Phenol removal by electro-Fenton process using a 3D electrode with iron foam as particles and carbon fibre modified with graphene http://dx.doi.org/10.5599/jese.1806 537 J. Electrochem. Sci. Eng. 13(3) (2023) 537-551; http://dx.doi.org/10.5599/jese.1806 Open Access : : ISSN 1847-9286 www.jESE-online.org Original scientific paper Phenol removal by electro-Fenton process using a 3D electrode with iron foam as particles and carbon fibre modified with graphene Hind H. Thwaini and Rasha H. Salman Department of Chemical Engineering, College of Engineering, University of Baghdad, Baghdad, Iraq Corresponding authors: hind.abd2107m@coeng.uobaghdad.edu.iq; Tel.: +964-772-453-0704 Received: April 3, 2023; Accepted: June 2, 2023; Published: June 21, 2023 Abstract The 3D electro-Fenton technique is, due to its high efficiency, one of the technologies suggested to eliminate organic pollutants in wastewater. The type of particle electrode used in the 3D electro-Fenton process is one of the most crucial variables because of its effect on the formation of reactive species and the source of iron ions. The electrolytic cell in the current study consisted of graphite as an anode, carbon fiber (CF) modified with graphene as a cathode, and iron foam particles as a third electrode. A response surface methodology (RSM) approach was used to optimize the 3D electro-Fenton process. The RSM results revealed that the quadratic model has a high R2 of 99.05 %. At 4 g L-1 iron foam particles, time of 5 h, and 1 g of graphene, the maximum efficiency of phenol removal of 92.58 % and chemical oxygen demand (COD) of 89.33 % were achieved with 32.976 kWh kg-1 phenol of consumed power. Based on the analysis of variance (ANOVA) results, the time has the highest impact on phenol removal efficiency, followed by iron foam and graphene dosage. In the present study, the 3D electro-Fenton technique with iron foam partials and carbon fiber modified with graphene was detected as a great choice for removing phenol from aqueous solutions due to its high efficiency, formation of highly reactive species, with excellent iron ions source electrode. Keywords Wastewater treatment; organic pollutants; removal efficiency; 3D-electrode system; iron foam; response surface methodology; analysis of variance Introduction Phenol is one of the most significant bio-recalcitrant pollutants found in numerous chemical and biochemical industries. It is created during the operation of manufacturing facilities for resins, coke, pharmaceuticals, olive groves, and other food-related businesses [1,2]. Pesticides, herbicides, dye molecules, phenolic compounds, antibiotics, medicines, and surfactants are a few examples of organic pollutants that are typically exceedingly difficult to break down [3]. Phenolic compounds are http://dx.doi.org/10.5599/jese.1806 http://dx.doi.org/10.5599/jese.1806 http://www.jese-online.org/ mailto:hind.abd2107m@coeng.uobaghdad.edu.iq J. Electrochem. Sci. Eng. 13(3) (2023) 537-551 PHENOL REMOVAL BY ELECTRO-FENTON PROCESS 538 among the substances that should be the most concerned due to their tendency to persist in the environment for long periods and their potentially dangerous effects [4]. The presence of phenol in drinking water and irrigation poses substantial health hazards to people as a possible carcinogen, even at low dosages [5]. The health of people and all other living things are highly impacted by water quality. Therefore, surface and groundwater contamination by harmful or persistent contaminants became a significant issue [6,7]. Several aromatic and aliphatic intermediates that are more toxic than phenol are produced during the electrochemical oxidation of phenol [8]. Since it is a clean process that produces high concentrations of hydroxyl radicals (OH•) which can be used to oxidize, degrade, and mineralize a variety of organic compounds, the electro-Fenton process has been