Photoreductive Mechanism of CCl4 Degradation on TiO2 Particles and Effects of Electron Donors

Isotope Techniques in Chemical Wastewater Treatment: Opportunities and Uncertainties

A comprehensive and in-depth analysis of reaction mechanisms is essential for advancing chemical water treatment technologies. However, due to the limitations of conventional experimental and analytical methods, the types of reactive species and their generation pathways are commonly debatable in many aqueous systems. As highly sensitive diagnostic tools, isotope techniques offer deeper insights with minimal interference from reaction conditions. Nevertheless, precise interpretations of isotope results remain a significant challenge. Herein, we first scrutinized the fundamentals of isotope chemistry and highlighted key changes induced by the isotope substitution. Next, we discussed the application of isotope techniques in kinetic isotope effects, presenting a roadmap for interpreting KIE in sophisticated systems. Furthermore, we summarized the applications of isotope techniques in elemental tracing to pinpoint reaction sites and identify dominant reactive species. Lastly, we propose future research directions, highlighting critical considerations for the rational design and interpretation of isotope experiments in environmental chemistry and related fields.

Sunlight-Driven Direct/Mediated Electron Transfer for Cr(VI) Reductive Sequestration on Dissolved Black Carbon-Ferrihydrite Coprecipitates

Surface runoff horizontally distributed chromium (Cr) pollution into various surface environments. Sunlight is a vital factor for the Cr cycle in the surface environment, which may be affected by photoactive substances such as ferrihydrite (Fh) and dissolved black carbon (DBC). Herein, sunlight-driven transformation dynamics of Cr species on DBC-Fh coprecipitates were studied. Under sunlight, the removal of aqueous Cr(VI) by DBC-Fh coprecipitates occurred through sunlight-driven reductive sequestration including adsorption, followed by surface reduction (pathway 1) and aqueous reduction, followed by precipitation (pathway 2). Additionally, coprecipitates with a higher DBC content exhibited a more effective reduction of both adsorbed (kapp,S_red) and aqueous Cr(VI) (kapp,A_red). Photoelectrons facilitated Cr(VI) reduction through direct electron transfer; notably, electron donating DBC promoted the production of photoelectrons by consuming photogenerated holes. Photogenerated Fe(II) species (mineral-phase and aqueous Fe(II)) mediated electron transfer for Cr(VI) reduction, which was reinforced via a ligand-to-metal charge transfer (LMCT) process between DBC-organic ligands and mineral Fe(III). Furthermore, ·O2- also mediated Cr(VI) reduction, although this impact was limited. Overall, this study demonstrates that photoelectrons and photogenerated electron mediators play a crucial role in Cr(VI) reductive sequestration on DBC-Fh coprecipitates, providing new insights into the geochemical cycle of Cr pollution in sunlight-influenced surface environments.

Effect of low molecular weight organic acids on the lead and chromium release from widely-used lead chromate pigments under sunlight irradiation

Lead chromate pigments are commonly used yellow inorganic pigments. They can pose environmental risks as they contain toxic heavy metals lead and chromium. Low molecular weight organic acids (LMWOAs), as widespread dissolved organic matter (DOM), affect the lead and chromium release from the pigment in water. In this work, the role of LMWOAs in the photodissolution of commercial lead chromate pigment was investigated. The pigment underwent significant photodissolution under simulated sunlight exposure with LMWOAs, and subsequently released Cr(III) and Pb(II). The photodissolution process is caused by the reduction of Cr(VI) by photogenerated electrons of the lead chromate pigment. The LMWOAs promoted photodissolution of the pigment by improving the electron-hole separation. The formation of Cr(III)-contained compounds leads to a slower release of chromium than lead. The photodissolution kinetics increase with decreasing pH and increasing LMWOAs concentration. The photodissolution of lead chromate pigment was basically positively related to the total number of hydroxyl and carboxyl groups in LMWOAs. The LMWOAs with stronger affinity to lead chromate pigment, lower adiabatic ionization potential (AIP) and higher energy of the highest occupied molecular orbital (EHOMO) are favorable to Cr(VI) reduction by photogenerated electrons and pigment photodissolution. 2.39% of chromium and 10.34% of lead released from the lead chromate pigment in natural conditions during a 6-h sunlight exposure. This study revealed the photodissolution mechanism of lead chromate pigment mediated by LMWOAs with different molecular structures, which helps understand the environmental photochemical behavior of the pigment. The present results emphasize the important role of DOM in the heavy metals release from commercial inorganic pigments.

