A Comparison of the Mechanism of TOC and COD Degradation in Rhodamine B Wastewater by a Recycling-Flow Two- and Three-dimensional Electro-Reactor System

: Dye wastewater, as a kind of refractory wastewater (with a ratio of biochemical oxygen demand (BOD) and chemical oxygen demand (COD) of less than 0.3), still needs advanced treatments in order to reach the discharge standard. In this work, the recycling-ﬂow three-dimensional (3D) electro-reactor system was designed for degrading synthetic rhodamine B (RhB) wastewater as dye wastewater (100 mg / L). After 180 min of degradation, the removal of total organic carbon (TOC) and chemical oxygen demand (COD) of RhB wastewater were both approximately double the corresponding values in the recycling-ﬂow two-dimensional (2D) electro-reactor system. Columnar granular activated carbon (CGAC), as micro-electrodes packed between anodic and cathodic electrodes in the recycling-ﬂow 3D electro-reactor system, generated an obviously characteristic peak of anodic catalytic oxidation, increased the mass transfer rate and electrochemically active surface area (EASA) by 40%, and rapidly produced 1.52 times more hydroxyl radicals ( · OH) on the surface of CGAC electrodes, in comparison to the recycling-ﬂow 2D electro-reactor system. Additionally, the recycling-ﬂow 3D electro-reactor system can maintain higher current e ﬃ ciency (CE) and lower energy consumption (Es).


Introduction
Nowadays, the unsafe disposal of dye wastewater, which still contains lots of complex pollutants and toxic matter, such as aromatic, chloric, and azo compounds, is seriously threating environmental and ecological systems and human health [1][2][3][4][5]. The reason is that dye wastewater, as a kind of refractory wastewater (with a ratio of biochemical oxygen demand (BOD) and chemical oxygen demand (COD) of less than 0.3), is severely difficult to degrade in order to fall under discharge standards in the activated sludge process using traditional and biological wastewater treatment methods [6]. Therefore, there is a definite urgent need for methods which efficiently degrade dye wastewater after biological treatment [7][8][9][10].
In comparison to the two-dimensional (2D) electro-reactor, consisting of an anode electrode, cathode electrode, and electrolyzer, the three-dimensional (3D) electro-reactor contains a certain number of small granular substances, such as activated carbon particles and diatomite particles, In this recycling system, one peristaltic pump (Youji Keyi ZQ000S, Baoding, Hebei, China) was used to pump the influent RhB wastewater from a beaker into the bottom of the electro-reactor. Then the RhB wastewater flowed out from the upper outlet of the electro-reactor and was collected in an Erlenmeyer flask. Simultaneously, the other peristaltic pump was used to pump the effluent RhB wastewater from the Erlenmeyer flask back to the beaker.
Prior to commencing the electrochemical oxidation treatment, CGAC electrodes adsorbed RhB solution until becoming saturated in order to minimize the effect of adsorption on TOC and COD removal. All experiments used a digital DC power supply (DC 30 V/5 A; DH1716-6D). After completing the experiment, all treated samples were collected and filtered through 0.45 mm filters. The filtrate was then analyzed, as described in the next sub-sections.

Analytical Methods
TOC was measured by a Vario TOC analysis device. According to the instructions of the Vario TOC analysis device, when carbonaceous compounds are burned in an oxygen-rich environment, the carbon is completely converted into CO2, and then the non-scattering infrared detector (NDIR) detects the amount of CO2 and converts it into total carbon (TC) in the sample. After the sample is acidified by phosphoric acid (H3PO4) (1% v/v) and pH decreasing, the carbonate and bicarbonate in the sample are converted into CO2, which is blown out and enters the NDIR, and then the detected amount of CO2 is converted into total inorganic carbon (TIC). The value of TOC is TC minus TIC. COD was measured by a microwave digestion method. According to the instructions of the Kedibo microwave digestion device, mercury(II) sulfate (HgSO4) as a masking agent, 0.05 mol/L potassium dichromate (K2Cr2O7) as a digestion solution, a mix of 10 g silver sulfate (Ag2SO4) and 1 L sulfuric acid (H2SO4) as a catalyst, ferroin solution as an indicator, and potassium ferrocyanide (K4Fe(CN)6) as a standard solution were used.
Electrochemical measurements were performed using a conventional three-electrode cell and a CHI 660E electrochemical workstation (CHI, Beijing, China). Ag/AgCl and Ti/RuO2/TiO2 board electrodes served as the reference and counter electrodes, respectively. CE (%) is the rate of the efficient current and total current in a period and Es (KW·h/kg TOC) is the electricity consumption of 1 kg TOC degradation. They were calculated according to the following equations [25][26][27]: In this recycling system, one peristaltic pump (Youji Keyi ZQ000S, Baoding, Hebei, China) was used to pump the influent RhB wastewater from a beaker into the bottom of the electro-reactor. Then the RhB wastewater flowed out from the upper outlet of the electro-reactor and was collected in an Erlenmeyer flask. Simultaneously, the other peristaltic pump was used to pump the effluent RhB wastewater from the Erlenmeyer flask back to the beaker.
Prior to commencing the electrochemical oxidation treatment, CGAC electrodes adsorbed RhB solution until becoming saturated in order to minimize the effect of adsorption on TOC and COD removal. All experiments used a digital DC power supply (DC 30 V/5 A; DH1716-6D). After completing the experiment, all treated samples were collected and filtered through 0.45 mm filters. The filtrate was then analyzed, as described in the next sub-sections.

