Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 201–205

Dependence of the mechanism of phase transformation of Fe(III)hydroxide on pH

Hui Liua,b, Yu Weib,∗, Yuhan Suna, Wei Weia

a Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Chinab College of Chemistry, Hebei Normal University, Shijiazhuang 050016, China

Received 25 March 2004; accepted 21 October 2004Available online 28 November 2004

Abstract

The phase transformation from Fe(OH)3 gel to�-Fe2O3 particles at different initial pHs at about 100◦C was studied. The time necessary forcompleting the above process was determined. The results showed that the time of phase transformation from Fe(OH)3 gel to�-Fe2O3 particlesshortened with the increase of initial pH at pH < 4.5. In this pH range,�-FeOOH, as an intermediate product, was obtained and hematitew ged withi ays.O the point ofz©

K

1

ipbnsmtttctith

e-o inech-hipical

or-

lyfrom

plet-

1 dayzkoost

e de-sory

0d

as formed by dissolution/reprecipitation mechanism. However, in the pH range from 4.5 to 9.0, the transformation time prolonncreasing pH. In this pH range, no intermediate product was found. From Fe(OH)3 gel to hematite, there are two transformation pathw

ne is the dissolution/reprecipitation mechanism and the other is the solid state transformation mechanism. With the pH close toero charge (pzc) of Fe(OH)3 gel, the later mechanism gradually predominated.2004 Elsevier B.V. All rights reserved.

eywords: Fe(OH)3 gel; Phase transformation; Mechanism; Hematite;�-FeOOH

. Introduction

Iron oxides are common compounds that are widespreadn nature. The studies on the preparation and the formationrocess of iron oxides have been an area of active researchecause of two reasons. The first reason is of an academicature, because iron oxides can be used as model systems intudying the fundamental colloid and surface properties ofetal oxides. The second reason for these researches is that

hese compounds are important chemical materials in indus-ry that can be applied extensively in many fields. The phaseransformation process of Fe(III) in solutions is very compli-ated and is influenced by many factors such as the concen-ration of iron salt, the type of anion, the presence of foreignons and molecules, pH, temperature and time of crystalliza-ion. According to the available literature data, great attentionas devoted till now to the preparation and characterization

∗ Corresponding author. Tel.: +86 311 6268342; fax: +86 311 5893425.E-mail address:weiyu@mail.hebtu.edu.cn (Y. Wei).

of iron oxides[1–6] while very little attention has been dvoted to the explanation of their formation mechanism. Sthis field there are still many uncertainties about the manism of the formation of iron oxides and the relationsbetween the method of their preparation and their chemand physical properties.

Sugimoto et al.[7] studied in detail the phase transfmation from ferric hydroxide gel into pseudocubic�-Fe2O3particles. They found that the�-Fe2O3 particles were actualformed through a distinct two-step phase transformationFe(OH)3 to�-FeOOH and from�-FeOOH to�-Fe2O3. Theyalso found that Fe(OH)3 gel was totally transformed into�-FeOOH in a few hours while the times necessary for coming transformation of�-FeOOH into�-Fe2O3 at 100◦C wereabout 7 days at pH 1.9, about 2 days at pH 3.9 and aboutat pH 7.7, respectively. According to the report of Sapieset al. [8], Sugimoto and co-workers thought that the mprobable precursor responsible for the formation of�-Fe2O3is Fe(OH)2+ and each unit process proceeds through thposition of the solute with the dissolution of each precur

927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2004.10.105

202 H. Liu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 201–205

solid, that is, through dissolution/reprecipitation mechanism.In our preliminary experiment conducted under the conditionof boiling and refluxing in the open system, we found someresults differing from Sugimoto’s.

2. Experimental

2.1. Materials

Ferric chloride hexahydrate (FeCl3·6H2O), sodium hy-droxide (NaOH) of analytical purity and distilled water wereused. All the chemicals are supplied by Kaitong ChemicalCom. (Tianjin, China). The ferric salt solutions were filteredthrough a 0.22�m Millipore filter to remove any particulatecontaminants before use.

2.2. Preparation

The standard condition of the procedure is as follows. Anappropriate amount of NaOH solution (7.5 M) was slowlyadded to 100 ml of well-stirred 1.0 M FeCl3 solution to bringthe pH to a desired value. Subsequently, the agitation wascontinued for an additional 10 min. The total volume ofeach system was adjusted to 200 ml and the concentrationo ctedi ed toa vig-o xingf oh akeno o de-t )h ifugeda ed.

