THE INFLUENCE OF ALUMINIUM ON IRON OXIDES: XIII. PROPERTIES OF GOETHITES SYNTHESISED IN 0-3 M KOH AT 25~

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1 Clay Minerals (1987) 22, THE NFLUENCE OF ALUMNUM ON RON OXDES: X. PROPERTES OF GOETHTES SYNTHESSED N 0-3 M KOH AT 25~ D. G. SCHULZE AND U. SCHWERTMANN* Agronomy Department, Purdue University, West LaJayette, ndiana 47907, USA, and *nstitut J~r Bodenkunde, Technische Universiti~t Miinchen, 8050 Freising-Weihenstephan, FRG (Received 20 May 1986; revised 16 September 1986) A B S T R A C T : Aluminium-substituted goethites were synthesised in 0.3 M KOH at either 25 ~ or 70~ The 25~ goethites had unit-cell a and c dimensions about ~ larger than the 70~ goethites of comparable A1 substitution. Unit-cell b dimensions were similar regardless of synthesis temperature. The 25~ goethites contained larger amounts of A1 in their structures than the 70~ goethites synthesised at the same initial A1/(A1 + Fe) mole ratio in solution. The 25~ goethites had lower dehydroxylation temperatures and smaller differences between the two OH-bending bands than comparable 70~ goethites. These differences between the two sets of goethites are believed to be caused by the presence of more structural defects in the 25~ goethites than in the 70~ goethites. Goethites with the lowest A substitutions consisted of crystals with several coherently diffracting domains, while at high A1 substitutions the crystals were mainly monodomainic. Surface areas of the 25~ goethites varied in a complex manner as a function of thickness along [100], character of the domains, and physical size. Al-substituted goethites can be synthesised in a few days by using high OH concentrations and temperatures of 70~ (Lewis & Schwertmann, 1979a,b). The properties of these goethites have been studied in considerable detail (Schulze & Schwertmann, 1984). Goethite forms at much lower temperatures during weathering and soil formation. Not much is known about synthetic Al-substituted goethites formed at lower temperatures. Goethites have previously been synthesised at 25~ only by oxidation of mixed Fe()-A1 solutions. These goethites usually have very small crystals and more structural defects than goethites formed from Fe() at higher temperatures and OH concentrations (Fey & Dixon, 1981; Goodman & Lewis, 1981; Schulze & Schwertmann, 1984). The purpose of this study was to produce and characterize somewhat better crystalline Agoethites synthesised by transforming freshly precipitated ferrihydrite in an alkaline aluminate solution (0.3 M KOH) at 25~ The goethites are compared to Al-goethites made by the same procedure but at 70~ MATERALS AND METHODS An aluminate solution was prepared by slowly pouring 1.51 of 0.5 N A(NO3) 3 into 900 ml of 5 M KOH while constantly stirring. The required amounts of this aluminate solution plus additional 5 M KOH (Table 1) were poured into 5 1 polypropylene bottles. Then, for each bottle, 225 ml of 1 M Fe(NO3) 3 was quickly added, the bottle was swirled to thoroughly mix The Mineralogical Society

2 84 D. G. Schulze and U. Schwertmann TABLE 1. Volumes of aluminate and 5 M KOH used for each sample (ml). Sample Aluminate 5 M KOH 35/o / / / / / / / / / the solutions, then quickly filled to with deionized H20 from another large bottle, and then stirred with a large plastic spoon. The elapsed time from the beginning of the Fe(NO3)3 addition to the achievement of the final volume was < 30 s. The solutions were then placed in an oven at 70~ for 14 days (Series 34) or in a constant-temperature room at 25~ for 1310 days (Series 35). The solutions at 70~ were stirred once each day and those at 25~ were stirred at irregular intervals every few weeks. At the end of the synthesis period the samples were washed with deionized H20 and then dried at 50~ Total Fe (Fet) was determined after dissolving the sample in concentrated HC. Oxalateextractable Fe (Feo) was determined after treatment with NH4-oxalate at ph 3 in the dark for 2 h (Schwertmann, 1964). ron was determined by the e,e'-sulfosalicylic acid procedure (Koutler-Anderson, 1953). Samples for X-ray diffraction were mixed with either 25~o (Series 35) or 50~o (w/w) c~-a1203 (Series 34) as an internal standard. nstrument settings and preparation of the powder mounts were as described by Schulze (1984). The Series 35 samples were scanned at a rate of 1/2 ~ 20/min and the patterns recorded on a strip chart. The Series 34 samples were step-scanned and a curve-fitting procedure used to determine line positions (Schulze, 1984) which, for both series were measured relative to the positions of nearby c~-a1203 lines. A check using three samples showed that both methods gave essentially the same results. Widths at half-height (WHH) were corrected for instrumental broadening by a folding procedure (H. Stanjek, nstitut ftir Bodenkunde, Technische Universit~it Mfinchen, unpublished). Calcined hematite showing no particle-size broadening was used to obtain the instrument broadening profile. Mean crystallite dimensions (MCD) of both series were calculated from the 110 and l 1 l lines using the Scherrer formula (Klug & Alexander, 1974). Goethite unit-cell dimensions were calculated from the 110, 130 and 111 line positions (Schulze, 1984). Hematite a dimensions were calculated from the 110 and 300 lines. The procedures used for infrared absorption spectroscopy (R), differential thermal analysis (DTA), transmission electron microscopy (TEM), and specific surface area determinations have been described by Schulze & Schwertmann (1984).

