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Special Issue "Photon-involving Purification of Water and Air"

A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Photochemistry".

Deadline for manuscript submissions: closed (10 January 2017)

Special Issue Editor

Guest Editor
Dr. Pierre Pichat

Photocatalyse et Environnement, CNRS/ Ecole Centrale de Lyon (STMS), 69134 Ecully CEDEX, France
E-Mail
Interests: heterogeneous photocatalysis, advanced oxidation processes, environmental chemistry, catalysis, photochemistry

Special Issue Information

Dear Colleagues,

In September 2015, the Editorial Office of Molecules launched a section on Photochemistry. To help develop this new section, a feature paper issue entitled “Photon-involving Purification of Water and Air” is proposed.

This feature paper issue will contain articles on the various photon-driven processes that can be used for the disinfection and decontamination of water and air. Photons of sufficiently high energy are known to inactivate microorganisms and to produce active species—principally hydroxyl radicals—by either directly cleaving H2O, H2O2, O3 and chemical complexes or exciting semiconductors. For each type of these processes, this feature paper issue is expected to provide updated information on the fundamentals, the optimal application conditions, the limitations, the devices, the modeling, the economic aspects, the use of solar irradiation, and foreseeable developments. Gathering these processes and these topics in a unique feature paper issue should give readers a broad survey of the assets and drawbacks of the use of photons to clean water and air. For the authors, it will be a good opportunity to make their results, analyses and surveys more visible than when they are dispersed in many journals.

This feature paper issue is meant to contain contributions about all the technologies and aspects broadly mentioned above, and also indicated in the list of keywords below. Review articles by experts in these diverse fields will particularly be welcome.

Dr. Pierre Pichat
Guest Editor

Submission

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. Papers will be published continuously (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are refereed through a peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Molecules is an international peer-reviewed Open Access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript.

Keywords

  • UV processes
  • VUV processes
  • UV/H2O2 processes
  • UV/O3 processes
  • UV/H2O2/O3 processes
  • photo-Fenton processes
  • photocatalytic processes over semiconductors
  • sun-driven processes

Published Papers (18 papers)

