Indian Journal of Chemical Technology
Vol.18, March 2011, pp. 107-112
Facile precipitation and phase formation of iron oxide with in situ Fe(II) in
Fe2(SO4)3-NaOH-(N2H4)2H2SO4-H2O medium
Rasmita Barik & Mamata Mohapatra*
Hydrometallurgy Department, Institute of Minerals and Materials Technology, CSIR, Bhubaneswar 751 013, India
Received 9 March 2010; accepted 10 January 2011
Present study deals with the precipitation of iron oxides in NaOH-H2O and NaOH-H2O-NH2NH2H2SO4 system. The
precipitates at different pH are collected. Total iron and ferrous analysis of both the filtrate and residue are reported.
Sulphate content of the samples are also included. All the precipitates at different pH are characterized by XRD for phase
identifications. The precipitation of ferric sulphate in absence and in presence of hydrazinium sulphate (reducing agent) is
investigated. The results indicate that the reducing agent has an influence not only on the species, the amount as well as the
type of products but also on the transformation rate of ferrihydrite. XRD patterns consistent with a mixed ferrihydrite and
schwertmannite when the precipitation of ferric sulphate is carried out in absence of hydrazinium sulphate. However by
adding only 0.05 M of hydrazinium sulphate, the natures as well as the composition of precipitates change. At a pH of 3 6line ferrihydrite is obtained. The precipitate at a pH of 7 contain only goethite phase where as at a pH of 10 both goethite as
well and magnetite phases are observed.
Keywords: Precipitation, phase identifications, Iron oxides, Ferric sulphate, Hydrazinium sulphate, Ferrihydrite, XRD
The precipitation chemistry of iron hydroxides,
oxyhydroxides and oxides has been the subject of
various investigations. Iron oxides were often used as
model systems in general studies of colloid and
surface properties of metal oxides because they can
be prepared as very stable sols and owing to
their desirable acid/base behaviour at the metal
oxide/H2O interface. The kinetics and mechanisms of
crystallization from iron (III)-hydroxide, as well as
the phase transformations of iron (III)-oxyhydroxides
in aqueous suspensions were also investigated from
various standpoints. The physico-chemical conditions
and kinetics of iron oxide precipitation may
significantly modify the properties of the obtained
precipitate, and in some cases the phase composition
of the precipitate may be completely changed. For
this reason, researchers and engineers focused their
attention on the relations between physico-chemical
conditions of the iron oxide precipitation on the one
hand and their surface and bulk properties on the
other. The precipitation of iron is extremely important
in extractive metallurgy, as the dissolved iron has
to be discarded prior to treatment of leach solutions
for desired metal recovery. Due to this reason a
considerable amount of R and D effort has been put in
investigating the precipitation behaviour of iron
——————
*Corresponding author (E-mail: [email protected])
oxides/oxy-hydroxides from synthetic solutions
during the last several decades. A variety of processes
known for iron removal are (a) hematite process,
(b) goethite process and (c) jarosite process1-5.
The final form of iron precipitate is dependent on
conditions like pH, temperature, iron concentration
and its oxidation state in solution, nature of anion,
cations and ageing time. It has been observed that pH
and temperature have more influence on the formation
of ferrihydrite and Fe2O3 whereas anion influences
the formation of different isomers of FeOOH6. The
presence of sulphate in solution helps in faster ageing
of the precipitate7-8. The formation of dimeric species
(FeOH)24- occurs more rapidly in the presence of
sulphate6-9. Lepentsiotis10 observed that a high excess
of sulphate (103-104) suppresses the precipitation
in Fe (III) solutions at a pH of < 4.5. The different
iron phases that could be produced in a sulphate
containing environment by varying the pH and
temperature was suggested by Babcan11.
Besides that in some environments, it was
shown that phases such as ferrihydrite12,13 which
are metastable towards goethite are produced.
