Study of TiO2-coated Ultrafiltration with UV irradiation for Natural
Organic Matter Removal
*Arie Dipareza Syafei1, Cheng-Fang Lin2
Graduate Institute of Environmental Engineering, National Taiwan University, Taipei, Taiwan
Key words: ultrafiltration, TiO2, photocatalysis, humic acid, membrane fouling, flux, NOM
ABSTRACT
Ultrafiltration membrane nowadays has been increasingly used in drinking water treatment
as alternative technology to conventional filtration to remove Natural Organic Matter (NOM).
Efforts on how to minimize fouling, maximize flux and rejection are always on motion. This
study is aimed to improve the performance of membrane by combining membrane coated
photocatalyst TiO2 under UltraViolet exposure. The experiments are carried out using ceramic
disc membrane which is high temperature resistant, using humic acid solutions as model
substances representative of naturally occuring organic matter, they are aimed to identify the
significance performance between TiO2 coated membrane and naked membrane.
A commercial humic solution was subject to UF Fractionation to study about molecular
weight distribution affected during the operation of membrane. 1 kD, 15 kD and 50 kD
membranes were used to demonstrate the effectivity of the coating and UV irradiation. For all
TiO2-UV254 membranes used in this research exhibit the more flux decline with similar quality
compare to naked membrane. Although the UF system is able to remove a significant portion
amount of humic acid particles, the combination with photocatalysis exerts low performance in
terms of flux. Thus, TiO2 coating combined with UV254 irradiation are not better for operation in
removing natural organic matter.
INTRODUCTION
One of the critical issues for good implementation of ultrafiltration in water treatment is
membrane fouling due to natural organic matter (NOM). NOM as often represented by dissolved
organic matter will negatively affects productivity, product quality and process cost (Katsofidou
1
2
Graduate Student
Professor
et al., 2004; Lin et al., 1999). It also will react with the most commonly used disinfectant,
chlorine, to form disinfection by-products (Thomson et al., 2002). Thus it is crucial to consider
removing NOM as primary step to secure water distribution towards consumer. A number of
membrane researches for particular subject are present, but these usually focused on water
chemistry instead from the membrane modification.
NOM molecular weight (MW) distribution is also known to contribute towards membrane
performance as shown by Lin et al (1999) that the fraction with the largest apparent molecular
weight 6.5-22.6 kD present in humic substances exhibits the worst decline with the best
permeate quality. On the other hand, humic particles ranging 160-650 Dalton exerted little effect
on flux decline.
Many approaches have been studied to minimize membrane fouling; these include pretreatment of feed water, hydrodynamic cleaning, optimization water chemistry such as pH, but
still rare modification of the membrane surface on application for organic matter removal. Thus
that is what this research themed. Membrane surface modification may be introduced to enhance
membrane performance and minimize membrane fouling. These include the adsorption of
surfactants and soluble polymers on the membrane surface.
Photocatalysis has received considerable attention due to its several reasons. Porous Teflon
Sheets (PTS) coated with TiO2 can be used to prevent snow from becoming attached to the
surfaces of electric wires. Another reason is the ability for self-cleaning in terms of
photocatalytic degradation of organic pollutants (Yamashita et al., 2003). Photocatalysis is also
able to remove trichlorophenol, recalcitrant pollutant and toxic organic removals (Tanaka et al.,
1994; Xi et al., 2001; Molinari et al., 2000; Molinari et al., 2001; Molinari et al., 2002)).
However, main application of photocatalysis is located on its ability to produce oxidation.
When photocatalyst, TiO2, is illuminated with light of which wavenumber below 400 nm, an
electron is promoted from the valence band to the conduction band of the semiconducting oxide
to give an electron/hole pair. The valence band potential is positive enough to generate hydroxyl
radicals at the surface and the conduction band potential is negative enough to reduce molecular
O2. The hydroxyl radical is a powerful oxidizing agent to attack organic pollutants present at
near the surface of TiO2 particles. This will result their complete oxidation to CO2 (Byrne et al.,
1998).
