Designing and modeling of a novel electrolysis reactor using porous cathode to produce H2O2 as an oxidant

Graphical abstract

The entry of toxic organic pollutants and resistant to biodegradation has increased the concern about human health. The use of advanced oxidation (AO) processes to degrade these pollutants has been developing. One of the AO processes is based on the use of hydrogen peroxide in removing resistant organic pollutants.
This study aimed to develop a new reactor capable of producing H 2 O 2 in the solution. Therefore, a porous electrode made of stainless steel with the capability of air injection in the electrode center was used. The 30 cm rod graphite electrodes were also used as an anode electrode in a 4000 ml reactor. The effects of variables, including current density (30-40 mA/cm 2 ), time (10-30 min), and electrolyte concentration (12-17 mM/L) on the amount of H 2 O 2 production were evaluated by Box behenken design under response surface methodology using Design expert software. The results of this study showed that H 2 O 2 can be produced at the electrode surface of porous cathode under optimal conditions of 36 mA/cm 2 current density, 16 mM/l electrolyte concentration, in 23 min, and in the amount of 34 ppm. Using a porous cathode electrode causes the maximum contact among the solution, water, and air, and increases the production of H 2 O 2 .

Method details
Recently, human societies development and the use of various compounds have led to the presence of micro-pollutants in the environment, which, in addition to the difficult detection of contaminants, are sometimes resistant to bio-degradation [5][6][7][8][9]. On the other hand, using biological processes based on genetically modified bacteria and algae to eliminate toxic compounds is associated with challenges [10][11][12]. Therefore, the use of physicochemical methods in removing resistant pollutants has been accepted [13,14]. The use of peroxide is an AO process for the removal of resistant pollutants from the environment [15].
The EO process as a simple, economical, safe, and eco-friendly technology is capable of degrading compounds that are resistant to biodegradation by anodic direct and indirect oxidation [16,17]. Indirect oxidation in the EO process is due to the oxidizing compounds produced by the EO process, and these compounds can non-selectively oxidize organic compounds [18,19]. H 2 O 2 is used as an oxidant in oxidation processes and as a propellant for hydroxyl radical production in Fenton processes [20]. This compound can be produced at the cathode surface by the correct design of the reactor and it can oxidize organic compounds.
One of the major parameters affecting the type and amount of produced oxidants is the shape and type of electrode [21]. Using stable electrodes reduces costs; moreover, increasing the electrode surface increases the level of wastewater contact with electrodes, which results in more organic compounds degradation [22]. In order to increase the contact between the electrode and wastewater, the use of porous electrodes has been developed in wastewater treatment industry [23]. Furthermore, by changing the type of cathode electrode, in addition to increasing the active surface, it is possible to improve oxidizing compounds production, which is the purpose of designing and constructing the modified electrolysis reactor in this study.
Referring to Fig. 1 and the explanations provided, a better understanding of the EO reactor can be achieved. The components in Fig. 1 do not necessarily require scaling, and instead, the principles of the reactor are emphasized. Fig. 1 is a transverse perspective view of the reactor and its components. The technical map provided by various components of the system is presented as follows. In this map, the referenced numbers indicate the relevant parts in the technical map.
This reactor includes -1-an electrical current supply connected to -2-the rod graphite electrodes as an anode electrode and -3-a steel porous electrode as a cathode electrode by -4-the probe and interface cable. In the center of the steel porous electrode, there is -5-a porous air injection tube. The air is injected through -6-a blower pump to the cathode electrode center, which affects the H 2 O 2 production by the electrolysis process. The reactor contents are mixed by -7-a magnetic stirrer.
In this reactor, H 2 O 2 can be produced by reducing 2 electrons from oxygen (either purely or by injecting air into the solution) at the cathode surface under acidic or normal conditions (with E o = 0. 68 V/SHE) according to Eq. (1) [24].
Eq. (1) is simpler than reducing 4 electrons from water oxygen with E o = 1. 23 V/SHE [25]. The advanced oxidation (AO) process performed by the oxidants produced by the electrochemical process is called anodic oxidation with electro-generated H 2 O 2 [26]. In order to maintain the H 2 O 2 efficiency produced by the electrochemical method, oxygen, cathode, and water contact must be maintained at its maximum value. Therefore, using porous cathodes having a high specific surface is preferable to produce higher amounts of H 2 O 2 [27]. In this study, porous stainless steel was used as a cathode electrode. Porosity increases the contact between the electrode and the water. The aeration process was also used to increase the oxygen contact with water and the electrode by injecting air into the cathode electrode center. Using porous electrode and air injection into the electrode center maintains oxygen, cathode, and water contact at its maximum, resulting in high H 2 O 2 production.

