Journal of Alloys and Compounds 559 (2013) 49–56

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Journal of Alloys and Compounds

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The synthesis and microstructure of goethite particles precipitated in highlyalkaline media

Mira Ristic ⇑, Ivana Opacak, Svetozar MusicRu -der Boškovic Institute, Bijenicka 54, P.O. Box 180, HR-10002 Zagreb, Croatia

a r t i c l e i n f o

Article history:Received 13 December 2012Received in revised form 3 January 2013Accepted 4 January 2013Available online 17 January 2013

Keywords:Iron choline citrateGoethiteHematiteMössbauer spectroscopyElectron microscopy

0925-8388/$ - see front matter � 2013 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jallcom.2013.01.027

⇑ Corresponding author. Tel.: +385 1 4680 107; faxE-mail address: ristic@irb.hr (M. Ristic).

a b s t r a c t

Iron choline citrate was used as the starting chemical in the synthesis of long goethite rods in highly alka-line media. Goethite particles were characterized by 57Fe Mössbauer, XRD, FT-IR, UV/Vis/NIR and FE-SEM.The phase composition and morphology of goethite particles did not change for up to 14 days of heatingtheir suspension at 90 �C. However, upon autoclaving the suspension at 160 �C, the phase transformationgoethite ? hematite occurred through the dissolution/recrystallization mechanism, as confirmed by FE-SEM. Upon heating the goethite particles in air at 300 �C for 2–4 h hematite was formed with a preservedmorphology of the original goethite particles. The hematite particles formed at 800 �C showed a changesin morphology due to the common effect of the solid-state rearrangement involving a loss of residual OHgroups and sintering.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Goethite (a-FeOOH) is a widespread mineral in nature and canbe found in many soils, sediments and rocks. It plays an importantrole in the geochemistry of plant nutrients and can also be used fordecontamination of toxic metals or radioisotopes. This is due toexcellent acid–base surface properties at the goethite/water inter-face, which facilitates the adsorption of many cations and anions.For this reason goethite particles have been used in many academicstudies of the adsorption/desorption phenomena. The colour ofgoethite particles may vary from lemon yellow to dark brownand this property has been utilized in practical application as a pig-ment. Goethite particles can be used as the starting compound inthe production of magnetic materials. Besides magnetite (Fe3O4)and hematite (a-Fe2O3) goethite is the third most important orein raw iron production.

Owing to its superior properties, researchers have been focusedfor many years on the precipitation chemistry of goethite. Forexample, Prélot et al. [1] compared the morphology and surfaceheterogeneities of one commercial goethite sample and one goe-thite sample prepared using the procedure introduced by Cornelland Schwertmann [2]. Schwertmann et al. [3] investigated the syn-thesis of goethite from ferrihydrite in 0.7 M KOH between 4 and90 �C. Kosmulski et al. [4] also precipitated goethite at alkalinepH by adding a concentrated 2.5 M KOH solution to the 0.15 MFe(NO3)3 solution. Krehula et al. [5] synthesized the acicular

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goethite particles of good uniformity at a very high pH startingwith the FeCl3 solution and using TMAH (tetramethylammoniumhydroxide) as the precipitating agent. The same precipitationmethod was used [6] to monitor the microstructural changes dur-ing the autoclaving of the acicular goethite particles between 2 and168 h at 160 �C. The FE SEM inspection of these particles showedchanges in their dimensions in accordance with a-, b- and c-axisdetermination by XRD. Zic et al. [7] investigated the crystallizationof condensed ferrihydrite suspension into goethite or hematite athigh pH. Rod-type goethite or platelet-type a-Fe2O3 particles wereobtained. The phase composition of the precipitates strongly de-pended on the concentration of the FeCl3 and NaOH reactants,the temperature and ageing time of the suspensions.

