Treatment of o-tert-Butyl phenol micro-polluted water with electro-oxidation and microporous aeration: Method development, performance evaluation and mechanism study

Article abstract of DOI:10.14233/ajchem.2016.19540

This study investigated the treatment of organic micro-pollutants in drinking water using a combination of electro-oxidation and microporous aeration (EOMA) technique. Results indicated that microporous aeration enhanced the turbulence of reaction solution and improved the efficiency of organic contaminant removal two-fold versus electro-oxidation alone. o-tert-Butyl phenol (OTBP) was used as a representative pollutant. 1600 mL OTBP solution contained 160 mg sodium sulfate and 2 mL 30 % hydrogen peroxide. When the current density was 5 mA cm^-2, 1 and 2 mg L^-1 o-tert-butyl phenol was removed up to 98.0 and 75.1 %, respectively. The major intermediate products included trimethylacetic acid, succinic acid and other acid. These have much less toxicity than o-tert-butyl phenol. After 30 min, the organics were mineralized completely. Electro-oxidation and microporous aeration was applied to actual source water that was contaminated by complicated organics. No toxicity was shown to algae growth after 15 min of treatment and total organic carbon was removed completely after 30 min.

Full text of DOI:10.14233/ajchem.2016.19540

Asian Journal of Chemistry; Vol. 28, No. 2 (2016), 450-454
A
SIAN
J
OURNAL OF HEMISTRY
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http://dx.doi.org/10.14233/ajchem.2016.19540
Treatment of o-tert-Butyl Phenol Micro-Polluted Water with Electro-Oxidation and Microporous
Aeration: Method Development, Performance Evaluation and Mechanism Study
Z. CHEN
Hangzhou No. 2 High School of Zhejiang Province, 76 Dongxin Avenue, Binjiang District, Hangzhou, Zhejiang Province, P.R. China
Corresponding author: Fax: +86 571 88320882; Tel: +86 571 88320881; E-mail: zhichen98@hotmail.com
Received: 30 July 2015;
Accepted: 22 September 2015;
Published online: 3 November 2015;
AJC-17627
This study investigated the treatment of organic micro-pollutants in drinking water using a combination of electro-oxidation and microporous
aeration (EOMA) technique. Results indicated that microporous aeration enhanced the turbulence of reaction solution and improved the
efficiency of organic contaminant removal two-fold versus electro-oxidation alone. o-tert-Butyl phenol (OTBP) was used as a representative
pollutant. 1600 mL OTBP solution contained 160 mg sodium sulfate and 2 mL 30 % hydrogen peroxide. When the current density
was 5 mA cm-2, 1 and 2 mg L-1 o-tert-butyl phenol was removed up to 98.0 and 75.1 %, respectively. The major intermediate products
included trimethylacetic acid, succinic acid and other acid. These have much less toxicity than o-tert-butyl phenol. After 30 min, the
organics were mineralized completely. Electro-oxidation and microporous aeration was applied to actual source water that was contaminated
by complicated organics. No toxicity was shown to algae growth after 15 min of treatment and total organic carbon was removed
completely after 30 min.
Keywords: Water treatment, Electro-oxidation, Microporous aeration, Organic micro-pollution, Removal mechanism.
for certain organic pollutants. The addition of large amounts
of carbon also requires long processing times. Pre-oxidation
usually involves using chemicals such as potassium perman-
ganate or chlorine oxidation and ozone–these chemicals
can lead to other problems. Potassium permanganate-based
oxidation is weak and overdoses of potassium permanganate
can cause colour changes and manganese contamination. For
chlorine oxidation, chlorine reacts with organic matter in water
and produce large amounts of harmful chlorination byproducts.
Though ozone is a strong oxidant and can oxidize most organic
materials, it is difficult to store and transport. Thus, it must
be prepared on site, which limits its application in many
emergencies.
Electro-oxidation can deplete organic contaminant via the
gain or loss of electrons on an electrode surface. It has been
used widely applied for industrial wastewater treatment [7].
Our study was based on the electro-oxidation mechanism,
which uses the microporous aeration technique to enhance
electro-oxidation for the organic micro-pollutant water. This
study investigated the efficiency of this technique to remove
o-tert-butyl phenol (OTBP) and studied the critical parameters
of the method followed by a hypothetical mechanism. The
results will be useful for additional applications of microporous
aeration that improve electro-oxidation.
