Photocatalytic decomposition of 4-chlorophenol over oxide catalysts

Unraveling the role of Mn(V)/Mn(III) in the enhanced permanganate oxidation under Vis-LED radiation

A novel approach of visible light-emitting diode (Vis-LED) radiation was employed to activate permanganate (Mn(VII)) for efficient organic micropollutant (OMP) removal. The degradation rates of OMPs by Vis-LED/Mn(VII) were 2-5.29 times higher than those by Mn(VII) except for benzoic acid and atrazine. Increasing wavelengths (445-525 nm) suppressed the degradation of diclofenac (DCF) and 4-chlorophenol (4-CP) owing to the decreased quantum yields of Mn(VII). Comparatively, light intensity and Mn(VII) dosage had a positive effect on the degradation of DCF and 4-CP. Experimental data revealed that Mn(V) dominated the DCF degradation whereas Mn(III) was the active oxidant in the 4-CP degradation. Mn(V) and Mn(III) formed from the photo-decomposition of Mn(VII), meanwhile, Mn(III) also formed from the Mn(V) photo-decomposition. The increase in solution pH inhibited DCF degradation but had a positive impact on 4-CP degradation, mainly due to the changing speciation of DCF and 4-CP. Inorganic anions (Cl- and HCO3-) had little impact on DCF and 4-CP degradation, while humic acid (HA) showed a positive impact because of the π-π interaction between HA and DCF/4-CP. The transformation products of DCF and 4-CP were identified and transformation pathways were proposed. Finally, the Vis-LED/Mn(VII) exhibited great degradation performance in various authentic waters. Overall, this study boosts the development of Mn(VII)-based oxidation processes.

Photocatalytic degradation of p-chlorophenol by hybrid H₂O₂ and TiO₂ in aqueous suspensions under UV irradiation

In this study, TiO2 particles were used as photocatalysts for the degradation of aqueous p-chlorophenol (p-CP) under UV irradiation. The effect of TiO2 dose (0-3 g/L), initial p-CP concentration, H2O2 concentration (2-45 mM), solution pH (4.6-9.5), and UV light intensity on the degradation of p-CP were examined. Four oxidative degradation processes, which utilized UV alone (direct photolysis), H2O2/UV, TiO2/UV, and H2O2/TiO2/UV, were compared in a batch photoreactor with a 100-W high-pressure mercury lamp. The photodegradation of p-CP could be described by the pseudo-first-order kinetics according to the Langmuir-Hinshelwood model. Moreover, the apparent degradation rate constants increased considerably from 3.5 × 10(-)(3) min(-)(1) (direct photolysis) to 19.9 × 10(-)(3) min(-)(1) (H2O2/TiO2/UV system).

Photolytic and Photocatalytic Degradation of Phenols in Aqueous Solution

Photolytic and photocatalytic degradation of phenols in aqueous solution were investigated under the UV irradiation of 254 nm light and VUV irradiation of 254 nm and 185 nm light. The decomposition and mineralization of 4-CP in UV/TiO2 process were both faster than in UV process. VUV irradiation led to the most efficient degradation of 4-CP. The initial concentration of 4-CP had little effect on the decomposition rate in UV and UV/TiO2 processes while in VUV and VUV/TiO2 processes the pseudo first-order rate constants decreased with the increase of the initial concentration. In UV/TiO2 process gas bubbling and liquid circulation could enhance the formation of oxidative species (HO2•) and mass transfer efficiency. The optimal air flow rate and liquid circulation flow rate were 300 mL/min and 1200 mL/min respectively in our reactor. Photocatalytic decomposition and mineralization of 4-CP were highly dependent on the nature of the organic substrate treated. Adding salt into aqueous solution could enhance photocatalytic degradation of phenols but after long operation with aqueous solution containing salt TiO2 catalyst would lose activity.

