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All content in this area was uploaded by Abdelrahman Amer on Jun 25, 2019
Content may be subject to copyright.
JOURNAL OF ENGINEERING AND APPLIED SCIENCE, VOL. 63, NO. 4, AUG. 2016, PP. 277-292
FACULTY OF ENGINEERING, CAIRO UNIVERSITY
A COMPARATIVE STUDY OF METHODS USED FOR FE AND MN
OXIDATION AND REMOVAL FROM GROUNDWATER
M. A. ELSHEIKH
1
, H. S. GUIRGUIS
2
AND A. FATHY
3
ABSTRACT
This study focuses on the removal of iron “Fe+2” and manganese “Mn+2” from
groundwater using oxidation methods namely: aeration, dosing of chlorine or
potassium permanganate and/or ozone followed by filtration. The concentrations were
1.50 and 1.0 mg/l for Fe+2, Mn+2 respectively for the experiments. Results show that
aeration alone can remove up to 90% of iron and only 30% of manganese. Chlorine
can remove up to 100% of iron and 90% of Manganese at pH greater than 9.0. Without
increasing pH, higher dosages of chlorine are required to remove about 95% of iron
and 78% of Manganese. Using aeration with chlorine enhanced the removal process
but high chlorine dose is still needed to remove high ratio of Mn+2. Potassium
permanganate (PP) gives the best removal efficiency. By using PP dose equals 2.0
ppm, it is possible to remove up to 100% and 90% of iron and manganese respectively
at pH=7.0. Using aeration with PP does not enhance the Fe+2 and Mn+2 removal
process. Although ozone is considered very effective oxidant, it removes about 93%
of iron and only 58% of manganese when used at pH=7.0.
KEYWORDS: Iron, manganese, aeration, chlorine, potassium permanganate, ozone.
1. INTRODUCTION
Groundwater is considered as the third water source for irrigation and other
human uses in Egypt [1]. Groundwater may contain some constituents like iron and
manganese, hardness, salinity and others that should be treated before water use [2].
Existence of iron and manganese in water causes problems of water coloring and taste
[3]. The secondary maximum contaminant levels for iron and manganese are 0.3 mg/l
1
Head of Civil Engineering Department, Professor of Sanitary and Environmental Engineering, Menoufia
University. Email: mshafy2@yahoo.com
2
Lecturer of Sanitary and Environmental Engineering, Civil Engineering Department, Menoufia University.
Email: hanyguirguis@yahoo.com
3
Teaching Assistant, Civil Engineering Department, Menoufia University. Email: engabdo_1990@yahoo.com
M. A. ELSHEIKH ET AL
278
and 0.05 mg/l respectively [4]. When Fe+2 and Mn+2 are exposed to an oxidant, they
are oxidized to a more stable state “Fe+3 and Mn+4” [5] and so form iron hydroxide and
manganic dioxide that form a relatively large, sticky floc, which is easy to filter out of
treated water [6].
Fe+2 and Mn+2 oxidation by aeration is determined by the ideal theoretical
stoichiometric ratio of oxygen to metal for each reaction [7]. The volumetric air-to-
water ratio is reported to be around 0.75 to 1.0 [6]. However, larger volumes are
required to overcome the inefficiencies of the aeration systems [8].
Chlorine is historically used for iron and manganese oxidation. Oxidation of
one mg Fe+2 requires about 0.64 mg chlorine while one mg Mn+2 oxidation requires
1.29 mg chlorine [9]. Fe+2 and Mn+2 are generally oxidized by chlorine as follows [6]:
22223223 6)(2)()(2 COCaClOHFeHCOCaClHCOFe
(1)
OHCOCaClMnOHCOCaClHCOMn 222223223 342)()(
(2)
Potassium permanganate “PP” is considered a stronger oxidant than chlorine
and has many advantages over chlorine. Oxidation chemistry of iron and manganese
by PP can be described as following [10]:
HKMnOOHKMnOMn 425223 224
2
(3)
HMnOKOHFeOHKMnOFe 53)(373 224
2
(4)
The stoichiometric doses of KMnO4 required to oxidize one mg of iron and one
mg of manganese are about 0.94 mg and 1.92 mg respectively [9]. But in practice, the
actual amounts have been found to be less than these quantities. This may be due to
the catalytic influence of MnO2 on the reactions [9]. And so, the amount of KMnO4
should be determined accurately as the excess amount of it can produce additional
manganese [8]. Ozone is considered a very effective oxidant and it needs a short
oxidation time, but it is expensive and needs more advanced technology than others
[6]. Removal of iron and manganese by ozone can be described by the following
equations [8]:
HOOHFeOHOFe 43)(252 223
2
(5)
A COMPARATIVE STUDY OF METHODS USED FOR FE AND MN …
279
HOMnOOHOMn 2
2223
2
(6)
The theoretical ozone dose required to oxidize one mg of iron and one mg of
manganese are 0.43 mg and 0.87 mg respectively [8].