Furthermore, the morphologies of these metastable
phases differ considerably from each other14 and from
that of goethite. Depending on the conditions, various
iron oxides are interconverted from one form to
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INDIAN J. CHEM. TECHNOL., MARCH 2011
another in aqueous environment. The transformation
is affected by such factors as temperature,
concentration, pH value and anionic type as well as
all kinds of additives. Generally, the conversion of
iron oxide is slow in the absence of catalysts. Due to
strong reducing capacity, Fe(II) is used for induction
of the dissolution and the subsequent conversion of
ferrihydrite15,16. Jang et al.15 investigated the effects
of Zn(II), Fe(II), NO3 - or SO42- on the transformations
of hydrous ferric oxide. Lie group had explored the
transformation of ferrihydrite in the presence of
trace Fe(II)16-19. It was found that Fe(II) can catalyze
the transformation of ferrihydrite even at a low
temperature (e.g. room temperature (RT)). The
transformation time as well as the species and the
amount of products have a close relationship with
the initial pH, the temperature as well as anionic
media. More interestingly, they indicated that a
minor change in the preparing condition of
ferrihydrite can cause great changes in the
transformation product of ferrihydrite. However,
the effect of in situ generation of Fe(II) on the
phase formation and precipitation of iron in sulphate
media is not reported. The aim of present paper
is to investigate the precipitation and transformation
of iron oxides in the presence of Fe(II) generated in
situ by adding hydrazine sulphate as reducing agent.
Materials and Methods
Materials
The chemicals used in the synthesis were
Fe2(SO4)3.6H2O (Merck), NaOH and N2H6SO4
(Merck) (Name as HS). All other chemicals were of
AnLar grade. Standard solutions of metal salts were
prepared and these solutions were used in different
stoichiometric amounts for precipitation work.
Instrumental techniques
Atomic absorption spectrophotometer (Perkin
Elmer Model 3100) was used to analyze Fe in filtrate.
The X-ray diffractograms (XRD) of the powder
samples were taken using a Philips PW1730 or a
Philips PW 1830 X-ray diffractometer. The diffracted
X-ray intensities were recorded as a function of 2θ by
using Fe filtered Co targets (Cu Kα radiation with
λ=1.5404Å and Co Kα radiation with λ=1.7494Å) in
a range of 10 to 70° (2θ) at a scanning rate of 2o/min.
For pH measurements, an ELICO LC II digital pH
meter equipped with a single composite electrode
was used. Standard buffer tablets of pH 4.0, 7.0 and
9.2 (BDH India Ltd.) were used to calibrate the
instrument. The pH of solution was measured at least
twice and the average value was recorded.
Total iron, ferrous and ferric was estimated
using BDAS indicator20. Sulphate content in solid
was estimated by EDTA titrative method using
Eriochrome Black-T indicator20.
Result and Discussion
Effect of pH on iron precipitation and iron and sulphate
content of synthesized samples
Figure 1 shows the %Fe precipitated in presence
or absence of hydrazinium sulphate at different pH
values. In presence of hydrazine sulphate the
percentage of precipitation of Fe increases gradually
where as in the absence of hydrazine sulphate
precipitation is completed at a pH of 3. A distinct
difference in the colour profile of precipitated iron
was observed. The colour of the precipitated
compound changed from yellow to dark grey and
black in presence of hydrazine sulphate where as in
other case the colour was brown up to a pH of 7 then
it changed to yellowish red. Iron analysis of the
In-situ generation of Fe(II) and precipitation procedures
To carry out the present study, a series of samples
were prepared using 0.1M Fe2(SO4)3.6H2O solution,
1M NaOH solution in the presence or absence of
0.05M NH2NH2H2SO4. For first series of samples
2 L of 0.1M ferric sulphate solution was taken and
the pH was adjusted to desired values by drop wise
addition of 1M sodium hydroxide solution. The pH
of the suspension was monitored with a pH meter,
and 50 mL each of precipitated iron(III)
hydroxide/oxide suspensions were collected at
different pH. The precipitates were immediately
filtered and dried at 333 K.
Fig. 1—%Fe precipitated from the solution with and without the
addition of 0.05 M HS.