Efficient removal of TBBPA with a Z-scheme BiVO4-(rGO-Cu2O) photocatalyst under sunlight irradiation

In this study, reduced graphene oxide (rGO) was used to fabricate a Z-scheme BiVO₄-(rGO-Cu₂O) photocatalyst for the degradation of Tetrabromobisphenol A (TBBPA) under sunlight irradiation. The photocatalyst was synthesized using a three-step method BiVO₄-(rGO-Cu₂O) photocatalyst with an rGO loading of 1% and (rGO-Cu₂O) to BiVO₄ ratio of 50% achieved the best degradation effect for TBBPA removal. Electron paramagnetic resonance spectroscopy (EPR) confirmed that the charge transfer path of BiVO₄-(rGO-Cu₂O) follows that of Z-scheme photocatalysts. Moreover, the addition of rGO increases the charge transfer efficiency. High performance liquid chromatography-mass spectrometry (HPLC-MS) was used to detect and analyze intermediate products, allowing the proposal of the main degradation pathway of TBBPA. Photogenerated electrons of BiVO₄-(rGO-Cu₂O) were then transferred into the conduction band of Cu₂O. Cu₂O is located in the surface layer, which has the most effective contact area with pollutants, and therefore has a good outcome for the photocatalytic reduction of TBBPA. Photogenerated electrons (e^-) and hydroxyl radicals (∙OH) are the main factors affecting TBBPA degradation. The degradation process of TBBPA includes electron reduction debromination, hydroxylation, and β-cleavage. In our work, BiVO₄-(rGO-Cu₂O) was successfully synthesized to degrade TBBPA; this study brings forth a novel approach for the degradation of halogenated organic pollutants using a Z-scheme photocatalytic composite.

Titanium dioxide nanoparticles as multifunctional surface-active materials for smart/active nanocomposite packaging films

Environmental issues such as plastic packaging and high demand for fresh and safe food has increased the interest for developing smart/active food packaging films with colloidal nanoparticles (NPs). Titanium dioxide nanoparticles (TNPs) are cost effective and stable metal oxide NPs which could be used as a functional nano-filler for biodegradable food packaging due to their excellent biocompatibility, photo catalyzing, and antimicrobial properties. This article has comprehensively reviewed the functional properties and advantages of TNPs-containing smart/active films. The advantage of adding TNPs for ameliorating food packaging materials such as their physical, mechanical, moisture/light barrier, optical, thermal resistance, microstructure and chemical properties as well as, antibacterial, and photocatalytic properties are discussed. Also, the practical and migration properties of administrating TNPs in food packaging material are investigated. The ethylene decomposition activity of TNPs containing active films, could be used for increasing the shelf life of fruits/vegetables after harvesting. TNPs are safe with negligible migration rates which could be used for fabrication of multifunctional smart/active packaging films due to their antimicrobial properties and ethylene gas scavenging activities.

Lead release and species transformation of commercial minium pigments in aqueous phase under UV-irradiation

This study reports the lead ions release and species transformation of minium pigment under UV-irradiation in aqueous phase. The effects of fulvic acid (FA) and pH on lead ions release were investigated. Lower pH and higher FA concentration facilitate the release of lead ions. During photoreaction, electron donors (FA) are provided to scavenge photogenerated holes and enhance the charge separation, leading to the increased lead ions release. When significant amounts of FA exist in the water, they will complex with the released lead ions preferentially. Then, lead ions react with CO₂ and proton resulting in the formation of insoluble Pb₃(CO₃)₂(OH)₂ after FA consumed. Correspondingly, the dissolved lead ion concentration in water showed a trend of increasing first and then decreasing. Based upon characterization and experimental results, lead ions release and lead species transformation mechanisms of minium were eventually speculated. This research is helpful for better understanding the environmental behavior and risk of semiconductor pigments in natural water.

