THE DEGRADATION MECHANISM OF WASTEWATER CONTAINING MDEA USING UV/H2O2 ADVANCED OXIDATION PROCESS
The Degradation Mechanism of Wastewater
Containing MDEA using UV/H2O2 Advanced
Oxidation Process
Sabtanti Harimurti
Anisa Ur Rahmah
Department of Chemical Engineering
Universiti Teknologi Petronas
Seri Iskandar, Perak, MALAYSIA
sabtanti@gmail.com
Department of Chemical Engineering
Universiti Teknologi Petronas
Seri Iskandar, Perak, MALAYSIA
anisa.urrahmah@gmail.com
Abul Aziz Omar
Thanpalan Murugesan
Department of Chemical Engineering
Universiti Teknologi Petronas
Seri Iskandar, Perak, MALAYSIA
aaziz_omar@petronas.com.my
Department of Chemical Engineering
Universiti Teknologi Petronas
Seri Iskandar, Perak, MALAYSIA
murugesan@petronas.com.my
Abstract—Alkanolamines such as MEA, DEA, MDEA and DIPA
in aqueous solutions is frequently used for scrubbing carbon
dioxide from natural gas. High quantity of alkanolamine appears
in the wastewater during cleaning and maintenance as well as
shutdown of the absorption and desorption columns. The
alkanolamines waste is toxic to the environment and this
wastewater cannot be treated in the conventional wastewater
treatment. Advanced Oxidation Process (AOP) named UV/H2O2
is introduced to solve this problem. A laboratory set up of
degradation MDEA waste using UV/H2O2 was conducted and the
degradation mechanism has been studied. Glycine, oxalic acid,
acetic acid, nitrate ion, nitrite ion and ammonium ion are
identified as degradation intermediate product.
Keywords- MDEA; AOP’s; UV/H2O2; TOC
I.
INTRODUCTION
Natural gas is a major energy source in the world. It is one
of the cleanest, safe, and most useful of all energy sources.
World natural gas consumption rose by 2.5% from 2938.0
billion cubic meters in 2007 to 3018.7 billion cubic meters in
2008 [1]. Consequently, the production of natural gas is in
rise. Natural gas has a wide range of acid gas concentrations,
from parts per million to 50 volume percent and higher,
depending on the nature of the rock formation from which it
comes. Because of the corrosiveness of H2S and CO2 in the
presence of water and because of the toxicity of H2S and the
lack of heating value of CO2, sales gas is required to be
sweetened.
Alkanolamine in water solution are extensively used for
scrubbing certain acidic gases in the natural gas production.
The most utilized alkanolamines for scrubbing acidic gases are
978-1-4577-1884-7/11/$26.00 ©2011 IEEE
monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA) and di-isopropanolamine (DIPA)
[11]. The alkanolamines are regenerated in stripping tower for
recycling back to the absorber. During shutdown and
maintenance of these facilities, huge concentrations of residual
alkanolamine may be carried over into the wastewater. As a
result, they able disturb the biological treatment system of the
plant.
In the recent two decade, advanced oxidation processes
(AOP’s) have proved to be very important in the degradation
of high concentrations of organics which may be difficult to
treat in a conventional biological oxidation unit. This process
is involving a very reactive species named hydroxyl radical
(•OH). Fenton’s treatment, UV/O3 and UV/H2O2 are the
popular technique for generation of hydroxyl radical. In this
paper will be explained concerning the MDEA degradation
mechanism by hydroxyl radical in the water solution. The
UV/H2O2 process in evaluation to other methods of water
treatment has additional advantages such as no formation of
sludge during the treatment, high ability on producing of
hydroxyl radical and applicable in the wide range of pH. In
UV/H2O2 process oxidation, Equation (1) is generally
accepted to generate the hydroxyl radical •OH [6, 8, 16, and
23]:
H2O2 + hv → 2•OH
(1)
The reaction rate of organic contaminant degradation is
usually controlled by the rate of generation of •OH radicals,
which depend upon the concentrations of H2O2 and UV dose.
