Novel Photocatalytic Pervious Composites for Removing Multiple Classes of Toxins from Water

 

The goal of my research was to develop a green, sustainable, and economically viable technique for water purification. My innovative methodology integrates filtration with an enhanced photocatalytic advanced oxidation process. I synthesized novel silver (Ag) doped photocatalytic pervious composites using uniformly graded sand, Portland cement, titanium dioxide (TiO2) and silver nitrate (AgNO3). The ratio of TiO2: cement: sand was 1: 5: 20 by weight. The optimum amount of Ag was 0.04% by weight of the composite. This composition was determined from photodegradation studies of the organic indicator dye methylene blue, using UV-Vis spectroscopy. The photodegradation of methylene blue conformed to pseudo-first order kinetics according to the Langmuir–Hinshelwood model. Bacterial inactivation studies with the Ag-doped photocatalytic pervious composite showed 98% reduction in total coliform bacteria immediately after filtration. Subsequent exposure of the filtered water to sunlight inside a beaker containing an Ag-doped photocatalytic composite disc resulted in 100% inactivation of total coliform bacteria in just 15 minutes. In the future I will focus on studying the feasibility of removing different types of pathogens, organics, pesticides, and heavy metals. Safety, durability and long-term performance will also be investigated in detail before the filter is deployed in the real-world. My project opens numerous possibilities for safe, cost-effective, and eco-friendly water purification.

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My name is Deepika Kurup and I am a senior at Nashua High School South in Nashua, NH, USA. I was exposed to the water problem at a very early age during my summer visits to India. The plight of children drinking polluted water from nearby streams greatly troubled me. I later learned that one-ninth of the population lacks access to clean water, and this unacceptable social injustice compelled me to find a solution to the world’s water problem. I enjoy the thrill of finding solutions to real world problems, and for the past three years I have been conducting sustained research to help solve the global water crisis. I also give talks around the world, and encourage students to participate in science, technology, engineering and math (STEM). I strongly believe that STEM education has the power to lead us to revolutionary new discoveries that can solve global challenges.

For my environmental stewardship, I was honored by the White House and the Environmental Protection Agency with the “President’s Environmental Youth Award”, and was named one of 2015 Forbes’ 30 Under 30. This fall I will be attending Harvard College, where I will continue to pursue my passion for scientific research and solving the global water crisis. I believe that the Google Science Fair prize will provide me with the platform to share my ideas, and help me deploy my invention in places around the world.

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Can a filter that removes multiple classes of toxins from water be engineered using novel photocatalytic composites?

The overall objective of my research was to synthesize and evaluate novel photocatalytic pervious composites for water purification. I hypothesized that an Ag-doped photocatalytic pervious composite would be more effective in removing bacteria and degrading organics than a plain pervious composite (without any photocatalyst). I developed a simple method to synthesize the photocatalytic pervious composite using sand, TiO2, Portland cement, and silver nitrate (AgNO3). My composite is envisioned to have potential applications in wastewater treatment plants, point-of-use water (POU) purification systems, and surface layer coatings for pervious concrete pavements.

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Solar Disinfection (SODIS)

Currently in many developing countries, SODIS is used to purify water. Clear plastic bottles are filled with the contaminated water, and exposed to sunlight for 8 hours or more. Ultraviolet (UV) radiation from the sun disrupts the DNA of harmful pathogens by creating pyrimidine dimers around thymine and cytosine base pairs. This prevents DNA replication and eventually rids the water of pathogens. While the SODIS process is easy to use and cost-effective, it is often very slow.


