J. Int. Environmental Application & Science, Vol.

3(5): 409-414 (2008)

Air Oxidation of Ferrous Iron in Water#

Ahmet Alıcılar1,∗, Göksel Meriç2, Fatih Akkurt3, Olcay Şendil4

1
Gazi University, Engineering & Architecture Faculty, Department of Chemical Engineering, 06570, Ankara,
TURKEY; 2General Managership of Bank of Provinces, Ankara, TURKEY; 3 Republic of Turkey Ministry of
Environment and Forestry, Turkish State Meteorological Service, 06120, Ankara, Turkey; 4Gazi University, Arts
& Science Faculty, Department of Chemistry, 06500, Ankara, TURKEY

Abstract: Air oxidation of ferrous iron in water was studied. It was worked at
three different values of pH and concentration. Oxidation was firstly carried
out at stationary atmosphere. Thereafter, the experiment was successively
repeated by blowing air to the solution without and with inert packing. Lastly,
the catalytic effect of ferric hydroxide was investigated. While the maximum
yield of 86 % is catalytically achieved by blowing air at a neutral medium, the
oxidation was almost completed in an alkaline solution even at stationary
atmosphere. The reaction was first order with respect to Fe2+.
Keywords: Iron, water pollution, oxidation

Introduction
Oxygenated water will have only low levels of iron. The problem is most likely to develop in
water from wells with high carbonate and low oxygen. Iron carbonates in an oxygen poor environment
are relatively soluble and can cause high levels of dissolved iron (http://www.ext.nodak.edu). In
addition to groundwater supplies, acidic mine waters are often characterised by elevated
concentrations of metal ions such as Fe2+ and Mn2+(Banks et al., 1997). On the other hand, alkaline
mine discharges are generally not that polluting in terms of metal toxicity, but can be highly
ferruginous (Younger & Banwart, 1999)
Iron in water does not present a health hazard. However, its presence may cause taste, staining
and accumulation problems. The treatment can be performed by various techniques. Chemical
oxidation followed by filtration is the accepted method for treatment of iron when its concentration is
greater than 10 ppm (http://www.ext.nodak.edu).
Oxidation of iron is achieved by addition of chemical oxidants. However, it can be easily and
low cost carried out by contact with air (Wong, 1984). During oxidation of Fe2+ salt aqueous solutions,
poor soluble compounds including Fe3+ oxides are formed (Domingo et al., 1994). The composition of
precipitate formed depends on numerous parameters such as temperature, pH, concentration, feed rate
and anion nature (Das & Anand, 1995; Tolchev et al., 2002).
For acidic discharges, lime dosing is often used to raise the pH. In this way, the rate of oxidation
and the subsequent precipitation of Fe2+ oxide minerals are increased. Aeration will also assist O2
transfer for the oxidation of Fe2+ to Fe3+ and the subsequent removal by precipitation (Burke &
Banwart, 2002). Additionally, the reaction is catalyzed by the reaction product ferric hydroxide
(Sarıkaya, 1990). In this work, air oxidation of Fe2+ ions in water is studied. Firstly, oxidation is
performed at stationary atmosphere. Thereafter, the experiment is repeated by blowing air to the
solution without and with packing. Additionally, the reaction is tried to catalyze by ferric hydroxide.
Effects of concentration and pH on oxidation are investigated.

Experimental
The oxidation kinetics of Fe(II)(aq) species has been previously reviewed by many workers
(Wehrli, 1990; Zhang et al., 1992). The stoichiometry for the overall oxidation of Fe2+ ions by O2 is
given by Eq. (1) (Burke & Banwart, 2002).

O2(aq) + 4Fe2+ + 6H2O ↔ 4FeOOH(s) + 8H+ (1)

∗
Corresponding: E-mail: aalicilar@gazi.edu.tr; Tel: +90 3122317400/2524, Fax: +90 312230 8434
#
This study has been presented at BIES’08, Giresun-Turkey

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Eq. (2) gives the most commonly verified rate law for the oxidation of Fe(II) (Davison & Seed,
1983; Tamura et al., 1976) . It has been, however, reported that the oxidation kinetics appear to change
depending on pH, acidic compounds and ions in solution (Miles & Brezonik, 1981; Millero, 1985;
Sung & Morgan, 1980), and oxygenation is greatly accelerated in the presence of surfaces (Barry et
al., 1994).

