Development of an O₃-assisted photocatalytic water-purification unit by using a TiO₂ modified titanium mesh filter

Abstract

An ozone-assisted photocatalytic water-purification unit using a TiO₂ modified titanium-mesh sheet (TMiP) was investigated. Significant decomposition of biological and chemical contaminants has been achieved by highly active intermediates formed by catalytic decomposition and photocatalysis.

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Water purification is one of the most important technologies for human life. The strong oxidation ability of TiO₂ has received growing attention.^1–3TiO₂ generates hydroxyl radicals and superoxide ions by UV light irradiation. These are highly reactive with organic compounds. Thus, a TiO₂ photocatalyst is expected to be effective in a water purification system. However, there are several limitations such as electron–hole recombination, low efficiency, and difficulty in decomposition of large amounts of pollutants. In the previous work, the fabrication method of easy-to-handle photocatalytic filter material, TiO₂ nanoparticles impregnated titanium mesh (TMiP^TM), and its ability for air- and water-purification were investigated.⁴ Based on these results, here we reported an ozone-assisted photocatalytic water-purification unit using a TMiP. Biological and chemical purification efficiencies of the reactor were examined by a decomposition test of waterborne pathogens and phenol, respectively.

Fig. 1 shows the schematic illustration of a water-purification system which consists of a water-purification unit, a reservoir, a pump, and an O₃ production unit. For comparison of the effect of UV-C, TMiP, and O₃ bubbling, four types of units were investigated (Fig. 2): (a) UV-C + TMiP + O₃, (b) O₃ only, (c) UV-C + TMiP, and (d) UV-C only. An acrylic tube was equipped with two ports for water flow inlet and outlet. The irradiation was provided by a UV-C lamp (Cnlight, 18 W) in the tube. The UV-C lamp was wrapped using a corrugated TMiP sheet. An O₃ production unit consists of the following parts: oxygen source, flow meter, ozone generator using a discharge cell, and bubbler. One litre of an aqueous solution containing a large number of waterborne pathogens or phenol was circulated through the units by a pump at a flow rate of 1 L min⁻¹ and was treated under each condition. The experiments were carried out at room temperature and atmospheric pressure.

Fig. 1 Schematic view of a water-purification system using the ozone-assisted photocatalytic water-purification unit.

Fig. 2 Schematic illustrations of water-purification units (inside the broken rectangle in Fig. 1). (a) UV-C + TMiP + O₃, (b) O₃ only, (c) UV-C + TMiP, (d) UV-C only. Sample vol.: 1 L, flow: 1 L min⁻¹, UV intensity: 15 mW cm⁻² at 254 nm.

E. coli (NBRC 13965) and Qβ phage were used as the main test waterborne pathogens to assess the biological purification efficiency of the units. E. coli and Qβ phage used in this study were obtained from the Biological Resource Center of the National Institute of Technology and Evaluation (NITE-BRC). E. coli was consecutively cultivated on nutrient broth (NB) agar at 37 °C for 18–20 h; then, the live cells were collected and suspended in PBS solution at a bacterial density of approximately 10⁹ colony-forming units (CFU) per mL. The suspensions of E. coli were then used as the biologically contaminated water sample. Qβ phage was prepared following a similar method,⁵ with a density of approximately 10¹¹ plaque-forming units (PFU) per mL.

The viability of E. coli in the artificially contaminated water samples was analyzed in NB agar medium. The water samples were diluted with PBS in a 10-fold dilution series. One-hundred microlitre aliquots of each dilution stage of the water samples were plated onto NB agar in a 10 cm Petri dish and were incubated at 37 °C for 24 h to determine the number of CFUs. The bacterial survival rate was calculated as: bacterial survival rate (α min) = CFU (α min) × 100/CFU (0 min). Similarly, plaque assays to determine phage titer were performed by individually mixing 100 μL of an overnight culture of E. coli, 900 μL of the water samples at each dilution stage, and 3 mL of top layer agar (Bacto-Agar 0.5%), at 45 °C. The mixture was pour plated onto a NB agar plate and allowed to solidify. The plates were incubated overnight at 37 °C for 24 h to determine the number of PFUs. The survival rate was calculated as: survival rate (α min) = PFU (α min) × 100/PFU (0 min). On the other hand, one litre of an aqueous solution containing 273 mg L⁻¹phenol was supplied to the units and was treated under each condition. The concentration of dissolved phenol as a function of irradiation time was measured with a Digital pack test for phenol (Kyoritsu Chemical-Check Lab., Corp) to assess the chemical purification efficiency of the units.

Fig. 3 shows the time course of the log survival rate of E. coli with the units. The survival rate of E. coli in the artificially contaminated water was decreased by 10⁵–10⁷ (below the detection limits of colony-forming method) for each unit. By using the UV-C only (crosses) or the UV-C + TMiP (solid circles) unit, more than 5 min of treatment time was required for total inactivation of E. coli. In contrast, the O₃ only (solid triangles) or the UV-C + TMiP + O₃ (open circles) unit takes a shorter treatment time (less than 1 min) for the inactivation. There is no difference between the O₃ only unit and the UV-C + TMiP + O₃ unit. On the other hand, Qβ phage could not be inactivated by the UV-C + TMiP unit efficiently (Fig. 4, solid circles). The difference of inactivation efficiency between E. coli and Qβ phage by the UV-C + TMiP unit may be due to the size of the waterborne pathogens. Because the size of Qβ phage (ca. 20 nm) is smaller than that of E. coli (c.a. 1–2 μm), it becomes difficult for Qβ phage to contact the TiO₂ surface efficiently. However, O₃ only (Fig. 4, solid triangles) or UV-C + TMiP + O₃ (Fig. 4, open circles) units could inactivate Qβ phage efficiently. Ozone could attack the bacterial cell wall, membrane, and the surface of waterborne pathogens and then disrupt their integrity by its extremely strong oxidation potential.⁵

Fig. 3 Time course of the log survival rate of E. coli with the units. Crosses: UV-C only (Fig. 2d); solid circles: UV-C + TMiP (Fig. 2c); solid triangles: O₃ only (Fig. 2b); open circles: UV-C + TMiP + O₃ (Fig. 2a).

