Decolourization Kinetics Research Articles

Scale-up of an electrochemical reactor for treatment of industrial wastewater with an electrochemically generated redox mediator

This paper presents the results of a study on the scale-up, from a batch to a continuous flow unit, of an electrochemical reactor applied for the treatment of textile wastewater. Decolourisation of the wastewater bearing a reactive dye Red Procion H-EXGL proceeded via indirect electro-oxidation, mediated by “active chlorine”. The kinetics of decolourisation in a single-cell reactor under different operating conditions were second order, with the highest apparent rate constant (k = 0.523 l mol−1 s−1) achieved at 40 °C. A low Hatta number (Ha = 0.03) indicated that the reaction occurred totally in the bulk solution, hence homogeneous reaction kinetics were used successively to scale-up a continuous flow electrochemical unit: a once-through filter-press reactor. Its hydrodynamic characteristics were defined by the residence time distribution (E(t)) function using a pulse injection method. The decolourisation efficiency experimentally determined in a continuous flow reactor was further compared with that predicted on the basis of the knowledge of the E(t) of the reactor and the homogeneous phase kinetic expression, obtained from the batch study and corrected for the geometric parameters of the reactors. A complete segregation of the fluid inside the flow reactor was assumed. For different applied flow rates, the experimentally defined conversion of the dye was close to the calculated value.

Decolourization of naphthalene-containing sulfonated azo dyes by Kerstersia sp. strain VKY1

The dye-decolourizing bacterium was isolated from a coconut coir sample and identified as a Kerstersia sp. by 16S rRNA sequencing. It decolourized aerobically seven different naphthalene-containing sulfonated azo dyes such as Amaranth, Fast Red E, Ponceau S, Congo Red, Orange II, Acid Red 151 and Acid Orange 12. The bacterium decolourized Amaranth, Fast Red E, Congo Red and Ponceau S by 100% (100 mg L −1) and the remaining three dyes Orange II, Acid Orange 12 and Acid Red 151 were decolourized by 84%, 73% and 44%, respectively in 24 h. About 1 g L −1 concentration of Amaranth and Ponceau S were decolourized within 24 h, whereas only 0.6 g L −1 of Fast Red E was decolourized in 24 h. On the other hand a mixture of all the three dyes at 1 g L −1 concentration was decolourized within 26 h. The oxygen insensitive azoreductase activity of cell extract with respect to Amaranth, Fast Red E and Ponceau S were 0.069, 0.061 and 0.038 units mg −1 of protein respectively. The decolourization kinetics of the dyes with azoreductase enzyme supports the ping-pong mechanism. Further, the bacterium capable of decolourizing these naphthalene-containing sulfonated azo dyes required 0.5% of peptone, yeast extract and temperature of 38 °C for growth.

Electrochemical studies on textile dye naphthol yellow S with particular reference to colour removal

Aromatic nitro dyes are resistant to chemical and biological oxidation and to hydrolysis because of electron withdrawing nitro groups. Consequently they are environmentally persistent and remediation of waste streams and contaminated water is difficult. In the present work, the electrochemical study has been carried out on the nitro acid dye naphthol yellow S predominantly used in textile dyeing. On the basis of results of the electrochemical study experimental conditions are optimized for the controlled potential electrolysis or the model dye. Complete decolourisation resulted after 1 to 3 h of controlled potential electrolysis. Kinetics of decolourisation was monitored and half-life period determined. Chromatographic study revealed formation of single decomposition product and COI) decreased substantially in the decolourised solution.

Kinetics of oxidative decolourisation of Acid Orange 7 in water by ultraviolet radiation in the presence of hydrogen peroxide

Wastewater from textile processing plants can be highly coloured and difficult to decolourize. The present study describes the photo-catalytic decolourisation of an azo dye, namely Acid Orange 7 (AO7), in aqueous solution by combined UV and hydrogen peroxide. The reactor used in this study is a continuous laboratory-scale photo-reactor equipped with a low-pressure mercury lamp. Different initial dosages of hydrogen peroxide at variable initial concentrations of dye were used. The rate of colour removal was monitored spectrophotometrically at the visible maximum absorption wavelength. Decolourisation was complete in 15 min and follows apparent first-order kinetics. It has been found that the decolourisation rate increased up to an optimum value, beyond which the reagent exerted an inhibitory effect. Indeed, for each dye solution there is an optimum value for H 2O 2 initial dosage at which the pseudo-first order reaction rate constants have the maximum values. The apparent rate constant ( k) values for each case have been determined.

