Cu(II), and Cr(VI)

water

Article Removal Mechanism and Eﬀective Current of Electrocoagulation for Treating Wastewater Containing Ni(II), Cu(II), and Cr(VI)

Chien-Hung Huang 1, Shan-Yi Shen 2 , Cheng-Di Dong 3 , Mohanraj Kumar 2 and Jih-Hsing Chang 2,* 1 Department of Health Wellness and Marketing, Kainan University, Taoyuan 33857, Taiwan; [email protected] 2 Department of Environmental Engineering and Management, Chaoyang University of Technology, 168, Jifeng E. Rd., Taichung 41349, Taiwan; [email protected] (S.-Y.S.); [email protected] (M.K.) 3 Department of Marine Environmental Engineering, National Kaohsiung University of Science and Technology, Kaohsiung 81157, Taiwan; [email protected] * Correspondence: [email protected]

Received: 7 August 2020; Accepted: 15 September 2020; Published: 18 September 2020

Abstract: This study aims to clarify the removal mechanism and to calculate the effective current of electrocoagulation (i.e., EC) for treating wastewater containing Ni(II), Cu(II), and Cr(VI). The adsorption behavior of various heavy metals onto Al(OH)3 coagulant generated by the EC process was investigated and the estimating method of the corresponding current was established. Results indicate that adsorption of single Ni(II) and Cu(II) by Al(OH)3 coagulant can be simulated by the Langmuir isotherm, while Cr(VI) adsorption fits the Freundlich isotherm better. As treating single heavy metal of wastewater, the removal mechanism of the EC process is the adsorption reaction. Under the coexisting condition, the Ni(II) and Cu(II) will compete for the same active sites on the Al(OH)3 surface and Cu(II) suppresses Ni(II) adsorption. As treating the coexisting heavy metals, Ni(II) removal not only associates with adsorption but also with the coprecipitation. In contrast, Cr(VI) does not compete with other metal ions for the same type of adsorption sites. Whether single or coexisting conditions, the adsorption capacity of heavy metals onto Al(OH)3 coagulants can be used to compute the necessary current to effectively remove heavy metals in the EC system.

Keywords: adsorption; aluminum hydroxide; electrocoagulation; heavy-metal wastewater

1. Introduction Wastewater from metal-finishing facilities is troublesome for bio-treatment due to its complexity and toxicity. A typically waste stream from metal-finishing facilities usually contains different kinds of metal ions such as copper, nickel, cadmium, and chromium ions, along with chelating agents, oil and grease, organic solvents, and suspended solid particulates [1]. In addition to the complexity, the concentration of these heavy metals in wastewater is so high that further biological processes fail. Two significant reviews in recent years have summarized traditional and emerging techniques that apply to treating such wastewater [2,3]. Among the methods reviewed are chemical precipitation, coagulation/flocculation, adsorption, electro-coagulation, flotation, ion exchange, and membrane filtration. Chemical precipitation, especially hydroxide precipitation, was adapted early and is still recommended by US EPA for metal ion removal [1,4]. Coagulation is employed for the removal of contaminants in suspended or colloidal forms. Adsorption is the binding mechanism between soluble contaminants and solids that can provide a significant amount of active surface.The electro-coagulation (i.e., EC)

Water 2020, 12, 2614; doi:10.3390/w12092614 www.mdpi.com/journal/water Water 2020, 12, 2614 2 of 14 integrates the chemical precipitation, coagulation, and adsorption. The EC technique is based on the electrochemical reaction at the anode under alkaline pH condition, which leads to the removal function of hydroxide precipitation, coagulation, and adsorption (i.e., soluble pollutants of wastewater are adsorbed on the coagulant). Due to its high removal efficiency and easy operation, the EC system has been employed as the pretreatment process for the metal-finishing industry in Taiwan. In an EC system, one pair of electrodes is installed in the electrochemical reactor and the anode (e.g., iron or aluminum) is used to produce the coagulant. With aluminum as the sacrificial 3+ anode, aluminum ions (Al ) and the hydroxyl group (OH−) are produced at the anode and cathode, respectively. The precipitation of aluminum hydroxides will present under the alkaline pH condition. Then, aluminum hydroxide precipitate acts as an adsorbent at pH between 5.5 and 8 to adsorb heavy metal ions and acts as a coagulant above pH 8 to promote the coagulation process. In past research, extensive work has been conducted to explore the potential of EC for industrial wastewater treatment [5–8]. Some efforts focused on the removal of heavy metal ions [9–11]. Huang et al. reported the effect of coexisting anions on the formation of aluminum coagulant, resulting in the removal of metal ions [11]. Another study was conducted for the US Air Force to evaluate the effectiveness of EC for the removal of chromium, cadmium, nickel, and fluoride from leachate generated in the processing of spent abrasive blast media [12]. Recently, several researchers have studied the removal efficiencies of multiple heavy metals from different wastewater by EC [13–18]. Through different operating parameters, the EC is capable of obtaining the satisfactory removal of heavy metals and the optimal operating conditions were documented. However, rare studies clarified the competitive adsorption of coexisting heavy-metal ions during the EC process. Heidmann and Calmano proposed that the formation of heavy-metal hydroxides coupled with the coprecipitation with aluminum hydroxide is the main EC mechanism to remove zinc, copper, nickel, and silver ions [19]. However, Adhoum et al. reported that the optimal EC condition for heavy-metal removal was between pH 4 and 8 [20]. Accordingly, it needs reasonable theory to explain why the concentration of heavy-metal ions starts dropping as the pH is less than 7 during the EC process. In addition, the EC took 15 min to reduce Zn(II), Ni(II), and Cu(II) concentrations to acceptable levels, in contrast, the removal for Ag(I) and Cr(VI) was only 30% under 15-min treatment. In terms of removal kinetics, Adhoum et al. found that the removal of Cr(VI) was about five times slower than that of Cu(II) and Zn(II). Apparently, the adsorption behavior of various heavy metals in the EC system plays a key role in removal. Akbal and Camci reported that with aluminum sulfate coagulation, 99.9% removal of Cu(II), Ni(II), and Cr(VI) was achieved by dosages of 500, 1000, and 2000 mg/L, respectively. They proposed that the removal of these metal ions was due to the combination of coagulation, adsorption, and coprecipitation [21]. Although significant progress has been made, the quantitative adsorption relationship between heavy metal ions and the EC coagulant is still unclear, which results in the difficulty of applying the EC process. The EC process is usually employed to treat industrial wastewater containing complex heavy-metal compounds and organic chemical agents [22–24]. Before the EC application, the adsorption capacity of the coagulant for removing target heavy metals should be calculated correctively. Based on this adsorption capacity, the operational parameter of EC electricity can be computed to produce the appropriate amount of coagulant. Consequently, the EC process can effectively achieve the removal efficiency. In order to construct the above quantitative association, the adsorption model of coexisting nickel, copper, and chromium in the EC process (aluminum as the sacrificed anode) was established to clarify the competitive adsorption behavior of the above three heavy metals. Moreover, this adsorption model with the EC process was applied to treat wastewater samples collected from an electroplating facility to verify its practicability. Water 2020, 12, 2614 3 of 14

2. Materials and Methods

2.1. Metal Removal of EC System At the cathode of the EC system, local hydroxyl ions can be generated due to water and oxygen reductions as the following equations [25]:

2H O + 2e− 2OH− +H (1) 2 → 2

2H O + O + 4e− 4OH− (2) 2 2 → With an aluminum sheet as the consumable anode, Al3+ ions will be produced by the oxidation reaction.

