Trapping and decomposing of color compounds from recycled water by TiO(2) coated activated carbon

Five types of commercially available activated carbons (ACs) were coated with TiO(2) nanoparticles prepared using a sol-gel method. Color and trace organics remaining in the actual treated effluent were adsorbed by TiO(2) coated ACs. The absorbed organic compounds were then decomposed using a photocatalytic process, and the ACs were regenerated for reuse. The efficiency of the process was assessed by the characterization of true color and A(254) (the organics absorption at the wavelength of 254nm) at the beginning and the end of the experiment. The effects of UV light source, UV irradiation time, hydrogen peroxide and ultrasound on the efficiency of photocatalytic regeneration were also investigated. Significant differences in the efficiency were observed between uncoated ACs and TiO(2) coated samples. Among the 5 types of ACs tested, AC-3, AC-4 and their coated ones achieved better efficiency in color and A(254) removal, with around 90% or more color and A(254) being removed within 1h of treatment. The data obtained in this study also demonstrated that the photocatalytic process was effective for decomposing the adsorbed compounds and regenerating the spent TiO(2)/AC-3. Finally, it was found that this regeneration process could be greatly enhanced with the assistance of H(2)O(2) and ultrasound by reducing the required regeneration time.

Effect of particles on ultraviolet light penetration in natural and engineered systems

The effect of light scattering on measurement of UV absorbance and penetration of germicidal UVC irradiance in a UV reactor were studied. Using a standard spectrophotometer, absorbance measurements exhibited significant error when particles that scatter light were present but could be corrected by integrating sphere spectroscopy. Particles from water treatment plants and wastewater effluents exhibited less scattering (20%-30%) compared with particles such as clay (50%) and alumina (95%-100%). The distribution of light intensity in a UV reactor for a scattering suspension was determined using a spherical chemical actinometry method. Highly scattering alumina particles increased the fluence rate in the reactor near the UV lamp, whereas clay particles and absorbing organic matter reduced the fluence rate. A radiative transfer fluence rate model reasonably predicted the fluence rate of absorbing media and highly scattering suspensions in the UV reactor.

Visible light photocatalysis with platinized rutile TiO2 for aqueous organic oxidation

Platinized rutile TiO2 samples containing varying concentrations of Pt were synthesized using Kemira (KE, BET surface area 50 m2/g, from Finland), and Toto HT0270 (HT, BET surface area 2.9 m2/g, from Japan) as the starting materials by solution mixing followed by sintering the precursors. Photocatalytic activities were established for phenol oxidation under visible light (wavelength >400 nm). Our results show optimal performance for 8 wt % platinized KE (8 wt % Pt/KE) and (1/2) wt % platinized HT rutile samples. The specific roles of O2 and visible light were examined using the 8 wt % Pt/KE sample in either N2 gas ambient or no illumination. Separately, 8 wt % platinized SiO2 was tested to compare its performance with that of platinized rutile TiO2. Several other chemicals containing different functional groups (formic acid, salicylic acid, 4-chlorophenol, 2,4,6-trichlorophenol, diethyl phosphoramidate) were selected for photooxidation tests with (1/2) wt % platinized HT rutile. X-ray diffraction reveals Pt metal clusters segregating on the surface of rutile TiO2 particles with increasing Pt weight percent. The Pt cluster surface area broadly increases, while the effective optical band gap steadily decreases with platinization of the rutile samples. These results suggest that Pt clusters on the surface of rutile TiO2 particles serve to mediate electron transfer from rutile to O2, thus facilitating photooxidation of organic chemicals.

Enhanced photocatalytic inactivation of bacterial spores on surfaces in air

TiO₂ photocatalysis with ultraviolet (UV-A) light has proven to be a highly effective process for complete inactivation of airborne microbes. However, the overall efficiency of the technology needs to be improved to make it more attractive as a defense against bio-terrorism. The present research investigates the enhancement in the rate of destruction of bacterial spores on metal (aluminum) and fabric (polyester) substrates with metal (silver)-doped titanium dioxide and compares it to conventional photocatalysis (TiO₂ P25/+UV-A) and UV-A photolysis. Bacillus cereus bacterial spores were used as an index to demonstrate the enhanced disinfection efficiency. The results indicate complete inactivation of B. cereus spores with the enhanced photocatalyst. The enhanced spore destruction rate may be attributed to the highly oxidizing radicals generated by the doped TiO₂.

