Mechanism of phenol photodegradation in the presence of pure and modified-TiO(2): A review

In recent years, the application of heterogeneous photocatalytic water purification processes has gained wide attention due to its effectiveness in degrading and mineralizing the recalcitrant organic compounds as well as the possibility of utilizing the solar UV and visible-light spectrum. By far, titania has played a much larger role in this scenario compared to other semiconductor photocatalysts due to its costly effectiveness, inert nature and photostability. A substantial amount of research has focused on the enhancement of TiO₂ photocatalysis by modification with metal, non-metal and ion doping.This paper aims to review and summarize the recent works on the titanium dioxide (TiO₂) photocatalytic oxidation of phenol and discusses various mechanisms of phenol photodegradation (indicating the intermediates products) and formation of OH radicals. Phenol degradation pathway in both systems, TiO₂/UV and doped-TiO₂/Vis, are described.     

Electrochemical degradation of chlortetracycline using N-doped Ti/TiO2 photoanode under sunlight irradiations

The appearance and the persistence of pharmaceutical products in the aquatic environment urgently call for the development of an innovative and practical water treatment technology. This study deals with the development of nanostructured nitrogen-doped TiO₂ photoanodes and their subsequent use for chlortetracycline (CTC) photoelectrocatalytic oxidation under visible light. The N-doped TiO₂ photoanodes with different nitrogen contents were prepared by means of a radiofrequency magnetron sputtering (RF-MS) process, with the objective to tune shift their optical absorption from the UV towards the visible. The N-doped TiO₂ consist of nanostructured anatase phase with average TiO₂ nanocrystallite size of 29 nm. The nitrogen doping is clearly shown to produce the desired red shift of the absorption onset of the TiO₂ coatings (from ∼380 nm to ∼550 nm). Likewise, the N-doped TiO₂ are found to be highly photo-electroactive not only under the UV light but most interestingly under the visible light as well. Using the optimal N-doped photoanodes, 99.6% of CTC (100 μg/L) was successfully degraded after 180 min of treatment time with a current intensity of 0.6 A. Under these conditions, a relatively high mineralization of CTC (92.5% ± 0.26% of TOC removal and 90.3% ± 1.1% of TN removal) was achieved.     

Photocatalytic degradation of 2,4-dinitrophenol (DNP) by multi-walled carbon nanotubes (MWCNTs)/TiO2 composite in aqueous solution under solar irradiation

Nanosized multi-walled carbon nanotubes (MWCNTs)/TiO₂ composite and neat TiO₂ photocatalysts were synthesized by sol-gel technique using tetrabutyl titanate as a precursor. The as prepared photocatalysts were characterized using XRD, SEM, FTIR and UV-vis spectra. The samples were evaluated for their photocatalytic activity towards the degradation of 2,4-dinitrophenol (DNP) under solar irradiation. The results indicated that the addition of an appropriate amount of MWCNTs could remarkably improve the photocatalytic activity of TiO₂. An optimal MWCNTs:TiO₂ ratio of 0.05% (w/w) was found to achieve the maximum rate of DNP degradation. The effects of pH, irradiation time, catalyst concentration, DNP concentration, etc. on the photocatalytic activity were studied and the results obtained were fitted to the Langmuir-Hinshelwood model to study the degradation kinetics. The optimal conditions were an initial DNP concentration of 38.8 mg/L at pH 6.0 with catalyst concentration of 8 g/L under solar irradiation for 150 min with good recyclisation of catalyst. The degree of photocatalytic degradation of DNP increased with an increase in temperature. The MWCNTs/TiO₂ composite was found to be very effective in the decolorization and COD reduction of real wastewater from DNP manufacturing. Thus, this study showed the feasible and potential use of MWCNTs/TiO₂ composite in degradation of various toxic organic contaminants and industrial effluents.     