A large number of experimental works on the application of
advanced oxidation processes (AOP’s) to treat high-strength of
waste have been reported in the literature. The use of Fenton’
reagent in the AOP’s was conducted to degrade MEA
(monoethanolamine) waste [20, 21, and 22]. Putri et al. [17]
also used the Fenton’s reagent to study the degradation of
salfolen. In addition, Raihan et al. [18] studied the effect of
continuous addition mode of Fenton’s reagent on
mineralization of di-isopropanolamine (DIPA). The continuous
addition mode of Fenton’s reagent was better than the one time
addition mode. Meanwhile, the used of UV/H2O2 in the
advance oxidation process had been also examined. Idhzam et
al. [5] conducted the degradation of MEA. Malik and Sanyal
[12] and Muruganandham and Swamithan [15] studied the
The
effectiveness of UV/H2O2 to decolorize azo-dye.
degradation of 4-nitrophenol using UV/H2O2 was reported by
Daneshvar et al. [3]. Lopez et al. [10] successfully degraded the
4-chloro-3,5-dinitrobenzoic acid (CDNBA) in aqueous
solution. Degradation and detoxification of formaline
wastewater using UV/H2O2 was greatly achieved as reported
by Kajitvichyanukul et al. [7]. Mamane H et al. [13] studied
inactivation of microbiocidal (E. coli and B. subtilis spores)
and virucidal (MS2, T4, and T7 phage) using UV/H2O2. The
effect of presence of inorganic salt on advanced oxidation was
investigated by Riga et al. [19]. However, Fürhaker et al. [4]
reported that MDEA is non-biodegradable in the batch
bioreactor during the test period of 28 days. There is a well
recognized suggestion to treat the MDEA to become
biodegradable. It is suggested that advance oxidation processes
(AOP’s) are effective to degrade or reduce the toxicity of
organic contaminant. Further, the UV/H2O2 process in
evaluation to other methods of water treatment has additional
advantages such as no formation of sludge during the treatment
and high removal rate of organic contaminant. Consequently, a
study on the degradation process of MDEA using UV/H2O2
that may possible as pre-treatment of the MDEA waste before
biological oxidation was conducted in this research.
II.
EASE MATERIAL AND METHOD
A. Reagent
MDEA, KMnO4, H2SO4 and Hydrogen Peroxide (H2O2)
were obtained from Merk (German) and NaOH was obtained
from RM Chemicals (Malaysia).
B. Experiment
The experiment is conducted in 400 ml stirred jacketed
glass reactor to monitor the progress mineralization. The
photoreactor is equipped with the low pressure Hg vapor lamp,
a currents-voltage control unit and hole for collect the sample.
A solution of the amine in desired concentration is prepared
and required amount of H2O2 is added in the amine solution.
The pH adjustment is carried out with NaOH and H2SO4. The
Process starts immediately and than the temperature is
maintained by circulating cooling water through the jacket.
Samples of the liquid are withdrawn from time to time and the
TOC of the samples are measured using TOC analyzer from
Shimadzu, while the H2O2 concentration is measured using
KMnO4 titration [14]. Furthermore, the nitrate ion is
determined using a Metrohm of 761 Compact Ion
Chromatography with Metrosep A supp 5-150 column and 3.2
mM Na2CO3/1.0 mM NaHCO3 as eluent. NH4 ion is
determined using Ammonia Nessler Method (Method 8038
Standard Methods for Examination of Water and wastewater
4500-NH3 B&C). After the UV/H2O2 process, the monitoring
of degradation product and un-reacted MDEA was determined
using high performance liquid chromatography (HPLC-Model
Agilent series 1100). YMC-Pack PolymerC18 reverse phase
column was used. Elucidation was using a mixture between
100mM Na2HPO4 and 100mM NaOH with ratio 60% of
Na2HPO4 and 40% of NaOH at pH of 12. UV light at 215 nm
was used as detector. Another column special for organic acid
determination was also used. The column was obtained from
Transgenomic (Model ORH801) and 0.01 N H2SO4 was used
as mobile phase. Refractive index was used as detector.
Degradation product determination was performed by
comparison of the sample with the standard compound.
III.
