1,4-Benzoquinone and 1,4-hydroquinone based determination of electron and superoxide radical formed in heterogeneous photocatalytic systems

Phototransformation of tetrabromobisphenol A in saline water under simulated sunlight irradiation

The widespread use of halogenated flame retardants in recent years has led to the accumulation of TBBPA in water, which may cause potential harm to living organisms. The phototransformation of the flame retardant TBBPA in alkaline saline water under simulated sunlight irradiation was investigated. The effects of abiotic factors such as the initial concentration of TBBPA, chloride ion concentration, solution pH, inorganic anions and cations, dissolved organic matter (DOM) were studied. The results showed that the phototransformation rate of TBBPA accelerated with the decrease of the initial concentration of TBBPA, the increase of chloride ion concentration and solution pH. The scavenging experiments showed that •OH, ¹O₂, O₂•^- and ³TBBPA* all participated in the phototransformation of TBBPA. The presence of NO₃^-, CO₃^2-, SO₄^2-, Mg²⁺, Ca²⁺, Fe³⁺ and fulvic acid (FA) all inhibited the phototransformation of TBBPA in the present study. The phototransformation products of TBBPA were detected by liquid chromatography-mass spectrometry (LC-MS), and the phototransformation pathways were proposed. This is the first report on the photo-induced generation of halogen exchange products from TBBPA in saline solutions, which will contribute to a better understanding of the environmental behavior and risks of BFRs in water.

A microbubble-assisted rotary tubular titanium cathode for boosting Fenton's reagents in the electro-Fenton process

To improve cathodic H₂O₂ accumulation and Fe³⁺ reduction synchronously in the electro-Fenton (EF) process, a microbubble-assisted rotary tubular titanium cathode (MRTTC) was designed for the first time. By utilizing this MRTTC, H₂O₂ accumulation improved by 4.05-fold, along with a 200% enhancement in iron reduction compared to the conventional EF process. This promotion is mainly attributed to a considerably higher oxygen mass transfer, which reduces the thickness of the adhered diffusion layer. The oxygen mass transfer coefficient (K_La) also improved from 0.0073 s^-1 to 0.012 s^-1 at a rotational speed of 300 rpm. In addition, the microbubble-assisted cathode further improved the K_La to 0.047 s^-1. The synergistic effect between the rotating and microbubble-assisted cathodes further intensified H₂O₂ accumulation in MRTTC. Apart from H₂O₂ promotion, the iron reduction rate was elevated because the newly formed O₂^-• provided an additional reduction pathway for Fe³⁺ reduction in addition to the cathodic path. The effectiveness of MRTTC was confirmed by treating a benchmark organic pollutant, sulfamerazine (SMR), where approximately 100% SMR decay was obtained in 3 h. The results show that MRTTC is a novel and promising design in EF for antibiotic wastewater treatment.

TiO2-Acetylacetone as an Efficient Source of Superoxide Radicals under Reduced Power Visible Light: Photocatalytic Degradation of Chlorophenol and Tetracycline

Visible light-sensitive TiO2-based nanomaterials are widely investigated for photocatalytic applications under high power (≥300 W) UV and visible light. The formation of charge transfer complexes (CTCs) between bidentate ligands and nanocrystalline TiO2 promotes visible light absorption and constitutes a promising alternative for environmental remediation under reduced visible light power. However, the efficiency of photodegradation, the volatilization profile of bidentates, and the role of reactive oxidizing species (ROS) are not fully understood. In this study, thermogravimetric analyses coupled with mass spectroscopy (TGA-MS) were performed on TiO2-Acetylacetone (ACAC) CTC. TiO2-ACAC CTC calcined at 300 °C (TiO2-A300) was applied for the photocatalytic degradation of chlorophenol (4-CP) and tetracycline (TC) under low power visible light (26 W). Furthermore, the ROS scavengers isopropanol and benzoquinone were added for studying the photocatalytic role of •OH and •O2− radicals. The TGA-MS showed the release of ACAC fragments, such as ethyl ions and acetone, in the range between 150 °C and 265 °C, while between 300 °C and 450 °C only CO2 and H2O were released during oxidation of ACAC. The photocatalytic abatement of tetracycline (68.6%), performed by TiO2-A300, was ~two times higher than that observed for chlorophenol (31.3%) after 6 h, indicating a distinct participation of ROS in the degradation of these pollutants. The addition of the ROS scavenger revealed •O2− radicals as primarily responsible for the high efficiency of TiO2-ACAC CTC under reduced visible light. On the other hand, the •OH radicals are not efficiently generated in the CTC. Therefore, the development of heterostructures based on TiO2-ACAC CTC can increase the generation of ROS through coupling with semiconductors capable of generating •OH under visible light.