Synergetic effect between photocatalytic degradation and adsorption processes on the removal of phenolic compounds from olive mill wastewater

The photocatalytic degradation of two phenolic compounds, p-coumaric acid and caffeic acid, was performed with a suspended mixture of TiO₂ and powdered activated carbon (PAC) (at pH = 3.4 and 8). Adsorption, direct photolysis and photocatalytic degradation were studied under different pH and UV light sources (sunlight vs. 365 nm UV lamps). The potential for reusing this catalyst mixture in sequential photocatalytic runs was examined as well. Quantum yields for the direct photolysis of caffeic acid under solar and artificial 365 nm light were calculated (for the first time) as 0.005 and 0.011, respectively.A higher removal rate of contaminants by either adsorption or photocatalysis was obtained at a low pH (pH 4). Furthermore, the addition of PAC increased the removal efficiency of the phenolic compounds. Fast removal of the pollutants from the solution over three sequential runs was achieved only when both TiO₂ and PAC were present. This suggests that at medium phenolic concentrations, the presence of PAC as a co-sorbent reduces surface poisoning of the TiO₂ catalyst and hence improves photocatalysis degradation of phenolic pollutants.The adsorption equilibrium of caffeic acid or p-coumaric acid on TiO₂, PAC and the combined mixture of TiO₂ and PAC follows the Langmuir isotherm model. Experiments with PAC TiO₂ mixture and olive mill wastewater (anaerobically treated and diluted by a factor of 10) showed higher removal of polyphenols than of chemical oxygen demand (COD). 87% removal of total polyphenols, compared to 58% of COD, was achieved after 24 h of exposure to 365 nm irradiation (7.6 W/m²) in the presence of a suspended mixture of TiO₂ and PAC, indicating “self-selectivity” of polyphenols.     

Antifungal capability of TiO2 coated film on moist wood

Photocatalytic oxidation by TiO₂ has been shown to deactivate biological pollutants. Most previous studies evaluated TiO₂'s antimicrobial performance using bacteria, with Escherichia coli most commonly applied as the test microbe. There have not been concentrated studies focusing on the photocatalytic disinfection of fungi which widely exist in buildings and cause health problems. In this study, the antifungal activity of TiO₂ photocatalytic reaction against Aspergillus niger was investigated for moist wood boards during periods of several weeks. TiO₂ coated film in the presence of UVA (365 nm) irradiation exhibited antifungal capability. No visible growth was observed on specimens during the photo-process. Re-growth appeared in subsequent dark, indicating that the photocatalytic reaction was not sufficient for total disinfection against mold fungi but did suppress fungi growth. The study sheds light on conditions and potential applications of photocatalytic deactivation of fungi.     

Effect of sunlight irradiation on photocatalytic pyrene degradation in contaminated soils by micro-nano size TiO2

The enhanced catalytic pyrene degradation in quartz sand and alluvial and red soils by micro-nano size TiO₂ in the presence and absence of sunlight was investigated. The results showed that the synergistic effect of sunlight irradiation and TiO₂ was more efficient on pyrene degradation in quartz sand and red and alluvial soils than the corresponding reaction system without sunlight irradiation. In the presence of sunlight irradiation, the photooxidation (without TiO₂) of pyrene was very pronounced in alluvial and red soils and especially in quartz sand. However, in the absence of sunlight irradiation, the catalytic pyrene degradation by TiO₂ and the photooxidation (without TiO₂) of pyrene were almost nil. This implicates that ultra-violet (UV) wavelength range of sunlight plays an important role in TiO₂-enhanced photocatalytic pyrene degradation and in photooxidation (without TiO₂) of pyrene. The percentages of photocatalytic pyrene degradation by TiO₂ in quartz sand, alluvial and red soils under sunlight irradiation were 78.3, 23.4, and 31.8%, respectively, at 5 h reaction period with a 5% (w/w) dose of the amended catalyst. The sequence of TiO₂-enhanced catalytic pyrene degradation in quartz sand and alluvial and red soils was quartz sand > red soil > alluvial soil, due to different texture and total organic carbon (TOC) contents of the quartz sand and other two soils. The differential Fourier transform infrared (FT-IR) spectra of degraded pyrene in alluvial soil corroborate that TiO₂-enhanced photocatalytic degradation rate of degraded pyrene was much greater than photooxidation (without TiO₂) rate of degraded pyrene. Based on the data obtained, the importance for the application of TiO₂-enhanced photocatalytic pyrene degradation and associated organic contaminants in contaminated soils was elucidated.     