identified as a particularly attractive technology. It is a promising and emerging advanced oxidation process (AOPs) [9,10]. The use of transition metals as heterogeneous catalysts will prevent the production of iron sludge and increase the range of potential catalysts that may be employed in AOPs [11]. The proposed heterogeneous electro-Fenton system exhibited high efficiency, stability and economy [12]. The electro-Fenton method has many advantages; it is an appealing technology for wastewater treatment because it does not create secondary pollutants. The power used is clean and pollution- free, it uses no hazardous chemicals, which make this method to be an environmentally benign procedure [13,14]. Electro-Fenton's fundamental idea is summarized in equation (1), where the oxidation of Fe2+ to Fe3+ makes it easier for H2O2 to become the highly oxidizing OH•. The OH• radical is a strong, non-selective oxidant that takes part in the degradation of organic contaminants [15]. Fe2+ and H2O2 are necessary for OH• to be continuously produced [16]. If Fe3+ ions do not form hydroxide precipitates in the solution phase, the Fe2+ ions can be created by reducing the Fe3+ at the cathode (equation (2)), providing a nearly constant supply of Fe2+. The H2O2 is produced by the two- electron reduction of dissolved oxygen, as shown in equation (3) [17]. The rate constant (k) for each reaction is illustrated as follows [18]: Fe2+ + H2O2 + H+ → Fe3+ + OH• + H2O; k = 55.0 M-1 s-1 (1) Fe3+ + e- → Fe2+; k = 0.1 s-1 (2) O2 + 2H+ + 2e- → H2O2; k = 5.9 nM s-1 (3) Due to the comparatively easy handling of the reagents needed for this process and the efficient performance that can be obtained at a low cost, the electro-Fenton process has been widely used in wastewater treatment [19]. Furthermore, no hazards are associated with the handling, storage, or transportation of H2O2 [20]. The electro-Fenton process is affected by several factors, including the initial contaminant concentration, pH, current intensity, and reagent dosage [21]. Carbonaceous electrodes are often employed as anodes in wastewater treatment due to their large surface area per volume [22]. To enhance the efficiency of electrochemical processes for eliminating organic pollutants, the selection of cathode material is considered an essential factor. The good characteristics of carbon fiber (CF), like high specific surface and low cost, make it a good choice as a cathode for H2O2 generation. The modification of carbon electrodes with graphene was confirmed to exhibit better performance in producing H2O2 [23]. The modified electrode demonstrated higher electron-transfer ability than the raw carbon felt electrode since the redox current and charge-transfer resistance are increased [24]. Among sophisticated oxidation methods, the three-dimensional (3D) electrode reactor has received attention for efficiently degrading organic pollutants due to its high efficiency, ease of operation, and environmental compatibility, among other factors [25]. In comparison to standard two-dimensional (2D) electrode technology, the electrocatalytic property of 3D particle electrodes H. H. Thwaini and R. H. Salman J. Electrochem. Sci. Eng. 13(3) (2023) 537-551 http://dx.doi.org/10.5599/jese.1806 539 is the core technology of the 3D electrode reactor [26]. In terms of current efficiency, energy consumption (EC), and pollutant removal efficiency, the 3D electrode technology outperforms the 2D electrode technology. The overall 3D electrode system offers a large effective contact area as well as a large number of micro-electrolysis cells, and the electrochemical reaction takes place not only on the surface of the main electrode but also on the surface of each particle electrode [27,28]. On the other hand, using iron foam has two benefits. Iron foam functions as a 3D particle to offer active sites and increase reaction efficiency and