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Research

Jump to: Review

Open AccessArticle Transparent Nanotubular TiO2 Photoanodes Grown Directly on FTO Substrates
Molecules 2017, 22(5), 775; doi:10.3390/molecules22050775
Received: 17 March 2017 / Revised: 27 April 2017 / Accepted: 4 May 2017 / Published: 10 May 2017
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Abstract
This work describes the preparation of transparent TiO2 nanotube (TNT) arrays on fluorine-doped tin oxide (FTO) substrates. An optimized electrolyte composition (0.2 mol dm−3 NH4F and 4 mol dm−3 H2O in ethylene glycol) was used for
[...] Read more.
This work describes the preparation of transparent TiO2 nanotube (TNT) arrays on fluorine-doped tin oxide (FTO) substrates. An optimized electrolyte composition (0.2 mol dm−3 NH4F and 4 mol dm−3 H2O in ethylene glycol) was used for the anodization of Ti films with different thicknesses (from 100 to 1300 nm) sputtered on the FTO glass substrates. For Ti thicknesses 600 nm and higher, anodization resulted in the formation of TNT arrays with an outer nanotube diameter around 180 nm and a wall thickness around 45 nm, while for anodized Ti thicknesses of 100 nm, the produced nanotubes were not well defined. The transmittance in the visible region (λ = 500 nm) varied from 90% for the thinnest TNT array to 65% for the thickest TNT array. For the fabrication of transparent TNT arrays by anodization, the optimal Ti thickness on FTO was around 1000 nm. Such fabricated TNT arrays with a length of 2500 nm exhibit stable photocurrent densities in aqueous electrolytes (~300 µA cm−2 at potential 0.5 V vs. Ag/AgCl). The stability of the photocurrent response and a sufficient transparency (≥65%) enables the use of transparent TNT arrays in photoelectrochemical applications when the illumination from the support/semiconductor interface is a necessary condition and the transmitted light can be used for another purpose (photocathode or photochemical reaction in the electrolyte). Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Open AccessArticle Electrochemical Enhancement of Photocatalytic Disinfection on Aligned TiO2 and Nitrogen Doped TiO2 Nanotubes
Molecules 2017, 22(5), 704; doi:10.3390/molecules22050704
Received: 17 February 2017 / Revised: 24 April 2017 / Accepted: 26 April 2017 / Published: 28 April 2017
Cited by 2 | PDF Full-text (1841 KB) | HTML Full-text | XML Full-text
Abstract
TiO2 photocatalysis is considered as an alternative to conventional disinfection processes for the inactivation of waterborne microorganisms. The efficiency of photocatalysis is limited by charge carrier recombination rates. When the photocatalyst is immobilized on an electrically conducting support, one may assist charge
[...] Read more.
TiO2 photocatalysis is considered as an alternative to conventional disinfection processes for the inactivation of waterborne microorganisms. The efficiency of photocatalysis is limited by charge carrier recombination rates. When the photocatalyst is immobilized on an electrically conducting support, one may assist charge separation by the application of an external electrical bias. The aim of this work was to study electrochemically assisted photocatalysis with nitrogen doped titania photoanodes under visible and UV-visible irradiation for the inactivation of Escherichia coli. Aligned TiO2 nanotubes were synthesized (TiO2-NT) by anodizing Ti foil. Nanoparticulate titania films were made on Ti foil by electrophoretic coating (P25 TiO2). N-doped titania nanotubes and N,F co-doped titania films were also prepared with the aim of extending the active spectrum into the visible. Electrochemically assisted photocatalysis gave higher disinfection efficiency in comparison to photocatalysis (electrode at open circuit) for all materials tested. It is proposed that electrostatic attraction of negatively charged bacteria to the positively biased photoanodes leads to the enhancement observed. The N-doped TiO2 nanotube electrode gave the most efficient electrochemically assisted photocatalytic inactivation of bacteria under UV-Vis irradiation but no inactivation of bacteria was observed under visible only irradiation. The visible light photocurrent was only a fraction (2%) of the UV response. Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Open AccessFeature PaperArticle Photocatalytic and Adsorption Performances of Faceted Cuprous Oxide (Cu2O) Particles for the Removal of Methyl Orange (MO) from Aqueous Media
Molecules 2017, 22(4), 677; doi:10.3390/molecules22040677
Received: 25 February 2017 / Revised: 19 April 2017 / Accepted: 19 April 2017 / Published: 23 April 2017
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Abstract
Particles of sub-micron size possess significant capacity to adsorb organic molecules from aqueous media. Semiconductor photocatalysts in particle form could potentially be utilized for dye removal through either physical adsorption or photo-induced chemical process. The photocatalytic and adsorption capabilities of Cu2O