-d[Fe(II)]/dt=k[Fe(II)][OH-]2[O2] (2)

In the light of this, it may be thought that the oxygenation is affected by numerous factors, e.g.
solution pH, ferrous ion and oxygen concentrations, and catalyzed by solid surfaces.
In experiment, it was tried to study these effects. For this purpose, it was conducted at three
different values of pH and concentrations of Fe2+ (Table 1). The oxidant was air. Aqueous solutions of
Fe(NH4)2(SO4)2.6H2O were prepared as an initial reagent and the required pH value was achieved by
adding NH3 or H2SO4. The distilled water used to prepare solutions was saturated to air by resting at
open atmosphere for a long time prior to use.
Oxidation was carried out in a batch system was used for oxidation. The experiment was firstly
made at stationary atmosphere and the solution of 100 mL was rested at 20oC in a thermostatic bath
for 15 minutes. The second part of experiment was also performed as similar to the first one.
Differently, it was worked at 35oC in order to study the effect of temperature. The third part
experiment was conducted by blowing air to the centre of the solution in order to support the transfer
of O2. Air flow rate was determined as 20.3 mL/s with a flow meter. To increase the gas-liquid contact
efficiency and catalyze the reaction, last studies were completed in a solution with inert packing (8
mm spherical glass bead) and Fe(OH)3 respectively.
For each case, variations happened on oxidation of Fe2+ were observed. Amounts of unoxidized Fe2+ in
the solution were measured with bipyridine method (Fresenius, 1988). Oxidation yields were
calculated by using initial and final concentrations of ferrous ion in the solution. The results were
discussed from several points of view.

Result and Discussion

Reaction order can be calculated from experimental results. If concentrations of OH- ion and
oxygen are kept constant, the oxidation rate changes only depending on the concentration of Fe2+ in the
solution according to the equation 2. If a logarithmic graph is drawn as oxidized amount versus
concentration of Fe2+, the slope of obtained line gives the reaction order with respect to Fe2+.
An example graph drawn for this purpose has been shown in Fig. 1. This graph can also be
formed for different conditions (Table 2). The slope of line can be calculated as 1.0 from Fig.1. The
values obtained from other graphs ranges from 0.9 to 1.1 by which the mean value can be found as
1.0, i.e. air oxidation of ferrous ion in water is first order with respect to Fe2+.

Table 1: Experimental Conditions

System Batch
Initial reagent Fe(NH4)2(SO4)2.6H2O
Solution volume, m3 0.1
Initial concentration of Fe2+, kg m-3 0.025, 0.050, 0.075
pH 3, 7, 9
pH adjuster NH3 or H2SO4
Temperature of solution, oC 20, 35
Oxidant Air
Operation time, sec 900
Air flow rate, m3 s-1 0.0000203
Inert packing Spherical glass
Diameter of inert packing, m 0.008
Catalyst Fe(OH)3
Determination method of Fe2+ Bipyridine

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4.5

ln[(Co-C)x10 3]
4
3.5
3
2.5
2
3 3.5 4 4.5
ln(Cx103)

Figure 1: An example graph drawn to calculate the reaction order (pH=9, stationary atmosphere,
20oC, 15 minutes resting)

Figures 2 to 5 show oxidation yields in different cases. As seen from Fig. 2, in the case of
resting in stationary atmosphere for 15 minutes, the minimum conversion of Fe2+ to Fe3+ is 96 % in
basic solution while the yield is maximally 88 % in acidic medium. The yields obtained in neutral
medium are around 30 % only. This result is agreed to those in the literature (Eroğlu, 1984; Lorenz et
al., 1988; Stauffer, 1987).

Table 2. Line slopes calculated from graphs

Ventilation pH Line slope

Stationary atmosphere 3 1.1
Stationary atmosphere 7 0.9
Stationary atmosphere 9 1.0
Air blowing 3 1.1
Air blowing 7 0.9
Glass packing + Air blowing 3 1.0
Glass packing + Air blowing 7 1.0

Co=0.075 kg/m3 Co=0.050 kg/m3 Co=0.025 kg/m3

100
90
Oxidation Yield (%)

80
70
60
50
40
30
20
2 3 4 5 6 7 8 9 10
pH

Figure 2. Oxidation yields (20oC, solution without packing, stationary atmosphere, 15 minutes resting)

As seen, it is enough for oxidation of Fe2+ in basic medium to rest the solution at open atmosphere.
It may be shown as a reason to this that Fe(OH)3 formed after oxidation easily precipitates in basic
medium and catalyzes the reaction. The solution has to be more effectively ventilated in nonbasic
medium, especially at neutral one.