supplies the reaction with Fe2+, which reacts with H2O2 to create OH• that oxidizes phenol [29]. The goal of the current study is to detect the enhancement of phenol removal efficiency by utilizing CF modified with graphene as cathode and iron foam as the third electrode in a 3D electro- Fenton system and optimizing results using Box-Behnken design. Experimental Materials All aqueous solutions were prepared using distilled water. All reagents were of analytical grade and no additional purification was needed. The chemicals were: phenol crystals (C6H5OH, 99.5 % purity, Alpha Chemical Reagent Company, India), SDFCL or ferrous sulfate heptahydrate FeSO4.7H2O (CDH Company, India), sodium sulfates (Na2SO4, purity ≥99.0 %, SDFCL), and sulfuric acid (H2SO4, with a concentration of 98 %, Sigma-Aldrich). CF (99.5 % Carbon content, China) was used as a cathode that has a 0.111 mm thickness and a 12K yarn size purchased from Jiaxing ACG composites Co. Ltd. (China) and iron foam was purchased from Xiamen Top new energy Technology (China) with porosity of 110 (pores per cm) Experimental methods The electro-Fenton reaction was conducted in 1 L cylindrical beaker. The batch reactor contained 150 mg L-1 of a phenol solution and three electrodes that were fully submerged in the solution. A cathode was made of carbon fiber felt modified with graphene (186 cm), and the anode was a graphite plate (1860.5 cm). Iron foam particles (111 cm) served as a third electrode and the distance between anode and cathode was 3 cm. The iron foam was washed with 0.1 M H2SO4 and deionized water in sequence to remove the oxide layer on its surface. To enhance the amount of dissolved oxygen, the solution was aerated with (ACO-001 electromagnetic, China) for 20 minutes before the reaction and continued until the completion of the experiment at a rate of 10 L h-1. Before each experiment, a pH value of 3 was achieved by adding 0.1 M H2SO4 acid to the solution, and the pH value was measured with a pH meter of HANNA (Romania). To achieve the desired concentration of iron ions, they were released in a specific amount from iron foam particles into the solution. To keep the ionic strength constant and increase the medium conductivity, 0.05 M of Na2SO4 was added to the solution. Electrolytes improve the effectiveness of electrodes and processes, such as increasing the conversion of oxygen to hydrogen peroxide at the cathode surface due to an acceleration in the speed of ion exchange and the electrostatic resistance between electrodes and ions [30]. The electrodes were connected to a DC power supply type (UNI-T, UTP3315PE) which operated at a constant current density of 4 mA cm-2. A digital voltmeter (FLOWTECH, China) was used to detect voltage and current values. Before the pH correction, a first sample of 10 ml of aqueous solution with phenol was obtained. Before analysis, periodic samples of treated solution were filtered through 0.45 µm filter papers to eliminate any suspended contaminants. The temperature throughout the 3D electro-Fenton process was kept constant at 27 ±1 °C. Samples http://dx.doi.org/10.5599/jese.1806 J. Electrochem. Sci. Eng. 13(3) (2023) 537-551 PHENOL REMOVAL BY ELECTRO-FENTON PROCESS 540 were taken and analyzed to determine COD and phenol concentration by the RD125, Lovibond, and UV-9200 spectrometers, respectively. A schematic drawing of the 3D electro-Fenton system is shown in Figure 1. The efficiency of phenol removal was evaluated using equation (4) [31]. REphenol = [(C1-C2)/C1]100 (4) where REphenol / % is phenol removal efficiency, C1 and C2 / mg L-1 represent the original and final phenol concentrations, respectively. The energy consumption (EC / kWh kg-1 phenol) that defines the amount of energy required to digest one kilogram of phenol was estimated by equation (5) [31]: EC = 1000IUt/ΔCphenolV (5) where I / A is the operating current intensity, U is voltage, V / L is the volume of the solution, t / h is the electrolysis time, and ΔCphenol / mg L-1 is the difference in experimental phenol concentrations. Figure 1. Schematic drawing of the electro-Fenton system: 1) power supply, 2) multimeter, 3) magnetic stirrer, 4) iron particles, 5) cathode, 6) anode, 7) flow meter and 8) pump Carbon fiber modification Before each process, the commercial CF used as one of the working electrodes (cathode) was cut into an 186 cm2 rectangle piece. This CF piece was activated for 30 minutes at 80 °C with 5 % HNO3, cleaned, and stored in distilled water. The CF cathode was then modified using a certain amount of graphene. The slurry was created by combining 0.14 ml of polytetrafluoroethylene (PTFE), 3 ml of ethanol, and 2 ml of deionized water. Using a brush, this mixture was applied to the two sides of the CF. The CF was then allowed to dry at ambient temperature before being calcined for 30 minutes at 360 °C [32,33]. Characterization of electrodes Graphite and CF modified with graphene were used as the anode and cathode, respectively. The crystallographic characterization of the graphite, graphene, pure CF, and modified CF was examined by X-ray diffraction (XRD) as shown in Figure 2. The XRD device characteristics were Model, XRD 6000, and Shimadzu, Japan. A scan speed of 5 degrees per minute was used with a 40 kV voltage and 30 mA current for the X-ray tube. A scanning electron microscope (SEM) was employed to investigate the electrodes surfaces as shown in Figure 3. Energy dispersive X-ray spectroscopy (EDX) H. H. Thwaini and R. H. Salman J. Electrochem. Sci. Eng. 13(3) (2023) 537-551 http://dx.doi.org/10.5599/jese.1806 541 data was also measured at a voltage of 25 kV for iron foam before and after the reaction, as shown in Figure 4. Figure 2 demonstrates that CF modified with graphene structure can be identified as the source of the sharp diffraction peak visible at 2 of 26.99°, corresponding to the diffraction line C (002). There is a weaker abrupt peak at 2  of 18.81° related to the PTFE as it is used in the modification process. The pure graphite shows a sharp and tight peak at 26.5°, corresponding to the diffraction line C (002) [34]. a b c d Figure 2. XRD patterns of a - graphite, b - pure CF, c - graphene and d - CF modified with graphene By utilizing SEM, the morphology of graphene on the CF was revealed. Figure 3 displays the SEM images of the pure and modified cathode. The graphene flakes were thin sheets, and they were all aligned on the CF surface. Figure 3a illustrates the CF without modification, while Figure 3b shows the transparent layer of graphene developed on the CF. A definite penetration of graphene between the CF threads can be detected in Figures (3a and b) which indicates an increase in the probability of solution mixture penetration between the CF threads. This penetration of graphene between the CF threads increases the surface area, enhances electrode performance, and produces more H2O2. Figures 3c and d show the structure of the third electrode (the iron foam). 0 10 20 30 40 50 60 0 2000 4000 6000 8000 10000 12000 In te n s it y 2 /° 0 10 20 30 40 50 60 0 200 400 600 800 1000 In te n s it y 2 /° 0 10 20 30 40 50 60 -200 0 200 400 600 800 1000 1200 1400 1600 In te n s it y 2 /° 0 10 20 30 40 50 60 0 200 400 600 800 1000 1200 1400 In te n s it y 2 /° http://dx.doi.org/10.5599/jese.1806 J. Electrochem. Sci. Eng. 13(3) (2023) 537-551 PHENOL REMOVAL BY ELECTRO-FENTON PROCESS 542 Figure 3. SEM images of: a - CF before modification, b to d - CF after modification with graphene and iron foam before the electro-Fenton process Design of experiment With the aid of mathematical and statistical data collection techniques, including response surface methodology (RSM), it is feasible to ascertain the relationship between a process response and its contributing elements [35]. In order to detect the best