Available online at http://ijcpe.uobaghdad.edu.iq and www.iasj.net Iraqi Journal of Chemical and Petroleum Engineering Vol. 24 No.1 (March 2023) 51 – 63 EISSN: 2618-0707, PISSN: 1997-4884 *Corresponding Author: Name: Zainab M. Issa, Email: zainab.issa1607m@coeng.uobaghdad.edu.iq IJCPE is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License. Electrosorption of Hexavalent Chromium Ions by MnO2/Carbon Fiber Composite Electrode: Analysis and Optimization of the Process by Box-Behnken Design Zainab M. Issa a, *, Rasha H. Salman a, and Prashant Basavaraj Bhagawati b a Chemical Engineering Department, College of Engineering, University of Baghdad, Baghdad, Iraq b College of Engineering and Technology, Ashta, Maharashtra, India Abstract A nano manganese dioxide (MnO2) was electrodeposited galvanostatically onto a carbon fiber (CF) surface using the simple method of anodic electrodeposition. The composite electrode was characterized by field emission scanning electron microscopy (FESEM), and X-ray diffraction (XRD). Very few studies investigated the efficiency of this electrode for heavy metals removal, especially chromium. The electrosorption properties of the nano MnO2/CF electrode were examined by removing Cr(VI) ions from aqueous solutions. NaCl concentration, pH, and cell voltage were studied and optimized using the Box-Behnken design (BDD) to investigate their effects and interactions on the electrosorption process. The results showed that the optimal conditions for the removal of Cr(VI) ions were a cell voltage of 4.6 V, pH of 2 and NaCl concentration of 1.5 g/L. This work indicated that MnO2/CF electrode was highly effective in removing Cr(VI) ions and the BBD approach was a feasible and functional method for evaluating the experimental data. Keywords: electrosorption process, hexavalent chromium ions, composite electrode, electrodeposition, nanostructured MnO2, Box-Behnken design. Received on 13/12/2022, Received in Revised Form on 24/01/2023, Accepted on 25/01/2023, Published on 30/03/2023 https://doi.org/10.31699/IJCPE.2023.1.7 1- Introduction Development of industries to serve human needs increases as the population grows. This rise has a negative impact on the environment, human health, and aquatic life [1, 2]. Heavy metals are a source of pollution even at trace levels, with toxicity ranging from high to carcinogenicity due to repeated exposure [3, 4]. Metals are extremely dangerous since they are not biodegradable and may concentrate in biological organisms, getting denser as they go into the food chain. [5]. In industrial wastewater treatment, toxic heavy metals of particular concern are zinc, copper, nickel, mercury, cadmium, lead, and chromium [2, 6]. Chromium is a primary concern for the Environmental Protection Agency (EPA) especially when present in the hexavalent form, Because of its widespread application and high toxicity [7]. In industry, chromium compounds are utilized in electroplating, leather tanning, metal polishing, chemical manufacturing, pigments, electronic and electrical equipment [8]. Chromium typically enters the aquatic environment in two forms: Cr(III) and Cr(VI). In general, Cr(VI) is more dangerous than Cr (III). Furthermore, Cr(VI) is a strong oxidant that may be absorbed by the skin and is highly mobile in soeil and aquatic systems [9, 10]. The acceptable limits of Cr(VI) and total chromium (TCr) released to surface water should be less than 0.05 and 0.1 mg/l, respectively, according to the United States Environmental Protection Agency (USEPA) [2, 11]. Many strategies for removing Cr(VI) ions, including ion exchange, chemical precipitation, coagulation, chemical reduction, adsorption [4], and capacitive deionization (CDI) or electrosorption considered efficient methods [12]. Electrosorption is an electrochemical method of removing metals under the influence of an electric field. CDI is an electrosorption process in which ions in an aqueous solution are adsorbed on the electrical double layers of porous electrodes after being exposed to a low- voltage electric field [13, 14]. When compared to other existing desalination systems, CDI offers the benefits of being environmentally sustainable, utilizing less energy and having lower running costs than other desalination methods, and being simpler to regenerate and maintain [12]. The most important component of any