SODIS Enhancement Using Photocatalytic CementBy: 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 disease-

causing 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 80-

mL 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).

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0.4 0.6 0.8 1 1.2 1.4 1.620

25

30

35

40

45

grams of TiO2 per Liter of MB Solution

Perc

ent R

educ

tion

of M

B in

3 H

ours

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.

2 3 4 5 6 710

12

14

16

18

20

22

24

Grams of Tio2 mixed with 80 g Cement

Perc

ent R

educ

tion

of M

B in

3 h

rs

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.

0 30 60 90 120100

110

120

130

140

150

160

170

180

TiO2Control

Time (min)

E.Co

li/Co

lifor

m C

ount

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.

0 30 60 90 120100

110

120

130

140

150

160

170

180

190

TiO2Control

Time (min)

E.Co

li/Co

lifor

m C

ount

Figure 7 - Average E.Coli/Coliform count present in 1 mL of water over a period of 2 hours when exposed to natural sunlight.

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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.

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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.

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14. 3M. 3M Petrifilm E.Coli/Coliform Count Plate. Interpretation Guide. [Online] 2008. http://multimedia.3m.com/mws/mediawebserver?66666UuZjcFSLXTt4XMa4xTaEVuQEcuZgVs6EVs6E666666--.

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.

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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)0.5 1 1.5

Tim

e (m

in)

0 0.94 0.94 0.94

Optical Density

15 0.89 0.88 0.930 0.86 0.85 0.8745 0.82 0.8 0.8560 0.78 0.75 0.890 0.73 0.7 0.76

120 0.7 0.63 0.72150 0.67 0.57 0.7180 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%

0 20 40 60 80 100 120 140 160 180 2000.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

0.5 g/L1.0 g/L1.5 g/L

Time (min)

Opti

cal D

ensit

y

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.

TiO2:Cement Ratio3:80 3:80 (Ball Mill) 4:80 5:80 6:80 7:80

Tim

e

0 0.929 0.929 0.954 0.969 0.958 0.952

Optical Density

15 0.897 0.899 0.932 0.933 0.929 0.93530 0.892 0.896 0.909 0.928 0.920 0.91645 0.871 0.875 0.902 0.882 0.879 0.89260 0.864 0.867 0.900 0.871 0.865 0.87690 0.850 0.848 0.884 0.832 0.828 0.860

120 0.823 0.825 0.866 0.823 0.787 0.831150 0.803 0.802 0.841 0.811 0.762 0.785180 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

Reduction14.7% 14.9% 13.2% 16.3% 22.5% 18.9%

0 20 40 60 80 100 120 140 160 180 2000.700

0.750

0.800

0.850

0.900

0.950

1.000

3g:80g3g:80g (B)4g:80g5g:80g6g:80g7g:80g

Time (min)

Opti

cal D

ensit

y

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 TrialTiO2 Trial 1 Trial 2 Average Control Trial 1 Trial 2 Average

Tim

e (m

in)

0 187 163 175 0 187 163 175

Bacteria Colony Count

15 165 148 156.5 15 177 175 17630 155 139 147 30 167 159 16345 148 145 146.5 45 172 161 166.560 140 131 135.5 60 164 158 16190 133 136 134.5 90 159 144 151.5

120 122 118 120 120 151 149 150% MB

Reduction 31.4% 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 TrialTiO2 Trial 1 Trial 2 Average Control Trial 1 Trial 2 Average

Tim

e (m

in)

0 7 8 7.5 0 7 8 7.5 E.Coli Colony Count

15 4 4 4 15 6 5 5.530 3 2 2.5 30 6 6 645 3 0 1.5 45 5 3 460 4 3 3.5 60 6 6 690 2 0 1 90 3 7 5

120 2 1 1.5 120 5 7 6% Reduction 80.0% 20.0%

16

0 20 40 60 80 100 1200

1

2

3

4

5

6

7

8

ControlTiO2

Time (min)

E.Co

li Co

lony

Cou

nt

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 TrialTiO2 Trial 1 Trial 2 Average Control Trial 1 Trial 2 Average

Tim

e (m

in)

0 187 163 175 0 187 163 175

Bacteria Colony Count

15 177 162 169.5 15 179 182 180.530 171 169 170 30 171 164 167.545 168 144 156 45 178 175 176.560 156 155 155.5 60 173 169 17190 152 146 149 90 166 165 165.5

120 146 131 138.5 120 156 167 161.5% Reduction 20.9% 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 TrialTiO2 Trial 1 Trial 2 Average Control Trial 1 Trial 2 Average

Tim

e (m

in)

0 7 8 7.5 0 7 8 7.5 E.Coli Colony Count

15 4 6 5 15 5 7 630 7 8 7.5 30 5 8 6.545 5 8 6.5 45 6 7 6.560 4 6 5 60 6 6 690 6 7 6.5 90 4 6 5

120 5 6 5.5 120 6 6 6% Reduction 26.7% 20.0%

0 20 40 60 80 100 1200

1

2

3

4

5

6

7

8

ControlTiO2

Time (min)

E.Co

li Co

unt

FigureC2 - E.Coli levels from Table C4 were recorded with respect to time.

18