A Comparison of Solar Photocatalytic Inactivation of Waterborne E. coli Using Tris (2,2′-bipyridine)ruthenium(II), Rose Bengal, and TiO2

Abstract

Background. The development of alternative processes to eliminate pathogenic agents in water is a matter of growing interest. Current drinking water disinfection procedures, such as chlorination and ozonation, can generate disinfection by-products with carcinogenic and mutagenic potential and are not readily applicable in isolated rural communities of less-favored countries. Solar disinfection processes are of particular interest to water treatment in sunny regions of the Earth. Solar light may be used to activate a photocatalyst or photosensitizer that generates, in the presence of molecular oxygen dissolved in water, reactive oxygen species (ROS), such as the HO• radical, singlet oxygen (O21), or superoxide (O2•), which are toxic to waterborne microorganisms. Method of Approach. Wild and collection-type Escherichia coli have been selected as model bacteria. Inactivation of such bacteria by either TiO2 nanoparticles, water-soluble tris(2,2′-bipyridine)ruthenium(II) dichloride or Rose bengal (RB) subject to simulated sunlight have been compared. Although TiO2 is the prototypical material for heterogeneous photocatalysis, the other two dyes are known to generate significant amounts of O21 by photosensitization but have different chemical structures. The concentration of dye, illumination time, photostability, presence of scavengers, and post-treatment regrowth of bacteria have been investigated. Results. After 1hr of solar illumination the Ru(II) complex produced a strong loss of E. coli culturability monitored with solid selective agars. Both the collection- and wild-type bacteria are sensitive to the treatment with 2-10mgL−1 of dye. This photosensitizer showed a better inactivation effect than TiO2 and the anionic organic dye RB due to a combination of visible light absorption, photostability, and production of O21 and other ROS when bound to the bacterial membrane. A complete loss of culturability was observed when the initial concentration was 103CFUmL−1, with no bacteria regrowth detected after 24hr of the water treatment. At higher initial microorganism levels, culturability still remains and regrowth is observed. Scavengers show that the HO• radical is not involved in bacteria inactivation by photosensitization. Conclusions. A higher quantum yield of ROS generation by the sensitizing dyes compared to the semiconductor photocatalyst determines the faster sunlight-activated water disinfection of photodynamic processes. The homogeneous nature of the latter determines a more efficient interaction of the toxic intermediates with the target microorganisms. Solid supporting of the Ru(II) dye is expected to eliminate the potentials problems associated to the water-soluble dye.

Phototreatment of Water by Organic Photosensitizers and Comparison with Inorganic Semiconductors

“…It is expected that the inactivation rate increases with increase in photosensitizer concentration. This behavior is actually observed by many [40,42,84,96,97]. Acher et al [40,42] have studied the photoactivity of MB and RB in water treatment.…”

“…However, Jemli et al [81] have reported the better activity of RB (di anionic) than MB (mono cationic) against fecal coliforms referring the enhanced activity of the former to better photophysical properties (high quantum yield of singlet oxygen and better absorption). Nonetheless, more recent studies proving that cationic dyes are more effective in destroying Gram-negative bacteria are published [50,52,84]. Caminos et al [82] have studied the effect of an anionic and four cationic porphyrins with different pattern of meso-substitution by 4-(3-N, N, N-trimethylammoniumpropoxy) phenyl (A) and 4-(trifluoromethyl) phenyl (B) groups.…”

“…Many have reported the successful inactivation of Gramnegative bacterial strains with TiO 2 , although with differing photosensitivities [1] unlike organic sensitizers which require positive charged moieties on the molecule in order to break the lipopolysaccharide membrane in Gram-negative bacteria. Comparing the efficiencies of organic photosensitizers and TiO 2 , Rengifo-Herrera et al [84] have shown that better inactivation was achieved by the sensitizing dyes compared to TiO 2 . They have attributed this enhanced activity to better attack on the bacterial membrane, higher quantum yield of ROS, greater absorption of the incident photonic flux by the dyes than TiO 2 , and improved photostability of the dyes.…”

“…The photodisinfection efficiencies of these materials and the type of irradiation used are also displayed. Photodynamic destructions of different kinds of bacteria using TiO 2 as a catalyst are reported by many [14,84,96,106]. Mostly E. coli is considered under study as it is used as biological indicator of disinfection efficiency in water systems [100].…”

“…Another factor limiting the reuse of photosensitizers is their degradation due to prolonged exposure to light. MB, RB, and some porphyrins have been reported to undergo bleaching thereby reducing their photoinactivation efficiencies [81,84]. Photosensitizers with excellent light stability and capable of absorbing visible light should be employed.…”