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Wireless Electrocoagulation in Water Treatment Based on Bipolar Electrochemistry
Electrochimica Acta 229 (2017) 96–101
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Wireless Electrocoagulation in Water Treatment Based on Bipolar Electrochemistry Zhenlian Qi, Shijie You* , Nanqi Ren State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, PR China
A R T I C L E I N F O
Article history: Received 18 November 2016 Received in revised form 11 January 2017 Accepted 23 January 2017 Available online 24 January 2017 Keywords: Electrocoagulation Bipolar electrochemistry Wireless Iron
A B S T R A C T
The present study reports the wireless electrocoagulation (WEC) based on bipolar electrochemistry where the iron sheets were used as sacrificial bipolar electrodes (BPEs) and two graphite plates served as driving electrodes. Driven by the interfacial potential difference induced by electric field in solution, the iron dissolution started at the anodic pole, achieving the generation of iron coagulant and thus turbidity removal. The total iron concentration produced was found to be more dependent on the geometrical configuration of BPE rather than electrochemical parameters. That is, placing the BPE with the length in parallel to electric field could generate iron concentration of 84.6% higher than that when placed vertically. Besides, increasing the number of BPE from one to three led to the increase in coagulant production by 2.1 times. Moreover, to maximize the voltage drop and minimize the material used, the BPE was designed into different geometrical shapes. The “H-shaped” BPE could achieve almost the same iron concentration as the unmodified BPE even the relative cathode/anode surface area was as low as 0.36. This accounted for efficient coagulant production by saving as great as 40% of the electrode material and cost. The wireless operation can not only solve the problems caused by electrode connection, but also allow arbitrary number of sacrificial BPEs working simultaneously in a very simple setup. In addition, the WEC performance can be easily controlled by the geometrical configuration of BPE. All these factors will be expected to make the electrocoagulation process much easier, more economical and more reliable in water treatment. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Electrocoagulation (EC) is a classic and widely used method to remove a broad range of contaminants (e.g. suspended particles, dyes, soluble inorganic and organic matter) from the polluted water [1–3]. A typical EC system contains an anode (sacrificial electrode) and a cathode (counter electrode) connected via an external circuit. Driven by positive potential applied to anode (e.g. aluminum or iron), the anodic dissolution leads to the formation of positively charged ions, hydroxide, and polyhydroxide compounds. As those formations are strongly affinitive toward removal of particulate, colloids and counter ions [4], the pollutants then can be separated and removed by in-situ produced coagulant via the processes of charge neutralization, flocculation, adsorption, sedimentation, and even flotation [5].
* Corresponding author at: P. O. Box 2603#, No. 73, Huanghe Road, Nangang District, Harbin 150090, PR China. E-mail address: [email protected] (S. You). http://dx.doi.org/10.1016/j.electacta.2017.01.151 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
In conventional EC, the electrochemical parameters are controlled by external power supply, and the amount of coagulant electro-produced is positively proportional to the current passing the electrode according to Faraday’s Law. In this context, the electrode connection is of particular importance, but it is often ignored in lab-scale studies and little attention has been paid to the impact of electrical connection on EC performance [6]. It is common practice to create connection between electrode and external circuit via ohmic contact (i.e. conductive wire). When the electrode and wire are in contact with one another, the contact interface is subject to have much higher corrosion stress, rendering the sacrificial electrode more susceptible to corrosion at the joint regions [7]. This will cause high contact resistance, which may add the complexity of manufacturing operation and maintenance in practical applications [8]. The three-dimensional (3D) electrodes may offer a possible solution, but the diminution in performances may be in association with the ohmic contact among the particles, short-circuit current [9], and blocking phenomenon [10]. Also, particle electrode that can play as the charged microelectrodes in the presence of electric field [9] may lose its adsorption capacity and catalytic activity due
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to the accumulation of pollutants on particle surface in continuous runs [11]. Moreover, the mechanisms of electro-coagulation based on 3D electrode are ambiguous rather than definitive because of irregular potential distribution within the complex electrode matrix, which may further limit the development of this technology for wide utilization in various wastewater treatments. There are still many works to be done to forward from fundamental mechanism to reactor scale for application [12]. Recently, bipolar electrochemistry has gained a growing popularity in a broad range of applications such as sensing and screening [13], generating nanowires [14], creating molecular gradient [15], and concentrating and separating charge analytes [16–18]. The bipolar electrochemistry works based on bipolar electrode (BPE) where faradaic reactions are induced at its opposite ends driven by electrode/solution interfacial potential difference in the internal electric field of solution produced from driving electrodes [19]. This eliminates the need for the ohmic connection between working electrode and power supply, which allows controlling arbitrary number of “wireless” electrodes having definable dimension simultaneously in a very simple setup [20]. The BPEs that work individually do not interfere with one another, making it possible for better understanding, diagnosing, manipulating and optimizing the reaction process [21]. To the best of our knowledge, most of bipolar electrochemistry is restricted to micro- and/or nano-scale systems, but these small-size systems are inappropriate to handling larger-volume water stream. Herein, we develop a lab-scale wireless electrocoagulation (WEC) system based on the concept of bipolar electrochemistry. In WEC, the driving electrodes produce electric field in solution, giving the sacrificial BPE to float to equilibrium interfacial potential. If equilibrium potential is sufficiently high, it will drive electrochemical dissociation of sacrificial electrode at anodic pole and simultaneous hydrogen evolution at cathodic pole in the absence of ohmic contact (Scheme S1 in Supporting Information) [22]. Compared with conventional EC (Scheme S2 in Supporting Information), the WEC may offer several unique advantages such as simple design, wireless operation, ease of management, and simultaneous working with many electrodes [23–25]. We will show that the iron coagulant production was more dependent on the geometrical configuration of BPE rather than electrochemical parameters.
