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Titanium dioxide ( TiO2 ) has been found to be a popular pick of stuff in moving as a photocatalyst in the purification of H2O. There have been legion scientific surveies on the photocatalytic inactivation of micro-organisms in H2O utilizing TiO2. In this reappraisal, the current position and future chances of utilizing TiO2 immobilized on a surface in H2O will be studied and discussed by looking at a figure of different surveies.

Each of the surveies will be scrutinised in item and the writers ‘ positions will be compared for similitude and differences. In add-on, they will be assessed to find if the articles make an original part to this field.

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2.0 Introduction

Photocatalysis can be defined as the acceleration of a photoreaction in the presence of a accelerator, the accelerator being Titanium Dioxide ( TiO2 ) . It was Frank and Bard ( 1977 ) , who were the first scientists to analyze the consequence of utilizing TiO2 to interrupt down nitrile in H2O, and of all time since research workers have been utilizing TiO2 for environmental redress. The advantages that this compound has in the environment are that it is comparatively inexpensive, it is chemically extremely stable and the photogenerated holes that it produces are extremely oxidising, hence are able to bring forth the photoreactive substances that can finally demobilize micro-organisms.In photocatalytic H2O intervention there are two ways in which the TiO2 can be applied, either in a pulverization signifier or in an immobilized signifier.

The trouble in retrieving TiO2 pulverization from treated H2O is a major obstruction for the instrumentality. In order to avoid utilizing TiO2 pulverization, which involves later separation from the H2O, scientists began to work on ways of immobilising TiO2 atoms, for case in thin movie signifier or as a coating on assorted types of supports such as ceramic tiles. Coating the TiO2 with silicon binder is one of the most normally used immobilisation methods, Si being the discriminatory pick due to its stableness to photocatalysis.

As the sum of binder is increased, this provides a greater physical strength to the coated bed, but its photocatalytic efficiency is decreased ( Kaneko and Okura, 2002 ) .

2.1 Purposes

* Understanding the history behind this procedure and when it foremost originated.* Understanding how photocatalysis with TiO2 plants and what photogenerated substances are produced.

* Investigating the different methodological analysis used in photocatalysis and which was found to be the most effectual in destructing microorganisms nowadays in H2O.* Looking for the optimal conditions with which immobilized TiO2 can be combined to accomplish the best consequences.* The future chances of this procedure and how it can be farther explored.


Organic affair and micro-organisms can undergo oxidization when negatrons are removed and decrease when negatrons are added. The negatron hole braces originate on the TiO2 surface once it is excited by visible radiation. These electron holes in the valency set so respond with species that are adsorbed on their surface such as H2O or hydrated oxide ions to bring forth hydroxyl groups ( OHa?™ ) which are highly reactive along with the negatron holes when they come into contact with organic affair. The negatrons in the conductivity set on the other manus can originate the decrease of O which consequences in the production of superoxide ions ( O2- ) .

The cell membranes are indispensable in the violent death mechanism for free groups. Cell membranes are composed of unsaturated lipoid molecules which are susceptible to oxidative harm by free groups as they undergo lipid peroxidation. Deoxyribonucleic acid in the chondriosome of a cell is particularly affected and unlike atomic Deoxyribonucleic acid which can be fixed rather easy, the Deoxyribonucleic acid in a chondriosome can non be as easy repaired. This Deoxyribonucleic acid harm accumulates over a period of clip and interrupt down the chondriosome which can no longer bring forth energy for the cells to map and therefore causes cell decease.In 1985 Matsunaga and his squad thought that it was the passage of the intracellular coenzyme A to its dimeric signifier, due to direct photochemical oxidization, that was the cause for the decreased respiratory rate that caused cells to decease.

