An Enhanced Electrocoagulation Process for the Removal of Fe and Mn from Municipal Wastewater Using Dielectrophoresis (DEP)

water

Article An Enhanced Electrocoagulation Process for the Removal of Fe and Mn from Municipal Wastewater Using Dielectrophoresis (DEP)

Abdulkarim Almukdad, Alaa H. Hawari * and MhdAmmar Haﬁz

Department of Civil and Architectural Engineering, College of Engineering, Qatar University, Doha 2713, Qatar; [email protected] (A.A.); [email protected] (M.H.) * Correspondence: [email protected]

Abstract: In this study the removal of Fe and Mn from primary treated municipal wastewater using a new electrode configuration in electrocoagulation was evaluated. The used electrode configuration induced a dielectrophoretic (DEP) force in the electrocoagulation process. The impact of the electrolysis time, electrodes spacing and applied current on the removal of Fe and Mn was evaluated. The maximum removal percentages of Fe and Mn were obtained using an electrolysis time of 60 min, an electrode spacing of 0.5 cm and an applied current of 800 mA. Under these operating conditions and using the new electrodes configuration, the Fe and Mn removals were 96.8% and 66%, respectively. The main advantage of using the DEP-induced electrode configuration was the minimal consumption of the electrodes. The new electrode configuration showed 42% less aluminum content in the reactor compared to the aluminum electrodes with no DEP effect. The energy consumption at the selected operation conditions was 4.88 kWh/m3. The experimental results were comparable with the simulation results achieved by the COMSOL software.

Citation: Almukdad, A.; Hawari, Keywords: dielectrophoresis; electrocoagulation; heavy metal; municipal wastewater A.H.; Hafiz, M. An Enhanced Electrocoagulation Process for the Removal of Fe and Mn from Municipal Wastewater Using 1. Introduction Dielectrophoresis (DEP). Water 2021, 13, 485. https://doi.org/10.3390/ Municipal wastewater is known to be rich with multiple pollutants, including heavy w13040485 metal. Heavy metals are considered highly toxic elements as they can be easily absorbed by living organisms due to their high solubility in the aqueous environment. Therefore, it Academic Editor: Maria Gavrilescu is necessary to remove heavy metal from wastewater before its discharge to water bodies. Received: 17 January 2021 Heavy metals are usually removed from wastewater using conventional processes, such as Accepted: 9 February 2021 chemical precipitation and ion exchange. The removal of heavy metal using conventional Published: 13 February 2021 processes has several disadvantages, such as the production of toxic sludge and high energy consumption. Several advanced technologies, such as electrocoagulation (EC), Publisher’s Note: MDPI stays neutral have been used for the removal of heavy metal from wastewater. Electrocoagulation is with regard to jurisdictional claims in an electro-chemical process whereby chemical coagulants are generated by connecting published maps and institutional affil- sacrificial anodes to an electric current. Figure1 summarizes the main processes that occur iations. in electrocoagulation systems [1,2]. In the EC process, the main reactions occurring at the aluminum electrodes are as follows. At anode: Al → Al3++3e− (1) Copyright: © 2021 by the authors. + − 2H2O → 4H +O2(g)+4e (2) Licensee MDPI, Basel, Switzerland. (aq) This article is an open access article At cathode: distributed under the terms and − − 2H2O + 2e → H2(g)+2(OH)(aq) (3) conditions of the Creative Commons n+ Attribution (CC BY) license (https:// Metal cations (M ) may be reduced at the cathode surface electrochemically [3]: creativecommons.org/licenses/by/ n+ − 4.0/). M +ne → nM (4)

Water 2021, 13, 485. https://doi.org/10.3390/w13040485 https://www.mdpi.com/journal/water Water 2021, 13, x FOR PEER REVIEW 2 of 19

+ 2H2O→4H(aq)+O2(g)+4e (2) At cathode: 2H2O+2e →H2(g)+2(OH)(aq) (3) Metal cations (M) may be reduced at the cathode surface electrochemically [3]:

Mn++ne−→nM (4) The production of hydroxide ions at the cathode induces the metal ions’ precipitation by increasing the pH of the wastewater. This occurs in parallel with the precipitation of aluminum hydroxides [3]:

n+ − M +nOH →M(OH)n↓ (5) In addition, Al and Al(OH) are produced by the electrolytic dissolution of the aluminum anode at low pH values. Both of the cationic monomeric species transform Water 2021, 13, 485 2 of 18 initially into Al(OH), then polymerize to Al(OH) at appropriate pH values [4]: 3+ + The production of hydroxide ions at theAl(aq) cathode+3 inducesH2O the→ metalAl(OH) ions’ precipitation3+3H(aq) (6) by increasing the pH of the wastewater. This occurs in parallel with the precipitation of aluminum hydroxides [3]: nAl(OH)3→Aln(OH)3n (7) n+ − M +nOH → M(OH)n ↓ (5) However, other ionic species might be present in the solution depending on the + In addition, Al3+ and Al(OH)2 are produced by the electrolytic dissolution of the pH values of the medium, such as dissolved Al(OH) , Al(OH) and Al(OH) hy- aluminum anode at low pH values. Both of the cationic monomeric species transform initiallydroxide into Alcomplexes.(OH)3, then polymerize The pollutants to Aln(OH )can3n at appropriatebe removed pH values from [4 ]:the wastewater using the aluminum hydroxides by sorption, co-precipitation or electrostatic attraction, followed by Al3+ +3H O → Al(OH) +3H+ (6) coagulation [3]. Several(aq) studies2 have 3demonstrated(aq) the high removal efficiency of heavy

metal using electrocoagulanAl(OHtion)3 → (TableAln(OH) 3n1). (7)

FigureFigure 1. A schematic1. A schematic sketch of the sketch electrocoagulation of the electrocoagulation process. process.

However, other ionic species might be present in the solution depending on the pH 2+ 4+ − values of the medium, such as dissolved Al(OH) , Al2(OH)2 and Al(OH)4 hydroxide complexes. The pollutants can be removed from the wastewater using the aluminum hydroxides by sorption, co-precipitation or electrostatic attraction, followed by coagulation [3]. Several studies have demonstrated the high removal efficiency of heavy metal using electrocoagulation (Table1). Although electrocoagulation has shown high removal efficiency for different pollutants from wastewater, the energy and electrode consumption of EC need to be further improved. Recently, increased attention has been placed on the use of dielectrophoretic force (DEP) in electrocoagulation [14–16]. Alkhatib et al. (2020) evaluated the impact of using DEP force in the EC process for the removal of total phosphorus (TP) and chemical oxygen demand (COD) from secondary treated wastewater [14]. The removal of TP and COD increased by 24% and 18%, respectively, using an electrode configuration that induces

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a DEP force comparable to the conventional EC process. In addition, it was found that electrode corrosion was reduced by 87% when using DEP-inducing electrodes in the EC process. Hawari et al. (2020) investigated the impact of DEP-inducing electrodes in an EC process for the enhanced harvesting of marine microalgae [15]. It was found that the major significance of using the DEP-inducing electrodes in the EC process was that the aluminum content in the harvested biomass decreased by 52% compared to the conventional EC electrodes. Moreover, Hawari et al. (2020) studied the impact of the new electrodes configuration on the removal of total organic carbon (TOC) from primary treated municipal wastewater [16]. Using the new electrode configuration increased the removal of TOC by 7.2% in comparison to the conventional EC process. Furthermore, the energy consumption when using the new electrode configuration was 14% less than the regular EC process [16].