Oxygen vacancy engineering of cerium oxide for the selective photocatalytic oxidation of aromatic pollutants

The engineering of oxygen vacancies in CeO₂ nanoparticles (NPs) allows the specific fine-tuning of their oxidation power, and this can be used to rationally control their activity and selectivity in the photocatalytic oxidation (PCO) of aromatic pollutants. In the current study, a facile strategy for generating exceptionally stable oxygen vacancies in CeO₂ NPs through simple acid (CeO₂-A) or base (CeO₂-B) treatment was developed. The selective (or mild) PCO activities of CeO₂-A and CeO₂-B in the degradation of a variety of aromatic substrates in water were successfully demonstrated. CeO₂-B has more oxygen vacancies and exhibits superior photocatalytic performance compared to CeO₂-A. Control of oxygen vacancies in CeO₂ facilitates the adsorption and reduction of dissolved O₂ due to their high oxygen-storage ability. The oxygen vacancies in CeO₂-B as active sites for oxygen-mediated reactions act as (i) adsorption and reduction reaction sites for dissolved O₂, and (ii) photogenerated electron scavenging sites that promote the formation of H₂O₂ by multi-electron transfer. The oxygen vacancies in CeO₂-B are particularly stable and can be used repeatedly over 30 h without losing activity. The selective PCOs of organic substrates were studied systematically, revealing that the operating mechanisms for UV-illuminated CeO₂-B are very different from those for conventional TiO₂ photocatalysts. Thus, the present study provides new insights into the design of defect-engineered metal oxides for the development of novel photocatalysts.

Photoreductive degradation of CCl4 by UV-Na2SO3: influence of various factors, mechanism and application

Due to the strong electron-withdrawing nature of Cl atom in CCl₄, CCl₄ could not readily be degraded by oxidation process. In the present study, aqueous electron (e_aq ^-), a powerful reducing agent generated in UV-Na₂SO₃ system, was applied to reductively degradation of CCl₄. The effects of several crucial factors (e.g. Na₂SO₃ concentration, solution pH, inorganic ions and NOM) on CCl₄ degradation as well as degradation mechanism and pathway were systematically investigated. Results indicated that CCl₄ was efficiently degraded in UV-Na₂SO₃ system and the process could be well described by pseudo-first order kinetic model. The degradation rate increased with the elevated Na₂SO₃ concentration (0-10 mmol/L) and solution pH (6.0-8.0), while remained approximately constant in alkaline conditions (pH = 8.0, 9.0 and 10.0). Nevertheless, O₂, inorganic ions and NOM exerted a negative effect on CCl₄ degradation and the removal efficiency of CCl₄ in groundwater was only 31.7%. Mechanistic study implied that degradation of CCl₄ was primarily induced by e_aq ^-. CCl₄ (10 mg/L) was almost completely dechlorinated within 60 min and the predominant intermediate products were CHCl₃, C₂Cl₄ and C₂HCl₃. CHCl₃ and CH₂Cl₂ were also rapidly degraded in the UV-Na₂SO₃ system.

Sunlight-Mediated Lead and Chromium Release from Commercial Lead Chromate Pigments in Aqueous Phase

Lead chromate pigments are included in a group of the most widely used pigments, which account for 3% of worldwide lead consumption. This study reports the photoactivity of commercial lead chromate pigment (i.e., chrome yellow) under simulated sunlight. It underwent photodissolution in the presence of organic acid and dissolved organic matter in the aqueous phase, releasing Pb(II) and Cr(III). Pb(II) was released more readily than Cr(III) which mainly formed hydroxides and oxides. The photodissolution can be activated by light with a wavelength <514 nm. The reaction is mediated by the reduction of Cr(VI) in the pigment by self-generated electrons. The kinetics were mainly affected by the electron-hole separation efficiency which can be enhanced by electron donors. The reaction rate decreases with increasing solution pH as the photodissolution process consumes protons. The photodissolution of the chrome yellow pigment was further confirmed in a river water sample under natural sunlight, with 11.28% of lead and 2.56% of chromium released in 7 h. This study highlights the importance of considering photochemical processes in risk assessments and regulations of commercial semiconductor pigments, which are currently based on their solubility.