[...] Read more.
Particles of sub-micron size possess significant capacity to adsorb organic molecules from aqueous media. Semiconductor photocatalysts in particle form could potentially be utilized for dye removal through either physical adsorption or photo-induced chemical process. The photocatalytic and adsorption capabilities of Cu2O particles with various exposed crystal facets have been studied through separate adsorption capacity test and photocatalytic degradation test. These crystals display unique cubic, octahedral, rhombic dodecahedral, and truncated polyhedral shapes due to specifically exposed crystal facet(s). For comparison, Cu2O particles with no clear exposed facets were also prepared. The current work confirms that the surface charge critically affects the adsorption performance of the synthesized Cu2O particles. The octahedral shaped Cu2O particles, with exposed {111} facets, possess the best adsorption capability of methyl orange (MO) dye due to the strongest positive surface charge among the different types of particles. In addition, we also found that the adsorption of MO follows the Langmuir monolayer mechanism. The octahedral particles also performed the best in photocatalytic dye degradation of MO under visible light irradiation because of the assistance from dye absorption. On top of the photocatalytic study, the stability of these Cu2O particles during the photocatalytic processes was also investigated. Cu(OH)2 and CuO are the likely corrosion products found on the particle surface after the photocorrosion in MO solution. By adding hole scavengers in the solution, the photocorrosion of Cu2O was greatly reduced. This observation confirms that the photocatalytically generated holes were responsible for the photocorrosion of Cu2O. Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Open AccessArticle Degradation of Methyl 2-Aminobenzoate (Methyl Anthranilate) by H2O2/UV: Effect of Inorganic Anions and Derived Radicals
Molecules 2017, 22(4), 619; doi:10.3390/molecules22040619
Received: 24 February 2017 / Revised: 3 April 2017 / Accepted: 6 April 2017 / Published: 12 April 2017
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Abstract
This study shows that methyl 2-aminobenzoate (also known as methyl anthranilate, hereafter MA) undergoes direct photolysis under UVC and UVB irradiation and that its photodegradation is further accelerated in the presence of H2O2. Hydrogen peroxide acts as a source
[...] Read more.
This study shows that methyl 2-aminobenzoate (also known as methyl anthranilate, hereafter MA) undergoes direct photolysis under UVC and UVB irradiation and that its photodegradation is further accelerated in the presence of H2O2. Hydrogen peroxide acts as a source of hydroxyl radicals (·OH) under photochemical conditions and yields MA hydroxyderivatives. The trend of MA photodegradation rate vs. H2O2 concentration reaches a plateau because of the combined effects of H2O2 absorption saturation and ·OH scavenging by H2O2. The addition of chloride ions causes scavenging of ·OH, yielding Cl2· as the most likely reactive species, and it increases the MA photodegradation rate at high H2O2 concentration values. The reaction between Cl2· and MA, which has second-order rate constant k C l 2 + M A = (4.0 ± 0.3) × 108 M−1·s−1 (determined by laser flash photolysis), appears to be more selective than the ·OH process in the presence of H2O2, because Cl2· undergoes more limited scavenging by H2O2 compared to ·OH. While the addition of carbonate causes ·OH scavenging to produce CO3· ( k C O 3 + M A = (3.1 ± 0.2) × 108 M−1·s−1), carbonate considerably inhibits the photodegradation of MA. A possible explanation is that the elevated pH values of the carbonate solutions make H2O2 to partially occur as HO2, which reacts very quickly with either ·OH or CO3· to produce O2·. The superoxide anion could reduce partially oxidised MA back to the initial substrate, with consequent inhibition of MA photodegradation. Fast MA photodegradation is also observed in the presence of persulphate/UV, which yields SO4· that reacts effectively with MA ( k S O 4 + M A = (5.6 ± 0.4) × 109 M−1·s−1). Irradiated H2O2 is effective in photodegrading MA, but the resulting MA hydroxyderivatives are predicted to be about as toxic as the parent compound for aquatic organisms (most notably, fish and crustaceans). Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Open AccessArticle Growth, Structure, and Photocatalytic Properties of Hierarchical V2O5–TiO2 Nanotube Arrays Obtained from the One-step Anodic Oxidation of Ti–V Alloys
Molecules 2017, 22(4), 580; doi:10.3390/molecules22040580
Received: 29 January 2017 / Revised: 30 March 2017 / Accepted: 1 April 2017 / Published: 5 April 2017
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Abstract