Goethite particles were also produced by oxidation of Fe(OH)2

suspensions [8,9]. Subbanna and Sudakar [10] prepared aciculargoethite particles (a-FeOOH�xH2O), 0.1 < x < 0.22 by air oxidationof Fe(OH)2 suspension, with Pd2+ or Rh3+ addings, serving to con-trol the morphology of these particles. Krehula and Music [11–15] precipitated acicular goethite particles with TMAH in highlyalkaline media and investigated the influence of Ru, Pd, Rh, Pt orIr on the phase transformation of goethite and the correspondingchanges in the microstructure.

Near spherical aggregates of spindle-shape goethite particleswere produced by a simple oxidation of the 0.1 M FeSO4 solution(suspension) with pure oxygen at 90 �C [16]. Wide and thin goe-thite particles elongated along the crystallographic c-axis wereformed in parallel. The concentration regions for the precipitationof ammonium jarosite, NH4Fe3(OH)6(SO4)2, or superparamagneticgoethite by the forced hydrolysis of NH4Fe(SO4)2 solutions at 90

Table 1Experimental conditions for the preparation of samples.

Sample ICC (g) H2O (ml) 3 M NaOH (ml) T (�C) Time

S1 0.5 45 5 90 24 hS2 1 40 10 90 24 hS3 1 40 10 90 3 dS4 1 40 10 90 5 dS5 1 40 10 90 7 dS6 1 40 10 90 14 dS7 0.8 32 8 160 2 hS8 0.8 32 8 160 6 hS9 0.8 32 8 160 24 hS10 0.8 32 8 160 3 d

Key: h = hour; d = day; ICC = iron choline citrate.

-10 0 10

S6

S4

S3

S2

Cou

nt ra

te /

a.u.

Velocity / mm s-1

S1

Fig. 1. 57Fe Mössbauer spectra of samples S1–S4 and S6, recorded at 20 �C. Thesespectra correspond to goethite as single phase.

Table 2Calculated Mössbauer parameters at 20 �C.

Sample Spectralline

d(mm s�1)

D or Eq(mm s�1)

HMF orhBhfi

C(mm s�1)

Area(%)

S1 M 0.37 �0.26 35.3 0.25 100S2 M 0.37 �0.27 35.6 0.23 100S3 M 0.37 �0.27 35.2 0.23 100S4 M 0.37 �0.26 36.3 0.25 100S5 M 0.37 �0.27 35.6 0.22 100S6 M 0.37 �0.27 35.6 0.23 100

S7 M1 0.37 �0.27 35.9 0.24 95M2 0.37 �0.21 51.2 0.31 5

S8 M1 0.37 �0.26 36.0 0.23 95M2 0.37 �0.20 51.4 0.37 5

S9 M1 0.37 �0.26 36.0 0.22 87M2 0.37 �0.20 51.3 0.29 13

S10 M1 0.36 �0.26 38.0 0.32 27M2 0.37 �0.21 51.4 0.27 73

S5-300-2 M 0.37 �0.22 51.0 0.30 100S5-300-4 M 0.37 �0.21 51.0 0.29 100S5-800-4 M 0.37 �0.21 51.4 0.27 100

Errors: d = ± 0.01 mm s�1, D or Eq = ± 0.01 mm s�1; HMF = ± 0.2 T.d = isomer shift; D or Eq = quadrupole splitting; HMF = hyperfine magnetic field.Isomer shift, d, is given relative to a-Fe at 20 �C.Samples S1–S9 were fitted by using distribution of HMF (hBhfi).Samples S10, S5-300-2, S5-300-4 and S5-800-4 were not fitted by using distributionof HMF.

50 M. Ristic et al. / Journal of Alloys and Compounds 559 (2013) 49–56

or 120 �C were determined [17]. Music et al. [18] also investigatedthe forced hydrolysis of Fe2(SO4)3 solutions at 90 or 120 �C. Theconcentration regions of Fe2(SO4)3 were determined for theprecipitation of hydronium jarosite H3OFe3(OH)6(SO4)2 or super-paramagnetic goethite. The forced hydrolysis of 0.1 M Fe2(SO4)3

solutions at 120 �C yielded H3OFe3(OH)6(SO4)2 and basic sulphateFe4(OH)10SO4.