Several methods had been conducted such as ion assisted deposition (Yamashita et al.,
2003), direct filtration of nanoparticle aqueous solution (Moliner et al., 2002) and TiO2/polymer
thin film composite (TFC) reverse osmosis membrane to mitigate biofouling by
photobactericidal (Kwak et al., 2001). Summary of coating types are describe elsewhere (Byrne
et al., 1998). However, no studies have been done to understand about the effect of TiO2 coating
towards NOM removal.
The aim of the study is to understand the effect of membrane surface modification by TiO2
coating and the efficiency of the technique employed. The purpose of present work was to
investigate basic performance and rejection on TiO2 coating on a ceramic membrane to remove
NOM. This study was mainly focused on (1) flux performance and fouling mitigation on TiO2UV254 membrane compare to naked membrane (2) natural organic matter removal in terms of
dissolved organic carbon (DOC) (3) the effect of photocatalysis batch process towards molecular
weight distribution of organic particles. Flux and rejection experiments are carried out with
ceramic membrane, using humic acid solutions as model substances representative of naturally
occurring organic matter.
MATERIALS AND METHODS
Feed water
The commercial humic acid (Aldrich) was used as the feed water. A stock solution was
prepared by dissolving 1 gram humic acid in 1 L deionized water (MilliQ) and filtering through
0.45 µm membrane filter (CA, Whatman). The filtrate was stored at 4oC for subsequent use.
Before each experiment, feed water was adjusted to pH 7 with addition of NaOH or H2SO4.
UF membrane
To observe flux decline within a reasonable time, a single ceramic flat sheet membrane was
used. The characteristics of the membrane used are shown in Table 1.
Table 1 Characteristics of ceramic membrane
Support
Manufacturer
Tami Industries
Material
Alumina, Titania, Zirconia (ATZ)
Operating maximum pressure
4 bars
pH operating range
0-14
Solvents
Insensitive
Operating temperature
<350oC
MWCO
1, 15 and 50 kD
Disc Holders
Material
Stainless steel 316 L
pH operating range
0-14
Operating temperature
<130oC
The set up comprises of ceramic disc UF membrane module. Ultrafiltration is typical UF
membrane used in water and wastewater treatment process. Figure 1 displayed the set up of the
experimentations.
Pressure gauge
Valve
UV254 lamp
Membrane module
Retentate
Pressure
feed
Permeate
Feed patch
Feed tank
Fraction
collector
Nitrogen
cyclinder
Recirculation
Digital
Weighter
Figure 1 Apparatus diagram of ceramic disc membrane employed in the filtration experiments
The set up consisted of a ceramic membrane, from TAMI industries, described in Table 1
above. The feed tanks are 3 cylinders which each is able to accommodate 3 L humic acid
solutions, made total feed become 9 L. Feed tank is a closed tank, air is then flowed inside of
feed tank, this will push the water to flow towards membrane reactor. Trans Membrane Pressure
(TMPs) for all experiments was set constant at 10 ±0.2 psi under room temperature. No
backwashing were employed for these experiments.
The experimental procedure consisted of several cycles (5-6) of cross-flow filtration (0.05
m/s). The duration of each cycle was roughly 1 hour 20 minutes. The feed solution passed
through the ceramic membrane installed on the membrane reactor and permeate was collected on
the outside of the membrane reactor utilizing gravitation. At a constant pressure and velocity,
only concentrate (retentate) was recycled to the feed batch to simulate the actual UF plant
operation. After 8 hours of operation, each membrane was cleaned for subsequent use using
NaOH 1 M for 8 hours, citric acid 1 M for 1 hour, NaOH 1 M for 1 hour, and last ultrasonic for
10 minutes for subsequent use.
Before experiment began, membrane was compacted for 5 minutes, continued with
measuring membrane permeability. Pure water was fed to membrane module with incremental
increase and decrease step of TMP for 5-6 minutes from 5; 7,5; 10; 12,5; 15; 15; 12,5; 10; 7,5;
and 5. Membrane permeability is useful to determine range of UF filtration, this also useful to
compare each membrane. The measurement of flux began after 3 minutes started when constant
TMP 10 psi achieved. Membranes used were 1, 15 and 50 kD.