They besides proposed that the sum of killing decreased as the cell wall thickness and complexness increased. In 1992, Saito and his squad so progressed a small farther with Matsunagas findings to suggest that photocatalytic inactivation with TiO2 led to the break of the cell membrane and cell wall of the Streptococcus sobrinus AHT, due to intracellular K ions leaking out of the cell doing cell decease. Other biological molecules that can be affected include RNA, and proteins which were found to be released from the cells at a decreased rate due to a longer reaction clip.Then in 1998, Sunada et Al, were analyzing the effects of TiO2 photocatalysis on E.coli cell construction, and found that it destroyed an indispensable portion of the outer cell membrane, the endotoxin. In 1999, Maness et Al, utilizing E.

coli K-12 cells investigated the underlying mechanism of decease utilizing UV visible radiation and found that there was an exponential addition in the polyunsaturated phospholipid constituent of the cell membrane which is the chief site of onslaught by reactive O species, and a 77 to 93 % decrease in cell respiratory activity.In 2007, Cheng et Al, studied the consequence of the procedure on strains of the Legionella pneumophila group of beings. Using transmittal and scanning negatron microscopy, they were able to observe lipid peroxidation of the outer and cytoplasmatic membrane, which triggered hole formation and allowed entry to A·OH into the cell. This free group was so able to oxidize the internal cellular constituents and do decomposition of the cell. The alteration in the fatty acids composing of the membrane was besides suggested due to the seeable decrease in the sheen of the surface of the settlements that were formed.



Harmonizing to BelhaA‰ova . et Al, photocatalysis is a better method compared to chlorination as this produces chlorinated hydrocarbons which causes H2O to go toxic. It is besides a cheaper method in comparing to the application of the ozone and UV radiation entirely. Ozonation and chlorination can besides bring forth potentially carcinogenic disinfection byproducts, among the most unsafe of which are the trihalomethanes.Photocatalytic inactivation utilizing TiO2 works via the photovoltaic consequence. The theory of this procedure is rather complex, therefore it is necessary to explicate what this involves before get downing to understand how TiO2 is incorporated in this procedure ( Kaneko and Okura, 2002 ) .The procedure can be explained by looking at photons which are packages of energy and can arise from sunshine. When a photon in sunlight hits the surface of the semiconducting material, which in this instance would be the TiO2, it can do an array of possibilities.

If it is a low energy photon, it can go through directly through, and some photons can even reflect off the surface. If the photon energy is higher than the set spread value ( which is defined as the energy difference between the top of the valency set and the underside of the conductivity set ) of the semiconducting material so it can go absorbed by the semiconducting stuff. When the photon is absorbed, this generates an negatron hole brace and sometimes heat depending on the construction.

When a photon is absorbed, its energy is given to an negatron in the crystal lattice construction in the semiconducting material, normally situated in the valency set. This negatron will be bound in covalent bonds between neighboring atoms and hence unable to travel far. The energy given to the negatron by the photon ‘excites ‘ it into the conductivity set where it is free to travel about within the semiconducting material. The covalent bond which the negatron was released from now has one less negatron and is referred to as a ‘hole ‘ . The presence of this losing covalent bond allows bonded negatrons of neighboring atoms to travel into the hole, go forthing another hole behind and it is in this manner that a hole can travel through a lattice. These complementary positive charges or “ holes ” flow in the opposing way to the negatrons. Therefore it can be deduced that photons absorbed in semiconducting materials creates mobile electron hole braces.When a bed of n-type semiconducting material is in direct contact with a p-type semiconducting material, negatrons diffuse from a part of high negatron concentration, which is the n-type side of the junction, to a part of low negatron concentration which is the p-type side of the junction.

When negatrons diffuse across the p-n junction, they recombine with holes on the p-type side. Charges can construct up on either side of the junction, and this creates an electric field which creates a rectifying tube that encourages charge flow that opposes and finally balances out the diffusion of negatrons and holes. If semiconducting materials were attached to both the p- and n-type sides of the solar cells and electrodes connected to an external burden, negatrons created on the n-type side or negatrons that have been collected by the junction and swept over to the n-type side, may go through the wire, power the external burden, and continue through the wire until they reach the p-type semiconducting material metal contact. Here they can recombine with the hole that was created as an negatron hole brace on the p-type side or a hole that was swept across the junction from the n-type side after being created at that place. This procedure can make a possible gradient at the interface of the semiconducting material and a liquid stage. If alternatively of this agreement, the semiconducting material was present as solid atoms submerged in a liquid such as H2O, one time the atoms were excited so this would do redox reactions to happen in the interfacial part around each atom.