RESULT AND DISCUSSION
Degradation of MDEA in water solution by using
UV/H2O2 has been studied.
MDEA containing the
elements C, H, O and N, alkanolamines oxidation by hydroxyl
radical HO • is expected to be CO2, H2O, NH4 +, NO2- and
NO3- Klare et al. [2] presenting a possible mechanism of
secondary amine degradation by hydroxyl radical. NH3/NH4+
was identified as intermediate product in partially degraded
secondary amine. Low et al. [24] found ammonium ion (NH4+)
and nitrate ion (NO3-) in the degradation of organic containing
nitrogen atom using TiO2 mediated photo-catalytic.
In this study, the nitrate ion (NO3-), nitrite ion (NO2-) and
ammonia nitrogen (NH3-N) is generated during the oxidation
process of MDEA by using UV/H2O2. The profile of the
nitrate (NO3-) and nitrite (NO2-) was evaluated by using ion
chromatography (Metrohm-761 Compact IC) which was
equipped with Metrosep A Supp 5-150 column. The ammonia
nitrogen (NH3-N) was evaluated according to Ammonia
Nessler (Method 8038 Nitrogen). Figure 1 shows the profile of
inorganic by products during UV/H2O2 oxidation process.
Identification of the organic intermediate products after
UV/H2O2 process was made. HPLC was used to characterize
the intermediates. An Agilent series 1100 brand of HPLC was
used to monitor the degradation products and un-reacted
MDEA after UV/H2O2. YMC-Pack PolymerC18 reverse phase
column was used with 100mM Na2HPO4/100mM NaOH
(60/40, pH 12) as eluent and UV at 215 nm as detector. Flow
rate of the eluent was 1 ml/minute. Another specific column
for organic acid determination was also used. The column was
obtained from Transgenomic (Model ORH801) and 0.01 N
H2SO4 was used as mobile phase. Refractive index was used
as detector. Degradation product determination was performed
by comparison of the sample with the standard.
200
nitrate
nitrite
ammonia
Concentration (ppm)
180
160
140
120
100
80
60
40
20
0
0
50
100
Time (minute)
150
200
Figure 1. Inorganic by product profile during the UV/H2O2 process. MDEA =
2000 ppm (≈1000 ppm TOC), [H2O2]o concentration = 0.22 M, initial pH =
10.17, temperature = 30˚C and with intensity UV was 12.06 mW/cm2 for
radiation time = 180 minute.
Figure 3. Chromatogram from Transgenomic column of formic acid and
acetic acid during UV/H2O2 process. MDEA = 2000 ppm (≈1000 ppm TOC),
[H2O2]o concentration = 0.22 M, initial pH = 10.17, temperature = 30˚C and
with intensity UV was 12.06 mW/cm2 for radiation time = 180 minute.
Concentration (ppm)
formic acid
500
oxalic acid
450
acetic acid
400
350
300
250
200
150
100
50
0
0
50
100
150
200
Time (minute)
Figure 2. Chromatogram from YMC-PolymerC18 column of MDEA, partially
degraded MDEA, Glycine and Oxalic Acid. MDEA = 2000 ppm (≈1000 ppm
TOC), [H2O2]o concentration = 0.09 M, initial pH = 7.2, temperature = 30˚C
and with intensity UV was 12.06 mW/cm2 for radiation time = 180 minute.
Figure 4. Organic by product profile during the UV/H2O2 process. MDEA =
2000 ppm (≈1000 ppm TOC), [H2O2]o concentration = 0.22 M, initial pH =
10.17, temperature = 30˚C and with intensity UV was 12.06 mW/cm2 for
radiation time = 180 minute.
The chromatogram from YMC-Pack PolymerC18 column
is depicted in Figure 2. A few peaks are shown. At 30 minute
radiation time, one of intermediate products was identified as
glycine which had retention time at 3.9 minute. Peak for
MDEA appeared at 7.1 minute. At longer radiation time,
quantity of glycine was decreasing but another intermediate
product was appearing, which was identified as oxalic acid
with retention time at 3.7 minute. In addition, Figure 3 shows
the chromatogram from the Transgenomic column. Two
organic acids were detected. Those were acetic acid and
formic acid. Peak of formic acid appeared at 8.1 minutes and
peak for acetic acid appeared at 8.9 minutes. Organic by
product profile during UV/H2O2 process are presented in
Figure 4.