Permeability
Membranes used during the experiment were new from manufactures. Permeability test
were done by incremental increasing TMP as described above. The permeability was drawn from
the slope of each TMP, Figure 2 is example for determination permeability for 50 kD membrane.
After the coating process, there appears that after coating, membranes had slight lower
permeability as shown inTable 2. This gives brief idea of possible tighter pore size that might
occur due to coating of TiO2 on the membrane surface. Tighter MWCO after coating outweigh
the characteristic of TiO2 coating which can modify membrane surface to higher affinity to water
(Bae et al., 2005)
Flux (gr/cm2/min)
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
10
20
30
40
50
60
Time (min)
TMP 5
TMP 10
TMP 15
TMP 12.5
TMP 7.5
Linear (TMP
Linear (TMP
Linear (TMP
Linear (TMP
Linear (TMP
5)
10)
15)
12.5)
7.5)
TMP 7.5
TMP 12.5
TMP 15
TMP 10
TMP 5
Linear (TMP
Linear (TMP
Linear (TMP
Linear (TMP
Linear (TMP
7.5)
12.5)
15)
10)
5)
Figure 2 Determination of permeability of 50 kD naked membrane
Table 2 Permeability of membrane operation
Membrane
Permeability
MWCO
(gr/cm2.min.psi)
50 kD
0.0129
15 kD
0.0131
1 kD
0.0055
Combined
50 kD
0.01
with
15 kD
0.0078
photocatalysis
1 kD
0.0052
Description
Without
photocatalysis
TiO2 coating
TiO2 powder used was P25 Degussa. An aqueous suspension containing 1.25% Degussa P25
(99.5% purity and about 80% anatase), 3.75% acetyl-acetone and 5 drops of Triton-X are stirred
for 1 hour. While stirring the coating solution, membrane was prepared in such position that the
latitude is flat to ensure equal distribution of solution within the membrane surface. After 1 hour,
the solution was then poured carefully on top of membrane. These methods are called sol-gel
methods.
Temperature was initially set to 100oC achieved for 1 hours, gradually increased
temperature up to 450oC was to ensure that equal evaporation of acetyl-acetone to build up
membrane pore size was equally achieved.
TiO2-UV254 batch
Humic acid was obtained from Aldrich. The photocatalyst was titanium dioxide powder,
P25 Degussa. The pyrex photoreactor used in batch experiments. TiO2 in aqueous solution was
kept in suspension in the reaction vessel with a magnetic stirrer to ensure uniform distribution of
photocatalysis. The reaction vessel was also jacketed with water for cooling purposes and to
control the solution temperature (20oC). The UV lamp was placed at the inner periphery of the
quartz glass tube. UV mercury lamp used was Philips 254 nm, TUV 8W/G8 T5. The reactor is
shown on Figure 3.
A solution of 9 ppm humic acid concentration was used for all batch experiments, because it
is a representative level of organic presence in water surface in Taiwan, and it also allows
appropriate analysis. All humic acid for this experiment were screened by 0.45 microgram
membrane filter (Whatman, cellulose acetate). 0.1 gram (0.125%) of TiO2 was used as
photocatalyst and suspended by stirring in 3 L of humic acid solution. The solution was aerated
by an air pump and the constant air flow rate. The pH is measured for humic acid solution,
adjusted to 7 by adding sodium hydroxide or sulphuric acid. The illumination started after the
addition of TiO2. Samples were collected after 1 hour, and screened with 0.45 µm membrane to
filter TiO2, and then measured for Dissolved Organic Carbon (DOC) by TOC analyzer.
Molecular Weight Distribution
UF fractionation and Gel Filtration Chromatography was conducted to determine the
molecular weight distribution. UF fractionation was done after TiO2-uv batch experiment. In
order to determine the apparent molecular weight distribution (AMW) of this humic acid, UF
fractionation was employed. UF membrane types used were 1 kD, 15 kD and 50 kD types
(TAMI Industries). The method used was the same with the point membrane systems mentioned
above except for the capacity.