V2O5-TiO2 mixed oxide nanotube (NT) layers were successfully prepared via the one-step anodization of Ti-V alloys. The obtained samples were characterized by scanning electron microscopy (SEM), UV-Vis absorption, photoluminescence spectroscopy, energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (DRX), and
[...] Read more.
V2O5-TiO2 mixed oxide nanotube (NT) layers were successfully prepared via the one-step anodization of Ti-V alloys. The obtained samples were characterized by scanning electron microscopy (SEM), UV-Vis absorption, photoluminescence spectroscopy, energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (DRX), and micro-Raman spectroscopy. The effect of the applied voltage (30–50 V), vanadium content (5–15 wt %) in the alloy, and water content (2–10 vol %) in an ethylene glycol-based electrolyte was studied systematically to determine their influence on the morphology, and for the first-time, on the photocatalytic properties of these nanomaterials. The morphology of the samples varied from sponge-like to highly-organized nanotubular structures. The vanadium content in the alloy was found to have the highest influence on the morphology and the sample with the lowest vanadium content (5 wt %) exhibited the best auto-alignment and self-organization (length = 1 μm, diameter = 86 nm and wall thickness = 11 nm). Additionally, a probable growth mechanism of V2O5-TiO2 nanotubes (NTs) over the Ti-V alloys was presented. Toluene, in the gas phase, was effectively removed through photodegradation under visible light (LEDs, λmax = 465 nm) in the presence of the modified TiO2 nanostructures. The highest degradation value was 35% after 60 min of irradiation. V2O5 species were ascribed as the main structures responsible for the generation of photoactive e and h+ under Vis light and a possible excitation mechanism was proposed. Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Open AccessArticle Sulfur-Doped Carbon Nitride Polymers for Photocatalytic Degradation of Organic Pollutant and Reduction of Cr(VI)
Molecules 2017, 22(4), 572; doi:10.3390/molecules22040572
Received: 18 February 2017 / Revised: 29 March 2017 / Accepted: 29 March 2017 / Published: 1 April 2017
Cited by 4 | PDF Full-text (3683 KB) | HTML Full-text | XML Full-text
Abstract
As a promising conjugated polymer, binary carbon nitride has attracted extensive attention as a metal-free and visible-light-responsive photocatalyst in the area of photon-involving purification of water and air. Herein, we report sulfur-doped polymeric carbon nitride microrods that are synthesized through thermal polymerization based
[...] Read more.
As a promising conjugated polymer, binary carbon nitride has attracted extensive attention as a metal-free and visible-light-responsive photocatalyst in the area of photon-involving purification of water and air. Herein, we report sulfur-doped polymeric carbon nitride microrods that are synthesized through thermal polymerization based on trithiocyanuric acid and melamine (TM) supramolecular aggregates. By tuning the polymerization temperature, a series of sulfur-doped carbon nitride microrods are prepared. The degradation of Rhodamine B (RhB) and the reduction of hexavalent chromium Cr(VI) are selected as probe reactions to evaluate the photocatalytic activities. Results show that increasing pyrolysis temperature leads to a large specific surface area, strong visible-light absorption, and accelerated electron-hole separation. Compared to bulk carbon nitride, the highly porous sulfur-doped carbon nitride microrods fabricated at 650 °C exhibit remarkably higher photocatalytic activity for degradation of RhB and reduction of Cr(VI). This work highlights the importance of self-assembly approach and temperature-control strategy in the synthesis of photoactive materials for environmental remediation. Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Open AccessArticle Self-Organized TiO2–MnO2 Nanotube Arrays for Efficient Photocatalytic Degradation of Toluene
Molecules 2017, 22(4), 564; doi:10.3390/molecules22040564
Received: 3 February 2017 / Revised: 27 March 2017 / Accepted: 28 March 2017 / Published: 31 March 2017
Cited by 2 | PDF Full-text (7676 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Vertically oriented, self-organized TiO2–MnO2 nanotube arrays were successfully obtained by one-step anodic oxidation of Ti–Mn alloys in an ethylene glycol-based electrolyte. The as-prepared samples were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), UV-Vis absorption, photoluminescence spectroscopy, X-ray
[...] Read more.