The synthesis of goethite nanorods was also reported [19–21].Yuwono et al. [22] explained the formation of goethite nanorodsfrom ferrihydrite via self-assembly of nanocrystal intermediates.Yu et al. [23] reported the preparation of uniform goethite nano-tubes, and it was supposed that the reverse micelles generatedby mixing oleic acid, xylene and water played a key role in the for-mation of these nanotubes.

This brief introduction clearly shows how the type of iron salt,the type of alkali, pH, temperature and ageing influences the for-mation of goethite and its microstructure. In continue of our inves-tigations in the precipitation chemistry of iron oxides (group name)we used for the first time iron choline citrate as the starting chem-ical for the precipitation of goethite at a very high pH. The micro-structure of thus obtained goethite particles was characterized.The effect of high temperature goethite to hematite transformationon the microstructure of hematite particles was also examined.

2. Experimental

2.1. Preparation of samples

Iron choline citrate (C33H57Fe2N3O24; Mw = 991.5) supplied by SIGMA was used.Sodium hydroxide granules were supplied by Kemika. Twice-distilled water wasused. After adding a predetermined volume of the 3 M NaOH aqueous solution tothe aqueous iron choline citrate solution in an alkali-resistant plastic bottle the ob-tained suspension was vigorously shaken, then the bottle was closed and left at90 �C for an appropriate length of time. A Teflon-lined, non-stirred pressure vessel(autoclave) manufactured by Paar Instruments (model 4744) was used for autoclav-ing the samples at 160 �C. The autoclaves were heated at 160 �C in a Cole-Parmer DX300 gravity oven (Yamato; temperature uniformity ±1.9 �C at 100 �C or ±3 �C at200 �C). The autoclaving times were corrected for the time needed for the autoclaveto reach the predetermined temperature. Upon expiry of the autoclaving time, theautoclaves were abruptly cooled with cold water. The mother liquor was separatedfrom the precipitate using the ultra-speed centrifuge (Sorvall, model Super T21).The precipitates were subsequently washed with twice-distilled water to remove‘‘neutral’’ electrolyte, then dried. Table 1 shows experimental conditions for thepreparation of samples S1–S10. Furthermore, sample S5 (goethite) was heated inair at 300 �C for 2 h (sample S5-300-2) and 4 h (sample S5-300-4) and at 800 �Cfor 4 h (sample S5-800-4).

2.2. Instrumentation

XRD patterns were recorded at 20 �C with an APD 2000 powder diffractometerdeveloped by ItalStructures (GNR-Analytical Instruments Group, Italy).

57Fe Mössbauer spectra were recorded at 20 �C in the transmission mode. Thestandard configuration of Mössbauer spectrometer was set up using WissEl GmbH(Starnberg, Germany) products. The 57Co in a Rh matrix was used as the Mössbauersource. The velocity scale and the Mössbauer parameters refer to the metallic a-Feabsorber at 20 �C. Deconvolution of Mössbauer spectra was performed using theMossWin program.

-10 0 10

S7

S8

S9

Cou

nt ra

te /

a.u.

S10

Velocity / mm s-1

Fig. 2. 57Fe Mössbauer spectra of samples S7, S8, S9 and S10, recorded at 20 �C. Theinner sextet corresponds to goethite, whereas the outer sextet corresponds tohematite.

20 40 60

170301

330112

061320

002151160

231060

240

221

141211

041131020200

121

140

101

040111

021

130120

110

Inte

nsity

/ a.

u.

S6

S7

2 θ / o (CuKα)

S8

H

H

020

Fig. 3. Characteristic parts of XRD patterns of samples S6, S7 and S8, recorded at20 �C. The arrow H shows the position where the hematite diffraction line (012)should be present.

-10 0 10

Cou

nt ra

te /

a.u.

Velocity / mm s-1

S5

S5-300-4

S5-800-4

Fig. 4. 57Fe Mössbauer spectra of starting sample S5 and S5-300-4 and S5-800-4,recorded at 20 �C. The spectra of S5-300-4 and S5-800-4 correspond to hematite, asa single phase.

