A Novel Method for Ecient Electrochemical Treatment of Actual Dyeing Wastewater With Energy Saving

: Electro-oxidation is a promising technology for wastewater treatment with 13 bio-refractory organic and nitrogen pollutants; however, the high energy-demanding 14 hinders its wide application. In this study, a novel method by regulating the significant 15 parameter during electro-oxidation process timely for actual dyeing wastewater 16 treatment with energy saving was studied. Operating factors (i.e., flow rate, initial pH 17 value, electrode distance, and current density) were investigated for chemical oxygen 18 demand (COD) and ammonia removal, and results indicated that current density was 19 the key factor which obviously influenced the electrochemical performance. Indirect 20 oxidation by active chlorine was then confirmed as the main reaction pathway for 21 pollutants oxidation, and the relationship between the current density and the generation 22 of active chlorine was established, suggesting that a large part of the generated active 23 chlorine was not utilized effectively. Subsequently, a novel method by variation of 24 current density timely based on the reaction mechanism was proposed; results indicated 25 that, with similar pollutant removal efficiency, energy consumption could be reduced 26 from 31.6 kWh/m 3 to 20.5 kWh/m 3 . Additionally, the novel system was further 27 optimized by Box-Behnken design: COD and ammonia removal efficiencies could 28 reach 71.8% and 100% respectively, and energy-demanding could be reduced by 45.6%.


Introduction
are also proposed for treating such wastewater; however, the potential secondary by-44 7 COD and ammonia were measured by the dichromate method and Nessler reagent 116 spectrophotometry, respectively. Active chlorine and chloramines were measured using 117 the DPD standard method (Yao et al., 2021). 118 The current efficiency CE (%) was estimated as: 119 time 0 and t, respectively; I is the applied current; F is the Faraday constant (96 485 122 C/mol); 8 is the oxygen equivalent mass (g/eq); 14 is the atomic mass of N; 3 is the 123 electron transfer number from ammonia to N2; V is the solution volume; t is the reaction 124

time. 125
The energy consumption E (kWh/m 3 ) was calculated as follows: 126 where U is the voltage. 128 Response surface methodology based on Box-Behnken design (BBD) was selected 129 as an experimental design to investigate the effect of significant parameter on pollutant 130 removal and energy consumption. The three current densities were set as explanatory electrochemical performance were investigated individually to determine the optimal 137 conditions. Fig. 1a shows that, with the increase of flow rate, pollutant removal 138 increased gradually at the beginning of the experiment, and then reached a maximum 139 value with a flow rate of 150 mL/min. This finding implies that mass transfer limitation 140 of pollutants existed at flow rate lower than 150 mL/min, and then reaction limitation 141 hindered increasing pollutants degradation with further increase of flow rate (Huang et 142 al., 2016). The effect of initial pH value is illustrated in Fig. 1b Although excellent ammonia removal efficiency was also obtained in acidic condition, 147 initial pH value of 9 was also favorable for ammonia removal; the explanation was 148 connected with the direct electron transfer which had been reported in our previous 149 work ( Yao et al., 2016b). Fig. 1c displays that an apex existed for the COD and 150 ammonia removal efficiencies at electrode distance of 1 cm. Shortening the electrode 151 distance can not only increase the potential between the solution phase and the electrode, 152 but also reduce the mass transfer resistance; however, too small distance may cause 153 electrode breakdown or short circuit, resulting in reduction of electro-oxidation 154 performance (Kahraman and Şimşek, 2020). Additionally, Fig. 1d indicates that there 155 was always an upward trend for COD removal with the increase of current density. The 156 same phenomenon was observed on ammonia oxidation: it was completely removed as 9 current density of 20 mA/cm 2 was applied. These results were consistent with other 158 studies: high current density could accelerate the generation of active radicals and thus 159 promote the pollutant removal (Li et al., 2020). 160 According to the results shown in Fig. 1, an inflection point always existed for the 161 pollutant removal with flow rate, initial pH value, and electrode distance, that is, these 162 parameters could be easily optimized. However, the optimization of current density 163 would be further investigated combining reaction mechanism, current efficiency, 164 energy consumption, etc. 165

