1-Rahmani

JRHS 2009; 9(1): 1-6

Copyright © Journal of Research in Health Sciences

Photo catalytic Disinfection of Coliform Bacteria Using Uv/Tio2

Rahmani AR( PhD),  Samarghandi MR( PhD),  Samadi MT( PhD) Nazemi F

Department of Environmental Health Engineering, Faculty of Public Health and Center for Health Research, Hamadan University of Medical Sciences, Iran

Corresponding Author: Dr Mohammad Taghi Samadi, , E-mail: samadi@umsha.ac.ir

Received: 7 October2008; Accepted: 19 April 2009

Abstract

Background: There are great interests in photocatalytic oxidation of contaminants using titanium di­oxide in recent years. The main objective of this research was to study photocatalytic disinfection of Coli­form bacteria as water microbial pollution index using TiO2 and a low pressure UV lamp in a batch re­actor.

Methods: The polluted water was prepared by adding a colony of Coliform in raw water and in sepa­rate stages was contacted with UV, TiO2 and combination of them and various parameters such as con­tact time, pH and amount of TiO2 were studied in terms of their effect on reaction progress.

Results: The results showed that in simultaneous presence of both UV ray and TiO2, there was the most effective disinfection of Coliform. This study showed that 100% of Coliform was killed by irra­diation for 60-75 min. in the presence of 0.8 gr l-1 TiO2 in pH=7.0.

Conclusion: Based on the results, UV/TiO2 process may be effectively applied for disinfection of pol­luted water and can be suggested as a effective purifying method for water disinfection. 

Keywords: Water treatment, Disinfection, Coliform, Photocatalyst, TiO2, UV

Introduction

The practice of eliminating harmful microor­gan­isms in water dates back to ancient times. The most common methods for water dis­infec­tion are using chemicals, Ozona ion, Ul­tra Violet ray, Membrane Processes etc (1-3).

In the past, the primary emphasis of disinfec­tion was prevention of water-born diseases by controlling some bacteria such as Coli­form. A new finding made in 1970 resulted in significant reevaluation of this long estab­lished disinfection practice was about dis­infec­tion by products. This disinfection by pro­ducts, formed via the reaction between dis­infectants and certain organic matters in water, may be harmful to human health. Thus, the new methods for decreasing the side effects of disinfectants must be im­proved (4-8).

The application of photo catalysis in the treat­ment of water and wastewater is an inter­esting alternative and is the object of a great inter­est over the last years by many re­searchers (9, 10). In these researches, atten­tion has been drawn toward an alternative tech­nique where the pollutants are degraded by irradia­tion suspension of metal oxide semi­conduc­tor particles such as TiO2 or ZnO (2, 4, 9). TiO2 is an excellent photocatalyst for com­plete mineralization of pollutants in water and wastewater. It has been known as a chemi­cal that is non-toxic, insoluble in wa­ter, stable under UV radiation and compara­tively cheap (6, 11).

TiO2 is capable to be activated by UV-light with wavelengths less than 400 nm (6, 11, 12). Light energy from ultraviolet radiation (light) in the form of photons, excites the elec­trons on the surface of titanium atoms sus­pended in the contaminated water, mov­ing them from the valence band to the conduc­tance band. The result of this energy change is the for­mation of holes in the sur­face of titanium atom, and free electrons, which are now available to form hydroxide (OH) or other radicals, which can used as a powerful oxidizing agent to con­vert organic pollutants into CO2 or inacti­vated microorgan­isms (7, 9, 11, 13).

The antimicrobial activity of UV/TiO2 has been essayed in several microbes including Entero­bacter caloacae (12), Escherichia coli (2, 4, 8, 10-12, 14, 15), Pseudomonas aerugi­nosa (10, 12), Cyanobacteria and Unice­llular algae (3), Lactobacillus helveti­cus (4), Legionella pneumophila (14), Clos­tridium perfringens and Coliphages (7).

In this work, we have compared the effec­tive­ness of TiO2, UV and UV/TiO2 for the disinfection of Coliform bacteria as a wa­ter microbial pollution index.