Preparation of DNA-adsorbed TiO2 particles — Augmentation of performance for environmental purification by increasing DNA adsorption by external pH regulation

We have previously developed a novel photocatalyst, DNA-attached titanium dioxide (DNA-TiO₂), useful for the recovery and decomposition of chemicals [Suzuki et al. Environ. Sci. Technol. 42, 8076, 2008]. Chemicals accumulated in DNA near the surface of TiO₂ and were degraded under UV light. The efficiency of their removal was dependent on the amount of DNA adsorbed on TiO₂, indicating the attachment of larger amounts of DNA to result in higher efficiency. In this study, we succeeded in improving the performance of DNA-TiO₂ by increasing the amount of DNA adsorbed by regulating the external pH. The adsorption of DNA by TiO₂ dramatically increased at pH2, to about fourfold that at other pH values (pH4-10). Repeating the process of DNA addition increased the adsorption further. The attached DNA was stable on the surface of TiO₂ at pH2-10 and 4-56 °C, the same as DNA-TiO₂ prepared at pH7. As the DNA-TiO₂ prepared at pH2 retained much DNA on its surface, chemicals (methylene blue, ethidium bromide, etc.) which could intercalate or react with DNA were effectively removed from solutions. The photocatalytic degradation was slow at first, but the final degradation rate was higher than for non-adsorbed TiO₂ and DNA-TiO₂ prepared at pH7. These results indicated that preparation of DNA-TiO₂ at pH2 has advantages in that much DNA can be attached and large amounts of chemicals can be concentrated in the DNA, resulting in extensive decomposition under UV light.     

Photocatalytic transformation and mineralization of 2,4,6-trinitrotoluene (TNT) in TiO2 slurries

An analysis of the photodegradation of TNT in a TiO₂ slurry reactor is presented. The rates and extent of TNT transformation and mineralization are compared for photocatalytic and direct photolytic reactions under conditions of varying light energies and in the presence and absence of oxygen. Certain initial organic transformation products are identified for both photocatalytic and photolytic reactions. Nitrate, nitrite, and ammonium ions are analyzed and the possibility of semiconductor sensitization by colored compounds is considered. TNT was transformed rapidly under each set of photochemical conditions but destruction was faster and more complete with TiO₂ photocatalysis. Transformation by-products were destroyed readily under oxygenated photocatalytic conditions and were observed to be more refractory under direct photolytic conditions. Mass balances performed on carbon and nitrogen revealed that when the TiO₂ photocatalyst was utilized in the presence of oxygen and near u.v. radiation (λ > 340 nm) approx. 90% of the TNT was mineralized and 35% of the total nitrogen was recovered as ammonium ion after 120 min. Among the large number of organic transformation products produced photocatalytically, trinitrobenzoic acid, trinitrobenzene and trinitrophenol have been identified as oxidative intermediate species and dinitroaniline as a reduction product. The photocatalytic transformation of TNT appears to involve both oxidative and reductive steps and sensitization by colored compounds plays no detectable role in degradation.     

Synergistic effect of nickel ions on the coupled dechlorination of trichloroethylene and 2,4-dichlorophenol by Fe/TiO₂ nanocomposites in the presence of UV light under anoxic conditions