INTRODUCTION
Quality water is a very important and basic issue that
ensures the safety of urban living. The safety of drinking water
is one of the top concerns of all countries. The technologies
used to treat drinking water have been developed very fast.
New techniques such as using ozone-biological activated
carbon for deep treatment have overcome limitations of
conventional methods that cannot efficiently remove organic
pollutants [1]. However, as more pollution accidents occur,
there is a further need to improve the safety of the water supply
[2]. Moreover, there have been many reports about the
abnormal smell of drinking water in some areas in China in
recent years. Many of these were caused by leakage during
chemical transportation or illegal industry wastewater dis-
charge. Such pollution accidents cause major damage to the
society, environment and economy.
Deep treatment process can only control regular organic
pollutants with very limited control of water source conta-
mination. Additional strategies are needed during these
incidences. Some usual methods include introducing large
amount of activated carbon or chemical pre-oxidation, but all
of those methods have pros and cons [3-6]. The efficiency of
activated carbon is very limited with poor or no absorbance

Vol. 28, No. 2 (2016)
Treatment of o-tert-Butyl Phenol Micro-Polluted Water with Electro-Oxidation and Microporous Aeration 451
total organic carbon (TOC) with an analyzer (Shimadzu,
TOCVCPH, Japan). A diluting plate count method was used
to investigate the method disinfection efficiency. The study
also checked the toxicity of a post-processing solution on chlorella
to evaluate the ecological safety of the technique.
EXPERIMENTAL
Fig. 1 shows a device designed in-house combining
electro-oxidation and microporous aeration (EOMA). The
device consists of a plexiglass reaction tank, a pump and a
power supply. The plexiglass reaction tank is divided into two
regions–a bottom region was installed with the aeration unit
and was connected to the pump by a glass rotameter. The rest
of the region was for electrolysis. This was installed with 3
cathode plates and 5 anode plates alternatively. The cathode
and anode plates were connected via a wire and a power converter.
The power converter convertsAC power to DC constant output
for an electro-oxidation reaction.
Analytical method: The key HPLC parameters included
a C18 column (4.6 mm × 250 m, 5 µm), column temperature
of 25 °C, mobile phase with a methanol-to-water ratio of 85/
15 (v/v), flow rate of 1.0 mL/min, detection wavelength of
230 nm and a sample loading volume of 20 µL. GC-MS was
used to measure OTBP products via SPME-GC/MS sample
preparation. The SPME extraction head was CAR/PDMS with
extraction and dissociation temperatures of 80 and 250 °C for
15 and 3 min, respectively. Chromatography employed a
capillary column DB-5MS (30 m × 0.25 mm × 0.25 µm) and
an injection temperature of 250 °C followed by a programmed
temperature gradient at 50 °C for 2 min. This was dropped to
10 °C and then increased to 250 °C for 5 min. The ion source
temperature was 230 °C, transfer tube was 250 °C and the
electron collision energy was 70 eV. The ecological toxicity test
used chlorella and was based on algae inhibition test methods
from the National Environment Protection Department [8].
Methods
o-tert-Butyl phenol (OTBP) electrolysis test: The effi-
ciency of three different techniques on the removal of OTBP
was tested. The OTBP solution (1.6 L; 5 mg L-1) was added
with sodium sulfate to a final concentration of 100 mg L-1.
This was then mixed with 2 mL 30 % hydrogen peroxide.
Three treatment methods were tested: microporous aeration,
electro-oxidation and both microporous aeration and electro-
oxidation. The OTBP concentration was measured by a high
performance liquid chromatography (HPLC) (USA Agilent
Technologies, 1200 series).
RESULTS AND DISCUSSION
Multiple experimental conditions were tested for the effi-
ciency of microporous aeration to enhance electro-oxidation
in a solution containing 1 mg L-1 OTBP. The variables included
the sodium sulfate concentration (50, 100, 150 and 200 mg L-1),
aeration rate (2, 3 and 4 L min-1) and the amount of hydrogen
peroxide (1.5, 2 and 2.5 mL). The final OTBP concentration
was measured via HPLC.
The oxidation products after microporous aeration treat-
ment were analyzed further. Electrolysis reaction was performed
with a 1 mg/L OTPB solution under optimized experimental
conditions obtained in the above optimization tests. A GC-
MS (USAWaters, GCT Premier) was employed for the product
analysis.