As reported by many researcher [2, 25, and 26],
degradation of compound containing amino functional by
hydroxyl radical is highly dependent on pH. In the low or
neutral pH, amino functional is protonated than resulting the
deactivation of α – CH bond [2 and 27]. Hence, a further
located C atom of the amino functional is oxidized.
Meanwhile, in the alkaline condition, free electron pair of
nitrogen atom can be attacked by hydroxyl radical [28 and 29].
This is due to the un-protonated of amino functional. Here the
sterical effect drives the ability of hydroxyl radical to attack the
electron pair of nitrogen atom [2]. Regarding to the degradation
of MDEA by hydroxyl radical, a degradation mechanism has
proposed. Since MDEA is a tertiary alkanolamine which are
containing two group of ethanol and one methyl, the hydroxyl
radical could not attack directly to the free electron pair even
though the pH condition was at alkaline (pH ≈ 10). This is due
to the sterical effect inhibition. Initially, hydroxyl radical
attacks α – CH from atom nitrogen to form water and organic
compound radical. This step called hydrogen abstraction.
MDEA has three α – CH bonds. Base on the observation of
organic by product profile which is depicted in Figure 4, the
first organic acid by product which was appearing is formic
acid. Hence, the mechanism hypothesis of MEDA degradation
by hydroxyl radical started from methyl group by hydrogen
abstraction and then continued by electrophilic addition for
further degraded to form the end products. Figure 5 shows the
MDEA degradation mechanism by hydroxyl radical.
IV.
CONCLUSION
UV/H2O2 is able to remove the TOC of wastewater
containing MDEA to a certain level. Glycine, oxalic acid,
acetic acid, formic acid, nitrite (NO2-), nitrate (NO3-) and
ammonia were identified as intermediate product during
UV/H2O2 oxidation of MDEA.
ACKNOWLEDGMENT
The authors are thankful to Universiti Teknologi Petronas
for the financial support and laboratory.
REFERENCES
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Containing MDEA using UV/H2O2 Advanced
Oxidation Process
Sabtanti Harimurti
Anisa Ur Rahmah
Department of Chemical Engineering
Universiti Teknologi Petronas
Seri Iskandar, Perak, MALAYSIA
sabtanti@gmail.com
Department of Chemical Engineering
Universiti Teknologi Petronas
Seri Iskandar, Perak, MALAYSIA
anisa.urrahmah@gmail.com
Abul Aziz Omar
Thanpalan Murugesan
Department of Chemical Engineering
Universiti Teknologi Petronas
Seri Iskandar, Perak, MALAYSIA
aaziz_omar@petronas.com.my
Department of Chemical Engineering
Universiti Teknologi Petronas
Seri Iskandar, Perak, MALAYSIA
murugesan@petronas.com.my
Abstract—Alkanolamines such as MEA, DEA, MDEA and DIPA
in aqueous solutions is frequently used for scrubbing carbon
dioxide from natural gas. High quantity of alkanolamine appears
in the wastewater during cleaning and maintenance as well as
shutdown of the absorption and desorption columns. The
alkanolamines waste is toxic to the environment and this
wastewater cannot be treated in the conventional wastewater
treatment. Advanced Oxidation Process (AOP) named UV/H2O2
is introduced to solve this problem. A laboratory set up of
degradation MDEA waste using UV/H2O2 was conducted and the
degradation mechanism has been studied. Glycine, oxalic acid,
acetic acid, nitrate ion, nitrite ion and ammonium ion are
identified as degradation intermediate product.
Keywords- MDEA; AOP’s; UV/H2O2; TOC
I.
INTRODUCTION
Natural gas is a major energy source in the world. It is one
of the cleanest, safe, and most useful of all energy sources.