electrochemical system is the electrode material [15]. Carbon aerogel, activated carbon, carbon fiber, ordered mesoporous carbon, carbon nanotubes, and graphene are examples of nanoporous carbon materials that have been suitable electrodes for the electrosorption process [16], [17]. Carbon fiber (CF) is a porous kind of activated carbon that has a strong electrosorptive capability owing to its larger specific surface area [14]. Carbon fiber has gotten attention because of its potentially inexpensive manufacturing cost, significant uptake capacity, high strength, high electrical conductivity, high stability, and consistent pore size [18, 19]. The capacitance of the http://ijcpe.uobaghdad.edu.iq/ http://www.iasj.net/ mailto:zainab.issa1607m@coeng.uobaghdad.edu.iq http://creativecommons.org/licenses/by-nc/4.0/ https://doi.org/10.31699/IJCPE.2023.1.7 Z. M. Issa et al. / Iraqi Journal of Chemical and Petroleum Engineering 24,1 (2023) 51 - 63 52 electrosorption process is a strong indicator of how effectively porous materials remove ions [20]. The electrochemical storing mechanism of metal ions in capacitive deionization operates in the same manner as an electrochemical supercapacitor. It is based on either the electrical double-layer capacitance (EDL) from pure electrostatic adsorption when porous carbon materials are used or the pseudo capacitance from reversible and fast faradic reactions when redox-active materials (pseudo capacitive materials) are used [21]. In surface-redox pseudo capacitive materials, ions are electrochemically adsorbed onto the surface of a material accompanied by a faradaic charge transfer [22]. An example of pseudo capacitive material is manganese dioxide, MnO2 stores ions by the surface redox reaction between oxidation states of Mn4+ and Mn3+. The mechanism is based on the adsorption of electrolyte cations (C+) onto the surface of MnO2 as shown in Eq. 1 [23]: (𝑀𝑛𝑂2)𝑠𝑢𝑟𝑓𝑎𝑐𝑒 + 𝐶 + + 𝑒− ↔ (𝑀𝑛𝑂2 − 𝐶 +)𝑠𝑢𝑟𝑓𝑎𝑐𝑒 (1) Where: C+ represents an alkali metal cation (H+, Li+, Na+, K+) in the electrolyte. Electrochemical pseudo capacitors could be constructed from a wide range of materials, including conducting polymers, noble metal oxides which including RuO2 and IrO2, and transition metal oxides like MnO2, NiO, Co2O3, FeO, TiO2, SnO2, V2O5 and MoO [24]. Transition metal oxides offer pseudo-capacitance behavior, with a higher specific capacitance than electrical double layers of carbonaceous materials due to the faradaic redox reaction in the active materials. These active materials are loaded onto the surface of carbon to enhance the electrosorption capability of the carbon materials [25]. MnO2 is a perspective electrode material for supercapacitors among metal oxide compounds because of its low cost, environmentally friendly nature, and relatively high theoretical pseudo capacitance value, 1370 F.g-1, which is much greater than that of carbon-based materials. So, a lot of interest has been focused on developing MnO2-based materials as an electrode for electrochemical supercapacitors and as effective adsorbents to remove heavy metals from water. Numerous investigations have focused on connecting MnO2 to the surface of carbonaceous materials, such as activated carbon, carbon fibers, carbon nanotubes, and graphene-based materials, to overcome this difficulty [26, 27]. Currently, relatively few articles have explored heavy metal removal by electrosorption onto MnO2/CF electrodes, and no previous works have investigated the applicability of this electrode for chromium ion removal by electrosorption. The current study targets to synthesize and characterize MnO2/carbon fiber electrode, the produced electrode's performance in electrosorption of Cr(VI) ions from an aqueous solution was examined by changing three variables cell voltage, pH, and ionic strength (NaCl concentration). The effect of these selected operational factors and their interactions on removal efficiency were studied and optimized using the response surface methodology and the Box-Behnken design. 