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experiments, the rectangular iron sheets were cut by into “H-shape” or “T-shape” with various relative cathode/anode surface area. The iron sheets were mechanically polished using abrasive paper, and then washed with ethanol and DI-water before they were used as bipolar electrodes. During the WEC tests, the BPEs were placed at the center of the cell using two alligator clips, located at 30 mm from the bottom of the cell, and connected with DC power supply using copper wire. During each cycle of experiment, the cell was filled with 500 mL of Na2SO4 solution, giving the conductivity of 1.67 mS cm1 for the electrolyte. 2.3. Measurements and Analyses At the beginning of the WEC operation, the polarization potential along the solution/BPE interface was measured by connecting a voltmeter, and the positive and negative terminals of BPE were connected to a mechanical pencil lead (6B) separately, which acted as the new probe to prevent the corrosion of metal probe in the electric field. Following the formation of electric field in the solution induced by driving potential on graphite plates, the two mechanical pencil lead probes nearby the surface of the BPE acted as the negative and positive terminal of the voltmeter. By fixing the cathodic pencil lead probe near the marginal part of the BPE anode, and moving the anodic pencil lead probe directly along the BPE in the solution, the potential difference between the two new probes could be measured. Before sedimentation of iron floc in WEC system, the samples of mixed solution were stirred evenly, and then collected and dissolved in 4 M HNO3 (A.R.) for total iron measurement. To determine total iron concentration, the water samples were diluted to 30 times using 4 M HNO3 solution to ensure total solubility of the iron [27]. The total iron concentration was measured by using inductively coupled plasma optical emission spectrometer (ICP-OES, Perkin-Elmer Optima 5300DV, U.S.) [28]. The turbidity of water samples was measured using a portable turbidimeter (VELP Scientifica, Italy) according to standard methods described in the literature [28]. The conductivity and pH of solution were measured by using conductometer (Type CON2700, Eutech, U.S.) and pH meter (Type MT-5000, China), respectively. 3. Results and discussion
2. Materials and methods 3.1. WEC Can Be Driven by Internal Electric Field of Electrolyte 2.1. Chemicals and Reagents All the aqueous solutions were prepared using deionized water (DI-water). The sodium sulfate (Na2SO4, A.R.) was dissolved in DIwater for the required concentration. The water used in the experiment was suspension of river sand (collected from the Songhua River, Harbin, China), and the ratio of river sand (g) to DIwater (mL) was 100:1000 with the initial turbidity of 44.10 NTU. Following stirring, the sample of river sand suspension was placed for 1 h at room temperature, and then volume of 500 mL was taken for the experiment [26]. 2.2. Bipolar Electrochemistry WEC Setup The wireless electrocoagulation (WEC) experiments were carried out in a cubic plexiglas cell (100 mm 100 mm 100 mm). A direct current (DC) power supply (HSPY-36-03, China) was used to perform bipolar electrochemistry experiments. Two graphite plates (60 mm 70 mm 10 mm) were used as driving electrodes with the interspacing distance of 75 cm. The BPEs were made of pure iron sheets (0.5 mm thick). The iron sheets were cut into rectangles with the dimensions of 70 mm 60 mm. In some
It is the BPE/solution interfacial potential induced by electric field in solution that drives the electrochemical reactions on BPE (Scheme S2 in Supporting Information) [29]. From the thermodynamic point of view, the faradaic reaction on two terminates of BPE requires the voltage drop along the length of BPE (DEBPE) to be higher than conditional potential difference of two redox couples included [16]. During the operation of WEC at pH-neutral condition (pH7.2), the dissolution took place on the anodic pole with the standard potential of E0Fe/Fe2+ = 0.44 V (vs standard hydrogen electrode; SHE). At the same time, H2 was produced on the cathodic pole at E0H+/H2 = 0.414 V vs SHE [30]. That is, the equilibrium interfacial potential has to be at least in excess of |E0H 0 +/H2 E Fe/Fe2+| = 0.854 V otherwise no faradaic reaction will be attained on BPE. It was worth noting that the actual voltage applied to driving electrodes (EApp) may be much higher than the theoretical value so as to overcome the overpotential resulting from charge transfer, electrolyte, diffusion, and even the threshold effect of electrode [31]. EApp should constitute the most significant parameter in WEC because it not only determines the extent to which iron is electrochemically oxidized, but also is a measure of energy consumption [32]. To determine the practically accessible
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minimum EApp for iron dissolution, total iron concentration was measured with respect to EApp at the reaction time of 60 min. On the basis of given sodium sulphate solution conductivity of 0.14 mS cm1, total iron concentration remained low (<21 mg L1) for EApp < 5.0 V, but underwent a sudden ascent for EApp > 5.0 V, reaching the maximum of 250 mg L1 at EApp = 9 V (Fig. 1A). These results accorded well with the change of clear and colorless solution to yellow color, and the turbidity removal tendency of river water suspensions as revealed in Fig. 1B. The turbidity removal was increased steeply when the iron concentration reached the critical point of 5 V. The turbidity removal achieved in WEC was the same as that attained in conventional coagulation process (above 80%) [3], where the negatively charged colloids are destabilized and flocculated by ions, hydroxide, and polyhydroxides produced from the corrosion of BPE. In WEC, the sacrificial BPE is a conductor, when reaching electric balance in electric fields, the whole BPE is an equal potential body, which point (plane) can represent the electric potential of the whole BPE [33]. Instead, the interfacial potential difference between the BPE and the solution drives electrochemical reactions starting at the pole of BPE [34]. By using the methods described by Termebaf et al [35], the potentials of anodic and cathodic pole were measured to be +1.8 V and 1.8 V vs the solution, respectively, forming the electric field in solution with linear potential gradient at EApp of 9 V. That is, more than 85% loss of voltage in the electric field of solution here can be in line with approximately 10-15% voltage drop at BPE as reported in literatures [36]. This means that the iron dissolution began firstly at the edge of anodic pole, and then evolved in the direction of cathodic pole to the critical point where the reaction terminates [37,38]. This could be seen from the SEM morphology (Fig. 2B–E), showing the gradient of extent to which BPE corroded along the length, i.e. the severe corroded surfaces on the anodic pole while no corrosion observed on the cathodic pole. At the same time, hydrogen gas was produced at the cathodic pole to accomplish charge neutralization during WEC.
