SODIS Enhancement Using Photocatalytic Cement
By: Erica Rapp, advisor - Dr. Yan Wu
Abstract
This study determined the optimal combination of cement and titanium dioxide (a common
photocatalyst) for use in speeding the solar water disinfection (SODIS) process. For varying
TiO2 to cement ratios, the reduction capabilities of the composite were measured using a UV
lamp and methylene blue, a redox indicator. The optimal TiO2 to cement weight ratio according
to this study is 6:80. To verify the lab results, contaminated water was exposed to the SODIS
process in both a control group and a test group exposed to the photocatalytic cement. In each
trial, a significant decrease in bacterial contamination, up to 50%, was observed in the test
groups as compared to the control. Given the successful results in both a lab and trial setting,
photocatalytic cement is a viable option for future use in solar water decontamination.
Table of Contents
Introduction………………………………………………………………………………..........2-4
Materials………………………………………………………………………………..……….4-5
Procedure………………………………………………………………………….…………….5-7
Results and Discussion……………………………………………………………………...…7-11
Conclusions……………………………………………………………………………….……11
References………………………………………………………………………………...….12-13
Appendix A – Methylene Blue Reduction using Aggregate TiO2……………………………….14
Appendix B – Methylene Blue Reduction using Photocatalytic Composites……………………15
Appendix C – Petrifilm Trial Bacterial Colony Counts……………………………………...16-18
1
Introduction
In a vast number of developing countries, there is a shortage of clean water to use in
sanitation and consumption. More than 3.4 million people die each year due to water-borne
illnesses – a majority of them are children who have been forced to rely on unclean water
sources. (1; 2). Due to the overwhelming need for potable water, a great deal of study has gone
into the science of water decontamination (3; 4; 5; 6; 7). This study focuses on one method in
particular: solar water disinfection (SODIS). The SODIS method promotes the inactivation of
microbes present in water through UV radiation and by creating a high temperature environment
(7). UV rays alter bacterial DNA and create an oxidizing environment detrimental to diseasecausing organisms (2; 7). Most communities that use SODIS implement clear plastic bottles
(PET bottles), fill them with water, and let them sit in the sun anywhere from 6 hours to 2 days.
The amount of time needed depends on the amount of sunlight present and the turbidity of the
water. Benefits of SODIS include a significant decrease in water-borne illnesses, simple
implementation, a low rate of recontamination, no additives, and little to no alteration of water
taste (2). SODIS has been hailed as one of the most cost-effective water purifications methods –
the only monetary requirements involve acquiring PET bottles (2; 4). Analysts predict that
ultimately eliminating disease caused by unclean water would reap a benefit of $3-$34 USD for
every $1 spent in treatment implementation (1).
Despite this, there are drawbacks which prevent SODIS from being used on a larger
scale. Unfortunately, SODIS can only purify a finite amount of water at once. In large, this is
due to the length of time needed for purification and the limited volume allowed by plastic
bottles. One attempt to alleviate this concern employs the use of photocatalysts to speed the
SODIS process. A photocatalyst is a substance which accelerates a chemical reaction using
2
sunlight – in this experiment, titanium dioxide (TiO2) was used. As shown in Figure 1, UV light
reaches the photocatalyst and creates hydroxyl radicals which then inactivate harmful
microorganisms (3).
Figure 1 - The mechanism by which TiO2 breaks down harmful microbes (8)
Past experiments using photocatalysts to speed SODIS have used either a thin film of TiO2 lining
the inner wall of the bottle or left it suspended in the water. With the first method, TiO2
eventually washes away leaving nothing but the plastic bottle. Taking the time to deposit the
titanium dioxide in a thin film also requires expertise, time, and money that many developing
countries do not have. Leaving the photocatalyst suspended in water only allows for one use and
must be filtered out before consumption (5). To alleviate these concerns, the work presented in
this study examines the long-term effectiveness of a cement and TiO2 mixture in enhancing the
SODIS method.
Photocatalytic cement is not a new idea; it simply has not seen widespread use in water
purification. Presently, buildings and roads made of cement mixed with TiO2 are being used in
Europe to cut down on pollution and dirt buildup. A study conducted in the Netherlands
3
demonstrated that nitrogen oxide levels on roadways made of photocatalytic cement were
reduced by 25-45% (9). Since pollution is a prominent issue in the modern day, there have been
entire businesses founded on the production of photoactive cement (9; 10).
