Homogeneous photocatalysis by transition metal complexes in the environment

Catalytic Photodegradation of Rhodamine B in the Presence of Natural Iron Oxide and Oxalic Acid under Artificial and Sunlight Radiation

Natural iron oxide was used as catalyst for a heterogeneous photo-Fenton-like process and it was characterized by X-ray diffraction, X-ray fluorescence, Raman spectroscopy, scanning electron microscopy and N2adsorption volumetry measurement. Natural iron oxide consists mainly of hematite (76 %). Rhodamine B was mineralized by iron oxide/oxalic acid/UV system, involving creation of dissolved Fe-oxalate and adsorption of Fe-oxalate on the iron oxide surface. The effects of the initial concentration of oxalic acid and pH value, amount of natural iron oxide and concentration of dye, temperature and sunlight irradiation on the kinetics of photodegradation of rhodamine B were investigated. Excellent degradation rate was achieved with 5 mmol.L−1of oxalic acid at a pH around 2–4. During the process, the formation of Fe2+, H2O2and the pH of the solution were strongly dependent on the initial concentration of oxalic acid. Use oft-butanol (2.0 %) confirms that hydroxyl radicals are the entities responsible for the rhodamine B photodegradation. The use of the natural iron oxide as a catalyst in wastewater treatment is very interesting; because it is an abundant mineral and easy to separate from the solution in the end of treatment.

A survey of photogeochemistry

The participation of sunlight in the natural chemistry of the earth is presented as a unique field of study, from historical observations to prospects for future inquiry. A compilation of known reactions shows the extent of light-driven interactions between naturally occurring components of land, air, and water, and provides the backdrop for an outline of the mechanisms of these phenomena. Catalyzed reactions, uncatalyzed reactions, direct processes, and indirect processes all operate in natural photochemical transformations, many of which are analogous to well-known biological reactions. By overlaying photochemistry and surface geochemistry, complementary approaches can be adopted to identify natural photochemical reactions and discern their significance in the environment.

A New Insight of Graphene oxide-Fe(III) Complex Photochemical Behaviors under Visible Light Irradiation

Graphene oxide (GO) contains not only aromatic carbon lattice but also carboxyl groups which enhanced the aqueous solubility of GO. To study the transformation of GO nanosheets in natural environments, GO aqueous dispersion was mixed with Fe3+ ions to form photoactive complex. Under visible light irradiation, Fe(III) of the complex would be reduced to Fe(II) which could subsequently reduce highly toxic Cr(VI) to Cr3+. The electron of the reduction was contributed by the decarboxylation of carboxyl groups on GO and iron was acting as a catalyst during the photoreduction. On the other hand, the consumption of carboxyl groups may convert GO to rGO which are tend to aggregate since the decreased electrostatic repulsion and the increased π-π attraction. The formed Cr3+ may be electrostatically adsorbed by the rGO sheets and simultaneously precipitated with the aggregated rGO sheets, resulting the effective removal of chromium and GO nanosheets from the aqueous environment. This study may shed a light on understanding the environmental transformation of GO and guide the treatment of Cr(VI).

Photochemical decomposition of 1H,1H,2H,2H-perfluorooctane sulfonate (6:2FTS) induced by ferric ions

Perfluorooctane sulfonate (PFOS) had wide applications, such as in the electroplating industry, but its use was restricted in 2009 by the Stockholm Convention, due to its environmental persistence and potential hazards. As the most common PFOS alternative, 1H,1H,2H,2H-perfluorooctane sulfonic acid (6:2FTS) and its salts have been increasingly used. However, little is known about its photochemical decomposition. This paper reports the ferric ion-induced efficient decomposition and defluorination of 6:2FTS under 254nm ultraviolet (UV) irradiation; the underlying mechanisms were also investigated. In the presence of 100μmol/L ferric ion and at pH3.0, the first-order decomposition rate constant of 6:2FTS (10mg/L) was 1.59/hr, which was 6 times higher than for direct UV photolysis. The effects of the ferric ion concentration and the solution pH on the 6:2FTS photodecomposition were investigated and the optimal reaction conditions were determined. In addition to fluoride and sulfate ions, shorter-chain PFCAs (C2-C7) were detected as major intermediates. The addition of hydrogen peroxide or oxalic acid accelerated the decomposition of 6:2FTS under UV irradiation, but not its defluorination, indicating that hydroxyl radicals can directly react with 6:2FTS but not with the shorter-chain PFCAs. Accordingly, a mechanism for 6:2FTS photochemical decomposition in the presence of ferric ion was proposed, which comprises two reaction pathways. First, hydroxyl radicals can directly attack 6:2FTS, leading to CC bond cleavage. Alternatively, 6:2FTS coordinates with ferric ion to form Fe(III)-6:2FTS complexes, which can undergo ligand-to-metal charge transfer under UV irradiation, causing CS bond cleavage.

