Burapha University International Conference 2014
Burapha University, Thailand
July 3-4, 2014
Application of Fenton Reaction with Subsequent Hydroxide
Precipitation for Derusting Wastewater Treatment
Piseth Som and Tongchai Sriwiriyarat
Department of Chemical Engineering, Faculty of Engineering, Burapha University,
169 Long-Hard Bangsaen Road, Saensook, Muang, Chonburi 20131, Thailand
E-mail: sriwiri@buu.ac.th
ABSTRACT
Chemical cleaning of iron pipes and boilers generates complex wastewater so-called
derusting wastewater containing high concentrations of organic chelating agents such as
ethylenediamine tetraacetic acid (EDTA) and metals, primarily iron. Complex wastewater
makes it difficult to be treated by conventional methods due to metal complexation.
However, both the organic chelating agents and the metals in derusting wastewater must be
removed. This study investigated the use of Fenton reaction followed by hydroxide
precipitation for this type of industrial wastewater treatment. The optimum conditions
including initial pH, hydrogen peroxide (H2O2) to ferrous iron (Fe2+) molar ratio (H2O2: Fe2+),
and precipitation pH were also determined in a Jar Test apparatus at the room temperature.
The results indicated that removal efficiencies of total chemical oxidation demand (total
COD) and iron depended on initial pH, H2O2: Fe2+ molar ratio, and precipitation pH. At the
optimum initial pH of 3.0, the H2O2: Fe2+ molar ratio of 40:1, and the precipitation pH of 9.0,
the removal efficiencies of total COD and total iron were as high as 90% and 94.4 %,
respectively, after 20 min of reaction time. It was found that total COD was removed by
Fenton reaction while precipitation was responsible for the total iron removal. In summary,
Fenton reaction with subsequent hydroxide precipitation can be feasibly applied for the
removals of total COD and total iron from the complex derustiong wastewater.
Keywords: Fenton reaction, derusting wastewater, metal complexation, chelating agent
INTRODUCTION
Chemical cleaning of iron pipes and boilers is operated to remove the rust, deposit,
and scale for reactivation and performance improvement (Bansal, 2012). Various types of
chemicals have been used for cleaning depending on the equipment including inorganic acids,
organic acids, chelating agents, alkali agents and aids agents. During the cleaning operation,
the organic acids are extensively used. The chelating agents including EDTA and citric acid
are the most common organic acids used for dissolving encrustation or rust (Fe2O3,/Fe3O4)
removal (Huang et al., 2000; Bansal, 2012). As a result, this chemical cleaning process
generates complex industrial wastewater often containing large amounts of iron, copper, and
chelating agents. Iron is the most prevalent cation, generally presents at a concentration of
1000-10000 mg/L. Copper is the second most abundant metal with minor level of nickel,
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chromium, and zinc, which typically present at the concentration less than 100 mg/L (Kim et
al., 2010; Bansal, 2012). The organic chelating agent, EDTA, causes the complexation and
immobilization of heavy metals. Indeed, metal-EDTA complexation is biologically persistent
(Ghiselli et al., 2004; Chitra et al., 2011). Lime or caustic treatment, hydroxide precipitation,
ion exchange are reported to be inhibited when metal existed as metal-EDTA complexes due
to dramatically decrease of heavy metal solubility (Fu et al., 2009; Lan et al., 2012). Metal
chelated wastewater has been treated by electrochemical reduction (Huang et al., 2000) and
interior micro-electrolysis (Lan et al., 2012). Both processes can successfully remove metals;
however, interior microelectrolysis cannot remove or degrade chelating EDTA. In addition,
electrochemical reduction can achieve EDTA recovery for reuse. To remove metal and
mineralize the metal-EDTA complexes, Fu et al. (2009; 2012) feasibly applied Fenton and
Fenton-like reactions followed by hydroxide precipitation for synthetic Ni-EDTA complex
wastewater. However, the real complex wastewater is suggested for further research;
therefore, application of Fenton reaction for real derusting wastewater is of interest.
For last few decades, advanced oxidation processes (AOPs) are known for destruction
of a wide range of recalcitrant organic pollutants (Poyatos et al., 2010; Ameta et al., 2012).
Among AOPs, Fenton process are adopted for wastewater treatment in terms of organic
pollutant destruction, toxicity reduction, biodegradability improvement, COD, odor and color
removals due to its economic advantage, ease of application, and effectiveness (Matthew Tarr,
2003; Bautista et al., 2008).
