### 1. Introduction

_{2}O

_{2}/Fe

^{2+}and H

_{2}O

_{2}/O

_{3}), homogeneous and/or heterogeneous phase photocatalysis (H

_{2}O

_{2}/UV, O

_{3}/UV, Fe

^{2+}/H

_{2}O

_{2}/UV and TiO

_{2}/UV), sonochemical oxidation and electrochemical oxidation, allowing more reactive oxidants to be generated under softer conditions than conventional oxidants. These powerful oxidants designated by highly reactive radicals which include the oxygen superoxide radical ( ${\text{O}}_{2}^{\u2022-}$), the hydroperoxyl ion radical ( ${\text{HO}}_{2}^{\u2022-}$) and especially the hydroxyl radical (HO

^{•}) which, when compared to the others, has a greater non-selective reactivity making it responsible for the destruction of organic pollutants and competitive surrounding anionic species. The production rate of hydroxyl radicals depends mainly on various parameters, including the characteristics of the UV lamps (emission spectrum, power) and the physico-chemical properties of the medium (pH, turbidity, H

_{2}O

_{2}concentration, among others).

_{3}/UV system, to affect the mineralizing activity and to form by-products such as bromates and trihalomethanes [12]. The global equations of the reactions concerning the formation of hydroxyl radicals from the photolysis of H

_{2}O

_{2}(Eq. (4)), the action of bromides on HO

^{•}[13] (Eq. (5)), the reduction of ozone by hypobromite ions (Eq. (6)) resulting from the oxidation of bromides by O

_{3}[14] (Eq. (7)) and the inter-species reactions discussed previously are summarized as follows:

##### (3)

$${\text{HOC}}_{3}^{-}+{\text{HO}}^{\u2022}\to {\text{CO}}_{3}^{\u2022-}+{\text{H}}_{2}\text{O}$$##### (4)

$${\text{H}}_{2}{\text{O}}_{2}\stackrel{\text{h}v}{\to}2{\text{HO}}^{\u2022}\mathrm{\hspace{0.17em}\u200a\u200a}\mathrm{\hspace{0.17em}\u200a\u200a}\mathrm{\hspace{0.17em}\u200a\u200a}5.5\times {10}^{9}\hspace{0.17em}{\text{mol}}^{-1}\hspace{0.17em}\text{L\hspace{0.17em}}{\text{s}}^{-1}$$^{−}are rapidly reduced by the hydrogen peroxide present in the H

_{2}O

_{2}/UV system, in order to prevent significant formation of brominated species like bromides (Eq. (8) and Eq. (9)) [14]. Furthmore, chlorides are present in large quantities in a marine environment (pH > 8), they are reactive towards HO

^{•}giving rise to an anion radical ClOH

^{•}(Eq. (10)) whose transformation would depend on the pH of the medium (Eq. (11) and Eq. (12)) [15]. The global equations are given bellow:

##### (8)

$${\text{BrO}}^{-}+{\text{H}}_{2}{\text{O}}_{2}\to {\text{Br}}^{-}+{\text{O}}_{2}+{\text{H}}_{2}\text{O}$$##### (11)

$${\text{HOCl}}^{\u2022-}\to {\text{Cl}}^{-}+{\text{HO}}^{\u2022}\mathrm{\hspace{0.17em}\u200a\u200a}\mathrm{\hspace{0.17em}\u200a\u200a}\mathrm{\hspace{0.17em}\u200a\u200a}\text{at\hspace{0.17em}pH}>7.2$$##### (12)

$${\text{HOCl}}^{\u2022-}+{\text{H}}^{+}\to {\text{Cl}}^{\u2022}+{\text{H}}_{2}\text{O\hspace{0.28em}}\mathrm{\hspace{0.17em}\u200a\u200a}\mathrm{\hspace{0.17em}\u200a\u200a}\text{at\hspace{0.17em}pH}<7.2$$_{2}O

_{2}process on PS photocatalytic removal in artificial seawater. The factors retained and which affect the phenomena were the PS concentration, the hydrogen peroxide dose and the irradiated volume. The experiments are planned using the central composite design under the response surface methodology to determine the main effects of the above factors with their interactions and to achieve the combination of the optimum operating conditions.

### 2. Materials and Methods

### 2.1. Materials

_{22}H

_{12}N

_{4}Na

_{4}O

_{13}S

_{4}; molecular weight: 760.6 g/mol) was obtained from REACTIFS RAL. The hydrogen peroxide (H

_{2}O

_{2}, 50%) was manufactured by PROCHILABO. The ultrapure water with a resistivity of 0.055, obtained using a (VWR PURANITY TU), was used to prepare all stock and working solutions.

### 2.2. Methods

#### 2.2.1. Preparation of the artificial seawater

_{3}, CaSO

_{4}, SrCO

_{3}, or SrSO

_{4}, the chloride solution (Table 1. B) and the first solution (Table 1. A) were prepared in two separate containers, after the two solutions were thoroughly mixed, they were combined while stirring.

#### 2.2.2. Photocatalytic oxidation experiments

_{2}O

_{2}were expressed as function of the irradiated volume (mL). The reactor was operated with the working volume ranging from 197.73 to 702.27 mL. All the experiments were carried out in an aerated cylindrical reactor with 1L capacity in which we immersed a double walled quartz sleeve containing a high-pressure mercury lamp (250 W, manufactured by Ingelec). The continuous circulation of water through an internal cooling tube in the sleeve was used to keep a constant temperature in the treated solution. The temperature during the experiment was equal to 23°C. The solutions were stirred at the same agitation speed with a magnetic stirrer placed at the reactor base. The lamp has been turned on to initiate the reaction after adding the hydrogen peroxide.