The redox reactions can happen with adsorbed species such as H2O, hydroxide ions ( OH- ) , organic compounds, or O. The charges can respond straight with adsorbed pollutants, but reactions with H2O are far more likely since there are far more H2O molecules than contaminant molecules. Oxidation of H2O or OH­­­a?’ by the hole produces the hydroxyl extremist ( OHA· ) , an highly powerful oxidant and other oxidative species such as H2O2, and superoxide groups such as O2A· are besides generated. OH groups quickly attack pollutants at the surface, and perchance in solution every bit good, and are normally the most of import groups formed in TiO2 photocatalysis ( Guimaraes and Barretto, 2003 ) .



The applications of photocatalysis have been turning since Fujishima and Honda ( 1972 ) discovered the photocatalytic decomposition of H2O into H and O utilizing TiO2 electrodes, which was so labelled the “ Honda-Fujishima consequence ” . Applications of this technique have been employed in many Fieldss including imbibing H2O intervention, industrial, and wellness applications, many of which are described below.It is besides utile in the devastation of organic compounds, inorganic compounds, natural organic affair but most significantly it can be used to destruct bacteriums and viruses ( Al-Rasheed, 2005 ) . Some of the beings that it can demobilize include Streptococcus mutans, Streptococcus natuss, Streptococcus Cricetus, Escherichia coli, Saccharomyces cerevisiae, Lactobacillus acidophilus, and the poliovirus.



1 Photocatalytic inactivation of micro-organisms utilizing a flow-through photoreactor with an immobilized TiO2 bed.

In this experiment a flow-through reactor with glass home base coated with bed of TiO2 was used to demobilize a strain of Escherichia coli called DH5I± and bacteriophage I»NM1149. The glass home base, made out of polymethylmethacrylate, was of length 60 centimeter and width 30 centimeter, and was prepared by the deposit of aqueous suspension of TiO2 of the type P-25 ( Degussa ) , at a concentration of 5 g dm-3.

The solution was made more acidic by the add-on of azotic acid. The home base was so allowed to dry before being heated at 300A°C for 3 hours so that the TiO2 could put. The home base was situated at an angle of 10A° above the land so that a movie of liquid could run along it at a velocity of 62 cm3s-1. The temperature of the liquid was kept controlled at 25A°C.

UV lamps with a light strength of 0.9 to 6.2 ten 10-9 Einstein cm-2s-1 were used as the beginning of radiation and these were situated sheer to the glass home base with differing wavelengths between 300 nanometers to 400 nanometers. The volume of the liquid that was circulated throughout the system was 5000 cm3.The purpose of this experiment was to happen the magnitude of inactivation that TiO2 can do when it is attached and immobilized to a surface instead than being present as a pulverization, which can do jobs when it comes to separation at a ulterior phase. This experiment was designed to demobilize the bacteriums and viruses present in the H2O sample by the production of hydroxyl groups which are capable of destructing the cell construction of these micro-organisms.

This method has been developed to supply better ways to handle drinking H2O instead than utilize antecedently used techniques such as chlorination which can bring forth chlorinated hydrocarbons which are unsafe if ingested or utilizing the ozone or UV radiation entirely which can be rather expensive.Sing the E coli strain DH5 I± , after 6 hours of being under the experiment, 99.6 % of the cells were found to be damaged for good.The controls which were done without UV radiation, showed no difference in the endurance of micro-organisms after the 6 hours under experimentation.

The writer is cognizant that UV radiation entirely can do inactivation of micro-organisms and it was shown in the experiment UV radiation entirely caused the figure of feasible beings to drop by 50 % or the original concentration, this can be seen by looking at the figures given in Figure 2, where it states that with radiation the N/N0 count ( N is the existent concentration and N0 is the initial concentration of bacteriums ) is 81000 CFU cm-3 whereas without UV radiation the count was 43000 CFU cm-3. For the bacteriophage I»NM1149, after 3 hours under experimentation, the concentration was reduced by more than four orders of magnitude and was besides affected by the UV radiation.Insert Fig 2 from article – put at terminal as appendix.The consequences seem just as they take into history every factor that could impact the experiment, for illustration the ionic strength of the aqueous solution could hold affected the rate invariable so these were adjusted utilizing assorted solutions so that the solution in the experiment mirrored that of the H2O in the environment. The lessening in clip for the bacteriophage was explained due to its size compared to bacteriums which are much larger and would therefore take more clip to interrupt down. However, the sum of ‘dead infinite ‘ on the glass home base was unaccounted for, and this could hold affected the consequences, volumes of H2O that were non in contact with the TiO2 and some parts may hold had a higher concentration of the compound than other countries, doing the surface less equally spread and hence some micro-organisms may hold escaped being damaged due to the absence of sufficient TiO2 or merely damaged to a certain point when they could retrieve once more. Some farther suggestions for this experiment to be explored farther would be to prove the home base at different decline angles to see if a slower flow rate or a larger surface country would impact the inactivation rate ( BelhaA‰ova , L.

et Al. 1999 ) .