Analytical Methods
TOC was measured by a Vario TOC analysis device. According to the instructions of the Vario TOC analysis device, when carbonaceous compounds are burned in an oxygen-rich environment, the carbon is completely converted into CO 2 , and then the non-scattering infrared detector (NDIR) detects the amount of CO 2 and converts it into total carbon (TC) in the sample. After the sample is acidified by phosphoric acid (H 3 PO 4 ) (1% v/v) and pH decreasing, the carbonate and bicarbonate in the sample are converted into CO 2 , which is blown out and enters the NDIR, and then the detected amount of CO 2 is converted into total inorganic carbon (TIC). The value of TOC is TC minus TIC. COD was measured by a microwave digestion method. According to the instructions of the Kedibo microwave digestion device, mercury(II) sulfate (HgSO 4 ) as a masking agent, 0.05 mol/L potassium dichromate (K 2 Cr 2 O 7 ) as a digestion solution, a mix of 10 g silver sulfate (Ag 2 SO 4 ) and 1 L sulfuric acid (H 2 SO 4 ) as a catalyst, ferroin solution as an indicator, and potassium ferrocyanide (K 4 Fe(CN) 6 ) as a standard solution were used.
Electrochemical measurements were performed using a conventional three-electrode cell and a CHI 660E electrochemical workstation (CHI, Beijing, China). Ag/AgCl and Ti/RuO 2 /TiO 2 board electrodes served as the reference and counter electrodes, respectively. CE (%) is the rate of the efficient current and total current in a period and Es (KW·h/kg TOC) is the electricity consumption of 1 kg TOC degradation. They were calculated according to the following equations [25][26][27]: where TOC 0 (g/L) and TOC t (g/L) correspond to the total organic carbon at t = 0 min and t = t min, respectively. I is the average current (A), F is the Faraday constant (96,485 C/mol), Q is the flow rate of water (L/min), and U is the applied electric voltage (V).

Electrochemical Properties of TOC and COD Degradation in the Recycling-Flow Elecro-Reactor System
As shown in Figure 2, the TOC removal of RhB wastewater in the recycling-flow 3D electro-reactor system was always higher than that in the recycling-flow 2D electro-reactor system, and the highest TOC removal was 72.0%, which was 1.98 times that in the recycling-flow 2D electro-reactor system (36.3%). From the perspective of COD, the COD removal of RhB wastewater in the recycling-flow 3D electro-reactor system was much higher than that in the recycling-flow 2D electro-reactor system and the highest COD removal was up to 86.9%. By the 30th minute, COD removal in the recycling-flow 3D electro-reactor system had already reached 63.4%, meanwhile, the recycling-flow 2D electro-reactor system just reached 41.9% by the 180th minute.
where TOC0 (g/L) and TOCt (g/L) correspond to the total organic carbon at t = 0 min and t = t min, respectively. I is the average current (A), F is the Faraday constant (96,485 C/mol), Q is the flow rate of water (L/min), and U is the applied electric voltage (V).