##### (13)

$$\%\text{Decolorization\hspace{0.17em}efficiency}=\left(\frac{{\text{A}}_{0}-{\text{A}}_{\text{t}}}{{\text{A}}_{0}}\right)*100$$_{0}is the initial absorption of PS, and A

_{t}is the absorption of PS at reaction time.

#### 2.2.3. Experimental design

_{2}O

_{2}process. The CCD was performed by the Design Expert 7.0 (DX) software to design the trials and analyze the data. The number of experiments required is defined by the Eq. (14) [17].

##### (15)

$$\text{Y}={\text{b}}_{0}+{\mathrm{\Sigma}}_{\text{i}=1}^{\text{n}}{\text{b}}_{\text{i}}{\text{x}}_{\text{i}}+{\mathrm{\Sigma}}_{\text{i}=1}^{\text{n}-1}{\mathrm{\Sigma}}_{\text{j}=\text{i}+1}^{\text{n}}{\text{b}}_{\text{ij}}{\text{x}}_{\text{i}}{\text{x}}_{\text{j}}+{\mathrm{\Sigma}}_{\text{i}=1}^{\text{n}}{\text{b}}_{\text{ii}}{{\text{x}}_{\text{i}}}^{2}$$_{0}is the constant coefficient, b

_{ii}is the quadratic coefficient, bij is the interaction coefficient and xi, xj are the coded values of the factors.

### 3. Results and Discussions

### 3.1. Model Fitting and Analysis of Variance (ANOVA)

#### 3.1.1. Mathematical model equation

##### (16)

$$\begin{array}{c}\text{Decolorization\hspace{0.17em}efficiency\hspace{0.17em}}(\%)=+62.80+7.85\text{A}+2.52\text{B}\\ -3.00\text{C}-0.090\text{AB}-0.042\text{AC}-0.042\text{BC}\\ -2.02{\text{A}}^{2}+0.71{\text{B}}^{2}-2.21{\text{C}}^{2}\end{array}$$^{2}and adjusted-R

^{2}. In the reported results, R

^{2}and adjusted-R

^{2}were equal to 0.9612 and 0.9266 respectively. The R

^{2}of 0.9612 means that the selected model can describe 96.12 % of the response data. In addition, the predicted-R

^{2}of 0.7543 completely agree with the adjusted-R

^{2}that indicates high correlation between the experimental results and the predicted model.

#### 3.1.2. Analysis of variance (ANOVA)

^{2}, C

^{2}are the significant model terms. All the interaction effects (AB, AC and BC), between the studied factors were not significant. The negative effect of the PS concentration on the PS decolorization efficiency makes his interaction with the other factors non-significant. The low interaction between the H

_{2}O

_{2}concentration and the irradiated volume could be justified by the complex composition of seawater in several ions that act as scavengers of active hydroxyl radicals, such as chlorides, sulphates, carbonates and bromides, thereby reducing the degradation efficiency of PS in the seawater matrix by the UV/H

_{2}O

_{2}process.

### 3.2. Model Accuracy Check

### 3.3. Response Analysis

_{2}O

_{2}concentration and the irradiated volume. In other words, increasing the H

_{2}O

_{2}concentration from 1.46 mM to 2.24 mM and the irradiated volume from 300 mL to 600 mL has a positive effect on the PS decolorization efficiency.

_{2}O

_{2}. Hence, the formation of plentiful hydroxyl radicals in the treated solution. Similarly, the effect of increasing hydrogen peroxide concentration is positive for efficient PS decolorization. That can be attributed to generation of more hydroxyl radicals since the H

_{2}O

_{2}concentration increased. However, increasing the PS concentration despite increasing both the hydrogen peroxide concentration and the irradiated volume decreases the PS decolorization efficiency as appears in Fig. 3(b) and 3(c). At a high PS concentration, the dye may inhibit light penetration and minimize the photolysis.

### 3.4. Process Optimization

##### (17)

$$\begin{array}{c}\text{Decolorization\hspace{0.17em}efficiency\hspace{0.17em}}(\%)=+63.78+7.65\text{A}+2.73\text{B}\\ -2.84\text{C}+1.43\text{AC}-1.87{\text{A}}^{2}-2.59{\text{C}}^{2}\end{array}$$_{2}O

_{2}process. All the statistical indicators were improved, namely: The R

^{2}that was increased slightly to 0.9654 and the predicted-R

^{2}of 0.8155 that still in reasonable agreement with the adjusted-R

^{2}of 0.9294.

### 4. Conclusions

_{2}O

_{2}process using to decolorize the PS in the artificial seawater. The quadratic equation of the PS decolorization efficiency was evaluated as function of three factors, as follows: the hydrogen peroxide concentration, the irradiated volume and the PS concentration. The coefficient of regression R

^{2}for the considered equation was equal to 0.9612, showing a good agreement between the independent factors and the output data. The process variables were optimized by using RSM, the hydrogen peroxide of 2.24 mM, the irradiated volume of 600 mL and the PS concentration of 50.91 mg/L exhibited a maximum PS decolorization of 72.47%. The H

_{2}O

_{2}concentration was determined as the most positive factor affecting the response with a coefficient of 7.65 followed by the irradiated volume. Nevertheless, the individual effect of the PS concentration has a negative effect on the PS decolorization efficiency.