Fig. 2. (A) Distribution of equilibrium floating potential and corrosion morphology along the length of (B) 0 mm, (C) 20 mm, (D) 35 mm and (D) 70 mm of BPE. (EApp of 9 V, reaction time of 20 min and electrolytic conductivity of 1.67 mS cm1).
3.2. Impact of Placement Angle of BPE Considering the feature of potential distribution on BPE illustrated in Fig. 2, the WEC performances were studied at angle of various degrees (u = 0 , 45 and 90 ) for BPE to the horizontal electric field line formed in the solution. As shown in Fig. 3, for the
Fig. 1. Effect of supply voltage on (A) iron electrodissolution rate and (B) turbidity removal of river water suspensions during WEC process. (Electrolytic conductivity of 1.67 mS cm1 and reaction time of 60 min).
Fig. 3. Total iron concentration as function of the angle to the horizontal electric field line formed in solution at EApp of 9 V. The error bars S.D. represent the measurement in triplicate.
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However, it worth noting that the equation (Eq. (1)) can only be used for estimation on a qualitative basis, because it is simplified by incorporating a number of assumptions that may be possible to ignore. One example is the occurrence of dissolution for vertically placed BPE with minimum effective length (u = 90 ), and the similar phenomenon was also reported by Jerome Duval et al. [42,43]. 3.3. Impact of BPE Number
Fig. 4. Total iron concentration as function of the number of BPE at applied driving voltage of 9 V. The error bars S.D. represent the measurement in triplicate.
BPE having the fixed length, the iron concentration was impacted significantly by the placed angle. Based on the same reaction time (20 min), placing the BPE in parallel to electric field line (u = 0 ) produced the highest iron concentration of 52.8 mg L1, representing a value 84.6% higher than that placed vertically (u = 90 , 28.6 mg L1). As expected, the placement at u = 45 (39.2 mg L1) resulted in the iron concentration falling between the two limits. That is, the amount of coagulant produced can be increased substantially by just simply altering the placing angle of BPE in WEC system. This should be the result of difference in the driving force (i.e. interfacial potential) for iron dissolution at different placing angles [39]. Since the voltage is always dropped along the length of BPE in parallel to the electric field line (horizontally projected length) [40], DEBPE depends on just two experimental variables, i.e. the magnitude of EApp and the horizontally projected length of BPE (lHPL) at constant length of channel (lTotal), which can be estimated as (Eq. (1)) [41]. l DEBPE ¼ EApp HPL ð1Þ lTotal Given the constant EApp of 9 V and lTotal of 80 cm and lHPL = lBPE cosu, the DEBPE can be maximized for horizontal placement (u = 0 ), which is in good consistence with the potential measured experimentally (Fig. S1 in Supporting Information).
Wireless operation without ohmic connection to power supply makes it technically possible to implement many sacrificial BPEs simultaneously in bipolar electrochemistry [20,44,45]. Based on this consideration, we operated WEC by employing one, two, and three pieces of iron sheets (i.e. N = 1, 2, 3) under constant EApp of 9.0 V and solution conductivity of 1.67 mS cm1 in 20 min reaction time. As shown in Fig. 4, the iron concentration was 50.67 mg L1, 81.99 mg L1, and 102.60 mg L1 for N = 1, 2, 3, respectively. In other words, increasing the number of BPE from 1 to 3 led to the increase in coagulant production from 0.47 mol L1 to 0.96 mol L1 (increased by 2.1 times). Such increase can be easily understood by considering the working feature of bipolar electrochemistry where electrochemical reaction takes place individually driven by the electric field in solution (Scheme S2) [46]. This result suggests a new strategy for manipulating the coagulant dosage in water treatment by coagulation process. Unlike either conventional coagulation or electrocoagulation, the WEC provides an easy-tohandle and cost-effective manner to control the amount of generated coagulants by doing nothing but in-situ altering the number of electrodes. On the other hand, it is noticed that the iron concentration for N = 3 is approximately 2.1 times as much as that for N = 1, which is lower than triplicate value estimated theoretically. We attribute such discrimination to the assumption of no interference between the electric field produced by BPEs and formed in solution, but which is often not the case. In detail, the increase in BPE number is equivalent to the addition of a parallel branch to the electric field [47], which may lead to the decrease in the current passing through each branch [36,48]. 3.4. Impact of Geometrical Configuration of BPE As seen from the equilibrium floating potential and corrosion feature given in Fig. 2, the iron corrosion and dissolution only occurs at the anodic pole of BPE in the length region of 0–30 cm. For
Table 1 Summary of WEC performances of BPE with various geometrical configurationsa . Geometrical configuration of BPEb
a b
Area of cathodic pole (cm2)
Relative cathode/anode surface area
Fe concentration (mg L1)
42
1:1
52.06 3.34
29.82
0.71:1
50.81 2.87
23.1
0.55:1
53.37 1.66
15.12
0.36:1
51.51 3.51
7.14
0.17:1
26.73 4.53
The experiments were carried out at driving voltage of 9 V, solution conductivity of 1.67 mS cm1 and reaction time of 20 min. The surface area of anodic pole was fixed 42 cm2 measured from the middle point of the BPE.