Low temperature curable titanium-based sols for visible light photocatalytic coatings for glass and polymeric substrates

Titanium oxide (TiO2) is a widely used material in photocatalytic coatings in which efficiency generally lies in the ultraviolet (UV) spectrum of light. Sol-gel method provides a simple and versatile...

Merging the chemistry of metal-organic and polyoxometalate clusters into an enhanced photocatalytic material

The combination of a zirconium metal-organic cluster and a Keggin type polyoxotungstate into a compound of formula [Zr6(µ3-O)4(µ3-OH)4(µ-OOCC6H5)8(H2O)8][SiW12O40] led to a chemically and photochemically stable porous material in which a...

Surface ligands enhance the catalytic activity of supported Au nanoparticles for the aerobic α-oxidation of amines to amides

The catalytic aerobic α-oxidation of amines in water is an atom economic and green alternative to current methods of amide synthesis. The reaction uses O2 as terminal oxidant, avoids hazardous...

CsPbBr3 perovskite quantum dots as a visible light photocatalyst for cyclisation of diamines and amino alcohols: an efficient approach to synthesize imidazolidines, fused-imidazolidines and oxazolidines

We report the synthesis of highly stable CsPbBr3QD based photocatalysts using dibromoisocyanuric acid (DBI) as a benign non-toxic bromide precursor.

Arsenite photo-oxidation and removal by ferrihydrite in the presence of oxalate: a pH dependence and surface-mediated process

As(iii) is removed via a surface-mediated process in a ferrihydrite/oxalate system, and the sources of HO˙ depend strongly on pH.

Wavelength Dependence of the Transformation Mechanism of Sulfonamides Using Different LED Light Sources and TiO2 and ZnO Photocatalysts

The comparison of the efficiency of the commercially available photocatalysts, TiO2 and ZnO, irradiated with 365 nm and 398 nm light, is presented for the removal of two antibiotics, sulfamethazine (SMT) and sulfamethoxypyridazine (SMP). The •OH formation rate was compared using coumarin, and higher efficiency was proved for TiO2 than ZnO, while for 1,4-benzoquinone in O2-free suspensions, the higher contribution of the photogenerated electrons to the conversion was observed for ZnO than TiO2, especially at 398 nm irradiation. An extremely fast transformation and high quantum yield of SMP in the TiO2/LED398nm process were observed. The transformation was fast in both O2 containing and O2-free suspensions and takes place via desulfonation, while in other cases, mainly hydroxylated products form. The effect of reaction parameters (methanol, dissolved O2 content, HCO3- and Cl-) confirmed that a quite rarely observed energy transfer between the excited state P25 and SMP might be responsible for this unique behavior. In our opinion, these results highlight that "non-conventional" mechanisms could occur even in the case of the well-known TiO2 photocatalyst, and the effect of wavelength is also worth investigating.

Synergistic Photocatalytic-Adsorption Removal of Basic Magenta Effect of AgZnO/Polyoxometalates Nanocomposites