Vertically oriented, self-organized TiO2–MnO2 nanotube arrays were successfully obtained by one-step anodic oxidation of Ti–Mn alloys in an ethylene glycol-based electrolyte. The as-prepared samples were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), UV-Vis absorption, photoluminescence spectroscopy, X-ray diffraction (XRD), and micro-Raman spectroscopy. The effect of the applied potential (30–50 V), manganese content in the alloy (5–15 wt. %) and water content in the electrolyte (2–10 vol. %) on the morphology and photocatalytic properties was investigated for the first time. The photoactivity was assessed in the toluene removal reaction under visible light, using low-powered LEDs as an irradiation source (λmax = 465 nm). Morphology analysis showed that samples consisted of auto-aligned nanotubes over the surface of the alloy, their dimensions were: diameter = 76–118 nm, length = 1.0–3.4 μm and wall thickness = 8–11 nm. It was found that the increase in the applied potential led to increase the dimensions while the increase in the content of manganese in the alloy brought to shorter nanotubes. Notably, all samples were photoactive under the influence of visible light and the highest degradation achieved after 60 min of irradiation was 43%. The excitation mechanism of TiO2–MnO2 NTs under visible light was presented, pointing out the importance of MnO2 species for the generation of e and h+. Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Open AccessFeature PaperArticle A Model Study of the Photochemical Fate of As(III) in Paddy-Water
Molecules 2017, 22(3), 445; doi:10.3390/molecules22030445
Received: 27 January 2017 / Revised: 3 March 2017 / Accepted: 6 March 2017 / Published: 11 March 2017
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Abstract
The APEX (Aqueous Photochemistry of Environmentally-occurring Xenobiotics) software previously developed by one of us was used to model the photochemistry of As(III) in paddy-field water, allowing a comparison with biotic processes. The model included key paddy-water variables, such as the shielding effect of
[...] Read more.
The APEX (Aqueous Photochemistry of Environmentally-occurring Xenobiotics) software previously developed by one of us was used to model the photochemistry of As(III) in paddy-field water, allowing a comparison with biotic processes. The model included key paddy-water variables, such as the shielding effect of the rice canopy on incident sunlight and its monthly variations, water pH, and the photochemical parameters of the chromophoric dissolved organic matter (CDOM) occurring in paddy fields. The half-life times (t1/2) of As(III) photooxidation to As(V) would be ~20–30 days in May. In contrast, the photochemical oxidation of As(III) would be much slower in June and July due to rice-canopy shading of radiation because of plant growth, despite higher sunlight irradiance. At pH < 8 the photooxidation of As(III) would mainly be accounted for by reaction with transient species produced by irradiated CDOM (here represented by the excited triplet states 3CDOM*, neglecting the possibly more important reactions with poorly known species such as the phenoxy radicals) and, to a lesser extent, with the hydroxyl radicals (HO). However, the carbonate radicals (CO3•−) could be key photooxidants at pH > 8.5 provided that the paddy-water 3CDOM* is sufficiently reactive toward the oxidation of CO32−. In particular, if paddy-water 3CDOM* oxidizes the carbonate anion with a second-order reaction rate constant near (or higher than) 106 M−1·s−1, the photooxidation of As(III) could be quite fast at pH > 8.5. Such pH conditions can be produced by elevated photosynthetic activity that consumes dissolved CO2. Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Open AccessArticle Influence of Se/N Codoping on the Structural, Optical, Electronic and Photocatalytic Properties of TiO2
Molecules 2017, 22(3), 414; doi:10.3390/molecules22030414
Received: 9 January 2017 / Revised: 13 February 2017 / Accepted: 27 February 2017 / Published: 7 March 2017
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Abstract
Se4+ and N3− ions were used as codopants to enhance the photocatalytic activity of TiO2 under sunlight irradiation. The Se/N codoped photocatalysts were prepared through a simple wet-impregnation method followed by heat treatment using SeCl4 and urea as the
[...] Read more.
Se4+ and N3− ions were used as codopants to enhance the photocatalytic activity of TiO2 under sunlight irradiation. The Se/N codoped photocatalysts were prepared through a simple wet-impregnation method followed by heat treatment using SeCl4 and urea as the dopant sources. The prepared photocatalysts were well characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-diffuse reflectance spectroscopy (UV-DRS), scanning electron microscopy (SEM) and Raman spectroscopy. The codoped samples showed photoabsorption in the visible light range from 430 nm extending up to 580 nm. The photocatalytic activity of the Se/N codoped photocatalysts was evaluated by degradation of 4-nitrophenol (4-NP). The degradation of 4-NP was highly increased for the Se/N codoped samples compared to the undoped and single doped samples under both UV-A and sunlight irradiation. Aiming to determine the electronic structure and dopant locations, quantum chemical modeling of the undoped and Se/N codoped anatase clusters was performed using Density Functional Theory (DFT) calculations with the hybrid functional (B3LYP) and double-zeta (LanL2DZ) basis set. The results revealed that Se/N codoping of TiO2 reduces the band gap due to mixing of N2p with O2p orbitals in the valence band and also introduces additional electronic states originating from Se3p orbitals in the band gap. Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Open AccessArticle Determination of the Clean Air Delivery Rate (CADR) of Photocatalytic Oxidation (PCO) Purifiers for Indoor Air Pollutants Using a Closed-Loop Reactor. Part I: Theoretical Considerations
