Performance of Fluidized bed Fenton process in Degrading Acid Blue 113

The performance of a fluidized bed Fenton process in degrading Acid Blue 113 (AB 113) was investigated. Fluidized bed Fenton process is a modification of conventional Fenton oxidation, aimed at reducing sludge generation and improving process performance. Response surface methodology was used to study the effects of operational parameter on the color removal from the dye. Dimensionless factors, Dye/Fe2+, H2O2/Fe2+ and pH were used as the independent variables in Box-Behnken Design (BDD). Reduced quadratic model was developed to predict the color removal. The process could remove up to 99 % of the initial color. The most significant factor for color removal was found to be Dye/Fe2+, followed by H2O2/Fe2+. Unlike conventional Fenton, the initial pH of the solution does not have a significant effect on the color removal.


Introduction
The environmental pollution arising from the use of synthetic dyes in textile and other industries has become a source of concern globally. Textile industry produces large quantity of dyes-containing effluents which are highly polluting, posing a threat to the environment [1]. Organic dyes have complex structures and high stability, making them difficult to degrade through the conventional methods [2]. Consequently, the treatment of dyes-containing effluents has attracted researchers' interest.
Various technologies such as adsorption [3], coagulation-flocculation [4], biological process [5], membrane technology [6] have been investigated for dye removal. These methods have some limitations in textile wastewater treatment. The physicochemical methods usually concentrate the pollutants into solid/liquid side streams [1] whereas biological process is relatively slow and usually convert the dye to some intermediates [7]. Thus, the search for effective treatment technologies for dye removal has been intensified.
Advanced oxidation processes (AOPs), which are based on the generation of reactive oxidants, are considered promising technologies for the degradation of recalcitrant dyes [8]. Among AOPs, Fenton oxidation has received wide attention due to its effectiveness, simplicity of the process and availability of Fenton's reagent [9]. The conventional Fenton oxidation is based on a catalytic reaction between Fe 2+ and H2O2. In a general form, the reaction can be written as shown in equation 1 [10].
Despite the effectiveness of Fenton oxidation in degrading recalcitrant pollutants, it faces some limitations which have hindered its large-scale applications. Fenton oxidation is pH dependent and favors a narrow range of operational pH (3 -4). This is because the production of HO, which are responsible for oxidizing the pollutant, is enhanced at lower pH [11]. Another limitation is the production of ferric sludge which requires further treatment and disposal [12]. Many approaches, such as heterogeneous Fenton and Fenton-like processes, have been taken to address these limitations. Recently, the use of fluidized bed technology to reduce sludge generation in conventional Fenton has received interest [13][14][15][16]. In fluidized bed Fenton (FBF), solids are used which provide surface for the precipitation of iron oxide and hence, lower sludge generation.
In this work, the performance of FBF process in the degradation of an azo dye, AB113 was investigated. Response Surface Methodology (RSM) was used to design the experiment and investigate the effects of operational parameters. RSM is a multivariate statistical analysis tool that can be used to design experiment, develop empirical models and study the effects of operational parameter [17]. Among the design approaches in the RSM, Box-Behken Design (BBD) is considered suitable as it has a low number of required experimental runs [18]. Thus, BBD was employed in this work to design the experiments and develop the RSM model for color removal from the Acid Blue 113.

Experimental setup
The schematic diagram of the FBF process is shown in Figure 1. It consists of a reservoir, recirculation section, magnetic pump and a flow meter. The reactor is a glass column of 135ml capacity and 30 cm height. The magnetic pump was used to pump the dye solution from the storage tank to the reactor at a flow rate of 0.21 mL/min. Glass beads were packed at the bottom of the reactor to a height of 4 cm to act as distributors. SiO2 was placed above the glass beads to a height of 6 cm. Internal recirculation was used to control the fluidization of the carriers. The desired amount of Fe 2+ was added into the system in form of FeSO4.7H2O. The Fenton process was initiated by the addition of H2O2. Samples were collected at pre-determined intervals and the pH of the sample was raised above 10 with NaOH to stop the reaction.

Analysis
The analysis was done according to the Standard Methods for the Examinations of Water and Wastewater [19]. Color removal from the dye solution was measure using a UV Spectrophotometer (Spectroquant ® Pharo 300, Merck, Germany). The color was measure as absorbance at 464 nm wavelength. The percent color removal is reported as the difference between the initial absorbance ( ) and absorbance at a time, t ( ) as shown in equation 2. (2)

Model fitting and ANOVA
The BBD suggested a total of 17 runs with 5 center points and 12 non-center points. Table 2 shows the experimental and predicted response based on the BDD. The highest experimental color removal (99.51 %) is slightly lower than the highest predicted color removal (100.53 %). However, it can be observed that experimental and predicted responses are nearly the equal.

Effects of operational parameters
One of the most challenging aspects of Fenton process is the selection and optimization of operational parameters. Thus, understanding the effects of operational parameters on the performance of FBF is necessary for process control. In this section, the effects of operational parameters on COD and color removals by the FBF process are discussed.
The effects of Dye/ Fe 2+ , H2O2/Fe and pH on the color removal were investigated. Figure 3 shows the 3D plots of the color removal as a function of (a) Dye/Fe and H2O2/Fe and (b) Dye/ Fe 2+ and pH. It can be seen that the color removal decreases as Dye/ Fe 2+ increases from 10 to 50 (Fig 3 a). This is because the amount of Fe decreases as the Dye/ Fe 2+ increases since the dye concentration was kept constant. Thus, there is insufficient Fe 2+ to catalyze the production of OH. Also, the color removal increases as the H2O2/Fe 2+ increases from 5 to 15. However, when the H2O2/ Fe 2+ was increased from 15 to 25, the color removal decreases, albeit slightly. This could be due to complexation causes by excess H2O2. These complexes can increase the color of the solution. On the other hand, the effect of solution pH is not very significant. From Figure 3 (b), it can be seen that the color removal did not change significantly when the pH was increased from 3 to 9. This could be due to the heterogeneous reaction induced by the iron crystallization. Figure 4 shows the perturbation plot for the color removal. The perturbation plots compare the effects of all factors at a particular point in the design space. A steep slope or curvature in a factor shows that the response is sensitive to that factor. A relatively flat line shows insensitivity to change in that particular factor. In this case, the perturbation is plotted at the center point of the design space. From figure 4, it can be seen that the color removal is most sensitive to Dye/ Fe 2+ , followed by H2O2/ Fe 2+ . On the other hand, the perturbation of pH is nearly flat, indicating that the color removal is not sensitive to the solution pH.

Conclusion
This study investigated the performance of FBF process in the degradation of a model dye, AB113. RSM was found to be a suitable tool for designing the FBF process and studying the effects of operational parameters. The FBF process could remove up to 99 % of the initial color of the dye. The color removal can be predicted by a reduced quadratic model developed through the BBD-RSM. The most significant parameter affecting the performance of the FBF process is Dye/Fe 2+ . The effect of solution pH on the color removal was found to be insignificant. This study shows that FBF process can be a promising technology for the degradation of textile effluents.