Braiding kinetics and spectroscopy in photo-catalysis: the spectro-kinetic approach

The combination of kinetic and spectroscopic tools has become a key scientific methodology for the understanding of catalytic behavior but its application in photocatalysis has inherent difficulties due to the nature of the energy source of the reaction. This review article provides an overview of its use by, first, presenting mechanistically derived kinetic formulations and spectroscopic data handling methods including intrinsic expressions for light and, second, highlighting representative examples of application. To do it we consider universal catalytic systems, particularly (although not exclusively) titania-based materials, and the most frequent hole and/or electron triggered reaction schemes. This review also provides a general framework to pave the way for the future progress of the spectro-kinetic approach in the photocatalysis area.

Dissolved Mineral Ash Generated by Vegetation Fire Is Photoactive under the Solar Spectrum

Vegetation fire generates vast amounts of mineral ash annually that can be readily mobilized by water or wind erosion. Little is known about the photoactivity of dissolved mineral ash in aquatic systems and its ability to mediate redox reactions of environmental pollutants. This study reports that dissolved mineral ash derived from pyrolysis of biomass is photoactive under simulated sunlight, generating reactive oxygen species. It can mediate the photoreduction of hexavalent chromium (Cr(VI)) in the presence of electron donors; for example, phenols and dissolved organic matter, at pH 4.7. The reaction kinetics followed the Langmuir-Hinshelwood model, suggesting a heterogeneous photocatalytic reaction. The enhancement of reduction efficiency was linearly correlated with the one-electron reduction potential of phenols. The synergy between dissolved mineral ash and phenols is attributed to the inhibition of electron-hole recombination. The reduction rate decreases with increasing solution pH, owing to the decreased reduction potential and surface adsorption of Cr(VI). The silicon and silicon carbide components are most likely responsible for the photocatalytic activity of dissolved mineral ash. Our results suggest that dissolved mineral ash is a natural photocatalyst that can mediate redox reactions of pollutants in sunlit aquatic systems, playing an overlooked role in natural attenuation and aquatic photochemistry.

Location of photocatalytic oxidation processes on anatase titanium dioxide

The detailed mechanism of photocatalytic oxidation processes on the TiO₂ surface is still not completely clear, particularly the location of degradation processes.

Bacterial disinfection in a sunlight/visible-light-driven photocatalytic reactor by recyclable natural magnetic sphalerite

A 5-L reactor was designed and used to enhance the sunlight/visible-light-driven (VLD) photocatalytic disinfection efficiency towards Gram-negative bacterium (Escherichia coli). Natural magnetic sphalerite (NMS) was used as the photocatalyst, which could be easily recycled by applying a magnetic field. Results showed that NMS with irradiation by the blue light emitting diode (LED) lamp could completely inactivate 1.5 × 10⁵ cfu/mL of E. coli within 120 min in the first three runs. However, the inactivation efficiency of E. coli started to decrease in the 4th Run, while in the 5th run, the E. coli with the initial concentration of 5 logs was inactivated to 3.3 (blue-light) and 3.5 logs (sunlight), respectively. Moreover, the stability and deactivation mechanism of NMS during subsequent runs were also studied. The results showed that the decline of the photocatalytic activity was possibly attributed to adsorption of the bacterial decomposed compounds on the active sites. In addition, photocatalytic bactericidal mechanism of the NMS in the photocatalytic system was investigated by using multiple scavengers to remove the specific reactive species. Moreover, various Gram-positive bacteria including Staphylococcus aureus, Microbacterium barkeri, and Bacillus subtilis could also be efficiently inactivated in the photocatalytic system.

Microbial inactivation kinetics and mechanisms of carbon-doped TiO2 (C-TiO2) under visible light

In this study, titanium dioxide nanoparticles doped with carbon (C-TiO2) were synthesized by means of sol-gel methods, and the synthesis was verified by means of X-ray photoelectron spectroscopy. The nanoparticles' photocatalytic disinfection activity of Listeria monocytogenes was tested under UV and visible light. The observed inactivation levels for 150min of visible light exposure with and without UV cutoff filters were 2.10 and 2.45 log, respectively. We also found that traditional reactive oxygen species had insignificant actions on C-TiO2 photocatalysts and that L. monocytogenes inactivation in the C-TiO2 system under visible light was induced in large part by the midgap states (hmid(+)) that was produced photochemically from the visible light response. C-TiO2 was found to accelerate bacterial inactivation (of L. monocytogenes) in the presence of visible light. Our data suggests that the C-TiO2 may be useful in the development of alternative disinfectants for environmental applications.