The bifunctional photocatalytic-adsorbent AgZnO/polyoxometalates (AgZnO/POMs) nanocomposites were synthesized by combining AgZnO hybrid nanoparticles and polyoxometalates [Cu(L)₂(H₂O)]H₂[Cu(L)₂(P₂Mo₅O₂₃)]⋅4H₂O (HL = C₆H₆N₂O) into nanostructures via a sonochemical method. Transmission electron microscopy (TEM) indicated that AgZnO/POMs nanocomposites were uniform with narrow particle size distribution and without agglomeration. X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis confirmed the nanostructure and composition of AgZnO/POMs nanocomposites. The ultraviolet-visible spectra (UV-Vis) and photoluminescence spectra (PL) confirmed excellent optical properties of the AgZnO/POMs nanocomposites. 94.13% ± 0.61 of basic magenta (BM) in aqueous solution could be removed using the AgZnO/POMs nanocomposites through adsorption and photocatalysis. The kinetic analysis showed that both the adsorption and photocatalysis process conform to pseudo-second-order kinetics. In addition, the removal rate of AgZnO/POMs nanocomposites was found to be almost unchanged after 5 cycles of use. The bifunctional photocatalytic-adsorbent AgZnO/POMs nanocomposites with high stability and cycling performance have broad application prospects in the treatment of refractory organic dye wastewater containing triphenylmethane.

Synthesis of a plasmonic AgCl and oxygen-rich Bi24O31Cl10 composite heterogeneous catalyst for enhanced degradation of tetracycline and 2,4-dichlorophenoxy acetic acid

In this study, a AgCl/Bi24O31Cl10 composite heterostructure was constructed. Varying ratios of AgCl nanoparticles were immobilised onto the Bi24O31Cl10 rod-like structure. The physical and optical properties of the synthesised catalysts were characterised using a range of techniques. The photocatalytic activity of the catalysts was investigated by the degradation of 2,4-dichlorophenoxy acetic acid (2,4-D) and tetracycline (TC) under visible light irradiation. The performance of the composite photocatalysts was 18 and 3.4 times better in 2-4,D and TC photodegradation when compared to Bi24O31Cl10 alone. The improved photocatalytic performance was due to the surface plasmon resonance (SPR) effects of the Ag nanoparticles deposited on the surface of the Xwt%AgCl/BOC thereby improving the separation of the electron-hole pair. The effects of the initial contaminant concentration, pH, photocatalyst loading were investigated. Trapping experiments were also carried out to deduce the reactive species responsible for the degradation process and a preliminary mechanism of degradation was proposed. Successful mineralisation of 2,4-D and TC at 65% and 63% efficiency was also measured after 24 h and the potential for reusability of the as-synthesised photocatalyst was established. This work reports a promising heterogeneous photocatalyst for the removal of pollutants such as TC and 2,4-D from wastewater.

Electrochemistry of Quinones with Respect to their Role in Biomedical Chemistry

Quinones are ubiquitous in nature and form one of the largest class of antitumor agents approved for clinical use. They are known to be efficient in inhibiting cancer cells growth. Under physiological conditions they can undergo non-enzymatic one-electron reduction to give the moderately toxic species of semiquinone radical-anion. Thus, electrochemical study of quinones might provide a basic knowledge on semi-quinone radicals formation in both in vivo and in vitro under different media. Several processes are outlined briefly and discussed in the present article. Previously we investigated the electrochemical and spectral properties of ω-N-quinonyl amino acids. Such quinone-bearing peptides are known to be cytotoxic and of potential clinical significance. We were able to prove that the ω-amino quinonyl compounds are very effective in producing stable semiquinone radicals. Moreover, a direct relation was found between the first reduction potentials of the quinonyl moiety and their reactivity towards the ω-amino acids. In order to increase our knowledge of such amino quinonyl compounds and enlarge the arsenal of such cytotoxic compounds, a series of N,N-diquinonyl amines (1-6) bearing an internal proton (stems from the NH moiety) were synthesized. Their electron-transfer capabilities were probed by cyclic voltammetry measurements, in dichloromethane. It was found that the acidic NH group linking the two quinonyl moieties undergoes an initial electrochemical reduction step and generates a nitride anion. This step is followed by further reductions to yield quasi-stable semiquinone radicals and polyanions, Since these acidic diquinones (1-6) serve also as a source of internal proton donors even in non-polar medium, they might cause protonation of basic radical-anions and polyanion intermediates during the various electrochemical stages. The processes are demonstrated and discussed by analyzing different mechanistic schemes. The successful generation of relatively stable semiquinone radicals is a prerequisite for the manifestation of site directed antitumor activity by these bis-quinonyl amino derivatives. Based on the values of their redox potentials some of them could be promising candidates for clinical development.