Comparison of the three methods: The o-tert-butyl
phenol (OTBP) solution was treated using the electro-oxidation
and microporous aeration (EOMA) device shown in Fig. 1
with an applied current density of 5 mA cm-2 and an aeration
rate of 4 L min-1. The sample was collected at 0, 7.5, 15, 22.5
and 30 min and the test results are shown in Table-1. The
efficiency of only the OTBP microporous aeration technique
was not clear–only 15 % of the OTBP was removed within 30
min. In comparison, using electro-aeration or microporous
aeration assisted electro-aeration showed a much more
pronounced change. The latter even showed as much change
as twice of the former indicating that the microporous aeration
enhanced electro-oxidation. This is because microporous
aeration enhanced the reactive fluid turbulence and promoted
transfer of material while improving the concentration of the
dissolved oxygen [9]. Therefore, the microporous aeration
technique can assist the electro-oxidation to remove the organic
pollutant.
Electrolysis testing on a mixed organic-contaminated
water sample:A contaminated water sample was used to make
1 mg L-1 OTBP and 1,4-dichloroethane. The tests were
performed with the optimized conditions but with different
treatment times. The results were evaluated by measuring the
Microporous aeration tube
Cathode plate
Anode plate
Na2SO4
H2O2
Power
Pump
Fig. 1. Diagram of EOMA

452 Chen
Asian J. Chem.
TABLE-1
REMOVAL EFFICIENCY OF OTBP BY MA, EO AND EOMA
density. Meanwhile, greater power was consumed at higher
current density. Considering multiple factors such as the current
efficiency, OTBP removal rate and effluent quality, the optimum
concentration of electrolyte sodium concentration was 100 mg L-1.
Microporous aeration flow rate: Increasing the amount
of microporous aeration will speed up the mass transfer rate
of dissolved oxygen and enhance the exchange rate of the
electrode surface material. This improves the removal of OTBP.
However, excessive aeration will make the reactants interact
with the electrode surface with insufficient time. This moves
more OTBP from the liquid phase into the gas phase, which is
not favourable for removal. Therefore studying the impact
of microporous aeration on OTBP removal is particularly
important.
Time (min)
7.5
MA
4.3
8.9
12.6
15.4
EO
25.4
42.3
75.2
100.0
EOMA
54.7
85.4
100.0
100.0
15.0
22.5
30.0
MA = Microporous aeration; EO = Electro-oxidation
Factors influencing removal of OTBP with EOMA:
The concentration of organic micro-pollution is typically very
low in water and thus the concentration of OTBP was only 1
mg L-1 in these experiments. A single factor method was used
to select one experimental condition at a time for testing while
keeping all other factors unchanged. HPLC was used for OTBP
measurement of a sample at different sampling times.
Effect of sodium sulfate concentration:The current density
directly affected the electrolysis during electro-oxidation and
the current density was affected by electrolyte concentration.
Because the electrolyte concentration in water is low and limited
the electrolysis, additional sodium sulfate was added as an
electrolyte. According to China’s “Standards of Healthy
Drinking Water” (GB5749-2006), the sulfate concentration
should not exceed 250 mg L-1. Therefore, our study tested 50,
100 and 200 mg L-1 sodium sulfate to not affect water quality.
Other parameters remained unchanged (aeration rate 4 L min-1;
2 mL hydrogen peroxide; sampling time of 10 min).
Fig. 2 shows that the removal of OTBP increased
significantly as the concentration of sodium sulfate increased
from 50 to 100 mg L-1. However, when the sodium concen-
tration was further increased to 150 mg L-1, the removal of
OTBP did not change. The removal decreased when the sodium
concentration increased to 200 mg L-1. The current density
was measured under different sodium sulfate concentrations
and was 2.5, 5.0, 9.5 and 15 mA cm-2. In theory, the increasing
current density is in favour of the removal of organic contami-
nants. But when current density is too high, it may cause an
oxygen evolution side reaction that reduces the removal
efficiency [7]. This experiment used a microporous aeration
technique that promoted the oxygen mass transfer. The oxygen
evolution side reaction could occur at an even lower current
Aeration flow rates of 2, 3 and 4 L min-1 were tested with
other parameters remaining constant (sodium sulfate 100 mg
L-1; 2 mL of hydrogen peroxide). The sampling time was 10
min and the results were shown in Fig. 3. With aeration alone,
the rate of aeration increase caused more OTBP removal due
to stripping. This was because with fast airflow, the OTBP
concentration between the gas phase and gas-liquid phase
becomes large. This leads to an increase in the mass transfer
driving force [10] and resulted in more OTBP turning into the
vapour phase. Although the increase in aeration can reduce
the OTBP in the aqueous phase, it causes a pollutant transfer
between the different phases. This results in airborne pollutants.
