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Titanium Dioxide Mediated Photocatalytic Degradation of Methylene Blue in a Fixed Film-Type Photoreactor

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AATCC Review
DOI:
10.14504/ar.14.1.3
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January, 2014
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Student Research

Titanium Dioxide Mediated Photocatalytic
Degradation of Methylene Blue in a Fixed
Film-Type Photoreactor
By Gayathri RamMohan, Paul Rosenberger, Samriddhi Buxy, and Pratap Pullammanappallil (advisor),
University of Florida; and Tushar Goswami (advisor), Innovative Materials Development Company
Note: This paper tied for 1st place in the Herman and Myrtle Goldstein Student Paper Competition at the
AATCC International Conference, April 9-11, 2013, in Greenville, SC, USA.
Abstract
A novel, cost effective and reusable TiO2-coated nonwoven media was used to investigate the photocatalytic
degradation of methylene blue (MB)-containing wastewater. A falling film-type photoreactor with
recirculation was designed and operated inside a black box set up with provision for three 15 W UV 350 nm
lamps. Photocatalytic experiments were conducted at an initial pH= 7 and 25 °C. Dye photodegradation was
studied by the change in absorbance at 660 nm using a UV-Vis spectrophotometer. Time period of exposure,
initial dye concentrations (5, 10, and 15 mg/L), and mixing were investigated. More than 95% decolorization
was obtained within 4, 5, and 9 h for 5, 10, and 15 mg/L of dye-containing wastewater, respectively. Results
were compared with experiments using TiO2-powdered catalysts.
Key Terms
Decolorization, Methylene Blue, Photocatalysis, Titanium Dioxide, UV

Introduction
Conventional wastewater treatments usually remove
organic dyes using coagulants or adsorbents, as
biological treatment is ineffective for degradation of
synthetic dyes. However, changes in environmental
laws have led to categorizing such spent adsorbents
or sludge as hazardous waste that require further
processing. An attractive, cost effective alternative is
the advanced oxidation process (AOP). The objective of an AOP design is to generate OH* radicals
that non-selectively attack compounds that may
otherwise be recalcitrant to conventional oxidation
techniques. AOPs may involve combining chemical
reagents such as ozone, hydrogen peroxide, Fe; nton’s
reagent, and auxiliary energy sources such as ultraviolet or solar radiations.1,2
In recent times, titanium dioxide (TiO2)-mediated
photocatalysis has been extensively investigated for
environmental purification applications.3-7 Heterogeneous TiO2 photocatalysis is an attractive technique
for complete degradation of organic pollutants in
wastewaters. Various advantages of this process
include the stability and nontoxicity of TiO2, the use
of solar irradiation for catalyst activation, and low
energy consumption due to the ability to operate the
process at ambient temperatures.6,8
40 | AATCC Review

Vol. 14, No. 1

January/February 2014

In the literature, advanced oxidation studies using
TiO2 have mainly been carried out with powdered
catalyst suspended and agitated in the wastewater.4
Some modifications of this process include coating inert solid substrates such as activated carbon,
silica gel, or zeolite with TiO2 powder. As most of
these adsorbents are granular, such coated particles
are also suspended and agitated in wastewaters,
thus requiring an additional separation step such
as filtration.9 Slurry-type reactors pose two serious limitations in practical applications. First, they
suffer from the need for longer retention times due
to smaller diameter of aggregated TiO2 particles
that reduce their settling velocity. Second, the use
of higher dosage of TiO2 powder to achieve higher
rates of photocatalysis affects UV penetration due to
the “shadowing effect.”6 On the other hand, immobilizing TiO2 powder on support materials such as
sand particles or ceramic membranes suffers from
limitations of mass transfer. Studies have shown that
a part of the porous structure is lost through the
heating process that is used to fix the photocatalyst
on the support material. However, treating wastewaters using a fixed film apparatus may be a better
system for easily scaling up the technology. A number of immobilized systems have been investigated
for photocatalytic treatment including fixed bed,

Student Research

tubular annular, elliptical, film type, and fluidized
bed photoreactors.5,6,8,9

exposed to UV. Wastewater samples treated in slurry
type systems were mixed using a magnetic stirrer.

In the present research, a falling film-type photoreactor was designed to successfully carry out
photocatalytic degradation of methylene blue (MB)
dye using TiO2-coated nonwoven media and an
artificial UV light source. MB is an organic molecule
that has less absorption at the absorption edge of
anatase phase of TiO2 and is relatively strong and
stable under UV radiation.10 Therefore, it was chosen
as a model compound in the present study.

A weighing balance was used to measure the mass of
the tray holding the TiO2 sheets and wastewater. Any
loss of water due to evaporation was compensated by
adding DI water.