Photocatalytic Advanced Oxidation Process (AOP)

In recent years, photocatalyst such as titanium dioxide (TiO2) has been used to accelerate the SODIS process. When UV-A radiation strikes the photocatalyst, electrons (e-) from the valence band are energized into the conduction band, thereby leaving positive holes (h+) in the valence band. Some of the electrons and holes may recombine, but most combine with oxygen and water to create reactive oxygen species (ROS) such as super oxides (O2-), hydroxyl radicals (·OH) and hydrogen peroxide (H2O2) (Figure 1, Equations 1-5). Through the advanced oxidation process (AOP), organics such as methylene blue are oxidized by ·OH radicals, into carbon dioxide and water. The reactive oxygen species also destroy bacteria by damaging their cell structure and disrupting their DNA, thereby preventing them from replicating.

 

Disadvantages of Current Photocatalytic Methods

  1. The photocatalyst is coated on the inside of clear bottles; this blocks UV radiation & diminishes photocatalysis (Figure 2). 
  2. The coatings are not tightly bound to the plastic bottle and wash off into the water (Figure 3).                     
  3. It is difficult to recover photocatalytic nanoparticles from the slurry.
  4. Traditional photocatalytic SODIS uses only UV-A radiation (UV-A is 3%, while visible light is 44% of solar energy).

 


Enhanced Photocatalytic AOP

To enhance and extend the photocatalytic activity from the UV into the visible light spectrum, researchers have tried doping the photocatalyst with silver (Ag).

  1. Ag introduces an intermediate band, which reduces the band gap energy and allows visible light absorption (Figure 4).     
  2. Ag acts as an e- scavenger, and prevents e--h+ recombination. This creates more ROS, and results in greater bacterial inactivation (Figure 5).
  3. Ag has inherent bactericidal properties (Figure 6). It binds to and alters bacterial nucleic acids, disrupts the cell membrane, and deactivates enzymes.

 


Pervious Concrete

Pervious concrete is a type of high porosity concrete that is permeable to water. It is becoming increasingly popular in the construction of pavements for parking lots and pedestrian walkways, and its applications are recognized by the U.S. Environmental Protection Agency (EPA) as best management practices for storm water mitigation. Pervious concrete reduces runoff from paved areas, permits the use of smaller storm water sewer, and filters water and prevents pollutants from being discharged into water bodies and ground water. However, the void size of pervious concrete is typically large (> 0.45μm) and unable to filter bacteria efficiently.

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Synthesis of Preliminary Compositions

To evaluate the efficiency of Ag-doped TiO2 for degrading organics, six photocatalytic compositions were prepared (Figure 7). From permeability and strength tests, a ratio of cement: sand of 1: 4 was selected for the pervious composites. A ratio of TiO2: cement: sand of 1: 5: 20 was selected for the photocatalytic compositions. For the Ag-doped photocatalytic compositions, six samples were prepared using various amounts of Ag (Table 1, Figure 8). A seventh sample served as the control and was a plain mixture of cement and sand (in the ratio 1:4) with no Ag or TiO2

To synthesize the preliminary compositions,

  1. TiO2 and sand were proportioned and mixed in the ratio 1: 20 by weight to form photocatalytic sand.

  2. An AgNO3 solution was prepared in deionized water at various concentrations to achieve the desired wt.% Ag.

  3. The AgNO3 solution was uniformly mixed with the photocatalytic sand and exposed to sunlight for one hour. 

  4. The samples were calcined at 300oC for 3 hours to form the Ag-doped photocatalytic sand.

  5. The Ag-doped photocatalytic sand was mixed with cement to achieve a TiO2: cement: sand ratio of 1: 5: 20 by weight. 

  6. Deionized water was added at a water-cement ratio of 0.3, mix uniformly to form wet mortars. 

  7. Wet mortars were placed inside 5 cm diameter, 3.8 cm deep containers, and lightly compacted to a thickness of 0.3 cm.

  8. Samples were moist cured for 28 days.

 


Degradation of Methylene Blue

The photocatalytic activity of the seven preliminary test samples was evaluated by the photodegradation of methylene blue under sunlight (Figure 9). The color change (from blue to clear) was quantitatively determined from samples taken at various time intervals and analyzed using an Agilent 8453 UV-Vis spectrophotometer at the University of Massachusetts Lowell. The degradation of methylene blue was the fastest for sample 4 (with 0.04% Ag in composite), and this ratio was selected for fabricating the photocatalytic pervious composite filter. 