2- Experimental Work 2.1. Reagents and Chemicals All reagents were of analytical grade and were utilized without additional purification. For the electrodeposition of MnO2, Manganese (II) sulfate monohydrate (MnSO4.H2O) (98% purity, THOMAS BAKER) and sulfuric acid (H2SO4) (98% purity, alpha chemika, India) were used in the electrolytic solution. For the experiment of chromium ions removal, potassium dichromate (K2Cr2O7) (99% purity, alpha chemika, India), sodium chloride (NaCl) (99% purity, HiMedia Laboratories Pvt.Ltd.), sodium hydroxide (NaOH) (98% purity, alpha chemika, India), diphenylcarbazide (C13H14N4O, Fluka Chemika, Switzerland), and HNO3 (69% purity, Central Drug House (P) Ltd.) were utilized. All solutions were prepared with deionized water of 5µS/cm conductivity. The pure carbon fiber purchased form Jiaxing AGG composites Co. Ltd., China. Fig. 1 shows the FESEM-EDX characterization of the purchased pure carbon fiber electrode. FESEM analysis depicts the smooth surface of pure carbon fiber electrode as shown in Fig. 1a and Fig. 1b. EDX analysis demonstrates that 86.76% of electrode material is composed of carbon as shown in the Fig. 1c. 2.2. Preparation of Electrodes To synthesize nanostructured MnO2, experiments of anodic electrodeposition of MnO2 onto CF were carried out utilizing commercial CF as a substrate and current collector. A rectangular slice of CF (16.5cm × 5 cm) was activated for 30 minutes at 80°C with 5% HNO3, then cleaned and stored in distilled water. The electrodeposition of MnO2 onto CF was carried out galvanostatically for 4 h at a current density of (0.3 mA/cm2) using a regulated DC power supply (UNI-T: UTP3315TF-L). A 2L beaker containing 1.8L of aqueous solution (0.64M H2SO4 + 0.35M MnSO4) was used as the electrolytic solution. The solution was agitated and retained at 90 ℃ by a hot plate magnetic stirrer (Heidolph: MR Hei-standard, Germany). Three electrodes were used; an activated CF piece served as an anode, and two graphite served as counter electrodes. After the completion electrodeposition process, the composite electrode of MnO2/CF was washed and rinsed multiple times with distilled water and then dried. The prepared electrode would be used later in the electrosorption process of chromium ions. The electrodeposition experimental setup is schematically shown in Fig. 2. 2.3. Characterization and Analysis The crystal structure of deposited MnO2 films were characterized by X-ray diffraction (XRD, Phillips: PW1730, USA), and the diffraction data were collected over 2θ. Cu-Kα radiation utilized as the X-ray source with λ = 1.54056 ̊A. The morphology and composition of the prepared MnO2/CF electrode was tested using Field Z. M. Issa et al. / Iraqi Journal of Chemical and Petroleum Engineering 24,1 (2023) 51 - 63 53 emission scanning electron microscopy (FESEM, Tescan Mira3, Czech) equipped with an energy dispersive X-ray spectrometry (EDX ) analyzer at an operating conditions of 25kV and 100𝜇𝐴. The amount of Cr(VI) ions that remained in the aqueous solution was determined at the maximum wavelength of 540 nm using a UV spectrophotometer (Thermo UV- Spectrophotometer, USA). In acidic environments, diphenylcarbazide interacts with Cr(VI) ions to generate a dark violet-colored complex [28, 29]. Fig. 1. Characterization of the Pure Carbon Fiber Electrode (a, b) FESEM (c) EDX Fig. 2. Setup Scheme of the Electrodeposition Process 2.4. Electrosorption experiment The electrosorption process was tested in a batch experiment consisting of a glass beaker comprising of an aqueous solution of 0.8L with a constant starting Cr(VI) ions concentration of 100 ppm to study the electrosorption characteristics of the nanostructured MnO2/carbon fiber. The CDI system used two electrodes: a working electrode of MnO2/CF and counter electrode of stainless steel plate (17cm x 5 cm x 3 mm), with a 1.5cm space between the 1 2 3 4 5 6 7 8 9 10 keV 0 2 4 6 8 10 12 14 16 cps/eV C O N Mn Mn Z. M. Issa et al. / Iraqi Journal of Chemical and Petroleum Engineering 24,1 (2023) 51 - 63 54 electrodes. The aqueous solution was agitated with a constant stirring rate of 250 rpm during the experiment using a hot plate magnetic stirrer (Heidolph: MR Hei- standard, Germany) to guarantee ionic diffusion, and all tests were carried out at 25°C ±1 room temperature. A direct-current power supply was coupled to the electrodes to supply the electrical current. Before each experiment, the pH was adjusted to the desired value by adding 1M of sodium hydroxide or 1M of sulfuric acid, and it was detected with a pH meter equipment (HANNA instrument, pHep: HI98107). Eqs. 2 and 3 were used to compute the removal efficiency of Cr(VI) ions and the equilibrium adsorption capacity qe (mg/g), respectively [17]: 𝑅𝑒% = (𝐶0−𝐶𝑒) 𝐶𝑜 × 100 (2) 𝑞𝑒 = 𝐶𝑜−𝐶𝑒 𝑚 × 𝑣 (3) Where 𝐶0 (mg/l) and 𝐶𝑒 (mg/l) are the initial and equilibrium chromium ion concentrations in the solution, respectively, m is the total mass of the electrode, and V is the volume of solution. The CDI experimental setup is schematically shown in Fig. 3. Fig. 3. Schematic Diagram of the Electrosorption Setup 2.5. Preliminary experiment To determine the equilibrium time required for the MnO2/carbon fiber electrodes to reach their saturation point, two preliminary batch electrosorption experiments were performed at pH 2, 4.1V, and 1g/L NaCl concentration with two initial concentrations (100 and 150 ppm of Cr ions). These two experiments were carried out before the set of experiments determined by the experimental design. 2.6. Experimental design Response surface methodology (RSM) is a software program that consists of a set of a statistical and mathematical methods that are considered to be very effective for modeling and analyzing in which a response of interest is affected by numerous variables in order to investigate the optimal conditions to achieve the desired response [30]. A statistical design called Box-Behnken design (BBD) was used in this study to optimize and investigate the impact of specified operating variables on Cr(VI) ions removal using the electrosorption method. A Box- Behnken design of three-factors with three levels would result in a set of 15 tests with three replicates at the center point [31]. The factors were pH (X1), NaCl concentration (X2), and cell voltage (X3), with the removal efficiency (Y) of Cr(VI) ions being the response. Generally, each variable in the Box-Behnken design has three levels as coded values, denoted by the numbers -1, 0 and 1. Levels 1 and -1 represent the maximum and minimum values for each input variable, respectively, while level 0 represents the variable's center value [17]. Table 1 displays the variables of the process with their coded and real values. According to Eq. 4, the Box-Behnken design establishes a model of a polynomial second-order equation to analyze the influence of the specified variables on the produced response: 𝑌 = 𝛽𝑜 + ∑ 𝛽𝑖 𝑘 𝑖=1 𝑋𝑖 + ∑ 𝛽𝑖𝑖 𝑘 𝑖=1 𝑋𝑖 2 + ∑ ∑ 𝛽𝑖𝑗 𝑘 𝑗>1 𝑘 𝑖=1 𝑋𝑖 𝑋𝑗 (4) As previously stated, Y represents the achieved response (removal efficiency of Cr(VI) ions), 𝛽𝑜 is the intercept term, 𝛽𝑖 is the linear term, 𝛽𝑖𝑖 is the quadratic term, and 𝛽𝑖𝑗 is the interaction term, Xi, Xj, …, Xk are the process input variables [32]. Z. M. Issa et al. / Iraqi Journal of Chemical and Petroleum Engineering 24,1 (2023) 51 - 63 55 Table 1. Coded and Real Values of Variables Run Blk Coded values Real values X1 X2 X3 pH NaCl voltage 1 1 0 -1 1 4 0.5 4.6 2 1 1 0 1 6 1 4.6 3 1 0 0 0 4 1 4.1 4 1 0 0 0 4 1 4.1 5 1 0 1 -1 4 1.5 3.6 6 1 1 0 -1 6 1 3.6 7 1 0 -1 -1 4 0.5 3.6 8 1 -1 0 1 2 1 4.6 9 1 0 1 1 4 1.5 4.6 10 1 0 0 0 4 1 4.1 11 1 -1 1 0 2 1.5 4.1 12 1 -1 -1 0 2 0.5 4.1 13 1 -1 0 -1 2 1 3.6 14 1 1 -1 0 6 0.5 4.1 15 1 1 1 0 6 1.5 4.1 3- Result and Discussion 3.1. Characterization of MnO2/carbon Fiber Composite Electrode X-ray diffraction (XRD) analysis was used to determine the presence of deposited manganese dioxide nanoparticles on the carbon fiber surface, as well as their phases and crystal structure at 0.3 mA/cm2, applied current density, and 0.64 M H2SO4 concentration. Fig. 4 shows the patterns of XRD analysis for MnO2 nanostructured electrodeposited onto the carbon fiber surface. The intensity of most diffraction peaks was for nanostructured MnO2 with two diffraction peaks of pure carbon fiber, positioned approximately at 24o and 43o, which correspond to the amorphous carbon (002) and (100) lattice planes [23]. All of the electrodeposited MnO2 diffraction peaks attributed to the pure phase of orthorhombic γ- MnO2, and the peaks correspond with the standard JCPDS card (NO.14-0644) [33]. The diffraction peaks were at 2θ (34.2o, 37.16o, 38.24o, 42.44o, 44.04o, 56.32o, 57.8o, 61.69o, 65.24o, 69.08o, 71.96o, 75.72o, and 78.88o) as shown in Fig. 4. Numerous studies have shown that electrode efficiency, capacity, and stability are greatly influenced by electrode morphology [34]. MnO2 nanostructures electrodeposited on the surface of carbon fiber have the morphology of spherical brushes consisting of straight nanorods grown normally to the surface of carbon fiber, this has also