The effect of operational parameters on UV/H 2O 2 decolourisation of Acid Blue 74

In the present study, advanced oxidation treatment, the UV/H 2O 2 process was applied to decolourisation of the indigoid dye C.I. Acid Blue 74 in aqueous solution. The UV radiation was carried out with a 15 W low-pressure mercury lamp. The rate of colour removal was studied by measuring of the absorbency at characteristic wavelength. The effects of H 2O 2 dosage, dye initial concentration and pH on decolourisation kinetics in the continuous circulation photoreactor were investigated. The highest decolourisation rates were observed at pH range between 3.5 and 5.5. The optimal levels of H 2O 2 needed for the process were examined. It appears that high levels of H 2O 2 could reduce decolourisation by scavenging the OH. The colour degradation rate decreases as the dye concentration increases.

Photochemical oxidation of reactive azo dye with UV–H 2O 2 process

The photooxidation of a chlorotriazine reactive azo dye Reactive Orange 4 has been carried out in the presence of H 2O 2 using UV-A light. The effects of reaction pH, applied H 2O 2 dose, UV light power have been studied. These parameters strongly influence the decolourisation and degradation. The increase of initial dye concentration decreases the removal rate. The kinetics of decolourisation and degradation follow pseudo-first order. The solar–H 2O 2 process is also able to oxidise the dye and its efficiency is comparable with UV–H 2O 2. The influence of dye assisting chemicals such as NaOH, NaCl and Na 2CO 3 on photodecolourisation has been investigated. Addition of these chemicals inhibit the removal rate. The optimum operating conditions of the method are reported.

Homogenous photocatalytic degradation of a disperse dye and its dye bath analogue by silicadodecatungstic acid

A new method for disperse dye decolourization using the photochemically active silicadodecatungstic acid (H 4SiW 12O 40)+isopropanol (electron donor) redox system has been described. Results of a series of control experiments have indicated that the one-electron reduced form of the silicadodecatungstic acid, SiW 12O 40 5−, was responsible for reductive cleavage of disperse dye as evidenced by photochemical heteropoly blue generation upon H 4SiW 12O 40+isopropanol UV-illumination. A threshold (0.09 mM) and an optimum (0.5 mM) heteropoly acid dose existed for minimum and maximum decolourization rates, respectively. It was also evident that decolourization kinetics were first-order with respect to dye concentration for concentrations up to 75 mg/l, and the photochemical degradation rate became UV-light penetration limited (zero order) above 100 mg/l dye concentration.

Kinetics of decolourisation of azo dyes in wastewater by UV/H2O2 process

Abstract Hydrogen peroxide and UV light were used in the decolourisation of azo dyes in spent dyeing wastewater. A kinetic study was carried out taking into account the contribution of the UV light alone and the combined reaction of UV light and H 2 O 2 . An empirical decolourisation rate expression was suggested by considering the two reactions taking place in parallel. Pseudo-first-order decolourisation rate constants were obtained from batch experimental data. Decolourisation rate was not significantly influenced by the initial pH of the wastewater in the range of 4–8. As the initial concentration of H 2 O 2 increased, the rate of decolourisation increased. Consequently, the relationship of decolourisation rate constant ( k obs ) and initial concentration of hydrogen peroxide at constant UV light intensity was determined as k obs =8.11[H 2 O 2 ] 0 0.63 . Nearly completely decolourisation was achieved in 10 min with 5.88×10 −3 M H 2 O 2 dose while only 18.9% of the organic carbon was mineralised.