VUV/UV light inducing accelerated phenol degradation with a low electric input

This study presents the first evidence for the accelerated degradation of phenol by Fenton's reagent in a mini-fluidic VUV/UV photoreaction system (MVPS). A low-pressure mercury lamp used in the MVPS led to a complete degradation of phenol within 4-6 min. The HO˙ and HO₂˙ originating from both Fenton's reagent and VUV photolysis of water were identified with suitable radical scavengers. The effects of initial concentrations of phenol, H₂O₂ and Fe³⁺ as well as solution pH on phenol degradation kinetics were examined. Increasing the initial phenol concentration slowed down the phenol degradation, whereas increasing the initial H₂O₂ or Fe³⁺ concentration accelerated the phenol degradation. The optimal solution pH was 3.7. At both 254 and 185 nm, increasing phenol concentration enhanced its absorption for the incident photons. The reaction mechanism for the degradation of phenol was suggested consistent with the results obtained. This study indicates that the VUV/UV photo-Fenton process has potential applications in the treatment of industrial wastewater containing phenol and related aromatic pollutants.

Ultrafast Photochemistry of Copper(II) Monochlorocomplexes in Methanol and Acetonitrile by Broadband Deep-UV-to-Near-IR Femtosecond Transient Absorption Spectroscopy

Photochemistry of copper(II) monochlorocomplexes in methanol and acetonitrile solutions is studied by UV-pump/broadband deep-UV-to-near-IR probe femtosecond transient absorption spectroscopy. Upon 255 and 266 nm excitation, the complexes in acetonitrile and methanol, respectively, are promoted to the excited ligand-to-metal charge transfer (LMCT) state, which has a short (sub-250 fs) lifetime. From the LMCT state, the complexes decay via internal conversion to lower-lying ligand field (LF) d-d excited states or the vibrationally hot ground electronic state. A minor fraction of the excited complexes relaxes to the LF electronic excited states, which are relatively long-lived with lifetimes >1 ns. Also, in methanol solutions, about 3% of the LMCT-excited copper(II) monochlorocomplexes dissociate forming copper(I) solvatocomplexes and chlorine atoms, which then further react forming long-lived photoproducts. In acetonitrile, about 50% of the LMCT-excited copper(II) monochlorocomplexes dissociate forming radical and ionic products in a ratio of 3:2. Another minor process observed following excitation only in methanol solutions is the re-equilibration between several forms of the copper(II) ground-state complexes present in solutions. This re-equilibration occurs on a time scale from sub-nanoseconds to nanoseconds.

Concentration dependent effect of CuCl2 on the photocatalytic degradation of phenol over anatase, rutile and brookite TiO2

A mechanism for the positive and negative effect of CuCl₂ on the TiO₂-photocatalytic degradation of phenol is proposed.

Recent advances in the synthesis and application of photocatalytic metal–metal oxide core–shell nanoparticles for environmental remediation and their recycling process

Metal–metal oxide core–shell nanoparticles have received enormous research attention owing to their fascinating physicochemical properties and extensive applications. In this review we have discussed the challenges and recent advances in their synthesis and application.

Wastewater remediation using a spiral shaped reactor for photochemical reduction of hexavalent chromium

The application of photochemical reduction of Cr(vi) in the treatment of wastewater from the electroplating industry was studied, and a spiral shaped reactor was designed and constructed (SSR).

Review of the bulk and surface chemistry of iron in atmospherically relevant systems containing humic-like substances

The current state of knowledge and future research directions of the bulk and surface chemistry of iron relevant to atmospheric surfaces are reviewed.

Zeolite encapsulated active metal composites and their photocatalytic studies for rhodamine-B, reactive red-198 and chloro-phenols

Niobium (Nb), tantalum (Ta) and palladium (Pd) were impregnated in the cavities of a zeolite by the ion exchange method.