3+ Al Al + 3e− (3) → In the bulk solution, aluminum hydroxide will precipitate out as colloidal suspended particles in an EC system. 3+ Al + 3OH− Al(OH) (4) → 3 Al(OH)3 is one kind of amphoteric hydroxide, that is, most of Al(OH)3 will be formed between pH 5 and 8 (the lowest solubility at pH 6.4). The existence of Al(OH)3 serves as the coagulant to remove the heavy metal ions in the wastewater. As the EC operation prolongs, the solution pH will increase beyond 8 and Al(OH)3 will gradually dissolve in the alkaline solution. Under the high pH status of the EC system (pH above 8), precipitates of metal hydroxides such as Ni(OH)2, Cu(OH)2, and Cr(OH)3 are formed. In contrast to Cr(OH)3, hexavalent chromium (Cr(VI)) ion does not react with OH− at a high pH condition. Its removal is most likely due to adsorption by the coagulant of Al(OH)3. The other possible removal method of Cr(VI) in an EC process is that the Cr(VI) ion may be reduced to Cr(III) in the vicinity of the cathodes [19]. Then, the Cr(III) ions react with OH− at a high pH to precipitate in the wastewater. 6+ 3+ Cr + 3e− Cr (5) → 2.2. Adsorption Isotherms of Metal Ions As described in the previous section, heavy-metal ions can be adsorbed on the surface of aluminum hydroxide at a pH below 8. In the industrial wastewater, the EC process (aluminum as the sacriﬁced anode) should remove the nickel, copper, and chromium ions simultaneously. Consequently, the quantitative association between Ni, Cu, Cr ions and aluminum hydroxide should be established to clarify competitive adsorption behaviors. In this study, Freundlich and Langmuir adsorption isotherms were deployed to ﬁt experimental data, respectively. The Freundlich isotherm describes adsorption as the following equation: q = Kyn (6) where q is the adsorption capacity of heavy metal ions on the adsorbent (mg/g), and y is the equilibrium concentration of heavy metal ions in the bulk solution (mg/L). The constants, n and K, are constants that depend on the nature of the heavy-metal ions and the adsorbent at a particular temperature. These two constants can be determined by a log-log plot of q versus y. The Langmuir isotherm has a strong theoretical basis. The basis relies on a postulated chemical reaction between solute and vacant sites on the absorbent surface. The model is expressed as:

q = qo K y/(1 + K y) (7) Water 2020, 12, x; doi: FOR PEER REVIEW 4 of 13

2.3. Adsorption Experiments in an EC System A bench-scale electro-coagulation system, as shown in Figure 1, was constructed to study the adsorption features of heavy-metal ions onto the Al(OH)3 and the effectiveness of EC treatment for heavy metal removal. In addition, the schematic of the surface adsorption of metal ions in the electrocoagulation system is shown in Figure 2. The details of the chemicals information and the Water 2020, 12, 2614 4 of 14 specification of the EC system can be found in a previous report [11]. Two series of experiments, i.e., indirect and direct EC tests, were designed to respectively distinguish the contribution of adsorption andwhere coprecipitationqo is the adsorption mechanism capacity on the of heavy removal metal of heavy ions on metal the adsorbent ions from (mg wastewater./g). The way For tothe obtain indirect the 1 1 1 ECvalue test, of Al(OH)qo and K3 wasis to plotgenerated (q− )versus in the (yEC− ),reactor as the followingand transferred equation, to a the test intercept tube where is at (aq ocertain− ) and 1 concentrationthe slope is (q oofK− Ni,). Cu, and Cr ions was prepared to react with the coagulant (i.e., Al(OH)3 adsorption reaction). The pH was controlled1/q = in1/ qao ti+ght1/q rangeoK(1/y from) 5.0 to 7.0 during whole the electro- (8) coagulation process, to avoid the occurrence of hydroxide precipitation. The sulfuric acid (>95% of purity)2.3. Adsorption and sodium Experiments hydroxide in an (>97% EC System of purity) were used for acidic or basic adjustment of the solution.A bench-scale Four groups electro-coagulation of test tubes were system,prepared; as each shown group in Figure contained1, was different constructed concentrations to study the of single Ni, single Cu, single Cr, and mixed Ni, Cu, and Cr ions, respectively. After a 3-min vigorous adsorption features of heavy-metal ions onto the Al(OH)3 and the effectiveness of EC treatment for heavy mixingmetal removal. and 1 h of In settling, addition, the the pH schematic and heavy-metal of the surface concentration adsorption of of metal the supernatant ions in the electro-coagulation was determined. Thesystem heavy is shown metal inadsorption Figure2. The capacity details of of the the coagulant chemicals informationwas calculated and based the specification on the heavy-metal of the EC concentrationsystem can be of found supernatant. in a previous At last, report the adsorpti [11]. Twoon series relationship of experiments, between i.e.,heavy-metal indirect and ions direct and the EC coagulanttests, were was designed quantified to respectively by Freundlichdistinguish and Langmuir the contribution adsorption ofisotherms, adsorption respectively. and coprecipitation For the direct EC test, the system was operated at a constant current of 1 ampere for 10 min. mechanism on the removal of heavy metal ions from wastewater. For the indirect EC test, Al(OH)3 Solutionswas generated of Ni(II), in the Cu(II), EC reactorand Cr(VI), and transferredwith a 1.75 tog/L a testof sodium tube where chloride a certain (NaCl) concentration as the supporting of Ni, electrolyte, were prepared and treated in the EC process. Once the Al(OH)3 is generated in the EC Cu, and Cr ions was prepared to react with the coagulant (i.e., Al(OH)3 adsorption reaction). The pH reactorwas controlled it would in react a tight immediately range from with 5.0to heavy 7.0 during metal wholeions in the wastewater. electro-coagulation Treated wastewater process, to avoidof 10 mLthe was occurrence taken out of hydroxideevery minute precipitation. and let to settle The sulfuricfor 1 h, acidthen, ( >the95% pH of and purity) heavy-metal and sodium concentration hydroxide of(> the97% supernatant of purity) were was used determined. for acidic The or basic same adjustment volume of offresh the wastewater solution. Four was groups recharged of test into tubes the were EC reactorprepared; after each each group sampling contained to dicompensateﬀerent concentrations for the volume of single loss. Ni, singleIn addition Cu, single to Cr,the andsynthetic mixed wastewater,Ni, Cu, and the Cr wastewater ions, respectively. collected After from a an 3-min electr vigorousoplating factory mixing was and treated 1 h of settling,by the EC the process pH and as well.heavy-metal The concentrations concentration of Ni, of the Cu, supernatant Cr in all wastew was determined.ater sample were The heavydetermined metal adsorptionby an IRIS Intrepid capacity IIof inductively the coagulant coupled was calculated plasma optical based onemission the heavy-metal spectrometer concentration (ICP-OES) ofproduced supernatant. by Thermo At last, Electronthe adsorption Corporation, relationship Franklin, between MA. heavy-metal Experimental ions details and theof the coagulant above analysis was quantiﬁed are described by Freundlich in the Standardand Langmuir Methods adsorption for the Examination isotherms, respectively. of Water and Wastewater [26].