Sampling
port
Air inlet
Outflow
UV
lamp
Inflow
Stirrer
Figure 3 The photoreactor used in the experiment
TiO2-UV batch was using 3 L of humic acid solution. After 1 hour, this solution was fed to
membrane module with the help of pressure from nitrogen cylinder. The TMP used was 10 psi,
and visually monitored. Humic acid concentrations were determined by TOC Analyzer and UV
absorbance measurements at wavelength 254 nm in a UV/VIS Cintra spectrophotometer.
Molecular weight (MW) distributions were determined by Gel Filtration Chromatography.
The operating system consisted of a C 26/70 column (Pharmacia). The carrier solvent consisted
of a 20 mM phosphate buffer (sodium phosphate, MW=141.96) adjusted to an ionic strength of
0.1 M with sodium chloride (MW=58.44), with a pH of 7. A flow rate of 2.0 mL/min was used.
Gel bead matrix was using Sephadex G-75, comprises of 75% slurry under phosphate buffer
solutions.
Volume sample was adjusted to 2% Vt. The system was calibrated with PEG standards of
the following molecular weights: 1500 Da, 6000 Da, 15000, 35000 and 72000 Da (Merck),
prepared at 1 g/L concentration. The PEG standards and NOM were detected at 254 nm, and
measured as DOC as well. A linear equation of the form log (MW) = a – b(Ve) was obtained
(R=0.98), where MW is the molecular weight and Ve is the eluted volume.
Analysis
DOC were analyzed on filtered samples (0.45 µm) in an organic carbon analyzer TOC
analyzer O. I. Analytical 1010. UV absorbance was measured with a UV/Vis spectrophotometer
(Cintra) at a single wavelength 254 nm with a 1-cm path length, and measuring pH with SensIon
pH meter.
RESULTS AND DISCUSSION
Flux Performance
Unless for 15 kD membrane, it appears that for 1 kD and 50 kD membrane, both were
producing similar initial flux compare to naked membrane as shown in Figure 4. TiO2-UV254
membranes used for this experiment seem to give more severe decline towards flux. It can be
observed than for 50 kD membrane, TiO2-UV254 membrane operation within first 2 hours
declined until it reached steady state flux in 0.0897 gr/cm2/min. On the other hand for same
molecular weight cut off (MWCO) membrane 50 kD, nice steady decreasing slope occurred.
This result was found to be similar with the other remaining MWCO membrane.
0.15
0.10
TiO2-UV membrane
0.05
naked membrane
0.12
0.09
0.06
naked membrane
0.03
TiO2-UV membrane
0.00
0.00
0
60
120
180
240
300
360
420
480
0
50 100 150 200 250 300 350 400 450 500
Time (min)
Time (min)
Flux (gr/cm2/min)
0.15
Flux (gr/cm2/min)
Flux (gr/cm2/min)
0.15
naked membrane
0.12
TiO2-UV membrane
0.09
0.06
0.03
0.00
0
50 100 150 200 250 300 350 400 450 500
Time (min)
(a)
(b)
(c)
Figure 4 Flux of (a) 50 kD, (b) 15 kD (c) 1 kD membrane, naked membrane and TiO2-UV254
membrane
The decline of 15 kD TiO2-UV254 membrane took place within 6 hours while for 1 kD
approximately within 3 hours. These results are able to give very rough conclusion that during
the photocatalyst coating on the membrane surface coupled with UV254 irradiation gave more
fouling than naked membrane. This can be approximately estimated by looking at the
permeability value for each membrane. However, interesting fact that the coating process seems
to produce tighter of membrane MWCO which cause more organic adsorption can take place on
both the surface of membrane and TiO2 particles. The coating process also gives lower steady
state flux as shown by Figure 5 and higher flux loss (Table 3).