World natural gas consumption rose by 2.5% from 2938.0
billion cubic meters in 2007 to 3018.7 billion cubic meters in
2008 [1]. Consequently, the production of natural gas is in
rise. Natural gas has a wide range of acid gas concentrations,
from parts per million to 50 volume percent and higher,
depending on the nature of the rock formation from which it
comes. Because of the corrosiveness of H2S and CO2 in the
presence of water and because of the toxicity of H2S and the
lack of heating value of CO2, sales gas is required to be
sweetened.
Alkanolamine in water solution are extensively used for
scrubbing certain acidic gases in the natural gas production.
The most utilized alkanolamines for scrubbing acidic gases are
978-1-4577-1884-7/11/$26.00 ©2011 IEEE
monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA) and di-isopropanolamine (DIPA)
[11]. The alkanolamines are regenerated in stripping tower for
recycling back to the absorber. During shutdown and
maintenance of these facilities, huge concentrations of residual
alkanolamine may be carried over into the wastewater. As a
result, they able disturb the biological treatment system of the
plant.
In the recent two decade, advanced oxidation processes
(AOP’s) have proved to be very important in the degradation
of high concentrations of organics which may be difficult to
treat in a conventional biological oxidation unit. This process
is involving a very reactive species named hydroxyl radical
(•OH). Fenton’s treatment, UV/O3 and UV/H2O2 are the
popular technique for generation of hydroxyl radical. In this
paper will be explained concerning the MDEA degradation
mechanism by hydroxyl radical in the water solution. The
UV/H2O2 process in evaluation to other methods of water
treatment has additional advantages such as no formation of
sludge during the treatment, high ability on producing of
hydroxyl radical and applicable in the wide range of pH. In
UV/H2O2 process oxidation, Equation (1) is generally
accepted to generate the hydroxyl radical •OH [6, 8, 16, and
23]:
H2O2 + hv → 2•OH
(1)
The reaction rate of organic contaminant degradation is
usually controlled by the rate of generation of •OH radicals,
which depend upon the concentrations of H2O2 and UV dose.
A large number of experimental works on the application of
advanced oxidation processes (AOP’s) to treat high-strength of
waste have been reported in the literature. The use of Fenton’
reagent in the AOP’s was conducted to degrade MEA
(monoethanolamine) waste [20, 21, and 22]. Putri et al. [17]
also used the Fenton’s reagent to study the degradation of
salfolen. In addition, Raihan et al. [18] studied the effect of
continuous addition mode of Fenton’s reagent on
mineralization of di-isopropanolamine (DIPA). The continuous
addition mode of Fenton’s reagent was better than the one time
addition mode. Meanwhile, the used of UV/H2O2 in the
advance oxidation process had been also examined. Idhzam et
al. [5] conducted the degradation of MEA. Malik and Sanyal
[12] and Muruganandham and Swamithan [15] studied the
The
effectiveness of UV/H2O2 to decolorize azo-dye.
degradation of 4-nitrophenol using UV/H2O2 was reported by
Daneshvar et al. [3]. Lopez et al. [10] successfully degraded the
4-chloro-3,5-dinitrobenzoic acid (CDNBA) in aqueous
solution. Degradation and detoxification of formaline
wastewater using UV/H2O2 was greatly achieved as reported
by Kajitvichyanukul et al. [7]. Mamane H et al. [13] studied
inactivation of microbiocidal (E. coli and B. subtilis spores)
and virucidal (MS2, T4, and T7 phage) using UV/H2O2. The
effect of presence of inorganic salt on advanced oxidation was
investigated by Riga et al. [19]. However, Fürhaker et al. [4]
reported that MDEA is non-biodegradable in the batch
bioreactor during the test period of 28 days. There is a well
recognized suggestion to treat the MDEA to become
biodegradable. It is suggested that advance oxidation processes
(AOP’s) are effective to degrade or reduce the toxicity of
organic contaminant. Further, the UV/H2O2 process in
evaluation to other methods of water treatment has additional
advantages such as no formation of sludge during the treatment
and high removal rate of organic contaminant. Consequently, a
study on the degradation process of MDEA using UV/H2O2
that may possible as pre-treatment of the MDEA waste before
biological oxidation was conducted in this research.
II.