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Similar operations were repeated at 35oC by changing temperature only. As shown in Fig. 3, the
values observed are approximately equal to those at 20oC, i.e. increasing of the temperature does not
affect the conversion remarkably. This result is perhaps sourced from two different effects of
temperature. Increasing of temperature accelerates the reaction and depending on this, the conversion
percent in a certain time increases. However, the solubility of oxygene in water decreases with
increase in temperature (Tchobanoglous & Schroeder, 1987). This decrease results in decreasing of the
conversion percent. Which one of these two effects dominates will change depending on the
conditions. In this study, a change on the oxidation yield has not been observed when temperature is
increased. The effect of temperature is neglected because of discussions above and subsequent studies
are conducted at 20oC only.
The yields observed in basic medium are very high even at stationary atmosphere. Therefore,
latter parts of experiment are performed in nonbasic solutions only.
In this stage, the mass transfer of oxygene was supported by blowing air to the solution and it was
tried to raise the contact efficiency of oxygene with the solution. The results obtained in acidic and
neutral mediums as comparing to previous ones are shown in Fig. 4.

Co=0.075 kg/m3 Co=0.050 kg/m3 Co=0.025 kg/m3

100
90
Oxidation Yield (%)

80
70
60
50
40
30
2 3 4 5 6 7 8 9 10
pH

Figure 3: Oxidation yields (35oC, solution without packing, stationary atmosphere, 15 minutes resting)

As seen from the figure, the yields in neutral medium are low still. However, in the case
of air blowing, an important increase reaching to the 12 % has been observed as comparing to
the stationary atmosphere. The increase in gas-liquid contact efficiency by air-blowing may
be shown as a reason of this increase in yield.
In addition to air blowing, the gas-liquid contact can also be improved by solid surfaces in
the solution. The results obtained from experiments performed in this manner have been
shown in Fig. 5 as comparing to the previous ones.
pH=3 resting pH=7 resting pH=3 blowing pH=7 blowing

100
Oxidation Yield (%)

80
60

40
20
20 30 40 50 60 70 80
C0 x 103 (kg m-3)

Figure 4: Oxidation yields (20oC, solution without packing, 15 minutes operation)

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pH=3 without packing pH=7 without packing

pH=3 with packing pH=7 with packing

100

Oxidation Yield (%)

80

60

40
20 40 60 80
C0 x 103 (kg m-3)

Figure 5: Oxidation yields (20oC, 15 minutes air blowing)

It is seen from the figure that in the solution with glass packing, especially at neutral medium, an
improvement in yield of 24 % has been observed as comparing to that without packing. It is clear that
this increase in yield results from the increase obtained in gas-liquid contact by using of packing
(Alıcılar et al., 1994). This increase although less has been observed in acidic medium too. Although
the increases in oxidation yield for acidic solutions are recorded as 4 % mean, the yield in this case is
minimally 92 % and reaches to 96 % for the solution with high concentration. Hence, the last part of
experiment has been carried out at neutral medium only and the catalytic effect of Fe(OH)3 has been
investigated. The results obtained are shown in Table 3.

Table 3. The oxidation yields at catalytical conditions (pH=7, 20oC, solution with glass bead/Fe(OH)3, 15
minutes air blowing)

Concentration, kg m-3 Yield, %

0.025 84
0.050 86
0.075 84

The yield increases in the range of 8 to 12 % as comparing to that without Fe(OH)3. This result
can be explained so that Fe(OH)3 catalyzes the air oxidation of Fe2+, which is agreed to the literature
(Hafsi, 2001; Tamura et al., 1976). It has been stated that Fe(OH)2(aq) and the Fe(II) species adsorbed
on the Fe(III) oxide mineral surface are significantly more reactive than Fe2+ [9] and iron oxygenation
is greatly accelerated in the presence of surfaces [19]. Similar discussions are also made by other
researchers and it is offered that the oxidation of Fe(II) by molecular O2 is a result of adsorption and
surface catalyzed auto-oxidation (Burke & Banwart, 2002; Tamura et al., 1976; Hafsi, 2001).

Conclusion
It can be conclusively said that Fe+2 ions in basic solution can be easily oxidized even at
stationary air. Different techniques must be tried in acidic medium to contact gas and liquid phases
more effectively. Support operations such as air blowing, packing usage may be applied for this
purpose. High conversion in neutral medium can be catalytically achieved and catalysts different from
Fe(OH)3 may be studied. Nevertheless, if water hardness is low, a basic medium can be easily and low
cost formed by dosing lime, where the yield for air oxidation of Fe2+ is minimally 96% even at
stationary atmosphere.