 


Synthesis of Pervious Filters

Two pervious composites were fabricated: a plain pervious composite filter using the composition of sample 7 (control), and a photocatalytic pervious composite filter using the composition of sample 4. The composite filters (7.0 cm diameter and 2.5 cm thick) were formed by placing the wet mortar inside 10.0 cm long plastic tubes. Physical properties determined include bulk dry unit weight, porosity and permeability (Figure 10).

 


Bacterial Inactivation Studies

Wastewater for testing was obtained from the Nashua Wastewater Treatment Facility (NWTF) located in New Hampshire, USA. Two 200 mL samples of wastewater were taken in glass beakers. The first sample was filtered using the plain pervious composite (control), while the second was filtered using the photocatalytic pervious composite. The filtered water was collected in two glass beakers containing disks (5.0 cm diameter, 0.3 cm thick) made of the respective composite mixtures. The unperturbed water samples were exposed to sunlight for 4 hours. Total coliform counts (TCC) were determined with 3M Petrifilms before filtration (initial bacteria count), and after filtration at 0h, 0.25h, 0.5h, 1h, 2h, 3h, 4h.

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Physical Properties

Physical properties of the photocatalytic and plain pervious composite filters are given in Table 2.

 


Photocatalytic Degradation of Methylene Blue

Methylene blue absorbance spectra for the six preliminary photocatalytic composites and the control after 2 hours of sunlight exposure are shown in Figure 11. The degradation was studied by monitoring the absorbance of the methylene blue solution at the peak absorption wavelength of 664 nm. It can be seen that the control with no photocatalyst showed the least, and sample 4 with 0.04% Ag in the photocatalytic composite, showed the most degradation of methylene blue.

 

Kinetic Studies for Modeling the Photocatalytic Degradation of Methylene Blue

Figure 12 shows the photodegradation efficiency of the plain pervious composite and photocatalytic pervious composite (using the optimum composition of sample 4) calculated from the UV-Vis spectra of the methylene blue solution at its maximum absorption wavelength (664 nm). 

 

The photocatalytic AOP is a heterogeneous process since the reaction occurs at the surface of the catalyst. The Langmuir–Hinshelwood model (Equation 6) can be used to describe the kinetics of the photocatalytic degradation of aqueous methylene blue on a heterogeneous catalytic system.

Equation 6: Equation X 

r is the degradation rate, C is the concentration of aqueous methylene blue at time t, kr is the rate constant, and Kad is the adsorption equilibrium constant. 

When the adsorption is weak and/or methylene blue concentration is low, the Langmuir–Hinshelwood model simplifies to a pseudo-first order kinetic model (Equation 7).

Equation 7: 

ka is the apparent first order rate constant

Figure 13 shows linear relationships between ln(Co/C) and sunlight irradiation time for the plain pervious composite and photocatalytic pervious composite. This linear relationship confirms that the photocatalytic degradation of methylene blue obeys pseudo-first order kinetics according to the Langmuir–Hinshelwood model. The apparent first order rate constants of photocatalytic degradation by the plain pervious composite and photocatalytic pervious composite are 0.1784 h-1 and 0.9780 h-1 respectively. The higher rate constant of the photocatalytic pervious composite signifies greater photocatalytic activity in degrading organics, when compared with the plain pervious composite.

 


Bacterial Inactivation Studies

Figure 14a shows the average of the total coliform counts (TCC) for the photocatalytic pervious composite filter and the plain pervious composite filter. The TCC for the photocatalytic pervious composite filter dropped from an initial average value of 225 to 5 colony forming units per milliliter (cfu/mL) immediately after filtration. This value further dropped to 0 cfu/mL after 15 minutes of sunlight exposure following filtration. The TCC for the plain pervious composite filter dropped from an initial average value of 191 to 152 cfu/mL immediately after filtration. It took 3 hours of sunlight exposure following filtration for this value to drop to 0 cfu/mL. Figure 14b shows the normalized TCC values. The average TCC at different time intervals (t hours) were normalized with respect to their initial counts (before filtration) in order to easily compare the effectiveness of the photocatalytic pervious composite and the control.