Evaluation of the efficiency of the photo Fenton disinfection of natural drinking water source during the rainy season in the Sahelian region

The photo-disinfection of water from two different wells (W1, pH: 4.6-5.1 ± 0.02) and (W2 pH: 5.6-5.7 ± 0.02) was carried out during the rainy season at Ouagadougou-Burkina Faso, West Africa. The weather variation during the rainy season significantly affects the photo-disinfection processes (solar disinfection and photo-Fenton). The dilution of the water by rainwater highly affected the chemical composition of the wells' water used in this study; very low iron contents Compared to the ones recorded during the dry season were recorded in all water samples. Both photo-disinfection processes were used to treat 25 L of water in a compound parabolic collector (CPC). None of them have shown the total inactivation of both wild enteric bacteria strains (total coliforms/E. coli and Salmonella spp.) involved in the treatment. However, the total coliforms/E. coli strains were totally inactivated during the exposure under most of the photo-Fenton treatment. Also, the remaining strains, especially those of Salmonella spp. were achieved during the subsequent 24h of dark storage under the action of the Fenton process. Under uniquely solar radiation, total inactivation was recorded only in the total coliforms/E. coli strains. The impact of the available irradiance on the efficiency of the photo-Fenton disinfection of natural water was highlighted during the exposure under high intermittent solar radiation. The impact of the HCO3(-) concentration of both wells' water on the evolution of the pH during the photo-disinfection was recorded. Drastic decrease was noticed after the initial fast increase in presence of low HCO3(-) concentration while a steady state was observed after the increase in presence of higher concentration. The redox activities of the nitrogen components of the water during both photo-disinfection processes have led to increased concentration of nitrite in all the cases and variations were noticed in that of nitrate and ammonia.

Ferric ion mediated photodecomposition of aqueous perfluorooctane sulfonate (PFOS) under UV irradiation and its mechanism

Perfluorooctane sulfonate (PFOS) recently has received much attention due to its global distribution, environmental persistence and bioaccumulation. The methods for PFOS decomposition are very limited due to its inertness. In this report we first found the photodecomposition of PFOS under UV was greatly accelerated by addition of ferric ions. In the presence of ferric ion (100 μM), PFOS (20 μM) decreased to below the detection limit within 48 h, with the rate constant of 1.67 d(-1), which was 50 times higher than that by direct photolysis (0.033 d(-1)). Besides fluoride and sulfate ions, C2-C8 perfluorocarboxylic acids (PFCAs) were identified as the main intermediates. It was found that addition of PFOS into the FeCl3 aqueous solution led to reduction of UV absorption, and the presence of ferric ion reduced the response of PFOS as analyzed by UPLC-MS/MS, which indicated that PFOS formed a complex with ferric ion. The ESR detection indicated that the electronic state of Fe(3+)-PFOS complex changed during reaction. And the role of oxygen and hydroxyl radical on the defluorination of PFOS was investigated. Accordingly the mechanism for PFOS photodecomposition in the presence of ferric ion was proposed.

Different recycle behavior of Cu2+ and Fe3+ ions for phenol photodegradation over TiO2 and WO3

Photocatalytic degradation of organic pollutants on TiO2 and WO3 have been widely studied, but the effects of Cu(2+) and Fe(3+) ions still remain unclear. In this work, we have found that the recycle behavior of Cu(2+) and Fe(3+) are greatly dependent on the photocatalytic activity of metal oxide used. With TiO2 (P25, anatase, and rutile), all the time profiles of phenol degradation in water under UV light well fitted to the apparent first-order rate equation. On the addition of Cu(2+), phenol degradation on anatase, rutile and WO3 also followed the first-order kinetics. On the addition of Fe(3+), the initial rate of phenol degradation on each oxide was increased, but only the reactions on three TiO2 became to follow the first order kinetics after half an hour. The relevant rate constants for phenol degradation in the presence of Cu(2+) or Fe(3+) were larger than those in the absence of metal ions. Under visible light, phenol degradation on WO3 was also accelerated on the addition of Fe(3+) or Cu(2+). Moreover, several influencing factors were examined, including the metal ion photolysis in solution. It becomes clear that as electron scavengers of TiO2 and WO3, Fe(3+) is better than Cu(2+), while they are better than O2. We propose that Fe(3+) recycle occurs through H2O2, photogenerated from TiO2, not from WO3, while Cu(2+) regeneration on a moderate photocatalyst is through the dissolved O2 in water.

A review of chemical, electrochemical and biological methods for aqueous Cr(VI) reduction

Hexavalent chromium is of particular environmental concern due to its toxicity and mobility and is challenging to remove from industrial wastewater. It is a strong oxidizing agent that is carcinogenic and mutagenic and diffuses quickly through soil and aquatic environments. It does not form insoluble compounds in aqueous solutions, so separation by precipitation is not feasible. While Cr(VI) oxyanions are very mobile and toxic in the environment, Cr(III) cations are not. Like many metal cations, Cr(III) forms insoluble precipitates. Thus, reducing Cr(VI) to Cr(III) simplifies its removal from effluent and also reduces its toxicity and mobility. In this review, we describe the environmental implications of Cr(VI) presence in aqueous solutions, the chemical species that could be present and then we describe the technologies available to efficiently reduce hexavalent chromium.