Materials
Four materials in particular were used in the course of this study: TiO2, methylene blue,
portland cement, and 3M petrifilmTM. To create the photocatalytic compound, TiO2 was mixed
with cement. P-25 Degussa, an industrial standard TiO2 powder consisting of anatase and rutile
phases in a 3:1 ratio was used for this study. The effectiveness of the compound was then
measured using methylene blue. Once the most effective compound was ascertained, the
petrifilms were used to measure bacterial contamination of river water upon exposure to the
enhanced SODIS process. Each material is discussed in detail in this section.
Although there are many photocatalysts available, TiO2 displays optimal properties for
use with SODIS. It is chemically non-reactive, easily obtainable due to heavy use in industry
and research, and non-poisonous. The anatase phase of TiO2 in particular performs well in
photoactive applications (3; 11).
During exposure to UV light, TiO2 reacts by absorbing a
photon and releasing an electron-hole pair in response. To do this, the photon must be greater
than the bandgap energy of the material it comes in contact with (anatase TiO2 has a bandgap
energy of 3.25 eV). The hole is then free to oxidize organics and the electron goes on to react
with reducible molecules as seen in Figure 1 (8; 5). No other photocatalyst considered for this
study was able to offer the same versatility and flexibility.
Methylene blue is a common redox indicator. During exposure to reducing agents, a
solution of methylene blue begins to turn colorless. The subsequent change in absorption
spectrum is then measured using a spectrometer. Generally, the change in optical density of the
4
peak wavelength (664 nm for methylene blue) is recorded. The rate of change in the MB
absorption spectrum reflects the rate at which SODIS would reduce organics (12).
Portland cement was used as a binding agent in this study. It was chosen because it is
cost effective, chemically non-reactive, porous for increased surface area, and simple to work
with (13).
3M PetrifilmTM is used as a culture template. Each plate contains nutrients, a gelling
agent, and an indicator which turns either red or blue in the presence of bacterial colonies. Red
coloring is apparent in coliforms and a blue color indicates E.coli. One petrifilm plate is able to
sample up to 1 mL of water (14).
Procedure
A methylene blue solution was created using MB powder and deionized water. Each test
within this study used a 10 ppm methylene blue solution as an initial indicator. UV light used to
degrade the MB solution was provided by a Mineralight Multiband UVGL-25 lamp with light
wavelengths of 254/366 nm. The absorption spectrum was measured using a SpectraSuite
spectrometer.
Maximum effectiveness of the TiO2 powder was ascertained by mixing it directly into the
MB solution, exposing it to UV light, and measuring the absorption spectrum change over 3
hours. Tested concentrations of TiO2 included 0.5, 1.0, and 1.5 g/L. These were based off of
previous studies which found that the optimal titanium dioxide concentration was centered about
1.0 g per liter of MB solution (12; 15).
Thorough mixing of the TiO2 and cement was an important consideration in the
effectiveness of the compound. Two methods were compared to determine whether manual
mixing detracted from the end results: a planetary ball mill and mixing by hand. The planetary
5
ball mill used ceramic balls and rotational motion to thoroughly mix the composite. Manual
mixing was conducted by carefully measuring both cement and TiO2 powder into a plastic bag
and proceeding to shake the bag for ten minutes. If clumps of either material were present
afterwards, additional mixing was employed until a powder with consistent texture and color was
attained. Due to very little change in results, the manual method was used for each tested ratio.
Tested weight ratios of TiO2 to cement were: 3:80, 4:80, 5:80, 6:80, and 7:80. These
ratios were initially based upon previous photocatalytic composite studies with optimal results
centered about 5 grams of photocatalyst per 125 grams of binding agent (16).
Once water was added to the powder, the composite was applied to the lower portion of a
t-plate for each mixture (see in Figure 2). After the composite dried, it was lowered onto an 80mL beaker filled with methylene blue. In this manner, a set amount of the MB solution could be
exposed to UV light with varying composite mixtures. This setup eliminated variations in
acquired data by keeping the amount of MB the same, keeping composite surface area consistent,
and allowing for multiple tests of the same composite sample.
Figure 2 - T-plate to which the wet cement mixture was applied.
6
Once the optimal ratio was determined, a rod made of the photocatalytic cement was
created for use in a 16.9 oz. PET bottle. The rod was modeled as a cylinder with a length of 12
cm and diameter of 1.5 cm.
To test the rod, two 16.9 oz. plastic bottles were filled with water from the Rountree
Branch of the Little Platte River in Platteville, WI. One bottle contained the cylinder while the
other acted as a control. At given time intervals, 1 mL of water from each bottle was deposited
on a petrifilm. The petrifilms required an incubation period of 48 hours at which time the
number of bacterial colonies were recorded. This process was repeated twice; once using natural
sunlight, and once using the UV lamp. During the natural sunlight trial, the bottles were placed
on a black metal surface during sunny conditions.