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Scheme 1. Schematic illustration of (A) mechanisms and (B) potential application mode of WEC based on bipolar electrochemistry.
the length greater than 30 cm, however, the thermodynamic barrier emerges as a consequence of insufficient interfacial potential to drive the anodic dissolution [49]. Obviously, this means negligible contribution of cathodic pole to coagulant production, accounting for nearly 50% of the electrode material wasted unproductively. Considering the maximization of voltage drop and electron transportation on BPE, we re-designed the sacrificial BPE by cutting the abdomens on two sides of cathodic part and remaining the central axis, giving a variety of geometrical configurations of BPE shown in Table 1. Clearly is visible that all the configurations produced iron concentration on a similar magnitude (50.81 2.87-53.37 1.66 mg L1) only except for “T-shaped” BPE (26.73 4.53 mg L1). This should result from the electrochemical activation limitation associated with the lack of enough surface area for water reduction at the cathodic pole [50,51]. In such case, the overall WEC performances were dominated by the cathodic reaction. Modifying the BPE into “H-shape” was shown effective to mitigate such limitation; even the relative cathode/ anode surface area was as low as 0.36. In such case, the WEC performance was impacted slightly by their shapes at the same horizontally-projected length. This suggests a technical possibility to optimize the design of geometrical configuration of electrode for coagulant production by saving as great as 40% of the electrode material and cost. 4. Conclusions Electrocoagulation has long been adopted as an effective technology for water and wastewater treatment [1]. Nevertheless, conventional EC process requires the viable connection between each sacrificial electrode and external power supply, otherwise the corrosion of electrode/wire conjunction will cause a sharp increase in contact resistance, complexity of maintenance and operational cost in practice. The three-dimensional (3D)-electrode technology
can be a alternative, but its performances are commonly limited by ohmic contact among the granular electrodes, short-circuit current [9], and blocking phenomenon [10]. Besides, the irregular potential distribution within the complex electrode matrix makes it quite difficult optimize the design and operation of the system [12]. We herein demonstrate that all these aspects can be addressed, at least partially, using wireless electrocoagulation (WEC) based on the concept of bipolar electrochemistry. WEC allows wireless operation, which eliminates the concern on problems associated with ohmic connection and works upon arbitrary number of sacrificial BPEs simultaneously in a very simple setup. Notably, it is interesting to find that, driven by the electric field in solution, the total iron concentration produced is more dependent on the geometrical configuration of BPE rather than electrochemical parameters, suggesting a possibility of optimizing WEC performance by simply designing proper length, angle, number and shape of sacrificial BPEs. The corresponding WEC reaction system can be modified into various configurations for potential applications as illustrated in Scheme 1. Taken together, this study provides a proof-in-concept demonstration of bipolar electrochemistry for wireless electrocoagulation (WEC). Being similar as the conventional electrocoagulation, the WEC also accomplishes pollutants removal via mechanisms of charge neutralization, flocculation, and adsorption by iron coagulants produced. By virtue of several unique advantages, it may offer a new strategy to design electrocoagulation process based on sacrificial BPE, making the electrocoagulation treatment much easier, more economical and more reliable. Moreover, it will be also desirable for this mode to find applications in electrochemical oxidation, reduction, or adsorption systems. Acknowledgements Project supported by the National Natural Science Foundation of China (No. 51678184, 51378143), Natural Science Foundation of Heilongjiang Province of China (No. E2016034), and HIT Environment and Ecology Innovation Special Funds (No. HSCJ201610). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2017.01.151. References [1] O. Sahu, B. Mazumdar, P.K. Chaudhari, Treatment of wastewater by electrocoagulation: A review, Environ. Sci. Pollut. R. 21 (2014) 2397–2413. [2] M.Y.A. Mollah, R. Schennach, J.R. Parga, D.L. Cocke, Electrocoagulation (EC)— science and applications, J. Hazard. Mater. 84 (2001) 29–41. [3] P. Cañizares, C. Jiménez, F. Martínez, A. Cristina Sáez, M.A. Rodrigo, Study of the electrocoagulation process using aluminum and iron electrodes, Ind. Eng. Chem. 46 (2007) 6189–6195. [4] D. Vandamme, K. Muylaert, I. Fraeye, I. Foubert, Floc characteristics of chlorella vulgaris: Influence of flocculation mode and presence of organic matter, Bioresour. Technol. 151 (2014) 383–387. [5] K.L. Dubrawski, M. Mohseni, In-situ identification of iron electrocoagulation speciation and application for natural organic matter (NOM) removal, Water Res. 47 (2013) 5371–5380. [6] J.R. Parga, D.L. Cocke, V. Valverde, J.A. Gomes, M. Kesmez, H. Moreno, M. Weir, D. Mencer, Characterization of electrocoagulation for removal of chromium and arsenic, Chem. Eng. Technol. 28 (2005) 605–612. [7] U. Erb, H. Gleiter, G. Schwitzgebel, The effect of boundary structure (energy) on interfacial corrosion, Acta Metall. 30 (1982) 1377–1380. [8] H. Yasuda, Q. Yu, M. Chen, Interfacial factors in corrosion protection: An EIS study of model systems, Prog. Org. Coat. 41 (2001) 273–279. [9] L. Yan, H. Ma, B. Wang, Y. Wang, Y. Chen, Electrochemical treatment of petroleum refinery wastewater with three-dimensional multi-phase electrode, Desalination 276 (2011) 397–402. [10] M. Mastali, E. Samadani, S. Farhad, R. Fraser, M. Fowler, Three-dimensional multi-particle electrochemical model of LiFePO4 cells based on a resistor network methodology, Electrochim. Acta 190 (2016) 574–587.