20 40 60

301

104 116

024113

006

110

012 214 300018

S5170

330112

018

018 214 300

300214

116

024

024

202

113

113

006

006

110

110

104012

104

012

061320002

160151231

060240

221

141211

041131220

140121200

111

040101021

130

120020

S5-300-2

S5-300-4Inte

nsity

/ a.

u.

2 θ / o (CuKα)

S5-800-4

110

Fig. 5. Characteristic parts of XRD patterns of sample S5 (goethite) and samples S5-300-2, S5-300-4 and S5-800-4 corresponding to hematite.

M. Ristic et al. / Journal of Alloys and Compounds 559 (2013) 49–56 51

FT-IR spectra were recorded at RT with a Perkin–Elmer spectrometer (model2000). Powders were mixed with spectroscopically pure KBr, then pressed into tab-lets using a Carver press.

Optical spectra were recorded at 20 �C with Shimadzu UV/Vis/NIR spectrometer(model UV-3600) using the integrated sphere. Anallytically pure BaSO4 powder waspressed and used to record the background spectrum.

1000 800 600 4001000 800 600 400

895 795

380400

455

500

666795

630

S9

Rel

ativ

e tra

nsm

ittan

ce

666

350

381

381

381

381

458

458

666

795895

895 795

666

637

637

637

544

500

500

500

458

399

399

396

895

895 795

666637

550 465

400

S6

S7

S8

S10

Wave number / cm-1

795

398

388377

377458

500

458

500

398

378

458499 380

458499380

458499666

666

666

665

665

640

640

640

640

643

795

795

795

795

895

895

895

895S5

S4

S3

S2

S1

Fig. 6. Characteristic parts of the FT-IR spectra of samples S1 to S10, recorded at RT. The intense band parallel to the c-axis at 380 cm�1 for sample S1 is shifted to 396 cm�1 forsample S6.

1000 900 800 700 600 500 400 300

440526

648

648

397

397

305

470

470

526440

305

354

305

397

440

481

525590

S5-800-4

S5-300-4

Rel

ativ

e tra

nsm

ittan

ce

Wave number / cm-1

S5-300-2

630

Fig. 7. Characteristic parts of the FT-IR spectra of samples S5-300-2, S5-300-4 andS5-800-4, recorded at RT. These spectra correspond to hematite as a single phase.

200 400 600 800 1000 1200

680

S5-800-4

S5-300-4

S5-300-2

230

215

230

888330

330

335

540

507

507

661

890

890

475

325

Abso

rban

ce /

a.u.

Wave length / nm

S5

380

Fig. 8. Characteristic parts of the UV/Vis/NIR spectra of samples S5, S5-300-2, S5-300-4 and S5-800-4, recorded at 20 �C.

52 M. Ristic et al. / Journal of Alloys and Compounds 559 (2013) 49–56

3. Results and discussion

3.1. 57Fe Mössbauer spectroscopy and XRD

The samples prepared in the present work were analysed byMössbauer spectroscopy. This technique is particularly useful inthe analysis of iron oxyhydroxides and oxides [24]. For example,it can be used in phase analysis, determination of the oxidationstate, particle superparamagnetism and other magnetic properties.Fig. 1 shows the Mössbauer spectra of samples S1–S4 and S6; thecalculated parameters (Table 2) of these spectra can be assignedto goethite. The spectra were fitted by using a distribution ofhyperfine magnetic fields. This broadening of spectral lines canbe assigned to imperfect crystallization of goethite particles. More-over, the relative intensity of the 2nd and 5th spectral lines is high-er than for other lines and deviates from the ratio 3:2:1:1:2:3. Inthe present case this could be assigned to strongly elongated goe-thite particles in one crystallographic direction. The RT Mössbauer

spectrum of goethite may vary from one paramagnetic doublet to awell-shaped sextet. Generally, this effect depends on the particlesize and distribution, crystallinity, defects and the like. Goethiteparticles smaller than �15 to �20 nm show a superparamagnetictype of the Mössbauer spectrum at RT, whereas goethite particles

(a)

(b)

(c)

Fig. 9. FE-SEM images of samples (a) S1, (b) S2 and (c) S6.