conditions for higher phenol removal efficiency in the 3D E-Fenton process, a Box-Behnken design (BBD) was employed. Using preliminary data, this study used three main independent variables and three levels. Iron particles, graphene dosage, and electrolysis time were selected as variables in the performed experiments. Table 1 displays the experimental range and levels of independent variables. Table 2 displays the Box- Behnken design. Table 1. Variables and different levels of experimental design Independent variable levels -1 0 1 X1 - mass of iron particles, g 2 3 4 X2 - mass of graphene, g 0.5 0.75 1 X3 - time, h 3 4 5 The empirical quadratic polynomial model depicted by equation (6) can represent the mathematical relationship between independent factors and response [36,37]. Y = a0 + aixi +  aii xi2 + aij xixj (6) H. H. Thwaini and R. H. Salman J. Electrochem. Sci. Eng. 13(3) (2023) 537-551 http://dx.doi.org/10.5599/jese.1806 543 Table 2. Design of experiments with the level of each factor using Box-Behnken designs, Bk. = 1 Run Coded values Real values x1 x2 x3 Mass of iron particles, g X1 Mass of graphene, g X2 Time, h X3 1 -1 1 0 2 1.00 4 2 1 0 -1 4 0.75 3 3 0 -1 -1 3 0.50 3 4 -1 0 1 2 0.75 5 5 0 -1 1 3 0.50 5 6 0 1 1 3 1.00 5 7 1 1 0 4 1.00 4 8 -1 0 -1 2 0.75 3 9 0 0 0 3 0.75 4 10 0 1 -1 3 1.00 3 11 0 0 0 3 0.75 4 12 1 0 1 4 0.75 5 13 0 0 0 3 0.75 4 14 1 -1 0 4 0.50 4 15 -1 -1 0 2 0.50 4 Where Y represents the response (REphenol), i and j are the index numbers for independent variables, 𝑎0 is the intercept term, x1, x2 … xk are the process variables (independent variables) in coded form. ai is the first-order (linear) main effect, aii is the second-order main effect, and aij is the interaction effect [36,37]. The Minitab-18 software was used to examine the results of phenol removal efficiency. Iron foam characterization Figure 4a and b shows the surface compositions of the iron before and after the reaction which were analyzed by EDX. The results showed that the highest elemental peak is for the Fe element with a content of 72.32% before the reaction, and there are other peaks for other elements with (Co of 24.67 wt.% and C of 4.01 wt.%). The results show a reduction in the Fe element to 69.32 wt.% after the reaction due to the release of iron ions needed for the reaction. Figure 4. EDX results for iron foam: (a) before reaction, (b) after reaction Iron foam is characterized by high surface porosity, where fluid can move easily through the connections between porous structures, and it plays a vital role in the 3D electro-Fenton system as a source of iron ions [29]. Based on the images of SEM for iron foam after the reaction, as illustrated in Figures 5a and 5b, the structure was varied due to the iron ions releasing. http://dx.doi.org/10.5599/jese.1806 J. Electrochem. Sci. Eng. 13(3) (2023) 537-551 PHENOL REMOVAL BY ELECTRO-FENTON PROCESS 544 Figure 5. SEM images of iron foam after reaction Results and discussion Statistical analysis The following quadratic model of the phenol removal efficiency in terms of real values of process parameters was obtained by evaluating the phenol removal effectiveness by the Minitab-18 program. REphenol = -39.3 + 8.10X1 + 18.0X2 + 27.19X3 - 0.960(X1)2+ 20.1(X2)2- 2.940(X3)2 - - 9.90X1X2 + 2.794X1X3- 0.40X2X3 (7) Table 3 displays the experimental phenol removal efficiency (REphenol) and energy consumption. Table 3. Experimental results for phenol removal efficiency and power consumed obtained using the Box-Behnken design, Bk. = 1 Run Mass, g Time, h REphenol / % Voltage, V EC / kWh kg-1 phenol Iron particles Graphene Actual Predicted 1 2 1.00 4 75.460 73.8532 3.98 31.012 2 4 0.75 3 62.290 60.5942 4.07 28.813 3 3 0.50 3 55.110 55.1990 4.25 34.009 4 2 0.75 5 70.216 71.9117 3.98 41.661 5 3 0.50 5 79.820 78.8905 4.25 39.134 6 3 1.00 5 87.220 87.1310 4.06 34.911 7 4 1.00 4 80.312 81.0782 4.09 29.944 8 2 0.75 3 