3 Properties of synthetic goethites 85 RESULTS AND DSCUSSON Samples 35/0 through 35/5 of the 25~ series had an Feo/Fet ratio of < at the end of the 1310 day period (Table 2), indicating essentially complete conversion to well-crystallized phases. Samples 35/6 through 35/9, however, still contained appreciable quantities of oxalate-soluble material. The greater the A1 addition, the greater was the Feo/Fet ratio, reflecting the increasing inhibition by A1 of the transformation of ferrihydrite to goethite or hematite. The same trend held for Feo/Fet of the 70~ series but the values at higher A1 substitution were much lower, indicating more complete conversion. More A was incorporated into the goethite structure at 25~ than at 70~ at the same A/(A1 + Fe) mole ratio in the initial solution. This indicates that the goethite crystals can accommodate more A if they grow slowly than if they grow quickly. Substitutions of > 30 mole~ A1 are common under natural conditions, perhaps in part due to the slow growth of the crystals. Schulze (1984) showed that the unit-cell c dimension of 81 synthetic goethites decreased linearly with increasing A substitution. The relationship Al(mole~) = c (dashed line in Fig. 1) was found to predict A1 substitution to within mole~ at the 95~o confidence level. This equation predicts A1 substitution of the goethites in this study within the stated accuracy. A closer examination of the cell dimensions, however, shows that the c dimensions are slightly different depending on synthesis temperature (Fig. 1). The c dimensions of the goethites synthesised at 25~ are slightly above the line given by Schulze (1984), whereas the goethites synthesised at 70~ fall slightly below the line. Linear ! ' goethites "~',,~ -- ""',~ C= t8t5 A1 "~~'~7, r=-o ' goethites - ~-,,~ ~"-~ c=_3.o2 -o.oo 2 AZ n=7, r=-o.993 "~ Schu lze" " b..... {5 " " A1 (mole %) FG. 1. Unit-cell c dimensions vs. A substitution.

4 86 D. G. Schu~e and U. Schwertmann ~ ~ ~ ~ ~ ' ~!= o ~.~. ~o. o. o. ~m. ~. r~,..~(~.~l~.r~.~l ~.. ~2.o 0 e~ ',.7 ~ ~.....,-._, ~ ~ " ~ 2.-, ~ ~ 9 9 ~.o e-i,.o ~1 ~'~Z "~ ~,~ ~.o ~..

5 Properties of synthetic goethites ' 0.03' 25' goethi W w "-" 0.02' r ~a 0.0t o o 'o ab 'o A1 (mole %) FG. 2. Aa vs. A substitution. regression analysis shows that the intercepts of the two lines differ by A. This difference is significant at the 95% level. The slopes, however, are not significantly different. Thus, some of the uncertainty in estimating A substitution from the c dimension using the relationship given by Schulze (1984) is due to small variations in the c dimension itself and not due to measurement error, f Schulze's (1984) regression line is used, the c dimension over-estimates A substitution by about 1 mole% for the 25~ goethites, while it underestimates it by about the same amount for the 70~ goethites within the range of 0 to ~ 10 mole% A1. Chemically determined A1 is used in subsequent discussions for the samples which contained only goethite. Samples 34/7, 34/8, 34/9, 35/7, 35/8, and 35/9 contained hematite. The A1 substitution of these samples was determined by extrapolation of the two linear relationships given in Fig. 1. The unit-cell a dimensions (Table 2) were A larger for the samples synthesised at 25~ than for those synthesised at 70~ The larger a dimensions may indicate that the samples synthesised at 25~ contain more structural defects than those synthesised at 70~ (Schulze, 1984). Schulze & Schwertmann (1984) proposed A a, the deviation of the observed a dimension from the a dimension predicted by the Vegard Rule, as a measure of structural defects. Aa is defined by the relationship Aa = aob~ -- ( A1), where ao~ is the observed a dimension, and A is A substitution in mole%. The value of A is for a wellcrystallized goethite without A substitution (ASTM card (starred)). This equation differs slightly from the equation given by Schulze & Schwertmann (1984) because the previous ASTM card for goethite, (starred), has been replaced by card (starred).