Fe2+ + H2O2 → Fe3+ + OH• + OH−
k = 70 M-1s-1
(1)
RH + OH• → R•+ H2O
k =107 -1010 M-1s-1
(2)
R• + Fe3+ → R+ + Fe2+
-
(3)
Fe2++ OH• → Fe3++ OH−
k = 3.2 108 M-1s-1
(4)
H2O2 + OH•→ HO2• + H2O
k =3.3 107 M-1s-1
(5)
As shown in reaction (1), the ferrous iron (Fe2+) initiates and catalyses the
decomposition of hydrogen peroxide (H2O2) to generate the hydroxyl radicals (OH•). The
reaction (1) is commonly known as the main reaction of Fenton process (Neyens & Baeyens,
2003). The generated hydroxyl radical reacts immediately with organic substances (RH)
resulting in a free organic radicals (R•) as indicated by reaction (2). These radicals are
subsequently oxidized by ferric ion to generate other oxidation products as shown in reaction
(3) (Matthew Tarr, 2003). In addition, ferrous iron and hydrogen peroxide are possibly
involved with competitive or scavenging reactions with hydroxyl radicals as listed in
reactions (4) and (5), respectively.
This study discusses the feasibility of Fenton reaction for derusting wastewater
monitored in terms of total COD and total iron removal efficiencies. Since the complex
characteristics of the industrial wastewater, total COD represented all organic compound
existed in wastewater. Total iron was the predominant metal in wastewater and used as
objective parameter for this study. The objectives of the study were to investigate the effects
of initial parameters of Fenton reaction including initial pH and H2O2 concentration for the
treatment of derusting industrial wastewater and to determine the optimum precipitation pH
after the Fenton reaction for the treatment of derusting wastewater.
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July 3-4, 2014
MATERIALS AND METHODS
Derusting Wastewater
The derusting wastewater used in this study was obtained from a cleaning service
company located in Rayong Province, Thailand. The characteristics of the wastewater were
provided in Table 1.
Table 1. Characteristics of derusting wastewater used in the study
Parameters
Value
Limited effluent
pH
10.3
6.5-8.5
Total COD (mg/L)
22257
< 400
Total Iron (mg/L)
3920
< 0.5
3+
Ferric (Fe ) (mg/L)
3683
TDS (mg/L)
9840
< 5000
TSS (mg/L)
68
< 150
Conductivity (mS/cm)
19.7
The limited effluent is based on Thai industrial effluent standard in Pollution Control Department
(PCD). www.pcd.go.th (Retrieved: September 20, 2013)
Materials
The following analytical-grade reagents including hydrogen peroxide (H2O2-35%
w/w), ferrous sulfate (FeSO4-7H2O), manganese dioxide (MnO2), sodium hydroxide (NaOH),
and sulfuric acid (H2SO4) were obtained from the Thai chemical supply company. The FeSO4
solution was prepared daily and used as a catalyst for the Fenton reaction. Deionized water
(DI water) was used throughout the experiment.
Analytical Methods
The analytical methods for all parameters were analyzed according to the Standard
Methods for the Examination of Water and Wastewater (APHA, 2005). Furthermore, the
total COD and total iron were determined by the close reflux titrimetric method (Method
5520) and the phenanthroline method (Method 3500), respectively. A pH meter (EUTECH
Model 510) was used to determine pH value of solution.
Experimental Procedure
The experiment was conducted by using a Jar Test apparatus (Model JR-6A,
Metrology Technical, Co., Ltd., Thailand) equipped with six beakers of 1-L each under the
room temperature (28 °C). Firstly, every beaker was added with 500 mL of wastewater taken
from a large tank storing the industrial wastewater. The wastewater was mixing while the
wastewater was transferred to the beakers so that the wastewaters in all beakers had the same
characteristics. Subsequently, the wastewaters were mixed with 50 rpm mixing speed for a
period of 15 min, and then adjusted with the H2SO4 (99.8% pure) to the pH value of 3.0.