Table 3 Initial flux of naked membrane and TiO2-UV254 membrane
Description
Naked
membrane
TiO2-UV254
membrane
Membrane
Initial flux
Steady state flux
Percentage flux
MWCO
(gr/cm2.min)
(gr/cm2.min)
loss (%)
50 kD
0.01198
0.0909
24.13
15 kD
0.01044
0.0838
19.76
1 kD
0.0623
0.0503
19.32
50 kD
0.01199
0.0868
27.60
15 kD
0.0748
0.0748
20.68
1 kD
0.0631
0.0462
26.73
2
Steady State Flux (gr/cm /min)
0.1000
Naked membrane
TiO2-UV membrane
0.0800
0.0600
0.0400
0.0200
0.0000
50 kD
15 kD
1 kD
MWCO
Figure 5 Steady state flux for naked and TiO2-UV254 membrane
Higher flux decline during TiO2-UV254 membrane operation due to the clogging of some
pores on membrane surface. While large molecules are rejected by concentration polarization,
smaller molecules are attached on the TiO2 particles and adsorbed within membrane pores. The
results of this study is interestingly not in agreement with previous study under different feed
solution (Bae et al., 2005). The author revealed that TiO2 entrapped membrane by phase
inversion could reduce fouling. Possible reason is that different material membrane used that will
affect overall hydrodynamic condition, especially chemical interaction between foulants and
membrane.
Removal Characteristics
Dissolved organic carbon (DOC) removal for both naked membrane and TiO2-UV
membrane are shown in Figure 6 below. DOC removal for 50 kD naked membrane was ranging
from 51 to 71% while for TiO2-UV membrane was 57-73%. Slight higher removal for TiO2-UV
membrane compare to naked membrane. However, this result is slightly reiterated for 15 kD
membrane, when 15 kD TiO2-UV membrane gave DOC removal from 72-92% while the
removal for naked membrane is 79-88%.
On average (data not shown), the DOC removal performance for naked membrane was
proven to be slightly better than TiO2-UV membrane. Possible reason is the alteration of organic
structure, in another term is molecular weight (MW) distribution. The organic particles between
15-50 kD was transformed to below 15 kD by bond breaking process caused by the presence of
OH radicals. These OH radicals are produced as a result of reaction between photocatalyst TiO2
and humic acid particles with the help of UV254 irradiation. However, the figure also gives
another perspective. While on one side the photocatalyst transform organics into smaller MW
distribution, it appears that the presence of TiO2 over time especially after 6 hours as shown on
Figure 6 part b adsorb these organics within the pores, as the result is decreasing pores within the
membrane, thus more solutes are rejected. DOC removal for 1 kD naked membrane was 81-87%
100
100
80
80
80
60
40
naked membrane
TiO2-UV
20
DOC rejection (%)
100
DOC rejection (%)
DOC rejection (%)
while for TiO2-UV254 membrane was 79-87% which shows similar result with 50 kD membrane.
60
40
naked membrane
20
60
40
20
naked membrane
TiO2-UV membrane
0
0
100
200
300
Time (min)
400
500
TiO2-UV
0
0
0
100
200
300
Time (min)
400
500
0
100
200
300
400
500
Time (min)
(a)
(b)
(c)
Figure 6 DOC removal of (a) 50 kD, (b) 15 kD (c) 1 kD membrane, naked membrane and TiO2UV254 membrane
From the flux and DOC removal perspective, it appears that the alteration of MW
distribution within feed humic acid solution was mainly taking place for organics in the range 150 kD. The path of transformation occurred towards below 1 kD and above 50 kD. The result of
this present work is slight in agreement with the work done by Bae et al (2005), where rejections
of membrane with TiO2 entrapped on the surface only give similar rejection with neat membrane.
These phenomena might be derived from intrinsic nature of TiO2 particles where TiO2 can
increase water-solvent interdiffusion velocity and make membrane more porous because of its
hydrophilic nature. However, this experiment shows the opposite. Even though the presence of
TiO2 on the membrane surface is able to make the surface become more hydrophilic and increase
affinity between water and membrane, still denser surface pore is formed. This is due to the
presence of TiO2 particles attached on membrane surface (confirmed by SEM picture Error!