Electrochemical Properties of TOC and COD Degradation in the Recycling-Flow Elecro-Reactor System
As shown in Figure 2, the TOC removal of RhB wastewater in the recycling-flow 3D electroreactor system was always higher than that in the recycling-flow 2D electro-reactor system, and the highest TOC removal was 72.0%, which was 1.98 times that in the recycling-flow 2D electro-reactor system (36.3%). From the perspective of COD, the COD removal of RhB wastewater in the recyclingflow 3D electro-reactor system was much higher than that in the recycling-flow 2D electro-reactor system and the highest COD removal was up to 86.9%. By the 30th minute, COD removal in the recycling-flow 3D electro-reactor system had already reached 63.4%, meanwhile, the recycling-flow 2D electro-reactor system just reached 41.9% by the 180th minute. Total organic carbon (TOC) and chemical oxygen demand (COD) removal of rhodamine B (RhB) wastewater in the recycling-flow 3D and 2D electro-reactor systems (RhB wastewater initial concentration is 100 mg/L; volume is 500 mL; initial pH is 7; Na2SO4 as an electrolyte, initial concentration is 2 g/L; voltage is 5 V; hydraulic retention time (HRT) is 20 min).
It is normally considered that the mass transfer and EASA of the electrodes play major roles. Therefore, the higher TOC and COD removal in the recycling-flow 3D electro-reactor system is due to the presence of CGAC as conductive particles packed in the 3D electro-reactor and constitute a number of microelectrodes, which dramatically increase the area of the reaction electrode and benefit organic matter degradation easily and quickly in oxidative processes [28,29].
In order to obtain the mass transfer rate and EASA of the electrodes in the 2D and 3D electroreactors, respectively, cyclic voltammograms (CVs) of the 2D and 3D electro-reactors were measured in a 0.05 mol/L K4Fe(CN)6 + 0.45 mol/L Na2SO4 solution at different scan rates from 0.015 V/s to 0.1 V/s by a CHI 660E electrochemical workstation (CHI, China). The 3D electro-reactor had a characteristic peak of anodic catalytic oxidation, as illustrated in Figure 3. Taking a sweep rate of 0.1 V/s (red line) as an example, when the potential was 0.7 V, it should be 0.2 A if the potential and the corresponding current were in a linear relationship (demonstrated in Figure 3b). However, the Figure  3a curve shows that the corresponding current is as high as 0.35 A at the potential of 0.7 V. This was because the corresponding current incurred a mutation at 0.7 V, which is called a characteristic peak of anodic catalytic oxidation.  Total organic carbon (TOC) and chemical oxygen demand (COD) removal of rhodamine B (RhB) wastewater in the recycling-flow 3D and 2D electro-reactor systems (RhB wastewater initial concentration is 100 mg/L; volume is 500 mL; initial pH is 7; Na 2 SO 4 as an electrolyte, initial concentration is 2 g/L; voltage is 5 V; hydraulic retention time (HRT) is 20 min).
It is normally considered that the mass transfer and EASA of the electrodes play major roles. Therefore, the higher TOC and COD removal in the recycling-flow 3D electro-reactor system is due to the presence of CGAC as conductive particles packed in the 3D electro-reactor and constitute a number of microelectrodes, which dramatically increase the area of the reaction electrode and benefit organic matter degradation easily and quickly in oxidative processes [28,29].
In order to obtain the mass transfer rate and EASA of the electrodes in the 2D and 3D electro-reactors, respectively, cyclic voltammograms (CVs) of the 2D and 3D electro-reactors were measured in a 0.05 mol/L K 4 Fe(CN) 6 + 0.45 mol/L Na 2 SO 4 solution at different scan rates from 0.015 V/s to 0.1 V/s by a CHI 660E electrochemical workstation (CHI, China). The 3D electro-reactor had a characteristic peak of anodic catalytic oxidation, as illustrated in Figure 3. Taking a sweep rate of 0.1 V/s (red line) as an example, when the potential was 0.7 V, it should be 0.2 A if the potential and the corresponding current were in a linear relationship (demonstrated in Figure 3b). However, the Figure 3a curve shows that the corresponding current is as high as 0.35 A at the potential of 0.7 V. This was because the corresponding current incurred a mutation at 0.7 V, which is called a characteristic peak of anodic catalytic oxidation.  It was found that both the anodic peak current and the cathodic peak current increased as the scan rate increased, indicating a reversible electrochemical reaction of the [Fe(CN)6] 4− /[Fe(CN)6] 3− redox couple. At the same time, Figure 3 (insets) shows a brilliant linear relationship between the oxidation peak current (Ip) and the square root of the scan rate (v 1/2 ), according to the following equation [21,30,31]: where n is the number of transferred electrons, A is the EASA (cm 2 ), DR is the diffusion coefficient of the reduced species (cm 2 /s), CR is the bulk reduced species concentration (mmol/L), and v is the scan rate (mV/s). The above equation can be simplified into the following equation: where k is a coefficient only relevant to A and DR because n and CR are constant in this study. As shown in the insets of Figure 3a,b, the slopes of the linear relationship between Ip and v 1/2 , named as the value of k, representing the mass transfer rate, were obtained according to the linear fitting of the plots. As expected, the k value of the 3D electro-reactor (0.3953) was larger than the corresponding value of the 2D electro-reactor (0.2816). This demonstrates that the 3D electro-reactor had greater mass transfer properties than the 2D electro-reactor [32,33].
In addition, the EASA can also be derived from the k value by assuming that the DR value of [Fe(CN)6] 4− is constant in this study. The obtained EASA value of the 3D electro-reactor is also higher than that of the 2D electro-reactor. In particular, it was 1.40 times the corresponding value of the 2D electro-reactor. The higher EASA of the 3D electro-reactor means that the 3D electro-reactor will provide much more electrochemically active sites for RhB oxidation, and thus will be beneficial in improving the oxidation of organics on the electrode surface [34].
Based on the mass transfer rate and EASA result and discussion above, the 3D electro-reactor has been demonstrated to possess a much higher electro-catalytic activity for degrading organic matter (RhB) than the 2D electro-reactor.
It is well known that the hydroxyl radical (·OH), of which the oxidation potential (2.8 eV) is the second highest and regarded as a powerful oxidizing chemical in nature, plays an important role in the electrochemical oxidation of RhB wastewater [35]. Therefore, the production of ·OH was detected by using the electron paramagnetic resonance (EPR) technique by adding the ·OH scavenger 5,5dimenthyl,1-pyrroline-N-oxide (DMPO) to further reveal the underlying mechanism of the TOC and COD degradation of RhB wastewater. As expected, typical EPR spectra of the DMPO-·OH adduct with a 1:2:2:1 quartet were acquired in the 3D and 2D electro-reactors when current and voltage were applied, as shown in Figure 4. It is worth noting that the 3D electro-reactor achieved the higher EPR intensity, which was nearly 2.52 times higher than the corresponding value of the 2D electro-reactor It was found that both the anodic peak current and the cathodic peak current increased as the scan rate increased, indicating a reversible electrochemical reaction of the [Fe(CN) 6 ] 4− /[Fe(CN) 6 ] 3− redox couple. At the same time, Figure 3 (insets) shows a brilliant linear relationship between the oxidation peak current (I p ) and the square root of the scan rate (v 1/2 ), according to the following equation [21,30,31]: where n is the number of transferred electrons, A is the EASA (cm 2 ), D R is the diffusion coefficient of the reduced species (cm 2 /s), C R is the bulk reduced species concentration (mmol/L), and v is the scan rate (mV/s). The above equation can be simplified into the following equation: where k is a coefficient only relevant to A and D R because n and C R are constant in this study. As shown in the insets of Figure 3a,b, the slopes of the linear relationship between I p and v 1/2 , named as the value of k, representing the mass transfer rate, were obtained according to the linear fitting of the plots. As expected, the k value of the 3D electro-reactor (0.3953) was larger than the corresponding value of the 2D electro-reactor (0.2816). This demonstrates that the 3D electro-reactor had greater mass transfer properties than the 2D electro-reactor [32,33].
In addition, the EASA can also be derived from the k value by assuming that the D R value of [Fe(CN) 6 ] 4− is constant in this study. The obtained EASA value of the 3D electro-reactor is also higher than that of the 2D electro-reactor. In particular, it was 1.40 times the corresponding value of the 2D electro-reactor. The higher EASA of the 3D electro-reactor means that the 3D electro-reactor will provide much more electrochemically active sites for RhB oxidation, and thus will be beneficial in improving the oxidation of organics on the electrode surface [34].
Based on the mass transfer rate and EASA result and discussion above, the 3D electro-reactor has been demonstrated to possess a much higher electro-catalytic activity for degrading organic matter (RhB) than the 2D electro-reactor.
It is well known that the hydroxyl radical (·OH), of which the oxidation potential (2.8 eV) is the second highest and regarded as a powerful oxidizing chemical in nature, plays an important role in the electrochemical oxidation of RhB wastewater [35]. Therefore, the production of ·OH was detected by using the electron paramagnetic resonance (EPR) technique by adding the ·OH scavenger 5,5-dimenthyl,1-pyrroline-N-oxide (DMPO) to further reveal the underlying mechanism of the TOC and COD degradation of RhB wastewater. As expected, typical EPR spectra of the DMPO-·OH adduct with a 1:2:2:1 quartet were acquired in the 3D and 2D electro-reactors when current and voltage were applied, as shown in Figure 4. It is worth noting that the 3D electro-reactor achieved the higher EPR intensity, which was nearly 2.52 times higher than the corresponding value of the 2D electro-reactor and demonstrated that the electro-generation of ·OH occurred on the CGAC electrodes and the usage of the CGAC electrodes could enhance ·OH generation effectively in electrochemical oxidation processes [36].
Water 2020, 12, x FOR PEER REVIEW 6 of 15 and demonstrated that the electro-generation of ·OH occurred on the CGAC electrodes and the usage of the CGAC electrodes could enhance ·OH generation effectively in electrochemical oxidation processes [36]. Overall, the degradation of organic matter (RhB) by the 3D electro-reactor depends on more ·OH being adsorbed on the surface of CGAC electrodes, forming more electrochemically active sites for RhB oxidation. However, the 2D electro-reactor can just undergo less ·OH adsorption and the degradation strength is also reduced due to the absence of CGAC as conductive particles.
The CE and Es of the recycling-flow 3D and 2D electro-reactor systems are shown in Figure 5. Overall, the CE of the recycling-flow 3D and 2D electro-reactor systems both went up from 8.3% and 2.5% to 14.4% and 7.6%, respectively, and the Es of the recycling-flow 3D and 2D electro-reactor systems both decreased from 202.6 KW·h/kg TOC and 658.1 KW·h/kg TOC to 116.3 KW·h/kg TOC and 219.9 KW·h/kg TOC, respectively, during the electrochemical processes. The CE of the recyclingflow 3D electro-reactor system was always nearly twice as much as the corresponding value of the recycling-flow 2D electro-reactor system in the treatment period. Additionally, the lowest CE value of the recycling-flow 3D electro-reactor system, by the 30th minute, had reached 8.3%, which approximately equaled the highest CE value of the recycling-flow 2D electro-reactor system (7.6%).