BARIK & MOHAPATRA: FACILE PRECIPITATION & PHASE FORMATION OF IRON OXIDE
precipitated compound at each pH was estimated and
is given in Fig. 2. It is observed that in the presence
of hydrazine sulphate the % Fe (total as well as
Fe(II)) in the solid increased as the pH increased,
where as in case of the solids prepared at various
pH without hydrazine sulphate, the variation of
% Fe was in the range of 37.41-24.57% only. From
this data it is inferred that presence of hydrazine
sulphate provides a reductive environment. %
Sulphate analysis in the solid samples prepared both
in absence and presence of hydrazine sulphate was
reported in Fig. 3. It is observed that % sulphate in
precipitate obtained in absence of HS were increased
in the solid as pH of the solution increases where as
in presence of HS it cross through a minima at pH 7.
109
amorphous solids over poorly crystalline two-line
ferrihydrite to a more ordered six-line ferrihydrite.
X-ray diffraction (XRD) patterns of two-line
ferrihydrite are characterized by two broad peaks,
while for six-line ferrihydrite six peaks are
visible. This variety of crystallinities is associated
with a poorly defined stoichiometry and a not well
established atomic structure. The generally accepted
XRD studies for the samples prepared at various pH with and
without addition of hydrazine sulphate
The XRD patterns of the samples prepared at
various pH in the absence of hydrazine sulphate are
shown in Fig. 4. All the samples prepared at pH
values varying in the range of 3 to 10 show broad
peaks situated at d-spacing of 3.04 2.81, 2.54, 2.12 Å
corresponding to more or less (002), (003), (110)
and (112) plane of ferrihydrite JCPDS Card No
(46-1315) (JCPDS 2002)21. However, the intensity
of (003) plane is 100% rather than (005) plane which
has the maximum relative intensity (RI) in the
referred spectra. Besides that one peak at 3.34 (100)
and 2.34 Å was found to be present which correspond
to Schwertmannite (according to PDF 47-1775)
(ICDD, 1997)22 up to pH 6 then these peaks are
vanished. The ferrihydrite phase has very broad
peak at all the pH. Ferrihydrite is an interesting
iron oxide phase, which is not even easily accessible
as a bulk material. The degree of crystallinity
of ferrihydrite is variable and ranges from quasi-
Fig. 2—%Fe as total as well as Fe(II) present in the solid obtained
at different pH (with and without the addition of 0.05 M HS).
Fig. 3—%sulphate present in the solid obtained at different pH
(with and without the addition of 0.05 M HS).
Fig. 4—XRD patterns of solid samples obtained at various
pH values in absence of hydrazine sulphate (HS) S:
Schwertmannite, f: 2 line-ferrihydrite
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INDIAN J. CHEM. TECHNOL., MARCH 2011
chemical formulas for ferrihydrite are Fe5HO8. 4H2O
and 5Fe2O3.9H2O23,24. Regarding the crystal structure,
it is believed that it corresponds to that of hematite12.
It was proposed that the Fe3+ ions are octahedrally
coordinated within a hexagonal unit cell; whereas
the Fe3+ ions located on the surface (about 30%)
have a tetrahedral coordination25. Schwertmannite
(Fe16O16 (OH)12(SO4)2) is a nanoscale and poorly
crystalline iron oxyhydroxy sulphate mineral
found in acidic water systems, acid mine drainage
and mineral processing wastes. Schwertmann et al.26
demonstrated the precipitation of ferrihydrite and
its more crystalline analogues to be governed
by crystallization kinetics, rather than any
other factor (i.e. thermodynamics). Although the
ferrihydrite precipitation mechanism is dominated
by fast nucleation, control of the rate permits the
formation of ferrihydrite with a range of structural
ordering and sizes. The ferrihydrite nucleation rate
is created by very high super saturations, due to the
very low equilibrium solubility of iron and hence
ferrihydrite, which at a fixed temperature is dictated
by pH. XRD patterns of Figs 3 and 4 demonstrate
that ferrihydrite was not discretely formed. It appears
that intermediate structures between 2- and 6-line
ferrihydrite were precipitated. These XRD patterns
also indicate these reflections become more
representative of schwertmannite admixed with 2-line
ferrihydrite up to a pH of 6. This is exhibited in the
alternate symmetry and relative intensity of the major
ferrihydrite reflection (41° 2θ) appearing to split
into two separate reflections (alternate intensity ratio
to 6-line ferrihydrite); and, the weak reflections
(45° 2θ) being more similar in intensity and position
to schwertmannite than 6-line ferrihydrite.