Visible light Cr(VI) reduction and organic chemical oxidation by TiO2 photocatalysis

Here we report the simultaneous Cr(VI) reduction and 4-chlorophenol (4-CP) oxidation in water under visible light (wavelength > 400 nm) using commercial Degussa P25 TiO2. This remarkable observation was attributed to a synergistic effect among TiO2, Cr(VI), and 4-CP. It is well known that TiO2 alone cannot remove either 4-CP or Cr(VI) efficiently under visible light. Moreover, the interaction between Cr(VI) and 4-CP is minimal if not negligible. However, we found that the combination of TiO2, Cr(VI), and 4-CP together can enable efficient Cr(VI) reduction and 4-CP oxidation under visible light. The specific roles of the three ingredients in the synergistic system were studied parametrically. It was found that optimal concentrations of Cr(VI) and TiO2 exist for the Cr(VI) reduction and 4-CP oxidation. Cr(VI) was compared experimentally with other metals such as Cu(ll), Fe(lll), Mn(IV), Ce(IV), and V(V). Among all these metal ions, only Cr(VI) promotes the photocatalytic oxidation of 4-CP. The amount of 4-CP removed was directly related to the initial concentration of Cr(VI). The system was also tested with four other chemicals (aniline, salicylic acid, formic acid, and diethyl phosphoramidate). We found that the same phenomenon occurred for organics containing acid and/or phenolic groups. Cr(VI) was reduced at the same time as the organic chemicals being oxidized during photoreaction under visible light. The synergistic effect was also found with pure anatase TiO2 and rutile TiO2. This study demonstrates a possible economical way for environmental cleanup under visible light.

Kinetics of Liquid Phase Photocatalyzed Reactions: An Illuminating Approach†

Analysis of photocatalyst kinetics to date have relied largely on Langmuir-Hinshelwood rate forms, which assume equilibrated adsorption of reactants and, correspondingly, a slow, rate-controlling surface step. Alternatively, and more generally, a pseudo-steady state analysis based upon the stationary state hypothesis for reaction intermediates may be applied. We show here that only this second approach is consistent with the reported intensity dependence of apparent adsorption (and desorption) binding constants, as well as the catalytic rate constant. In consequence, we show that for at least some photocatalyzed reactions, adsorption/desorption reaction equilibria are not established during reaction, because the substantial reactivity of an adsorbed active species (e.g., hole (h+), radical (*OH), etc.) causes a continued displacement from equilibrium of the adsorbed reactant concentration.

Highly active meso–microporous TaON photocatalyst driven by visible light

TaON was found to be ca. 20 times more active as a visible light activated photocatalyst for the oxidation of methanol, when compared to the well studied UV-visible activated TiO2 (P25) catalyst.

Effect of oxygen in a thin-film rotating disk photocatalytic reactor

A novel experimental procedure was developed to measure oxygen mass transfer during the oxygenation of water in a thin film of a rotating disk photocatalytic reactor (RDPR). The increase in dissolved oxygen (DO) of initially deaerated water was monitored with time in the reactor vessel at different disk angular velocities after exposure of the reactor to the atmosphere. Oxygenation was predominantly achieved by oxygen mass transport through the thin liquid film carried by the disk and to a much lesser extent by direct oxygenation of the water in the reactor vessel via a surface renewal mechanism. A mathematical model was developed to simulate the phenomenon considering both cases of presence and absence of oxygen mass transport limitations. In the latter case, the model considered that the amount of liquid carried by the disk was saturated with oxygen when returning to the reactor vessel. On the basis of the model and the experimental data, it was proven that mass-transfer limitations existed until the water in the reactor vessel became saturated with oxygen. Results obtained from the model were validated by an alternative analysis using dimensionless groups characteristic to the system. The study revealed that the mass-transfer coefficient increased linearly with disk angular velocity and thus disk Reynolds number. The results showed that oxygen mass-transfer limitations decreased with increasing disk angular velocity, mainly due to an increase in the overall mass-transfer coefficient. In the presence of UV radiation, the influence of oxygen on the photocatalytic oxidation of 4-chlorobenzoic acid was investigated in the RDPR operated in batch and continuous mode. The photocatalytic reactions occurred in a thin film of liquid carried by the disk in the presence of UV radiation and ST-B01 composite spherical ceramic (SiO2/Al2O3) balls coated with anatase TiO2 catalyst. It was found that the initial degradation rate followed Langmuir kinetics with respect to oxygen concentration in the gas phase. When the oxygen concentration in the gas phase surpassed that in air, the degradation rates did not improve significantly, suggesting that operation with air instead of oxygen is most probably a more realistic practical choice. Measurements of DO during the presence and absence of UV radiation suggested that the photocatalytic reactions were mainly oxygen concentration-limited rather than oxygen mass-transfer-limited.