1 TiO2 Solar photocatalysis

This technique has proved to be successful in H2O disinfection, which is of import particularly in developing states. In this experiment, compound parabolic concentrators ( CPC ) were used and the TiO2 was fixed on a solid support and besides suspended individually for comparing. The bacterial strain used was Escherichia coli K-12. The consequences showed a degree of inactivation of bacteriums with merely the CPC solar aggregator and no accelerator but this was found to be uncomplete as regrowth of the E. coli suspension was detected.

Complete inactivation of the bacterium was merely achieved when the reactor was combined with TiO2 either in the suspended or fixed signifier ( Fernandez, P. 2005 ) .


2 Photocatalysis utilizing UV visible radiation

A big figure of experiments that investigate photocatalysis usage sunlight as the energy beginning to excite negatrons. Sunlight is made up of a spectrum of electromagnetic radiation, which is composed of UV A ( UV-A ) , ultravliolet B ( UV-B ) , ultraviolet C ( UV-C ) , infrared and seeable visible radiation. The part that the procedure photocatalysis is concerned most with is the UV and seeable parts which have the lowest wavelength compared to the other parts. Out of the three types of UV visible radiation, it is merely UV-A, which has a longer wavelength between 320 and 400 nanometer, and some UV-B, which has a wavelength between 290 to 320 nanometers, that can make the Earth ‘s surface. The ozone bed is the barrier that filters out any UV-C and some UV-B which are the most detrimental to both aquatic and tellurian beings. UV visible radiation does non supply H2O molecules with adequate energy to divide the molecule up but UV-B can bring forth two molecules of OHA· for every molecule of H2O2 nowadays. These free groups can damage the Deoxyribonucleic acid of beings and can do more hurtful mutants.

UV-B can besides be absorbed by T or C and create aroused provinces that are capable of responding with H2O to bring forth pyrimidine hydrates or with an next pyrimidine to bring forth cross-links to bring forth pyrimidine dimers. These dimmers can do a alteration in form of the DNA which can do jobs in DNA reproduction. The cells can seek to mend this alteration but this in bend can take to mutants as good. UV visible radiation besides leads to oxidization of polyunsaturated fatty acid ( PUFA ) side ironss and cholesterin in the membrane of micro-organisms. Typically as the wavelength of light additions, the energy emitted lessenings. This is why most of the experiments use around 360 nanometer for the wavelength of UV visible radiation and 380 nanometer for seeable visible radiation for the photocataysis procedure as these wavelengths are the most effectual at destructing micro-organisms. If utilizing UV radiation entirely, it was found that a wavelength scope of 240 to 280 nanometer was the most effectual in bring forthing irreversible harm to bacterial cells ( Wolfe, 1990 ) . If utilizing sunshine which has a UV radiation wavelength of about 280 nm alternatively of a UV lamp so this works merely every bit good and the added heat besides aids in the violent death procedure.

In 1994, Zhang, et Al found that 99 % of the E.coli beings were inactivated in TiO2 suspensions when exposed to sunlight for 23 min.What wavelengths are most effectual at killing? Low wavelength has more energy than higher wavelengths so can transgress set spread, some experiments try and change the TiO2 so that the set spread can be breached so they can utilize higher wavelengths.Since there are several jobs in traditional UV disinfection techniques, a extremely efficient, dependable and economical method, utilizing quartz optical fibers to present UV visible radiation is proposed which provides a promising attack to disinfection intervention of imbibing H2O.

The rule of the experimental apparatus is that ultraviolet beams are gathered by a reflector and converge on a light point, the diameter of about 5 millimeters. In this manner UV visible radiation can be transferred into H2O to kill the bacteriums in the H2O. Its suitableness for application is shown in experiment with E. coli ( ATCC8099 ) as trial micro-organism. The distribution of the optical fibres has been optimised in the H2O in bench-scale survey but the consequences can supply counsel for pilot-scale and field-scale survey of this new technique. The consequences show that this UV irradiation technique offers an attractive option to conventional UV disinfection methods for imbibing H2O because the UV irradiation transferred by optical fibres caninactivate E. coli.