2

ublyd p ofs were

Fig. 1. XRD patterns of the phase transformation products of Fe(OH)3 gel.

obtained with a Hitachi H-600 transmission electron micro-scope.

2.4. Infrared spectrophotometry

The sample powder (4 mg) was uniformly mixed with160 mg of ground KBr powder in an agate mortar with apestle. The mixture was pressed to produce a disk. IR spectrawas conducted over the range 400–3500 cm−1 with a FTIR-8900 Fourier transform infrared spectroscopy to monitor thechange in composition of the solid phase.

2.5. X-ray diffractometry

X-ray diffraction (XRD) pattern was obtained with aBruker diffractometer D8 ADVANCE using a Cu K� radia-tion.

3. Results and discussion

3.1. Effect of the initial pH of Fe(III) hydroxide on thetime of the phase transformation

The times necessary for completing the phase transfor-m ti d atd M.T

oducts

f Fe(III) was 0.5 mol/l. The above process was condun a water bath about 25◦C .In this mixed system, gel-likeposit (Fe(OH)3) formed. Then this slurry was poured innother reaction vessel with a reflux condenser. Underrous stirring the slurry was heated to boil and kept reflu

or a certain time until the Fe(OH)3 gel transformed intematite completely. The samples of about 10 ml were tut of the suspension at different reaction times so as t

ermine whether the phase transformation from Fe(OH3 toematite has been completed. The samples were centrnd washed thoroughly with distilled water and then dri

.3. Transmission electron microscopy

For TEM exam the samples were dispersed in doistilled water using an ultrasonic treatment and a drouspension was dried on a bronze grid. TEM images

Fig. 2. TEM images the phase transformation pr

ation from Fe(OH)3 gel to �-Fe2O3 particles at differennitial pHs were determined. The final products obtaineifferent initial pHs were characterized by XRD and TEhe results are shown inFigs. 1–3respectively.

of Fe(OH)3 gel (A, pH = 2.34; B, pH = 4.5; C, pH = 9.0).

H. Liu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 201–205 203

Fig. 3. The time for completing the phase transformation from Fe(OH)3 tohematite changes with the initial pH of the system.

The results both inFigs. 1 and 2indicate that the final prod-ucts are�-Fe2O3 particles. FromFig. 3 it can be seen thatthe reaction rate increases with increase of pH at pH < 4.5.However, in pH range from 4.5 to 9.0, the changing trendof the reaction rate with pH was in contrast to the result atpH < 4.5. Sugimoto and co-workers thought that the mostprobable precursor responsible for the formation of�-Fe2O3is Fe(OH)2+ because Fe(OH)2

+ ions, which are one species offerric hydroxo complexes, predominated in their reaction sys-tem. Therefore, in order to explain our experimental results,it is necessary to make sure the species distribution for ferrichydroxo complexes in the current system. It is well knownthat there are such species as Fe3+, Fe(OH)2+, Fe(OH)2+ andFe2(OH)24+ in Fe(III) aqueous solution. Among these speciesof ferric hydroxo complexes, there are the following equa-tions:

Fe3+ + H2O = Fe(OH)2+ + H+ K1 (1)

Fe3+ + 2H2O = Fe(OH)2+ + 2H+ K2 (2)

2Fe3+ + 2H2O = Fe2(OH)24+ + 2H+ K22 (3)

Sapieszko et al.[8] determined the equilibrium constants forthe above equations at 25, 55 and 80◦C. They also gave thespecies distribution for ferric hydroxo complexes under theabove conditions within pH range from 0.7 to 2.2. The tem-p -d ich u-l sen-s antsgec en-t tem-p

K2 = 4.52× 10−3 andK22 = 5.87× 10−3, respectively. FromEqs.(1)–(3)the concentrations of species for ferric hydroxocomplexes can be written as

[Fe(OH)2+] = K1[Fe3+]/[H+],

[Fe(OH)2+] = K2[Fe3+]/[H+]

2,

[Fe2(OH)24+] = K22[Fe3+]

2/[H+]

2

It means that the concentrations of all species can becalculated if the total concentration of all species of Fe(III)ions ([Fe(III)]total = [Fe3+] + [Fe(OH)2+] + [Fe(OH)2+] +[Fe2(OH)24+]) and pH of the system are known. Practically,it is unnecessary to calculate the absolute value of concen-tration of species. It is enough to give a relative value toshow the species distribution for ferric hydroxo complexesin the current system. The relative concentration values ofthe Fe(III) species are listed inTable 1.