observed by Xie et al. [35] and illustrated in Fig. 5. These several nanorods have approximately a smaller diameter (35.11 nm), they are merged to form a smooth and thin film. This film has a uniform structure and high capacity as a result of its large surface area. Furthermore, rod-like shapes can supply both ions and electrons with short diffusion channel lengths as well as adequate porosity for electrolyte penetration, resulting in rapid charge/discharge rates [36]. Fig. 4. Results of XRD Analysis of MnO2 Nanoparticles Deposited on Carbon Fiber Prepared at (0.3mA/cm 2, 0.64M H2SO4) Fig. 5. Results of FESEM Image of MnO2 Nanoparticles Deposited on Carbon Fiber Prepared at (0.3mA/cm 2, 0.64M H2SO4) with Different Magnifications (a) 10 𝜇𝑚, (b) 500nm and (c) 200 nm Z. M. Issa et al. / Iraqi Journal of Chemical and Petroleum Engineering 24,1 (2023) 51 - 63 56 3.2. Result of preliminary experiment At pH = 2, cell voltage = 4.1V, and NaCl concentration = 1g/L, two preliminary experiment were accomplished. Adsorption for MnO2/carbon fiber electrode was allowed to proceed for up to 5 h with periodic measurements, and it was revealed that adsorption for 4 h was sufficient to achieve equilibrium with high removal efficiency as shown in Fig. 6. Since the equilibrium time for the two concentrations (100 and 150 ppm) were the same, 100 ppm concentration has been used as the initial concentration in all experiments that have been studied in the experimental design. MnO2/carbon fiber electrode could accelerate the electrosorption process to complete in 3 h according to its faradic-capacitive behavior. While due to the capacitive behavior of carbon fiber electrode, the electrosorption process require 4 h to reach the equilibrium time, that result has been achieved in our previous study [37]. Fig. 6. The Removal Efficiency of Cr(VI) Ions vs. Time on MnO2/Carbon Fiber Electrode at Conditions of pH 2, 4.1V, and 1 g/L NaCl Concentration 3.3. Analysis for electrosorption process using Response surface methodology 3.3.1. The Box–Behnken model Table 2 displays the actual values achieved from the experimental work as well as the predicted values measured using RSM methods, which illustrate the electroporation process's response to the chromium ion removal efficiency [38]. Minitab-19 software was used to analyze experimental data. To establish the regression equation, the experimental data were fitted into a second- order model. Equation 5 gives the final empirical equation that shows the relation between the removal efficiency of Cr(VI) ions and the three specified variables in terms of coded units: 𝑌% = 245.0 − 39.42𝑋1 − 115.2𝑋2 − 38.0𝑋3 + 3.223𝑋12 + 5.66𝑋22 + 2.56𝑋32 + 1.492𝑋1 ∗ 𝑋2 + 1.829𝑋1 ∗ 𝑋3 + 28.59𝑋2 ∗ 𝑋3 (5) Where Y represents the removal % of Cr(VI) ions, i.e. the response, and the operational variables are pH (X1), NaCl concentration (X2), and cell voltage (X3). The analysis of variance (ANOVA) based on the BBD was studied to evaluate the significance and suitability of the recommended polynomial quadratic models [39]. The degree of freedom (DOF), percentage contribution%, sum of squares (SS), mean of squares (MS), adjusted sum of squares (Adj. SS), adjusted mean of squares (Adj. MS), F- value, and P-value were used to specify the ANOVA analysis. Table 3 displays the ANOVA results. ANOVA provides the percentage contribution for each variable, as shown in Table 3. The greater a variable's percentage contribution, the greater its impact to the final results. Any small change in its value will have a significant impact on the reaction [30]. ANOVA analysis shows that the contributions of pH, NaCl concentration and cell voltage are the same approximately, which mean that their impact on the response were convergent. The contribution of linear term was 71.48%, while the terms of square and two-way interaction have contributed with 20.42% and 7.52%, respectively. The model is highly significant, as demonstrated by lower P-values (p model = 0.000) and higher F-values (F model = 96.06). The ratio of mean square model (MS) to the error mean square represent the F-value. The higher the ratio, the higher the F-value [40]. In the model, the Probability value (P-value) is used to evaluate statistically significant effects. A P-value of less than 0.05 refers that the impact of the variable is statistically significant at a 