 0.008 22 11 45 60 52 0.0 0.007 23 16 54 42 37 0-0 0.008 35 26 71 30 24 0.0 0.005 31 29 80 22 22 0"0 0.001 32 36 80 19 19 0.0 0.001 34 38 80 18 18 0"0 0"001 31

GV1 21 58 34 30 0.0 0-006 40 GV2 24 61 31 27 0.0 0.005 36 GV3 17 62 48 34 0.0 0.005 25 GV4 20 34 28 37 7.6 0.005 19

GT2 32 56 38 23 18.7 0.006 8 GT4 10 42 63 57 0.0 0-010 25

35/0 17 52 40 36 0"0 0"000 30 35/3 24 104 32 24 4.9 0-000 19 35/4 33 70 27 21 7'9 0"000 13 35/5 47 95 26 15 11'6 0'005 9

* Dissolution rate per unit of surface.

For hematite, if we assume that the specific gravity of the pure mineral is 5.26 g cm -3 and that of corundum is 4.05 gcm -3 and the particles are hexagonal prisms whose width is equal to MCDa and whose height is equal to MCDc, it can be shown that the specific surface is:

(760/MCD~) + (380/MCDc) SShm =

1 - 0-00228A

Specific surface was measured by water sorption at 20~ relative humidity. This method gives usually surfaces about 30~ lower than those measured with EGME (Carter et al., 1965).

The oxalate-extractable Fe (Feo) was determined after Schwertmann (1964) and the dithionite-soluble Fe (F%) after Mehra & Jackson (1960) as modif ied by Torrent & G6mez- Mart in (1985). The dissolved Fe and A1 were determined by atomic absorption spectrophotometry (AAS).

332 J. Torrent et al.

TABLE 2. General characteristics of the hematites.

Specific surface Hematite MCD a MCDc measured calculated AI substitution b/SS*

no. (nm) (nm) (m2/g) (mZ/g) (mol %) Feo/Fed (nmol Fem -2 min -1)

A2 67 40 28 21 2.0 0.007 25 A3 75 54 29 17 4-5 0-016 15 A4 80 37 95 20 8.5 0.051 10 A5 44 16 138 42 11-5 0.310 12

B1 67 56 23 18 0.0 0.002 37 B2 55 33 96 26 14.3 0.025 6

C1 44 47 37 26 0.0 0.003 29 C2 69 35 91 22 7.5 0.064 14 C3 47 23 101 34 14.2 0.047 8 C4 68 48 73 20 10-2 0-032 10 C5 55 41 98 24 13.4 0.061 7

D1 80 67 21 15 0-0 0-008 13 D2 76 61 20 17 7.5 0.012 15 D3 39 17 57 44 14.0 0.052 9

E1 94 88 11 13 4.1 0.005 17 E2 88 85 6 13 7-8 0.002 21 E3 77 56 23 17 12.6 0.022 11

M3 40 13 93 49 12.6 0-194 9 M4 40 12 100 53 12.8 0.189 9

MH22 90 81 11 13 1.7 0.019 36 MH27 80 42 12 19 12.5 0.006 32 MH41 37 42 43 29 0.0 0-012 50 MH47 28 27 67 42 8-2 0.005 14

S1 55 44 44 23 0.0 0.001 40 $2 55 38 36 24 0.0 0.001 29 $3 38 23 27 37 0.0 0-001 48

* As for Table 1.

For investigation of the kinetics of reductive dissolution, 60 ml polyethylene bottles were filled with 50 ml of a solution of sodium citrate (0.25 M)/sodium bicarbonate (0.1 M). This solution was continuously stirred with a magnetic bar while bubbling N2 at a rate of ~ 1 ml s-1 and kept at 25~ After N 2 had bubbled for at least 15 min, 250 mg of solid sodium dithionite was added. In the meant ime, 10 mg of the sample of goethite or hematite were suspended in 1 ml of water and treated ultrasonically for 5 min. This suspension was added to the stirred solution 5 min after all the solid dithionite had dissolved. Then 1 ml portions of the final stirred suspension were taken at selected times and reduction was immediately stopped by adding 0.050 ml of 30% hydrogen peroxide. Finally, the dissolved Fe was analysed by AAS in the clear supernatant after centrifuging the suspensions. All determinat ions were carried out in duplicate or triplicate and the coefficient of var iat ion was < 15%.

Reduction of goethite and hematite 333

RESULTS AND DISCUSSION

Goethites

For the goethites the plots of dissolved Fe against time were essentially linear for the first 30 min of reaction, when 9~ (for GT2) to 55~ (for 39/4) of the total Fe had dissolved. Departure from linearity, when it occurred, was due to a small initial curvature of either decreasing or increasing slope (Fig. 1), which is typical of first order and S-shaped curves, respectively. Fitting straight lines to the 0-30 rain plots gave small positive or negative intercepts (< 2~ of total Fe). In several cases, positive intercepts corresponded to samples having significant amounts of Feo, which would indicate some preferential, quick dissolution of poorly-crystalline material. The slope, b, of these fitted lines is, obviously, a measure of the initial dissolution rate which, in turn, can be taken as a measure of the 'reducibility' of the goethites. The initial dissolution rate per unit of surface area can be obtained by dividing that slope by the specific surfaces (SS). The resulting b/SS values (in nmol Fem -2 min-1), shown in Table 1, are negatively correlated with A1 substitution (Fig. 2). Hence A1 appears to be a major factor determining the dissolution rate.