Figure 5.
Energy consumption (Es) and current efficiency (CE) of the recycling-flow 3D and 2D electro-reactor systems (RhB wastewater initial concentration is 100 mg/L; volume is 500 mL; initial pH is 7, Na2SO4 as an electrolyte, initial concentration is 2 g/L; voltage is 5 V; HRT is 20 min; current density is 60 mA/cm 2 ). Overall, the degradation of organic matter (RhB) by the 3D electro-reactor depends on more ·OH being adsorbed on the surface of CGAC electrodes, forming more electrochemically active sites for RhB oxidation. However, the 2D electro-reactor can just undergo less ·OH adsorption and the degradation strength is also reduced due to the absence of CGAC as conductive particles.
The CE and Es of the recycling-flow 3D and 2D electro-reactor systems are shown in Figure 5. Overall, the CE of the recycling-flow 3D and 2D electro-reactor systems both went up from 8.3% and 2.5% to 14.4% and 7.6%, respectively, and the Es of the recycling-flow 3D and 2D electro-reactor systems both decreased from 202.6 KW·h/kg TOC and 658.1 KW·h/kg TOC to 116.3 KW·h/kg TOC and 219.9 KW·h/kg TOC, respectively, during the electrochemical processes. The CE of the recycling-flow 3D electro-reactor system was always nearly twice as much as the corresponding value of the recycling-flow 2D electro-reactor system in the treatment period. Additionally, the lowest CE value of the recycling-flow 3D electro-reactor system, by the 30th minute, had reached 8.3%, which approximately equaled the highest CE value of the recycling-flow 2D electro-reactor system (7.6%).
From the perspective of Es, by the 30th minute, the Es of the recycling-flow 3D electro-reactor system was 202.6 KW·h/kg TOC, which was less than one third the corresponding value of the recycling-flow 2D electro-reactor system (658.1 KW·h/kg TOC). Although the Es of the recycling-flow 2D electro-reactor system reduced slightly, the minimum Es was still as high as 219.9 KW·h/kg TOC, which was almost double the corresponding value of the recycling-flow 3D electro-reactor system (116.3 KW·h/kg TOC).
This verified that the existence of CGAC electrodes in the recycling-flow 3D electro-reactor system increased the mass transfer rate and EASA, rapidly generated much more ·OH on the surface of CGAC electrodes, and improved CE and reduced Es. systems both decreased from 202.6 KW·h/kg TOC and 658.1 KW·h/kg TOC to 116.3 KW·h/kg TOC and 219.9 KW·h/kg TOC, respectively, during the electrochemical processes. The CE of the recyclingflow 3D electro-reactor system was always nearly twice as much as the corresponding value of the recycling-flow 2D electro-reactor system in the treatment period. Additionally, the lowest CE value of the recycling-flow 3D electro-reactor system, by the 30th minute, had reached 8.3%, which approximately equaled the highest CE value of the recycling-flow 2D electro-reactor system (7.6%). Figure 5. Energy consumption (Es) and current efficiency (CE) of the recycling-flow 3D and 2D electro-reactor systems (RhB wastewater initial concentration is 100 mg/L; volume is 500 mL; initial pH is 7, Na2SO4 as an electrolyte, initial concentration is 2 g/L; voltage is 5 V; HRT is 20 min; current density is 60 mA/cm 2 ). electro-reactor systems (RhB wastewater initial concentration is 100 mg/L; volume is 500 mL; initial pH is 7, Na 2 SO 4 as an electrolyte, initial concentration is 2 g/L; voltage is 5 V; HRT is 20 min; current density is 60 mA/cm 2 ).

Mechanism of TOC and COD Degradation with Different Electrolytes in the Recycling-Flow Electro-Reactor System
An electrolyte is frequently added to wastewater to enhance the conductivity of the solution and reduce impedance in an electrochemical reaction. It is actually necessary to research the effect of the electrolyte on the degradation of RhB wastewater, as shown in Figure 6. From the perspective of Es, by the 30th minute, the Es of the recycling-flow 3D electro-reactor system was 202.6 KW·h/kg TOC, which was less than one third the corresponding value of the recycling-flow 2D electro-reactor system (658.1 KW·h/kg TOC). Although the Es of the recycling-flow 2D electro-reactor system reduced slightly, the minimum Es was still as high as 219.9 KW·h/kg TOC, which was almost double the corresponding value of the recycling-flow 3D electro-reactor system (116.3 KW·h/kg TOC).
This verified that the existence of CGAC electrodes in the recycling-flow 3D electro-reactor system increased the mass transfer rate and EASA, rapidly generated much more ·OH on the surface of CGAC electrodes, and improved CE and reduced Es.