Table 1. Previous studies and the present study on the removal of heavy metals using the electrocoagulation process and electrocoagulation-enhanced dielectrophoresis, respectively.

Initial Con- Electrolysis Current Electrodes Electrodes Electrodes Removal Feed Water centration Time Density Spacing Ref. No Material Percentage (mg/L) (min) (mA/cm2) (cm) Cu: 45 Cu: 100% Metal Anode: Fe 3 sets Cr: 44.5 20 10 1 Cr: 100% [5] plating Cathode: Al Ni: 394 Ni: 100% Mn:250 Mn: 72.6% Anode: Fe Zn:250 Zn: 96% Model 3 sets 50 25 0.3 [6] Cathode: Fe Cu:250 Cu: 96% Ni: 250 Ni: 96% Synthetic Anode: Al 1 set Mn: 22.5 60 6.2 1 Mn: 39.6% [7] solution Cathode: Al Cu:100% Cu: 50–200 Synthetic Anode: Al Zn: 100% 2 sets Zn: 50–200 35 15 0.5 [8] solution Cathode: Al Mn: Mn: 50–200 80–85% Synthetic Anode: Al 1 set Mn: 100 30 6.2 1 Mn: 75% [9] solution Cathode: Al Fe (II): Drinking Anode: Al Fe (II): 20 1 set 60 4.31 1 100% [10] water Cathode: Al F: 10 F: 96% Anode: Al Fe (II): Groundwater 1 set Fe (II): 25 20 10 1 [11] Cathode: Al >97% Drinking Anode: Al Fe (II): 2 sets Fe (II): 20 20 1.5 0.5 [12] water Cathode: Al 98.5% Drinking Anode: Al Fe 1 set Fe (II): 20 45 2 1 [13] water Cathode: Al (II):98.6% Municipal Anode: Al Fe (II): 0.124 Fe (II): 94% Present 1 set 3060 22.86 0.5 wastewater Cathode: Al Mn: 0.118 Mn: 66% study

According to the ﬁndings from our previous studies, the DEP force can improve the removal of pollutants from wastewater and can reduce the corrosion of electrodes. This study proposes a more enhanced electrodes conﬁguration for the removal of Fe and Mn from primary treated municipal wastewater. From our previous studies, it was found that one electrode with rods could induce the required DEP force independently of the other electrode. Therefore, in this study we propose the use of two symmetrical electrodes with built-in rods. As such, each electrode is expected to produce a DEP force. The impact of the applied current, electrode spacing, and electrolysis time were investigated. Water 2021, 13, 485 4 of 18

2. Materials and Methods 2.1. Characteristics of the Wastewater Samples Primary treated municipal wastewater was used in the electrocoagulation process. The samples were collected from a municipal wastewater treatment plant located in the northern district of Doha, Qatar. The samples were collected after the grit removal stage and stored at a temperature of 2 ◦C to preserve the water quality. The sample was kept at room temperature before the experiment. The initial water quality of the water sample was the same for all experiments. The characteristics of the collected samples are summarized in Table2. The conductivity and pH of samples were measured using an OAKTON PCD650 multi-meter. The turbidity was measured using a turbidity meter (Hach 2100p). Heavy metal concentration was measured using ICP-MS (Nexion 300D).

Table 2. Characteristics of primary treated municipal wastewater.

Parameters Value Standard Method Temperature (◦C) 22.5 ± 0.3 APHA.2550 Temp pH 7.35 ± 0.01 APHA 4500-H+ B. Electro-metric Method Turbidity (NTU) 158 ± 2 APHA 2130 B. Nephelometric Method Conductivity (mS/cm) 2.20 ± 0.01 APHA 2510 B. Conductivity Fe (mg/L) 0.124 ± 0.001 Mn (mg/L) 0.118 ± 0.001 Al (mg/L) 0.038 ± 0.001 Zn (mg/L) 0.003 ± 0.001 Ni (mg/L) 0.003 ± 0.001 EPA Method 200.8 Cu (mg/L) 0.001 ± 0.001 Cd

2.2. Experimental Setup Two electrode configurations were used in this study. The first configuration used two symmetrical flat sheet aluminum electrodes with dimensions of 7 cm × 5 cm connected to an alternative power supply. This configuration will be referred to as AC-EC. The second configuration used two symmetrical aluminum electrodes with rods connected to an alternative current power supply; this configuration will be referred to as AC-DEP. The elemental composition of the aluminum alloy used to fabricate the electrodes is shown in Table3. In the AC-DEP configuration, seven aluminum rods with a diameter of 2 mm were attached to the 7 × 5 cm aluminum sheets. The distance between each rod was 1 cm center to center, with a distance of 0.5 cm from the electrode’s edge. Figure2 shows a schematic sketch of the experimental setup used in this study. A VARIAC transformer was used to generate the AC current with a voltage between 0 and 250 V and a frequency of 50 Hz. A TEKTRONIX oscilloscope was used to measure the voltage and the current in the system. The experiments were performed at room temperature. A magnetic stirrer was used at a stirring speed of 200 rpm to mix the solution in the reactor. After each experiment, the samples were left to settle for 1 h and then stored in the fridge at 4 ◦C before analysis. All samples were filtered using a 0.45 µm Millipore filter (HAWP04700). Then, the filtrate was analyzed for heavy metal concentration. The electrodes were washed with water and cleaned using sandpaper after each experiment.

Table 3. Elemental composition of the aluminum alloy (Alloy-1070) used to fabricate the electrodes. The values indicate the maximum limits unless indicated as minimum.

Element Si Fe Cu Mn Mg Cr Zn Ti Al (min) Wt. % 0.2 0.25 0.04 0.03 0.03 - 0.04 0.03 99.7 Water 2021, 13, x FOR PEER REVIEW 5 of 19

electrodes were washed with water and cleaned using sandpaper after each experiment.

Table 3. Elemental composition of the aluminum alloy (Alloy-1070) used to fabricate the electrodes. The values indicate the maximum limits unless indicated as minimum.