Reference source not found.), this physically even possible that TiO2 particles are attached in
both surface or inside the membrane pores, thus give lower permeability. However, since lower
permeability usually give higher rejection, and the work shows the opposite, it seems that there is
MW distribution change during the operation. Thus, in one side, membrane pose size become
smaller, but on the other side, possible transformation from bigger MWCO to smaller MWCO
will allow more passages pass through TiO2 coated membrane.
The removal characteristics of humic acid solution with TiO2-UV254 membrane shows that
the operation converted the degree of unsaturated C-C bonds of NOM, as Specific Ultraviolet
Absorbance (SUVA) represented by ratio between UV254 and DOC can be indicative of
complexity resulting from the presence of aromaticity and other unsaturated chemical bonds.
Interestingly, better conversion from complex aromaticity and unsaturated chemical bonds
was observed with membrane treatment without TiO2-UV (Figure 7 and Figure 8) for membrane
15 and 1 kD membrane. While the SUVA in retentate was ranging 0.08 to 0.09 l/(mg.cm) (15 kD
and 1 kD) the permeate observed was found to be in the range 0.005 to 0.06 l/(mg.cm)in the
permeate. On the other hand, lower conversion was observed after TiO2-UV membrane treatment
for 15 and 1 kD (Figure 7 and Figure 8). The retentate SUVA was similar with those for naked
membrane, but slight different for permeate, ranging from 0.04 to 0.07 l/(mg.cm). It appears that
oxidation reaction occurred during the photocatalysis inhibit further conversion towards simpler
form of humic groups.
0.15
0.15
Retentate
Retentate
0.12
0.09
SUVA
SUVA
0.12
Permeate
0.06
0.03
Permeate
0.09
0.06
0.03
0.00
0.00
0
60
120
180
240
300
Time (min)
360
420
480
0
60
120
180
240
300
360
420
480
Time (min)
(a)
(b)
Figure 7 SUVA for 15 kD membrane (a) naked membrane, (b) TiO2-UV254 membrane
0.15
0.15
Retentate
Retentate
0.12
0.09
SUVA
SUVA
0.12
Permeate
0.06
0.03
Permeate
0.09
0.06
0.03
0.00
0.00
0
60
120 180 240 300 360 420 480
0
60
120
Time (min)
180
240
300
360
420
480
Time (min)
(a)
(b)
Figure 8 SUVA for 1 kD membrane (a) naked membrane, (b) TiO2-UV254 membrane
Molecular Weight Distribution
Figure 9 below shows more details information about molecular weight distribution. It must
be noted that each of the graph was run under different initial DOC concentration, therefore these
graphs only give pictures about the distribution of MW after reaction with TiO2 as photocatalyst
and irradiated with UV254. Initial DOC concentration of UF fractionation for 1 kD, 15 and 50 kD
membranes were 8.73, 9.04 and 9.39 mg/L respectively. Another note is time operation was 1
hour to easier identify MW behaviour within photocatalysis process.
The MW of raw water below 1 kD was around 25.17% and above 1 kD was 74.83%. After 1
hour of the TiO2-UV batch process, there was removal of DOC as much as 15.8% for 0.45 µm
membrane screened sample and 8.52% for unscreened sample. The concentration of screened
and unscreened sample was 7.351 mg/L and 7.896 mg/L (data not shown). This evidence showed
that roughly 8% of the removal was due to adsorption of humic acid particles on TiO2 particles.
The MW distribution after photocatalysis was 58.9% below 1 kD and 25.29% above 1 kD as
shown at Table 4.
