Degradation of aniline by the combined process of ultrasound and hydrogen peroxide (US/H2O2)

Graphical abstract


Protocol data
The presented data demonstrated that the combined ultrasonic (US) and hydrogen peroxide (H 2 O 2 ) process can be applied efficiently for the removal of aniline. Data on the degradation kinetics, and the effect of operating parameters were provided. The dataset will also serve as reference material to any researcher in this field.

Chemicals and materials
Aniline (C 6 H 5 NH 2 ) of purity: 99.5% with a molecular weight: 93.13 g/mol and maximum adsorption (l max ): 280 nm, and 30% w/w hydrogen peroxide (H 2 O 2 ) were purchased from Sigma-Aldrich Chemical Company (USA).A stock solution of aniline (concentration of 1000 mg/L) was prepared by dissolving the required amount in 1 L of deionized water. Other aniline concentrations used in the study were prepared from the stock solution. All chemicals used in this study were of analytical grade.

Pilot ultrasonic
Reactor of determined surface including a digital ultrasonic appliance that is made of Plexiglas with a volume of 3.7 L, input energy per unit of 2.5 W/cm 2 , and input power of 500 W including 100 mL of the water samples in the bath with ultrasonic (US) waves. The schematic illustration of the sonochemical process is shown in Fig. 1.

Data analysis
The batch experiments were carried out to optimize aniline removal under different pH (3, 5, 7, 9 and 11), contact time (15,30,45,60, and 90 min), H 2 O 2 concentration (0.01, 0.04, and 0.07 mol/L), and initial aniline concentration (20, 40, 60, 80, and 120 mg/L). The pH of the aniline solution was adjusted by adding 0.1 mol/L hydrochloric acid (HCl) or 0.1 mol/L sodium hydroxide (NaOH) solutions. The pH was measured through a MIT65 pH meter. Each experiment involves preparing a 100 mL of aniline solution with a desired initial aniline concentration and varying the pH and H 2 O 2 concentration. The sample was transferred into an ultrasonic bath (ElMA-Germany) generating supersonic waves at 240 W and 50 Hz. The samples were treated in the US bath at different contact time and the residual concentrations were measured afterward.
Aniline concentration was determined using a UV-vis spectrophotometer (Shimadzu Model: CE-1021-UK). The removal efficiency, R (%) was calculated based on the following formula [1][2][3]: Where C 0 and C f represent the initial and final (after the degradation process) aniline concentrations, respectively.

Effect of pH
One of the most important parameters to be investigated in a chemical process is the pH of the reaction medium (Fig. 2). It affects the characteristics of the contaminant, the extent of decomposition of the organic matter and the efficiency of the degradation process. In order to determine the optimal pH, 100 mL samples were prepared and subjected to treatment in the chamber of the ultra-sonication On the other hand, molecular ionization of aniline was maximized in the acidic pH range due to the increased electrostatic attraction between the anionic and cationic species. At high pH values, OH À contributes to the formation of dissolved compounds in the form of water [4], thereby hindering the adsorption process. Moreover, in an acidic media, the available amount (that is, stability) of OH is somewhat larger than that in a basic media, hence oxidation processes give better results in acidic media rather than basic ones, which is in agreement with the report by Zarrabi et al. [5].

Effect of initial aniline concentration
In order to determine the optimum initial aniline concentration, the H 2 O 2 concentration and pH were kept constant (Fig. 3). Since the number of hydroxyl radicals produced was constant, the rate of decomposition was reduced with higher aniline concentration, thereby lowering the removal  efficiency [6], while at higher aniline concentrations (since H 2 O 2 concentration and ultrasonic waves were unchanged), the ultrasonic waves did not reach the surfaces of all particles, reducing the extent to which the particles were oxidized [7]. In most studies on the oxidation of organic compounds, an increase in the concentration of the considered contaminant has been associated with a reduction on the removal efficiency [8].

Effect of H 2 O 2 concentration
Effect of H 2 O 2 concentration on aniline removal was investigated at the optimum aniline concentration (20 mg/L) and the optimal pH of 3 (Fig. 4)  acted as an interfering agent and reacted with hydroxyl radicals, OH in the aqueous medium, inhibiting their attack to the contaminant molecules [12]. The higher the number of hydroxyl radicals, the higher the rate of decomposition and oxidation of the organic matter, and given that the number of radicals obtained from H 2 O 2 of the hybrid system is higher than that of an isolated sonication system; the rate of oxidation will be higher in the hybrid system rather than the isolated sonication system [11].

Effect of contact time
Higher removal efficiency was obtained with increasing contact time (while keeping all other conditions constant) (Fig. 5). Maximum removal efficiency was attained at a contact time of 45 min. Indeed, with lengthening the contact time, more hydroxyl free radicals were produced and contributed to the oxidation of the aniline molecules, thereby lowering the aniline concentration. Another explanation for the high rate of removal in the stage was the high concentration of aniline during this period, which enhanced the collisions between the aniline molecules and hydroxyl free radicals, thereby eliminating a larger amount of aniline. However, as time passed, aniline concentration became lower, so that the hydroxyl free radicals in the pilot were used to oxidize the aniline metabolites, lowering the removal rate [13,14].

Degradation kinetics
The kinetic studies were carried out by taking decolorization into consideration under the optimum conditions for the process. To determine the characteristics of m and b, Eq. (8) was applied [15]: Where C 0 is the initial concentration of aniline (mg/L); C is the aniline concentration at time, t (mg/L); m and b are the two dimensionless characteristic constants of the model relating to the initial removal rate and maximum oxidation capacities, respectively. A straight line was obtained by plotting t/ (1 À C/C 0 ) against contact time, t (Fig. 6), where m and b were obtained from the slope and intercept of the straight line, respectively. According to the obtained results (Table 1 and Fig. 6), this study is more compatible with the degradation equation. The correlation coefficient (R 2 ) of the degradation of aniline was high (R 2 = 0.9997). The loss of aniline was observed as a function of irradiation time and the experimental data were fitted to a pseudo-first-order rate model according to the following equation [15,16]: Where C o denotes the initial concentration in milligrams per liter, and C is the concentration value in milligrams per liter at time, t. The slope of the plot of Ln c c 0 versus time (Fig. 7) gives the value of the rate constant, k 1 (min À1 ). The process follows the pseudo-first-ordermodel (R 2 = 0.857) ( Table 1). Table 1 The coefficients of determination and the characteristic constants of the kinetic model.