The XRD patterns (Fig. 5) of the precipitates
obtained from solutions containing hydrazine
sulphate at a pH of 3 showed broad peaks situated at
d-spacing of 2.55, 2.21, 2.03, 1.92 Å corresponding to
the (110), (200), (113) and (114) planes of the 6-line
ferrihydrite JCPDS Card No (29-0712). However,
the peaks are very broad and sample is amorphous in
nature. Similarly at a pH of 4 and 5 only broad peaks
for ferrihydrite and 4.20, 2.46, 2.25 and 1.71 Å peaks
corresponding to major (110), (111), (112) and (122)
goethite were identified. At a pH of 7 the peaks for
goethite became sharper and more peaks of same
phase were observed. At a pH of 7, all the peaks
matched with those of goethite and no other phase
was present. Further when the pH was increased to
9, some new, sharp peaks appeared at 2.97 and 2.53 Å
corresponding to (220) and (311) major planes of
magnetite. It can be seen that the intensities of
the above mentioned characteristic peaks increase
gradually when the pH was raised up to 10. This is
indicative of increase in magnetite content with
increasing pH. From the chemical analysis it was
observed that, at pH of 7 the sulphate content was
lowest implying that the adsorbed sulphate is going
back to solution form and again in increasing the pH
it adsorb on the surface as ferrous hydroxyl sulphate
form and assist the formation of magnetite.
The XRD data demonstrate conclusively that
hydrazinium sulphate catalyze the formation of six
line ferrihydrite in sulphate media. In-situ release of
Fe(II) (aq) can be attributed to the transformation of
ferrihydrite to goethite (in addition to formation of
some minor Magnetite) (Fig. 5). This data also
confirm absence of schwertmannite which is formed
in the absence of hydrazinium sulphate (Fig. 4), and
at pH < 6.0. Again earlier researcher reported Fe (II)
can catalyze the transformation of ferrihydrite even
at a low temperature (e.g. room temperature)15-18. The
transformation time as well as the species and the
Fig. 5—XRD patterns of solid samples obtained at various
pH values in presence of hydrazine sulphate (HS). F: 6-line
ferrihydrite, G: Goethite (∝-FeOOH), M: Magnetite (Fe3O4).
BARIK & MOHAPATRA: FACILE PRECIPITATION & PHASE FORMATION OF IRON OXIDE
amount of products have a close relationship with
the initial pH, the temperature as well as anionic
media. Jang et al.27 reported lepidocrocite and
goethite were formed in Cl- medium and the
formation of lepidocrocite was restrained in SO42medium. The transformation from ferrihydrite to
various iron oxides and/or iron oxyhydroxides in
sulphate-rich system was investigated by Liu et al.17
Their results indicated that anionic species has
an influence not only on the species, the amount as
well as the morphology of products but also on the
transformation rate of ferrihydrite. Lepidocrocite
predominates in the product at room temperature in
Cl- medium while goethite is the main product in
SO42- medium. However, the transformation rate in
the latter system was slower than that in the former
system, which is attributed to the difference in the
ionic strength between the two systems.
In our earlier communication28, we reported when
iron precipitation was carried out in the presence
of hydrazine sulphate, only ferrihydrite phase is
formed at a pH of 2. A mixture of ferrihydrite,
schwertmannite and goethite were formed at pH of
3. Formation of goethite starts at a pH of 3 in the
presence of hydrazine sulphate and pure acicular
goethite with particles of 50-150 nm length with a
high axial ratio of >10 were formed at a pH of 7. With
further increase of pH to 9 or 12, mixtures of goethite
and magnetite were formed. The differences between
present and other results should be attributed to the
formation condition of ferrihydrite. We are generating
Fe(II) in-situ via the addition of hydrazinum sulphate.