95% confidence level [41]. Analysis of F-value and P-value indicated that pH (X1), NaCl (X2), voltage (X3) and pH*pH (X1*X1) terms are the most controlling terms in the model. Furthermore, high determination coefficient (R2) which equal to 99.42% indicating good model fitting with experimental data. 3.3.2. Main effect plots The relationship between the response and the selected operational variable could be illustrated using the main effect plots. Fig. 7 demonstrates the impacts of pH solution, cell voltage, and ionic strength on the removal efficiency of Cr(VI) ion in the electrosorption process using MnO2/carbon fiber electrode, respectively. The maximum removal efficiency was observed when the pH value was reduced to 2. The influence of pH was investigated from 2 to 6. Fig. 7 also demonstrates an obvious improvement in removal % when NaCl concentration increases from 0.5 to 1.5 g/L and applied voltage increases from 3.6 to 4.6V. The experimental results reveal that reducing the pH value improves the reduction of Cr(VI), whereas increasing the pH decreases the reduction of Cr(VI) due to the smaller amount of H+ ion in the solution [42]. Because the presence of H+ ions induces the cathodic reduction of Cr(VI) to Cr(III) under highly acidic environment, as indicated in Eqs. 6 and 7 [43]: Cr2O7 2- + 8H+ + 6e- → Cr2O3 + 4H2O E°= 1.33 V (6) HCrO4 - + 7H+ + 3e- → Cr3+ + 4H2O E°=1.35 V (7) Z. M. Issa et al. / Iraqi Journal of Chemical and Petroleum Engineering 24,1 (2023) 51 - 63 57 Table 2. Experimental Results of BBD for Cr(VI) Ions Removal Run Blk pH NaCl conc. Voltage Re% AC(mg/g) X1 X2 X3 Act.% Pred.% 1 1 4 0.5 4.6 64.510 64.493 83.23871 2 1 6 1 4.6 83.717 85.324 108.0219 3 1 4 1 4.1 69.901 69.804 90.19484 4 1 4 1 4.1 69.807 69.804 90.07355 5 1 4 1.5 3.6 64.914 64.931 83.76 6 1 6 1 3.6 63.027 62.768 81.32516 7 1 4 0.5 3.6 58.040 59.888 74.89032 8 1 2 1 4.6 99.990 100.249 129.0194 9 1 4 1.5 4.6 99.970 98.122 128.9935 10 1 4 1 4.1 69.703 69.804 89.93935 11 1 2 1.5 4.1 99.990 101.580 129.0194 12 1 2 0.5 4.1 85.469 85.227 110.2826 13 1 2 1 3.6 86.615 85.008 111.7613 14 1 6 0.5 4.1 65.251 63.661 84.19484 15 1 6 1.5 4.1 85.739 85.981 110.631 Table 3. Analysis of Variance for Cr(VI) Removal Using MnO2/Carbon Fiber Electrode Source DOF Seq. SS Contribution Adj. SS Adj.MS F-Value P-Value Model 9 2994.22 99.42% 2994.22 332.691 96.05 0.000 Linear 3 2152.61 71.48% 2152.61 717.537 207.15 0.000 pH 1 690.62 22.93% 690.62 690.619 199.38 0.000 NaCl 1 747.74 24.83% 747.74 747.742 215.87 0.000 voltage 1 714.25 23.72% 714.25 714.250 206.20 0.000 Square 3 615.04 20.42% 615.04 205.015 59.19 0.000 pH*pH 1 606.60 20.14% 613.83 613.834 177.21 0.000 NaCl*NaCl 1 6.93 0.23% 7.39 7.392 2.13 0.204 voltage*voltage 1 1.51 0.05% 1.51 1.512 0.44 0.538 2-Way Interaction 3 226.57 7.52% 226.57 75.523 21.80 0.003 pH*NaCl 1 8.90 0.30% 8.90 8.901 2.57 0.170 pH*voltage 1 13.38 0.44% 13.38 13.377 3.86 0.107 NaCl*voltage 1 204.29 6.78% 204.29 204.290 58.98 0.001 Error 5 17.32 0.58% 17.32 3.464 Lack-of-Fit 3 17.30 0.57% 17.30 5.766 587.86 0.002 Pure Error 2 0.02 0.00% 0.02 0.010 Total 14 3011.54 100.00% Model summery S R2 R2 (adj.) PRESS R2 (pred.) AICc BIC 1.86113 99.42% 98.39% 276.836 90.81% 154.72 74.51 Fig. 7. Main Effect Plot for Cr(VI) Ions Removal Using MnO2/Carbon Fiber Electrode Furthermore, at higher pH levels, the following reaction occurs (Equation 8), and chromium could also generate hydroxide in the form of Cr(OH)3. Cr3+ + 3OH → Cr(OH)3 (8) This reaction occurs quickly at higher pH levels and prevents Cr(VI) reduction [43]. Z. M. Issa et al. / Iraqi Journal of Chemical and Petroleum Engineering 24,1 (2023) 51 - 63 58 The positive influence of increasing NaCl concentration on removal efficiency could be attributed to an increases in system electric current, ion flow, and redox reaction. However, further increases in NaCl concentration may result in inhabitation behavior, lowering or even suppressing chromium adsorption due to competing forces [44]. These effects have been examined because both natural water and wastewater typically include ions that can affect and compete with the adsorption of other ions [45]. The effects of increasing the applied voltage in this Figure reveals that increasing the applied voltage has a significant effect on the efficiency of Cr(VI) ions removal. An increase in electron flow velocity at high applied voltage values leads to an increase in electrostatic attraction and as a result the removal efficiency increase [46]. 