J. of Supercritical Fluids 43 (2007) 106–111

Treatment of textile wastewater by SCWO in a tube reactor ¨ S¨og˘ u¨ t, Mesut Akg¨un ∗ Onur O. Yildiz Technical University, Chemical Engineering Department, Davutpasa Campus, No. 127, 34210 Esenler, Istanbul, Turkey Received 18 December 2006; received in revised form 13 April 2007; accepted 30 April 2007

Abstract Oxidation of aqueous solutions of a model azo dye pollutant (CI Disperse Orange 25) was studied in a continuous flow reactor, operated at temperatures between 400 and 600 ◦ C and at a fixed pressure of 25 MPa. The parameters used were the temperature, dye concentration, oxidant concentration and flow rate. The initial dye concentrations were in the range of 24.25 × 10−3 and 121.25 × 10−3 mol L−1 in terms of chemical oxygen demand (COD). Hydrogen peroxide (H2 O2 ) was used as a source of oxygen, and the initial oxidant concentrations were between 36.75 × 10−3 and 183.75 × 10−3 mol L−1 . The results demonstrated that the supercritical water oxidation (SCWO) process decreases the chemical oxygen demand up to 98.52% in very short reaction times (at a residence time of 4–12 s). Global rate expression according to wastewater and oxidant concentration was regressed from the complete set of data. The first-order global rate expression was determined with an activation energy of 27.8(±1.2) kJ mol−1 and a pre-exponential factor of 34.3(±1.5) s−1 to a 95% confidence level. © 2007 Elsevier B.V. All rights reserved. Keywords: SCWO; Oxidation kinetic; Textile wastewater; Disperse Orange 25; COD

1. Introduction Dyehouse effluents from the textile industry impose serious environmental problems because of their color and their high chemical oxygen demand content (COD). Synthetic azo dyes often receive considerable attention from researchers interested in textile wastewater treatment processes. Initial environmental efforts with dyes dealt with color pollution, which has a strong psychological effect. More recently, this interest has shifted to the potential toxicity of dyes and their degradation products, especially the suspected carcinogenicity of potential intermediate products. Azo dyes are known to be refractory pollutants, and slowly biodegraded, even with carefully selected microorganisms and under favorable conditions. Therefore, years of research have been aimed at the decolorization and concentration minimization of the azo dye effluent. Conventional methods of dyeing wastewater treatment include adsorption [1], flocculation [2], electrochemical methods [3,4], membrane separation [5], ozonation [2,6], advanced oxidation using UV/H2 O2 or UV/TiO2 [7–9] and biological oxidation [10,11]. Such approaches either are expensive for larger

∗

Corresponding author. Tel.: +90 212 449 1737; fax: +90 212 449 1895. E-mail address: [email protected] (M. Akg¨un).

0896-8446/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2007.04.007

effluent streams or provide only a relocation of the disposal problems. Some other advanced oxidation methods of dyeing wastewater treatment are wet air oxidation (WAO) and catalytic wet air oxidation (CWAO), which are performed at subcritical temperatures and pressures of water (