M. Ristic et al. / Journal of Alloys and Compounds 559 (2013) 49–56 53

smaller than 8 nm show a superparamagnetic type of theMössbauer spectrum down to 77 K [25].

Fig. 2 shows the Mössbauer spectra of samples S7–S10, whichwere prepared by autoclaving the precipitates at 160 �C from 2 hto 3 days. Upon 2 h of autoclaving the Mössbauer spectrumshowed an additional sextet of very low relative intensity. Thisadditional sextet of very low relative intensity was also presentin the spectrum of goethite precipitate after 6 h of autoclaving.Based on Mössbauer parameters, the additional sextet in thesesamples corresponds to hematite (�5%) (Table 2). With an in-creased time of autoclaving the precipitate for up to 3 days thehematite fraction increased to �73% with the corresponding de-crease of the goethite fraction. The Mössbauer spectrum of sample

S10 showed a decrease in the relative intensity of lines 2 and 5,thus indicating a changed morphology of hematite particles rela-tive to goethite particles. At a highly alkaline pH the goethite-to-hematite phase transformation is solution-mediated and, conse-quently, the original morphology of goethite particles is absentfrom hematite particles (see Section 3.3).

Fig. 3 shows characteristic XRD patterns of samples S6, S7 andS8. These diffractograms can be assigned to the goethite phase,while it is practically difficult to identify�5% of the hematite phasein samples S7 and S8, as found undoubtedly by Mössbauer spec-troscopy. The arrow shows the position where the hematite dif-fraction line (012) should be present. Also, the hematitediffraction line (024) is not visible in XRD patterns of samples S7and S8. This problem was discussed by Music and Popovic [26]within the analysis of the thermal decomposition products of nat-ural goethite.

In the present work the thermal decomposition of goethite(sample S5) to hematite was also monitored. Fig. 4 shows theMössbauer spectra of samples S5, S5-300-4 (4 h at 300 �C) andS5-800-4 (4 h at 800 �C). The Mössbauer spectrum of sample S5corresponds to goethite, whereas the other two correspond tohematite. The relative intensity of the lines 2 and 5 in the S5-300-4 spectrum is increased in relation to the 3:2:1:1:2:3 ratio,whereas this increase for sample S5-800-4 is smaller. In accor-dance with the previous discussion it can be assumed that themorphology of hematite particles in sample S5-300-4 is preservedin relation to that in the starting goethite. Fig. 5 shows character-istic XRD patterns of sample S5 and its thermal decompositionproducts which are in line with Mössbauer measurements.

Balek and Šubrt [27] noticed for precipitated goethite the loss ofadsorbed water at 130 �C and the release of structurally boundwater at 275 �C. At temperatures above 350 �C disappearance ofmicropores and their accumulation in macropores was noticed.The sintering of hematite particles between 720 and 850 �C causedsignificant decrease of the specific surface area. Schwertmann [28]interpreted the double dehydroxylation peak in goethite in termsof the degree of crystallinity. The low crystallinity of goethite leadsto a complete transformation to hematite before the change in unitcell size takes place, and for this reason, the high temperature peakis not visible. As the crystallinity of goethite increases, an increas-ing fraction of goethite may undergo the above-mentioned changein unit cell size, thus producing a high-temperature peak. Rendonet al. [29] heated goethite in air and observed three temperatureregions in which change of porosity occurred. In the region 200–250 �C, micropores �1.4 nm were formed and the phase analysisshowed the mixture of goethite and hematite. Micropores con-verted to closed spherical mesopores at 300–600 �C, whereasabove 600 �C interparticle sintering occurred. The exothermicpeaks slightly above 600 �C in DTA curves for precipitated akagane-ite and goethite were also noticed by Music et al. [30]. In the pres-ent case HMF = 51.0 T was recorded for hematite produced at300 �C, whereas HMF = 51.4 T was recorded for hematite producedat 800 �C. The increase of HMF value can be assigned to better crys-tal ordering in hematite with increase the heating temperature,and this is in accordance with above-mentioned thermal analyses.