Materials and Methods
MB-containing Wastewater

Test wastewater was prepared by dissolving 5, 10,
and 15 mg of analytical grade MB powder in 1 L of
deionized (DI) water in three different beakers. The
pH of the dye-containing wastewater was adjusted to
7 by adding 5 N NaOH.

Titanium Dioxide-coated Sheets

TiO2-coated nonwoven media, developed using
proprietary technology and supplied by Innovative Materials Development Company (IMDC),
Gainesville, FL, USA was used in the photocatalytic
experiments. The TiO2 content on these sheets was
3 mg TiO2/cm2 of the sheet. The TiO2-coated sheets
were reused in the photocatalytic experiments conducted to degrade MB.

UV Lamps

UV lamps were also provided by IMDC. Three 15
W, 350 nm ultraviolet lamps, G15T8, 18” long were
used in this study.

Photoreactor Assembly

The three UV lamps, spaced 5-cm apart, were
assembled together using a standard size holder. For
unmixed photocatalytic experiments, a 33 × 23 cm
tray was used to hold the TiO2-coated sheet and the
dye-containing wastewater. In the falling film-type
reactor set up, the tray was built with reservoirs on
both the ends and was raised by an inch on one end
to provide inclination for free flow of the treated
wastewater. A pump was connected to the reservoirs
for continuous recirculation. The entire assembly
(lamps and tray) was placed inside a box. The distance between the UV lamps and surface of tray was
about 5 cm and this was kept constant throughout
the study. In experiments using TiO2-powdered catalyst, a measured amount of TiO2 powder was mixed
with a known volume of sample and the mixture was

Method

All photocatalytic experiments were carried out in
batch mode. A TiO2-coated sheet was placed on
the tray. A known volume of the MB solution was
poured over the sheet and allowed to equilibrate in
the dark for less than an hour prior to UV exposure.
The pH of the MB solution was adjusted by dropwise
addition of 5 N sulfuric acid or 5 N sodium hydroxide and monitored using an Orion Benchtop pH
meter. The progress of decolorization of the sample
was monitored by analyzing samples exposed for
varying time periods for absorbance values at 660
nm using a UV-Vis spectrophotometer.
Control experiments were carried out with (i)
TiO2-coated sheets in the dark, and with (ii) UV
lamps in the absence of TiO2-coated sheets. Control
studies were carried out to ensure decolorization of MB occurred as a result of UV-activated
TiO2-mediated photocatalysis.
Percent decolorization was determined using Eq. 1.
% Decolorization = 100 × (Ao – A)/Ao

Eq. 1

Ao and A are the absorbance measurements at t = 0
and at time t, respectively.

Results and Discussion
Mechanism of MB Degradation

A number of possible reactions may occur on irradiation of TiO2 with UV. The mechanism by which
TiO2 catalyst degrades MB dye is described below:
(1) TiO2 + hϒ à TiO2 (e- + h+)
(2) TiO2 (e-) + O2 à O2*(3) TiO2 (h+) + H2O à TiO2 + OH* + H+
(4) O2*- + H+ à HO2*
(5) 2HO2* à H2O2 + O2
(6) TiO2 (e-) + H2O2 à TiO2 + OH- + OH*
The anatase phase of TiO2 has a band gap of 3.2 eV
and when UV light (350 nm) is incident on TiO2,
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Student Research

electron-hole pairs are generated.
Such photo-generated electronhole pairs participate in surface
redox reactions leading to formation of hydroxyl radicals. In the
scheme listed above, electrons
react with surface adsorbed
O2 to form superoxide radicals
and holes react with water to
form OH* radicals and H+ ions.
Following the generation of
superoxide, Steps 4 and 5 occur
leading to generation of hydrogen peroxide (H2O2). Finally,
H2O2 molecules react with free
electrons in the conduction
band, generating more hydroxyl
radicals. Hydroxyl radicals have
a high oxidation potential of 2.8
V and act as primary oxidants,
degrading the organic dye molecules as in Step 7.

Fig. 1. pH profile of photocatalytically-treated MB.

(7) MB + OH* à Degradation products
Control experiments carried out using aqueous MB
solution showed that when TiO2 alone was used in
the absence of UV, no significant decolorization (<
5% due to absorption) occurred and when UV was
used to degrade MB in the absence of TiO2, less than
17% decolorization occurred even after continuous
exposure for 6 h. However, TiO2 media in the presence of UV produced more than 95% decolorization
in the photocatalytic experiments.