53.330 54.0072 3.95 32.682 9 3 0.75 4 72.310 72.9467 4.25 43.559 10 3 1.00 3 62.910 63.8395 4.00 28.040 11 3 0.75 4 72.820 72.9467 4.25 34.317 12 4 0.75 5 90.350 89.6727 4.11 33.344 13 3 0.75 4 73.710 72.9467 4.23 33.743 14 4 0.50 4 75.980 77.5867 4.05 31.342 15 2 0.50 4 61.230 60.4637 3.94 37.845 The results show that the efficiency of phenol removal is in the range of 53.33 to 90.35 % and the specific energy consumption is in the range of 28.040 to 41.661 kWh kg-1 phenol. The impact of time on phenol removal was superior, as shown by the comparison of runs 2 and 12. Thus, at the constant graphene dosage of 0.75 g and iron particles of 4 g, phenol removal increased from 62.29 to 90.35 %, making a difference of 28.06 % as time increased from 3 to 5h. Also, based on this comparison, it is H. H. Thwaini and R. H. Salman J. Electrochem. Sci. Eng. 13(3) (2023) 537-551 http://dx.doi.org/10.5599/jese.1806 545 clear that increasing the electrolysis time from 3 to 5 h led to an increase in EC from 28.813 to 33.344 kWh kg-1 phenol. Phenol removal efficiency increased from 70.216 to 90.35 % with a difference of 20.134 %, as shown in runs 4 and 12 results by increasing the iron particles dosage from 2 to 4 g. This indicates that increasing the dosage of iron particles has the second influence on phenol removal. Also, by comparing the values of EC, it is obvious that by increasing the iron particles dosage from 2 to 4 g, the EC decreased from 41.661 to 33.344 kWh kg-1 phenol due to an increase in the removal of phenol by increasing the iron particles dosage. Additionally, when the graphene dosage increased from 0.5 to 1 g as shown in runs 15 and 1, an increase in phenol removal from 61.23 to 75.46 % with a 14.23% difference was attained. Also, based on this comparison, it is clear that increasing the graphene from 0.5 to 1g led to an increase in EC from 31.012 to 37.845 kWh kg-1 phenol due to a decrease in phenol removal. Analysis of variance (ANOVA) The influence of three variables (electrolysis time, graphene dosage, and amount of iron particles) and their interactions can be examined comprehensively by the ANOVA analysis. ANOVA was used to examine the model's significance. The results of ANOVA are presented in Table 4. The terms "sum of the square" (Seq. SS), "adjusted sum of the square" (Adj. SS), and "adjusted mean of the square" (Adj. MS) are used to represent statistical terms. Contr. denotes the contribution of each variable, and DF represents the degree of freedom of the model and its parameters [38]. Table 4 displays the low probability value (P value = 0.0001) and high F model value (57.87), these two values denote the significance of the obtained model [39]. The P-values lower than 0.05 specify that the terms of the model are significant [40]. The multiple regression models can be used to predict the effectiveness of phenol removal in the 3D electro-Fenton reactor, as evidenced by the high value of R2 (0.9905). High values of adjusted R2 and predicted R2 confirmed that the model fit is sufficient for the regression of experimental data. Electrolysis time had the highest significant effect on the phenol efficiency with a high F-value = 350.29 and Contr. of 66.6 %, the iron particles had lower Contr. = 17.89 %, and finally the graphene dosage had the lowest effect on the phenol removal efficiency with Contr. of 8.60 %. Table 4. ANOVA tests in removing phenol by electro-Fenton process Source DF Seq. SS Contr., % Adj. SS Adj. MS F-value P-value Model 9 1641.09 99.05 1641.09 182.34 57.87 0.0001 Linear 3 1542.60 93.10 1542.60 514.20 163.20 0.000 X1 1 296.41 17.89 296.41 296.41 94.08 0.000 X2 1 142.48 8.60 142.48 142.48 45.22 0.001 X3 1 1103.70 66.61 1103.70 1103.70 350.29 0.000 Square 3 42.74 2.58 42.74 14.25 4.52 0.069 X1X2 1 2.63 0.16 3.40 3.40 1.08 0.346 X2X2 1 8.19 0.49 5.85 5.85 1.86 0.231 X3X2 1 31.92 1.93 31.92 31.92 10.13 0.024 2-Way interaction 