The effect of pre-ozonation on the biocompatibility of reactive dye hydrolysates

Pre-ozonation of 14 different reactive dyestuff hydrolysates at alkaline pH was investigated to assess possible relationships between ozone transfer efficiency, first order decolourization kinetics, release of initially complexed heavy metals and relative changes in the biodegradability of the partially oxidized dye waste samples. Biocompatibility of the raw (untreated) and ozonated dye hydrolysates was comparatively tracked through specific oxygen uptake rate measurements from which the respirometric inhibition of biological activated sludge imparted by raw and ozonated reactive dye wastewater with respect to synthetic domestic wastewater was determined. It could be demonstrated that preliminary ozonation of reactive azo dyes increases their biological compatibility more significantly than formazan copper complex, copper complex azo and phythalocyanine dyes as a consequence of heavy metal release associated with the cleavage of associated chromophoric groupings right at the initial stages of pre-ozonation.

Advanced chemical oxidation of reactive dyes in simulated dyehouse effluents by ferrioxalate-Fenton/UV-A and TiO2/UV-A processes

Effective degradation of various mono- and bifunctional aminochlorotriazine reactive dyes in simulated dyehouse wastewater was achieved by the application of ferrioxalate-photo-Fenton [Fe(C2O4)33−/H2O2/UV-A; 300 nm>λ>400 nm] and titanium dioxide-mediated heterogeneous photocatalytic (TiO2/UV-A) treatment processes. These so-called advanced oxidation processes were studied in a novel batch photoreactor that was irradiated by a solar simulating installation. Decolorization by the ferrioxalate-photo-Fenton oxidation process was found to proceed three times faster than the photocatalytic process, while the latter was more efficient in reducing the optical density at 280 nm wavelength (UV280nm). Complete decolourization and partial mineralization with 17–23% total organic carbon (TOC) and 73–86% UV280nm removals were achieved by the ferrioxalate-Fenton/UV-A and TiO2/UV-A processes, respectively, within a 1-h treatment time. Emphasis was placed on the effect of dyehouse effluent strength on decolourization kinetics, along with possible advantages of the ferrioxalate-Fenton/UV-A process over the conventional photo-Fenton (Fe2+/H2O2/UV-C) process. The results of these experiments showed that the more dilute the dyehouse effluent the faster the decolourization rate. On the basis of spectrophotometric measurements, dye decomposition could be successfully fitted to the empirical Langmuir–Hinshelwood kinetic model.

Understanding anaerobic decolourisation of textile dye wastewater: mechanism and kinetics

Worldwide, some 280,000 tons of textile dyes are discharged per annum. Degradation of the predominant highly soluble reactive dyes is poor within activated sludge plants, and instead requires anaerobic pretreatment for reductive cleavage of the chromogenic azo bond to precede aerobic degradation. The mechanism for the anaerobic reductive cleavage, which results in decolourisation, is not well understood. Further, thermophilic anaerobic pre-treatment has not been reported, although the wastewater is produced at high temperature (50–80°C). This project therefore aimed to compare decolourisation kinetics under mesophilic (35°C) and thermophilic (55°C) conditions and elucidate biotic and abiotic factors in the decolourisation of the dye, Reactive Red 235. Preliminary experiments indicated that acclimation of the up-flow anaerobic sludge blanket (UASB) biomass was unnecessary and there was no dye toxicity at expected wastewater concentrations (0.05–0.1g/L). Decolourisation of Reactive Red 235 was studied under batch conditions with intact and autoclaved biomass, and also using the pre-reduced supernatant from a spent culture (0.2 μm filtered). Based on HPLC identification of the dye and reaction products, first-order kinetics was observed and rate constants of –0.0096/min (thermophilic) and –0.0034/min (mesophilic) were estimated for intact, viable biomass. Abiotic decolourisation was 60–80% lower for the autoclaved samples, and under 12–35% lower for the filtered supernatant. Hence it may be concluded that active anaerobic cells give the most efficient and complete decolourisation, especially under thermophilic conditions. Nonetheless, abiotic reduction does occur and has implications for the design of a novel uncoupled reactor system.