Photodecomposition of tetrabromobisphenol A in aqueous humic acid suspension by irradiation with light of various wavelengths

The reactive species generated in aqueous 3,3',5,5'-tetrabromobisphenol A (TBBPA)/humic acid (HA) suspensions above the TBBPA pKa (∼7.4), under various light-irradiation conditions, namely ambient and ultraviolet light, were investigated using electron paramagnetic resonance (EPR) spectroscopy and liquid chromatography-mass spectrometry (LC-MS). We confirmed that singlet oxygen and OH radicals are the key reactive oxygen species generated at wavelengths greater than 400 and 300 nm, respectively. The amount of 2,6-dibromo-p-benzosemiquinone anion radicals (2,6-DBSQ(•-)) formed under irradiation at 400 nm increased linearly with respect to irradiation time; the initial reaction rate was 7.03 × 10(-9) mol g(-1) HA s(-1). The rate increased with increasing pH and light intensity. LC-MS and EPR spectroscopy showed that tribromohydroxybisphenol A was formed under irradiation at 300 nm via reaction of OH radicals with TBBPA. This study, for the first time, shows that the main byproducts formed during irradiation at wavelengths above 300 nm are 2,6-DBSQ(•-) and tribromohydroxybisphenol A, generated from singlet oxygen ((1)O2) and OH radicals, respectively. Photodecomposition of TBBPA in the environment may occur by formation of (1)O2 and OH radicals.

A g-C3N4/nanocarbon/ZnIn2S4nanocomposite: an artificial Z-scheme visible-light photocatalytic system using nanocarbon as the electron mediator

Nanocarbon could be used as an electron mediator to construct a Z-scheme photocatalytic system with enhanced charge separation efficiency.

Sequential reduction-oxidation for photocatalytic degradation of tetrabromobisphenol A: kinetics and intermediates

C-Br bond cleavage is considered as a key step to reduce their toxicities and increase degradation rates for most brominated organic pollutants. Here a sequential reduction/oxidation strategy (i.e. debromination followed by photocatalytic oxidation) for photocatalytic degradation of tetrabromobisphenol A (TBBPA), one of the most frequently used brominated flame retardants, was proposed on the basis of kinetic analysis and intermediates identification. The results demonstrated that the rates of debromination and even photodegradation of TBBPA strongly depended on the atmospheres, initial TBBPA concentrations, pH of the reaction solution, hydrogen donors, and electron acceptors. These kinetic data and byproducts identification obtained by GC-MS measurement indicated that reductive debromination reaction by photo-induced electrons dominated under N(2)-saturated condition, while oxidation reaction by photoexcited holes or hydroxyl radicals played a leading role when air was saturated. It also suggested that the reaction might be further optimized for pretreatment of TBBPA-contaminated wastewater by a two-stage reductive debromination/subsequent oxidative decomposition process in the UV-TiO(2) system by changing the reaction atmospheres.

Comparative Study of Homogeneous and Heterogeneous Photocatalytic Redox Reactions: PW(12)O(40)(3-) vs TiO(2)