3 55.75 3.36 55.75 18.58 5.90 0.043 X1X2 1 24.49 1.48 24.49 24.49 7.77 0.039 X1X3 1 31.21 1.88 31.21 31.21 9.91 0.025 X2X3 1 0.04 0.00 0.04 0.04 0.01 0.915 Error 5 15.75 0.95 15.75 3.15 - - Lack-of-fit 3 14.75 0.89 14.75 4.92 9.79 0.094 Pure error 2 1.00 0.06 1.00 0.50 - - Total 14 1656.84 100.00 - - Model summary S R2 / % R2 (adj.) / % Press R2 (pred.) / % 1.77504 99.05 97.34 238.256 85.62 http://dx.doi.org/10.5599/jese.1806 J. Electrochem. Sci. Eng. 13(3) (2023) 537-551 PHENOL REMOVAL BY ELECTRO-FENTON PROCESS 546 Effect of studied factors RSM makes it feasible to identify the best available selection for the examined parameters and provides fundamental data on actual interactions between them. The contour and its related 3D surface plots can be investigated in order to more extensively understand the interaction between studied parameters on phenol reduction [41]. The effectiveness of phenol elimination was investigated by changing electrolysis time (3, 4 and 5 h) and amount of iron particles (2, 3 and 4 g) under constant graphene dosage, as shown in Figure 6. The contour plot in Figure 6a shows that high REphenol > 85 % is obtained over a narrow range of iron particles (3.5- 4 g). According to the 3D surface Figure 6b, when iron particulates increased from 2-4 g, the effectiveness of phenol removal increased progressively before essentially remaining constant as time increased to 5 h. The surface plot makes it obvious that at the time of 3 h, the efficiency of phenol removal dramatically dropped as the amount of iron particles dropped from 4 to 2 g. The effectiveness of phenol elimination increased linearly as the amount of iron particles raised to 4 g at a longer electrolysis time (5 h). So, the results showed that increasing the reaction time would improve the phenol removal efficiency. These results are in agreement with Zhang et al. [42], who also demonstrated that an increase in the reaction time improves COD removal from the landfill leachate by the EF process, and also Umar et al. [10], who described how the removal of landfill leachate is mostly dependent on H2O2 dose. The generation of OH• increased by increasing H2O2 in the electrolytic solution and the ferrous ions, which led to an increase in phenol removal efficiency [43]. Figure 7 shows the contour and 3D surface plots obtained by changing the graphene dosage over a range of electrolysis times (3, 4 and 5 h). The highest phenol removal efficiency (85 %) was attained at an electrolysis time of 5 h and 1 g of graphene. Graphene has a high surface area and strong conductivity, which boosts the electron transfer rate and enhances the formation of H2O2 from the oxygen reduction reaction that takes place on the cathode electrode (equation (3)) [44,45]. At modified cathodes, the effectiveness of phenol removal is increased. The rate of the Fenton reaction and, consequently, the rate of homogenous OH• production in the medium are controlled by the yield of H2O2 generation [46,47]. So, the results approved that increasing graphene dosage leads to an increase in phenol removal efficiency. a b Figure 6. Effect of time and amount of iron particles on REphenol at hold value of graphene 0.75 g: (a) contour plot and (b) 3D surface plot 2 3 60 70 80 2 3 4 4 5 80 90 Phenol RE % h ,emiT nori fo ssa ,particleM g REphenol / % H. H. Thwaini and R. H. Salman J. Electrochem. Sci. Eng. 13(3) (2023) 537-551 http://dx.doi.org/10.5599/jese.1806 547 a b Figure 7. Effect of time and graphene dosage on REphenol at hold value of iron particles of 3 g: (a) contour plot and (b) 3D surface plot Figure 8 shows the contour and 3D surface plots by changing iron particles (2, 3, and 4 g) over a range of graphene dosages (0.5, 0.75, and 1 g) at a constant electrolysis time of 4 h. The highest phenol removal efficiency (>85 %) was attained at iron dosage of 4 g, and 1 g of graphene. Graphene is very appropriate