When both microporous aeration and electro-oxidation were
applied, the removal of OTBP rate increased and then decreased
as the aeration rate increased. This suggests the presence of
an optimum aeration value. Conclusively, we selected 3 L min-1
as the best aeration rate.
100
MA
EOMA
80
60
40
20
0
100
80
60
40
20
0
2
5
8
Aeration quantity (mL min-1)
Fig. 3. Effect of aeration quantity on removal efficiency of OTBP
Amount of hydrogen peroxide: To increase the effect of
microporous aeration-assisted electro-oxidation, 30 % hydrogen
peroxide was used. Hydrogen peroxide can produce more
hydroxyl radicals (OH) in a relatively short period of time to
strengthen the removal of pollutants. Four hydrogen peroxide
volumes were studied–0, 1.5, 2.0 and 2.5 mL. Other parameters
remained unchanged: sodium sulfate 100 mg L-1, aeration rate
3 L min-1 and total volume 1600 mL.
50
100
Na2SO4 (mg L-1)
Fig. 2. Effect of Na2SO4 concentration on EOMA
200
150

Vol. 28, No. 2 (2016)
Treatment of o-tert-Butyl Phenol Micro-Polluted Water with Electro-Oxidation and Microporous Aeration 453
Fig. 4 shows that the addition of hydrogen peroxide
dramatically improved the removal of OTBP. The OTBP
removal rate was 54, 65, 82 and 76 % when the addition of
hydrogen peroxide was 0, 1.5, 2.0 and 2.5 mL, respectively.
This is because hydrogen peroxide forms radical •OH rapidly
on the electrode surface to facilitate oxidation. More OH is
produced when an excess of hydrogen peroxide is added, but
microporous aeration 3 L min-1, 2 mL of 30 % hydrogen
peroxide and a current density of 5 mA cm-2. As shown in
Fig. 5, 1 mg L-1 and 2 mg L-1 OTBP solutions had 98 and 75.1 %
lower OTBP concentrations, respectively. No signal was
detected after 30 min. o-tert-Butyl phenol is a phenol and
produces a slight phenolic odor. After treatment for 15 min
with EOMA, the odor was clearly reduced in both solutions
and was removed completely at 30 min.
•
too much H2O2 will react with OH to produce H2O and O2.
This not only reduces the OH concentration, but also consumes
H2O2 and makes hydrogen peroxide unable to participate in
electro-oxidation [11]. Thus, the optimum amount of hydrogen
peroxide was determined to be 2 mL.
2 mg L–1 5 mA cm–2
1 mg L–1 5 mA cm–2
2
1
0
0 mL H2O2
1.5 mL H2O2
2 mL H2O2
2.5 mL H2O2
100
80
60
40
20
0
0
10
20
30
Time (min)
Fig. 5. Effect of initial concentration on OTBP removal efficiency by EOMA
The formed products from the OTBP solution after EOMA
treatment were analyzed with headspace GC-MS (Table-2).
The results indicated that the electrolysis reaction time affects
the product type and amount directly. We proposed the
following degradation mechanism according to the product
eluting sequence and previous reports [14]. o-tert-Butyl phenol
was first converted to tert-butyl-o-benzoquinone, which then
generated trimethylacetic acid and succinic acid after ring
opening. Both of these can be further oxidized to formic acid,
acetic acid and other small organic molecules. These are
eventually mineralized to CO2 and H2O at 30 min.
0
5
10
15
20
Time (min)
Fig. 4. Effect of H2O2 amount on removal efficiency of OTBP
Mechanism of removal of OTBP by EOMA: Electro-
oxidation is an oxidation-redox reaction occurring on an
electrode surface at an applied current. Organic molecules
become harmless substances by losing electrons. Meanwhile,
nascent oxygen can also oxidize the organic pollutants in water.
The specific chemical reaction mechanisms are [12]:
4OH• + organic compounds → Other small
organic or inorganic compounds
TABLE-2
4OH• – 4e– → 2H2O + O2↑
MAIN INTERMEDIATES OF OTBP BY EOMA
WITH DIFFERENT TREATMENT TIMES
Organic compounds + O2 → Other small
Time (min)
10.0
Main intermediates
Butyl benzoquinone
Pivalic acid, succinic acid
Formic acid, acetic acid, etc.