Fig. 4 Time course of the log survival rate of Qβ phage with the units. Solid circles: UV-C + TMiP (Fig. 2c); solid triangles: O₃ only (Fig. 2b); open circles: UV-C + TMiP + O₃ (Fig. 2a).

Interestingly, inactivation efficiency of the UV-C + TMiP + O₃ unit (Fig. 4, open circles) is larger than that of the O₃ only unit (Fig. 4, solid triangles). The result of a phenol decomposition test shows this tendency more clearly (Fig. 5). The concentration of phenol after 20 min of treatment by the UV-C + TMiP + O₃ unit (Fig. 5, the rightmost brack bar) is smaller than that by the O₃ only or the UV-C + TMiP unit. This result indicates that the UV-C + TMiP + O₃ unit could decompose phenol more efficiently than the other units. On the other hand, O₃ concentration in the UV-C + TMiP + O₃ unit (Fig. 5, the rightmost gray bar) after 20 min of treatment is smaller than that in the O₃ only unit.

Fig. 5 The concentration of phenol (black bar) and ozone (gray bar) after 20 min of treatment by the units. Initial concentration of phenol and ozone were 273, 0 mg L⁻¹, respectively.

The log reduction of E. coli, the log reduction of Qβ phage, and the decomposition amount of phenol by each unit are summarized in Table 1. Purification efficiency of the UV-C + TMiP + O₃ unit against both biological and chemical contaminants is the most highest among the units in the present research.

Table 1 Comparison of the purification efficiency of the units

	log reduction of E. coli^a/CFU mL^−1	log reduction of phage^a/PFU mL^−1	decomposition amount of phenol^b/mg
a After 0.5 min of treatment. b After 20 min of treatment. c No data.
UV-C only	0.25	—^c	—^c
UV-C + TMiP	0.84	0.0064	17
O3 only	3.7	0.50	53
UV-C+TMiP+O3	3.7	1.9	143

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Ozone may play two roles in the system, i.e., to prevent carrier recombination in photocatalysis and to produce oxidative intermediates by photolytic and catalytic decomposition. Fig. 6 shows the schematic illustration of the reaction mechanism on the TiO₂ surface. When the TiO₂ is irradiated by UV light from a UV-C lamp, excitation of electrons into the conduction band takes place, resulting in formation of holes in the valance band. Both the holes and the electrons migrate to the TiO₂ surface, where they either recombine or react with adsorbed species such as H₂O and dissolved O₂. The holes oxidize adsorbed H₂O to ˙OH, which are the potential oxidants in photocatalysis, whereas the electrons reduce O₂ to O₂˙⁻.³ Here O₃ is a good acceptor of excited electrons, similar to O₂. Thus, ˙OH production and photocatalytic oxidation are enhanced by the presence of O₃, which prevents carrier recombination in photocatalysis.^6–9 Moreover, O₃˙⁻ can oxidize organics with a relatively long lifetime.^10,11 On the other hand, there are many reports about photolytic/catalytic decomposition of O₃.^12–15 Highly oxidative intermediates such as atomic oxygen could be produced from O₃ by UV-C irradiation and/or the presence of heterogeneous catalyst surfaces. In this case, UV-C irradiation and the large relative surface area of TMiP could decompose O₃ effectively.¹⁶ A smaller O₃ concentration in the UV-C + TMiP + O₃ unit in Fig. 5 is an evidence of decomposition of O₃. Moreover, a phenol decomposition test was carried out by using an additional unit, i.e., UV-C + O₃. The decomposition amount of phenol and ozone concentration after 20 min of treatment were 122 mg and 2.75 mg L⁻¹, respectively. This result indicates that the amount of residual phenol and O₃ in the UV-C + O₃ unit is larger than in the UV-C + TMiP + O₃ unit. Therefore, photolytic and catalytic decomposition of O₃ and prevention of career recombination by O₃ are critical for the present system. Loading of Pt onto the TiO₂ surface for stabilizing oxidative intermediates is worthy of further study.^10,11

Fig. 6 Schematic illustration of the reaction mechanism on the TiO₂ surface.

In conclusion, an ozone-assisted photocatalytic water-purification unit was investigated. Much higher efficiency for decomposition of biological and chemical contaminants was achieved in the ozone-assisted photocatalytic water-purification unit, compared with the photocatalysis or ozone treatment only. Ozone could prevent carrier recombination in photocatalysis and could produce oxidative intermediates by photolytic and catalytic decomposition. These roles are important for efficient water purification in the present system. Moreover, the purification efficiency of TMiP did not decrease by repeating the test. This result indicates the strong adhesion of TiO₂ nanoparticles onto the TMiP surface. Although we used a simple reactor with conventional methods and conditions, for example, atmospheric pressure, it would be attractive to develop a similar continuous-type water purification system for the practical treatment of a highly contaminated environment, such as sewage water.

Acknowledgements

The authors are grateful to Dr Etsuko T. Utagawa (National Institute of Infectious Diseases) and Dr Jitsuo Kajioka (KAST) for helpful discussion.

Notes and references

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