Results and Discussion
The concentration of aggregate titanium dioxide in solution which displayed optimal
reduction capabilities was 1 g/L. To support this, Figure 3 displays percent reduction after three
hours according to grams of TiO2 added to one liter of MB solution. Figure 4 depicts the
resulting spectral image from which changes in absorption spectrum data were taken. Percent
reduction was calculated through measuring the change in absorbance of MB’s peak wavelength
(664 nm) between the initial time and 3 hours (also see Appendix A).
7
Percent Reduction of MB in 3 Hours
45
40
35
30
25
20
0.4
0.6
0.8
1
1.2
grams of TiO2 per Liter of MB Solution
1.4
1.6
Figure 3 - Graphical evidence confirming 1.0 g/L as the optimal TiO2 concentration
Figure 4 - Spectrometer image of a MB solution degraded over 3 hours using 1 g TiO2 per L MB concentration.
8
The results were consistent with those found in previous studies (12; 15). This provided validity
to the process by which the photocatalytic composite efficiency was determined. It also defined
an accurate standard by which to compare the composite results.
Each composite mixture underwent multiple trials of methylene blue degradation to
determine which was optimal. The effectiveness of the composite was determined by looking at
the percent of MB reduced over the course of 3 hours in the same manner used for aggregate
TiO2 (see Figure 5). A TiO2 to cement ratio of 6:80 (3:40 or 7.5 wt%) proved to be the most
efficient in terms of reduction speed.
Percent Reduction of MB in 3 hrs
24
22
20
18
16
14
12
10
2
3
4
5
6
Grams of Tio2 mixed with 80 g Cement
7
Figure 5 - Each composite mixture and its reduction capability over the course of 3 hours.
Although the 7:80 composite performed adequately, there were qualitative problems with
it. The high concentration of titanium dioxide made the composite more prone to breaking and
crumbling. Higher concentrations of TiO2 were attempted in ratios of 8:80 and 9:80, but each
ended up cracking apart. It is possible that other materials could be added to the mixture to
prevent this phenomenon, but when using only TiO2 and cement, the ratio limit is 7:80.
9
The photocatalytic cement rod used to purify contaminated water was created using a
6:80 ratio of titanium dioxide and cement. Its reduction capabilities were measured using a
bacteria count instead of methylene blue. For each trial, the total bacteria count was lower in the
bottle containing the photocatalytic rod. The recorded bacteria counts including both E.coli and
coliforms for each trial can be seen in Figure 6 and 7.
190
180
E.Coli/Coliform Count
170
160
150
140
TiO2
130
Control
120
110
100
0
30
60
Time (min)
90
120
Figure 6 – Average E.Coli/Coliform count present in 1 mL of water over a period of 2
hours when exposed to a UV lamp.
190
180
E.Coli/Coliform Count
170
160
150
TiO2
140
Control
130
120
110
100
0
30
60
Time (min)
90
120
Figure 7 - Average E.Coli/Coliform count present in 1 mL of water over a period of 2
hours when exposed to natural sunlight.
10
While both graphs report a higher bacterial reduction rate for water exposed to the
photocatalytic composite, the rate is slower in the trial using natural sunlight. This can be
accounted for by taking into consideration lower temperatures and a UV index of only 4 for the
natural sunlight trial. The relevance of these results lies purely in their reduction speed relative
to that of the control trials. Each trial consistently concluded that the tested photocatalytic
cement composite increased the rate at which SODIS took place.
Conclusions
In many parts of the world, access to sanitary water is limited. Photocatalytic
enhancement of the SODIS method offers an inexpensive way to bolster water disinfection
processes already being used to counteract this disparity.
While the use of titanium dioxide to its full potential (in an aggregate solution) is able to
reduce 41.5% of pathogens in 3 hours, it is not a feasible plan for enhancing SODIS.
Photocatalytic cement offers a sanitary, inexpensive, and reusable alternative which, according to
this study, is able to reduce up to 22.5% of pathogens in the same time. The optimal ratio of
titanium dioxide and Portland cement used to create this composite is 3:40. Based on the results
of this study, photocatalytic cement is a viable tool for use in solar water disinfection.
In future studies, it is recommended that water in contact with the composite be tested to
see if aggregate TiO2 is escaping into the system. If this is the case, it is probable that the
composite actually has a lower reduction rate than that reported in this study. Another variable
to change in the future is the binding agent material.
11
References
1. Berman, Jessica. WHO: Waterborne Disease is World's Leading Killer. Voice of America. [Online]
October 29, 2009. http://www.voanews.com/content/a-13-2005-03-17-voa34-67381152/274768.html.