Element Si Fe Cu Mn Mg Cr Zn Ti Al (min) Water 2021, 13, 485 5 of 18 Wt. % 0.2 0.25 0.04 0.03 0.03 - 0.04 0.03 99.7

FigureFigure 2. AA schematic schematic sketch sketch for the for bench-scale the bench electrocoagulation-scale electrocoagulation setup used in this setup study. used in this study. The removal percentage of heavy metal was calculated using Equation (8). The removal percentage of heavy metal was calculated using Equation (8). Mi − M f M (%) = ×100 (8) MM (i%) = × 100 (8) where M (%) is the heavy metal removal efficiency, and Mi and M f are the initial and final whereheavy metalM (% concentrations) is the heavy in the metal wastewater removal (mg/L). efficiency, The specific and energy and consumption are the initial and finalwas calculatedheavy metal using Equationconcentration (9). s in the wastewater (mg/L). The specific energy con-

sumption was calculated using EquationU × I × (9t). E = (9) s V U × I × t = (9) 3 where Es is the specific energy consumption (Kwh/m ), UV is the electric potential (V), I is the applied current (A), t is the electrolysis time (h) and V is the sample volume (m3). where Es is the specific energy consumption (Kwh/m3), U is the electric potential (V), I is2.3. the Numerical applied Simulation current (A), t is the electrolysis time (h) and V is the sample volume (m3). The movement of a free particle in an inhomogeneous electric field is due to the 2.3.dielectrophoretic Numerical Simulation (DEP) force [17]. The placement of a particle in a solution with an inhomogeneous electric field will cause the redistribution of the charges in the particle. The redistributionThe movement of the charges of a depends free particle on the polarizability in an inhomoge of the mediumneous andelectric the particle field is due to the dielectrophoreticitself. The redistribution (DEP) of charges force on [17] the. surfaceThe placement of the particle of leads a particle to an induced in a solution dipole. with an in- homogeneousThe dipole will exertelectric a force field on thewill particle, cause named the redistribution the dielectrophoretic of the force charges [18]. The in the particle. dielectrophoretic force on the spherical particle can be calculated using the equation below: The redistribution of the charges depends on the polarizability of the medium and the 3 h i particle itself. The redistributionFDEP = 4πr εof0ε MchargesRe Ke (E ×on ∇ )theE surface of the particle (10) leads to an induced dipole. The dipole will exert a force on the particle, named the dielectrophoretic forceIn which [18]r is. theThe radius dielectrophoretic of the particle, ε M forceis relative on permittivity,the sphericalε0 is particle the free space can be calculated permittivity with a constant value of 8.854 × 10−12 (F/m), E is the electric field strength using the equationh i below: (V/m) and Re Ke is the Clausius Mossotti (CM) coefficient, calculated by the following equation [19]: F = 4π εε(E × ∇)E (10) ε p − ε M K = e f (11) e + In which r is the radius of theεe pparticle,2εfM is relative permittivity, is the free space permittivity with a constant valuejσ of 8.854 × 10 (F/m), E is the electric field ε = ε − (12) strength (V/m) and is the eClausiusω Mossotti (CM) coefficient, calculated by the following equation [19]:

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− K = (11) + 2 jσ ̃ = − (12) ω When an AC power supply is used, all the permittivity values will be replaced by the complex permittivity [18], whereby is the complex permittivity of the particle, is the complex permittivity of the medium, σ is the conductivity ( ), is the angular frequency ( ), ̃ is the complex permittivity and is the geometric gradient of the square of electric field (E), which can be calculated by [19]: Water 2021, 13, 485 6 of 18 1 = √−1 × (E × ∇)E = ∇|E| (13) 2 The direction ofWhen the anDEP AC powerforce supplydepends is used, on allthe the permittivity permittivity values of the will particle be replaced and by the me- complex permittivity [18], whereby εep is the complex permittivity of the particle, εfM is the dium. If the permittivity of the particle is higher than the permittivityS of the medium, complex permittivity of the medium, σ is the conductivity ( m ), ω is the angular frequency then the particle radwill move towards the stronger electric field, which is named positive ( s ), eε is the complex permittivity and j is the geometric gradient of the square of electric DEP. However, ﬁeldif the (E), permittivity which can be calculated of the byparticle [19]: is lower than the permittivity of the

medium, then the particle will move towards√ the weaker1 electric field, which is named j = −1 × (E × ∇)E = ∇|E| 2 (13) the negative DEP [18,20]. 2 DEP-inducing electThe directionrodes ofwere the DEP simulated force depends in the on the COMSOL permittivity Multiphysics of the particle and software. medium. The simulated electrodesIf the permittivity are shown of the particlein Figure is higher 3. As than noticeable the permittivity from of Equation the medium, (10 then), the DEP force is directlythe particle related will moveto the towards electric the field stronger squared. electric field,Therefore, which is namedthe square positive electric DEP. However, if the permittivity of the particle is lower than the permittivity of the medium, field was simulatedthen thein the particle model will move as an towards indicator the weaker of the electric DEP field, force. which Fixed is named boundary the negative conditions were appliedDEP [18 on,20 the]. surfaces of the electrodes: DEP-inducing electrodes were simulated in the COMSOL Multiphysics software. The simulated electrodes are shown inφ Figure = U3. As noticeable from Equation (10), the DEP (14) force is directly related to the electric field squared. Therefore, the square electric field was simulated in the model as an indicatorφ = of the0 DEP force. Fixed boundary conditions were (15) applied on the surfaces of the electrodes: U is the rms of the oscillating potential drop. The simulated medium was primary treated wastewater. The model was built in twoϕ1 dimensions,= U0 assuming that the width(14)

of the electrode is infinity. ϕ2 = 0 (15)

Figure 3. Illustration of the geometrical parameters of the simulated electrodes using three electrode spacings: (a) 0.5 cm, (b) 1 cm and (c) 1.5 cm.

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U0 is the rms of the oscillating potential drop. The simulated medium was primary treated wastewater. The model was built in two dimensions, assuming that the width of the electrode is inﬁnity.

2.4. Error Estimation All experimental runs were repeated three times. The reported result is the average of the experimental trials. The error shown represents the standard deviation of the results. All the standard deviation error bars did not exceed 3%.