Decomposition of 3-chlorophenol on nitrogen modified TiO2 photocatalysts

Photocatalytic activity of nitrogen modified TiO(2) calcined at temperatures of 100-350°C toward 3-chlorophenol (3-CP) degradation was studied. In the experiments the fluorescent UV lamp and the incandescent lamp emitting mainly Vis light were applied. The degradation efficiency was evaluated on a basis of changes of 3-CP, total organic carbon and Cl(-) concentration. A significant improvement of the photoactivity of the N-modified photocatalysts compared to the reference sample was observed. The rate of 3-CP degradation increased with the calcination temperature, and the highest efficiency was achieved for TiO(2) annealed at 350°C. After 5h of UV irradiation in the presence of TiO(2)/N-350 and reference TiO(2) the 3-CP concentration decreased for 77% and 36%, respectively. The 3-CP removal after 24h of Vis irradiation was 30% and 12% for the N-modified and reference samples, respectively. The 3-CP decomposition and mineralization were greatly influenced by pH of the solution, achieving the highest rate at pH 7 for the modified photocatalysts. An increase of the calcination temperature resulted in an increase of the rate of OH formation. The photocatalytic activity of the N-modified TiO(2) remained unchanged during repeated photocatalytic degradation cycles.

Comparative Study of the Adsorption of Aromatic Pollutants onto TiO2 (100) Surface via Molecular Simulation

The adsorption mode of benzoic acid onto the anatase TiO2 (100) surface has been studied through the semi-empirical self-consistent field molecular orbital method MSINDO and is compared with previously determined modes of four aromatic compounds: chlorobenzene, aniline, p-chlorophenol and nitrobenzene. The simulation results reveal that aniline and p-chlorophenol molecules are adsorbed with their aromatic ring positioned parallel to the surface although they are linked to a surface lattice titanium atom via the amino nitrogen and phenolic oxygen respectively. In contrast, chlorobenzene, nitrobenzene and benzoic acid are found in perpendicular configurations and they are attached to the surface via the chlorine and oxygen atoms of the NO2 and COOH groups respectively. The calculated substrate–surface interaction energy is influenced by the degree of basicity of the lone pair of the donating atoms, the number of linkages between the substrate and the surface and, further, the hydrogen bonding between the acidic hydrogen and lattice oxygen atom. The computed vibrational density of states for these adsorbed organic pollutants is in reasonably good agreement with available experimental data and previous theoretical results.

Study of the degradation behaviour of dimethoate under microwave irradiation

In this work, the degradation of dimethoate under microwave irradiation assisted advanced oxidation processes (MW/oxidants) were studied. The efficiencies of the degradation of dimethoate in dilute aqueous solutions for a variety of oxidants with or without MW irradiation were compared. The results showed that the synergistic effects between MW and K(2)S(2)O(8) had high degradation efficiency for dimethoate. Simultaneously, UV/TiO(2)/K(2)S(2)O(8) photocatalytic oxidation degradation of dimethoate was investigated. The experimental results indicated that the method of microwave degradation of organic pollutants in the presence of oxidant could reduce reaction time and improve product yield. Microwave irradiation was an advisable choice for treating organic wastewaters and has a widely application perspective for non- or low-transparent and fuscous dye wastewaters.

Degradation of 4-chlorophenol by a microwave assisted photocatalysis method

In this work, the degradation of 4-chlorophenol (4CP) under simultaneous microwave assisted UV (electrodeless discharge lamp) photocatalysis technique (MW/UV/TiO2) was investigated. Several factors affecting the degradation of 4CP by MW/UV/TiO2 method, such as the dosage of photocatalysts, the initial pH value of the solutions, gas bubbling, light intensity and addition of H2O2 oxidant, were studied in detail. The synergistic effects between microwave irradiation and TiO2 photocatalysis were also studied. The major intermediates were found to be chlorobenzene, phenol, hydroquinone, benzoquinone and 4-chlorocatechol. Based on the results, a general reaction pathway for the degradation of 4CP was proposed.

Degradation of 4-chlorophenol by microwave irradiation enhanced advanced oxidation processes

In this work the synergistic effects of several microwave assisted advanced oxidation processes (MW/AOPs) were studied for the degradation of 4-chlorophenol (4-CP). The efficiencies of the degradation of 4-CP in dilute aqueous solution for a variety of AOPs with or without MW irradiation were compared. The results showed that the synergistic effects between MW and H2O2, UV/H2O2, TiO2 photocatalytic oxidation (PCO) resulted in a high degradation efficiency for 4-CP. The potential of MW/AOPs for treatment of industrial wastewater is discussed.