6 88 D. G. Schulze and U. Schwertmann "E 40" r- 0 0 =K 20 Surface Area / Goethite Goethite + Hematite 0 l'0 2' A (mole %) FG. 3. MCD110, MCD111 and surface area vs. A substitution for the 25~ goethites (Series 35). 150,~ D1 E 1D lo0 ~ L D U '4-- f " O3 Fig. 2 shows that the greater the A substitution, the greater the value of Aa, suggesting that there are more structural defects at higher A1 substitutions than at lower ones. Aa is significantly larger for the 25~ goethites than for the 70~ goethites at the same level of A substitution. This is in agreement with the results of Schwertmann et al. (1985) for a synthesis series in which only temperature was varied. They found that the a dimension was about 0.02 A larger for goethites synthesised between 4 and 30~ than for goethites synthesised between 50 and 80~ The higher energy input during the synthesis procedure apparently reduces structural defects, resulting in a approaching the ideal value of A. The b dimensions were similar for both synthesis series, with the exception of sample 34/9 (Table 2). Schwertman et al. (1985) also found the b dimensions of goethites to be similar regardless of synthesis temperature. The EGME surface area of the 25~ series first decreased from 52 to 26 m 2 g-~ as A1 increased from 0 to 11-6 mole~o, then increased at higher A substitutions (Fig. 3). The decrease between 0 and 11-6 mole~ A1 can be explained by (i) an increase in domain size (and crystal thickness) along [100] and (ii) a decrease in number of domains per crystal. Electron microscopy shows lath-shaped crystals that have about the same width but become shorter as A substitution increases from 0 to 11.6 mole~o (Fig. 4, samples 35/0 to 35/5). Electron diffraction has shown that lath-shaped goethite crystals tend to orientate with the (100) face perpendicular to the electron beam (Cornell et al., 1983). Thus, only changes along [010] and [001] are normally observed by EM; changes in thickness along [100] are not observed. Both MCD, 10 and MCD~ ~ are, however, strongly dependent on the thickness of the coherently diffracting domains along [100] and are an indicator of the thickness along [100] not

7 Properties of synthetic goethites 89 FG.4. Selected transmission electron micrographs for the 25~ goethites (Series 35). observable by EM. Schulze & Schwertmann (1984) have shown that for similar acicular goethites, surface area is inversely related to MCD~, the thickness along [100]. The increase in MCD~0o and M C D ~ between 0 and 20 mole~o A1 (Fig. 3, Table 2) is accompanied by a decrease in surface area, at least up to 11.6 mole~ A1 substitution. Additional surface area decrease is due to the character of the domains in the crystals. At low A1 substitution each physical crystal consists of several domains (Fig. 4, sample 35/0) as observed by Schwertmann (1984), Schulze & Schwertmann (1985) and Schwertmann et al. (1985). These domains often grow to different lengths along the c axis to give crystals with jagged ends. Both the jagged ends and the gaps which often occur between adjacent domains (arrows in Fig. 4) can increase the surface area. The loss of the gaps and jagged ends as A1 substitution increases (Fig. 4, compare samples 35/0, 35/3 and 35/5) contributes to the decrease in surface area. Above 11.6 mole~ A1, each crystal consists of one domain, but the crystals become physically smaller (Fig. 4, sample 35/9) and surface area increases. The 70~ goethites also show a general increase in MCD with increasing A1 substitution, except for sample 34/9 (Table 2). The surface area is lower than for the 25~ goethites,

8 90 D. G. Schulze and U. Schwertmann 120 T E (J 110 "1- cz) "r *goethite goethites '0 2'0 A (mole %) 3O FG. 5. Difference between the two OH-bending bands (~OH and ~,OH) vs. A1 substitution. particularly at low substitution. There is a weak minimum at about 8 mole~ A substitution and higher values for the last two samples. The OH-stretching band positions (voh) were approximately 20 wavenumbers higher for the 25~ goethites than for the 70~ series for samples with < 12 mole~ A1 substitution. This increase in voh is consistent with the larger a dimensions. The separation of the two OHbending bands increased with increasing A1 substitution and was consistently lower for the 25~ goethites than for the 70~ goethites, except for sample 34/9 (Fig. 5). Band separation is determined mainly by the amount of A1 substitution, but it is also influenced to some extent by structural defects. More structural defects in the 25~ goethites leads to a slight decrease in the band separation as compared to the 70~ goethites. Pure goethites synthesised between 4 and 40~ were found to have a band separation of about 93 cm-i while goethites synthesised between 50 and 80~ had a separation of about 97 cm -1 (Schwertmann et al., 1985). The average dehydroxylation temperature was similar for both series and increased linearly with A1 substitution for samples with < 12 mole~ A1 substitution (Fig. 6). The hematite which was associated with goethite in samples 35/8 and 35/9 was also A1- substituted. A1 substitution in hematite was estimated from the a dimension using the relationship Al(mole~) = 676( a) (n = 7; r = 0.997; a in A) found for a series of hematites produced at 25~ and ph 7.0. Sample 35/8 had 2.8 and sample 35/9 had 4.4 mole~ A1 in the hematite structure. This is much less than the goethites in the same samples (22-9 and 27.3 mole~o A respectively). The (130) line of goethite and the (104) line of hematite are therefore clearly separated, although they coincide at 2.67 A when the two phases are