Methods

Bacteria culture and TiO2

The effect of photocatalytic on disinfection in a batch reactor was tested on Coliform. Bac­terial sample was taken from polluted wa­ter and grown in lactose broth (Merck) on an orbital shaker at 35 ºC for 24 to 48 h until was gotten turbid. From this culture, sample was inoculated into EMB agar (Merck). Plates were incubated at 37 ºC for 24 h. Then for pre­paration of polluted water, a colony of cul­ture produced in EMB agar was added to 1000 ml water. Most Probable Number (MPN) of Coliform in samples was meas­ured as MPN/ 100 ml before, during and after examinations by using the fermentation tech­nique in presumptive and confirmed tests (11, 15)

TiO2 (Degussa P-25) was used as the photo­catalyst in a batch-type reactor was a gift from Aeroxide (Evonik-Industries Ger­many). The sam­ple used in this particular work had a BET surface area of 50±15 m2 g-1, an aver­age par­ticle diameter of 21 nm and the den­sity was 130 g L-1. pH (Hach, Sen­sion 5) adjust­ments in samples were made us­ing 1 N HCl acid (Merck, 37%) and 1N NaOH (Merck 100%).

UV source and light intensity

A 40 W low pressure UV lamp, 0.8 m in length was installed 5 cm above the samples surface and the light intensity was 0.9 J s-1m-2 measured by a radiometer at 300 to 400 nm (Hagner EC1-UV-A). All analyze were made ac­cording to the standard methods (15).

Experimental Set-up and Measurements

In these experiments, the photochemical cell consisted of sterile 250 ml beakers and mag­netic stirrers that used for stirring the sam­ples. The temperature of reactors was con­trolled at 20 oC. In the first phase of run, the beakers were filled with 200 ml of polluted water and in separate stages were contacted with UV, TiO2 and combination of them. In the examinations various parameters such as pH (5.7, 7.0 and 8.1) and amount of TiO2 (0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9 gr L-1) were studied in terms of their ef­fect on reaction process for 60 min, respec­tively. The polished aluminum was used as the reflective material to protect the samples studied in absence of radiation. In the second phase of run based on the optimal values ob­tained from the first stage (pH and concen­tra­tion of TiO2), the examinations will be re­peated for determination of optimal of contact time (0, 10, 20, 30, 45, 60 and 90 min.). In the third phase of run based on the optimal values obtained from the previous stages, the effect of TiO2 concentration in disin­fection ef­ficiency was studied. The disin­fection effi­ciency, E, is calculated as:

 

 

Ci and Cf are the initial and final MPN/100 ml respectively. The experimental appara­tus as shown in Fig. 1 was set up in Hamadan Uni­versity of medical sciences in 2008.

Figure. 1: Schematic diagram of experimental apparatus

Results

The results from examinations as MPN/100ml in pH=5.7, 7.0 and 8.1 as the function of TiO2 concentration in the studied processes in 60 min. UV irradiation is shown in Fig. 2. It is found that disinfection effi­ciency is af­fected by TiO2 concentration. The results show that the variation on pH has no significant effect on Coliform inactiva­tion. The most ob­served yield is related to pH= 7.0.

The results from examinations in pH= 7.0 and TiO2= 0.8 gr L-1 as the function of ex­posed time in the studied processes are shown in Fig 3. It is found that Coliform re­moval is affected by contact time. By in­creas­ing the contact time, the percentage of disinfection is raised. Presence of TiO2 in these processes can promote removal rate, re­markably.

The effect of TiO2 concentration on samples exposed to UV light for 75 min at pH=7.0 is shown in Fig. 4. The Coliform removal effi­ciency decreased gradually with increasing amounts of TiO2. The maximum reduction was reached at around 0.8 gr L-1. Further in­crease in concentration caused a continuous rise in the present fraction. Accordingly, a TiO2 con­centration of 0.8 gr L-1 was used as the opti­mum concentration.