These results indicate that a minor change in the
preparing condition of ferrihydrite under sulphate
media can cause great changes in the transformation
product of ferrihydrite. On the other hand, these
results infer the complexity and diversity of products
of transformation of ferrihydrite. More interestingly,
present results are different from the precipitation of
iron oxide in sulphate media where Fe(II) is added
separately as we are also getting magnetite phase
which is not reported earlier.
Mechanistic aspects
The possible mechanism may be as follows:
In presence of hydrazine sulphate Iron (III) reduced
to iron (II) as equation (1).
N2H5+ + 4Fe3+ → N2 + 4Fe2+ + 5H+
...(1)
Again it has been observed that the pH of the
solution decreases during maintaining the pH 2 to 3.
111
This may be due to transformation of ferrihydrite
through dissolution re-precipitation process happens
in the current system. The reasons for the decrease in
pH are as follows: Firstly, some H+ ions are probably
adsorbed on the surface of ferrihydrite. As the
reaction proceeds, these ions are desorbed from
ferrihydrite into solution, which causes the decrease
of pH. Secondly, Fe(II) can catalyze the dissolution
of ferrihydrite. The species dissolved in solution
may be Fe(OH)2+ and/or Fe3+, depending on the pH
of the reaction system. Those monomeric Fe(III)
ions, such as Fe(OH)2+, dissolved in the solution can
condense to form polymers in which the Fe(III) ions
are bridged by OH- groups (Eq. (1) or O2- ions.
Equations (2) and (3) reveal that the formation of
polymers is accompanied by de-protonation.
…(2)
…(3)
The latter reason indicates that Fe(II) catalyzes
the dissolution of ferrihydrite. The dissolution
re-precipitation mechanism was restrained in this
process, thus the transformation and the formation
of magnetite is possible. The in-situ generated
Fe2+ ion in presence of hydrazine sulphate may
form Fe(OH)2 or a hydroxyl sulphate ferrous complex
as more hydroxyl ions will be added to the system.
Then they would grow on the FeOOH nuclei
(goethite) and finally convert into uniform magnetite
as Eqs (4) and (5).
Fe2++2OH−→Fe(OH)2
...(4)
2FeOOH+Fe(OH)2→Fe3O4+2H2O
...(5)
In summary it is a type of topotactic reaction
where on the goethite surface ferrous hydroxide or
hydroxyl complex would adsorb and help in
transformation of goethite to magnetite. Thus above
the pH of 8, formation of magnetite is observed in
present system. Transformation of pure and doped
goethite to magnetite in the presence of ferrous
sulphate under alkaline conditions has been reported
by Mohapatra et al.29,30. Thus above the pH of 7,
formation of magnetite is observed in present
system. It can be expected that if higher
concentrations of hydrazine sulphate are used, it
would lead to formation of pure magnetite phase.
112
INDIAN J. CHEM. TECHNOL., MARCH 2011
Conclusions
The present study has emphasis on the rate of the
in-situ generation of Fe (II) and Fe (II) catalyzed
precipitation as well as transformation of ferrihydrite
and may link this transformation pathway to
ferrihydrite fate. The precipitation of ferric sulphate in
absence and in presence of hydrazinium sulphate
(reducing agent) was investigated. The results indicate
that the reducing agent has an influence not only on
the species, the amount as well as the type of products
but also on the transformation rate of ferrihydrite.
XRD patterns consistent with admixed ferrihydrite
and schwertmannite when the precipitation of ferric
sulphate was done in absence of hydrazinium
sulphate. However by adding only 0.05 M of
hydrazinium sulphate, the nature as well as the
composition of precipitates changes. At a pH of 3
6-line ferrihydrite was obtained. The precipitate at
a pH of 7 contain only goethite phase where as at
a pH of 10 both goethite as well as magnetite
phase were obtained. The presence of hydrazinium
sulphate not only helps in formation of ferrihydrite
but also catalyze the kinetic of transformation of
ferrihydrite to goethite.