The aim of the study is to compare between using aeration, chlorine, PP and/or
ozone for Fe+2 and Mn+2 oxidation and removal by using filtration.
2. MATERIALS AND METHODS
2.1 Study Method
Simulated groundwater -which acts the groundwater in Delta Region, Egypt-
was prepared by adding salts of iron and manganese to tap water to be used as the
source of water for the study experiments. Every experiment lasts for 6 hours before
taking samples and then the filter is backwashed before the next one. The study
discusses the different factors “e.g. dosages, retention time, pH” that affect the
oxidation process by using one or more of the pre-mentioned oxidants followed by
direct filtration. Experiments are done for Fe+2 and Mn+2 concentrations of 1.50 and
1.0 mg/l respectively.
2.2 Pilot plant
Figure 1 show the pilot plant which is constructed for the study. It consists of
feeding tank, process tank for adding and mixing chemicals and a Rapid Sand Filter
“RSF”. The RSF is made of PVC pipe of internal Diameter = 100 mm and includes 35
cm gravel layer with diameter 6 to 25 mm and 75 cm of coarse sand with Diameter
1.18 to 1.60 mm. The Rate of Filtration is obtained by controlling the outlet filter
valve.
2.3 Chemicals
Ferrous sulfate heptahydrate “FeSO4.7H2O” and manganese sulfate mono-
hydrate “MnSO4.H2O” were used as source of iron and manganese salts. They are
obtained from AlNasr company for chemicals, Cairo. Commercial Calcium
hypochlorite “Ca(OCl)2” containing 65% of chlorine was used as the source of
M. A. ELSHEIKH ET AL
280
chlorine. Potassium permanganate “KMnO4” with 99.90% purity obtained from Local
supplier was used. Also, Sodium hydroxide “NaOH” was used to adjust pH.
Fig. 1. Pilot plant with mixing and filtration tanks.
2.4 Devices and Analyses
Iron and manganese measure devices were used to measure iron and manganese
concentrations “Hanna, USA”. Portable pH device was used to measure water pH. The
devices were calibrated before the study. An air blower with different capacities was
used for the aeration process. The used capacity was 0.5 m3 air/min which is the
minimum available capacity. An ozone generator obtained from Icon company, China
A COMPARATIVE STUDY OF METHODS USED FOR FE AND MN …
281
was used as a source of ozone. The device model is CFK-3A “with capacity of
generating 3.0 grams of ozone per hour at temperature 15oC”.
3. RESULTS AND DISCUSSIONS
All experiments are done for Fe+2 and Mn+2 concentrations = 1.50 and 1.0 mg/l
respectively, at constant ROF= 150 m3/m2/d. These experiments discuss the removal
efficiency of iron and manganese using oxidation by aeration, adding chlorine, adding
potassium permanganate and/or ozone, under different conditions.
3.1 Using Aeration
Figure 2 shows the results of using aeration with air flow equals about
0.5m3/min. The results show that aeration alone can remove up to 91% of Fe+2 after 20
min. Increasing aeration period does not enhance Fe+2 Removal Ratio “RR”. For
manganese, after aeration for 60 min., the RR is only 30%. This shows that manganese
is more difficult than iron in oxidation as the required power to convert Mn+2 into
“MnO2” is more than the required for conversion of Fe+2 into “Fe(OH)3” [7]. The
results are similar to that obtained in some other studies [5].
Fig. 2. Effect of using aeration on Fe+2 and Mn+2 removal “Initial
Fe+2 and Mn+2 conc. are 1.50 and 1.0 mg/l, pH=7.0”
M. A. ELSHEIKH ET AL
282
3.2 Using Chlorine “Cl”
The studied factors of oxidation of Fe+2 and Mn+2 by Cl are: Cl dosages,
retention times (RT), pH and using aeration in addition to Cl.