–
organic or inorganic compounds
Organic compounds – e– → Other small
15.0
22.5
30.0
organic or inorganic compounds
We used hydrogen peroxide, which is not only a strong
oxidant, but can also quickly generate other strong oxidizing
materials such as oxygen and ozone. The mechanism for ozone
production is below [13].
The contaminated water treated with EOMA is micro-
polluted drinking water and thus the toxicity after treatment
needs to be evaluated. Table-3 presents toxicity data for OTBP
and major intermediate products [15]. The data shows that the
toxicity of the intermediate products after treatment were lower
than the OTBP indicating that the treatment process can quickly
improve water safety and reduce toxicity.
Treatment of organic micro-pollutants in a practical
water sample: A real water sample was provided by a water
company in Hangzhou China. This was doped with 1 mg L-1
OTBP and 1 mg L-1 tetrachloroethane solution. After treating
with our method, the depletion of organic molecules was analyzed
3O2 → 2O3
O3 + Organic compounds → small organic acids or CO2 + H2O
Based on the above analysis, two major mechanisms for
removal of OTBP were involved in our study i.e., direct elec-
trolysis and electrolysis-produced strong active oxidant
reagents including ozone and oxygen.
We studied OTBP at two concentrations (1 and 2 mg L-1).
All the other major parameters were optimized based on
empirical tests. Ideal values were sodium sulfate 100 mg L-1,

454 Chen
Asian J. Chem.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
TABLE-3
TOXICITY OF OTBP AND ITS MAIN INTERMEDIATES
Control
0 min
5 min
10 min
15 min
20 min
30 min
Substance
OTBP
Pivalic acid
Acute toxicity
Adult mouse abdominal cavity LD50 = 82 mg kg-1
Adult mouse abdominal cavity LD50 = 900 mg kg-1
Succinic acid Adult mouse abdominal cavity LD50 = 2702 mg kg-1
Formic acid
Adult mouse abdominal cavity LD50 = 940 mg kg-1
Young rat oral LD50 = 1100 mg kg-1
Acetic acid
Young rat oral LD50 = 3310 mg kg-1
by the total organic carbon in the solution and the ecological
toxicity. The sample was assayed at 0, 7.5, 15, 22.5 and 30
min (Fig. 6). After 30 min, the original concentration of total
organic carbon 2.1 mg L-1 completely eliminated. This demons-
trated that the technique can treat complicated polluted water
by removing various organic contaminants.
1
2
3
4
5
6
7
Time (days)
Fig. 7. Effect of reaction time on growth of algae
A water sample provided by a water company was doped with
OTBP and tetrachloroethane to mimic organic micro-polluted
drinking water and used to further evaluate EOMA. After 30
min, the total organic carbon value of 2.1 mg L-1 at baseline
was completely gone. Most microorganisms were removed
indicating little or no disinfectant is needed. Ecological toxicity
testing showed no decrease in algae growth after treating the
solution for 15 min. This suggests that the solution was safe to
aquatic ecosystems.
2
1
0
ACKNOWLEDGEMENTS
The author thanks Mr. Suhao Lin and Mr. Yanlong Chen
from Hangzhou No. 2 High School of Zhejiang Province for
their support. Thanks are also due to College of Biological and
Environment Engineering, Zhejiang University of Technology
for providing experimental materials and facilities.
0.0
7.5
15.0
Time (min)
22.5
30.0
Fig. 6. Removal of OTBP and tetrachloroethane by EOMA
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Conclusion
We treated organic micro-polluted water with the EOMA
technique. The data show that the EOMA method is twice as
efficient in the removal of organic pollutants as electro-oxidation
alone. We empirically optimized the parameters to be: 1600 mL
total volume (1 mg L-1 or 2 mg L-1 OTBP) containing 100 mg L-1
sodium sulfate and 2 mL 30 % hydrogen peroxide with a micro-
porous aeration flow rate of 3 mL min-1 and a current density of
5 mA cm-2. o-tert-Butyl phenol solutions of 1 and 2 mg L-1 were
removed at 98.0 and 75.1 %, respectively. The major products
include trimethylacetic acid, succinic acid, formic acid and acetic
acid–these had much less toxicity than OTBP. After 30 min, the
organics were mineralized completely with no odor detected.