Practically, when pH > 4.0 the concentrations of thespecie Fe2(OH)24+ are so small that they are ignor-able. For example, when pH = 4.0, [H+] = 1× 10−4 mol/l.[Fe3+]pH 4 = Ksp/[OH−]3 = 1.1× 10−38/[OH−]3 ≈ 1.1× 10−8

mol/l becauseKsp for Fe(OH)3 is 1.1× 10−38. Therefore,[Fe2(OH)24+] = 5.87× 105[Fe3+]2pH4 = 7.10× 10−11 mol/l.From Table 1 it can be seen that the concentration ofF + + seo rr thed ultw ofF ught lls lutevi thatF test ayt n ofn ssaryt

aysf ei ri ismn housh tot atiteb ffect

TT us solu

p

4567

erature of the current system is about 101◦C. In order to unerstand the results inFig. 3, the species distribution for ferrydroxo complexes at 101◦C must be determined or calc

ated because the species distribution was exceedinglyitive to temperature and pH. With the equilibrium constiven by Sapieszko et al. at 25, 55 and 80◦C and Van’t Hoffquation, the equilibrium constants at 101◦C for Eqs.(1)–(3)an be approximately calculated if the thermodynamichalpy changes are thought as constants within the aboveerature range. The calculated results areK1 = 1.66× 10−2,

able 1he species distribution for ferric hydroxo complexes in Fe(III) aqueo

H [Fe3+] [Fe(OH)2+]

.0 [Fe3+]pH 4 1.66× 102[Fe3+]pH 4

.0 [Fe3+]pH 5 1.66× 103[Fe3+]pH 5

.0 [Fe3+]pH 6 1.66× 104[Fe3+]pH 6

.0 [Fe3+]pH 7 1.66× 105[Fe3+]pH 7

e(OH)2 ([Fe(OH)2 ]) also predominate with increaf pH, which means that Fe(OH)2

+ is also the precursoesponsible for the formation of hematite. However,issolution of Fe(OH)3 gel becomes more and more difficith increase of pH, which makes the concentratione(III) in solution becomes more and more small. Altho

he concentration of Fe(OH)2+ still predominates in a

pecies of Fe(III) in pH range from 4.5 to 9.0, the absoalue of the concentration of Fe(OH)2

+ is very small. Thisndicates that the formation of hematite through the pathe(OH)3 gel dissolves into the solution and reprecipita

o form hematite becomes more and more difficult. It make a longer time to attain the saturation concentratioucleation of the hematite particles. So the time nece

o form �-Fe2O3 particles is prolonged greatly.According to the literature data, there are two pathw

or the phase transformation from Fe(OH)3 to hematite. Ons the dissolution/reprecipitation mechanism[7] and the othes ‘solid state transformation mechanism’ or the mechanamed as ‘internal dehydration of the aggregated amorpydroxide’ [9–11]. Increasing temperature or a pH closehe point of zero charge (pzc) favors the formation of hemy the latter mechanism. The strong hematite favoring e

tion at different pHs at 101◦C

[Fe(OH)2+] [Fe2(OH)24+]

4.52× 105[Fe3+]pH 4 5.87× 105[Fe3+]2pH 4

4.52× 107[Fe3+]pH 5 Ignorable4.52× 109[Fe3+]pH 6 Ignorable4.52× 1011[Fe3+]pH 7 Ignorable

204 H. Liu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 201–205

at pH close to the pzc of Fe(OH)3 is most likely associ-ated with its solubility minimum in this pH range. So basedon the literature data and our experimental results, the twomechanisms should coexist in the pH range from 4.5 to 9.0.With increasing pH the formation of hematite by dissolu-tion/reprecipitation mechanism becomes more and more dif-ficult while that by the solid state transformation graduallypredominates. The pzc of Fe(OH)3 in the current system wasdetermined and the result was 9.2, which further supportedthe above viewpoint. In addition, the preliminary tests indi-cated that the formation condition of Fe(OH)3 gel had a greateffect on its transformation. It is probably because the mi-crostructure of Fe(OH)3 gel formed at different conditions isdifferent. The further studies are under way.

3.2. The determination of intermediate products

In order to further understand the relationship between thephase transformation mechanism of Fe(OH)3 gel and initialpHs, the samples of about 10 ml were taken out of the sus-pension at different times as the transformation reaction wasproceeding. The samples were characterized by Fourier trans-form infrared spectroscopy (FTIR) and XRD. The results areshown inFigs. 4 and 5.