Mechanism of TOC and COD Degradation with Different Electrolytes in the Recycling-Flow Electro-Reactor System
An electrolyte is frequently added to wastewater to enhance the conductivity of the solution and reduce impedance in an electrochemical reaction. It is actually necessary to research the effect of the electrolyte on the degradation of RhB wastewater, as shown in Figure 6. . TOC and COD removal of RhB wastewater in the recycling-flow 3D electro-reactor system (NaCl and Na2SO4 as electrolytes, initial concentration is 2 g/L; RhB wastewater initial concentration is 100 mg/L; volume is 500 mL; initial pH is 7; voltage is 7 V; HRT is 20 min).
From the beginning to the 75th min as Part 1, the TOC removal of RhB wastewater (NaCl) was higher than that of RhB wastewater (Na2SO4). The reason was that Cl − in the RhB wastewater (NaCl) dramatically generated lots of active chlorine (Cl), which reacted with ·OH in a synergistic process to increase TOC removal through electrochemical reaction processes. However, from the 75th min to the 180th min as Part 2, the TOC removal of RhB wastewater (Na2SO4) started to go beyond that of RhB wastewater (NaCl). The former and the latter peak rates reached 72.8% and 67.3%, respectively. As some active chlorine (Cl) was converted into chlorine (Cl2) in the RhB wastewater (NaCl), Cl2 partially escaped into the air. The electrons lost during the formation of Cl2 could no longer be used due to the balance of electron gain and loss. Therefore, RhB wastewater (NaCl) had a final TOC removal lower than that of RhB wastewater (Na2SO4) [37].
Additionally, the COD removal of RhB wastewater (NaCl) increased gradually and then reached 89.3%. Meanwhile, the COD removal of RhB wastewater (Na2SO4) first increased and then dropped to 43.9%. The highest value could just reach 77.6%. RhB decomposed into small molecules which could not be oxidized by potassium dichromate (K2Cr2O7) in the solution (Na2SO4) from the 60th min, Part 2 Figure 6. TOC and COD removal of RhB wastewater in the recycling-flow 3D electro-reactor system (NaCl and Na 2 SO 4 as electrolytes, initial concentration is 2 g/L; RhB wastewater initial concentration is 100 mg/L; volume is 500 mL; initial pH is 7; voltage is 7 V; HRT is 20 min).
From the beginning to the 75th min as Part 1, the TOC removal of RhB wastewater (NaCl) was higher than that of RhB wastewater (Na 2 SO 4 ). The reason was that Cl − in the RhB wastewater (NaCl) dramatically generated lots of active chlorine (Cl), which reacted with ·OH in a synergistic process to increase TOC removal through electrochemical reaction processes. However, from the 75th min to the 180th min as Part 2, the TOC removal of RhB wastewater (Na 2 SO 4 ) started to go beyond that of RhB wastewater (NaCl). The former and the latter peak rates reached 72.8% and 67.3%, respectively. As some active chlorine (Cl) was converted into chlorine (Cl 2 ) in the RhB wastewater (NaCl), Cl 2 partially escaped into the air. The electrons lost during the formation of Cl 2 could no longer be used due to the balance of electron gain and loss. Therefore, RhB wastewater (NaCl) had a final TOC removal lower than that of RhB wastewater (Na 2 SO 4 ) [37].
Additionally, the COD removal of RhB wastewater (NaCl) increased gradually and then reached 89.3%. Meanwhile, the COD removal of RhB wastewater (Na 2 SO 4 ) first increased and then dropped to 43.9%. The highest value could just reach 77.6%. RhB decomposed into small molecules which could not be oxidized by potassium dichromate (K 2 Cr 2 O 7 ) in the solution (Na 2 SO 4 ) from the 60th min, whilst a side reaction occurred in the solution (NaCl) to generate ClO − which had strong oxidizing properties, and enhanced the ability to degrade COD [38].
Obviously, the CE of the recycling-flow 3D electro-reactor system with NaCl was higher than the corresponding value of the recycling-flow 3D electro-reactor system with Na 2 SO 4 before the 75th minute, but the CE of the recycling-flow 3D electro-reactor system with Na 2 SO 4 was higher than the corresponding value of the recycling-flow 3D electro-reactor system with NaCl up to the 180th min, as illustrated in Figure 7. Overall, the CE of the recycling-flow 3D electro-reactor systems with NaCl and Na 2 SO 4 both went up smoothly and then got to the highest value (12.2% and 11.3%, respectively). Obviously, the CE of the recycling-flow 3D electro-reactor system with NaCl was higher than the corresponding value of the recycling-flow 3D electro-reactor system with Na2SO4 before the 75th minute, but the CE of the recycling-flow 3D electro-reactor system with Na2SO4 was higher than the corresponding value of the recycling-flow 3D electro-reactor system with NaCl up to the 180th min, as illustrated in Figure 7. Overall, the CE of the recycling-flow 3D electro-reactor systems with NaCl and Na2SO4 both went up smoothly and then got to the highest value (12.2% and 11.3%, respectively).

Figure 7.
Es and CE of the recycling-flow 3D electro-reactor system with NaCl and Na2SO4 as electrolytes (NaCl and Na2SO4 initial concentration is 2 g/L; RhB wastewater initial concentration is 100 mg/L; volume is 500 mL; initial pH is 7; voltage is 7 V; HRT is 20 min; current density is 60 mA/cm 2 ).
Meanwhile, the Es of the recycling-flow 3D electro-reactor system with Na2SO4 was higher than the corresponding value of the recycling-flow 3D electro-reactor system with NaCl up to the 80th min. In particular, the Es of the recycling-flow 3D electro-reactor system with Na2SO4 was 483.5 KW·h/kg TOC, which was 68.3% higher than the corresponding value of the recycling-flow 3D electro-reactor system with NaCl (287.2 KW·h/kg TOC) by the 30th min. However, the Es of the recycling-flow 3D electro-reactor system with Na2SO4 started to be lower than the corresponding value of the recycling-flow 3D electro-reactor system with NaCl from the 80th min. In addition, the Es of the recycling-flow 3D electro-reactor systems with NaCl and Na2SO4 both declined to the lowest value (192.7 KW·h/kg TOC and 206.7 KW·h/kg TOC, respectively) in this period.
These again indicate that NaCl, as an electrolyte, had a high conductive efficiency in the former period due to active chlorine (Cl) generation and then changed to low conductive efficiency due to Cl2 escaping into the air in the latter period from CE and Es [22].