EASE MATERIAL AND METHOD
A. Reagent
MDEA, KMnO4, H2SO4 and Hydrogen Peroxide (H2O2)
were obtained from Merk (German) and NaOH was obtained
from RM Chemicals (Malaysia).
B. Experiment
The experiment is conducted in 400 ml stirred jacketed
glass reactor to monitor the progress mineralization. The
photoreactor is equipped with the low pressure Hg vapor lamp,
a currents-voltage control unit and hole for collect the sample.
A solution of the amine in desired concentration is prepared
and required amount of H2O2 is added in the amine solution.
The pH adjustment is carried out with NaOH and H2SO4. The
Process starts immediately and than the temperature is
maintained by circulating cooling water through the jacket.
Samples of the liquid are withdrawn from time to time and the
TOC of the samples are measured using TOC analyzer from
Shimadzu, while the H2O2 concentration is measured using
KMnO4 titration [14]. Furthermore, the nitrate ion is
determined using a Metrohm of 761 Compact Ion
Chromatography with Metrosep A supp 5-150 column and 3.2
mM Na2CO3/1.0 mM NaHCO3 as eluent. NH4 ion is
determined using Ammonia Nessler Method (Method 8038
Standard Methods for Examination of Water and wastewater
4500-NH3 B&C). After the UV/H2O2 process, the monitoring
of degradation product and un-reacted MDEA was determined
using high performance liquid chromatography (HPLC-Model
Agilent series 1100). YMC-Pack PolymerC18 reverse phase
column was used. Elucidation was using a mixture between
100mM Na2HPO4 and 100mM NaOH with ratio 60% of
Na2HPO4 and 40% of NaOH at pH of 12. UV light at 215 nm
was used as detector. Another column special for organic acid
determination was also used. The column was obtained from
Transgenomic (Model ORH801) and 0.01 N H2SO4 was used
as mobile phase. Refractive index was used as detector.
Degradation product determination was performed by
comparison of the sample with the standard compound.
III.
RESULT AND DISCUSSION
Degradation of MDEA in water solution by using
UV/H2O2 has been studied.
MDEA containing the
elements C, H, O and N, alkanolamines oxidation by hydroxyl
radical HO • is expected to be CO2, H2O, NH4 +, NO2- and
NO3- Klare et al. [2] presenting a possible mechanism of
secondary amine degradation by hydroxyl radical. NH3/NH4+
was identified as intermediate product in partially degraded
secondary amine. Low et al. [24] found ammonium ion (NH4+)
and nitrate ion (NO3-) in the degradation of organic containing
nitrogen atom using TiO2 mediated photo-catalytic.
In this study, the nitrate ion (NO3-), nitrite ion (NO2-) and
ammonia nitrogen (NH3-N) is generated during the oxidation
process of MDEA by using UV/H2O2. The profile of the
nitrate (NO3-) and nitrite (NO2-) was evaluated by using ion
chromatography (Metrohm-761 Compact IC) which was
equipped with Metrosep A Supp 5-150 column. The ammonia
nitrogen (NH3-N) was evaluated according to Ammonia
Nessler (Method 8038 Nitrogen). Figure 1 shows the profile of
inorganic by products during UV/H2O2 oxidation process.
Identification of the organic intermediate products after
UV/H2O2 process was made. HPLC was used to characterize
the intermediates. An Agilent series 1100 brand of HPLC was
used to monitor the degradation products and un-reacted
MDEA after UV/H2O2. YMC-Pack PolymerC18 reverse phase
column was used with 100mM Na2HPO4/100mM NaOH
(60/40, pH 12) as eluent and UV at 215 nm as detector. Flow
rate of the eluent was 1 ml/minute. Another specific column
for organic acid determination was also used. The column was
obtained from Transgenomic (Model ORH801) and 0.01 N
H2SO4 was used as mobile phase. Refractive index was used
as detector. Degradation product determination was performed
by comparison of the sample with the standard.
200
nitrate
nitrite
ammonia
Concentration (ppm)
180
160
140
120
100
80
60
40
20
0
0
50
100
Time (minute)
150
200
Figure 1. Inorganic by product profile during the UV/H2O2 process. MDEA =
2000 ppm (≈1000 ppm TOC), [H2O2]o concentration = 0.22 M, initial pH =
10.17, temperature = 30˚C and with intensity UV was 12.06 mW/cm2 for
radiation time = 180 minute.