In Fig. 4 the band at 845 cm−1 can be ascribed to�-F[ r-m .5,w RDp edi-a waspw nismo rentf the� ans-

F (A,p

Fig. 5. XRD pattern of the product obtained at pH 2.34 as the reaction timeis 17 h (A for akaganeite and H for hematite).

formation from Fe(OH)3 to �-FeOOH and from�-FeOOHto �-Fe2O3. Because of the difference in structure betweenFe(OH)3 and�-FeOOH as well as between�-FeOOH and�-Fe2O3, the conversion must take place via solution[13].Therefore, the two-step phase transformation was completedby dissolution/reprecipitation mechanism. However, in pHrange from 4.5 to 9.0, the mechanism of phase transforma-tion from Fe(OH)3 to hematite has changed (see the commentonFig. 3).

The reason why�-FeOOH forms at pH < 4.5 can be ex-plained by the affinity between Fe(III) ions and Cl− ions. Itis well known that Fe(III) ions have strong affinity with suchions as OH−, PO4

3−, SO42−, F−, etc. The affinity between

Fe(III) ions and anions decreased successively in followingorder:

OH− > PO43−

(8.95)> F−

(5.16)> SO4

2−(2.94)

> Cl−(0.5)

> NO3−

(−0.23)> ClO4

−

The values in parentheses were the stability constants ofiron complex compounds[14]. Schwertmann’s[12] studiesshowed that Fe(III)oxyhydroxysalts formed in the systemcontaining Fe(NO3)3 or Fe2(SO4)3 when OH/Fe = 2.7–2.8.For the system containing FeCl3 Fe(III)oxyhydroxysalts cancertainly form when OH/Fe = 2.7–2.8 because the affinity be-tween Fe(III) ions and Cl− ions was weaker than that ofF 2− ndNi tiont sed.T rrents about4 y-m tiona thei neiter aneites[ 4.8,h -a t iscs he

eOOH and the bands at 578.6 and 474.5 cm−1 to �-Fe2O37,12], respectively. The results inFig. 4showed that the inteediate product�-FeOOH was found at the initial pH < 4hich is consistent with the results of Sugimoto et al. Xattern inFig. 5also revealed the presence of the intermte product�-FeOOH. When the phase transformationroceeding in pH range from 4.5 to 9.0, however,�-FeOOHas not found. These results also indicate that the mechaf phase transformations in pH range of pH < 4.5 is diffe

rom that in pH range from 4.5 to 9.0. That is, at pH < 4.5-Fe2O3 particles are formed through a two-step phase tr

ig. 4. IR spectra of intermediate products at different initial pHsH = 2.34; B, pH= 4.02; C, pH = 5.58; D, pH = 7.0; E, pH = 8.03).

e(III) and SO4 ions but stronger than that of Fe(III) aO3

− ions. Quirk and co-workers[15] suggested that Cl−ons be incorporated in the early stages of polymer formahough some Cl− ions were probably subsequently releahe experimental result showed that the pH of the cuystem at OH/Fe = 2.7–2.8 was between about 1.9 and.8. It implies that Cl− ions may incorporate in the poler of Fe(III) at pH < 4.8 so that influence both nucleand growth of crystals thus resulting in the formation of

ntermediate product akaganeite. The studies on akagaeveals that there are three-dimensional tunnels in akagtructure which are considered to be stabilized by Cl− ions16], which further supports the above concept. At pH >owever,�-FeOOH cannot form because Cl− has been probbly released from the polymer of Fe(III). Therefore, ilosely related to both the presence of Cl− and pH of theystem whether�-FeOOH is a intermediate product in t

H. Liu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 201–205 205

process of phase transformation from Fe(OH)3 to �-Fe2O3particles.

4. Conclusions

(1) The times necessary for completing the phase transfor-mation from Fe(OH)3 to �-Fe2O3 particles at differentinitial pHs are different. At pH < 4.5, the times becomeshorter with the increase of pH. At 4.5 < pH < 9.0, thetimes become longer with the increase of pH.

(2) At pH < 4.5,�-FeOOH was the intermediate product andat 4.5 < pH < 9.0,�-FeOOH was not be found. The resultsshow that it is closely related to both the presence of Cl−and pH of the system whether�-FeOOH is a intermedi-ate product in the process of phase transformation fromFe(III) hydroxide into�-Fe2O3 particles.

(3) At pH < 4.5, hematite was formed by the dissolu-tion/reprecipitation mechanism. In the pH range from 4.5to 9.0, there are two formation mechanisms for hematite.They are the dissolution/reprecipitation mechanism andthe solid state transformation mechanism, respectively.

Acknowledgment

inceF

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[ rk,

This work was supported by the Chinese Hebei Provoundation of Natural Science Research.

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