Application of response surface methodology and central composite design for the optimization of textile dye degradation by wet air oxidation

Muhammet Demirel 1 Berkant Kayan 0 0 Department of Chemistry, Arts and Sciences Faculty, Aksaray University , Aksaray 68100, Turkey 1 Department of Chemistry, Arts and Sciences Faculty, Mersin University , Mersin 33342, Turkey Background: The present study is aimed at investigating the degradation of azo dye solution of AR 274 by wet air oxidation conditions. The central composite design matrix and response surface methodology were applied in designing the experiments to evaluate the interactive effects of the three most important operating variables. Thus, the interactive effects of oxygen pressure (3.0 to 5.0 MPa), temperature (100C to 250C), and time (30 to 90 min) on the degradation of dye were investigated. Results: The predicted values were found to be in good agreement with the experimental values (R2 = 0.9981 and Adj-R2 = 0.9965), which define the propriety of the model and the achievement of CCD in the optimization of WAO process. Conclusions: Intermediates of dye degradation were detected by GC-MS, the possible degradation mechanism for the WAO of dye was discussed, and the probable degradation pathway was deduced. - Background Dye pollutants from textile and dyestuff industries are a major hazardous source of environmental contamination. The large quantity of dye wastewater has become a serious environmental problem owing to the characteristics of high color, high chemical oxygen demand, and fluctuating pH. The direct discharge of this wastewater into water bodies such as lakes and rivers causes pollution of the water and affects the flora and fauna. Effluent from textile industries contains different types of dyes, which show very low biodegradability owing to their high molecular weight and complex structures [1-3]. Some dyes, especially azo dyes, are known to be biorefractory pollutants even with carefully selected microorganism and under favorable conditions. Azo dyes are characterized by the presence of one or more azo bonds (N = N-) and account for 60% to 70% of all textile dyes used. It is estimated that approximately 8 105 tons (t) of dyes are produced annually worldwide, and about 50% of them are azo dyes [4-8]. Thus, azo dyes constitute a significant portion of dyes that are used in industries nowadays. The product obtained from dye degradation could be mutagenic and carcinogenic, thereby causing long-term health concerns. Therefore, the treatment of effluents containing such compounds is important for the protection of natural waters as well as the environment [9-12]. Conventional methods of dyeing wastewater treatment include adsorption, flocculation, ozonation, advanced oxidation using UV/H2O2 or UV/TiO2, and biological oxidation. Other advanced oxidation treatments for dyeing wastewater treatment are wet air oxidation (WAO) and catalytic wet air oxidation (CWAO), which are operated at subcritical water and pressures of water. Previous researches have shown that the treatment efficiencies for various dyes using WAO and CWAO are in the range of 50% to 90% at the operating times of 30 to 240 min in different types of reactors [13-17]. A variety of advanced oxidation process (AOPs) have been attempted for the degradation of dyes, among which WAO seems to be a clean method as it does not involve the use of any harmful chemicals and uses only the clean reagent of air [7,18]. By using WAO, organic pollutants are either partially oxidized into biodegradable intermediates or mineralized to carbon dioxide, water, and innocuous end products under elevated temperatures (100C to 300C) and pressures (0.5 to 20 MPa) using an oxidant such as oxygen. The enhanced solubility of oxygen in aqueous solutions at elevated temperatures and pressures provides a strong driving force for oxidation. The elevated pressures are required to keep water in the liquid state. Water also acts as a moderant by providing a medium for heat transfer and removing excess heat by evaporation. WAO has been demonstrated to oxidize organic compounds to CO2 and other innocuous end products. Carbon is oxidized to CO2; nitrogen is converted to NH3, NO3, or elemental nitrogen; and halogen and sulfur are converted to inorganic halides and sulfates. The degree of oxidation is mainly a function of temperature, oxygen partial pressure, residence time, and the oxidizability of the pollutants under consideration [19-22]. WAO is not only eco friendly but also economical when compared to other AOPs that use harmful and expensive oxidizing agents like ozone and hydrogen peroxide [23-25]. In this research, the aqueous solution of Acid Red 274 (AR 274) was selected as a model for textile wastewaters for evaluation under WAO conditions. Another part of this study involved the use of response surface methodology (RSM) and finding an applicable approximating function for predicting and determining the further response, and studying the optimum working state. The factors (variables) of oxygen pressure, temperature, and experimental time were investigated [26]. RSM is a kind of mathematical and statistical technique for designing experiments, building models, evaluating the relative significance of several independent variables, and determining the optimum conditions for desirable responses [5,27,28]. The two most common designs extensively used in RSM are the central composite design (CCD) and the Box-Behnken design (BBD). The CCD is ideal for sequential experimentation and allows a reasonable amount of information for testing lack of fit while not involving an unusually large number of design points [29-31]. Methods Analysis methods In the present study, AR 274 dye concentration was analyzed spectrophotometrically on a UVvis spectrometer (Shimadzu UV-160A, Shimadzu Corporation, Kyoto, Japan) at 527 nm by measuring the absorbance of the untreated samples at maximum wavelength, and the percentage of AR 274 degradation efficiency percentage was calculated using the following formula: where Co and Ct represent the initial and remaining AR 274 concentration at given time (t), respectively. The mineralization of AR 274 solution was monitored through the diminishment of the TOC, measured on a Tekmar-Dorhmann Apollo 9000 TOC analyzer (Teledyne Technologies, Inc., OH, USA). The mass analysis process, which is the same as the previous method, was performed for intermediates of dye degradation [9]. The gas chromatographymass spectrometry (GC-MS) analysis was performed using the 5890A Agilent model gas chromatograph (Agilent Technologies, Inc., CA, USA), interfaced with the ECD, NPD, and 5975C mass selective detector. The aqueous solutions were extracted three times with 15 mL dichloromethane. A 3-L sample was analyzed on GC-MS. A HP5-MS capillary column (30 m 0.25 mm 0.25 m) was used as the analytical column. Helium was used as the carrier gas with a flow rate of 2 mL/min. The GC injection port temperature was set at 250C (split mode = 1/5), and the column temperature was fixed at 70C for 5 min. Subsequent (...truncated)