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Wireless Electrocoagulation in Water Treatment Based on Bipolar Electrochemistry Zhenlian Qi, Shijie You* , Nanqi Ren State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, PR China
A R T I C L E I N F O
Article history: Received 18 November 2016 Received in revised form 11 January 2017 Accepted 23 January 2017 Available online 24 January 2017 Keywords: Electrocoagulation Bipolar electrochemistry Wireless Iron
A B S T R A C T
The present study reports the wireless electrocoagulation (WEC) based on bipolar electrochemistry where the iron sheets were used as sacrificial bipolar electrodes (BPEs) and two graphite plates served as driving electrodes. Driven by the interfacial potential difference induced by electric field in solution, the iron dissolution started at the anodic pole, achieving the generation of iron coagulant and thus turbidity removal. The total iron concentration produced was found to be more dependent on the geometrical configuration of BPE rather than electrochemical parameters. That is, placing the BPE with the length in parallel to electric field could generate iron concentration of 84.6% higher than that when placed vertically. Besides, increasing the number of BPE from one to three led to the increase in coagulant production by 2.1 times. Moreover, to maximize the voltage drop and minimize the material used, the BPE was designed into different geometrical shapes. The “H-shaped” BPE could achieve almost the same iron concentration as the unmodified BPE even the relative cathode/anode surface area was as low as 0.36. This accounted for efficient coagulant production by saving as great as 40% of the electrode material and cost. The wireless operation can not only solve the problems caused by electrode connection, but also allow arbitrary number of sacrificial BPEs working simultaneously in a very simple setup. In addition, the WEC performance can be easily controlled by the geometrical configuration of BPE. All these factors will be expected to make the electrocoagulation process much easier, more economical and more reliable in water treatment. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Electrocoagulation (EC) is a classic and widely used method to remove a broad range of contaminants (e.g. suspended particles, dyes, soluble inorganic and organic matter) from the polluted water [1–3]. A typical EC system contains an anode (sacrificial electrode) and a cathode (counter electrode) connected via an external circuit. Driven by positive potential applied to anode (e.g. aluminum or iron), the anodic dissolution leads to the formation of positively charged ions, hydroxide, and polyhydroxide compounds. As those formations are strongly affinitive toward removal of particulate, colloids and counter ions [4], the pollutants then can be separated and removed by in-situ produced coagulant via the processes of charge neutralization, flocculation, adsorption, sedimentation, and even flotation [5].
* Corresponding author at: P. O. Box 2603#, No. 73, Huanghe Road, Nangang District, Harbin 150090, PR China. E-mail address: [email protected] (S. You). http://dx.doi.org/10.1016/j.electacta.2017.01.151 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
In conventional EC, the electrochemical parameters are controlled by external power supply, and the amount of coagulant electro-produced is positively proportional to the current passing the electrode according to Faraday’s Law. In this context, the electrode connection is of particular importance, but it is often ignored in lab-scale studies and little attention has been paid to the impact of electrical connection on EC performance [6]. It is common practice to create connection between electrode and external circuit via ohmic contact (i.e. conductive wire). When the electrode and wire are in contact with one another, the contact interface is subject to have much higher corrosion stress, rendering the sacrificial electrode more susceptible to corrosion at the joint regions [7]. This will cause high contact resistance, which may add the complexity of manufacturing operation and maintenance in practical applications [8]. The three-dimensional (3D) electrodes may offer a possible solution, but the diminution in performances may be in association with the ohmic contact among the particles, short-circuit current [9], and blocking phenomenon [10]. Also, particle electrode that can play as the charged microelectrodes in the presence of electric field [9] may lose its adsorption capacity and catalytic activity due
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to the accumulation of pollutants on particle surface in continuous runs [11]. Moreover, the mechanisms of electro-coagulation based on 3D electrode are ambiguous rather than definitive because of irregular potential distribution within the complex electrode matrix, which may further limit the development of this technology for wide utilization in various wastewater treatments. There are still many works to be done to forward from fundamental mechanism to reactor scale for application [12]. Recently, bipolar electrochemistry has gained a growing popularity in a broad range of applications such as sensing and screening [13], generating nanowires [14], creating molecular gradient [15], and concentrating and separating charge analytes [16–18]. The bipolar electrochemistry works based on bipolar electrode (BPE) where faradaic reactions are induced at its opposite ends driven by electrode/solution interfacial potential difference in the internal electric field of solution produced from driving electrodes [19]. This eliminates the need for the ohmic connection between working electrode and power supply, which allows controlling arbitrary number of “wireless” electrodes having definable dimension simultaneously in a very simple setup [20]. The BPEs that work individually do not interfere with one another, making it possible for better understanding, diagnosing, manipulating and optimizing the reaction process [21]. To the best of our knowledge, most of bipolar electrochemistry is restricted to micro- and/or nano-scale systems, but these small-size systems are inappropriate to handling larger-volume water stream. Herein, we develop a lab-scale wireless electrocoagulation (WEC) system based on the concept of bipolar electrochemistry. In WEC, the driving electrodes produce electric field in solution, giving the sacrificial BPE to float to equilibrium interfacial potential. If equilibrium potential is sufficiently high, it will drive electrochemical dissociation of sacrificial electrode at anodic pole and simultaneous hydrogen evolution at cathodic pole in the absence of ohmic contact (Scheme S1 in Supporting Information) [22]. Compared with conventional EC (Scheme S2 in Supporting Information), the WEC may offer several unique advantages such as simple design, wireless operation, ease of management, and simultaneous working with many electrodes [23–25]. We will show that the iron coagulant production was more dependent on the geometrical configuration of BPE rather than electrochemical parameters.