2. Center for Disease Control. Household Water Treatment Options in Developing Countries: Solar
Disinfection (SODIS). Center for Disease Control. [Online] January 2008.
http://www.cdc.gov/safewater/publications_pages/options-sodis.pdf.
3. Photocatalytic Enhancement for Solar Disinfection of Water: A Review. Byrne, J. Anthony, et al., et al.
[ed.] Mohamed Sabry Abdel-Mottaleb. s.l. : Hindawi Publishing Corporation, December 24, 2010,
International Journal of Photoenergy, Vol. 2011.
4. Cost-effectiveness of Water Quality Interventions for Preventing Diarrhoeal Disease in Developing
Countries. Clasen, Thomas, et al., et al. 4, s.l. : IWA Publishing, 2007, Journal of Water and Health, Vol.
5.
5. Photocatalytic Destruction of Water Pollutants Using TIO2 Film in PET Bottles. Heredia, Manuel and
Duffy, John. Lowell, MA : s.n., 2006.
6. Kurup, Deepika, [perf.]. Discovery Education 3M Young Scientist Challenge. News Broadcast Network,
2012.
7. Optimizing the Solar Water Disinfection (SODIS) Method by Decreasing Turbidity with NaCl. Pearce,
Joshua M. and Dawney, Brittney. 2, s.l. : Journal of Water, Sanitation, and Hygiene for Development,
2012, Vol. 2.
8. Fujishima, Akira. Basic Mechanism of TiO2 Photocatalysis and a Recent Application. Photocatalyst
Group. s.l. : Kanagawa Academy of Science and Technology.
9. Self-Cleaning Concrete: Building a Better (Cleaner) World in the 21st Century. Concrete Technology.
[Online] 2013. http://www.cement.org/tech/self_cleaning.asp.
10. Photocatalytic Cements. TX Active. [Online] http://txactive.us/.
11. Background. Photocatalysis. [Online] 2012.
http://www3.nd.edu/~kamatlab/research_photocatalysis.html.
12. Experimental Study of Influencing Factors and Kinetics in Catalytic Removal of Methylene Blue with
TiO2 Nanopowder. Salehi, Marziyeh, Hashemipour, Hassan and Mirzaee, Mohammad. 1, s.l. : Scientific
& Academic Publishing, American Journal of Environmental Engineering, Vol. 2, pp. 1-7.
13. Burton, Maria Christina. Pervious Concrete With Titanium Dioxide as a Photocatalyst Compound for
a Greener Urban Road Environment. Civil Engineering, Washington State University. 2011. Thesis.
12
14. 3M. 3M Petrifilm E.Coli/Coliform Count Plate. Interpretation Guide. [Online] 2008.
http://multimedia.3m.com/mws/mediawebserver?66666UuZjcFSLXTt4XMa4xTaEVuQEcuZgVs6EVs6E66
6666--.
15. Titamium Oxide (TiO2) Assisted Photocatalytic Degradation of Methylene Blue. Madhu, G.M., Lourdu
Antony Raj, M.A. and Vasantha Kumar Pai, K. 2, October 16, 2007, Journal of Environmental Biology,
Vol. 30.
16. Discovery Education 3M Young Scientist Challenge. [Online] News Broadcast Network, 2012.
http://www.youtube.com/watch?v=71c95-LoBok.
17. Berman, Jessica. WHO: Waterborne Disease is World's Leading Killer. Voice of America. [Online]
October 29, 2009.
13
Appendix A – Methylene Blue Reduction using Aggregate TiO2
Table A1 - Various concentrations of aggregate TiO2 were tested to determine maximum reduction capabilities.
Amount of aggregate TiO2 within MB solution (g/L)
1
1.5
0
0.94
0.94
0.94
15
0.89
0.88
0.9
30
0.86
0.85
0.87
45
0.82
0.8
0.85
60
0.78
0.75
0.8
90
0.73
0.7
0.76
120
0.7
0.63
0.72
150
0.67
0.57
0.7
180
0.64
0.55
0.69
% Degradation
in 2 hours
25.5%
33.0%
23.4%
% Degradation
in 3 hours
31.9%
41.5%
26.6%
Optical Density
Time (min)
0.5
1
0.95
Optical Density
0.9
0.85
0.8
0.75
0.5 g/L
0.7
1.0 g/L
0.65
1.5 g/L
0.6
0.55
0.5
0
50
100
150
200
Time (min)
Figure A1 - TiO2 concentrations and the reduction over time.