Case study
The mentioned process was evaluated within a reactor with the following characteristics. The amount of produced H 2 O 2 was determined by Iodometric method and titration with sodium thiosulfate in accordance with Eqs. (2)-(5). Finally, according to the amount of thiosulfate consumed, it was measured by Eq. (6).
2 I À ! I 2 þ 2 e À ð3Þ 2 Na 2 S 2 O 3 þ I 2 ! Na 2 S 4 O 6 þ 2NaI ð4Þ The reactor used in this study was a 4000 ml batch reactor. In this reactor, two rod graphite electrodes of 30 cm in length and 0.8 cm in diameter as an anode electrode and a porous stainless steel electrode as a cathode electrode with an internal porous tube for air injection were placed. The input electrical current of the reactor was controlled by the DAZHENGPS-305D power supply. The mixing process was performed by a magnetic stirrer at a constant speed of 150 rpm. An aeration diffuser was used to inject 2 liters/min air into the center of porous steel electrode. Merck Company NaCl from Germany was used as electrolyte.
The amount of H 2 O 2 production was evaluated in the reactor at different levels of electrolyte concentrations, current intensity, and residence time (Table 1). Therefore, the number of experiments and optimal conditions for the three variables, including residence time, electrolyte concentration, and current density was determined using the Design Expert software (version 10) and Box Behnken Design. The range for each of the variables was performed at three levels. In order to analyze the experimental results, the response surface regression was used and optimal conditions for the H 2 O 2 production were presented by the Polynomial second order model according to Eq. (7) [28].
In this equation, Y is the amount of produced H 2 O 2 , Xi and Xj are coded variables, β0 is the model constant coefficient, βi, βii, and βij are linear, quadratic, and interaction coefficients, respectively.

Results and discussion
Based on the Box Behnken design, 17 runs was determined to examine the effect of the three variables, including residence time, electrolyte concentration, and current density.
Figs. 2-4 shows the amount of H 2 O 2 production based on current density and time, current density and electrolyte concentration, and time and electrolyte concentration, respectively. ANOVA analysis based on Quadratic Model showed that lack of fit was not significant. Moreover, the obtained values    According to Eq. (8), electrolyte concentration (A), current density (B) and time (C) coefficients are positive, indicating that these variables are directly related to H 2 O 2 production. The amount of real H 2 O 2 production and the predicted amount by the model, as well as the optimal predicted conditions, are presented in Table 2. Fig. 5 shows the actual and predicted amount of H 2 O 2 production.
Based on the results, production of hydrogen peroxide at the desired level by the designed reactor enables the reactor to produce hydrogen peroxide in aquatic environments and degrade organic compounds resistant to bio-degradation. Many studies have emphasized the use of peroxides in removing resistant organic compounds [15,29].

Concluding remarks
Developing a reactor that is capable of producing H 2 O 2 as a potential oxidizing agent in the aquatic environment through the electrochemical process was the main aim of this study. The results of this study showed that the use of porous stainless steel cathode as well as air injection to its center, causes the maximum contact surface of the electrode, air, and solution which produces significant amounts of H 2 O 2 .

Conflict of interest
The authors declare that they have no conflict of interests.