It was found that the figure of lasting E. coli decreased by increasing irradiation clip. The optimized distribution of optical fibres in a 500 milliliter UV reactor was 400 with 5 millimeters fiber spacing. At present the former method can non vie economically with the UV irradiation units presently used.

Further research is needed in order to obtain indispensable information on long-run behavior, scale-up, finding of irradiation denseness distribution, and cell devastation. Finally, improved energy use efficiency and a big figure of hazard micro-organism groups besides have to be tested ( Lu, et Al. 2008 ) .

6.3 Comparison of the Degussa P25, a hydrothermally prepared photocatalyst and a magnetic photocatalyst for disinfectant efficiency

The TiO2 pulverization used in this experiment called Degussa P25 is made up of around 79 % anatase and 21 % rutile and was immobilised on the interior wall of the reactor with a photodeposition of Ag nitrate.

There have been studies that Ag has a important disinfectant consequence on E.coli, as it can pin down the negatrons in the photocatalytic procedure, forestalling the recombination of negatrons and holes and hence advancing the production of reactive O species, therefore it has been included in this experiment to see if it causes an consequence on the inactivation of these beings ( Yamanaka, M. et Al.

2005 ) .Heating formless titanium dioxide in air at a specific temperature for specific sum of clip consequences in a single-phased nanocrystalline anatase pulverization which was used as the hydrothermally prepared photocatalyst.The magnetic photocatalyst ( MPC ) is made up of magnetic colloidal atoms which consists of a bed of Si dioxide ( SiO2 ) and so followed by a bed of TiO2.

The advantage that the magnetic photocatalyst has is that it can be used as suspension, which means there is a higher rate of contact between the photocatalyst and the E.coli micro-organism and it can besides be easy removed at the terminal one time a magnetic field is applied. The consequences of the experiment showed that Degussa P25 was the most effectual at inactivation E. coli, followed by HPC and so MPC. Silver was found to heighten the action of the Degussa P25 and hence proved to be an effectual bactericidal photocatalyst ( Coleman, et Al.

2005 ) .

6.4 Treatment of effluent utilizing UV/TiO2, RO and UF

This survey was carried out to seek and extinguish the production of harmful by-products of the chlorination procedure and besides to seek and extinguish the Cl immune pathogens, and the 1 that was looked at in this experiment was Strongyloides stercolaris parasites. To equal the photocatalytic procedure utilizing UV visible radiation and suspended TiO2 they employed other advanced intervention procedures such as ultrafiltration ( UF ) and rearward osmosis membrane procedures ( RO ) . All three procedures resulted in complete remotion of the parasite and the RO procedure produced the highest decrease in chemical O demand of the effluent which means that there was less organic stuff to be oxidized. However the advantage that the UV/ TiO2 procedure had was that it did non bring forth any waste whereas the UF and RO processes produced a watercourse of waste H2O incorporating contaminations and parasites which can do more jobs in footings of disposal. In add-on these two procedures require chemicals to be added to the system in order to protect the membranes, and acquiring rid of these can be harmful to the environment. However, the advantage that these two procedures have over the UV/ TiO2 procedure is that they consume less energy and are hence less expensive to run, but this increased cost can be balanced when looking at the fact that this procedure does non bring forth a contaminated waste watercourse, all that is needed is the TiO2 accelerator to be replaced every few old ages as it has a long life-time ( Al-Bastaki, 2004 ) .

6.5 Comparison of the TiO2 photocatalyst with polyoxometalates ( POM ) as a heterogeneous photocatalyst.

An experiment was done to compare the TiO2 photocatalyst with three types of polyoxometalates ( POM ) which are anions made up of three or more metal oxyanions joined by O atoms to organize a 3D-structure that is good organised. These all act likewise to the TiO2 photocatalyst to demobilize micro-organisms as they besides absorb visible radiation in a similar manner and are capable of bring forthing hydroxyl free groups when they are excited.