3. Results 3.1. Numerical Simulation 3.1.1. Impact of the Electrode Spacing The impact of electrode spacing on the DEP force was evaluated using three different electrodes spacings (i.e., 0.5 cm, 1 cm and 1.5 cm) at an applied current of 600 mA. Figure4 shows that the DEP force presented by the electric ﬁeld squared (∇E2) was the highest when the distance between the electrodes was 0.5 cm, compared to 1 cm and 1.5 cm. At an electrodes spacing of 0.5 cm, the maximum ∇E2 was 1.2 × 1011 v2/m3 at the electrode surface, and the minimum ∇E2 was at the midpoint, with a value of 4.0 × 1010 v2/m3. At an electrodes spacing of 1.0 cm, the maximum ∇E2 was 7.0 × 109 v2/m3 at the electrode surface, and it went down to zero at a distance 2 mm away from the electrode surface. It was also noticed that a zone with no DEP effect was present. The zone extended for a length of 2 mm. At an electrodes spacing of 1.5 cm, the maximum ∇E2 was 3.4 × 109 v2/m3 at the electrode surface, and this went down to zero at a distance of 3 mm away from the electrode surface. It was also noticed that a zone with no DEP effect was present. The zone extended for a length of 5 mm.

3.1.2. Impact of the Applied Current The impact of current density on the DEP force field was evaluated using four applied currents, 200 mA, 400 mA, 600 mA and 800 mA, corresponding to current densities of 5.71, 11.43, 17.14 and 22.86 mA/cm2, respectively. The electrode spacing was fixed at 0.5 cm. As shown in Figure5, the DEP force field increased as the applied current increased. The DEP force field was minimal when using an applied current of 200 mA. However, the DEP force field affected a larger area when using an applied current of 400 mA, and became significant when using an applied current of 600 mA and 800 mA. Figure6 shows the impact of the applied current on the squared electric field. The electric field squared (∇E2) at the surface of the electrodes was almost 1.5 × 1010 v2/m3 using an applied current of 200 mA. The ∇E2 at the surface of the electrodes increased significantly, by 70%, when using an applied current of 400 mA. As the applied current increased to 600 mA, the ∇E2 increased by 36%, and was further enhanced by 54% at an applied current of 800 mA. Water 2021, 13, 485 8 of 18

Figure 4. DEP force ﬁeld (∇|E|2) using current of 600 mA and different electrodes spacings of (a) 0.5 cm, (b) 1 cm and (cFigure) 1.5 cm. 4. DEP force field (∇|E|2) using current of 600 mA and different electrodes spacings of (a) 0.5 cm, (b) 1 cm and (c) 1.5 cm.

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2 WaterFigure 2021 5., 13DEP, x FOR force PEER ﬁeld REVIEW (∇|E| ) using electrode spacing of 0.5 cm and different applied currents: (a) 200 mA, (b) 40010 mA,of 19 (c) 600Figure mA and 5. DEP (d) 800 force mA. field (∇|E|2) using electrode spacing of 0.5 cm and different applied currents: (a) 200 mA, (b) 400 mA, (c) 600 mA and (d) 800 mA.

2.502.50E+11×1011 200 mA

400 mA 2.002.00E+11×1011 600 mA ) 3

/m 11

2 1.501.50E+11×10 800 mA (V 2 E 11 ∇ 1.001.00E+11×10

5.005.00E+10×1010

0.000.00E+00×100 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 Distance (mm)

Figure 6. Electric ﬁeld squared using electrode spacing of 0.5 cm and different applied currents of 200 mA, 400 mA, 600 mA Figure 6. Electric field squared using electrode spacing of 0.5 cm and different applied currents of 200 mA, 400 mA, 600 and 800 mA. mA and 800 mA.

3.2. Experimental Study 3.2.1. Impact of the Electrode Spacing To evaluate the impact of electrode spacing, the applied current was fixed at 600 mA and the electrolysis time was 30 min. As shown in Figure 7a,b, the Fe and Mn removal percentage decreased as the electrode spacing increased using both the AC-EC and AC-DEP configurations. The Fe removal percentage using AC-DEP was slightly higher than with using AC-EC. At an electrode distance of 0.5 cm, the removal percentages of Fe using AC-EC and AC-DEP were 78.8% and 80.3%, respectively. The Fe removal percentage decreased by 12.5% as the electrode spacing increased to 1 cm using AC-EC and AC-DEP. The Fe2 removal percentage decreased by 9.1% as the electrode spacing increased to 1.5 cm using AC-EC and AC-DEP. The Mn removal percentage using AC-DEP was slightly higher than when using AC-EC. At an electrode distance of 0.5 cm, the removal percentages of Mn using AC-EC and AC-DEP were 28.8% and 29.6%, respectively. The removal percentage of Mn decreased by 19.9% as the electrode spacing increased from 0.5 cm to 1 cm, using both AC-EC and AC-DEP. The Mn removal percentage decreased by 4.9% as the electrode spacing increased from 1 cm to 1.5 cm using both AC-EC and AC-DEP. The maximum removal percentage of Fe and Mn was obtained using an electrode spacing of 0.5 cm. The removal percentage of Fe and Mn was lower at higher electrodes spacings due to the increase in the electrical resistance in the solution. Higher resistance would reduce the dissolution of coagulants from the electrodes, thus reducing the removal efficiency of pollutants in the system [21,22]. In addition, the enhancement of the removal percentage of Fe and Mn in the AC-DEP configuration compared to the AC-EC configuration could be due to the effect of the DEP force. The DEP force is expected to enhance the interaction between particles and the formation of flocs, and thus enhance the removal percentage of Fe and Mn. As the distance between the electrodes decreases, the DEP force increases and higher removal efficiencies of Fe and Mn are obtained. These results are compatible with the simulation results, where it was found that the DEP force was the highest at an electrode distance of 0.5 cm, and as the distance between the electrodes increased the DEP force decreased. In addition, from the simulation studies, it was found that at electrode distances of 1 cm and 1.5 cm, a zone of no DEP effect existed.