3.3.3. Effect of interactive parameters Fig. 8a depicts the interaction between pH value and NaCl concentration and their impact on removal efficiency. As seen in the surface plot, increasing the pH value causes an obvious decrease in removal efficiency at 0.5 g/L NaCl concentration, whereas the decrease in removal efficiency is less at 1.5 g/L NaCl concentration. Furthermore, at pH 2, greater removal efficiency is obtained by increasing NaCl concentration to 1.5 g/L, however at pH 4, there is a significant reduction followed by a slight elevation at pH 6. The related contour plot shows a dark red region that represents the greatest removal effectiveness at pH 2 and NaCl concentration ranges of 0.8 to 1.5 g/L. The interaction between pH value and cell voltage and their impact on removal efficiency is depicted in Fig. 8b, which shows that at pH= 2, higher removal efficiency was achieved when the cell voltage was 4.6V, followed by a gradual decrease in removal efficiency when the applied voltage was reduced to 3.6V. At a cell voltage of 3.6V, the Fig. reveals a severe reduction in removal efficiency as the pH value increases, and this decrease appears to be reduced as the cell voltage rises from 3.6 to 4.6V. According to the related contour plot, the area of highest removal efficiency values appears to be confined to cell voltages of (3.9-4.5V) and pH values of 2. The effects of cell voltage and NaCl content on removal efficiency are depicted in Fig. 8c. At 1.5 g/L NaCl concentration, the removal efficiency of Cr(VI) ions increases linearly and reaches its maximum when the cell voltage is increased from 3.6 to 4.6V, however there is no significant influence of cell voltage at 0.5 g/L NaCl concentration. At a cell voltage of 3.6V, increasing NaCl concentration has no influence on removal efficiency, however at a cell voltage of 4.6V, increasing NaCl concentration from 0.5 to1.5 g/L increases removal efficiency. The resulting contour plot shows that the greatest removal efficiency values are found in the cell voltage range of 4.3-4.6V and the NaCl concentration range of 1.28-1.5g/L. 3.3.4. Optimization using desirability function and conformation test To analyze the results and attain the optimal conditions for reducing Cr(VI) ions from an aqueous solution, optimization utilizing overall desirability was devised. The desirability function offers information on the quality and acceptability of the process [47]. The function represent a mathematical strategy for simultaneously determining the optimum values of input variables and output (response) by applying the optimum input variable levels [48]. When the desirability function ranges from 0 to 1, the removal efficiency is maximized. If the desirability value is 0, the proposed value is unfavorable. If it is 1, it signifies that the system's response has improved and it is stable [49]. To achieve this purpose, the three examined variables were estimated on a specified range of values, with the response aiming for the maximum. The optimization results for MnO2/carbon fiber electrode are provided in Table 4, with the model having a desirability function of (1). Two conformation experiments were performed using the program's recommended optimal parameters. The results of these experiments are intended to fit within the parameters determined by the optimization analysis. Table 5 shows the confirmation test results on optimally specified variables. The average removal efficiency was 99.98%, which was extremely near to the suggested one. Table 4. Optimization of System Variables for the Maximum Removal of Cr(VI) Using MnO2/Carbon Fiber Electrode Response Goal Lower Target Upper Weight Importance Cr removal% Maximum 58.04 99.99 1 1 Solution of parameters Multiple response Prediction pH NaCl voltage Cr removal% (Fit) Composite Desirability SE Fit 95% CI 95% PI 2 1.5 4.6 116.986 1 2.20 (111.33, 122.64) (109.58, 124.39) Table 5. Confirmation Tests of Cr(VI) Ions Removal Using MnO2/ Carbon Fiber Electrode Run pH NaCl concentration Cell voltage Re% Actual Average AC (mg/g) 1 2 1.5 4.6 99.99% 99.98% 129.0194 2 2 1.5 4.6 99.98% 128.9806 Z. M. Issa et al. / Iraqi Journal of Chemical and Petroleum Engineering 24,1 (2023) 51 - 63 59 Fig. 8. Response Surface Plot and Contour