Figure. 2: Effect of TiO2 concentration and pH on Coliform removal as MPN/100 ml:

Irradiation time= 60 min

Figure. 3: Effect of irradiation time on Coliform removal: TiO2 = 0.8 gr L-1, pH=7.0

Figure. 4: Effect of TiO2 concentration on Coliform removal as MPN/100 ml:

Irradiation time = 75 min, pH=7.0

Discussion

In this work, the photocatalytic disinfection of Coliform by using a low pressure UV lamp intensity of 0.9 J s-1m-2 and effects of TiO2 concentration on 100% reduction time were investigated. The results obtained in this study for the photocatalytic destruction of Co­li­form are discussed below and similar to those reported by several other authors (1, 2, 4, 6, 8, 11-13, 16).

Effect of pH and TiO2 concentration

The effect of pH on Coliform inactivation, as the pH was changed from 5.6, 7.0 and 8.1 in UV/TiO2 process at 60 min and TiO2 con­cen­tration of 0 and 900 ppm is shown in Fig. 2. The results show that no significant pH effect on Coliform inactivation was ob­served at the pH conditions used in this study. Cho et al. reported that the cell sur­face charge of E. coli has a negative charge character and the TiO2 particle has a point of zero charge pH of about 6.3. Thus, it was ex­pected that electrostatic repulsion between the TiO2 surface and Coliform at high pH would be higher due to their both having the same negative charge. However, such a trend was not ob­served. Although the electrostatic interaction at the TiO2/water interface could be important in many cases of photocatalytic degradation of charged substrates, it is a rela­tively weak force, which can be domi­nated by other factors (2, 13, 17).

Effect of UV irradiation, TiO2 and contact time

The effect of UV irradiation and TiO2 on re­moval of Coliform shows in Fig. 3. The ex­periments were done at initial 240000 MPN/ 100 ml, TiO2 concentration of 800 ppm and pH= 7.0. The experiments were con­tinued up to 90 min. It is obvious that re­moval effi­ciency of Coliform in the absence of each of TiO2 or UV irradiation, with time was low, while in the presence of both of them, re­moval efficiency was increased. The 96% re­duction time at an optimal concentra­tion of 0.8 gr TiO2 L-1 was 10 min at 0.9 Js-1m-2. It can be concluded from these results that disinfection capability in the aspect of time using both TiO2 and UV light was more than 4.62 times as that by using only the UV light.

In the presence of both of TiO2 and UV irradia­tion, hydroxyl radicals (OH) are pro­duced, which increase the efficiency of the process. In the absence of UV irradiation, it may be TiO2 particles adsorbed some of mi­cro­organisms, which causes low decrease in the amount of Coliform with time (8, 10-12, 18).

Effect of amount of TiO2

Different amounts of TiO2, from 0 to 1.4 gr L-1, were added to the samples with initial 240000 MPN/100 ml and the results are shown in figure 4 at 75 min UV irradiation time. It can be seen that to some extent, in­creasing the TiO2 concentration increased the removal of Coliform. In this experiment, maximum disinfection capability occurred at 0.8 g TiO2 L-1. At higher TiO2 concentra­tions (>0.8 gr L-1), the disinfection capability decreases due to the absorption and scatter­ing of UV light by the suspended TiO2 parti­cles (2, 6, 10, 13). 

In conclusion, the results of this study showed that UV/TiO2 process could be effec­tively removed Coliform bacteria. The 100% re­moval efficiently at a UV intensity of 0.9 J s-1m-2 with 0.8 g TiO2 L-1 was 60 min. This process needed joint action of UV ir­radiation and TiO2. Increasing the concentra­tion of TiO2, first increased the Coli­form removal effi­ciency, but further in­crease in photocatalyst concen­tration causal re­sulted in a decrease in proc­ess efficiency. Also it is resulted that pH has no significant ef­fect on the disinfection by UV/ TiO2  proc­ess.

Acknowledgements

The authors of this paper are thankful to the Hamadan University of Medical Sciences for its financial and other supports of this work. The authors declare that there is no conflict of interests.

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