Advanced oxidation of a reactive dyebath effluent: comparison of O3, H2O2/UV-C and TiO2/UV-A processes
Introduction
Like many other industrial effluents, textile industry wastewater varies significantly in quantity, but additionally in composition [1]. Textile wastewater is strongly colored which creates an environmental as well as aesthetic problem. As regulations are becoming ever more stringent, the need for technically and economically more efficient means of decolorization and mineralization is obvious. Currently, there are no economically attractive technologies to achieve color removal. Existing physicochemical technologies such as membrane filtration or activated carbon adsorption are expensive and commercially unattractive. Furthermore, these processes just transfer pollutants from one phase to another rather than eliminating them from the water matrix. Recovery and reuse of certain chemical compounds present in dyebath effluents is currently under investigation [2], [3] but is not possible for the application of certain dyestuff classes, such as reactive dyes. In addition, the problem of color in dyehouse effluent has become identified particularly with the dyeing of cotton fibers that contributes to almost 40% of total fiber consumption annually worldwide, and the use of reactive dyestuffs since as much as 50% of these dyes end up in the exhausted dyebath in their hydrolyzed and unfixed form [4], [5].
The chemical limitations of conventional chemical oxidation techniques can be overcome by the development of so-called advanced oxidation processes (AOPs) which use strong oxidizing agents (O3, H2O2) and/or catalysts (Fe, Mn, TiO2) in the presence or absence of an irradiation source [6]. AOPs mainly involve the generation of a very powerful and non-selective oxidizing agent, the hydroxyl radical (OH), for the destruction of refractory and hazardous pollutants found in groundwater, surface water and industrial wastewater [7].
The treatment of reactive dyes in aqueous solutions via different AOPs has been extensively studied [8], [9], [10], [11], [12], [13], [14], [15]. However, the advanced oxidation of reactive dyebath effluents is rather limited to a few investigations [16], [17], [18]. From these studies it could be inferred that the dyebath constituents (sodium salts, detergents, wetting agents, sequestering agents etc.) seriously reduced the treatment efficiency of the selected AOP. Thus, the inhibiting effect of common reactive dye auxiliary chemicals, in particularly the OH scavenging effect of carbonate ions applied in the form of anhydrous Na2CO3 and used at substantially high concentrations in the reactive dyeing process, still needs to be further questioned for different AOPs. Moreover, a detailed assessment of the advanced oxidation conditions (i.e. reaction pH and oxidant dose) that are AOP- and pollutant-specific has to be undertaken to determine the most appropriate AOP for the treatment of the textile effluent in question.
With the aim to contribute to the clarification of the above mentioned points, the experimental work described herein evaluates comparatively the treatment of simulated exhausted reactive dyebath liquors prepared from a mixture of different dyeing formulations by two homogenous (O3, H2O2/UV-C) AOPs and one heterogeneous (TiO2/UV-A) AOP in batch experiments. Optimization of the reaction pH and oxidant (O3 and H2O2) dose were made in accordance with the highest rate in color (absorbance at 525 nm wavelength), aromaticity (absorbance at 280 nm wavelength) and TOC (total organic carbon) removal. Thereafter, the oxidative performance of the selected chemical oxidation systems was compared in terms of their electrical energy requirements.
Section snippets
Simulated reactive dyebath effluent
The investigated reactive dyes and dye assisting chemicals were kindly supplied by an integrated textile manufacturing plant. Synthetic spent dyebath wastewater was prepared weekly according to the cotton fiber dyeing procedure of the factory. The recipe mixture was used considering the international market share of their reactive dyestuff content and colors mostly applied to the cotton fabrics in the dyeing stage. The synthetic reactive dyebath effluent was prepared according to the
General aspects
Changes in pH were rather insignificant during any advanced oxidation of the simulated dyehouse effluent, which is easily explained since the reactive dyebath effluent was highly buffered. In all treatment cases, disappearance of UV-VIS absorbance exhibited a first order kinetic behavior, whereas no clear kinetic trend was observed for the TOC abatement rates.
O3 treatment
Ozone reacts with aromatic pollutants found in water and wastewater via two different pathways namely direct molecular and indirect
Conclusions
The comparison of different OH generation techniques is of interest, on the one hand, to determine the most efficient experimental conditions for the destruction of organic pollutants present in dyehouse effluents, and, on the other hand, to provide useful information for the understanding of the mechanism of advanced oxidation processes. The following conclusions can be drawn from the experimental work:
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Among the investigated AOPs, the quickest color (rid=11.8 1/m×min), UV280 (kUV280=0.35 1/min)
Acknowledgements
Idil Arslan is grateful to TUBITAK BAYG for the NATO A2 scholarship. The authors also acknowledge Bogaziçi Research Foundation for the financial support through grant 98Y-03.
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