H2/O2 ventilation Anode Cathode (aluminum) (graphite) Power supply Control & measurement Effluent

Flow measurement Electrochemical Sample Reservoir cell

Sludge disposal Flow control

Circulating pump

Pump FigureFigure 1. 1. FlowFlow diagram diagram of of the the electro-coagulation electro-coagulation test test system. system.

Water 2020, 12, 2614 5 of 14 Water 2020, 12, x; doi: FOR PEER REVIEW 5 of 13

FigureFigure 2. 2. TheThe schematic schematic of of the the surface surface adsorption adsorption of of meta metall ions ions in in the the electro-coagulation (EC) system.

For the direct EC test, the system was operated at a constant current of 1 ampere for 10 min. 3. Results and Discussion Solutions of Ni(II), Cu(II), and Cr(VI), with a 1.75 g/L of sodium chloride (NaCl) as the supporting 3.1.electrolyte, Adsorption were of Ni(II), prepared Cu(II), and and treated Cr(VI) in the EC process. Once the Al(OH)3 is generated in the EC reactor it would react immediately with heavy metal ions in wastewater. Treated wastewater of 10 mL wasFigure taken out3 shows every equilibrium minute and concentrations let to settle for of 1Ni(I h,I), then, Cu(II), the and pH andCr(VI) heavy-metal in the wastewater concentration under theofthe indirect supernatant EC procedure was determined. at different The pH same conditions. volume The offresh dashed wastewater lines represent was recharged the solubility into theof Ni(OH)EC reactor2 and after Cu(OH) each2 without sampling the to presence compensate of Al(OH) for the3 according volume loss.to the In USEPA addition report to the[4]. syntheticIt can be noticedwastewater, that the the dashed wastewater line is collected almost parallel from an to electroplating the solid line factory of Ni and was Cu, treated respectively. by the EC At process the same as pH,well. the The Cu(II) concentrations concentration of Ni, under Cu, Crthe in indirect all wastewater EC procedure sample werewas about determined one order by an of IRIS magnitude Intrepid lowerII inductively than the coupled solubility plasma of Cu(OH) optical2.emission The discrepancy spectrometer could (ICP-OES) be attributed produced to the by copper Thermo ions Electron being adsorbedCorporation, onto Franklin, the generated MA. Experimental Al(OH)3 in the details EC system. of the above Such disparity analysis are about described nickel inhydroxide the Standard was evenMethods more for significant, the Examination which of was Water about and Wastewaterthree orders [26 of]. magnitude. Heidmann and Calmano presented that the concentration of metal ions during an EC process declined as the pH was below 7 [19].3. Results Apparently, and Discussion the hydroxide precipitation cannot dominate the decrease of metal ions in a slightly acidic solution. However, most of Al(OH)3 can be formed between pH 5 and 8. Hence, the coagulant adsorption3.1. Adsorption is the of major Ni(II), mechanism Cu(II), and Cr(VI) to remove heavy metal ions in the EC system, which agrees with the literatureFigure3 shows research equilibrium [20,27,28]. concentrations Since hexavalent of Ni(II), chromium Cu(II), andcannot Cr(VI) precipitate in the wastewater with the underhydroxyl the group,indirect the EC decrease procedure of atCr(VI) diﬀerent is simply pH conditions. due to its Theadsorption dashed on lines the represent aluminum the hydroxide. solubilityof Ni(OH)2 and Cu(OH)2 without the presence of Al(OH)3 according to the USEPA report [4]. It can be noticed that the dashed line is almost parallel to the solid line of Ni and Cu, respectively. At the same pH, the Cu(II) concentration under the indirect EC procedure was about one order of magnitude lower than the solubility of Cu(OH)2. The discrepancy could be attributed to the copper ions being adsorbed onto the generated Al(OH)3 in the EC system. Such disparity about nickel hydroxide was even more signiﬁcant, which was about three orders of magnitude. Heidmann and Calmano presented that the concentration of metal ions during an EC process declined as the pH was below 7 [19]. Apparently, the hydroxide precipitation cannot dominate the decrease of metal ions in a slightly acidic solution. However, most of Al(OH)3 can be formed between pH 5 and 8. Hence, the coagulant adsorption is the major mechanism to remove heavy metal ions in the EC system, which agrees with the literature research [20,27,28]. Since hexavalent chromium cannot precipitate with the hydroxyl group, the decrease of Cr(VI) is simply due to its adsorption on the aluminum hydroxide.

Figure 3. Equilibrium concentrations of Ni(II), Cu(II), and Cr(VI) in the wastewater under the indirect EC procedure at different pH conditions.

Water 2020, 12, x; doi: FOR PEER REVIEW 5 of 13

Figure 2. The schematic of the surface adsorption of metal ions in the electro-coagulation (EC) system.

3. Results and Discussion

3.1. Adsorption of Ni(II), Cu(II), and Cr(VI) Figure 3 shows equilibrium concentrations of Ni(II), Cu(II), and Cr(VI) in the wastewater under the indirect EC procedure at different pH conditions. The dashed lines represent the solubility of Ni(OH)2 and Cu(OH)2 without the presence of Al(OH)3 according to the USEPA report [4]. It can be noticed that the dashed line is almost parallel to the solid line of Ni and Cu, respectively. At the same pH, the Cu(II) concentration under the indirect EC procedure was about one order of magnitude lower than the solubility of Cu(OH)2. The discrepancy could be attributed to the copper ions being adsorbed onto the generated Al(OH)3 in the EC system. Such disparity about nickel hydroxide was even more significant, which was about three orders of magnitude. Heidmann and Calmano presented that the concentration of metal ions during an EC process declined as the pH was below 7 [19]. Apparently, the hydroxide precipitation cannot dominate the decrease of metal ions in a slightly acidic solution. However, most of Al(OH)3 can be formed between pH 5 and 8. Hence, the coagulant adsorption is the major mechanism to remove heavy metal ions in the EC system, which agrees with Waterthe literature2020, 12, 2614 research [20,27,28]. Since hexavalent chromium cannot precipitate with the hydroxyl6 of 14 group, the decrease of Cr(VI) is simply due to its adsorption on the aluminum hydroxide.