Effect of pH on MB Degradation

The pH of the dye-containing wastewater plays
a very important role in determining the charge
on the surface of the catalyst. Since the zero point
of charge for TiO2 is at pH 6.8, the surface of the
catalyst is negatively charged. Presence of such
negatively-charged sites on the catalyst surface
causes electrostatic absorption of cationic dyes such
as MB. However, at acidic pH, electrostatic repulsion
occurs between the positively-charged TiO2 surface
and the cationic dye molecules. Studies by Joshi, et
al., have shown that degradation of MB increased
from 34.2% to 91.4% when the pH was increased
from 4 to 7 and later decreased to 77% when the pH
was increased further to 14.11 In this present work,
the initial pH of the dye-containing wastewater was
adjusted to neutral (pH 7) and the change in pH was
monitored throughout the course of the experi42 | AATCC Review

Vol. 14, No. 1

January/February 2014

ments. Fig. 1 shows the trend in pH achieved during
photocatalytic treatment of 5, 10, and 15 mg/L of
dye-containing wastewater. A similarity in pH trend
was observed in all the above-mentioned cases.
Although the initial pH was adjusted to 7, during
the course of the experiments, the pH dropped to a
little below 5.5 during the first 2-3 h and then stabilized around 6 ± 0.2.

Effect of Initial Dye Concentration

Three different dye concentrations; 5, 10, and 15
mg/L were chosen to investigate the rate of photocatalysis using reusable TiO2-coated nonwoven
media. About 2.7 L of dye containing wastewater
was treated photocatalytically in the falling film-type
photoreactor equipped with recirculation. Fig. 2
shows the degradation profile measured as change
in absorbance. The plot of A/A0 vs. time period of
exposure shows that dilute dye solutions with 5
mg/L initial dye concentration were photocatalytically treated to obtain 97% decolorization within
4 h, while 10 mg/L and 15 mg/L solutions needed
longer time periods of exposure (i.e., 5 and 9 h to
achieve 97% and 95% decolorization, respectively).
The absorbance data approximately followed firstorder kinetics and was fit with a first-order model
to determine the rate constants. The first-order rate
constants obtained from 5, 10, and 15 mg/L dye
containing wastewater treatments averaged to about
0.892, 0.613, and 0.237 h-1, respectively. On com-

Student Research

the falling film-type reactor.
In the unmixed system, 90%
decolorization was achieved
after 14 h of exposure to the
photoreactor setup. The firstorder rate constant obtained
from this study was 0.216 h-1.
The kinetics of the process was
four times faster due to the
complete mixing in the falling
film-type reactor achieved as a
result of recirculation.

Immobilized TiO2 vs.
TiO2-powdered Catalyst

The results obtained from
photocatalytic degradation of
MB in a falling film-type photoreactor set up was compared
with results from a slurry-type
reactor set up. As discussed
Fig. 2. Change in absorbance of photocatalytically-treated MB solution measured at 660 nm.
earlier, photocatalytic rates
are limited by catalyst dosage
paring the rate constants, it can be clearly seen that
used in slurry-type reactors. In this study, various
dilute wastewaters with 5 mg/L initial dye concencatalyst doses (0.1–1 g) were investigated for 5, 10,
tration had the fastest kinetics, followed by 10 and
and 15 mg/L dye-containing wastewaters. For dilute
15 mg/L dye-containing solutions, respectively.
wastewaters (5 mg/L dye), 85% decolorization was
As dye concentration increased, the process took
achieved after 4 h of exposure to TiO2/UV set up,
longer time periods to obtain greater extent of color
while 95% decolorization was achieved on increasremoval. Such a trend is in agreement with many
ing the TiO2 dosing from 0.02 g to 0.1 g per 200 mL
other studies carried out on photocatalytic degraof wastewater, respectively. Similarly, with 10 mg/L
dation of MB that investigated the effect of initial
and 15 mg/L MB solutions, more than 95% decolor5,6,9
dye concentration.
ization was obtained using 0.5–1 g of TiO2 per 200
Effect of Mixing On Dye Degradation
mL of wastewater after 4 h of treatment.
Photocatalytic experiments were carried out
Although the results obtained were similar to the
using the TiO2-coated nonwoven media in mixed
falling film-type photoreactors, slurry-type systems
and unmixed systems and the first-order kinetic
suffer from a few serious drawbacks. The volume
constants were compared to understand the imporof wastewater treated in slurry-type systems is less
tance of mixing in photocatalytic treatment. In
than one-tenth the volume treated in falling film
the unmixed experiments, there was a limit on the
reactors. Mainly, with the slurry-type systems, there
amount of wastewater that can be treated within a
is a limit to how high a TiO2 dosage can be applied.
given period of time. Since photocatalysis involves
On increasing the amount of catalyst to more than 1
reactions occurring on the surface of the catalyst,
g per 200 mL of sample, UV penetration is signifithe amount of bulk liquid that comes in contact
cantly reduced. This means that the very purpose of
with the TiO2 surface plays a significant role in
increasing the catalyst dosage is defeated as photodetermining the effectiveness of the treatment. In
catalytic rates are affected by the limited penetration
all the photocatalytic experiments, the depth of
of UV light. Secondly, since TiO2 is applied in
the sample treated was restricted to 3–4 mm above
powdered form, a post solid-liquid separation step
the surface of the media. This depth was found to
such as filtration or centrifugation is mandatory
be optimal based on previous research.2 Therefore,
to recover the treated water. Such drawbacks are
only 227 mL of dye-containing wastewater was
overcome in the falling film-type reactor. Since the
treated in the unmixed system compared to 2.7 L in
treated water is continuously recycled, more volume
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Vol. 14, No. 1