The coupled removal of priority pollutants by nanocomposite materials has recently been receiving much attention. In this study, trichloroethylene (TCE) and 2,4-dichlorophenol (DCP) in aqueous solutions were simultaneously removed by Fe/TiO₂ nanocomposites under anoxic conditions in the presence of nickel ions and UV light at 365 nm. Both TCE and DCP were effectively dechlorinated by Fe/TiO₂ nanocomposites, and the pseudo-first-order rate constants (k_obs) for TCE and DCP dechlorination were (1.39 ± 0.05)×10⁻² and (1.08 ± 0.05)×10⁻² h⁻¹, respectively, which were higher than that by nanoscale zerovalent iron alone. In addition, the k_obs for DCP dechlorination was enhanced by a factor of 77 when Fe/TiO₂ was illuminated with UV light for 2 h. Hydrodechlorination was found to be the major reaction pathway for TCE dechlorination, while DCP could undergo reductive dechlorination or react with hydroxyl radicals to produce 1,4-benzoquinone and phenol. TCE was a stronger electron acceptor than DCP, which could inhibit the dechlorination efficiency and rate of DCP during simultaneous removal processes. The addition of nickel ions significantly enhanced the simultaneous photodechlorination efficiency of TCE and DCP under the illumination of UV light. The k_obs values for DCP and TCE photodechlorination by Fe/TiO₂ in the presence of 20-100 μM Ni(II) were 30.4-136 and 13.2-192 times greater, respectively, when compared with those in the dark. Electron spin resonance analysis showed that the photo-generated electron-hole pairs could be effectively separated through Ni ions cycling, leading to the improvement of electron transfer efficiency of TCE and DCP by Fe/TiO₂.     

Development of a TiO2 modified optical fiber electrode and its incorporation into a photoelectrochemical reactor for wastewater treatment

Electrochemical advanced oxidation processes (EAOPs) are used to chemically burn non biodegradable complex organic compounds that are present in polluted effluents. A common approach involves the use of TiO₂ semiconductor substrates as either photocatalytic or photoelectrocatalytic materials in reactors that produce a powerful oxidant (hydroxyl radical) that reacts with pollutant species. In this context, the purpose of this work is to develop a new TiO₂ based photoanode using an optic fiber support. The novel arrangement of a TiO₂ layer positioned on top of a surface modified optical fiber substrate, allowed the construction of a photoelectrochemical reactor that works on the basis of an internally illuminated approach. In this way, a semi-conductive optical fiber modified surface was prepared using 30 μm thickness SnO₂:Sb films on which the photoactive TiO₂ layer was electrophoretically deposited. UV light transmission experiments were conducted to evaluate the transmittance along the optical fiber covered with SnO₂:Sb and TiO₂ showing that 43% of UV light reached the optical fiber tip. With different illumination configurations (external or internal), it was possible to get an increase in the amount of photo-generated H₂O₂ close to 50% as compared to different types of TiO₂ films. Finally, the electro-Fenton photoelectrocatalytic Oxidation process studied in this work was able to achieve total color removal of Azo orange II dye (15 mg L⁻¹) and a 57% removal of total organic carbon (TOC) within 60 min of degradation time.     

Enhanced arsenic removal using mixed metal oxide impregnated chitosan beads

Mixed metal oxide impregnated chitosan beads (MICB) containing nanocrystalline Al₂O₃ and nanocrystalline TiO₂ were successfully developed. This adsorbent exploits the high capacity of Al₂O₃ for arsenate and the photocatalytic activity of TiO₂ to oxidize arsenite to arsenate, resulting in a removal capacity higher than that of either metal oxide alone. The composition of the beads was optimized for maximum arsenite removal in the presence of UV light. The mechanism of removal was investigated and a mode of action was proposed wherein TiO₂ oxidizes arsenite to arsenate which is then removed from solution by Al₂O₃. Pseudo-second order kinetics were used to validate the proposed mechanism. MICB is a more efficient and effective adsorbent for arsenic than TiO₂-impregnated chitosan beads (TICB), previously reported on, yet maintains a desirable life cycle, free of complex synthesis processes, toxic materials, and energy inputs.     