Molecules 2017, 22(3), 407; doi:10.3390/molecules22030407
Received: 15 December 2016 / Revised: 15 February 2017 / Accepted: 2 March 2017 / Published: 6 March 2017
Cited by 1 | PDF Full-text (1179 KB) | HTML Full-text | XML Full-text
Abstract
This study demonstrated that a laboratory-scale recirculation closed-loop reactor can be an efficient technique for the determination of the Clean Air Delivery Rate (CADR) of PhotoCatalytic Oxidation (PCO) air purification devices. The recirculation closed-loop reactor was modeled by associating equations related to two
[...] Read more.
This study demonstrated that a laboratory-scale recirculation closed-loop reactor can be an efficient technique for the determination of the Clean Air Delivery Rate (CADR) of PhotoCatalytic Oxidation (PCO) air purification devices. The recirculation closed-loop reactor was modeled by associating equations related to two ideal reactors: one is a perfectly mixed reservoir and the other is a plug flow system corresponding to the PCO device itself. Based on the assumption that the ratio between the residence time in the PCO device and the residence time in the reservoir τPR tends to 0, the model highlights that a lab closed-loop reactor can be a suitable technique for the determination of the efficiency of PCO devices. Moreover, if the single-pass removal efficiency is lower than 5% of the treated flow rate, the decrease in the pollutant concentration over time can be characterized by a first-order decay model in which the time constant is proportional to the CADR. The limits of the model are examined and reported in terms of operating conditions (experiment duration, ratio of residence times, and flow rate ranges). Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Open AccessArticle Determination of the Clean Air Delivery Rate (CADR) of Photocatalytic Oxidation (PCO) Purifiers for Indoor Air Pollutants Using a Closed-Loop Reactor. Part II: Experimental Results
Molecules 2017, 22(3), 408; doi:10.3390/molecules22030408
Received: 16 December 2016 / Revised: 15 February 2017 / Accepted: 2 March 2017 / Published: 6 March 2017
Cited by 1 | PDF Full-text (3455 KB) | HTML Full-text | XML Full-text
Abstract
The performances of a laboratory PhotoCatalytic Oxidation (PCO) device were determined using a recirculation closed-loop pilot reactor. The closed-loop system was modeled by associating equations related to two ideal reactors: a perfectly mixed reservoir with a volume of VR = 0.42 m
[...] Read more.
The performances of a laboratory PhotoCatalytic Oxidation (PCO) device were determined using a recirculation closed-loop pilot reactor. The closed-loop system was modeled by associating equations related to two ideal reactors: a perfectly mixed reservoir with a volume of VR = 0.42 m3 and a plug flow system corresponding to the PCO device with a volume of VP = 5.6 × 10−3 m3. The PCO device was composed of a pleated photocatalytic filter (1100 cm2) and two 18-W UVA fluorescent tubes. The Clean Air Delivery Rate (CADR) of the apparatus was measured under different operating conditions. The influence of three operating parameters was investigated: (i) light irradiance I from 0.10 to 2.0 mW·cm−2; (ii) air velocity v from 0.2 to 1.9 m·s−1; and (iii) initial toluene concentration C0 (200, 600, 1000 and 4700 ppbv). The results showed that the conditions needed to apply a first-order decay model to the experimental data (described in Part I) were fulfilled. The CADR values, ranging from 0.35 to 3.95 m3·h−1, were mainly dependent on the light irradiance intensity. A square root influence of the light irradiance was observed. Although the CADR of the PCO device inserted in the closed-loop reactor did not theoretically depend on the flow rate (see Part I), the experimental results did not enable the confirmation of this prediction. The initial concentration was also a parameter influencing the CADR, as well as the toluene degradation rate. The maximum degradation rate rmax ranged from 342 to 4894 ppbv/h. Finally, this study evidenced that a recirculation closed-loop pilot could be used to develop a reliable standard test method to assess the effectiveness of PCO devices. Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Review