Polyoxometalates (POMs) as a homogeneous photocatalyst and semiconductor oxide as a heterogeneous photocatalyst share many aspects of similarity in their operating mechanisms. This study systematically compares various photocatalytic oxidation and reduction reactions of PW12O403- (a POM) and TiO2 in water to demonstrate that the two photocatalysts are very different in many ways. Both POM and TiO2 can photooxidize various organic compounds with comparable rates, but the POM-mediated mineralization is markedly slower than the mineralization with TiO2 under the experimental conditions employed in this study. Kinetic studies using tert-butyl alcohol as an OH radical scavenger suggest that OH radicals are the sole dominant photooxidant in POM-mediated degradations regardless of the kind of substrates tested, whereas both OH radicals and direct hole transfers take part in TiO2 photocatalysis. POM immobilization on silica support and surface fluorination of TiO2 significantly modified the kinetics and intermediate distribution. POM-mediated photoreductive dechlorination of CCl4 and trichloroacetate was negligible, whereas the dechlorination with TiO2 was markedly faster. The rate of electron transfer from POM- to reducible substrates seems to be significantly slower than the rate of conduction band electron transfer on TiO2 mainly due to the strong electron affinity of POM. The effects of H2O2 addition on photocatalytic reactivity are also very different between POM and TiO2. Detailed kinetic and mechanistic comparisons between PW12O403- and TiO2 photocatalysts are presented and discussed to understand the similarities and differences.

Photocatalytic degradation of perfluorooctanoic acid with beta-Ga2O3 in anoxic aqueous solution

Perfluorooctanoic acid (PFOA) is a new-found hazardous persistent organic pollutant, and it is resistant to decomposition by hydroxyl radical (HO*) due to its stable chemical structure and the high electronegativity of fluorine. Photocatalytic reduction of PFOA with beta-Ga2O3 in anoxic aqueous solution was investigated for the first time, and the results showed that the photoinduced electron (e(cb-)) coming from the beta-Ga2O3 conduction band was the major degradation substance for PFOA, and shorter-chain perfluorinated carboxylic acids (PFCAs, CnF2n+i1COOH, 1 < or = n < or = 6) were the dominant products. Furthermore, the concentration of F- was measured by the IC technique and defluorination efficiency was calculated. After 3 hr, the photocatalytic degradation efficiency was 98.8% and defluorination efficiency was 31.6% in the presence of thiosulfate and bubbling N2. The degradation reaction followed first-order kinetics (k = 0.0239 min(-1), t1/2 = 0.48 hr). PFCAs (CnF2n+1COOH, 1 < or = n < or = 7) were detected and measured by LC-MS and LC-MS/MS methods. It was deduced that the probable photocatalytic degradation mechanism involves e(cb-) attacking the carboxyl of CnF2n+1COOH, resulting in decarboxylation and the generation of CnF2n+1*. The produced CnF2n+1* reacted with H2O, forming CnF2n+1OH, then CnF2n+1OH underwent HF loss and hydrolysis to form CnF2n+1COOH.

Photoredox pathways of Cr(III)-tartrate complexes and their impacting factors

In the present study, exposure of Cr(III)-tar to full light of medium pressure mercury lamps and a xenon lamp was conducted in batch reaction systems at 25°C and different pH values to predict the potential for Cr(III) oxidation. The results indicated that the more intense irradiation and higher pH facilitated Cr(III)-tar oxidation. It appears that a ligand-to-metal charge-transfer occurs for Cr(III)-tar after irradiation, leading to the generation of Cr(II) and tar· or ·OH. The accompanying photochemical decomposition of tar·/or tar, together with O(2), further caused the formation of ·OH through multiple pathways, which ultimately converted Cr(II) to Cr(VI) step by step. H(2)O(2), a direct source of ·OH under irradiation, significantly enhanced photooxidation of Cr(III)-tar, but not obviously of aqueous Cr(III) or Cr(III)-tar in dark, implying that Cr(II) acts as a precursor of Cr(III) oxidization to Cr(VI).

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

Inactivation of MS-2 bacteriophage in a TiO(2) photoelectrocatalytic system was evaluated, wherein TiO(2) 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 TiO(2) 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 TiO(2) 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 TiO(2).

Photocatalytic oxidation mechanism of As(III) on TiO2: unique role of As(III) as a charge recombinant species