Table 4 Molecular weight (MW) distribution of raw water and TiO2-UV photocatalysis batch
MW distribution/water
Raw water (%)
TiO2-UV water (%)
<1 kD
25.17
58.9
> 1 kD
74.83
25.29
<15 kD
82.09
62.10
> 15 kD
17.91
18.13
<50 kD
85.64
38.08
> 50 kD
14.36
35.19
80
90
90
80
60
50
40
30
20
70
60
50
40
30
20
10
10
0
0
<1 kD
Raw water
TiO2-UV water
Raw water
TiO2-UV water
70
60
50
40
30
20
10
0
<15 kD
>1 kD
(a) Apparent Molecular Weight Distribution
80
Fractionation (%)
Raw water
TiO2-UV water
Fractionation (%)
Fractionation (%)
70
<50 kD
>15 kD
(b) Apparent Molecular Weight Distribution
>50 kD
(c) Apparent Molecular Weight Distribution
(a)
(b)
(c)
Figure 9 Apparent molecular weight distribution obtained by UF fractionation after 1 hour
photocatalysis batch (a) 1 kD, (b) 15 kD and (c) 50 kD membrane
Initial DOC of UF fractionation with 15 kD membrane was 9 mg/L and removal after 1 hour
photocatalysis process for screened sample and unscreened sample was 19.77 and 11.36%, with
the difference of 8.41% was probably due to adsorption of organic carbon on the surface of the
TiO2 particles attached (immobilized) on membrane surface. Table 4 also shows the MW
distribution for raw water and water after TiO2-UV photocatalysis batch. It can be seen that
before photocatalysis, there was approximately 82% of organic matter below 15 kD, that made
17.91% of organic matter was mainly above 15 kD. Alteration occurred due to photocatalysis
process towards MW distribution, when the position now the organic matter with MW below 15
kD was 62.10%, and above 15 kD was 18.13% as seen in the table.
The initial DOC for 50 kD UF fractionation was 9.39 mg/L composed of 85.64% below 50
kD and 14.36% above 50 kD. The removal of photocatalysis process was 26.73%. However,
there was a change in MW distribution after the photocatalysis process. MW below 50 kD was
become 38.08% and above 50 kD was now 35.19%.
0.06
UV absorbance
Raw
water
Hour 1
0.04
Hour 4
Hour 8
0.02
0
100
1,000
10,000
100,000
1,000,000
Molecular weight (Da)
Figure 10 Molecular weight distribution by GFC
Molecular weight distribution of UF fractionation is confirmed by gel filtration
chromatography that has been equilibrated with PEG giving R = 0.98. Figure 10 confirms that
molecular weight of permeate despite decreases, but significant peak at around 2 kD was
observed with highest peak was observed after 8 hour operation of membrane operation.
Results shown above gives a big picture that photocatalysis process alters the MW
distribution of organic matter contained in the water. Figure 9 displays general information that
most organic MW above 15 kD, either between 15-50 kD or above 50 kD, will be converted
during photocatalysis process to the direction of below 15 kD and a portion towards above 50 kD.
The result is strengthened by Figure 9 part c, when it appears that even the conversion move
below 1 kD. However, since the initial concentration and of each experiment was different, no
exact comparison can be made instead of overall view.
It may be concluded that from Figure 9 part b above 15 kD which is 62%, seems most of the
MW was actually below 1. Different result was obtained between Figure 9 part b and part c,
when the composition of organic MW is not the same for above 15 kD, this probably due to
overlap MWCO membrane which was used, thus different results were obtained. However, since
different membranes used will effect the fractionation process, it is difficult to exactly conclude
the MW distribution, therefore it still need another better method to determine MW distribution
during TiO2-UV254 membrane operation. However, GFC has provided excellent additional
information for molecular weight distribution as shown by Figure 10 above.
CONCLUSIONS
UF, although its membrane having a relatively small molecular weight cut off (MWCO), is
effective in reducing turbidity, organic matter and bacteria. However, the results of the present
study affirm that the presence of TiO2 coupled with UV254 irradiation combined with membrane
operation cannot effectively enhance flux and the removal of humic substances compare to
naked membrane.
1. Membrane characteristics after coating were changed for all MWCO by addition of TiO2.
The coating process can make the membrane pore size tighter due to the presence of TiO2
particles on membrane surface.