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3.2. Experimental Study 3.2.1. Impact of the Electrode Spacing To evaluate the impact of electrode spacing, the applied current was fixed at 600 mA and the electrolysis time was 30 min. As shown in Figure7a,b, the Fe and Mn removal percentage decreased as the electrode spacing increased using both the AC-EC and AC-DEP configurations. The Fe removal percentage using AC-DEP was slightly higher than with using AC-EC. At an electrode distance of 0.5 cm, the removal percentages of Fe using AC-EC and AC-DEP were 78.8% and 80.3%, respectively. The Fe removal percentage decreased by 12.5% as the electrode spacing increased to 1 cm using AC-EC and AC-DEP. The Fe removal percentage decreased by 9.1% as the electrode spacing increased to 1.5 cm using AC-EC and AC-DEP. The Mn removal percentage using AC-DEP was slightly higher than when using AC-EC. At an electrode distance of 0.5 cm, the removal percentages of Mn using AC-EC and AC-DEP were 28.8% and 29.6%, respectively. The removal percentage of Mn decreased by 19.9% as the electrode spacing increased from 0.5 cm to 1 cm, using both AC-EC and AC-DEP. The Mn removal percentage decreased by 4.9% as the electrode spacing increased from 1 cm to 1.5 cm using both AC-EC and AC-DEP. The maximum removal percentage of Fe and Mn was obtained using an electrode spacing of 0.5 cm. The removal percentage of Fe and Mn was lower at higher electrodes spacings due to the increase in the electrical resistance in the solution. Higher resistance would reduce the dissolution of coagulants from the electrodes, thus reducing the removal efficiency of pollutants in the system [21,22]. In addition, the enhancement of the removal percentage of Fe and Mn in the AC-DEP configuration compared to the AC-EC configuration could be due to the effect of the DEP force. The DEP force is expected to enhance the interaction between particles and the formation of flocs, and thus enhance the removal percentage of Fe and Mn. As the distance between the electrodes decreases, the DEP force increases and higher removal efficiencies of Fe and Mn are obtained. These results are compatible with the simulation results, where it was found that the DEP force was the highest at an electrode distance of 0.5 cm, and as the distance between the electrodes increased the DEP force decreased. In addition, from the simulation studies, it was found that at electrode distances of 1 cm and 1.5 cm, a zone of no DEP effect existed.

3.2.2. Impact of the Electrolysis Time Electrolysis times of 5, 10, 30 and 60 min were investigated. The applied current and the electrode spacing were fixed at 800 mA and 0.5 cm, respectively. As shown in Figure8a, the Fe removal percentage increased as the electrolysis time increased using both AC-EC and AC-DEP configurations. The removal percentage of Fe using AC-DEP was higher than when using AC-EC, where at an electrolysis time of 5 min, the removal percentages of Fe using AC-EC and AC-DEP were 61.3% and 69.3%, respectively. The Fe removal percentages increased by 14.7% (AC-EC) and 3.4% (AC-DEP) as the electrolysis time increased to 10 min. The Fe removal percentages increased by 14.5% (AC-EC) and 21.2% (AC-DEP) as the electrolysis time increased to 30 min. At an electrolysis time of 60 min, the Fe removal percentage increased by 15.4% using AC-EC, while the Fe removal percentage remained almost constant using AC-DEP. As shown in Figure8b, the Mn removal percentage increased as the electrolysis time increased using both AC-EC and AC- DEP modes. At an electrolysis time of 5 min, the removal percentages of Mn using AC-EC and AC-DEP were almost the same, with a value of 5%. As the electrolysis time increased to 10 min, the Mn removal percentages increased by 35.5% (AC-EC) and 56.8% (AC-DEP). At an electrolysis time of 30 min, the Mn removal percentage increased significantly by almost 80% using both AC-EC and AC-DEP configurations. At the electrolysis time of 60 min, the Mn removal percentage increased by 30.1% using AC-EC and 13.2% using AC-DEP. The electrolysis time had a major influence on the removal efficiency of Fe and Mn in the EC process, as it will affect the amount of metal ions produced by the electrodes [23]. As the electrolysis time increases more ions will be produced, and so higher removal efficiencies were expected [23]. It can be seen from Figure8a,b that the Water 2021, 13, 485 11 of 18

removal efficiency of Fe was higher than the removal efficiency of Mn. The higher removal efficiency of Fe could be attributed to the lower solubility of the formed iron hydroxides. The formation of manganese and iron hydroxides, and their precipitation, plays a dominant role in the removal mechanism of the corresponding metallic ions [9,24,25]. The solubility ◦ −13 −15 constants (Ksp) of manganese and iron hydroxides at 25 C are 1.9 × 10 and 2.0 × 10 , respectively. Figure8a,b show that at an electrolysis time of 60 min, the removal efficiency Water 2021, 13, x FOR PEER REVIEW of Fe and Mn in the AC-DEP configuration was less than that in the AC-EC configuration.11 of 19 This could be due to the fact that after a long period of DEP force application, the force could break the already-formed flocs and in turn reduce the removal efficiency.

80 (a)

Removal Removal (%) 30 Fe removal by AC-EC 20 Fe removal by AC-DEP 10

0 0.5 1 1.5 Spacing (cm)

30 (b)

15 Removal Removal (%) 10 Mn removal by AC-EC

5 Mn removal by AC-DEP

0 0.5 1 1.5 Spacing (cm)

Figure 7. TheFigure removal 7. The percentage removal of Fepercentage and Mn using of Fe variable and Mn electrode using spacing,variable electrolysis electrode timespacing, of 30 minelectrolysis and applied current 600time mA. (ofa) 30 Fe removalmin and percentage; applied current (b) Mn removal600 mA percentage.. (a) Fe removal percentage; (b) Mn removal percentage.

3.2.2. Impact of the Electrolysis Time Electrolysis times of 5, 10, 30 and 60 min were investigated. The applied current and the electrode spacing were fixed at 800 mA and 0.5 cm, respectively. As shown in Figure 8a, the Fe removal percentage increased as the electrolysis time increased using both AC-EC and AC-DEP configurations. The removal percentage of Fe using AC-DEP was higher than when using AC-EC, where at an electrolysis time of 5 min, the removal percentages of Fe using AC-EC and AC-DEP were 61.3% and 69.3%, respectively. The Fe removal percentages increased by 14.7% (AC-EC) and 3.4% (AC-DEP) as the electrolysis time increased to 10 min. The Fe removal percentages increased by 14.5% (AC- EC) and 21.2% (AC-DEP) as the electrolysis time increased to 30 min. At an electrolysis time of 60 min, the Fe removal percentage increased by 15.4% using AC-EC, while the Fe removal percentage remained almost constant using AC-DEP. As shown in Figure 8b, the Mn removal percentage increased as the electrolysis time increased using both AC-EC and AC-DEP modes. At an electrolysis time of 5 min, the removal percentages