Concurrent filtration and solar photocatalytic disinfection/degradation using high-performance Ag/TiO2 nanofiber membrane

A facile polyol synthesis was used for the deposition of Ag nanoparticles on electrospun TiO₂ nanofibers for the subsequent fabrication of Ag/TiO₂ nanofiber membrane. The permeate flux of the Ag/TiO₂ nanofiber membrane was remarkably high compared to commercial P25 deposited membrane. The Ag/TiO₂ nanofiber membrane achieved 99.9% bacteria inactivation and 80.0% dye degradation under solar irradiation within 30 min. The Ag/TiO₂ nanofiber membrane also showed excellent antibacterial capability without solar irradiation. Considering the excellent intrinsic antibacterial activity and high-performance photocatalytic disinfection/degradation under solar irradiation, this novel membrane proved to have promising applications in water purification industry.     

Solar Fenton and solar TiO2 catalytic treatment of ofloxacin in secondary treated effluents: Evaluation of operational and kinetic parameters

Two different technical approaches based on advanced oxidation processes (AOPs), solar Fenton homogeneous photocatalysis (hv/Fe²⁺/H₂O₂) and heterogeneous photocatalysis with titanium dioxide (TiO₂) suspensions were studied for the chemical degradation of the fluoroquinolone ofloxacin in secondary treated effluents. A bench-scale solar simulator in combination with an appropriate photochemical batch reactor was used to evaluate and select the optimal oxidation conditions of ofloxacin spiked in secondary treated domestic effluents. The concentration profile of the examined substrate during degradation was determined by UV/Vis spectrophotometry. Mineralization was monitored by measuring the dissolved organic carbon (DOC). The concentrations of Fe²⁺ and H₂O₂ were the key factors for the solar Fenton process, while the most important parameter of the heterogeneous photocatalysis was proved to be the catalyst loading. Kinetic analyses indicated that the photodegradation of ofloxacin can be described by a pseudo-first-order reaction. The rate constant (k) for the solar Fenton process was determined at different Fe²⁺ and H₂O₂ concentrations whereas the Langmuir-Hinshelwood (LH) kinetic expression was used to assess the kinetics of the heterogeneous photocatalytic process. The conversion of ofloxacin depends on several parameters based on the various experimental conditions, which were investigated. A Daphnia magna bioassay was used to evaluate the potential toxicity of the parent compound and its photo-oxidation by-products in different stages of oxidation. In the present study solar Fenton has been demonstrated to be more effective than the solar TiO₂ process, yielding complete degradation of the examined substrate and DOC reduction of about 50% in 30 min of the photocatalytic treatment.     

Novel, bio-based, photoactive arsenic sorbent: TiO2-impregnated chitosan bead

A novel sorbent for arsenic, TiO₂-impregnated chitosan bead (TICB), has been synthesized and successfully tested. Kinetic plots, pH dependence, isotherm data, and bead morphology are reported. Equilibrium is achieved after 185 h in batch experiments with exposure to UV light. The TICB system performs similarly to the mass equivalent of neat TiO₂ nanopowder. The point of zero charge (pzc) for TICB was determined to be 7.25, and as with other TiO₂-based arsenic removal technologies, the optimal pH range for sorption is below this pH_pzc. Without exposure to UV light, TICB removes 2198 μg As(III)/g TICB and 2050 μg As(V)/g TICB. With exposure to UV light, TICB achieves photo-oxidation of As(III) to As(V), the less toxic and more easily sequestered arsenic form. UV irradiation also results in enhanced arsenic removal, reaching sorption capacities of 6400 μg As/g TICB and 4925 μg As/g TICB, where arsenic is initially added as As(III) and As(V), respectively. Because the TICB system obviates filtration post-treatment, TICB is superior to TiO₂ nanopowder from the perspective of implementation for decentralized water treatment.     