Plot for the Effect of (a) pH and NaCl Concentration (g/L), (b) pH and Cell Voltage (V), and (c) NaCl Concentration (g/L) and Cell Voltage (V) on Cr(VI) Removal Using MnO2/ Carbon Fiber Electrode 4- Conclusion A Manganese dioxide/CF composite electrode has been efficiently synthesized by anodic electrodeposition process at 0.64 M H2SO4 concentration, 0.35 M MnSO4 concentration and 0.3 mA/cm2 current density. XRD and FESEM analysis confirm the formation of γ- MnO2 nanorods with diffraction peaks of sharp intensity and smaller diameter of about 35.11nm. The electrosorption ability of MnO2/carbon fiber electrode to adsorb Cr(VI) ions from an aqueous solution were investigated and the result showed a high removal efficiency of 99.99% and adsorption capacity of 129.0194 mg/g at concentration of 100 mg/l. From response surface methodology analysis, it has been detected that NaCl concentration has the major effect on removal efficiency with a contribution percentage of 24.83%. The optimization analysis of selected variables reveals that the optimal working conditions for electrosorption of Cr(VI) ions on MnO2/carbon fiber electrode were at pH=2, cell voltage= 4.6V, and NaCl concentration= 1.5 g/L, which was determined using the BDD. Nomenclature Symbols Description Units AC Adsorption capacity mg/g Ce Equilibrium concentration mg/l Co Initial concentration mg/l m Total mass of the electrode g qe Adsorption capacity mg/g R2 Determination coefficient - Re Removal efficiency % X1 Real pH value - X2 Real NaCl Concentration value g/L X3 Real applied voltage value V Xi input variables of the process - Xj input variables of the process - Xk input variables of the process - Z. M. Issa et al. / Iraqi Journal of Chemical and Petroleum Engineering 24,1 (2023) 51 - 63 60 Y Response (removal efficiency) % Greek Letters Description Units 𝛽 Phase of MnO2 - 𝛽𝑜 intercept term - 𝛽𝑖 linear term - 𝛽𝑖𝑖 quadratic term - 𝛽𝑖𝑗 interaction term - 𝛾 Phase of MnO2 - 𝜃 Angle radian 𝜆 wavelength A° v Volume of aqueous solution m3 Abbreviation Symbol Description Adj.MS Adjusted mean of square Adj.SS Adjusted sum of square AFM Atomic force microscopy ANOVA Analysis of variance BBD Box–Behnken design CDI Capacitive deionization CF Carbon fiber DC Direct current DOF Degree of freedom EDL Electrical double layer EDX Energy-dispersive X-ray spectrometry FESEM Field emission scanning electronic microscopy MS Mean of square RSM Response surface methodology SS Sum of Square UV Ultraviolet–visible XRD X-ray diffraction References [1] M. S. Gaikwad and C. Balomajumder, “Current Progress of Capacitive Deionization for Removal of Pollutant Ions,” Electrochem. 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Issa et al. / Iraqi Journal of Chemical and Petroleum Engineering 24,1 (2023) 51 - 63 63 2MnO نالكهربائي أليونات الكروم السداسية التكافؤ بواسطة القطب المركب م زازاالمت Box-Behnkenمن خالل تصميم وتحسينها ألياف الكربون: تحليل العملية / 2، وبراشانت باسافاراج 1 ، رشا حبيب سلمان* ،1زينب محمد عيسى قسم الهندسة الكيمياوية، كلية الهندسة، جامعة بغداد، العراق 1 جامعة بالكوت، الهند 2 الخالصة ( ، حيث تم ترسيب ثاني 2MnOتناولت هذة الدراسة طريقة تحضير قطب نانوي مركب ) ألياف الكربون / ب ( باستخدام طريقة بسيطة تدعى الترسيCFأكسيد المنغنيز ذو البنية النانوية على سطح ألياف الكربون ) ( FESEMالمركبة الناتجة بأستخدام الفحص المجهري اإللكتروني ) الكهربائي األنودي. تم تشخيص األقطاب يب ( لدراسة التركXRDلدراسة شكل المادة النانوية المتكونة على سطح القطب وفحص حيود األشعة السينية ) طب البلوري للمادة المتكونة على السطح. هنالك عدد قليل جًدا من الدراسات التي قامت بدراسة كفاءة الق ، وخاصة الكروم. تم فحص خصائص االمتزاز الكهربائي في إزالة المعادن الثقيلة CF 2MnO /ر المحض من محلول مائي. لدراسة تأثير العوامل Cr (VI)عن طريق إزالة أيونات CF 2MnO /للقطب النانوي لوريدكموضة وتركيز التشغيلية المختلفة والتفاعل فيما بينها على كفاءة عملية االزالة، تم اختيار درجة الح هرت النتائج أنه عند (، أظBDD) Box-Behnkenالصوديوم وجهد الخلية كظروف تشغيلية. باستخدام تصميم جم / لتر تم تحقيق 1.5فولط وتركيز كلوريد الصوديوم = 4.6، جهد الخلية = 2الرقم الهيدروجيني = Cr (VI)تم تحقيق كفاءة عالية في ازالة أيونات . في هذه الدراسةCr (VI)الظروف المثلى إلزالة أيونات كانت استراتيجية مجدية وفعالة لتقييم البيانات BBDوأن طريقة 2MnOبأستخدام قطب ألياف الكربون / التجريبية في ظروف مختلفة. نانوي تركيب ال، الالترسيب الكهربائيمركب، ، القطب الالكهربائي، أيونات الكروم سداسية التكافؤ عملية االمتزاز الكلمات الدالة: 2MnO تصميم ،Behnken-Box.