Figure 3. Equilibrium concentrations of Ni(II), Cu(II), and Cr(VI) in the wastewater under the indirect Figure 3. Equilibrium concentrations of Ni(II), Cu(II), and Cr(VI) in the wastewater under the indirect EC procedure at different pH conditions. EC procedure at different pH conditions. Figure4a shows the adsorption curve of individual Ni(II), Cu(II), and Cr(VI) onto aluminum hydroxide at the status of pH 7. The abscissa is the metal concentration in the supernatant and the ordinate represents the metal ions adsorbed on the Al(OH)3. Using Equation (6) can transform Figure4a to a log-log plot as shown in Figure4b, which deploys to gain adsorption parameters of Freundlich isotherm. It can be seen that the correlation coefficients are 0.82 and 0.86 for Ni(II) and Cu(II) adsorption, respectively. Experimental data of Figure4a were fitted by Langmuir isotherm as well, whose linearization of Langmuir isotherm for Ni(II) and Cu(II) adsorption is presented in Figure4c. Since correlation coefficients for Ni(II) and Cu(II) adsorption were above 0.9, Langmuir isotherm could describe the adsorption behavior of Ni(II) and Cu(II) ions onto the Al(OH)3 coagulant. According to the intercepts of 0.003 and 0.0027 from Figure4c, the adsorption capacity of Ni(II) and Cu(II) can be computed as 333 and 370 mg/g, respectively. In contrast to Ni(II) and Cu(II), the individual Cr(VI) adsorption onto the Al(OH)3 coagulant at the status of pH 7 could be suitable to Freundlich isotherm (the correlation coefficient was 0.9868) as shown in Figure4d. The di fferent adsorption behavior 2 from Ni(II) and Cu(II) may be attributed to Cr(VI) in an anion form of chromic acid (i.e., Cr2O7 −). The adsorption capacity of Cr(VI) onto the Al(OH)3 coagulant was estimated at around 200 mg/g. The adsorption capacity of each heavy metal is quite useful for environmental engineers to run the EC process. If we try to remove 100 mg Ni ions from the wastewater, for instance, the EC system should generate around 100 mg/333 mg/g = 0.3 g aluminum hydroxide. Since the molecular weight of Al(OH)3 is 78 (atomic weight of aluminum is 27), the aluminum anode of the EC system should release (0.3 g/78) 27 = 0.1 g aluminum. Based on Equation (5), we can obtain the electricity consumption of × 0.011 mole electrons (i.e., (0.1/27) 3 = 0.011). Consequently, the necessary current can be gained to × control the EC system effectively. Water 2020, 12, x; doi: FOR PEER REVIEW 6 of 13

Figure 4a shows the adsorption curve of individual Ni(II), Cu(II), and Cr(VI) onto aluminum hydroxide at the status of pH 7. The abscissa is the metal concentration in the supernatant and the ordinate represents the metal ions adsorbed on the Al(OH)3. Using Equation (6) can transform Figure 4a to a log-log plot as shown in Figure 4b, which deploys to gain adsorption parameters of Freundlich isotherm. It can be seen that the correlation coefficients are 0.82 and 0.86 for Ni(II) and Cu(II) adsorption, respectively. Experimental data of Figure 4a were fitted by Langmuir isotherm as well, whose linearization of Langmuir isotherm for Ni(II) and Cu(II) adsorption is presented in Figure 4c. Since correlation coefficients for Ni(II) and Cu(II) adsorption were above 0.9, Langmuir isotherm could describe the adsorption behavior of Ni(II) and Cu(II) ions onto the Al(OH)3 coagulant. According to the intercepts of 0.003 and 0.0027 from Figure 4c, the adsorption capacity of Ni(II) and Cu(II) can be computed as 333 and 370 mg/g, respectively. In contrast to Ni(II) and Cu(II), the individual Cr(VI) adsorption onto the Al(OH)3 coagulant at the status of pH 7 could be suitable to Freundlich isotherm (the correlation coefficient was 0.9868) as shown in Figure 4d. The different adsorption behavior from Ni(II) and Cu(II) may be attributed to Cr(VI) in an anion form of chromic acid (i.e., Cr2O72−). The adsorption capacity of Cr(VI) onto the Al(OH)3 coagulant was estimated at around 200 mg/g. The adsorption capacity of each heavy metal is quite useful for environmental engineers to run the EC process. If we try to remove 100 mg Ni ions from the wastewater, for instance, the EC system should generate around 100 mg/333 mg/g = 0.3 g aluminum hydroxide. Since the molecular weight of Al(OH)3 is 78 (atomic weight of aluminum is 27), the aluminum anode of the EC system should release (0.3 g/78) × 27 = 0.1 g aluminum. Based on Equation (5), we can obtain the electricity consumption of 0.011 mole electrons (i.e., (0.1/27) × 3 = 0.011). Consequently, the necessary currentWater 2020 can, 12 ,be 2614 gained to control the EC system effectively. 7 of 14

Figure 4. The adsorption curve and isotherm of diﬀerent metal ions. (a) Adsorption curve of individual Figure 4. The adsorption curve and isotherm of different metal ions. (a) Adsorption curve of Ni(II), Cu(II), and Cr(VI) onto aluminum hydroxide at the status of pH 7. (b) Linearization of Freundlich individual Ni(II), Cu(II), and Cr(VI) onto aluminum hydroxide at the status of pH 7. (b) Linearization isotherm for individual Ni(II) and Cu(II) adsorption. (c) Linearization of Langmuir isotherm for Ni(II) of Freundlich isotherm for individual Ni(II) and Cu(II) adsorption. (c) Linearization of Langmuir and Cu(II) adsorption. (d) Linearization of Freundlich isotherm for individual Cr(VI) adsorption. isotherm for Ni(II) and Cu(II) adsorption. (d) Linearization of Freundlich isotherm for individual 3.2. CompetitiveCr(VI) adsorption. Adsorption

Although the adsorption capacity of each heavy metal onto the aluminum hydroxide can be quantified by the above adsorption models, the complicated adsorption situation of coexisting heavy metals should be clarified as well. Figure5a shows the competitive adsorption between Ni(II) and Cu(II) onto Al(OH)3 at the status of pH 7. It could be observed that the Cu(II) adsorption behavior under the coexisting condition was similar to that under individual condition (shown in Figure4a). However, a collapsed adsorption of Ni(II) occurred. Figure5b shows the linearization of Langmuir isotherm for Ni(II) and Cu(II) competitive adsorption. With the competition between the two ions, Cu(II) adsorption was still consistent with the Langmuir isotherm, with a correlation coefficient of 1 0.95. However, there was no linear relationship that could be established between (qo− ) and the equilibrium concentration of Ni(II) in the solution. In addition, the adsorption capacity of Ni(II) and Cu(II) was around 50 and 450 mg/g, respectively (shown in Figure5a). The above phenomena indicate that the Cu(II) affinity to Al(OH)3 is much greater than Ni(II). Water 2020, 12, x; doi: FOR PEER REVIEW 7 of 13