Jump to: Research

Open AccessReview Participation of the Halogens in Photochemical Reactions in Natural and Treated Waters
Molecules 2017, 22(10), 1684; doi:10.3390/molecules22101684
Received: 18 September 2017 / Revised: 1 October 2017 / Accepted: 4 October 2017 / Published: 13 October 2017
PDF Full-text (2905 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Halide ions are ubiquitous in natural waters and wastewaters. Halogens play an important and complex role in environmental photochemical processes and in reactions taking place during photochemical water treatment. While inert to solar wavelengths, halides can be converted into radical and non-radical reactive
[...] Read more.
Halide ions are ubiquitous in natural waters and wastewaters. Halogens play an important and complex role in environmental photochemical processes and in reactions taking place during photochemical water treatment. While inert to solar wavelengths, halides can be converted into radical and non-radical reactive halogen species (RHS) by sensitized photolysis and by reactions with secondary reactive oxygen species (ROS) produced through sunlight-initiated reactions in water and atmospheric aerosols, such as hydroxyl radical, ozone, and nitrate radical. In photochemical advanced oxidation processes for water treatment, RHS can be generated by UV photolysis and by reactions of halides with hydroxyl radicals, sulfate radicals, ozone, and other ROS. RHS are reactive toward organic compounds, and some reactions lead to incorporation of halogen into byproducts. Recent studies indicate that halides, or the RHS derived from them, affect the concentrations of photogenerated reactive oxygen species (ROS) and other reactive species; influence the photobleaching of dissolved natural organic matter (DOM); alter the rates and products of pollutant transformations; lead to covalent incorporation of halogen into small natural molecules, DOM, and pollutants; and give rise to certain halogen oxides of concern as water contaminants. The complex and colorful chemistry of halogen in waters will be summarized in detail and the implications of this chemistry for global biogeochemical cycling of halogen, contaminant fate in natural waters, and water purification technologies will be discussed. Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Open AccessFeature PaperReview Solar or UVA-Visible Photocatalytic Ozonation of Water Contaminants
Molecules 2017, 22(7), 1177; doi:10.3390/molecules22071177
Received: 27 April 2017 / Accepted: 4 July 2017 / Published: 14 July 2017
Cited by 1 | PDF Full-text (969 KB) | HTML Full-text | XML Full-text
Abstract
An incipient advanced oxidation process, solar photocatalytic ozonation (SPO), is reviewed in this paper with the aim of clarifying the importance of this process as a more sustainable water technology to remove priority or emerging contaminants from water. The synergism between ozonation and
[...] Read more.
An incipient advanced oxidation process, solar photocatalytic ozonation (SPO), is reviewed in this paper with the aim of clarifying the importance of this process as a more sustainable water technology to remove priority or emerging contaminants from water. The synergism between ozonation and photocatalytic oxidation is well known to increase the oxidation rate of water contaminants, but this has mainly been studied in photocatalytic ozonation systems with lamps of different radiation wavelength, especially of ultraviolet nature (UVC, UVB, UVA). Nowadays, process sustainability is critical in environmental technologies including water treatment and reuse; the application of SPO systems falls into this category, and contributes to saving energy and water. In this review, we summarized works published on photocatalytic ozonation where the radiation source is the Sun or simulated solar light, specifically, lamps emitting radiation to cover the UVA and visible light spectra. The main aspects of the review include photoreactors used and radiation sources applied, synthesis and characterization of catalysts applied, influence of main process variables (ozone, catalyst, and pollutant concentrations, light intensity), type of water, biodegradability and ecotoxicity, mechanism and kinetics, and finally catalyst activity and stability. Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Open AccessReview Self-Sterilizing Sputtered Films for Applications in Hospital Facilities
Molecules 2017, 22(7), 1074; doi:10.3390/molecules22071074
Received: 4 May 2017 / Accepted: 23 May 2017 / Published: 28 June 2017
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Abstract
This review addresses the preparation of antibacterial 2D textile and thin polymer films and 3D surfaces like catheters for applications in hospital and health care facilities. The sputtering of films applying different levels of energy led to the deposition of metal/oxide/composite/films showing differentiated
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This review addresses the preparation of antibacterial 2D textile and thin polymer films and 3D surfaces like catheters for applications in hospital and health care facilities. The sputtering of films applying different levels of energy led to the deposition of metal/oxide/composite/films showing differentiated antibacterial kinetics and surface microstructure. The optimization of the film composition in regards to the antibacterial active component was carried out in each case to attain the fastest antibacterial kinetics, since this is essential when designing films avoiding biofilm formation (under light and in the dark). The antimicrobial performance of these sputtered films on Staphylococcus aureus (MRSA) and Escherichia coli (E. coli) were tested. A protecting effect of TiO2 was found for the release of Cu by the TiO2-Cu films compared to films sputtered by Cu only. The Cu-released during bacterial inactivation by TiO2-Cu was observed to be much lower compared to the films sputtered only by Cu. The FeOx-TiO2-PE films induced E. coli inactivation under solar or under visible light with a similar inactivation kinetics, confirming the predominant role of FeOx in these composite films. By up-to-date surface science techniques were used to characterize the surface properties of the sputtered films. A mechanism of bacteria inactivation is suggested for each particular film consistent with the experimental results found and compared with the literature. Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Open AccessFeature PaperReview Light-Assisted Advanced Oxidation Processes for the Elimination of Chemical and Microbiological Pollution of Wastewaters in Developed and Developing Countries
Molecules 2017, 22(7), 1070; doi:10.3390/molecules22071070
Received: 4 May 2017 / Revised: 22 June 2017 / Accepted: 23 June 2017 / Published: 26 June 2017
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Abstract
In this work, the issue of hospital and urban wastewater treatment is studied in two different contexts, in Switzerland and in developing countries (Ivory Coast and Colombia). For this purpose, the treatment of municipal wastewater effluents is studied, simulating the developed countries’ context,