Water 2021, 13, x FOR PEER REVIEW 12 of 19

of Mn using AC-EC and AC-DEP were almost the same, with a value of 5%. As the electrolysis time increased to 10 min, the Mn removal percentages increased by 35.5% (AC-EC) and 56.8% (AC-DEP). At an electrolysis time of 30 min, the Mn removal percentage increased significantly by almost 80% using both AC-EC and AC-DEP configurations. At the electrolysis time of 60 min, the Mn removal percentage increased by 30.1% using AC-EC and 13.2% using AC-DEP. The electrolysis time had a major influence on the removal efficiency of Fe and Mn in the EC process, as it will affect the amount of metal ions produced by the electrodes [23]. As the electrolysis time increases more ions will be produced, and so higher removal efficiencies were expected [23]. It can be seen from Figure 8a,b that the removal efficiency of Fe was higher than the removal efficiency of Mn. The higher removal efficiency of Fe could be attributed to the lower solubility of the formed iron hydroxides. The formation of manganese and iron hydroxides, and their precipitation, plays a dominant role in the removal mechanism of the corresponding metallic ions [9,24,25]. The solubility constants (Ksp) of manganese and iron hydroxides at 25 °C are 1.9 × 10−13 and 2.0 × 10−15, respectively. Figure 8a,b show that at an electrolysis time of 60 min, the removal efficiency of Fe and Mn in the AC- DEP configuration was less than that in the AC-EC configuration. This could be due to the fact that after a long period of DEP force application, the force could break the already-formed flocs and in turn reduce the removal efficiency. It was found that with the increase in electrolysis time, the pH value increased using both AC-EC and AC-DEP configurations (Figure 9). Initially, the pH was 7.35, and it increased to 8.58 at an electrolysis time of 60 min for AC-EC and 8.18 for AC- DEP. It was observed that the final pH value for AC-DEP was always lower than the final pH value for the AC-EC, especially at a longer electrolysis time. The higher pH value is an indicator of the higher production of hydroxide ions at the cathode, and the higher corrosion of electrodes. Even though the production of hydroxide ions was higher in the AC-EC configuration, the removal efficiency in the AC-DEP configuration was higher for Fe and the same for Mn with a 30 min electrolysis time. This could indicate that the removal efficiency using the AC-DEP configuration was enhanced by the exertion of the DEP force. At an electrolysis time of 60 min, the removal efficiency of Mn using the AC-DEP configuration was almost 17% less than the removal efficiency of Mn using the AC-EC configuration. The higher pH value using the AC-EC configu- Water 2021, 13, 485ration would decrease the solubility of the metal hydroxide; thus, a higher removal ef-12 of 18 ficiency is expected.

100 90 80 70 60 (a) 50 40 Removal Removal (%) 30 Fe removal by AC-EC 20 Fe removal by AC-DEP 10

Water 2021, 13, x FOR PEER REVIEW 0 13 of 19 0 10 20 30 40 50 60 Time (min)

50 (b)

Removal Removal (%) 30 Mn removal by AC-EC 20 Mn removal by AC-DEP 10

0 0 5 10 15 20 25 30 35 40 45 50 55 60 Time (min)

Figure 8. TheFigure removal 8. The percentage removal of percentage Fe and Mn usingof Fe variableand Mn electrolysis using variable time, electrodeelectrolysis spacing time, 0.5 electrode cm and applied spac- current 800ing mA. 0.5 (a) cm Fe removaland applied percentage; current (b) 800 Mn removalmA. (a) percentage. Fe removal percentage; (b) Mn removal percentage.

3.2.3. Impact of theIt wasApplied found Current that with the increase in electrolysis time, the pH value increased using both AC-EC and AC-DEP configurations (Figure9). Initially, the pH was 7.35, and it Four differentincreased applied to 8.58 currents at an electrolysis were tested time: 200, of 60 400, min 600 for and AC-EC 800 and mA 8.18, correspond- for AC-DEP. It ing to currentwas densities observed of that 5.71, the 11.43, final pH17.14 value and for 22.86 AC-DEP mA/cm was2 always, respectively. lower than The the elec- final pH trolysis time valuewas fixed for the at AC-EC, 30 min especially and the atelectrode a longer spacing electrolysis was time. 0.5 Thecm. higherAs shown pH value in is Figure 10, thean removal indicator percentage of the highers of production Fe and Mn of hydroxide increased ions as atthe the applied cathode, current and the in- higher creased. Whencorrosion using ofan electrodes. applied Evencurrent though of 200 the productionmA the Fe of hydroxideremoval p ionsercentage was higher was in the around 28% using both AC-EC and AC-DEP. The Fe removal percentage increased by 46.2% and 54.8% using an applied current of 400 mA compared to an applied current of 200 mA in the AC-EC and AC-DEP configurations, respectively. The Fe removal percentage increased to around 80% using an applied current of 600 mA in both the AC- EC and AC-DEP configurations. The maximum removal percentage of Fe was 94.3%, obtained using an applied current of 800 mA in the AC-DEP configuration. The removal percentage of Mn was 4.2% using an applied current of 200 mA in the AC-EC configuration. The Mn removal percentage increased by 72.3% and 93.3% using an applied current of 400 mA, compared to an applied current of 200 mA in the AC-EC and AC-DEP configurations, respectively. The Mn removal percentage increased to almost 30% using an applied current of 600 mA in both the AC-EC and AC-DEP configurations. The maximum removal percentage of Mn was 63.6%, obtained using an applied current of 800 mA in both the AC-EC and AC-DEP configurations. The production of Al3+ ions increases as the current density increases, due to the enhanced dissolution of the aluminum electrodes. Therefore, the amount of trapped heavy metal increased due to the increased amount of coagulants [26,27]. In addition, higher applied current means higher DEP force, as the DEP force is directly proportional to the applied voltage. Higher DEP force means greater interaction between particles, and hence more floc formation and higher removal efficiencies. Figure 11 illustrates the impact of the DEP force on the removal of heavy metals using the electrocoagulation process. The DEP force pushes the particles away from the electrodes exerting a negative DEP force. This force is expected to enhance the agglomeration of heavy metals, thus enhancing the removal efficiency. Moreover, the exertion of the DEP force would reduce the accumulation of flocs on the electrodes’ surfaces.

Water 2021, 13, 485 13 of 18

AC-EC configuration, the removal efficiency in the AC-DEP configuration was higher for Fe and the same for Mn with a 30 min electrolysis time. This could indicate that the removal efficiency using the AC-DEP configuration was enhanced by the exertion of the DEP force. At an electrolysis time of 60 min, the removal efficiency of Mn using the AC-DEP configuration was almost 17% less than the removal efficiency of Mn using the AC-EC Water 2021, 13, x FOR PEER REVIEW 14 of 19 configuration. The higher pH value using the AC-EC configuration would decrease the solubility of the metal hydroxide; thus, a higher removal efficiency is expected.