3.2. Competitive Adsorption Although the adsorption capacity of each heavy metal onto the aluminum hydroxide can be quantified by the above adsorption models, the complicated adsorption situation of coexisting heavy metals should be clarified as well. Figure 5a shows the competitive adsorption between Ni(II) and Cu(II) onto Al(OH)3 at the status of pH 7. It could be observed that the Cu(II) adsorption behavior under the coexisting condition was similar to that under individual condition (shown in Figure 4a). However, a collapsed adsorption of Ni(II) occurred. Figure 5b shows the linearization of Langmuir isotherm for Ni(II) and Cu(II) competitive adsorption. With the competition between the two ions, Cu(II) adsorption was still consistent with the Langmuir isotherm, with a correlation coefficient of 0.95. However, there was no linear relationship that could be established between (qo−1) and the equilibrium concentration of Ni(II) in the solution. In addition, the adsorption capacity of Ni(II) and Cu(II) was around 50 and 450 mg/g, respectively (shown in Figure 5a). The above phenomena indicate that the Cu(II) affinity to Al(OH)3 is much greater than Ni(II). In Figure 5a, it could be observed that the adsorption quantum of Ni(II) increased initially and declined gradually. Since vacant sites on the adsorbent (Al(OH)3) surface are limited, the coexisting metals will compete to occupy the limited sites. When the concentrations of both Cu(II) and Ni(II) were relatively low, there were plenty of vacant sites available for adsorption of both Cu(II) and Ni(II). When the Cu(II) concentration increased dramatically, Cu(II) ions dominated the adsorption process (i.e., occupy most of the vacant sites and even replaced the Ni(II)). Kang and colleagues studied the adsorption behavior of multiple ions on Amberlite IRN-77 cation exchange resins and reported that Cr(III) could replace Cu(II) and Ni(II) ions adsorbed onto the resin [29]. Such replacement demonstrated that Cu(II) not only has a stronger adsorption affinity than Ni(II) onto aluminumWater 2020, 12 hydroxide, 2614 but also competes for the same sites on the aluminum hydroxide. 8 of 14

(a) (b)

Figure 5. TheThe competitive competitive adsorption adsorption and and isotherm isotherm lin linearizationearization between between Ni(II) Ni(II) and Cu(II) onto aluminum hydroxide. ( (aa) )The The competitive competitive adsorption adsorption between between Ni(II) Ni(II) and and Cu(II) onto aluminum hydroxide at the status of pHpH 7.7. ( (bb)) Linearization Linearization of of Langmuir Langmuir is isothermotherm for for Ni(II) Ni(II) and and Cu(II) competitive adsorption.

FigureIn Figure 6a5 showsa, it could the becompetitive observed thatadsorption the adsorption among Ni(II), quantum Cu(II), of Ni(II) and Cr(VI) increased onto initially aluminum and hydroxidedeclined gradually. at the status Since of vacantpH 7. In sites Figure on the 6a, adsorbent Ni(II) and (Al(OH) Cu(II) adsorption3) surface are in the limited, presence the coexisting of Cr(VI) showsmetals similar will compete trends totooccupy those in the Figure limited 5a. sites. It is noticed When the that concentrations the adsorption of capacity both Cu(II) of Ni(II) and Ni(II) and wereCu(II) relatively declined low, from there 50 wereto 20 plenty mg/g ofand vacant from sites 450 available to 350 formg/g, adsorption respectively. of both According Cu(II) and to Ni(II). the Whenillustration the Cu(II) of Section concentration 3.1, the adsorption increased dramatically, sites of Al(OH) Cu(II)3 for ions Cr(VI) dominated were different the adsorption from those process for Ni(II),(i.e., occupy Cu(II). most This ofimplies the vacant that Cr(VI) sites and does even not replacedcompete thewith Ni(II)). Ni(II) Kangand Cu(II) and colleagues for the same studied types the of sites.adsorption Instead, behavior it interferes of multiple with the ions adsorption on Amberlite process IRN-77 through cation other exchange mechanisms. resins and In reportedaddition, thatthe Cr(III) could replace Cu(II) and Ni(II) ions adsorbed onto the resin [29]. Such replacement demonstrated that Cu(II) not only has a stronger adsorption affinity than Ni(II) onto aluminum hydroxide but also competes for the same sites on the aluminum hydroxide. Figure6a shows the competitive adsorption among Ni(II), Cu(II), and Cr(VI) onto aluminum hydroxide at the status of pH 7. In Figure6a, Ni(II) and Cu(II) adsorption in the presence of Cr(VI) shows similar trends to those in Figure5a. It is noticed that the adsorption capacity of Ni(II) and Cu(II) declined from 50 to 20 mg/g and from 450 to 350 mg/g, respectively. According to the illustration of Section 3.1, the adsorption sites of Al(OH)3 for Cr(VI) were different from those for Ni(II), Cu(II). This implies that Cr(VI) does not compete with Ni(II) and Cu(II) for the same types of sites. Instead, it interferes with the adsorption process through other mechanisms. In addition, the adsorption capacity of Cr(VI) declined from 200 (individual condition) to 100 mg/g (competitive condition), Figure6b shows the linearization of Langmuir isotherm for Ni(II) and Cu(II), and Cr(VI) competitive adsorption. In the presence of Ni(II) and Cu(II), Cr(VI) adsorption fits Langmuir well rather than the Freundlich isotherm, with a correlation coefficient of 0.99. The above results also demonstrate the interference mechanism among coexisting ions. Based on the adsorption capacity of each heavy metal under coexisting condition, like the example in Section 3.1, the necessary current can be obtained to control the EC system effectively. Water 2020, 12, x; doi: FOR PEER REVIEW 8 of 13

adsorption capacity of Cr(VI) declined from 200 (individual condition) to 100 mg/g (competitive condition), Figure 6b shows the linearization of Langmuir isotherm for Ni(II) and Cu(II), and Cr(VI) competitive adsorption. In the presence of Ni(II) and Cu(II), Cr(VI) adsorption fits Langmuir well rather than the Freundlich isotherm, with a correlation coefficient of 0.99. The above results also demonstrate the interference mechanism among coexisting ions. Based on the adsorption capacity of each heavy metal under coexisting condition, like the example in Section 3.1, the necessary current Water 2020, 12, 2614 9 of 14 can be obtained to control the EC system effectively.