Optimization of capacity and kinetics for a novel bio-based arsenic sorbent, TiO2-impregnated chitosan bead

The optimization of TiO₂-impregnated chitosan beads (TICB) as an arsenic adsorbent is investigated to maximize the capacity and kinetics of arsenic removal. It has been previously reported that TICB can 1) remove arsenite, 2) remove arsenate, and 3) oxidize arsenite to arsenate in the presence of UV light and oxygen. Herein, it is reported that adsorption capacity for TICB is controlled by solution pH and TiO₂ loading within the bead and enhanced with exposure to UV light. Solution pH is found to be a critical parameter, whereby arsenate is effectively removed below pH 7.25 and arsenite is effectively removed below pH 9.2. A model to predict TICB capacity, based on TiO₂ loading and solution pH, is presented for arsenite, arsenate, and total arsenic in the presence of UV light. The rate of removal is increased with reductions in bead size and with exposure to UV light. Phosphate is found to be a direct competitor with arsenate for adsorption sites on TICB, but other relevant common background groundwater ions do not compete with arsenate for adsorption sites. TICB can be regenerated with weak NaOH and maintain full adsorption capacity for at least three adsorption/desorption cycles.     

Gas‐phase removal of indoor volatile organic compounds and airborne microorganisms over mono‐ and bimetal‐modified (Pt,Cu, Ag) titanium(IV) oxide nanocomposites

The photocatalytic deactivation of volatile organic compounds and mold fungi using TiO₂ modified with mono‐ and bimetallic (Pt, Cu, Ag) particles is reported in this study. The mono‐ and bimetal‐modified (Pt, Cu, Ag) titanium(IV) oxide photocatalysts were prepared by chemical reduction method and characterized using XRD, XPS, DR/UV‐Vis, BET, and TEM analysis. The effect of incident light, type and content of mono‐ and bimetallic nanoparticles deposited on titanium(IV) oxide was studied. Photocatalytic activity of as‐prepared nanocomposites was examined in the gas phase using LEDs array. High photocatalytic activity of Ag/Pt‐TiO₂ and Cu/Pt‐TiO₂ in the reaction of toluene degradation resulted from improved efficiency of interfacial charge transfer process, which was consistent with the fluorescence quenching effect revealed by photoluminescence (PL) emission spectra. The photocatalytic deactivation of Penicillium chrysogenum, a pathogenic fungi present in the indoor environment, especially in a damp or water‐damaged building using mono‐ and bimetal‐modified (Pt, Cu, Ag) titanium(IV) oxide was evaluated for the first time. TiO₂ modified with mono‐ and bimetallic NPs of Ag/Pt, Cu, and Ag deposited on TiO₂ exhibited improved fungicidal activity under LEDs illumination than pure TiO₂.     

Enhance the photocatalytic activity for the degradation of organic contaminants in water by incorporating TiO2 with zero-valent iron

Titanium dioxide (TiO₂) has become the most popular photocatalyst in treating persistent organic pollutants. The main disadvantage of TiO₂ is the diminishing photocatalytic activity over time due to the electron-hole pair recombination. Many studies have aimed to prolong the photocatalytic life of TiO₂. Among them, incorporation of zero-valent iron (ZVI) is one of the approaches. In this study, a novel nano TiO₂/Fe⁰ composite (NTFC) was synthesized from a nano neutral TiO₂ sol and a nano zero-valent iron (nZVI), both prepared in our laboratory. The structure, composition and physical property of the NTFC are characterized. The photocatalytic activity of the NTFC was evaluated by the reductive decolourization of an azo dye, Acid Black-24 (AB-24), and was found superior to those of nZVI and nano neutral TiO₂ sol. Evidence suggests that the enhanced activity of NTFC is highly correlated to the ratio of ferrous to ferric ion in the system. The quantities of ferrous and ferric ions in the nZVI and NTFC systems were monitored separately. In the nZVI system, the concentration of ferric ions decreased significantly with time while a high level of ferrous ions was maintained in the NTFC suspension. The ferrous/ferric ratio of the NTFC suspension was substantially increased after irradiation by UV. Evidence from EPR analysis suggests that the excited electrons in the conduction band of the TiO₂ can be trapped by the half reaction of Fe³⁺/Fe²⁺, reducing the probability of electron-electron hole pair recombination and sustaining the catalytic life of TiO₂. Corrosion tests further proved that by incorporating TiO₂ with zero-valent iron the surface oxidation of nZVI can be effectively prevented.     