For the unsubstituted goethites the initial rate of dissolution per surface area varied less if referred to the calculated surface area than if referred to the surface area measured by H20 or EGME adsorption. In fact, there was a negative correlation between the dissolution rate and the difference SS~2 o - SScalc.. For the series 39 this difference increased from zero for goethite synthesised at 60~ to 31 m 2 g-1 for the goethite synthesised at 4~ As shown earlier by electron microscopy (Schwertmann et al., 1985) this extra surface may be attributed to an 'internal' surface due to fissures and cracks between single domains. In contrast to the

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Hematites

Many of the hematites used in this study had significant amounts of oxalate-soluble Fe (Table 2). In three cases (A5, M3, M4) the Feo/Fed ratio was > 0.1 (Table 2) although the products had been washed for 2 h with acid NH4-oxalate in amounts higher than those required for the stoichiometric removal of noncrystalline Fe-oxides. The presence of this noncrystalline Fe can explain, in part, why the measured specific surfaces were often much higher than the calculated ones, since noncrystalline Fe-oxides show high specific surfaces of about 400 mZ/g (Schwertmann & Fischer, 1973; Borggaard, 1984). However, for several samples having a Feo/F% ratio of < 0.05 (and, consequently, a specific surface attributable to noncrystalline oxides of < 400 x 0.05 = 20 m: g-l) the differences between calculated and measured specific surfaces were much higher than 20 m: g-1 (see e.g. A4, B2 and C3). This fact suggests that either the morphology of the particle is markedly different from the ideal plate assumed here, or that the particle is platy but the surface is irregular, or that the crystals have micropores to produce a marked increase in the specific surface (Schwertmann & K/~mpf, 1985). Irregular, grainy hematite crystals have been described specifically for hematites prepared in oxalate (Fischer & Schwertmann, 1975 ; Schwertmann, 1987) and this, indeed, is the case for some of the hematites of the present investigation, as for instance B2 and C3.

Reduction of goethite and hematite 335

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Most of the plots of dissolved Fe against time for the hematites were, after a few (0-5) rain close to linearity (Fig. 3) until, at least, the first 30 min, and similar, therefore, to those of the goethites. A few samples with high Feo/F% ratios (C3, B2, M3, M4 and A5) showed curved dissolution curves with a decreasing slope and no sharp breaks (Fig. 3), suggesting that noncrystalline oxides and hematite have not dissimilar dissolution rates (because in the opposite case sharp breaks close to the Feo values would have been observed).

As for the goethites, the initial rates of dissolution per unit surface area of the hematites (hISS) were also negatively correlated with the A1 substitution (Fig. 4). Comparison of Fig. 2 with Fig. 4 shows that the fitted quadratic curves are similar, i.e. the rate of dissolution per unit of surface of the two minerals is similar at similar A1 substitution.

Unsubstituted hematites showed markedly different values of b/SS. None of the mineralogical characteristics studied here were able to explain these differences. Particle morphology might affect dissolution. This would be in line with the anisotropy shown by hematites being etched by acids (Warren et al., 1969) although no proofs can be offered here.

CONCLUSIONS

Under the conditions of the present investigation, synthetic goethites and hematites having similar specific surfaces and A1 substitution showed similar dissolution rates in dithionite. Although this observation may not be valid in toto for natural goethites and hematites present in soils or sediments it suggests, nevertheless, that the apparent preferential dissolution of hematite in natural environments is due either to its smaller size (higher specific surface) or to

336 J. Torrent et al.

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its lower level of A1 substitution, or both. Although it has been frequently reported that hematite has larger crystal size than goethite (Schwertmann, 1987) this is not always true (Pefia & Torrent, 1984). As a consequence, particle size cannot be invoked as the main cause for preferential dissolution of hematite. In contrast, in natural environments A1 substitution is lower in hematite than in the coexisting goethite (Torrent et al., 1980; Pefia & Torrent, 1984; Schwertmann, 1985). This could be then the main reason for the preferential reductive dissolution of hematite in soil materials or sediments which have changed from red to yellow, and when only the pigmenting effect of the goethite remains.

ACKNOWLEDGMENT

This work was supported, in part, by the Comisi6n Asesora de Investigaci6n Cientifica y T6cnica (Spain) under Project No. 2010/83.

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MEHRA O.P. & JACKSON M.L. (1960) Iron oxide removal from soils and clays by dithionite-citrate systems buffered with sodium bicarbonate. Clays Clay Miner. 7, 317-327.

Reduction o f goethite and hematite 337

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SCl-tW~.RrMANN U. (1987) Some properties of soil and synthetic iron oxides. In: 1ton in Soils and Clay Minerals, Nato Advanced Institute (J. W. Stucki, B. A. Goodman & U. Schwertmann, editors). Reidel, Bad Windsheim, Germany.

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SCh'WER'rMAr,rN U. & FiSChER W. (1973) Natural 'amorphous' ferric hydroxide. Geoderma 10, 237-247. SCnWERTr~tANN U. & KL~u'F N. (1985) Properties of goethite and hematite in kaolinitic soils of Southern and

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TORRENT J. & GOMEZ MARTIN F. (1985) Incipient podzolization processes in Humic Acrisols of Southern Spain. J. Soil Sei. 36, 389-399.

TORRENT J., SCHWERTMA~ U. & SCHUnZE D.G. (1980) Iron oxide mineralogy of some soils of two river terrace sequences in Spain. Geoderma 23, 191-208.

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YAPP C.L (1983) Effects of A1OOH-FeOOH solid solution on goethite-hematite equilibrium. Clays Clay Miner. 31, 239-240.