9 360" Properties of," synthetic goethites 91 c~ :D 340 "0 t3.. E 320 g,j 4-J x o 300' 0 70* goethites 0 25* goethites 0 0 > 280: 2600 l'o 2'0 30 A (m01e %) FG. 6. Average dehydroxylation temperature vs. A1 substitution. unsubstituted. The lower A1 substitution of hematite than that of the associated goethite and, therefore, the line separation, has also been observed in soils (Schulze, 1981 ; Schwertmann & K~impf, 1985; Schwertmann & Taylor, 1984). CONCLUSONS Goethites synthesised from ferrihydrite in 0.3 M KOH at 25~ contain more structural defects than goethites synthesised at 70~ The structural defects give rise to larger unit-cell a and c dimensions, larger OH-stretching band positions, and smaller separations between the OHbending band positions. The goethites synthesised at low A1 additions consist of crystals which contain several coherently diffracting domains, while the goethites synthesised at higher A1 additions consist mainly of crystals which are single domains. The surface areas of the 25 ~ goethites varied as a function of both domain size and physical size. ACKNOWLEDGMENTS The technical assistance of Mrs B. Gallitscher and Mrs U. Maul from the nstitut fiir Bodenkunde, Technische Universit~it Mfinchen, in carrying out the experimental work, is gratefully acknowledged. This is journal article no of the Agricultural Experiment Station, Purdue University. REFERENCES CORNELL R.M., MANN S. ~r SKARNULS A.J. (1983) A high-resolution electron microscopy examination of domain boundaries in crystals of synthetic goethite. J. Chem. Soc., Faraday Trans. 1 79,

10 92 D. G. Schulze and U. Schwertmann FEY M.V. & DXON J.B. (1981) Synthesis and properties of poorly crystalline hydrated aluminous goethites. Clays Clay Miner. 29, GOODMAN B.A. & LEWS D.G. (1981) M6ssbauer spectra of aluminous goethites (~-FeOOH). J. Soil Sci. 32, KLUG H.P. & ALEXANDER L.E. (1974) X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials. Wiley, New York, 966 pp. KOUTLEg-ANOERSON E. (1953) The sulfosalicylic acid method for iron determinations and its use in certain soil analyses. Ann. Roy. Agric. Coll. Sweden 20, LEWS D.G. & SCHWERTMANN U. (1979a) The influence of A1 on iron oxides: Part. Preparation of A goethites in M KOH. Clay Miner. 14, LEWlS D.G. & SCHWEgTMANN U. (1979b) The influence of aluminum on the formation of iron oxides: V. The influence of [All, [OH], and temperature. Clays Clay Miner. 27, SCHULZE D.G. (1984) The influence of aluminum on iron oxides: V. Unit cell dimensions of Al-substituted goethites and estimation of A1 from them. Clays Clay Miner. 32, SCHULZE D.G. (1981) dentification of soil iron oxide minerals by differential X-ray diffraction. Soil Sci. Soc. Am. J. 45, SCHULZE D.G. & SCHWERTMANN U. (1984) The influence of aluminium on iron oxides: X. Properties of A1- substituted goethites. Clay Miner. 19, SCHWERTMANN U. (1964). Differenzierung der Eisenoxide des Bodens durch Extraktion mit Ammoniumoxalat-L6sung. Z. Pflanzenern. Diing. Bodenkunde 105, SCHWERTMANN U., CAMBER P. & MURAD E. (1985) Properties of goethites of varying crystallinity. Clays Clay Miner. 33, SCHWERTMANN U. & K~.MPF N. (1985) Properties of goethite and hematite in kaolinitic soils of southern and central Brazil. Soil Sci. 139, 34~350. SCHWERTMANN U & TAYLOR R.M. (1986) ron oxides: in Minerals in Soil Environments, 2nd ed. (J. B. Dixon & S. B. Weed, editors). Soil Sci. Soc. Am., Madison, Wisconsin (in press).