Using TiO(2) photocatalyst, arsenite, As(III), can be rapidly oxidized to arsenate, As(V), which is less toxic and less mobile in the aquatic environment. Therefore, the TiO(2)/UV process can be employed as an efficient pretreatment method for arsenic contaminated water. Since we first reported in 2002 that the superoxide (or hydroperoxyl radical) plays the role of main oxidant of As(III) in the TiO(2)/UV process, there has been much debate over the true identity of the major photooxidant among superoxides, holes, and OH radicals. The key issue is centered on why the much stronger OH radicals cannot oxidize As(III), and it has been proposed that the unique role of As(III) as an external charge recombination center on the UV-excited TiO(2) particle is responsible for this eccentric mechanism. Although the proposed mechanism has been supported by many experimental evidences, doubts on it were not clearly removed. In this study, we provided direct and undisputed evidence to support the role of As(III) in the charge recombination dynamics using time-resolved transient absorption spectroscopy. The presence of As(III) indeed mediated the charge recombination in TiO(2). Under this condition, the role of the OH radical is suppressed because of the null cycle, and the weaker oxidant, superoxide, should prevail. The role of the superoxide has been previously doubted on the basis of the observation that the addition of excess formic acid (hole scavenger that should enhance the production of superoxides) inhibited the photocatalytic oxidation of As(III). However, this study proved that this was due to the photogeneration of reducing radicals (HCO(2)·) that recycle As(V)/As(IV) back to As(III). It was also demonstrated that the 4-chlorophenol/TiO(2) system under visible light that cannot generate neither OH radicals nor valence band holes converted As(III) to As(V) through the superoxide pathway.

Photochemically controlled synthesis of anisotropic Au nanostructures: platelet-like Au nanorods and six-star Au nanoparticles

We report the shape-controlled synthesis of anisotropic Au nanostructures through TiO(2)-assisted photochemical reduction of HAuCl(4). By using this method, we have successfully synthesized the platelet-like Au nanorods and six-star Au nanoparticles. Importantly, the platelet Au nanorod exhibits the unique asymmetric five-twinned structure. The colloidal TiO(2) sols were used as both the photocatalyst to initiate the reaction and the stabilizing agent for the produced Au nanostructures. Significantly, in this photochemical process, the tunable irradiation intensity allows us to kinetically control the crystal evolution at various growth stages, leading to the shape difference of ultimate gold nanostructures. Our synthetic method shows a great potential as an alternative or supplement to the other wet chemical approaches for the shape-control of metallic nanostructures.

Photochemical decomposition of perfluorodecanoic acid in aqueous solution with VUV light irradiation

The photochemical decomposition of perfluorodecanoic acid (PFDeA) in water in the presence of persulfate ion (S(2)O(8)(2-)) and sulfur ion (S(2-)) was investigated under vacuum ultraviolet (VUV) light irradiation. PFDeA was decomposed under VUV light irradiation. With the addition of S(2)O(8)(2-) or S(2-), the photo-decomposition and defluorination of PFDeA were enhanced significantly. Sulfate radical anion (SO(4)(*-)) generated from photolysis of S(2)O(8)(2-) initiated PFDeA oxidation. While the S(2-) ion, acting as a *OH scavenger, enhanced the role of reduction pathway induced by aqueous electrons (e(aq)(-)). The shorter-chain perfluorocarboxylic acids (PFCAs), formed in a stepwise manner from longer-chain PFCAs, were identified as products by HPLC/MS.

Comparative photoactivity and antibacterial properties of C60 fullerenes and titanium dioxide nanoparticles

The production of reactive oxygen species (ROS) by aqueous suspensions of fullerenes and nano-TiO2 (Degussa P25) was measured both in ultrapure water and in minimal Davis (MD) microbial growth medium. Fullerol (hydroxylated C60) produced singlet oxygen (1O2) in ultrapure water and both 1O2 and superoxide (O2-*) in MD medium, but no hydroxyl radicals (OH*) were detected in either case. PVP/C60 (C60 encapsulated with poly(N-vinylpyrrolidone)) was more efficient than fullerol in generating singlet oxygen and superoxide. However, two other aggregates of C60, namely THF/nC60 (prepared with tetrahydofuran as transitional solvent) and aqu/nC60 (prepared by vigorous stirring of C60 powder in water), were not photoactive. Nano-TiO2 (also present as aggregates) primarily produced hydroxyl radicals in pure water and superoxide in MD medium. Bacterial (Escherichia coli) toxicity tests suggest that, unlike nano-TiO2 which was exclusively phototoxic, the antibacterial activity of fullerene suspensions was linked to ROS production. Nano-TiO2 may be more efficient for water treatment involving UV or solar energy, to enhance contaminant oxidation and perhaps for disinfection. However, fullerol and PVP/ C60 may be useful as water treatment agents targeting specific pollutants or microorganisms that are more sensitive to either superoxide or singlet oxygen.