(a) (b)

FigureFigure 6. The 6. The competitive competitive adsorption adsorption andand isotherm lineari linearizationzation among among Ni(II), Ni(II), Cu(II), Cu(II), and Cr(VI) and Cr(VI) onto onto aluminumaluminum hydroxide. hydroxide. (a) The(a) competitiveThe competitive adsorption adsorption among among Ni(II), Ni(II), Cu(II), Cu(II), and Cr(VI)and Cr(VI) onto aluminumonto aluminum hydroxide at the status of pH 7. (b) Linearization of Langmuir isotherm for Ni(II) and hydroxide at the status of pH 7. (b) Linearization of Langmuir isotherm for Ni(II) and Cu(II), and Cr(VI) Cu(II), and Cr(VI) competitive adsorption. competitive adsorption.

3.3. Direct3.3. Direct EC EC Treatment Treatment Figure 7 shows the removal of individual Ni(II), Cu(II), and Cr(VI) treated by a 10-min EC Figure7 shows the removal of individual Ni(II), Cu(II), and Cr(VI) treated by a 10-min EC process. It could be seen that Ni(II) and Cu(II) was eliminated completely after 4–5 min treatment, process.however, It could Cr(VI) be was seen decreased that Ni(II) from and 188.8 Cu(II) to 128. was0 mg/L. eliminated Since the completely volume of afterwastewater 4–5 min was treatment, 200 3 however,cm3 in Cr(VI) the EC was reactor decreased and the from initial 188.8 concentration to 128.0 mg of/L. Ni(II), Since Cu(II), the volume and Cr(VI) of wastewater was 119.9,was 96.0, 200 and cm in the EC188.8 reactor mg/L, and respectively, the initial the concentration total amount of Ni(II), Ni(II), Cu(II), Cu(II), and and Cr(VI) Cr(VI) was was around 119.9, 24.0, 96.0, 19.2, and and 188.8 37.8 mg/L, respectively,mg, respectively. the total The amount adsorption of Ni(II),capacity Cu(II), of Ni(II) and, Cu(II), Cr(VI) and Cr(VI) was around was obtained 24.0, from 19.2, Section and 37.8 3.1 mg, respectively.as 333, 370, The and adsorption 200 mg/g, respectively. capacity of Under Ni(II), 1.0 Cu(II), Ampere and and Cr(VI) 10 min wasEC operation, obtained the from total Section amount 3.1 as 333, 370,of generated and 200 Al(OH) mg/g,3 respectively. was around 0.16 Under g (i.e., 1.0 1.0 Ampere A × 60 × and10 = 600 10min coulombs, EC operation, 600/96,500 the = 0.0062 total amountmole of electrons, (0.0062/3) × 78 = 0.16 g of aluminum hydroxide). This EC operation could theoretically generated Al(OH)3 was around 0.16 g (i.e., 1.0 A 60 10 = 600 coulombs, 600/96,500 = 0.0062 mole remove 53.3 mg Ni(II), 59.2 mg Cu(II), and 32.0 mg× Cr(VI),× individually (i.e., 0.16 × 333 = 53.3 mg for electrons, (0.0062/3) 78 = 0.16 g of aluminum hydroxide). This EC operation could theoretically remove Ni(II), 0.16 × 370 ×= 59.2 mg for Cu(II), and 0.16 × 200 = 32.0 mg for Cr(VI)). Apparently, the theoretical 53.3 mg Ni(II), 59.2 mg Cu(II), and 32.0 mg Cr(VI), individually (i.e., 0.16 333 = 53.3 mg for Ni(II), removed Ni(II) was much greater than the initial Ni(II) content in the wastewater× (i.e., 53.3 mg > 24.0 0.16 370 = 59.2 mg for Cu(II), and 0.16 200 = 32.0 mg for Cr(VI)). Apparently, the theoretical removed ×mg). It is noticed that the initial amount× of Ni(II) was around half of the removed ability of Al(OH)3. Ni(II)Accordingly, was much greatera 5-min EC than operation the initial could Ni(II) adsorb content 24.0 inmg the Ni(II) wastewater as shown (i.e.,in Figure 53.3 7. mg Likewise,> 24.0 mg).the It is noticedtheoretical that the removed initial amount Cu(II) was of Ni(II) much was greater around than half the ofinitial the removedCu(II) content ability in ofthe Al(OH) wastewater3. Accordingly, (i.e., a 5-min59.2 EC mg operation > 19.2 mg). could A adsorb4-min EC 24.0 operation mg Ni(II) could as shown adsorb in Figure19.2 mg7. Cu(II) Likewise, completely. the theoretical The above removed Cu(II)derivation was much can greaterdemonstrate than that the the initial adsorption Cu(II) capacity content of in heavy the wastewater metals is beneficial (i.e., 59.2 to design mg > 19.2and mg). A 4-mincontrol EC the operation EC operation. could adsorb 19.2 mg Cu(II) completely. The above derivation can demonstrate that the adsorptionAccording to capacity Figure 7, of the heavy Cr(VI) metals concentratio is beneficialn was toreduced design from and 188.8 control to 127.9 the EC mg/L operation. for the 10-min EC treatment. The theoretical removed Cr(VI) was less than the initial Cr(VI) content in the According to Figure7, the Cr(VI) concentration was reduced from 188.8 to 127.9 mg /L for the wastewater (i.e., 32.0 mg < 37.8 mg). As a result, the Cr(VI) content could not be removed completely 10-min EC treatment. The theoretical removed Cr(VI) was less than the initial Cr(VI) content in the under such EC operation. Based on the adsorption capacity of Cr(VI), the removal efficiency of Cr(VI) wastewater (i.e., 32.0 mg < 37.8 mg). As a result, the Cr(VI) content could not be removed completely under such EC operation. Based on the adsorption capacity of Cr(VI), the removal efficiency of Cr(VI) should be around 85% (32.0/37.8 = 0.846). However, the Cr(VI) content removed only one-third of the initial content in the wastewater. This implies that some factors about Cr(VI) adsorption on the aluminum hydroxide during the indirect and direct procedures have not been clarified. Heidmann and Calmano reported a 50% removal of Cr(VI) in a 50 min treatment and proposed Cr(VI) reduced to Cr(III) at the cathode before precipitating as chromic hydroxide [19]. Yang and Kravets have reported that using a steel sheet as an anode, the EC process is very useful in removing Cr(VI) from a synthetic wastewater sample [9]. They confirmed that Cr(VI) was reduced to Cr(III) by ferrous ions (Fe2+) produced in the process. The adsorption of reduced Cr(III) needs to be quantified, which may interpret the removal of Cr(VI) in the EC system. Water 2020, 12, x; doi: FOR PEER REVIEW 9 of 13