Using different doses of Cl: Figure 3 shows the results of using different
concentrations of Cl. The results show that Fe+2 oxidation by Cl is also easier than
Mn+2. Dose of 2.50 ppm “which equals the theoretical calculated dose to oxidize Fe+2
and Mn+2” can oxidize 83% of total iron at neutral pH. Increasing dose leads to
slightly increase in iron oxidation and removal. For manganese, the removal process is
still difficult at pH=7.0. It is reported that the kinetics of oxidation of Mn+2 by oxygen
“O2” or free chlorine “Cl” are very slow relative to retention times typically used in
drinking water treatment systems when pH< 9 [11]. Cl dose greater 15 ppm is required
to obtain about 80% manganese removal. However, this high dose is rarely used to
prevent Disinfection by-products “DBPs” production especially if there are Natural
organic matters in water [5-8].
Fig. 3. Effect of using different Cl doses on Fe+2 and Mn+2 removal “Initial
Fe+2 and Mn+2 conc. are 1.50 and 1.0 mg/l, pH=7.0 and RT=20 min”
Retention time effect: Results for using Cl dose =5.0 ppm for different retention
times -10 to 40 min- are shown in Fig. 4. The results show that iron oxidation by
chlorine happens rapidly. About 86% of Iron is oxidized in just 10 min. Increasing
retention time more than 10 minutes leads to slight increase in the oxidation
A COMPARATIVE STUDY OF METHODS USED FOR FE AND MN …
283
efficiency. For manganese, after 40 minutes, the removal ratio is only about 56%.
Therefore, the oxidation process of manganese by using chlorine needs longer times
at pH=7.0 [8].
Fig. 4. Effect of using different retention time on Fe+2 and Mn+2 removal by Cl
dose=5.0 ppm “Initial Fe+2 and Mn+2 conc. are 1.50 and 1.0 mg/l, pH=7.0”.
For pH effect: two Cl concentrations “2.50 and 5.0 ppm” were used for RT=20
minutes and different pH values equal 7. 0, 8.0 and 9.0. Figure 5 shows these results
for Fe+2 and Mn+2. At pH greater than 7, about 83% of Fe+2 has been oxidized with
Cl dose =2.5 ppm. If water pH is increased to 9, about 100% of iron is oxidized. If
chlorine dosage is doubled, about 6% increase in RR is obtained at the same pH
values. Therefore, the oxidation of Fe+2 by using Cl is affected slightly by increasing
water pH. The process of manganese oxidation by Cl depends mostly on pH. When
Cl dose =2.5 ppm is used at pH=7.0, the removal ratio is about 11%. At pH=8 and 9,
the removal ratios are 50% and 86% respectively. Using Cl dose = 5.0 ppm at pH=9.0
gives similar results. It is concluded that manganese oxidation by minimum Cl dose
can be achieved by increasing pH to 9.0 [8-11].
Using aeration with chlorine: Fig. 6 shows results for using aeration for 20
minutes with different concentrations of chlorine “RT=20 min, pH=7.0”. It is shown
that iron oxidation by using aeration and chlorination is done by the minimum required
dose of chorine. About 96% of iron is removed after 20 minutes by using Cl dose of
2.5 ppm. For manganese, by using chlorine dose of 2.5 ppm, the RR is about 20%
M. A. ELSHEIKH ET AL
284
only. Chlorine dose of 10 ppm with aeration for 20 minutes is required to remove up to
80% of Mn+2 at pH=7.0. The lower RR using low cl dose is because Mn+2 oxidation by
O2 or Cl needs very longer times as mentioned before [11].
Fig. 5. Effect of pH on Fe+2/Mn+2 removal by Cl doses=2.5, 5.0 ppm “Initial
Fe+2/Mn+2 conc. are 1.50, 1.0 mg/l, RT=20 min”.
Fig. 6. Effect of using aeration for 20 minutes with chlorine on Fe+2 and Mn+2 removal
by different Cl dose “Initial Fe+2 and Mn+2 conc. are 1.50 and 1.0 mg/l, pH=7.0”.
A COMPARATIVE STUDY OF METHODS USED FOR FE AND MN …
285
3.2 Using Potassium Permanganate “PP”
The following section deals with using PP for iron and manganese oxidation
and removal. The studied factors are: different dosages of PP, contact time, effect of
pH, Effect of using aeration in addition to PP.