AATCC Review | 43

Student Research

can be treated within a given time frame while still
maintaining the 4 mm optimal thickness of wastewater applied on the TiO2-coated media. This helps
overcomes any limitation with UV penetration.
Using immobilized form of the catalyst also helps
overcome any need for post separation while being
able to achieve similar rates of photocatalysis.

Reusability of TiO2-coated Nonwoven Media

Table I presents the first-order rate constants for
various photocatalytic experiments conducted on
MB (5, 10, and 15 mg/L)-containing wastewaters
using TiO2-coated nonwoven media. Experiments
for each initial dye concentration were conducted in
duplicates, amounting to a total of six experiments
conducted using a single TiO2-coated nonwoven
media. Following these experiments, four successive
runs were conducted using 5 mg/L MB wastewater.
The photocatalytic efficiency of the photoreactor
setup was reduced by less than 10% by the end of 10
successive runs.
Table I.
First-order Kinetics of Photocatalytic Degradation of MB
TiO2 Catalyst
Form
Immobilized
TiO2-coated
nonwoven
media

Description

% Decolorization

First-order Rate
Constants (h-1)

Unmixed 5m/L

90

0.216

FFa 5mg/L

97

0.892

FF 10 mg/L

97

0.613

FF 15 mg/L

95

0.237

FF is falling film-type reactor.

one-tenth the volume treated in falling film design.
Such a design helped overcome the drawbacks such
as limitation on catalyst dosage and UV penetration
that significantly affect the rates of photocatalysis.

References
1.
2.

3.
4.
5.
6.
7.
8.
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Radjenovic, J., et al., Applied Catalysis B-Environmental, Vol.
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Mohan, G. R., An Integrated Technology for Recovery of
Energy, Nutrients and Clean Water from Cellulosic Ethanol
Stillage, Agricultural and Biological Engineering, University
of Florida, Gainesville, FL, USA, 2012, pp85–105.
Rajvanshi, A. K. and N. Nimbkar, Solar Detoxification of
Distillery Waste, Nimbkar Agricultural Research Institute,
Maharashtra, India, 2004.
Fotiadis, C., N. P. Xekoukoulotakis, and D. Mantzavinos,
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pp77–83.
Ling, C. M., A. R. Mohamed, and S. Bhatia, Chemosphere,
Vol. 57, No. 7, 2004, pp547–554.
Tehrani, F. M. K., et al., International Journal of
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Korean Journal of Chemical Engineering, Vol. 25, No. 1,
2008, pp64–72.
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and Technology, Vol. I, edited by Y. Wang, et al., 2007,
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pp280–283.
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Nano Dimension, Vol. 2, No. 4, 2011, pp241–252.

a

Corresponding Author (Advisor)

Conclusions
A novel, cost effective, reusable TiO2-coated nonwoven media developed and supplied by IMDC
was used to successfully investigate the photodegradation of MB-containing wastewater. A falling
film-type photoreactor with recirculation was
designed and operated inside a black box set up.
TiO2/UV-mediated photocatalytic experiments were
conducted at an initial pH 7 and temperature 25
°C. Effect of initial dye concentration, mixing, and
treatment time were investigated. More than 95%
decolorization was obtained within 4, 5, and 9 h of
treatment for 5, 10, and 15 mg/L dye-containing
wastewaters. The kinetics of the photocatalytic treatment were the fastest with the most dilute solution:
5 mg/L > 10 mg/L > 15 mg/L (0.892 h-1 > 0.613 h-1
> 0.237 h-1). The kinetics of the process was comparable with that of slurry-type reactors treating
44 | AATCC Review

Vol. 14, No. 1

January/February 2014

Pratap Pullammanappallil, University of Florida, 203
Frazier Rogers Hall, P. O. Box 110570, Gainesville,
FL 32611-0570, USA; phone +1.352.392.1864 ext.
203; fax +1.352.392.4092; pcpratap@ufl.edu.

DOI: 10.14504/ar.14.1.3