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In this work, the issue of hospital and urban wastewater treatment is studied in two different contexts, in Switzerland and in developing countries (Ivory Coast and Colombia). For this purpose, the treatment of municipal wastewater effluents is studied, simulating the developed countries’ context, while cheap and sustainable solutions are proposed for the developing countries, to form a barrier between effluents and receiving water bodies. In order to propose proper methods for each case, the characteristics of the matrices and the targets are described here in detail. In both contexts, the use of Advanced Oxidation Processes (AOPs) is implemented, focusing on UV-based and solar-supported ones, in the respective target areas. A list of emerging contaminants and bacteria are firstly studied to provide operational and engineering details on their removal by AOPs. Fundamental mechanistic insights are also provided on the degradation of the effluent wastewater organic matter. The use of viruses and yeasts as potential model pathogens is also accounted for, treated by the photo-Fenton process. In addition, two pharmaceutically active compound (PhAC) models of hospital and/or industrial origin are studied in wastewater and urine, treated by all accounted AOPs, as a proposed method to effectively control concentrated point-source pollution from hospital wastewaters. Their elimination was modeled and the degradation pathway was elucidated by the use of state-of-the-art analytical techniques. In conclusion, the use of light-supported AOPs was proven to be effective in degrading the respective target and further insights were provided by each application, which could facilitate their divulgation and potential application in the field. Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Open AccessFeature PaperReview Integral Design Methodology of Photocatalytic Reactors for Air Pollution Remediation
Molecules 2017, 22(6), 945; doi:10.3390/molecules22060945
Received: 22 March 2017 / Accepted: 31 May 2017 / Published: 7 June 2017
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Abstract
An integral reactor design methodology was developed to address the optimal design of photocatalytic wall reactors to be used in air pollution control. For a target pollutant to be eliminated from an air stream, the proposed methodology is initiated with a mechanistic derived
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An integral reactor design methodology was developed to address the optimal design of photocatalytic wall reactors to be used in air pollution control. For a target pollutant to be eliminated from an air stream, the proposed methodology is initiated with a mechanistic derived reaction rate. The determination of intrinsic kinetic parameters is associated with the use of a simple geometry laboratory scale reactor, operation under kinetic control and a uniform incident radiation flux, which allows computing the local superficial rate of photon absorption. Thus, a simple model can describe the mass balance and a solution may be obtained. The kinetic parameters may be estimated by the combination of the mathematical model and the experimental results. The validated intrinsic kinetics obtained may be directly used in the scaling-up of any reactor configuration and size. The bench scale reactor may require the use of complex computational software to obtain the fields of velocity, radiation absorption and species concentration. The complete methodology was successfully applied to the elimination of airborne formaldehyde. The kinetic parameters were determined in a flat plate reactor, whilst a bench scale corrugated wall reactor was used to illustrate the scaling-up methodology. In addition, an optimal folding angle of the corrugated reactor was found using computational fluid dynamics tools. Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Open AccessReview Photoactive Hybrid Catalysts Based on Natural and Synthetic Polymers: A Comparative Overview
Molecules 2017, 22(5), 790; doi:10.3390/molecules22050790
Received: 18 March 2017 / Revised: 5 May 2017 / Accepted: 7 May 2017 / Published: 12 May 2017
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Abstract
In the present review, we would like to draw the reader’s attention to the polymer-based hybrid materials used in photocatalytic processes for efficient degradation of organic pollutants in water. These inorganic–organic materials exhibit unique physicochemical properties due to the synergistic effect originating from
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In the present review, we would like to draw the reader’s attention to the polymer-based hybrid materials used in photocatalytic processes for efficient degradation of organic pollutants in water. These inorganic–organic materials exhibit unique physicochemical properties due to the synergistic effect originating from the combination of individual elements, i.e., photosensitive metal oxides and polymeric supports. The possibility of merging the structural elements of hybrid materials allows for improving photocatalytic performance through (1) an increase in the light-harvesting ability; (2) a reduction in charge carrier recombination; and (3) prolongation of the photoelectron lifetime. Additionally, the great majority of polymer materials exhibit a high level of resistance against ultraviolet irradiation and improved corrosion resistance. Taking into account that the chemical and environmental stability of the hybrid catalyst depends, to a great extent, on the functional support, we highlight benefits and drawbacks of natural and synthetic polymer-based photocatalytic materials and pay special attention to the fact that the accessibility of synthetic polymeric materials derived from petroleum may be impeded due to decreasing amounts of crude oil. Thus, it is necessary to look for cheap and easily available raw materials like natural polymers that come from, for instance, lignocellulosic wastes or crustacean residues to meet the demand of the “plastic” market. Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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Open AccessReview The Role of Molecular Modeling in TiO2 Photocatalysis
Molecules 2017, 22(4), 556; doi:10.3390/molecules22040556
Received: 7 January 2017 / Revised: 22 March 2017 / Accepted: 27 March 2017 / Published: 30 March 2017
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Abstract
Molecular Modeling methods play a very important role in TiO2 photocatalysis. Recent advances in TiO2 photocatalysis have produced a number of interesting surface phenomena, reaction products, and various novel visible light active photocatalysts with improved properties. Quantum mechanical calculations appear promising
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Molecular Modeling methods play a very important role in TiO2 photocatalysis. Recent advances in TiO2 photocatalysis have produced a number of interesting surface phenomena, reaction products, and various novel visible light active photocatalysts with improved properties. Quantum mechanical calculations appear promising as a means of describing the mechanisms and the product distributions of the photocatalytic degradation reactions of organic pollutants in both gas and aqueous phases. Since quantum mechanical methods utilize the principles of particle physics, their use may be extended to the design of new photocatalysts. This review introduces molecular modeling methods briefly and emphasizes the use of these methods in TiO2 photocatalysis. The methods used for obtaining information about the degradabilities of the pollutant molecules, predicting reaction mechanisms, and evaluating the roles of the dopants and surface modifiers are explained. Full article
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
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