Preliminary trial on degradation of waste activated sludge and simultaneous hydrogen production in a newly-developed solar photocatalytic reactor with AgX/TiO2-coated glass tubes

A solar fluidized tubular photocatalytic reactor (SFTPR) with simple and efficient light collector was developed to degrade waste activated sludge (WAS) and simultaneously produce hydrogen. The photocatalyst was a TiO₂ film doped by silver and silver compounds (AgX). The synthesized photocatalyst, AgX/TiO₂, exhibited higher photocatalytic activity than TiO₂ (99.5% and 30.6% of methyl orange removal, respectively). The installation of light collector could increase light intensity by 26%. For WAS treatment using the SFTPR, 69.1% of chemical oxygen demand (COD) removal and 7866.7 μmol H₂/l-sludge of hydrogen production were achieved after solar photocatalysis for 72 h. The SFTPR could be a promising photocatalysis reactor to effectively degrade WAS with simultaneous hydrogen production. The results can also provide a useful base and reference for the application of photocatalysis on WAS degradation in practice.     

Effects of water parameters on the degradation of microcystin-LR under visible light-activated TiO2 photocatalyst

A study was performed to determine the effect of pH, alkalinity, natural organic matter (NOM) and dissolved oxygen in the performance of nitrogen and fluorine doped TiO₂ (NF-TiO₂) for the degradation of hepatotoxin microcystin-LR (MC-LR) in synthetic and natural water under visible light irradiation. The initial degradation rate of MC-LR was fastest under acidic conditions (3.50 ± 0.02 × 10⁻³ μM min⁻¹ at pH 3.0) and decreased to 2.29 ± 0.07 × 10⁻³ and 0.54 ± 0.02 × 10⁻³ μM min⁻¹ at pH 5.7 and 7.1, respectively. Attractive forces between the opposite charged MC-LR and NF-TiO₂ are likely responsible for the enhancement in the photocatalytic decomposition of MC-LR resulting from increased interfacial adsorption. For carbonate buffered solutions, the photocatalytic activity of NF-TiO₂ was reduced when increasing the carbonate concentration up to 150 mg CaCO₃ L⁻¹. The scavenging of radical species by the bicarbonate ion at pH 7.1 is discussed. In the presence of NOM, the degradation rates decreased as pH and initial concentration of the NOM increased. The inhibition was higher with fulvic acid than humic acid under alkaline conditions. Oxygenated solution yields higher NF-TiO₂ photocatalytic degradation of MC-LR compared to nitrogen sparged solution at pH 5.7. The involvement of specific reactive oxygen species implicated in the photodegradation is proposed. Finally, no significant degradation is observed with various natural waters spiked with MC-LR under visible light (λ > 420 nm) but high removal was achieved with simulated solar light. This study provides a better understanding of the interactions and photocatalytic processes initiated by NF-TiO₂ under visible and solar light. The results indicate solar photocatalytic oxidation is a promising technology for the treatment of water contaminated with cyanotoxins.     

Inactivation and surface interactions of MS-2 bacteriophage in a TiO2 photoelectrocatalytic reactor

Inactivation of MS-2 bacteriophage in a TiO₂ photoelectrocatalytic system was evaluated, wherein TiO₂ particles were coated onto an indium tin oxide (ITO) electrode and an electrical potential was applied under black light blue (BLB) irradiation. MS-2 phage inactivation was greatly enhanced by anodic potential, whereas cathodic potential completely inhibited inactivation. Experiments performed with radical scavengers showed that inactivation was primarily caused by hydroxyl radicals, both in the bulk phase and on the TiO₂ surface. Application of positive potential to the electrode was found to result in two distinct beneficial effects: (i) electrostatic attraction between the negatively charged viral capsid and catalyst surface, causing improved usage of surface-bound hydroxyl radical, in comparison to conventional TiO₂ photocatalytic disinfection; and (ii) higher reactive oxygen species production. Results also suggest that inactivation of various microorganisms including Escherichia coli, Staphylococcus aureus, Klebsiella pneumoniae, Bacillus subtilis spores and Cryptosporidium parvum oocyst was enhanced via positive potential induction to TiO₂.