Acknowledgements
The authors are thankful to Prof. B K Mishra for
his encouragement during the study. We are thankful
to Dr R K Paramguru, Head of Hydro & ElectroMetallurgy Department.
References
1
2
3
4
5
Steinveit G in World Symposium on the Mining and
Metallurgy of Lead and Zinc, New York, AIME, (1970) 423.
Davey P T & Scott T R, Hydrometallurgy, 2 (1976) 25.
Gordon A R & Pickering R W, Met Trans, 6B (1975) 43.
Societe de la Vielle Montagne, Belg Pat 724 214, Nov 20,
1968.
Tsunoda S, Maeshiro I, Emi F & Sekine K, AIME Annual
Technical Meeting, Chicago, TMS paper, A 73 (1973).
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Jamieson E J, Ph.D Thesis, Murdoch University, Perth, 1995.
Dousma J D, Ottelander D & Bruyan P L, J Inorg Nucl
Chem, 41 (1979) 1565.
Solcova A, Subrt J, Hanousek F, Bechine K, Zapletal V,
Zolotovskii B P, Krivoruchko O P, Buyanov R A & Lipka J
Z, Anorg Allg Chem, 526 (1985) 191.
Wendt H, Z Elektrochem, 66 (1962) 235-237.
Lepentsiotis V, Diploma work, University of Dortmund,
Germany, April (1994).
Babcan J, Geol Zb, 22 (2) (1971) 299.
Jambor L & Dutrizac J E, Chem Rev, 98 (1998) 2549.
Loan M, Newman O M G, Cooper R M G, Farrow J B &
Parkinson G M, Hydrometallurgy, 81(2006)104.
Loan M, Parkinson G, Newman M, Farrow J, J Crystal
Growth, 235 (2002) 482.
Zhang X L, Liu H & Wei Y, J Mater Res, 20 (2005) 628.
Liu H, Wei Y & Sun Y H, Acta Chim Sin, 63 (5) (2005)
391.
Liu H, Guo H, Ping L & Wei Y, J Solid State Chem, 181
(2008) 2666.
Liu H, Wei Y, Li P, Zhang Y H & Sun Y H, Mater Chem
Phys, 102 (2007) 1.
Liu H, Wei Y & Sun Y H, J Mol Catal A: Chem, 226 (2005)
135.
Vogel A I, A Text Book of Quantitative Inorganic Analysis.
5th ed, (Longmans Green and Co), 2000.
Joint Committee on Powder Diffraction Standards (JCPDS),
Mineral Powder Diffraction File, International Center for
Diffraction Data, Swarthmore, PA, 2002.
ICDD, Powder Diffraction File; International Centre for
Diffraction Data, Newtown Square, 1997.
Towe K M & Bradley W F, J Colloid Interface Sci,
24(1967)384.
Cornell R M & Schwertmann U, (Wiley-VCH: Weinheim),
2003.
Zhao, J, Huggins F E, Feng Z G & Huffman P, Clays Clay
Miner, 42 (1994) 737.
Schwertmann U, Friedl J & Stanjek H, J Colloid Interface
Sci, 209 (1999) 215.
Jang J H, Dempsey BA, Catchen G L & Burgos W D, J
Colloids Surf A, 221 (2003) 55.
Mohapatra M, Gupta S, Satpati B, Anand S & Mishra B K,
Colloids Surf A: Physicochem Eng Aspects, 355 (2010) 53.
Mohapatra M, Anand S, Das R P, Upadhyay C & Verma H
C, Hydrometallurgy, 65 (2-3) (2002) 227.
Mohapatra M, Anand S, Das R P, Upadhyay C & Verma H
C, Hydrometallurgy, 66 (1-3) (2002)125.