Effect of PP dosages: Figure 7 shows the results of using different PP dosages for
Fe+2 and Mn+2 removal. The removal process of iron by using PP at normal conditions
of pH is easier and faster than other methods. Using dosage of 1 ppm of PP can
remove up to 97 % of iron just after 10 minutes. For manganese, using PP enhances
the removal process greatly at pH=7.0. Using PP dose = 2.0 ppm -which is near to half
of the theoretical calculated dose- can remove 66% of manganese after 10 minutes.
Using doses near to the theoretical dose remove up to 80% of Manganese in just 10
minutes. Increasing PP dosage than the theoretical one has bad effect on water. When
dosage of 4 ppm is used, the water is colored pink from the effect of increased PP
dosage. This probably happens as the increased PP quantity contains manganese
according to the following equation [5]:
OHKMneHKMnO 2
2
4458
(7)
Therefore, a special care for choosing the PP dosage should be taken into
consideration. These results agree with other studies that recommend using PP dosages
near the theoretical ones [5].
Fig. 7. Effect of using different PP doses on Fe+2 and Mn+2 removal “Initial Fe+2 and
Mn+2 conc. are 1.50 and 1.0 mg/l, pH=7.0, ROF= 150m/d and RT=10 min.”
M. A. ELSHEIKH ET AL
286
Effect of Retention Time: The results for using PP dose =2.0 ppm for RT=5 to
20 minutes are shown in Fig. 8. The oxidation process of iron using PP happens very
fast. Complete iron oxidation needs less than 5 minutes. The oxidation process of
manganese using PP also happens fast and is enhanced by increasing RT. when RT is
less than 5 minutes is used, about 66% of Manganese is oxidized. Using RT equals to
20 minutes increases the RR to 90%.
Fig. 8. Effect of retention time on Fe+2 and Mn+2 removal by PP “Initial Fe+2 and Mn+2
conc. are 1.50 and 1.0 mg/l, pH=7.0, P.P.=2.0 ppm and ROF=150 m/d”
Effect of pH: Figure 9 shows results of pH effect on Fe+2 and Mn+2 oxidation by PP
dose = 2.0 ppm. It is shown that iron is oxidized at pH greater than 7.0 and much of
manganese oxidation by using PP happens also at pH near to 7.0. Increasing pH to 8
and 9, increases the RR to 75% and 85% of Fe+2 and Mn+2 respectively. Therefore, pH
affects the process of Fe+2 and Mn+2 oxidation by PP slightly or by other words, the
oxidation of Fe+2 and Mn+2 just needs pH > 7.0.
Effect of using Aeration with PP: In these experiments, aeration was used in
addition to PP. The results are shown in Fig. 10. The results show that the removal of
iron and manganese by using aeration in addition to PP has improved the process of
oxidation and removal slightly. Iron is completely removed at dose of PP=1.0 ppm.
For manganese, using PP dose 2 ppm leads to increasing the RR to 75%. Using dose
of PP=3 ppm removes up to 87% of Manganese.
A COMPARATIVE STUDY OF METHODS USED FOR FE AND MN …
287
Fig. 9. Effect of pH on Fe+2 and Mn+2 removal by PP “Initial Fe+2 and Mn+2 conc. are
1.50 and 1.0 mg/l, P.P.=2.0 ppm ROF=150 m/d and RT=10 min.”
Fig. 10. Effect of using aeration with PP on Fe+2 and Mn+2 removal “Initial Fe+2 and
Mn+2 conc. are 1.50 and 1.0 mg/l, pH=7.0, P.P.=2.0 ppm, RT=10 min.”
The obtained results agree with other studies which found that the actual
amount of PP required to oxidize Mn+2 was less than that indicated by stoichiometry.
The reason may be when Mn+2 is separated on the filter, it starts to coat the filter sands
and convert it to work like green sand filter so the required dose becomes smaller [9].
The oxidation time ranges from 5 to 10 minutes, provided that the pH is over 7.0 [8-
10]. On the other hand, some studies found that the required dosage is slightly more
M. A. ELSHEIKH ET AL
288
than the required theoretical dose at pH less than 8.0 as shown before [5] and
therefore, the required dose should be determined accurately.
3.3 Using Ozone
This section discusses the oxidation of iron and manganese by ozone. The
studied factors are: different dosages of ozone and effect of pH. In these experiments,
the required ozone dose is injected to water by operating the device for definite time
with mixing with water for 10 minutes before moving to balance tank which feeds the
RSF to allow continuous work.