Nanomaterial-Based Antibacterial Paper

Antibacterial materials are widely used in everyday life and plays important roles in the public health system. There are a wide range of materials that have been known to prevent attachment and proliferation of microbes on material surfaces. These include antibiotics, metal ions and quaternary ammonium compounds. Given the availability of these various antibacterial materials, concerns about antibiotics-resistance, environmental pollution, relatively complex processing and high cost have been of much recent interest. Antimicrobial properties of nanomaterials have been explored to meet these challenges. Various nanomaterials including silver nanoparticles (AgNPs), titanium oxide nanoparticles and carbon nanotubes (CNTs) have been found to be highly effective bacterial-killing materials. The most well-known examples are silver and silver-based compounds, which were well known to be antiseptic to a spectrum of bacterium even in ancient times. AgNPs have proven to possess high antibacterial activity with minimal perturbation to human cells. Consequently, widespread applications of AgNPs have been found in medical and environmental areas [1]. More recently, carbon nanomaterials have emerged as a type of novel antibacterial nanomaterials, including CNTs and graphene. In this chapter, we aim to provide a review on the fabrication of nanomaterials-based paper-like films and their antibacterial applications.

Photodegradation of lambda-cyhalothrin and cypermethrin in aqueous solution as affected by humic acid and/or copper: intermediates and degradation pathways

The influence of coexisting humic acids (HA) or Cu²⁺ on the photodegradation of pesticides lambda-cyhalothrin (λ-CHT) and cypermethrin (CPM) in aqueous solution was studied under xenon lamp irradiation. The removal efficiency of pesticides λ-CHT and CPM were enhanced in the presence of either Cu²⁺ or HA but restrained in the presence of both Cu²⁺ and HA. The photodegradation of λ-CHT and CPM followed first-order reaction kinetics. The photodegradation intermediates of λ-CHT and CPM were determined using gas chromatography/mass spectrometry. Possible photodegradation pathways included decarboxylation, ester bond cleavage, dechlorination, and phenyl group removal.

Photodegradation of propranolol by Fe(III)-citrate complexes: kinetics, mechanism and effect of environmental media

Photogeneration of HO was optimized in Fe(III)-citrate solution within the pH range of 3.0-9.0 to investigate its photoreactivity at neutral pH without the addition of H(2)O(2) under simulated sunlight. The generation of HO decreased with increasing pH within the range of 6.0-9.0 at the Fe(III)-to-citrate ratio of 10:50 (10(-6)M). However, when the concentration of citrate increased to 1.5 × 10(-4)M, the formation rate of HO increased in the order of pH 9.0<3.0<7.0<4.0<5.0. The pH-dependent HO production was governed by the stability of Fe(II)/Fe(II)-citrate and the amount of O(2)(-) in the solution. Propranolol can be efficiently photodegraded in Fe(III)-citrate system at pH 7.0 with pseudo-first-order constant 3.1 × 10(-4)s(-1). HO was verified to be the main reactive oxygen species (ROS) responsible for the photodegradation of propranolol. The presence of metal ions inhibited the Fe(III)-cit-induced photodegradation in the order of Mn(2+)>Cu(2+)>Ca(2+)>Mg(2+). Both humic acid (HA) and fulvic acid (FA) markedly suppressed the degradation of propranolol. Moreover, the iron in Fe(III)-citrate system was reused by a simple addition of citrate to the reaction solution. By GC-MS analysis, the photoproducts of the propranolol were identified and the degradation pathway was proposed. This work suggests that Fe(III)-citrate complexes are good alternative for the advanced treatment of organic pollutants at neutral pH in aqueous solution.

Effects of Cu(II) and humic acid on atrazine photodegradation

This work was designed to explore the characteristics of photodegradation of herbicides in the copper-polluted water body. The results showed that Cu(II) alone could induce a photo Fenton-like reaction to enhance the degradation of atrazine, in which hydroxyl radical (*OH) was a main active species. Humic acids restrained atrazine degradation, nevertheless, when introducing Cu(II), the photodegradation was accelerated, in which singlet oxygen (1O2) replaced *OH acting as the prevailing species. A feasible mechanism for the photochemical process was also proposed, which is helpful for better understanding the environmental photochemistry of atrazine in the copper-polluted water.