should be around 85% (32.0/37.8 = 0.846). However, the Cr(VI) content removed only one-third of the initial content in the wastewater. This implies that some factors about Cr(VI) adsorption on the aluminum hydroxide during the indirect and direct procedures have not been clarified. Heidmann and Calmano reported a 50% removal of Cr(VI) in a 50 min treatment and proposed Cr(VI) reduced to Cr(III) at the cathode before precipitating as chromic hydroxide [19]. Yang and Kravets have reported that using a steel sheet as an anode, the EC process is very useful in removing Cr(VI) from a synthetic wastewater sample [9]. They confirmed that Cr(VI) was reduced to Cr(III) by ferrous ions (Fe2+) produced in the process. The adsorption of reduced Cr(III) needs to be quantified, which may interpret the removal of Cr(VI) in the EC system. For the actual condition of wastewater, regardless of the aluminum sheet or iron sheet used for electro-coagulation operation, various pollutants in wastewater can be removed through adsorption, coprecipitation, oxidation, and reduction mechanisms [30]. This study is focused on exploring the relationship between the removal mechanism of metal ions and competitive adsorption. The removal Water 2020, 12efficiency, 2614 of various pollutants in wastewater by different electrodes has been reported from Tahreen 10 of 14 et al. [31].

Figure 7.FigureTheremoval 7. The removal of individual of individual Ni(II), Ni(II), Cu(II),Cu(II), an andd Cr(VI) Cr(VI) treated treated by a 10-min by a 10-minEC process. EC process.

For the actualFigure condition8 shows the ofremoval wastewater, of coexisting regardless Ni(II) and of Cu(II) the aluminum throughout a sheet 10-min or EC iron process. sheet used for During the treatment, the pH was kept between 4.8 and 6.0 to ensure that hydroxide precipitation electro-coagulationwas minimized. operation, The 10 min various of treatment pollutants could inredu wastewaterce Cu(II) concentration can be removedfrom 96.0 to through 1.9 mg/L, adsorption, coprecipitation,which oxidation,was similar andto that reduction without mechanismsthe competition [30 from]. This Ni(II) study as shown is focused in Figure on 7. exploring In comparison the relationship between thewith removal the initial mechanism Cu(II) content of metaland the ions adsorption and competitive capacity of Al(OH) adsorption.3, this removal The removal phenomenon efficiency can of various pollutantsbe in estimated wastewater as the by same different as the electrodesprevious discussion. has been To reported consider fromthe Ni(II) Tahreen removal, et al.the [EC31 ].process reduced Ni(II) concentration from 111.3 to 12.6 mg/L. Such high removal efficiency was beyond our Figureexpectation8 shows since the the removal adsorption of coexistingcapacity of Ni( Ni(II)II) was and merely Cu(II) 50 mg/g. throughout If the removal a 10-min mechanism EC process. During thesolely treatment, depends on the the pH adsorption was kept by Al(OH) between3, the4.8 final and concentration 6.0 to ensure of Ni(II) that should hydroxide be around precipitation 90 was minimized.mg/L rather The than 10 min12.6 ofmg/L. treatment This high could removal reduce efficiency Cu(II) of Ni(II) concentration may be attributed from 96.0 to the to 1.9 mg/L, which wascoprecipitation similar to that mechanism without [18,19]. the competition from Ni(II) as shown in Figure7. In comparison with the initial Cu(II) content and the adsorption capacity of Al(OH)3, this removal phenomenon can be estimated as the same as the previous discussion. To consider the Ni(II) removal, the EC process reduced Ni(II) concentration from 111.3 to 12.6 mg/L. Such high removal efficiency was beyond our expectation since the adsorption capacity of Ni(II) was merely 50 mg/g. If the removal mechanism solely depends on the adsorption by Al(OH)3, the final concentration of Ni(II) should be around 90 mg/L rather than 12.6 mg/L. This high removal efficiency of Ni(II) may be attributed to the coprecipitation mechanismWater [18 2020,19,]. 12, x; doi: FOR PEER REVIEW 10 of 13

Figure 8. FigureThe removal 8. The removal of coexisting of coexisting Ni(II) Ni(II) and and Cu Cu(II)(II) throughout throughout a 10-min a 10-min EC process. EC process. Figure 9 shows the removal of coexisting Ni(II), Cu(II), and Cr(VI) throughout a 10-min EC Figureprocess.9 shows Basically the removalspeaking, the of removal coexisting curves Ni(II), of Ni(II) Cu(II), and Cu(II) and in Figure Cr(VI) 9 are throughout similar to those a 10-min in EC process. BasicallyFigure 8. This speaking, indicates the that removal the presence curves of Cr(VI) of Ni(II)influences and the Cu(II) removal in of Figure Ni(II) and9 are Cu(II) similar slightly. to those in Figure8. ThisIt could indicates be noticed that that the the presence Ni(II) concentration of Cr(VI) inﬂuencesremained unchanged the removal at the ofearly Ni(II) stage and of the Cu(II) EC slightly. It could beprocedure noticed (shown that the in Figures Ni(II) 8 concentrationand 9), then, it dropp remaineded down to unchanged 26.6 mg/L at the at end the of early the treatment. stage of the EC In contrast, the Cu(II) concentration dropped from 95.8 to 20 mg/L at the early stage. These different removal kinetics of Ni(II) and Cu(II) can be attributed to the fact that the Cu(II) ions dominated the adsorption process onto Al(OH)3 (discussion in Section 3.1). As stated in a previous section, Cr(VI) does not compete with Ni(II) and Cu(II) to occupy the same type sites on the Al(OH)3 surface. Instead, it may spatially or electrically block the sites or access to the sites. In Figure 9, the Cr(VI) concentration was reduced from 187.1 to 123.9 mg/L, which is almost identical to the EC treatment of single Cr(VI). This result was consistent with the hypothesis of Cr(VI) adsorption onto Al(OH)3.

Figure 9. The removal of coexisting Ni(II), Cu(II), and Cr(VI) throughout a 10-min EC process.

Water 2020, 12, x; doi: FOR PEER REVIEW 10 of 13

Figure 8. The removal of coexisting Ni(II) and Cu(II) throughout a 10-min EC process.