After that, the mixing speed was increased to 150 rpm and immediately added with 0.05 M of
Fe2+ into the solutions. The Fe2+ concentration of 0.05 M was determined from another set of
experiments. The solutions in all beakers were mixed for 15 min to allow homogenously
dissolution of Fe2+. Subsequently, the mixing speed was decreased to 50 rpm and then 2 M of
H2O2 was added. The Fenton reaction was taken place after the H2O2 was added. The Fenton
reaction was allowed to proceed for a period of 60 min. At the end of Fenton reaction, 10 mL
of sample was withdrawn from the beaker. A small amount of MnO2 was then added into the
samples as a catalyst to decompose the remaining H2O2 to water and oxygen. The remaining
H2O2 interfered with the COD measurement (Talinli, & Anderson, 1992). After Fenton
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July 3-4, 2014
reaction was completed, the Fenton treated effluents were continued for hydroxide
precipitation by adjusting solution to alkaline pH value by the NaOH (10 N) and were mixed
at the mixing speed of 150 rpm for a period of 15 min then allowed to precipitate for 30 min.
Finally, the sample was collected from the supernatant for total COD and residue total iron
determinations. The removal efficiency (R) was calculated by the following equation:
R(%) 
AB
x100
A
(6)
where A represents initial concentration of total COD or total iron
B represents final concentration of total COD or total iron
RESULTS AND DISCUSSION
Effects of initial pH
To examine the effects of initial pH on the total COD and iron removal efficiencies,
the experiments were conducted at different initial pH of 2.0, 2.5, 3.0, 3.5 4.0, 4.5, 5.0, 6.0,
and 7.0 with 2 M of H2O2 and 0.05 M of Fe2+. It is evident in Fig. 1 that Fenton reaction
provided high reactivity at the pH ranged from 2.0 to 5.0 as reported in US Peroxide (2012).
When pH increased from 2.0 to 3.0, the total COD and total iron removal efficiencies
increased from 85.2 to 93.4% and from 90.5 to 92.1%, respectively. However, the total COD
removal efficiency decreased from 91.9 to 25.9% with the increase of pH from 3.5 to 7.0
providing the optimum initial pH of 3.0. More importantly, the pH ranged from 6.0-7.0, the
residue total iron concentration remained up to 4526 mg/L, which was greater than initial total
iron concentration (3920 mg/L), leading to the negative removal efficiencies. Higher amount
of iron in solution was resulted from the addition of 0.05 M Fe2+ as the catalyst for Fenton
reaction.
COD and Iron Removal Efficiencies, %
120
100
80
60
Total COD
40
Total Iron
20
0
0.0
-20
1.0
2.0
3.0
4.0
Initial pH
5.0
6.0
7.0
8.0
-40
Fig. 1. Effect of initial pH on the total COD and iron removal efficiencies at the H2O2
concentration of 2 M, the Fe2+ concentration of 0.05 M, the reaction time of 60 min, and the
precipitation pH of 8.0
It is possibly to explain that lower removal efficiency of total COD at pH less than 3.0
is due to stabilization of H2O2 as oxonium ions (H3O2+). The reaction between OH• and H+
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also occurs. In addition, Fe2+ regeneration by the reaction of Fe3+ with H2O2 is inhibited at
more acidic pH value (Wang et al., 2011). The COD and iron removal efficiencies decreased
at high pH (pH > 6) is a result of the Fe3+ precipitation as Fe(OH)3. Fe(OH)3 functionally
catalyzes and decomposes H2O2 into O2 and H2O, providing less production of hydroxyl
radical (OH•). Also, if the pH is too high ( pH> 5), self-decomposition of H2O2 into oxygen
reduces its concentration in the solution (Bautista et al., 2008; Fu et al., 2009, 2012; Wang et
al., 2011). The results from this experiment are in agreement with those studies reported by
researchers (Neyens & Baeyens, 2003; Fu et al., 2009, 2012), who found that acidic pH level
about 3.0 is usually optimum for Fenton reaction.
Effects of H2O2:Fe2+ molar ratio
In the Fenton process, the molar ratio of H2O2:Fe2+ is very important in terms of
overall cost and removal efficiency of the process. Excessive or shortage of any of these two
reagents results in the occurrence of scavenging reactions. Various molar ratios of H2O2: Fe2+
were applied to the industrial wastewaters with the optimum initial pH of 3.0. The effects of
H2O2:Fe2+ on the removal efficiencies of total COD and iron are shown in Fig.2. The figure
indicates that the removal efficiencies of both total COD and iron increased with the increase
of the H2O2: Fe2+ molar ratios.