9.0

8.8

8.6

8.4

8.2

8.0

pH value 7.8

7.6 pH Value by AC-EC 7.4 pH Value by AC-DEP 7.2

7.0 0 5 10 15 20 25 30 35 40 45 50 55 60 Time (min)

Figure 9. 9.The The pH pH values values using using variable variable electrolysis electrolysis time, electrode time, electrode spacing 0.5spacing cm and 0.5 applied cm and current applied 800current mA. 800 mA. 3.2.3. Impact of the Applied Current 100Four different applied currents were tested: 200, 400, 600 and 800 mA, corresponding 2 to current90 densities of 5.71, 11.43, 17.14 and 22.86 mA/cm , respectively. The electrolysis time was fixed at 30 min and the electrode spacing was 0.5 cm. As shown in Figure 10, the removal80 percentages of Fe and Mn increased as the applied current increased. When using an applied current of 200 mA the Fe removal percentage was around 28% using 70 (a) both AC-EC and AC-DEP. The Fe removal percentage increased by 46.2% and 54.8% using an applied60 current of 400 mA compared to an applied current of 200 mA in the AC-EC and AC-DEP configurations, respectively. The Fe removal percentage increased to around 50 80% using an applied current of 600 mA in both the AC-EC and AC-DEP configurations. The maximum40 removal percentage of Fe was 94.3%, obtained using an applied current

of 800 Removal (%) mA in the AC-DEP configuration. The removal percentage of Mn was 4.2% using Fe removal by AC-EC an applied30 current of 200 mA in the AC-EC configuration. The Mn removal percentage increased20 by 72.3% and 93.3% using an applied current of 400 mA,Fe removal compared by AC-DEP to an applied current of 200 mA in the AC-EC and AC-DEP configurations, respectively. The Mn removal percentage10 increased to almost 30% using an applied current of 600 mA in both the AC-EC and AC-DEP0 configurations. The maximum removal percentage of Mn was 63.6%, obtained using an200 applied current of 800 mA400 in both the AC-EC and600 AC-DEP configurations. The800 production of Al3+ ions increases as the current density increases, due to the enhanced Current (mA) dissolution of the aluminum electrodes. Therefore, the amount of trapped heavy metal increased due to the increased amount of coagulants [26,27]. In addition, higher applied current means higher DEP force, as the DEP force is directly proportional to the applied

Water 2021, 13, x FOR PEER REVIEW 14 of 19

9.0

8.8

8.6

8.4

8.2

8.0

pH value 7.8

7.6 pH Value by AC-EC Water 2021, 13, 485 14 of 18 7.4 pH Value by AC-DEP 7.2

voltage.7.0 Higher DEP force means greater interaction between particles, and hence more ﬂoc formation0 and5 higher10 removal15 20 efﬁciencies.25 Figure30 35 11 illustrates40 45 the impact50 of55 the DEP60 force on the removal of heavy metals usingTime the electrocoagulation (min) process. The DEP force pushes the particles away from the electrodes exerting a negative DEP force. This force

is expected to enhance the agglomeration of heavy metals, thus enhancing the removal efﬁciency.Figure 9. The Moreover, pH values the exertionusing variable of the electrolysis DEP force would time, electrode reduce the spacing accumulation 0.5 cm and of ﬂocsapplied oncurrent the electrodes’ 800 mA. surfaces.

100 90 80 70 (a) 60 50 40 Removal Removal (%) 30 Fe removal by AC-EC 20 Fe removal by AC-DEP 10

Water 2021, 13, x FOR PEER REVIEW 0 15 of 19 200 400 600 800 Current (mA)

50 (b)

30 Removal Removal (%) 20 Mn removal by AC-EC

10 Mn removal by AC-DEP

0 200 400 600 800 Current (mA)

Figure 10.10. TheThe removalremoval percentage percentage of of Fe Fe and and Mn Mn using using variable variable applied applied current, current, electrolysis electrolysis time time30 min 30 and min electrode and electrode spacing spacing 0.5 cm. (0.5a) Fecm removal. (a) Fe percentage;removal percentage (b) Mn removal; (b) Mn percentage. removal percentage. The main advantage of using the DEP-inducing electrodes lies not only in the enhanced removal efﬁciency, but also in the amount of consumed Al in the EC process. This can reduce the electrode consumption and the amount of aluminum species in the treated efﬂuent. Figure 12 shows the aluminum content in the reactor with respect to the applied

Figure 11. Schematic diagram of the impact of DEP force on the enhancement of flocs formation in the EC process.

Water 2021, 13, x FOR PEER REVIEW 15 of 19

50 (b)

30 Removal Removal (%) Water 2021, 13, 485 20 15 of 18 Mn removal by AC-EC

10 Mn removal by AC-DEP current. The aluminum content increased as the applied current increased, using both AC-EC0 and AC-DEP configurations. However, the aluminum content obtained using the 200 400 600 800 AC-DEP configuration was much lower than with the AC-EC configuration. At an applied current of 400 mA, the aluminumCurrent content (mA) using AC-DEP was almost 31% less than with

the AC-EC. As the applied current increased, the difference in aluminum content also Figureincreased. 10. The Theremoval aluminum percentage content of Fe and using Mn AC-DEPusing variable was applied almost current, 42% less electrolysis than with AC-EC at timean 30 applied min and current electrode of spacing 800 mA. 0.5 cm. (a) Fe removal percentage; (b) Mn removal percentage.

Water 2021, 13, x FOR PEER REVIEW 16 of 19

The main advantage of using the DEP-inducing electrodes lies not only in the enhanced removal efficiency, but also in the amount of consumed Al in the EC process. This can reduce the electrode consumption and the amount of aluminum species in the treated effluent. Figure 12 shows the aluminum content in the reactor with respect to the applied current. The aluminum content increased as the applied current increased, using both AC-EC and AC-DEP configurations. However, the aluminum content obtained using the AC-DEP configuration was much lower than with the AC-EC configuration. At an applied current of 400 mA, the aluminum content using AC -DEP was almost 31% less than with the AC-EC. As the applied current increased, the difference Figure 11. Schematic diagram of the impact of DEP force on the enhancement of flocs formation Figure 11. Schematic diagram of the impact of DEP force on the enhancement of ﬂocs formation in inin the aluminum EC process. content also increased. The aluminum content using AC-DEP was almost 42%the ECless process. than with AC-EC at an applied current of 800 mA.

1.00

0.90

0.80 Al content using AC-EC 0.70 Al content using AC-DEP 0.60

0.50

0.40

0.30 Concentration (mg/L) 0.20

0.10

0.00 0 200 400 600 800 Current (mA)

FigureFigure 12. 12. AluminumAluminum content content in the in reactor the reactor using usingAC-EC AC-EC and AC and-DEP AC-DEP with 0.5 cm with electrodes 0.5 cm electrodes spacingspacing and and an an operational operational time time of of30 30min. min.

3.2.4. Energy Consumption The energy consumption of the system was evaluated for the four different applied currents (i.e., 200, 400, 600 and 800 mA). The electrode spacing and electrolysis time were fixed at 0.5 cm and 30 min, respectively. As shown in Figure 13, the energy consumption increased as the applied current increased. When using an applied current of 600 mA, the energy consumption in the AC-DEP was 3.13 kWh/m3, which is 2% lower than with AC-EC, and at an applied current of 800 mA the energy consumption obtained using the AC-DEP was 4.88 kWh/m3, which is 3% lower than with AC-EC. The energy consumption was almost the same using AC-EC and AC-DEP configurations. The minimal differences in the energy consumption between the two configurations are due to the differences in the resistance in each system. The shape of the electrodes configuration affected the resistance in the system, thereby affected the amount of consumed energy.