Reaction mechanism 166
Base on the results of optimization process, current density was selected as the key 167 factor to illustrate the oxidation mechanism during the electrochemical wastewater 168 was positively correlated with current density. As the current density ranging from 10 196 mA/cm 2 to 25 mA/cm 2 , the production of active chlorine increased exponentially. 197 However, compared with the phenomenon in Fig. 2, the COD/ammonia removal rate 198 contributed by active chlorine increased slowly with respect to the current density. Such 199 results suggested that the produced active chlorine was excessive, and a large part was 200 not utilized effectively in the electro-oxidation process. Thus, it seems that a feasible 201 way to enhance the electrochemical performance and reduce the energy consumption is 202 to conduct the chlorine evolution reaction and improve the utilization ratio of active 203 chlorine. 204

A novel method for wastewater treatment 205
Based on the above investigation, current density was undoubtedly determined as 206 the key factor to achieve the aims of high efficiency and low energy consumption for 207 wastewater treatment. A novel method by variation of current density (VCD) timely 208 was conducted, that is, the current density was controlled and decreased from 20 to 15, 209 and 10 mA/cm 2 gradually for each electrolysis time of 60 min. As shown in Fig. 4a, the 210 removal efficiencies of 73.0% and 100% were achieved in the VCD system for COD 211 and ammonia, respectively, which were higher than the efficiencies obtained by current 212 density of 15 mA/cm 2 (66.3% COD; 97.4% ammonia) and close to 20 mA/cm 2 (75.1% 213 COD; 100% ammonia). Besides, the current efficiency of VCD was compared with the 214 traditional electrochemical system as displayed in Fig. 4b. It indicated that the current 215 efficiencies decreased from 34.6% to 25.6%, and 21.2% with the increase of current 216 density from 10 mA/cm 2 to 20 mA/cm 2 , respectively. Fortunately, 27.7% current 217 efficiency was obtained by VCD. More significantly, the VCD system also had an 218 advantage in energy saving: the energy consumption was calculated as 20.5 kWh/m 3 219 which was approximately equal to the required energy with current density of 15 220 mA/cm 2 (20.2 kWh/m 3 ) and much lower than that of 20 mA/cm 2 (31.6 kWh/m 3 ). 221 The variation of active chlorine generation in electrolysis is shown in Fig. 5a. A 222 linear relationship between the concentration of active chlorine and electrolysis time 223 could be observed during the VCD, which was different with the situation in traditional 224 electrochemical process (Fig. 3). It indicated that the stable growth of active chlorine 225 concentration could ensure the efficient oxidation of pollutants, rather than to oxidize 226 pollutants by generating excessive active chlorine, suggesting that the energy utilization 227 efficiency could be greatly improved. especially for current density of 20 mA/cm 2 and VCD. Fig. 5c shows that the pH value 231 decreased obviously in electrolysis as the current density increased, which was in 232 accordance with the above results of chlorine evolution reaction (Eqs. (4) and (5)

Optimization of the novel method using BBD 238
Box-Behnken design was selected to provide an advisable way to regulate the  Table S1, where the range for current density A is 15−20 mA/cm 2 , for 243 current density B is 12.5−17.5 mA/cm 2 , and for current density C is 10−15 mA/cm 2 . 244 An electrolysis time of 60 min was arranged for each stage. And the current density was 245 set to decrease gradually from A to C. Besides, the above results indicated that ammonia 246 could be efficiently removed in this electrochemical system; thus, COD removal 247 efficiency was selected as the evaluation indicator of pollutant degradation. 248

Subsequently, seventeen runs of individual experiments with different current densities 249
were required to fit the three-factor BBD (Table S2). Moreover, normality of data was 250 estimated by means of normal probability plot (Fig. S1)  consumption. The first and second row in the figure refer to the removal efficiency and energy consumption, respectively. The change of color 469 from blue to red represents an increase of removal efficiency/energy consumption. 470 Table 1. The determination and verification of BBD for maximizing removal efficiency and minimizing energy-demanding. 471