POM can besides be immobilized on an oxide support. In this experiment E.coli and B.

subtilis were the beings that were studied under this procedure. The consequences from this experiment really showed a faster inactivation rate of E.coli and B. Subtilis organisms with POM under UV visible radiation.

PMo12 was recorded to demo the highest inactivation rate for E.coli and PW12 showed the highest inactivation rate for B. Subtilis. Another interesting discovery was that TiO2 had a higher inactivation efficiency for E.

coli than B. Subtilis which proved that the efficiency of each photocatalyst isdifferent harmonizing to the species that is used on ( Bae, et Al. 2008 ) .Following stairss for undertaking research:* Detailed account of how the groups affect the construction of the bacterial cell walls and cause cell lysis. – killing mechanism? Lipid peroxidation, affects phospholipids in membrane.* Research on PCD consequence on more different beings as the 1s most popularly studied is E.coli.* Preparation of crystalline TiO2 thin movies utilizing sol-gel and rearward micelle methods.

* More elaborate information and experimental methods on suspended TiO2 V immobilized TiO2. Immobilizing eliminates the demand for remotion of the titanium dioxide but is less efficient – suspended is better because more contact and higher surface country but besides because… … .

.. … .

. Better when the H2O is recycled, more opportunity of adhering to the accelerator.* Composite TiO2 movies grown by chemical vapor deposition – Silver/ Palladium oxide and nitrogen-doped Ti oxide* TiO2 immobilized on assorted stuffs – sintered glass cylinders, pumice rock* Use of Electrochemically assisted photocatalysis to heighten TiO2 photocatalysis – utilises the application of a little possible prejudice to a TiO2 electrode and has been reported to increase the rate of photocatalytic debasement of chemical and microbiological pollutants. EAP decreases electron-hole recombination and physically offprints charge bearer oxidation-reduction sites by taking photogenerated negatrons to the counter electrode.* PCD with Ti dioxide in air – Penicillium expansum, present in cold storage suites, affects fruits and veggies shelf life so of import in nutrient industry – merely reference, brief paragraph in debut.* PCD with Ti dioxide for ego cleansing* Comparison with other methods of remotion of micro-organisms utilizing UV entirely and chemical sterilization, ozonation, chlorination – merely reference, brief paragraph in debut.

* Table/chart of different systems used and the per centum efficiency of each, looking at the different methodological analysis such as different reactors, differentstrengths for the UV visible radiation, different sums of solution in the reactor, anddifferent surface countries for the reactors. But be critical of the fact that it is about impossible to compare many surveies as they have used different strains of bacteriums, different media every bit good as different trial systems.* Draw diagrams and simplify as it looks better.

Feedback from oral:

UV better than TiO2? How efficient is UV and TiO2 at killing?How are the free groups formed? Talk about semi music director non being a music director but becomes conductive when it is excited with energy from sunlight etc.What rays are in sunshine and what wavelengths?What wavelengths are most effectual? 360 for UV and about 380nm for seeable.

UVa longer wavelength.What wavelengths are most effectual at killing? Low wavelength has more energy than higher wavelengths so can transgress set spread, some experiments try and change the TiO2 so that the set spread can be breached so they can utilize higher wavelengths.What else affects the violent death of beings utilizing TiO2 – whether its imbibing, pre-treated or effluent. Wastewater contains a batch of organic stuff which needs to be oxidized so this has an consequence. Turbidity of the H2O, free groups are excessively many so they all overlap and ca n’t adhere decently. Other chainss such as phosphate ions compete with the accelerator in binding.Silver – stops negatrons recombining.

Hydroxyl free groups – extremely reactive but do n’t last really long.What ‘s efficiency of all these systems – eg certain per centum over how many hours?


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BelhaA?ova , 1999 2 3 4 5 6
Organism used E. coli ( DH5I± , I»NM1149 )
Size of TiO2 bed 60 centimeter ( cubic decimeter ) , 30 centimeter ( tungsten )
Volume of Liquid 5000 cm3
Light strength of UV 0.9 – 6.2 ten 10-9 Einstein cm-2 s-1
Duration of experiment 6h
Degree of harm ( % ) 99.6
Degree of harm with no UV or no TiO2 ( % )
Initial Concentration ( CFU/100 milliliter ) *
Concluding Concentration ( CFU/100 milliliter ) *

l – lengthw – breadth 321

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