Keiko Takashima

Adsorption of the diazo dye Direct Red 23 onto a zinc oxide surface: A spectroscopic study

The adsorption of the diazo dye Direct Red 23 onto a zinc oxide surface at 30 degrees C in the dark was investigated. The color reduction was monitored by spectrophotometry at 503 nm. The FTIR and Raman spectra of the Direct Red 23 adsorption as a function of ZnO concentration were registered. From the PM3 semi-empirical calculations of the atomic charge density and dipole moment of the Direct Red 23 molecule, it was demonstrated that the azo dye molecule may be adsorbed onto the ZnO surface through molecule geometry modifications, enhancing the interfacial area causing a variation in the bonding frequencies.

OPTIMIZATION AND EFFECTS OF SOME ELECTRON ACCEPTORS ON THE PHOTOCATALYTIC DEGRADATION OF DIRECT RED 23 AZO DYE

ABSTRACT The photocatalytic degradation of aqueous solution of commercial azo textile dye, direct red 23 (DR23), was carried out in TiO 2 suspension at 30oC with the use of artificial and solar light sources. The photoreaction followed the first-order behavior with respect to the azo dye as a function of irradiation time. A 2 3 factorial design was carried out, in order to obtain the best experimental conditions, using the rate constant of DR23 degradation as the analytical response. Seven chemical species were determined from the normalized UV-Vis spectra during the DR23 degradation through Imbrie Q mode factor analysis followed by varimax and Imbrie oblique rotations. The addition of electron acceptors, such as H 2 O 2 , S 2 O 82- , and ClO 3- on the optimized conditions, was carried out to increase the DR23 degradation rate. Comparison of degradation efficiencies under artificial and solar radiation was examined in the presence of oxidants. Keywords : degradation; direct red 23; inorganic oxidant; photocatalysis; factorial design.

COD removal and toxicity decrease from tannery wastewater by zinc oxide-assisted photocatalysis: a case study

This work reports the optimization of degradation conditions and toxicity decrease in the tannery wastewater, collected in the retanning and dyeing steps. This effluent was filtered, diluted in a 1:200 proportion, and investigated as a case study on a bench scale by heterogeneous photocatalysis. These conditions were attained when the suspension, containing 1 g L−1 of ZnO and effluent, was irradiated for 4 h at pH 8.0 and 30°C. Physico-chemical parameters such as chemical oxygen demand (COD) decreased from 15,023 to 350 mg O2 L−1; fifth-day biochemical oxygen demand (BOD5) from 4374 to 10 mg O2 L−1; total solids from 28,500 to 188 mg L−1; total organic carbon (TOC) from 4685 to 4.93 mg L−1, and turbidity from 331 to 1.15 NTU after 4 h of irradiation. The LC50 increase from 14.90% to 56.82% in the lethality assay of Artemia salina L. microcrustacean as well as the dissolved oxygen of 6.45 mg L−1 indicated efficiency in this treatment.

Fotocatálise eletroquímica de atrazina sobre dióxido de titânio: efeito de diferentes parâmetros experimentais

The atrazine photoelectrochemical degradation has been examined in solutions containing TiO2 on immobilized films under a variety of experimental conditions. It was possible to observe that the supporting electrolyte nature affects the intensity of the photocurrent, being an indicative of the adsorption process. The disappearance of the organic molecule follows approximately a pseudo-first order kinetic. As mineralization product, NH4+ and NO3- ion have been identified. These results indicated that the formation of NH4+ ion can be associated to the effect of atrazine adsorption, due to need of potential adaptation together with a variation in the supporting electrolyte concentration.

Efeitos dos parâmetros operacionais na fotodegradação do azo corante direct red 23 na interface dióxido de titânio/água

The decolorization and degradation of direct red 23 azo dye have been investigated in aqueous suspension of titanium dioxide under artificial irradiation. The effects of some operational parameters such as azo dye concentration, catalyst loading, and solution pH were investigated at 30.0 oC and optimized values were obtained. The first-order kinetic model was used to discuss the results. The UV-Vis spectra changes showed that the azo dye sample, collected after 6 h irradiation, was 98% decolorized while the residual total carbon was 97.9% degraded, indicating simultaneous photodecolorization and degradation.