2. TiO2-UV254 membrane showed more severe flux decline compared to naked membrane. The
decline was caused by either more dense MWCO of the membrane thus more adsorption
within membrane and TiO2 surface or the alteration of MW distribution of organics.
3. Specifically, during the photocatalysis process, organic particles ranging from 1 to 50 kD are
transformed to both below 1 kD and above 50 kD within 1 hour of photocatalysis process. In
specific operation for membrane, it is well concluded that during the process, transformation
of molecular weight distribution of organic takes place.
However, the advantage of DOC removal during photocatalysis process does not overweigh
the disadvantage of transforming humic particles into smaller molecular weight that cause easier
to penetrate the particular membranes used in this experiment, thus it is necessary to further
study the effect of decreasing velocity that can result higher contact between organic solution
with photocatalyst, and to modify the coating methods.
REFERENCES
Bae T.H., T.M. Tak. Effect of TiO2 nanoparticles on fouling mitiation of ultrafiltration
membranes for activated sludge filtration. J. Membrane Science 249 (205), 1-8.
Byrne J.A., B.R. Eggins, N.M.D. Brown, B. McKinney, and M. Rouse. Immobilisation of TiO2
powder for the treatment of polluted water. J. Membrane Science 249 (205), 1-8.
Katsoufidou K., S.G. Yiantsios, and A.J. Karabelas. A study of ultrafiltration membrane
fouling by humic acids and flux recovery by backwashing: Experiments and modelling. J.
Membrane Science 266 (2004), 40-50.
Lin, C.F., Y.J. Huang, and O.J. Hao. Ultrafiltration processes for removing humic substances:
Effect of molecular weight fractions and PAC treatment. Wat. Res. 33 (5) (1999), 1252-1264.
Lin, C.F., S.H. Liu, and O.J. Hao. Effect of functional groups of humic substances on UF
performance. Wat. Res. 35 (10) (2001), 2395-2402.
Kwak S.Y., S.H. Kim, and S.S. Kim. Hybrid organic/inorganic reverse osmosis (RO)
membrane for bactericidal anti-fouling. 1. Preparation and characterization of TiO2 selfassembled aromatic polyamide thin film composite (TFC) membrane. Environ. Sci. Technol.
35 (2001), 2388-2394.
Metsamuuronen, S., and M. Nystrom. Critical flux in cross-flow ultrafiltration of protein
solutions. Desalination 175 (2005), 37-47.
Molinari R., M. Mungari, E. Drioli, A.D. Paola, V. Loddo, L. Palmisano, and M. Schiavello.
Study on a photocatalytic membrane reactor for waste purification. Catal. Today 55 (2000),
71-78.
Molinari R., C. Grande, E. Drioli, L. Palmisano, and M. Schiavello. Photocatalytic membrane
reactors for degradation of organic pollutants in water. Catal. Today 67 (2001), 273-279.
Molinari R., L. Palmisano, E. Drioli and M. Schiavello. Studies on various reactor
configurations for coupling photocatalysis and membrane process in water purification. J.
Membr. Sci. 206 (2002), 399-415.
Tanaka S., and U.K. Saha. Effects of pH on photocatalysis of 2,4,6-trichlorophenol in
aqueous TiO2 suspensions. Wat Sci Tech 30 (9)(1994), 47-57.
Thomson J., F. Roddick and M. Drikas. Natural organic matter removal by enhanced photooxidation using low pressure mercury vapour lamps. Wat. Sci. Tech.: Water Supply 2 (5-6)
(2002), 435-443.
Yamashita H., H. Nakao, M. Takeuchi, Y. Nakatani, and M. Anpo. Coating of TiO2
photocatalysts on super-hydrophobic porous Teflon membrane by an ion assisted
deposition method and their self-cleaning performance. Nuclear Instruments and Methods in
Physics Research B 206 (2003), 898-901.
Xi W., S.U. Geissen. Separation of titanium dioxide from photocatalytically treated water
by cross-flow microfiltration. Wat Res 35 (2001), 1256-1262.