Mechanism of TOC and COD Degradation in Different Voltages in the Recycling-Flow Electro-Reactor System
As seen in Figure 8, the COD removal curve shows a smooth increase and then reaches 30.1% with the voltage of 3 V. In addition, the COD removal of RhB wastewater rose dramatically with the voltages of 5 V and 7 V up to the 60th min and then kept nearly flat between the 60th min and 120th min. Finally, COD removal with 5 V carried on increasing to 86.9%, whilst the corresponding value with 7 V started decreasing to 43.9%. This could be explained by the equation below: where COD0 is the initial COD of RhB wastewater and CODt is the COD of RhB wastewater after . Es and CE of the recycling-flow 3D electro-reactor system with NaCl and Na 2 SO 4 as electrolytes (NaCl and Na 2 SO 4 initial concentration is 2 g/L; RhB wastewater initial concentration is 100 mg/L; volume is 500 mL; initial pH is 7; voltage is 7 V; HRT is 20 min; current density is 60 mA/cm 2 ).
Meanwhile, the Es of the recycling-flow 3D electro-reactor system with Na 2 SO 4 was higher than the corresponding value of the recycling-flow 3D electro-reactor system with NaCl up to the 80th min. In particular, the Es of the recycling-flow 3D electro-reactor system with Na 2 SO 4 was 483.5 KW·h/kg TOC, which was 68.3% higher than the corresponding value of the recycling-flow 3D electro-reactor system with NaCl (287.2 KW·h/kg TOC) by the 30th min. However, the Es of the recycling-flow 3D electro-reactor system with Na 2 SO 4 started to be lower than the corresponding value of the recycling-flow 3D electro-reactor system with NaCl from the 80th min. In addition, the Es of the recycling-flow 3D electro-reactor systems with NaCl and Na 2 SO 4 both declined to the lowest value (192.7 KW·h/kg TOC and 206.7 KW·h/kg TOC, respectively) in this period.
These again indicate that NaCl, as an electrolyte, had a high conductive efficiency in the former period due to active chlorine (Cl) generation and then changed to low conductive efficiency due to Cl 2 escaping into the air in the latter period from CE and Es [22].

Mechanism of TOC and COD Degradation in Different Voltages in the Recycling-Flow Electro-Reactor System
As seen in Figure 8, the COD removal curve shows a smooth increase and then reaches 30.1% with the voltage of 3 V. In addition, the COD removal of RhB wastewater rose dramatically with the voltages of 5 V and 7 V up to the 60th min and then kept nearly flat between the 60th min and 120th Water 2020, 12, 1853 9 of 15 min. Finally, COD removal with 5 V carried on increasing to 86.9%, whilst the corresponding value with 7 V started decreasing to 43.9%. This could be explained by the equation below: where COD 0 is the initial COD of RhB wastewater and COD t is the COD of RhB wastewater after treating t minutes.
Water 2020, 12, x FOR PEER REVIEW 9 of 15 Figure 8. TOC and COD removal of RhB wastewater with different voltages in the recycling-flow 3D electro-reactor system (RhB wastewater initial concentration is 100 mg/L; volume is 500 mL; initial pH is 7; Na2SO4 as an electrolyte, initial concentration is 2 g/L; HRT is 20 min).
The COD0 value was smaller than the actual value due to some macromolecular substances being unable to be oxidized by K2Cr2O7 in the initial RhB wastewater, and then more macromolecules could be oxidized into smaller molecules with the voltage of 7 V set in this experiment, compared with the voltage of 5 V, which were easily oxidized by K2Cr2O7, resulting in a CODt value and COD removal that are greater simultaneously [39].
From the perspective of TOC, as shown in Figure 8, the TOC removal of RhB wastewater grew the most slowly and the final TOC removal just arrived at 61.0% when the voltage was 3 V. TOC removal was always higher with the voltages of 7 V and 5 V, and the highest values were basically equivalent (72.2% and 72.0%, respectively), which were nearly 1.18 times the corresponding value with the voltage of 3V.
Interestingly, from the 0th min to the 70th min, TOC removal with the voltage of 5 V was higher than that with the voltage of 7 V. TOC degradation processes are illustrated in Figure 9. First of all, TOC was converted into TIC and then TIC was converted into CO2 and H2O. The concentration of ·OH in the RhB wastewater was higher, so that oxidation was stronger between the board electrodes with 7 V. TIC was quickly oxidized into CO2 and H2O and TOC was converted into TIC as main processes. With the voltage of 5 V, the oxidation was weaker [40][41][42]. Electron transfer processes were normally used to convert TOC into TIC whilst only some of TIC was converted into CO2 and H2O. Hence, TOC removal was higher with 5 V. After the 70th min, TOC removal with the voltage of 7 V was beyond the corresponding value with the voltage of 5 V. TIC from TOC was basically converted into CO2 and H2O with the voltage of 7 V, while TIC just started being rapidly converted into CO2 and H2O as main processes with the voltage of 5 V. Hence, TOC removal was higher with 7 V. The final TOC removal with 7 V was slightly higher than the corresponding value with 5 V since the potential of 7 V was higher than 5 V up to the 180th min [43,44]. . TOC and COD removal of RhB wastewater with different voltages in the recycling-flow 3D electro-reactor system (RhB wastewater initial concentration is 100 mg/L; volume is 500 mL; initial pH is 7; Na 2 SO 4 as an electrolyte, initial concentration is 2 g/L; HRT is 20 min).
The COD 0 value was smaller than the actual value due to some macromolecular substances being unable to be oxidized by K 2 Cr 2 O 7 in the initial RhB wastewater, and then more macromolecules could be oxidized into smaller molecules with the voltage of 7 V set in this experiment, compared with the voltage of 5 V, which were easily oxidized by K 2 Cr 2 O 7 , resulting in a COD t value and COD removal that are greater simultaneously [39].
From the perspective of TOC, as shown in Figure 8, the TOC removal of RhB wastewater grew the most slowly and the final TOC removal just arrived at 61.0% when the voltage was 3 V. TOC removal was always higher with the voltages of 7 V and 5 V, and the highest values were basically equivalent (72.2% and 72.0%, respectively), which were nearly 1.18 times the corresponding value with the voltage of 3V.
Interestingly, from the 0th min to the 70th min, TOC removal with the voltage of 5 V was higher than that with the voltage of 7 V. TOC degradation processes are illustrated in Figure 9. First of all, TOC was converted into TIC and then TIC was converted into CO 2 and H 2 O. The concentration of ·OH in the RhB wastewater was higher, so that oxidation was stronger between the board electrodes with 7 V. TIC was quickly oxidized into CO 2 and H 2 O and TOC was converted into TIC as main processes. With the voltage of 5 V, the oxidation was weaker [40][41][42]. Electron transfer processes were normally used to convert TOC into TIC whilst only some of TIC was converted into CO 2 and H 2 O. Hence, TOC removal was higher with 5 V. After the 70th min, TOC removal with the voltage of 7 V was beyond the corresponding value with the voltage of 5 V. TIC from TOC was basically converted into CO 2 and H 2 O with the voltage of 7 V, while TIC just started being rapidly converted into CO 2 and H 2 O as main processes with the voltage of 5 V. Hence, TOC removal was higher with 7 V. The final TOC removal with 7 V was slightly higher than the corresponding value with 5 V since the potential of 7 V was higher than 5 V up to the 180th min [43,44]. Water 2020, 12, x FOR PEER REVIEW 10 of 15 CE with the voltage of 3 V rose the fastest from 8.0% by the 30th minute to 21.8% by the 180th minute, which increased by nearly two times, meanwhile, the CEs with 5 V and 7 V both showed gradually increasing trends to 14.1% and 12.2%, respectively, as indicated in Figure 10. Plus, the Es with different voltages (3 V, 5 V, and 7 V) gradually decreased by 63.1%, 42.6%, and 60.2%, respectively, from the 30th min up to the 180th min. In the whole electrolysis process, the higher the voltage was, the higher the Es was. At the 180th min, Es with the voltage of 3 V was 46.1 KW·h/kg TOC, which was 39.6% of the corresponding value with 5 V (116.3 KW·h/kg TOC) and 23.9% of the corresponding value with 7 V (192.7 KW·h/kg TOC). Figure 10. Es and CE of the recycling-flow 3D electro-reactor system with different voltages (RhB wastewater initial concentration is 100 mg/L; volume is 500 mL; initial pH is 7; Na2SO4 as an electrolyte, initial concentration is 2 g/L; HRT is 20 min; current density is 60 mA/cm 2 ).