3.2. FT-IR and UV/Vis/NIR spectra

Characteristic parts of FT-IR spectra are summarized in Figs. 6and 7. Fig. 6 shows the FT-IR spectra of samples S1–S10. Goethitesamples S1 to S6, prepared from the suspensions heated at 90 �Cfor different ageing times, showed the same type of the FT-IR spec-trum. The intense hydroxyl deformation bands observed at 895and 795 cm�1 can be assigned to the d(OH) and c(OH) vibrations,respectively. The lattice band at �640 cm�1corresponds to a tran-sition moment parallel to the a-axis and did not change the posi-

(a)

(b)

(c)

(d)

Fig. 10. FE-SEM images of samples (a) S7, (b) S8, (c) S9 and (d) S10.

54 M. Ristic et al. / Journal of Alloys and Compounds 559 (2013) 49–56

tion with altered ageing time. However, the intense band parallelto the c-axis at 380 cm�1 for sample S1 is shifted to 396 cm�1 forsample S6. On the basis of the FT-IR spectra of samples S7 and S8it is not possible to determine the presence of the hematite phase,as it was determined by Mössbauer spectroscopy, whereas in thecase of sample S9 indicative can be only the IR band at 544 cm�1.The FT-IR spectrum of sample S10 can be assigned to a hema-tite–goethite mixture. The IR spectra of goethite were discussedin reference literature [6,31,32].

Fig. 7 shows the FT-IR spectra of samples S5-300-2, S5-300-4and S5-800-4, all of which can be assigned to hematite. Infraredspectrum of hematite shows six active vibrations, two A2u (E k C)and four Eu (E \ C). The spectrum of sample S5-300-2 is character-ized with IR bands at 648 cm�1 (A2u), 526 cm�1 (Eu), 440 cm�1 (Eu),397 cm�1 (A2u) and 305 cm�1 (Eu). In the spectrum of sample S5-800-4 the intensity of IR band at 397 cm�1 is significantly in-creased. Also, for the same sample shoulders at 354, 481 and590 cm�1 were well pronounced and they can be related withchanges in the microstructure and much better crystallinity ofsample S-800-4 in relation to hematites produced at 300 �C. Thenature and various factors influencing the IR spectrum of hematitewere discussed in the reference literature [33–39].

Wolska [40] and Wolska and Szajda [41] investigated the phasetransformation of goethite to hematite. Thermal treatment of goe-thite at 180–250 �C yielded a cation-deficient compound Fe2.67(-OH)O2 with hematite crystal structure and hydroxyl group in theanionic sublattice. With further dehydroxylation hydrohematite(Fe1.83(OH)0.5O2.5) and hematite (a-Fe2O3) were obtained. The tem-peratures of all these transformations were related with goethitecrystallinity. The authors monitored the formation of hydrohema-

tite with XRD and IR band in the range 900–1000 cm�1 and shoul-der at 630 cm�1. In the present work in samples S5-300-2 and S5-300-4 we could not detect ‘‘hydrohematite’’ phase on the basis ofFT-IR spectroscopy, only some residual –OH groups can be present.However, with longer standing of hematite samples, due to theatmospheric moisture a possible presence of adsorbed water withCO2 can be detected by FT-IR spectroscopy.