for electrochemical methods, such as its application in the electro-Fenton systems to generate H2O2 by the reduction of dissolved oxygen. Using carbon or graphite felt coated with graphene as a support for cathodes is one of the most common approaches due to low cost, high surface area, excellent conductivity, and high porosity [32]. a b Figure 8. Effect of graphene dosage and amount of iron particles on REphenol at hold value of time 4h: (a) contour plot, and (b) 3D surface plot Optimization of 3D electro-Fenton process Outlining the optimal values of parameters to maximize phenol removal efficiency is the primary objective of the optimization. There are five options for the target field of the variables: maximize, objective, minimize, within the range, and none. The maximum removal of phenol with a corresponding weight of 1.0 was chosen as the goal. The results of the optimal values are presented in Table 5. Two confirmation experiments were conducted under optimal values of operating variables to obtain the highest value of phenol removal efficiency and the corresponding value of COD. The average phenol removal efficiency attained at optimum conditions was 92.83 %, as shown 2 3 60 70 2 0.50 4 0.75 1.00 80 enol RE %hP rg fo saM s g ,enehpa g ,elcitrap noassM of ir 0.50 0.75 60 70 80 3 1.00 4 5 80 90 E %R lonehP emiT h , g ,enehparg fo Mass REphenol / % REphenol / % http://dx.doi.org/10.5599/jese.1806 J. Electrochem. Sci. Eng. 13(3) (2023) 537-551 PHENOL REMOVAL BY ELECTRO-FENTON PROCESS 548 in Table 6, and the COD value at these conditions was 89.33 % with consumed energy of 32.976 kWh kg-1 phenol. Previous study by Zheng et al. [29], who utilized the 3D electrode with iron foam particles, gave 70.4 % of folic acid removal efficiency in 6 h, while in the present study, the modification of the cathode electrode increased the efficiency of the 3D electro-Fenton process in removing 92.58 % of phenol in 5 h, what which signifies the enhancement obtained in the present electrolytic cell. Table 5. Optimal results of system parameters for the maximum elimination of phenol Response Goal Lower Target Upper Weight Importance REphenol / % Maximum 53.33 90.35 1 1 Solution of parameters Multiple response prediction Mass of iron particles, g Mass of graphene, g Time, h REphenol / % fit Standard error fit Confidence interval 95 % Prediction Interval 95 % Composite desirability 4 1 5 92.58 2.10 (87.19; 97.97) (85.51; 99.64) 1 Table 6. Confirmation tests for phenol removal efficiency and consumed energy Run Mass, g Time, h Voltage, V EC / kWh kg-1 phenol REphenol, % Iron particles Graphene Actual Average 1 4 1 5 4.15 33.076 92.66 92.835 2 4 1 5 4.16 32.876 93.01 Conclusion 3D electro-Fenton reactor for phenol elimination with the carbon fibers (CF) modified by graphene cathode and graphite anode was assessed in the presence of iron foam particle electrode. The response surface methodology (RSM) was employed to ascertain the impact of electrolysis time, graphene dosage, and iron foam dosage on the phenol removal efficiency and how these factors interact. The predicted multiple regression correlation showed a high value of R2 (0.9905), which confirmed that the model sufficiently fits the regression of experimental data. High F-values and low P-values indicated that all variables influence phenol removal efficiency. The highest phenol removal efficiency of 92.835 % was achieved at 5 h, iron foam dosage of 4 g, and graphene of 1 g. CF approved its effectiveness in the present study as one of the most favored cathodes for H2O2 generation. This is due to the high specific surface and low cost of CF, which modification with graphene-enhanced its performance in producing H2O2. Iron foam functions were a good choice since 3D particles offer active sites, serve as iron ions source, and increase removal efficiency. 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