Mechanism of TOC and COD Degradation at Different HRTs in the Recycling-Flow Electro-Reactor System
HRT refers to the residence time of wastewater in the reactor, which can be calculated according to the following equation [45]: CE with the voltage of 3 V rose the fastest from 8.0% by the 30th minute to 21.8% by the 180th minute, which increased by nearly two times, meanwhile, the CEs with 5 V and 7 V both showed gradually increasing trends to 14.1% and 12.2%, respectively, as indicated in Figure 10. Plus, the Es with different voltages (3 V, 5 V, and 7 V) gradually decreased by 63.1%, 42.6%, and 60.2%, respectively, from the 30th min up to the 180th min. In the whole electrolysis process, the higher the voltage was, the higher the Es was. At the 180th min, Es with the voltage of 3 V was 46.1 KW·h/kg TOC, which was 39.6% of the corresponding value with 5 V (116.3 KW·h/kg TOC) and 23.9% of the corresponding value with 7 V (192.7 KW·h/kg TOC). CE with the voltage of 3 V rose the fastest from 8.0% by the 30th minute to 21.8% by the 180th minute, which increased by nearly two times, meanwhile, the CEs with 5 V and 7 V both showed gradually increasing trends to 14.1% and 12.2%, respectively, as indicated in Figure 10. Plus, the Es with different voltages (3 V, 5 V, and 7 V) gradually decreased by 63.1%, 42.6%, and 60.2%, respectively, from the 30th min up to the 180th min. In the whole electrolysis process, the higher the voltage was, the higher the Es was. At the 180th min, Es with the voltage of 3 V was 46.1 KW·h/kg TOC, which was 39.6% of the corresponding value with 5 V (116.3 KW·h/kg TOC) and 23.9% of the corresponding value with 7 V (192.7 KW·h/kg TOC). Figure 10. Es and CE of the recycling-flow 3D electro-reactor system with different voltages (RhB wastewater initial concentration is 100 mg/L; volume is 500 mL; initial pH is 7; Na2SO4 as an electrolyte, initial concentration is 2 g/L; HRT is 20 min; current density is 60 mA/cm 2 ).

Mechanism of TOC and COD Degradation at Different HRTs in the Recycling-Flow Electro-Reactor System
HRT refers to the residence time of wastewater in the reactor, which can be calculated according to the following equation [45]:  Figure 10. Es and CE of the recycling-flow 3D electro-reactor system with different voltages (RhB wastewater initial concentration is 100 mg/L; volume is 500 mL; initial pH is 7; Na 2 SO 4 as an electrolyte, initial concentration is 2 g/L; HRT is 20 min; current density is 60 mA/cm 2 ).