Water 2021, 13, 485 16 of 18

3.2.4. Energy Consumption The energy consumption of the system was evaluated for the four different applied currents (i.e., 200, 400, 600 and 800 mA). The electrode spacing and electrolysis time were fixed at 0.5 cm and 30 min, respectively. As shown in Figure 13, the energy consumption increased as the applied current increased. When using an applied current of 600 mA, the energy consumption in the AC-DEP was 3.13 kWh/m3, which is 2% lower than with AC-EC, and at an applied current of 800 mA the energy consumption obtained using the AC-DEP was 4.88 kWh/m3, which is 3% lower than with AC-EC. The energy consumption was almost the same using AC-EC and AC-DEP configurations. The minimal differences in the energy consumption between the two configurations are due to the differences in the Water 2021, 13, x FOR PEER REVIEW 17 of 19 resistance in each system. The shape of the electrodes configuration affected the resistance in the system, thereby affected the amount of consumed energy.

) 5 3

2 AC-EC AC-DEP Energy consumption Energy consumption (kWh/m 1

0 0 200 400 600 800 Current (mA)

Figure 13. AC-EC and AC-DEP energy consumption using 30 min of operational time and 0.5 cm electrodesFigure 13. spacingAC-EC and on four AC- differentDEP energy currents: consump 200,tion 400, using 600 and 30 min 800 mA.of operational time and 0.5 cm electrodes spacing on four different currents: 200, 400, 600 and 800 mA. 4. Conclusions 4. ConclusionsIn this study, a new electrocoagulation (EC) electrode configuration has been inves- tigatedIn this for thestudy, removal a new of electrocoagulation Fe and Mn from primary (EC) electrode treated configuration municipal wastewater. has been in- The effectvestigated of electrolysis for the removal time, electrodes of Fe and spacing Mn from and primary current ontreated the removal municipal of Fe wastewater. and Mn was investigated.The effect of electrolysis The experimental time, electrodes results showed spacing the and following: current on the removal of Fe and •Mn wasAs theinvestigated. electrolysis The time experimental increased, theresults removal showed of Fe the and following: Mn increased. As the elec-  trolysisAs the electrolysis time increased time more increased, ions were the removal produced, of Fe and and so higherMn increased. removal As efficiencies the were expected. The maximum removal percentages of Fe and Mn were obtained at electrolysis time increased more ions were produced, and so higher removal effi- an electrolysis time of 60 min. The removal of Fe was 99.2% using AC-EC and 96.8% ciencies were expected. The maximum removal percentages of Fe and Mn were using AC-DEP. The removal of Mn was 83.5% and 66% using AC-EC and AC-DEP, obtained at an electrolysis time of 60 min. The removal of Fe was 99.2% using respectively; AC-EC and 96.8% using AC-DEP. The removal of Mn was 83.5% and 66% using • As the distance between the electrodes decreased, the Fe and Mn removal increased. AC-EC and AC-DEP, respectively; This was due to the decrease in the electrical resistance in the solution. A lower  As the distance between the electrodes decreased, the Fe and Mn removal in- resistance would increase the dissolution of coagulants from the electrodes, thus creased. This was due to the decrease in the electrical resistance in the solution. A increasing the removal efficiency of pollutants in the system. The maximum Fe and lower resistance would increase the dissolution of coagulants from the electrodes, Mn removal was observed at an electrode distance of 0.5 cm; thus increasing the removal efficiency of pollutants in the system. The maximum • As the applied current increased, the removal efficiency increased. The production of Fe and Mn removal was observed at an electrode distance of 0.5 cm; Al3+ ions increased as the current increased due to the enhanced dissolution of the  As the applied current increased, the removal efficiency increased. The produc- aluminum electrodes. Therefore, the amount of trapped heavy metal increased due to tion of Al3+ ions increased as the current increased due to the enhanced dissolution of the aluminum electrodes. Therefore, the amount of trapped heavy metal increased due to the increased amount of coagulants. The maximum Fe and Mn removal percentage was obtained at an applied current of 800 mA;  The applied current had the highest impact on the removal efficiency of Fe and Mn ions when using AC-EC and AC-DEP. As the applied current increased from 200 mA to 800 mA using AC-EC, the removal efficiencies of Fe and Mn increased by 56% and 60%, respectively. Similarly, as the applied current increased from 200 mA to 800 mA using AC-DEP, the removal efficiencies of Fe and Mn increased by 66% and 63%, respectively;  The main advantage of using the new DEP-inducing electrodes lies not only in the enhanced removal efficiency, but also in the amount of consumed Al in the EC process. The aluminum content increased as the applied current increased

Water 2021, 13, 485 17 of 18

the increased amount of coagulants. The maximum Fe and Mn removal percentage was obtained at an applied current of 800 mA; • The applied current had the highest impact on the removal efficiency of Fe and Mn ions when using AC-EC and AC-DEP. As the applied current increased from 200 mA to 800 mA using AC-EC, the removal efficiencies of Fe and Mn increased by 56% and 60%, respectively. Similarly, as the applied current increased from 200 mA to 800 mA using AC-DEP, the removal efficiencies of Fe and Mn increased by 66% and 63%, respectively; • The main advantage of using the new DEP-inducing electrodes lies not only in the enhanced removal efficiency, but also in the amount of consumed Al in the EC process. The aluminum content increased as the applied current increased using both AC-EC and AC-DEP configurations. However, the aluminum content obtained using the AC-DEP configuration was much lower than with the AC-EC configuration. The aluminum content using AC-DEP was almost 42% less than the AC-EC at an applied current of 800 mA.

Author Contributions: Methodology, A.H.H. and M.H.; data curation, A.A. and M.H.; formal analysis, A.A., A.H.H. and M.H.; manuscript drafting, A.A.; manuscript review and edit: A.H.H. and M.H. All authors have read and agreed to the published version of the manuscript. Funding: This project is funded by Qatar National Research Fund (QNRF) under Qatar Research Leadership Program—Graduate Sponsorship Research award (GSRA5-2-0525-18072) and (GSRA6-1- 0509-19021). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: All data generated or analyzed during this study are included in this published article. Acknowledgments: This research was made possible by Awards (GSRA5-2-0525-18072) and (GSRA6- 1-0509-19021) from Qatar National Research Fund (a member of Qatar Foundation). The authors would like to thank the Central Laboratories Unit at Qatar University for the measurement of heavy metal. The authors also wish to thank Qatar Works Authority (Ashghal) for the supply of wastewater samples. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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