Photocatalytic oxidation of VX simulant 2-(butylamino)ethanethiol

Photocatalytic oxidation of 2-(butylamino)ethanethiol (BAET) was undertaken in aqueous suspension of TiO2 Hombikat UV 100 and Degussa P25 under different initial reaction conditions in order to determine the best parameters for the fastest mineralization of the substrate. BAET is considered to be a simulant for the VX chemical warfare agent. The application of ultrasound had only a small positive effect on the BAET photocatalytic degradation. The highest mineralization rate of 0.433 mg/(l min) was found in unbuffered TiO2 Degussa P25 suspension with initial pH value of about 9.4, TiO2 concentration 500 mg/l and the initial BAET concentration 1000 mg/l. Decreasing of the initial solution pH to 6.1 or below stops the mineralization of BAET while increasing of pH to about 11 drastically changed the degradation profile. At this initial pH, the first 100 min of reaction led to only oxidation of sulfur moiety and organic intermediates accumulated in the solution. Thereafter, mineralization of the products started. The main detected volatile product was butyl aldehyde and the main polar one was 2-(butylamino) acetic acid. In the case of TiO2 Hombikat UV 100, conversion of TOC at initial pH 11 exceeded that at initial pH 9.1. For Degussa P25, the starting pH 9.4 was the best for TOC conversion. The results can be used for treatment of water from pollutants with aliphatic nitrogen and sulfur atoms.

Photo-assisted oxidation of chlorophenols in aqueous solutions using hydrogen peroxide and titanium dioxide

In the present work, the efficiency of phenol and chlorophenol degradation under irradiation using hydrogen peroxide as oxidant and titanium dioxide powder as photo-catalyst was investigated. In the absence of titanium oxide, increased concentrations of hydrogen peroxide resulted in higher conversions. Generally, phenol was the most readily oxidized compound, whereas in excess of hydrogen peroxide, the more chlorine atoms were present in the ring, the less degradable the chlorophenol was, in terms of initial rate of oxidation. In the case of 4-chlorophenol, the catalytic photo-oxidation efficiency with hydrogen peroxide and titanium oxide was dependent on the catalyst concentration exhibiting a maximum at 0.025-0.05 g L(-1) titanium oxide. The combined use of titanium oxide and hydrogen peroxide resulted in higher degree of oxidation compared to results obtained when using hydrogen peroxide. Finally, the presence of Fe(III) proved to be beneficial for the photo-catalytic oxidation only in the presence of hydrogen peroxide.

Kinetic studies on UV-photodegradation of some chlorophenols using TiO2 catalyst

A number of chlorophenols, namely 2-chlorophenol, 2,4,-dichlorophenol and 2,4,6-trichlorophenol, were decomposed in aqueous solution by using TiO2 as photoactivated catalyst under UV radiation emitted by a 125W medium pressure Hg lamp in an immersion well-type quartz photoreactor. The organic-bound chlorine was converted into the environmentally harmless inorganic chloride. For catalyst doses between 0.1 and 0.5gl(-1) the reaction mechanisms are elucidated. The corresponding rate constants were obtained by periodically measuring the remaining chlorophenol, and converted chloride in solution. A theoretical model for the degradation pathway is proposed expressing the rate as a linear function of the concentrations of chlorophenol and catalyst. Aside from the model-calculated values, the pseudo-first order rate constants for a rough approximation of chlorophenols degradation as well as the kinetic parameters of Langmuir-Hinshelwood type decomposition are compared. The photodegradation rate of chlorophenols followed the order: C13 x Ph > Cl2 x Ph > Cl x Ph.

Modeling the kinetics of UV/hydrogen peroxide oxidation of some mono-, di-, and trichlorophenols

The decomposition of a number of chlorophenols (CPs), namely 2-CP, 2, 4-dichlorophenol and 2,4,6-trichlorophenol, has been studied in aqueous solution by UV-catalyzed oxidation with H(2)O(2) under UV radiation emitted by a 125-W medium pressure Hg lamp in an immersion well-type quartz photoreactor, and the organic-bound chlorine has been converted into the environmentally harmless inorganic chloride. For oxidant/CP mole ratios between 1:1 and 16:1, the reaction kinetics were modeled and the corresponding rate constants found by periodically measuring the remaining CP, hydrogen peroxide and converted chloride in solution. A theoretical model for the degradation pathway is proposed expressing the rate as a linear function of the concentrations of CP and oxidant. The rate constants for the pseudo-first order approximation of the CP degradation were compared. H(2)O(2), when combined with UV, is an effective photoactivated oxidant. The photodegradation order in terms of the initial rate of CPs destruction was: Cl(3).Ph>/=Cl(2).Ph>Cl.Ph.