Figure 9 shows the removal of coexisting Ni(II), Cu(II), and Cr(VI) throughout a 10-min EC Waterprocess.2020 ,Basically12, 2614 speaking, the removal curves of Ni(II) and Cu(II) in Figure 9 are similar to those11 of in 14 Figure 8. This indicates that the presence of Cr(VI) influences the removal of Ni(II) and Cu(II) slightly. It could be noticed that the Ni(II) concentration remained unchanged at the early stage of the EC procedure (shown in Figures8 and9), then, it dropped down to 26.6 mg /L at the end of the treatment. procedure (shown in Figures 8 and 9), then, it dropped down to 26.6 mg/L at the end of the treatment. In contrast, the Cu(II) concentration dropped from 95.8 to 20 mg/L at the early stage. These diﬀerent In contrast, the Cu(II) concentration dropped from 95.8 to 20 mg/L at the early stage. These different removal kinetics of Ni(II) and Cu(II) can be attributed to the fact that the Cu(II) ions dominated the removal kinetics of Ni(II) and Cu(II) can be attributed to the fact that the Cu(II) ions dominated the adsorption process onto Al(OH)3 (discussion in Section 3.1). As stated in a previous section, Cr(VI) adsorption process onto Al(OH)3 (discussion in Section 3.1). As stated in a previous section, Cr(VI) does not compete with Ni(II) and Cu(II) to occupy the same type sites on the Al(OH)3 surface. Instead, does not compete with Ni(II) and Cu(II) to occupy the same type sites on the Al(OH)3 surface. Instead, it may spatially or electrically block the sites or access to the sites. In Figure9, the Cr(VI) concentration it may spatially or electrically block the sites or access to the sites. In Figure 9, the Cr(VI) concentration was reduced from 187.1 to 123.9 mg/L, which is almost identical to the EC treatment of single Cr(VI). was reduced from 187.1 to 123.9 mg/L, which is almost identical to the EC treatment of single Cr(VI). This result was consistent with the hypothesis of Cr(VI) adsorption onto Al(OH)3. This result was consistent with the hypothesis of Cr(VI) adsorption onto Al(OH)3.

Figure 9. The removal of coexisting Ni(II), Cu(II), and Cr(VI) throughout a 10-min EC process.

3.4. Treatment of Electroplating Wastewater To fully understand the interaction among the metal ions in the EC process, it would be beneficial to test the theory on a real sample. A wastewater sample was collected from an electroplating facility,which is based on various processes, including hard chromium, black nickel, bronze, and brass plating. The initial pH was 2.7, and the concentration of Ni(II), Cu(II), and Cr(VI) were 7.6, 4.7, and 280.9 mg/L, respectively. The sample was treated using the electrochemical process with aluminum as anode and graphite (as the inert electrode and has excellent conductivity) as the cathode, under a constant current of 1.5 amperes, and with 1.75 g/L NaCl as the supporting electrolyte. Figure 10 shows the concentration change of Ni(II), Cu(II), and Cr(VI) as a function of treatment time. During the 10 min of treatment, the solution pH increased steadily from 2.7 to 6.0. The Cu(II) and Cr(VI) ions in the sample behaved like those in the synthetic wastewater, as described in a previous section. Cu(II) concentration dropped 88% in 2 min, while 44% Cr(VI) was removed in the 10 min treatment. The dashed line in Figure 10 represents the change of Ni(II) concentration in synthetic wastewater from a previous section. When compared with the data from the treatment for real wastewater, it shows that the competition from Cu(II) was diminished signiﬁcantly, while the interference from Cr(VI) was enhanced considerably. The above results demonstrate that the adsorption capacity of heavy metals plays a key role in the design and control of the EC operation. Water 2020, 12, x; doi: FOR PEER REVIEW 11 of 13

3.4. Treatment of Electroplating Wastewater To fully understand the interaction among the metal ions in the EC process, it would be beneficial to test the theory on a real sample. A wastewater sample was collected from an electroplating facility, which is based on various processes, including hard chromium, black nickel, bronze, and brass plating. The initial pH was 2.7, and the concentration of Ni(II), Cu(II), and Cr(VI) were 7.6, 4.7, and 280.9 mg/L, respectively. The sample was treated using the electrochemical process with aluminum as anode and graphite (as the inert electrode and has excellent conductivity) as the cathode, under a constant current of 1.5 amperes, and with 1.75 g/L NaCl as the supporting electrolyte. Figure 10 shows the concentration change of Ni(II), Cu(II), and Cr(VI) as a function of treatment time. During the 10 min of treatment, the solution pH increased steadily from 2.7 to 6.0. The Cu(II) and Cr(VI) ions in the sample behaved like those in the synthetic wastewater, as described in a previous section. Cu(II) concentration dropped 88% in 2 min, while 44% Cr(VI) was removed in the 10 min treatment. The dashed line in Figure 10 represents the change of Ni(II) concentration in synthetic wastewater from a previous section. When compared with the data from the treatment for real wastewater, it shows that the competition from Cu(II) was diminished significantly, while the

Waterinterference2020, 12, 2614from Cr(VI) was enhanced considerably. The above results demonstrate that12 ofthe 14 adsorption capacity of heavy metals plays a key role in the design and control of the EC operation.

Figure 10. The EC treatment of wastewater sample from an electroplating facility.

4.4. Conclusions BasedBased onon experimental experimental results results and and discussion, discussion, several several conclusions conclusions can be drawncan be as drawn the following. as the following. 1. For individual ions, adsorption of Ni(II) and Cu(II) by Al(OH)3 coagulant can be described by the 1. ForLangmuir individual isotherm, ions, adsorption while Cr(VI) of adsorption Ni(II) and ﬁtsCu(II) the by Freundlich Al(OH)3 isothermcoagulant better. can be described by 2. theTreating Langmuir a single isotherm, heavy while metal Cr(VI) of wastewater, adsorption the fits removal the Freundlich mechanism isotherm of the better. EC process is the 2. Treatingadsorption a single reaction. heavy metal of wastewater, the removal mechanism of the EC process is the 3. adsorptionUnder the coexisting reaction. condition, the Ni(II) and Cu(II) will compete for the same active sites on the 3. UnderAl(OH) the3 surface coexisting and Cu(II)condition, suppresses the Ni(II) Ni(II) and adsorption. Cu(II) will compete for the same active sites on 4. theTreating Al(OH) the3 coexistingsurface and heavy Cu(II) metals, suppresses Ni(II) Ni(II) removal adsorption. not only associates with adsorption but also 4. Treatingwith the coprecipitation.the coexisting heavy In contrast, metals, Cr(VI)Ni(II) remova does notl not compete only associates with other with metal adsorption ions for the but same also withtype the of adsorption coprecipitation. sites. In contrast, Cr(VI) does not compete with other metal ions for the same type of adsorption sites. 5. Whether single or coexisting conditions, the adsorption capacity of heavy metals onto Al(OH)3 5. Whethercoagulants single can beor usedcoexisting to compute conditions, the necessary the adsorption current capacity to eﬀectively of heavy remove metals heavy onto metals Al(OH) in3 coagulantsthe EC system. can be used to compute the necessary current to effectively remove heavy metals in the EC system.

Author Contributions: Conceptualization and methodology: J.-H.C., C.-D.D. and C.-H.H.; formal analysis, investigation and data curation: C.-H.H.; validation, J.-H.C.; writing—original draft preparation: C.-H.H. and S.-Y.S.; writing—review and editing: M.K., and S.-Y.S.; supervision: J.-H.C. and C.-D.D. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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