Citric acid oxidation by vanadium(V) in sulfuric acid medium: Kinetic and mechanistic study

The citric acid oxidation by vanadium(V) in sulfuric acid medium at 303 K is reported. The reaction rate was determined spectrophotometrically by monitoring the formation of vanadium(IV) at 760 nm. The oxidation showed a first-order dependence with respect to vanadium(V) concentration and fractional order with respect to citric acid concentrations, with no control and with constant ionic strength. The reaction is also first order with respect to sulfuric acid concentration with no control and of fractional order at constant ionic strength. The reaction rate is enhanced by an increase of ionic strength and increased by a decrease of the dielectric constant. The activation parameters were calculated based on the rate constants determined in the 293 to 313 K interval. The proposed oxidation mechanisms and the derived rate laws are consistent with the experimental rate laws.

Kinetic and mechanistic aspects for the tartaric acid oxidation by vanadium(V) in sulfuric acid medium

Tartaric acid oxidation by vanadium(V) in sulfuric acid medium was investigated spectrophotometrically at 760 nm and 30°C by appearance of the vanadium(IV), as vanadyl. The reaction rate was determined under pseudo-first-order conditions with an excess of hydroxyacid over the oxidant concentration. The oxidation showed a first-order dependence with respect to vanadium(V) concentration and fractional orders with respect to tartaric acid and sulfuric acid concentrations, with no control and with constant ionic strength. The reaction rate is enhanced by an increase of ionic strength, and slightly reduced by a decrease of the dielectric constant of the medium. The activation parameters were calculated based on the rate constants determined in the 293 to 313 K interval. The proposed oxidation mechanisms and the derived rate laws are consistent with the experimental rate laws.

OPTIMIZATION OF PHOTOCATALYTIC DECOLORIZATION OF THE AZO DYE DIRECT ORANGE 34 BY STATISTICAL EXPERIMENTAL DESIGN

abstract The photodecolorization of the azo dye Direct Orange 34 (DO34) was investigated in TiO 2 aqueous suspensions with artificial irradiation at 30oC. The experimental conditions were optimized by a 2 5 factorial design, using the decolorization percentage after 240 min irradiation as the response. The five factors considered were DO34 and TiO 2 concentrations, stirring speed, air saturation, and adsorption time. The addition of hydrogen peroxide and sodium periodate as oxidants increased the decolorization rate. Keywords: photocatalytic decolorization, Direct Orange 34, TiO 2 , oxidant, factorial design. e-mail: keiko@uel.br intrOductiOn Dyes are an abundant class of colored organic compounds that represent an increasing environmental hazard. During dye production and textile manufacturing processes a large amount of wastewater containing dyestuffs with intense color and toxicity can be introduced into aquatic systems and cause severe ecological and environmental problems. Azo dyes, which are aromatic compounds with one or more azo (–N=N–) groups, are the most important and largest class of synthetic dyes used in commercial applications, and they are characterized by azo chromophores

Preparation and characterization of zinc and cobalt (II, III) oxides mixture and direct red 23 diazo dye heterogeneous photocatalysis

The decolourization of the direct red 23 diazo dye, commonly used in textile industries, was investigated through the heterogeneous photocatalysis mediated by zinc oxide, n-type semiconductor combined with p-type cobalt oxide at 30 ∘C. The mixture of n- and p-type semiconductors may form a p–n junction, which can decrease the band gap energy and thus increase the photocatalytic activity. This work reports the preparation and characterization of cobalt oxide and oxide mixtures of Zn and Co in 1, 3 and 5% proportions, respectively, as well as their photocatalytic activity, measured by direct red 23 diazo dye decolourization rate constants. These oxides were characterized through the X-ray diffraction, scanning electron microscope, energy-dispersive X-ray spectroscopy, X-ray fluorescence spectroscopy and diffuse reflectance spectrophotometry. The suspension, formed from direct red 23 diazo dye solution and oxide mixtures, was kept in the dark for 1 h at 30 ∘C for diazo dye adsorption on the oxide surface and irradiated by UV light for diazo dye decolourization. The mixture of ZnO containing, respectively, 3 and 5% cobalt oxide showed higher decolourization rate constant in comparison to ZnO.

KINETIC BEHAVIOR OF SOME AZO DYES DECOLORIZATION BY VARIATION OF ZINC OXIDE AND TITANIUM DIOXIDE CONCENTRATIONS

The decolorization of three monoazo dyes (acid orange 7, direct orange 34, and methyl orange), one diazo dye (direct yellow 86) and one tetraazo dye (direct red 80) were mediated by n-type semiconductors as ZnO and TiO2 under pseudo-first order conditions at 30 oC. The decolorization rate constants of these azo dyes were determined, varying the semiconductor concentration for the majority of them from 1.0 to 10.0 g L. In general, the highest rate constants were displayed for ZnO. This work elucidates that the decolorization capacity depends on the charge, structure, and adsorption of the azo dye on the semiconductor surface as well as the agglomeration of the photocatalyst particles.