Figure 3. Chromatogram from Transgenomic column of formic acid and
acetic acid during UV/H2O2 process. MDEA = 2000 ppm (≈1000 ppm TOC),
[H2O2]o concentration = 0.22 M, initial pH = 10.17, temperature = 30˚C and
with intensity UV was 12.06 mW/cm2 for radiation time = 180 minute.
Concentration (ppm)
formic acid
500
oxalic acid
450
acetic acid
400
350
300
250
200
150
100
50
0
0
50
100
150
200
Time (minute)
Figure 2. Chromatogram from YMC-PolymerC18 column of MDEA, partially
degraded MDEA, Glycine and Oxalic Acid. MDEA = 2000 ppm (≈1000 ppm
TOC), [H2O2]o concentration = 0.09 M, initial pH = 7.2, temperature = 30˚C
and with intensity UV was 12.06 mW/cm2 for radiation time = 180 minute.
Figure 4. Organic by product profile during the UV/H2O2 process. MDEA =
2000 ppm (≈1000 ppm TOC), [H2O2]o concentration = 0.22 M, initial pH =
10.17, temperature = 30˚C and with intensity UV was 12.06 mW/cm2 for
radiation time = 180 minute.
The chromatogram from YMC-Pack PolymerC18 column
is depicted in Figure 2. A few peaks are shown. At 30 minute
radiation time, one of intermediate products was identified as
glycine which had retention time at 3.9 minute. Peak for
MDEA appeared at 7.1 minute. At longer radiation time,
quantity of glycine was decreasing but another intermediate
product was appearing, which was identified as oxalic acid
with retention time at 3.7 minute. In addition, Figure 3 shows
the chromatogram from the Transgenomic column. Two
organic acids were detected. Those were acetic acid and
formic acid. Peak of formic acid appeared at 8.1 minutes and
peak for acetic acid appeared at 8.9 minutes. Organic by
product profile during UV/H2O2 process are presented in
Figure 4.
As reported by many researcher [2, 25, and 26],
degradation of compound containing amino functional by
hydroxyl radical is highly dependent on pH. In the low or
neutral pH, amino functional is protonated than resulting the
deactivation of α – CH bond [2 and 27]. Hence, a further
located C atom of the amino functional is oxidized.
Meanwhile, in the alkaline condition, free electron pair of
nitrogen atom can be attacked by hydroxyl radical [28 and 29].
This is due to the un-protonated of amino functional. Here the
sterical effect drives the ability of hydroxyl radical to attack the
electron pair of nitrogen atom [2]. Regarding to the degradation
of MDEA by hydroxyl radical, a degradation mechanism has
proposed. Since MDEA is a tertiary alkanolamine which are
containing two group of ethanol and one methyl, the hydroxyl
radical could not attack directly to the free electron pair even
though the pH condition was at alkaline (pH ≈ 10). This is due
to the sterical effect inhibition. Initially, hydroxyl radical
attacks α – CH from atom nitrogen to form water and organic
compound radical. This step called hydrogen abstraction.
MDEA has three α – CH bonds. Base on the observation of
organic by product profile which is depicted in Figure 4, the
first organic acid by product which was appearing is formic
acid. Hence, the mechanism hypothesis of MEDA degradation
by hydroxyl radical started from methyl group by hydrogen
abstraction and then continued by electrophilic addition for
further degraded to form the end products. Figure 5 shows the
MDEA degradation mechanism by hydroxyl radical.
IV.
CONCLUSION
UV/H2O2 is able to remove the TOC of wastewater
containing MDEA to a certain level. Glycine, oxalic acid,
acetic acid, formic acid, nitrite (NO2-), nitrate (NO3-) and
ammonia were identified as intermediate product during
UV/H2O2 oxidation of MDEA.
ACKNOWLEDGMENT
The authors are thankful to Universiti Teknologi Petronas
for the financial support and laboratory.
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