References
Alıcılar A, Biçer A, Murathan A, 1994. The relation between wetting efficiency and liquid holdup in
packed columns. Chem. Eng. Commun. 128, 95-107.
Banks D, Burke SP, Gray C, 1997. The hydrogeochemistry of coal mine drainage and other
ferruginous waters in North derbyshire and south yorkshire. J. Engin.Geol. 30, 257-280.

413
 J. Int. Environmental Application & Science, Vol. 3(5): 409-414 (2008)

Barry RC, Schnoor JL, Sulzberger B, Sigg L, Stumm W, 1994. Iron oxidation kinetics in an acidic
alpine lake. Water Res. 28, 323-333.
Burke SP, Banwart SA, 2002. A geochemical model for removal of iron(II)(aq) from mine water
discharges. Appl. Geochem. 17, 431-443.
Das KP, Anand S, 1995. Precipitation of iron oxides from ammonia-ammonium sulphate solutions.
Hydrometallugry, 38, 161-173.
Davison, W, Seed G, 1983. The kinetics of the oxidation of ferrous iron in synthetic and natural
waters. Geochimica et Cosmochimica Acta, 47, 67-79.
Domingo C, Rodriguez-Clemente R, Blesa M, 1994. Morphological properties of α-FeOOH, γ-
FeOOH and Fe3O4 obtained by oxidation of aqueous Fe(II) solutions. Journal of Colloid and
Interface Science, 165, 244-252.
Eroğlu V, 1984. Su ve Kullanılmış Su Tasfiyesi, Vol.1, İstanbul Teknik Üniversitesi İnşaat Fakültesi
Yayınları, İstanbul, 276.
Fresenius W, 1988. Water Analysis (Eds: Fresenius W, Quentin KE, Schneider W). Springer-Verlag
Inc. Berlin, 303.
Hafsi M, 2001. Analysis of boujdour desalination plant performance. Desalination, 134, 93-104.
http://www.ext.nodak.edu
Lorenz W, Seifert K, Kasch K, 1988. Iron and manganese removal from rroundwater supply. Public
Works, 57.
Miles CJ, Brezonik PL, 1981. Oxygen consumption in humiccolored waters by a photochemical
ferrous-ferric catalytic cycle. Envir. Sci. Technol. 15, 1089.
Millero FJ, 1985. The effect of ionic interactions on the oxidation of metals in natural waters.
Geochim. Cosmochim. Acta, 49, 547-553.
Sarıkaya HZ, 1990. Contact aeration for iron removal-A theoretical assessment. .Water Res. 24, 329-
331.
Stauffer RE, 1987. A comparative analysis of ıron, manganese, silica, phosphorus, and sulfur in the
hypolimnia of calcareous lakes. Water Res. 21, 1009-1022.
Sung W, Morgan JJ, 1980. Kinetics and products of ferrous ıron oxgenation in aqueous systems.
Environmental Sscience and Technology, 14, 561-568.
Tamura H, Goto K, Nagayama M, 1976. The effect of ferric hydroxide on the oxygenation of ferrous
ions in neutral solutions. Corrosion Science, 16, 197-207.
Tchobanoglous G, Schroeder ED, 1987. Water Quality, 6th Ed, Addison Wesley, 768.
Tolchev AV, Kleschov DG, Bagautdinova RR, Pervushin VY, 2002. Temperature and pH effect on
composition of a precipitate formed in FeSO4-H2O-H+/OH--H2O2 system. Materials Chem. and
Phys. 74, 336-339.
Wehrli B, 1990. Redox Reactions of Metal Ions at Mineral Surfaces (Ed: Stumm W) Aquatic
Chemical Kinetics, Wiley-Interscience, New York.
Wong JM, 1984. Chlorination-Filtration for Iron and Manganese Removal. J Am Water Works Assoc.
76, 76-79.
Younger PL, Banwart SA, 1999. Mine waste and mine water pollution short course. The Mining
Institute, Newcastle upon Tyne.
Zhang Y, Charlet L, Schindler PW, 1992. Adsorption of protons, Fe(II) and Al(III) on lepidocrocite (
-FeOOH). Colloids and Surfaces, 63, 259-268.

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