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experiments, the rectangular iron sheets were cut by into “H-shape” or “T-shape” with various relative cathode/anode surface area. The iron sheets were mechanically polished using abrasive paper, and then washed with ethanol and DI-water before they were used as bipolar electrodes. During the WEC tests, the BPEs were placed at the center of the cell using two alligator clips, located at 30 mm from the bottom of the cell, and connected with DC power supply using copper wire. During each cycle of experiment, the cell was filled with 500 mL of Na2SO4 solution, giving the conductivity of 1.67 mS cm1 for the electrolyte. 2.3. Measurements and Analyses At the beginning of the WEC operation, the polarization potential along the solution/BPE interface was measured by connecting a voltmeter, and the positive and negative terminals of BPE were connected to a mechanical pencil lead (6B) separately, which acted as the new probe to prevent the corrosion of metal probe in the electric field. Following the formation of electric field in the solution induced by driving potential on graphite plates, the two mechanical pencil lead probes nearby the surface of the BPE acted as the negative and positive terminal of the voltmeter. By fixing the cathodic pencil lead probe near the marginal part of the BPE anode, and moving the anodic pencil lead probe directly along the BPE in the solution, the potential difference between the two new probes could be measured. Before sedimentation of iron floc in WEC system, the samples of mixed solution were stirred evenly, and then collected and dissolved in 4 M HNO3 (A.R.) for total iron measurement. To determine total iron concentration, the water samples were diluted to 30 times using 4 M HNO3 solution to ensure total solubility of the iron [27]. The total iron concentration was measured by using inductively coupled plasma optical emission spectrometer (ICP-OES, Perkin-Elmer Optima 5300DV, U.S.) [28]. The turbidity of water samples was measured using a portable turbidimeter (VELP Scientifica, Italy) according to standard methods described in the literature [28]. The conductivity and pH of solution were measured by using conductometer (Type CON2700, Eutech, U.S.) and pH meter (Type MT-5000, China), respectively. 3. Results and discussion
2. Materials and methods 3.1. WEC Can Be Driven by Internal Electric Field of Electrolyte 2.1. Chemicals and Reagents All the aqueous solutions were prepared using deionized water (DI-water). The sodium sulfate (Na2SO4, A.R.) was dissolved in DIwater for the required concentration. The water used in the experiment was suspension of river sand (collected from the Songhua River, Harbin, China), and the ratio of river sand (g) to DIwater (mL) was 100:1000 with the initial turbidity of 44.10 NTU. Following stirring, the sample of river sand suspension was placed for 1 h at room temperature, and then volume of 500 mL was taken for the experiment [26]. 2.2. Bipolar Electrochemistry WEC Setup The wireless electrocoagulation (WEC) experiments were carried out in a cubic plexiglas cell (100 mm 100 mm 100 mm). A direct current (DC) power supply (HSPY-36-03, China) was used to perform bipolar electrochemistry experiments. Two graphite plates (60 mm 70 mm 10 mm) were used as driving electrodes with the interspacing distance of 75 cm. The BPEs were made of pure iron sheets (0.5 mm thick). The iron sheets were cut into rectangles with the dimensions of 70 mm 60 mm. In some
It is the BPE/solution interfacial potential induced by electric field in solution that drives the electrochemical reactions on BPE (Scheme S2 in Supporting Information) [29]. From the thermodynamic point of view, the faradaic reaction on two terminates of BPE requires the voltage drop along the length of BPE (DEBPE) to be higher than conditional potential difference of two redox couples included [16]. During the operation of WEC at pH-neutral condition (pH7.2), the dissolution took place on the anodic pole with the standard potential of E0Fe/Fe2+ = 0.44 V (vs standard hydrogen electrode; SHE). At the same time, H2 was produced on the cathodic pole at E0H+/H2 = 0.414 V vs SHE [30]. That is, the equilibrium interfacial potential has to be at least in excess of |E0H 0 +/H2 E Fe/Fe2+| = 0.854 V otherwise no faradaic reaction will be attained on BPE. It was worth noting that the actual voltage applied to driving electrodes (EApp) may be much higher than the theoretical value so as to overcome the overpotential resulting from charge transfer, electrolyte, diffusion, and even the threshold effect of electrode [31]. EApp should constitute the most significant parameter in WEC because it not only determines the extent to which iron is electrochemically oxidized, but also is a measure of energy consumption [32]. To determine the practically accessible
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minimum EApp for iron dissolution, total iron concentration was measured with respect to EApp at the reaction time of 60 min. On the basis of given sodium sulphate solution conductivity of 0.14 mS cm1, total iron concentration remained low (<21 mg L1) for EApp < 5.0 V, but underwent a sudden ascent for EApp > 5.0 V, reaching the maximum of 250 mg L1 at EApp = 9 V (Fig. 1A). These results accorded well with the change of clear and colorless solution to yellow color, and the turbidity removal tendency of river water suspensions as revealed in Fig. 1B. The turbidity removal was increased steeply when the iron concentration reached the critical point of 5 V. The turbidity removal achieved in WEC was the same as that attained in conventional coagulation process (above 80%) [3], where the negatively charged colloids are destabilized and flocculated by ions, hydroxide, and polyhydroxides produced from the corrosion of BPE. In WEC, the sacrificial BPE is a conductor, when reaching electric balance in electric fields, the whole BPE is an equal potential body, which point (plane) can represent the electric potential of the whole BPE [33]. Instead, the interfacial potential difference between the BPE and the solution drives electrochemical reactions starting at the pole of BPE [34]. By using the methods described by Termebaf et al [35], the potentials of anodic and cathodic pole were measured to be +1.8 V and 1.8 V vs the solution, respectively, forming the electric field in solution with linear potential gradient at EApp of 9 V. That is, more than 85% loss of voltage in the electric field of solution here can be in line with approximately 10-15% voltage drop at BPE as reported in literatures [36]. This means that the iron dissolution began firstly at the edge of anodic pole, and then evolved in the direction of cathodic pole to the critical point where the reaction terminates [37,38]. This could be seen from the SEM morphology (Fig. 2B–E), showing the gradient of extent to which BPE corroded along the length, i.e. the severe corroded surfaces on the anodic pole while no corrosion observed on the cathodic pole. At the same time, hydrogen gas was produced at the cathodic pole to accomplish charge neutralization during WEC.