Fig. 8 shows UV/Vis/NIR spectra in the absorbance mode ofsample S5 and its thermal decomposition products at 300 and800 �C. The spectrum of sample S5 is characterized by an intensiveand broad band with two shoulders at 325 and 380 nm and oneadditional shoulder at 475 cm�1. A very broad band of low relativeintensity is centred at 661 cm�1. The low intensity band at 215 nmis due to a charge transfer transition (1T2u ? 2t2q). The peak at325 nm can be assigned to a 6A1 ! 4T1 transition whereas the peakat 380 nm can be assigned to a 6A1 ! 4Eð4DÞ transition. The shoul-der at 475 nm can be assigned to a 2ð6A1Þ ! 2ð4T1Þð4GÞ transition,whereas the broad band centred at 661 nm can be assigned to a6A1 ! 4T2ð4GÞ transition. These assignations correspond to litera-ture data [42–44]. Generally, the optical spectrum of goethite issensitive to particle morphology, size and crystallinity, as well asthe substitution of Fe3+ ions with some other metal ions in thecrystal structure [45]. The thermal decomposition products of goe-thite (sample S5) produced at 300 and 800 �C display general fea-tures known for hematite. The band at �230 nm is due to a chargetransfer, the band at 507–540 nm can be assigned to a2ð6A1Þ ! 2ð4T1Þ transition and the band at �900 nm can be as-signed to a 6A1 ! 4T1 transition. A very weak band at 680 nm is no-ticed for sample S5-300-4. The electronic transitions in a hematite-type crystal structure were discussed in detail by Music et al. [46].

M. Ristic et al. / Journal of Alloys and Compounds 559 (2013) 49–56 55

3.3. FE-SEM

Fig. 9 shows FE-SEM images of goethite samples S1, S2 and S6.Sample S1 shows very elongated rods with most particles rangingfrom 1.3 to 1.8 lm in length and 30–50 nm in width. The size ofthese particles did not change significantly up to the point of heat-ing the suspension for 14 days at 90 �C. Fig. 10 shows FE-SEMimages of the particles produced by autoclaving the goethite sus-pension at 160 �C from 2 h to 3 days. Besides the rod-like goethiteparticles the formation of a small quantity of hematite particlescan be seen (Fig. 10a–c). After 3 days of autoclaving, large

(a)

(b)

(c)

Fig. 11. FE-SEM images of samples (a) S5, (b) S5-300-4 and (c) S5-800-4. Hematiterods (b) preserved the morphology and size of goethite precursor (a).

micron-size hematite particles are formed (Fig. 10d). In the sameFE-SEM image (Fig. 10d), not completely dissolved goethite parti-cles of widely varying dimensions are well discernible. The disso-lution of these goethite particles evolved in three dimensions. Adifferent result is obtained by heating sample S5 (Fig. 11a) at300 �C for 4 h (Fig. 11b). The rod-like morphology of goethite par-ticles is preserved during their transformation to hematite parti-cles with virtually no change in size. Heating of rod-like goethiteparticles at 800 �C (Fig. 11c) yielded rod-like hematite particles lesselongated and with increased width (�100–�150 nm). The sinter-ing effect is noticeable at the ends of these particles. In the solidstate transformation a structural rearrangement of goethite takesplace with the loss of hydroxyl groups. Some laterally arrangedrod-like goethite particles were sintered at 800 �C thus producinghematite rods with increased width.

4. Conclusion

Goethite particles, precipitated in highly alkaline media by add-ing the NaOH solution to the iron choline citrate solution, were inthe form of long rods of micron size in length and nanosize inwidth. The microstructure and phase composition of these parti-cles did not change with ageing of their suspension for up to14 days at 90 �C. It can be argued that iron choline citrate had arole to play in forming the properties of goethite particles. The spe-cific properties influenced the characteristics of the correspondingMössbauer spectra. Autoclaving at 160 �C showed a goethite-to-hematite phase transformation where the yield was increasingwith a prolonged ageing time of the goethite suspension. This pro-cess was facilitated by the dissolution/recrystallization mechanismas visible with electron microscopy. The heating of goethite parti-cles in air for up to 4 h at 300 �C yielded hematite particles, where-by the original morphology of goethite particles was preserved.The changes in the morphology of hematite particles produced at800 �C were assigned to the common effect of the loss of residualOH groups during the solid state structural rearrangement andsintering.

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