Treatability of simulated reactive dye‐bath wastewater by photochemical and non‐photochemical advanced oxidation processes

A number of photochemical and non‐photochemical advanced oxidation processes were employed for the treatment of simulated dyehouse effluents containing six commercial reactive dyestuffs and various assisting chemicals at concentrations typically found in the textile dyeing and rinsing process stages. Effects of oxidant (H2O2) and catalyst (Fe2+‐ion) dose on decolourization kinetics, reduction in UV254nm and DOC were evaluated for each oxidation process. Treatment efficiencies were also assessed in terms of EE/O and EE/M values to compare electrical energy requirements of the investigated AOPs. UV‐light assisted treatment processes were found more effective in DOC and UV254nm removal, whereas the non‐photochemical O3/OH‐ and O3/H2O2 oxidation systems were significantly faster in decolourization of the dyehouse effluent than the H2O2/UV treatment process. Results clearly revealed that once optimal reaction conditions were established, the inhibiting effect of the complex wastewater matrix could be overcome and dyehouse effluent ingredients degraded successfully by all examined AOPs at feasible treatment times and electrical energy consumption.

Oxidative treatment of simulated dyehouse effluent by uv and near-UV light assisted Fenton's reagent

UV/Fenton, near-UV-visible/Fenton, dark Fenton, and H 2O 2/UV reactions have been used to treat simulated dyehouse effluents representing wastewater from the textile dyeing and rinsing process. Experiments were carried out in a lab - scale photochemical reactor using concentrations of 0.5–25 mM H 2O 2, 0.04-0.5 mM Fe 2+-ion and different dilutions of textile wastewater. To assess the extent of mineralization, decolourization kinetics and the effect of different fight sources on treatment efficiency, DOC, optical density at 254 nm and 600 nm wavelength and residual H 2O 2 concentrations were measured during the course of the advanced oxidation reactions. Comparative evaluation of the obtained results revealed that the decolourization rate increased with applied H 2O 2 and Fe 2+-ion dose as well as the strength of the synthetic textile wastewater. The best results were obtained by the near - UV/visible/Fenton process with a decolourization rate constant of 1.57 min −1, a UV 254nm reduction of 97% and a DOC removal of 41% at relatively low doses of the H 2O 2 oxidant and Fe 2+-ion catalyst within 60 min treatment time.

Advanced Oxidation of Synthetic Dyehouse Effluent by O3, H2O2/O3 and H2O2/UV Processes

Ozonation, hydrogen peroxide combination with ozone, and UV light processes were investigated for the treatment of synthetic dyehouse effluent containing six reactive dyestuffs and their assisting chemicals. The decrease in DOC, UV - absorbance at 254 nm, decolourization kinetics, and acute toxicity towards the bioluminescent marine bacteria Vibrio fischeri were used to examine the treatment performance of these oxidation processes. Data indicated that in all examined processes rapid and complete decolourization could be achieved, but optimum oxidation conditions such as H2O2 dose and reaction pH had to be established for effective treatment. Toxicity of the samples decreased abruptly to non - detectable levels during the first minutes of all advanced oxidation processes. However, none of the oxidation combinations was able to bring about effective mineralization of the wastewater within 60 min reaction period. For comparative purposes, the electrical energy requirements per order of pollutant removal were calculated for all treatment systems.

Photochromic transitions occurring in indolinospiropyrans in films of crosslinked polymers based on oligocarbonate methacrylates

A comparative study of spectral and photochemical properties and of kinetic characteristics has been carried out for films of crosslinked polymers based on oligocarbonate methacrylates containing indolinospiropyrans combined in the macromolecule. The latter were prepared by polymerizing oligomers containing reactive end-groups in the presence of an indolinospiropyran containing no polymerizable groups (the matrix mixtures). In contradistinction to the matrix mixtures, the decolourization kinetics of the photochromic copolymers in the dark do not obey the equation of a first order reaction. This behaviour is related to the presence of nonequivalent positions in the networks which, in view of the microheterogeneity of the network structure-differ in respect to steric hindrances affecting thermal closure of the pyranic ring. Low activation energies for activation of the dark reaction of decolourization of indolinospiropyrans in the copolymer films prove that the thermal processes depend on relaxation behaviour of the crosslinked polymer. It is shown that the photocolouration efficiency of crosslinked photochromic copolymer films does not differ from that of the matrix mixtures.