Fig. 2. (A) Distribution of equilibrium floating potential and corrosion morphology along the length of (B) 0 mm, (C) 20 mm, (D) 35 mm and (D) 70 mm of BPE. (EApp of 9 V, reaction time of 20 min and electrolytic conductivity of 1.67 mS cm1).
3.2. Impact of Placement Angle of BPE Considering the feature of potential distribution on BPE illustrated in Fig. 2, the WEC performances were studied at angle of various degrees (u = 0 , 45 and 90 ) for BPE to the horizontal electric field line formed in the solution. As shown in Fig. 3, for the
Fig. 1. Effect of supply voltage on (A) iron electrodissolution rate and (B) turbidity removal of river water suspensions during WEC process. (Electrolytic conductivity of 1.67 mS cm1 and reaction time of 60 min).
Fig. 3. Total iron concentration as function of the angle to the horizontal electric field line formed in solution at EApp of 9 V. The error bars S.D. represent the measurement in triplicate.
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However, it worth noting that the equation (Eq. (1)) can only be used for estimation on a qualitative basis, because it is simplified by incorporating a number of assumptions that may be possible to ignore. One example is the occurrence of dissolution for vertically placed BPE with minimum effective length (u = 90 ), and the similar phenomenon was also reported by Jerome Duval et al. [42,43]. 3.3. Impact of BPE Number
Fig. 4. Total iron concentration as function of the number of BPE at applied driving voltage of 9 V. The error bars S.D. represent the measurement in triplicate.
BPE having the fixed length, the iron concentration was impacted significantly by the placed angle. Based on the same reaction time (20 min), placing the BPE in parallel to electric field line (u = 0 ) produced the highest iron concentration of 52.8 mg L1, representing a value 84.6% higher than that placed vertically (u = 90 , 28.6 mg L1). As expected, the placement at u = 45 (39.2 mg L1) resulted in the iron concentration falling between the two limits. That is, the amount of coagulant produced can be increased substantially by just simply altering the placing angle of BPE in WEC system. This should be the result of difference in the driving force (i.e. interfacial potential) for iron dissolution at different placing angles [39]. Since the voltage is always dropped along the length of BPE in parallel to the electric field line (horizontally projected length) [40], DEBPE depends on just two experimental variables, i.e. the magnitude of EApp and the horizontally projected length of BPE (lHPL) at constant length of channel (lTotal), which can be estimated as (Eq. (1)) [41]. l DEBPE ¼ EApp HPL ð1Þ lTotal Given the constant EApp of 9 V and lTotal of 80 cm and lHPL = lBPE cosu, the DEBPE can be maximized for horizontal placement (u = 0 ), which is in good consistence with the potential measured experimentally (Fig. S1 in Supporting Information).
Wireless operation without ohmic connection to power supply makes it technically possible to implement many sacrificial BPEs simultaneously in bipolar electrochemistry [20,44,45]. Based on this consideration, we operated WEC by employing one, two, and three pieces of iron sheets (i.e. N = 1, 2, 3) under constant EApp of 9.0 V and solution conductivity of 1.67 mS cm1 in 20 min reaction time. As shown in Fig. 4, the iron concentration was 50.67 mg L1, 81.99 mg L1, and 102.60 mg L1 for N = 1, 2, 3, respectively. In other words, increasing the number of BPE from 1 to 3 led to the increase in coagulant production from 0.47 mol L1 to 0.96 mol L1 (increased by 2.1 times). Such increase can be easily understood by considering the working feature of bipolar electrochemistry where electrochemical reaction takes place individually driven by the electric field in solution (Scheme S2) [46]. This result suggests a new strategy for manipulating the coagulant dosage in water treatment by coagulation process. Unlike either conventional coagulation or electrocoagulation, the WEC provides an easy-tohandle and cost-effective manner to control the amount of generated coagulants by doing nothing but in-situ altering the number of electrodes. On the other hand, it is noticed that the iron concentration for N = 3 is approximately 2.1 times as much as that for N = 1, which is lower than triplicate value estimated theoretically. We attribute such discrimination to the assumption of no interference between the electric field produced by BPEs and formed in solution, but which is often not the case. In detail, the increase in BPE number is equivalent to the addition of a parallel branch to the electric field [47], which may lead to the decrease in the current passing through each branch [36,48]. 3.4. Impact of Geometrical Configuration of BPE As seen from the equilibrium floating potential and corrosion feature given in Fig. 2, the iron corrosion and dissolution only occurs at the anodic pole of BPE in the length region of 0–30 cm. For
Table 1 Summary of WEC performances of BPE with various geometrical configurationsa . Geometrical configuration of BPEb
a b
Area of cathodic pole (cm2)
Relative cathode/anode surface area
Fe concentration (mg L1)
42
1:1
52.06 3.34
29.82
0.71:1
50.81 2.87
23.1
0.55:1
53.37 1.66
15.12
0.36:1
51.51 3.51
7.14
0.17:1
26.73 4.53
The experiments were carried out at driving voltage of 9 V, solution conductivity of 1.67 mS cm1 and reaction time of 20 min. The surface area of anodic pole was fixed 42 cm2 measured from the middle point of the BPE.