COD and Iron Removal Efficiencies, %
120
Total COD
Total Iron
100
80
60
40
20
0
0
10
20
30
H2O2
40
:Fe2+
50
60
70
Ratio
Fig. 2 Effects of H2O2:Fe2+ molar ratio on the total COD and iron removal efficiencies at the
optimum initial pH of 3.0, the Fe2+ concentration of 0.05 M, the reaction time of 60 min, and
the precipitation pH of 8.
When the H2O2: Fe2+ molar ratio was 40, the total COD and total iron removal
efficiencies were 93.0% and 90.5 %, respectively. However, further increase of H2O2:Fe2+
molar ratios above 40 did not enhance the removal efficiencies of both total COD and total
iron due to quenching or scavenging effects reaction of hydroxyl radical by Fe2+ according to
reaction 4. It should be noted that Fenton reaction requires high amount of H2O2 in the
presence of EDTA in solution as previously reported by Ghiselli et al. (2004) and Fu et al.
(2012). The optimum molar ratio of 40:1 in this study is comparatively lower than those
studies reported in literatures (Ghiselli et al., 2004; Fu et al., 2009). Fu et al. (2009) have also
reported that the detrimental effects may be observed when greater than 500:1 of H2O2: Fe2+
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molar ratio is employed. However, in this study lower ratios were employed and detrimental
effects has not been observed.
Effects of precipitation pH
The experiments were conducted to study the effects of precipitation pH on the total
COD and total iron removal efficiencies. In this experiment, the total COD and iron removal
efficiencies were analyzed in both Fenton treated effluent and precipitation treated effluent to
investigate the extents of removal efficiency performed by individual process as shown in
Fig.3. After the solution was treated by Fenton reaction, the treated solution pH was adjusted
by adding hydroxide to form Fe(OH)3 precipitates as provided in the reactions (7)-(9)
according to Fu et al., (2009).
COD and Iron Removal Efficiencies, %
100
90
80
70
Total COD after Fenton Reaction
Total COD after Precipitation
Total Iron after Fenton Reaction
Total Iron after Precipitation
60
50
40
30
20
10
0
5
6
7
8
9
Precipitation pH
10
11
12
Fig. 3 Effects of precipitation pH on the total COD and iron removal efficiencies at optimum
initial pH of 3.0, Fe2+ concentration of 0.05M, H2O2 concentration of 2 M, and the reaction
time of 20 min.
The results indicated that about 15% of total iron was removed from the wastewater by
the Fenton reactions. High concentration of iron in Fenton treated effluent may be resulted
from low solution pH leading to high solubility of iron species (Fu et al., 2009; Lan et al.,
2012). However, as shown in Fig. 3, with precipitation pH values increasing from 6.0 to 9.0,
the removal efficiencies of total iron increased from 53.6 % to 94.4 % after 20 min of reaction
time. When pH increased further, the removal efficiencies were unchanged because the iron
precipitates as Fe(OH)3 at very high pH (Fu et al.,2009). This is apparently explained that at
the pH lower than 8.0, solubility and concentration of dissolved Fe2+ and Fe3+ remains high in
solution. However, when pH of solution is greater than 8.0, fraction of Fe2+ and Fe3+ are in
solid phase or form Fe(OH)2 and Fe(OH)3 as precipitates as shown in reactions below
(Morgan & Lahav, 2007; Fu et al., 2009). On the other hand, removal efficiencies of COD
remained almost unchanged (R = 90%) after Fenton reaction and precipitated in all pH values.
Organic degradation or total COD removal was accomplished by Fenton reaction before
precipitation process.
Fe2+ +
2OH−
→
Fe(OH)2↓
(7)
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July 3-4, 2014
4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)3↓
Fe3+
+
3OH− →
Fe(OH)3↓
(8)
(9)
In summary, precipitation does not improve the removal efficiency in terms of total
COD reduction; however, it is responsible for iron precipitation found in this study. As
reported, Fenton reaction is employed for organic degradation while the alkaline range of pH
is to stop Fenton reaction and precipitate irons (Lan et al., 2012; USperoxide, 2012).
ACKNOWLEDGEMENTS
The authors would like to thank the Research and Development Fund for Cooperation
Promotion with External Organizations of the Faculty of Engineering, Burapha University
(Contract No. 9/2556) and Kation Power, Co., Ltd. for financial supports. The authors are
also grateful for teaching assistance of Silchai Kuhakeaw and laboratory assistances of
undergraduate students including Siraprapa Pandaeng, Paphada Jirachonrat, Siwarin
Thongroe, and Prapromporn Klaichit.
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