Photo-assisted removal of fuel oil hydrocarbons from wood and concrete

A novel photo-treatment to decontaminate building structural elements polluted with fuel oil hydrocarbons as a result of spillage and/or a catastrophic flood was examined. A proof-of-concept study evaluating the photocatalytic removal of hydrocarbons (n-hexadecane and fuel oil #2) from contaminated wood (southern yellow pine) and concrete was conducted using scintillation counting (with (14)C-labeled n-hexadecane) and gas chromatography. Contaminated samples were irradiated by UV or fluorescent light in the absence or presence of a photocatalyst, TiO(2). As a result of the treatment, under various scenarios, up to 80-98% of the originally applied n-hexadecane was removed, within a wide range of contaminant concentrations (4-250 mg/g wood). The essential treatment time increased from 1-7 days for low concentrations to several weeks for high concentrations. Mass balance experiments showed that the only product formed from (14)C-labeled n-hexadecane in detectable amounts was (14)CO(2). For low amounts of applied hydrocarbon (4-20 mg/g wood), the overall process rate was limited by the contaminant transport/mobility whereas for high n-hexadecane concentrations (150-250 mg/g, corresponding to 50-80% filling of wood pores), the key factor was the photochemical reaction. Photodegradation experiments conducted with standard heating fuel oil #2 (a representative real-world contaminant) resulted in a significant (up to 80%) photochemical removal of mid-size hydrocarbons (C(13)-C(17)) in 3 weeks whereas heavier hydrocarbons (> C(17)) were not affected; light hydrocarbons (< C(12)) were removed by evaporation. These results point toward a promising technique to reclaim wooden and concrete structures contaminated with semi-volatile chemicals.

Photocatalytic decomposition of CCl4 on Zr-MCM-41

Photocatalytic decomposition of CCl4 (80 mg L(-1) in H2O) effected by Zr-MCM-41 (Zr incorporated in the amorphous wall of MCM-41) has been studied in the present work. Experimentally, photocatalytic decomposition of CCl4 on Zr-MCM-41 was enhanced by about 1.96 times over that on ZrO2. Photocatalytic decomposition of CCl4 may proceed via a two-electron transfer process that yields mainly CHCl3, Cl- and H2. Since little C2Cl2, C2Cl6 or CH2Cl2 was found, it is unlikely that CHCl3 involved in the secondary photocatalytic degradation process. In addition, photocatalytic splitting of H2O on Zr-MCM-41 was also enhanced. The yield of H2 was 6.5 mmol(gZrO2)(-1). About 68% of this hydrogen (6.5 mmol(gZrO2)(-1)) was consumed in the photocatalytic decomposition of CCl4.

Substrate-specific photocatalytic activities of TiO2 and multiactivity test for water treatment application

The multidimensional aspects of the photocatalytic activity were investigated in a systematic way. The photocatalytic activities of eight commercial TiO2 samples were quantified by employing 19 test substrates (phenols, organic acids, amines, chlorohydrocarbons, dyes, inorganic ions, etc.) in terms of their degradation or conversion rates in water. The measured activities exhibited a complex behavior that depends on the test substrate. The photocatalytic activities are roughly correlated only among structurally related compounds. The photocatalytic activities can be represented in many different ways, and even the relative activity order among the tested photocatalysts depends on what substrate is used. Each TiO2 (among eight samples) showed the best activity for at least one test-substrate. This highly substrate-specific activity of TiO2 photocatalysts hinders the straightforward comparison of which catalyst is better than others. Even the common belief that anatase is more photoactive than rutile cannot be fully supported on the basis of the present data set. Although there seems to be no simple correlation between the activity and the common physicochemical parameters of photocatalysts, the substrate-specific activity was analyzed and discussed in terms of various parameters such as surface area, crystallinity, surface charge, and substrate adsorption. Finally, the multivalue photocatalytic activity test in relation with water treatment application was proposed to take the substrate-specificity into account.