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Scheme 1. Schematic illustration of (A) mechanisms and (B) potential application mode of WEC based on bipolar electrochemistry.
the length greater than 30 cm, however, the thermodynamic barrier emerges as a consequence of insufficient interfacial potential to drive the anodic dissolution [49]. Obviously, this means negligible contribution of cathodic pole to coagulant production, accounting for nearly 50% of the electrode material wasted unproductively. Considering the maximization of voltage drop and electron transportation on BPE, we re-designed the sacrificial BPE by cutting the abdomens on two sides of cathodic part and remaining the central axis, giving a variety of geometrical configurations of BPE shown in Table 1. Clearly is visible that all the configurations produced iron concentration on a similar magnitude (50.81 2.87-53.37 1.66 mg L1) only except for “T-shaped” BPE (26.73 4.53 mg L1). This should result from the electrochemical activation limitation associated with the lack of enough surface area for water reduction at the cathodic pole [50,51]. In such case, the overall WEC performances were dominated by the cathodic reaction. Modifying the BPE into “H-shape” was shown effective to mitigate such limitation; even the relative cathode/ anode surface area was as low as 0.36. In such case, the WEC performance was impacted slightly by their shapes at the same horizontally-projected length. This suggests a technical possibility to optimize the design of geometrical configuration of electrode for coagulant production by saving as great as 40% of the electrode material and cost. 4. Conclusions Electrocoagulation has long been adopted as an effective technology for water and wastewater treatment [1]. Nevertheless, conventional EC process requires the viable connection between each sacrificial electrode and external power supply, otherwise the corrosion of electrode/wire conjunction will cause a sharp increase in contact resistance, complexity of maintenance and operational cost in practice. The three-dimensional (3D)-electrode technology
can be a alternative, but its performances are commonly limited by ohmic contact among the granular electrodes, short-circuit current [9], and blocking phenomenon [10]. Besides, the irregular potential distribution within the complex electrode matrix makes it quite difficult optimize the design and operation of the system [12]. We herein demonstrate that all these aspects can be addressed, at least partially, using wireless electrocoagulation (WEC) based on the concept of bipolar electrochemistry. WEC allows wireless operation, which eliminates the concern on problems associated with ohmic connection and works upon arbitrary number of sacrificial BPEs simultaneously in a very simple setup. Notably, it is interesting to find that, driven by the electric field in solution, the total iron concentration produced is more dependent on the geometrical configuration of BPE rather than electrochemical parameters, suggesting a possibility of optimizing WEC performance by simply designing proper length, angle, number and shape of sacrificial BPEs. The corresponding WEC reaction system can be modified into various configurations for potential applications as illustrated in Scheme 1. Taken together, this study provides a proof-in-concept demonstration of bipolar electrochemistry for wireless electrocoagulation (WEC). Being similar as the conventional electrocoagulation, the WEC also accomplishes pollutants removal via mechanisms of charge neutralization, flocculation, and adsorption by iron coagulants produced. By virtue of several unique advantages, it may offer a new strategy to design electrocoagulation process based on sacrificial BPE, making the electrocoagulation treatment much easier, more economical and more reliable. Moreover, it will be also desirable for this mode to find applications in electrochemical oxidation, reduction, or adsorption systems. Acknowledgements Project supported by the National Natural Science Foundation of China (No. 51678184, 51378143), Natural Science Foundation of Heilongjiang Province of China (No. E2016034), and HIT Environment and Ecology Innovation Special Funds (No. HSCJ201610). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2017.01.151. References [1] O. Sahu, B. Mazumdar, P.K. Chaudhari, Treatment of wastewater by electrocoagulation: A review, Environ. Sci. Pollut. R. 21 (2014) 2397–2413. [2] M.Y.A. Mollah, R. Schennach, J.R. Parga, D.L. Cocke, Electrocoagulation (EC)— science and applications, J. Hazard. Mater. 84 (2001) 29–41. [3] P. Cañizares, C. Jiménez, F. Martínez, A. Cristina Sáez, M.A. Rodrigo, Study of the electrocoagulation process using aluminum and iron electrodes, Ind. Eng. Chem. 46 (2007) 6189–6195. [4] D. Vandamme, K. Muylaert, I. Fraeye, I. Foubert, Floc characteristics of chlorella vulgaris: Influence of flocculation mode and presence of organic matter, Bioresour. Technol. 151 (2014) 383–387. [5] K.L. Dubrawski, M. Mohseni, In-situ identification of iron electrocoagulation speciation and application for natural organic matter (NOM) removal, Water Res. 47 (2013) 5371–5380. [6] J.R. Parga, D.L. Cocke, V. Valverde, J.A. Gomes, M. Kesmez, H. Moreno, M. Weir, D. Mencer, Characterization of electrocoagulation for removal of chromium and arsenic, Chem. Eng. Technol. 28 (2005) 605–612. [7] U. Erb, H. Gleiter, G. Schwitzgebel, The effect of boundary structure (energy) on interfacial corrosion, Acta Metall. 30 (1982) 1377–1380. [8] H. Yasuda, Q. Yu, M. Chen, Interfacial factors in corrosion protection: An EIS study of model systems, Prog. Org. Coat. 41 (2001) 273–279. [9] L. Yan, H. Ma, B. Wang, Y. Wang, Y. Chen, Electrochemical treatment of petroleum refinery wastewater with three-dimensional multi-phase electrode, Desalination 276 (2011) 397–402. [10] M. Mastali, E. Samadani, S. Farhad, R. Fraser, M. Fowler, Three-dimensional multi-particle electrochemical model of LiFePO4 cells based on a resistor network methodology, Electrochim. Acta 190 (2016) 574–587.
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