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CONCLUSIONS

My experimental results support my initial hypothesis, as my novel photocatalytic pervious composite was more effective in removing bacteria and degrading organics than a plain pervious composite. The plain pervious composite filter had a weight ratio of cement: sand of 1: 4 and a water-cement ratio of 0.3. The optimum composition of the Ag-doped photocatalytic pervious composite filter consisted of 0.04% Ag by weight of composite with a weight ratio of TiO2: cement: sand of 1: 5: 20.

The photocatalytic degradation of methylene blue obeyed pseudo-first order kinetics according to the Langmuir–Hinshelwood model. The apparent first order rate constants of photocatalytic degradation by the plain pervious composite and photocatalytic pervious composite were 0.1784 h-1 and 0.9780 h-1 respectively. This means that the photocatalytic pervious composite showed greater photocatalytic activity in degrading organics, when compared with the plain pervious composite. Filtration by the plain pervious composite showed only a 20% reduction in total coliform bacteria, whereas the photocatalytic pervious composite showed a 98% reduction in total coliform bacteria.  Subsequent exposure of the filtered water to sunlight inside beakers containing the composite discs made of their respective compositions, resulted in 100% inactivation of total coliform bacteria by the plain composite disc in 3 hours and by the photocatalytic composite disc in just 15 minutes.


SUSTAINABILITY 

The photocatalytic water purification technology developed in my research is safe and environmentally-friendly, as it does not produce any toxic byproducts. The purification process uses only solar energy, which is abundant in developing countries that lack large and expensive infrastructure for wastewater treatment. Also, the photocatalytic pervious composite is cost effective since the materials used to synthesize it are inexpensive and abundantly available.


DEPLOYABILITY

Large water tanks may be constructed with a pervious photocatalytic composite bottom, supported by a gravel drain (Figure 15). The filtered water may be led to a second tank containing photocatalytic rods where it is exposed to sunlight and further purified. Various types of point-of-use water purification systems can also be constructed for house-holds or industry to treat water on-site. Photocatalytic water purification panels (Figure 16) and pervious water filters (Figure 17) may also be fabricated for purifying water for potable use.

 


FUTURE

In the future, I would like to i) investigate the inactivation of different types of pathogens (including protozoa and viruses); ii) study the feasibility of degrading different types of organics and pesticides; iii) study the feasibility of removing heavy metals and other inorganic water pollutants; and iv) characterize the void size and its distribution; and v) evaluate the safety and performance of the composite filters over time.


IMPACT

The environmental and societal impact of my research is vast, considering the scarcity of clean water and the large number of people that are affected by the global water crisis. The technology I have developed has the potential to provide clean water for the 780 million people that lack safe drinking water, and could be a tremendous resource even in developed countries during periods of drought, and after natural disasters.

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Acknowledgements

I am grateful to Professor Ramaswamy Nagarajan for mentoring me and for providing me access to the laboratory facilities at the University of Massachusetts Lowell. I am also thankful to Mrs. Linda Polewarczyk, my chemistry teacher from Nashua High School South, for her constant encouragement and support. I would like to acknowledge Dr. Jim Jonza, 3M Center, St. Paul, MN for his guidance, and for providing me with 3M Solar Reflector Films, 3M Petrifilms and a 3M Petrifilm Plate Reader. Mrs. Geraldine Ciardelli and Ms. Noelle Osborne from the Nashua Wastewater Treatment Facility have also been very supportive of my research and have provided me with wastewater samples for testing. Finally, I would like to thank my parents and sister who have always been a constant source of inspiration during my research.


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