14
Appendix B – Methylene Blue Reduction using Photocatalytic Composites
Table B1 - Various TiO2:cement ratios were tested to determine their reduction efficiency.
0
15
30
45
60
3:80
0.929
0.897
0.892
0.871
0.864
3:80 (Ball Mill)
0.929
0.899
0.896
0.875
0.867
4:80
0.954
0.932
0.909
0.902
0.900
5:80
0.969
0.933
0.928
0.882
0.871
6:80
0.958
0.929
0.920
0.879
0.865
7:80
0.952
0.935
0.916
0.892
0.876
90
120
150
0.850
0.823
0.803
0.848
0.825
0.802
0.884
0.866
0.841
0.832
0.823
0.811
0.828
0.787
0.762
0.860
0.831
0.785
180
0.792
0.791
0.828
0.811
0.742
0.772
Δ OD
0.137
0.138
0.126
0.158
0.216
0.180
% MB
Reduction
14.7%
14.9%
13.2%
16.3%
22.5%
18.9%
Optical Density
1.000
0.950
0.900
Optical Density
Time
TiO2:Cement Ratio
3g:80g
3g:80g (B)
0.850
4g:80g
5g:80g
0.800
6g:80g
0.750
7g:80g
0.700
0
50
100
150
200
Time (min)
Figure B1 - Optical density was plotted with respect to time for each composite.
15
Appendix C – Petrifilm Trial Bacterial Colony Counts
Table C1 - Bacterial colonies present in 1 mL of water from both a control bottle and one containing the photocatalytic
cement were recorded over given time intervals in the trial using a UV lamp.
Coliform and E.Coli Count - UV Light Trial
TiO2
Trial 1
Trial 2
Average
Control
Trial 1
Trial 2
Average
163
175
0
187
163
175
15
30
45
165
155
148
148
139
145
156.5
147
146.5
15
30
45
177
167
172
175
159
161
176
163
166.5
60
90
140
133
131
136
135.5
134.5
60
90
164
159
158
144
161
151.5
120
122
118
120
120
151
149
150
Time (min)
187
% MB
Reduction
31.4%
Bacteria Colony Count
0
14.3%
Table C2 – The number of blue-colored E.Coli colonies present within 1 mL of water were recorded at set time intervals
during the UV light trial.
E.Coli Count - UV Light Trial
Trial 1 Trial 2 Average Control Trial 1 Trial 2 Average
0
7
8
7.5
0
7
8
7.5
15
4
4
4
15
6
5
5.5
30
3
2
2.5
30
6
6
6
45
3
0
1.5
45
5
3
4
60
90
4
2
3
0
3.5
1
60
90
6
3
6
7
6
5
120
2
1
1.5
120
5
7
6
% Reduction
80.0%
E.Coli Colony Count
Time (min)
TiO2
20.0%
16
8
7
E.Coli Colony Count
6
5
4
Control
3
TiO2
2
1
0
0
50
100
Time (min)
Figure C1 - E.Coli levels from Table C2 were recorded with respect to time.
Table C3 - Bacterial colonies present in 1 mL of water from both a control bottle and one containing the photocatalytic
cement were recorded over given time intervals in trial using natural sunlight.
E.Coli and Coliform Count - Natural Sunlight Trial
Trial 1
Trial 2
Average Control
Trial 1
Trial 2
Average
0
187
163
175
0
187
163
175
15
30
45
177
171
168
162
169
144
169.5
170
156
15
30
45
179
171
178
182
164
175
180.5
167.5
176.5
60
90
156
152
155
146
155.5
149
60
90
173
166
169
165
171
165.5
120
146
131
138.5
120
156
167
161.5
% Reduction
20.9%
Bacteria Colony Count
Time (min)
TiO2
7.7%
17
Table C4 – The number of blue-colored E.Coli colonies present within 1 mL of water were recorded at set time intervals
during the natural sunlight trial.
E.Coli Count - Natural Sunlight Trial
TiO2
4
6
5
15
5
7
6
30
45
60
90
7
5
4
6
8
8
6
7
7.5
6.5
5
6.5
30
45
60
90
5
6
6
4
8
7
6
6
6.5
6.5
6
5
120
5
6
5.5
120
6
6
6
Time (min)
15
% Reduction
26.7%
E.Coli Colony Count
Trial 1 Trial 2 Average Control Trial 1 Trial 2 Average
0
7
8
7.5
0
7
8
7.5
20.0%
8
7
E.Coli Count
6
5
4
Control
3
TiO2
2
1
0
0
50
Time (min)
100
FigureC2 - E.Coli levels from Table C4 were recorded with respect to time.
18