Mechanism of TOC and COD Degradation at Different HRTs in the Recycling-Flow Electro-Reactor System
HRT refers to the residence time of wastewater in the reactor, which can be calculated according to the following equation [45]: where V is the reactor volume or pool capacity and Q is the influent flow rate. In this experiment, the flow rate was controlled by operating the pumps in order to study the effect of HRT on the degradation of RhB wastewater.
COD removal curves were almost growing coincidently at different HRTs (20 min, 40 min, and 60 min), as illustrated in Figure 11. Finally, COD removal (86.9%) at HRT = 20 min was slightly higher than the corresponding values at HRT = 40 min (80.1%) and HRT = 60 min (83.4%). This indicated that HRT had little effect on the degradation of COD in the RhB wastewater.
where V is the reactor volume or pool capacity and Q is the influent flow rate. In this experiment, the flow rate was controlled by operating the pumps in order to study the effect of HRT on the degradation of RhB wastewater.
COD removal curves were almost growing coincidently at different HRTs (20 min, 40 min, and 60 min), as illustrated in Figure 11. Finally, COD removal (86.9%,) at HRT = 20 min was slightly higher than the corresponding values at HRT = 40 min (80.1%) and HRT = 60 min (83.4%). This indicated that HRT had little effect on the degradation of COD in the RhB wastewater. Figure 11. TOC and COD removal of RhB wastewater at different HRTs in the recycling-flow 3D electro-reactor system (RhB wastewater initial concentration is 100 mg/L; volume is 500 mL; initial pH is 7; Na2SO4 as an electrolyte, initial concentration is 2 g/L; voltage is 5 V).
TOC removal at different HRTs (20 min, 40 min, and 60 min) gradually increased and then reached the maximum value at the 180th min. The highest TOC removal (85.0%) at HRT = 60 min was 13.8% and 6.8% higher than the corresponding value (71.2% and 78.2%) at HRT = 20 min and HRT = 40 min, respectively. In the comparisons of TOC removal, it can be concluded that HRT = 60 min had a better removal effect.
However, HRT is equal to V/Q and the pool capacity of HRT = 60 min is three times that of HRT = 20 min, which means that the construction cost must be more, as the influent flow rate is constant. In addition, the initial concentration of RhB wastewater was 100 mg/L and the corresponding COD was 215 mg/L. The final COD removal (86.9%) at HRT = 20 min was that of the COD of the effluent, which was 28.2 mg/L after treatment, which reached the Grade A standard (< 50 mg/L) of the Pollutant Discharge Standard for Urban Sewage Treatment Plants [46]. In summary, HRT = 20 min, with less construction cost, is the optimal HRT.
TOC removal at different HRTs (20 min, 40 min, and 60 min) gradually increased and then reached the maximum value at the 180th min. The highest TOC removal (85.0%) at HRT = 60 min was 13.8% and 6.8% higher than the corresponding value (71.2% and 78.2%) at HRT = 20 min and HRT = 40 min, respectively. In the comparisons of TOC removal, it can be concluded that HRT = 60 min had a better removal effect.
However, HRT is equal to V/Q and the pool capacity of HRT = 60 min is three times that of HRT = 20 min, which means that the construction cost must be more, as the influent flow rate is constant. In addition, the initial concentration of RhB wastewater was 100 mg/L and the corresponding COD was 215 mg/L. The final COD removal (86.9%) at HRT = 20 min was that of the COD of the effluent, which was 28.2 mg/L after treatment, which reached the Grade A standard (< 50 mg/L) of the Pollutant Discharge Standard for Urban Sewage Treatment Plants [46]. In summary, HRT = 20 min, with less construction cost, is the optimal HRT.

Conclusions
In conclusion, CGAC, as micro-electrodes between anodic and cathodic electrodes in the recycling-flow 3D electro-reactor system, generated an obviously characteristic peak of anodic catalytic oxidation, increased the mass transfer rate and EASA by 40%, and rapidly produced 1.52 times more ·OH on the surface of CGAC electrodes so that the TOC and COD removal of RhB wastewater were both approximately double the corresponding values in the recycling-flow 2D electro-reactor system after 3 h of treatment in the same experimental conditions, with higher a CE and lower Es.
Treating RhB wastewater in the recycling-flow 3D electro-reactor system, Na2SO4 as an electrolyte was more beneficial for TOC degradation, and got higher CE and lower Es, in long electrolyzing times (more than 75 min and less than 180 min). On the contrary, NaCl as an electrolyte could improve COD removal more, and get higher a CE and lower Es in short electrolyzing times (less than 75 min). Plus, TOC and COD removal were the best in the proper voltage (5 V), not the highest one or lowest one. Normally, the higher the voltage, the lower the CE and the more the Es. The HRT condition had little effect on COD removal but high HRT was good for TOC degradation. Es and CE of the recycling-flow 3D electro-reactor system at different HRTs (RhB wastewater initial concentration is 100 mg/L; volume is 500 mL; initial pH is 7; Na 2 SO 4 as an electrolyte, initial concentration is 2 g/L; voltage is 5 V; current density is 60 mA/cm 2 ).

Conclusions
In conclusion, CGAC, as micro-electrodes between anodic and cathodic electrodes in the recycling-flow 3D electro-reactor system, generated an obviously characteristic peak of anodic catalytic oxidation, increased the mass transfer rate and EASA by 40%, and rapidly produced 1.52 times more ·OH on the surface of CGAC electrodes so that the TOC and COD removal of RhB wastewater were both approximately double the corresponding values in the recycling-flow 2D electro-reactor system after 3 h of treatment in the same experimental conditions, with higher a CE and lower Es.
Treating RhB wastewater in the recycling-flow 3D electro-reactor system, Na 2 SO 4 as an electrolyte was more beneficial for TOC degradation, and got higher CE and lower Es, in long electrolyzing times (more than 75 min and less than 180 min). On the contrary, NaCl as an electrolyte could improve COD removal more, and get higher a CE and lower Es in short electrolyzing times (less than 75 min). Plus, TOC and COD removal were the best in the proper voltage (5 V), not the highest one or lowest one. Normally, the higher the voltage, the lower the CE and the more the Es. The HRT condition had little effect on COD removal but high HRT was good for TOC degradation.