Effect of ozone doses: Fig. 11 shows the results of using different dosages of
ozone to remove Fe+2/Mn+2. The results show that ozone is affective to some limit on
iron oxidation. An ozone dose of 3.0 ppm can remove up to 88 % of iron at neutral
pH. Increasing dose to 7.0 ppm results slight increase in iron RR to reach about 93%
at the same pH. The oxidation of Manganese is difficult by using ozone at neutral
values of pH. At dose = 3.0 ppm, manganese RR equals 36%. When dose was
increased to 7.0 ppm, only 58% of manganese is removed.
Fig. 11. Effect of using ozone different doses on Fe+2/Mn+2 removal “Initial
Fe+2/Mn+2 conc. is 1.50/1.0 mg/l, pH=7.0 and mixing time =10 min”
Although it is known that ozone is a very strong oxidant compared to other
oxidants like chlorine or potassium permanganate, it removes only small ratio of
A COMPARATIVE STUDY OF METHODS USED FOR FE AND MN …
289
Fe+2/Mn+2 compared to them. The reason behind that may be: 1- The fact that ozone is
a gas and so tends to escape to the atmosphere through the used open tank. 2-The
ozone solubility in water is small when compared to other chemicals (e.g. chlorine or
PP). The solubility of ozone is 10 times less than solubility of chlorine [12]. A study
on using ozone in closed glass reactor found that the RR can reach 96% and 83% for
Fe+2 and Mn+2 respectively [4]. Therefore, the usage of ozone should be in special
closed tanks which are suitable and more common in small installations.
Effect of pH: Fig. 12 shows results of pH on Fe+2/Mn+2 oxidation by using
ozone dose = 3.0 ppm. The results show that pH affects the process of Fe+2/Mn+2
oxidation in an obvious way. Increasing water pH to over 9.0 leads to complete Fe+2
oxidation and removal by ozone dose. For Mn+2, increasing pH to 8.0 leads to increase
in RR to reach to 60%. When water pH is raised to 9.0, about 94% of Mn+2 is removed
by the same dose which agrees with other studies that recommend pH values to be 9 to
10 [4].
Fig. 12. Effect pH on Fe+2/Mn+2 oxidation by ozone “Initial Fe+2/Mn+2 conc. is
1.50/1.0 mg/l, ozone dose = 3.0 ppm, pH=7.0 and mixing time =10 min”
3.4 Comparing the Different Methods
Using aeration: aeration is considered a good choice when treating water
containing iron only or iron with low concentrations of manganese –less than the
M. A. ELSHEIKH ET AL
290
maximum allowable values set by local authorities-. Aeration should not be used for
Mn+2 removal as the removal ratio is very low.
Using chlorine: using chlorine at neutral pH is not recommended as it should
be fed in high dosages, needs long periods and/or high pH. Therefore, the
recommended procedure is to raise water pH to 9.0 and so can use lower dose.
Using PP: PP is considered a very good choice for the process of iron and
manganese removal, especially when manganese exists in high concentrations. The
experiments show that using dosages near to half of the required theoretical dose at
normal pH and retention time of 20 minutes can remove iron completely and 90%
of manganese. However, the dose should be determined very accurately to prevent
water coloring.
Using ozone: It is not recommended to use ozone in open tanks as it would
escape to the atmosphere and its cost will be high compared to other alternatives. It
could be used in small installations where closed tanks can be used.
4. CONCLUSION
Iron and manganese can be removed from groundwater by using oxidation by
aeration, chlorine, potassium permanganate and/or ozone followed by filtration. Every
alternative is suitable in some cases. Aeration alone can be used when water contains
iron alone or manganese with low concentrations. Chlorine is not recommended to be
used to remove iron and manganese as it needs longer time than other methods, raised
pH over 9.0 or high dosages. Using aeration with chlorine enhances the removal
process of iron and manganese but chlorine should be fed in high dosages. Potassium
permanganate is considered the best option. By using doses near to half of the
theoretically calculated dose, it can remove up to 100% and 90% of iron and
manganese at pH=7.0. Using aeration with potassium permanganate does not improve
the removal process. Ozone can remove 100 % and 94% of iron and manganese when
pH is raised to over 9.0.
A COMPARATIVE STUDY OF METHODS USED FOR FE AND MN …
291
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