Study of The Performance of A Cylindrical Flow-Through Electro-Fenton Reactor Using Different Arrangements of Carbon Felt Electrodes: Degradation of Amoxicillin In Aqueous Solution

In this work, a cylindrical flow-through electro-Fenton reactor integrated by graphite felt 21 electrodes and Fe(II) loaded resin was evaluated for the production of the Fenton 22 reaction mixture and for the degradation of amoxicillin (AMX) containing aqueous 23 solutions. First, the influence of several factors such as treatment time, current intensity, 24 flow rate and electrode position were investigated for the electrogeneration of H 2 O 2 and 25 the energetic consumption by means of a factorial design methodology using a 2 4 26 factorial matrix. Electric current and treatment time were found to be the pivotal 27 parameters influencing the H 2 O 2 production with respective contributions of 40.2% and 28 26.9%. The flow rate had low influence on the responses, however, 500 mL min -1 (with 29 an average residence time of 1.09 min obtained in the residence time distribution 30 analysis) allowed to obtain a better performance due to the high mass transport to and 31 from the electrodes. As expected, polarization was also found to play an important role, 32 since for cathode-to-anode flow direction, lower H 2 O 2 concentrations were determined 33 when compared with anode-to-cathode flow arrangement, indicating that part of the H 2 O 2 34 produced in the cathode could be destroyed at the anode. A fluorescence study of 35 hydroxyl radical production on the other hand, showed that higher yields were obtained 36 using an anode-to-cathode flow direction (up to 3.88 µM), when compared with 37 experiments carried out using a cathode-to-anode flow direction (3.11 µM). The removal 38 of a commercial formulation of the antibiotic amoxicillin (AMX) was evaluated in terms of 39 total organic carbon, achieving up to 57.9 % and 38.63% of the pollutant mineralization 40 using synthetic and real sanitary wastewater spiked, respectively. Finally, the efficiency 41 of the process on the inactivation of fecal coliforms in sanitary wastewater samples was 42 assessed, reducing 90% of the bacterium after 5 min of electrolysis.


Abstract 20
In this work, a cylindrical flow-through electro-Fenton reactor integrated by graphite felt 21 electrodes and Fe(II) loaded resin was evaluated for the production of the Fenton 22 reaction mixture and for the degradation of amoxicillin (AMX) containing aqueous 23 solutions. First, the influence of several factors such as treatment time, current intensity, 24 flow rate and electrode position were investigated for the electrogeneration of H2O2 and 25 the energetic consumption by means of a factorial design methodology using a 2 4 26 factorial matrix. Electric current and treatment time were found to be the pivotal 27 parameters influencing the H2O2 production with respective contributions of 40.2% and 28 26.9%. The flow rate had low influence on the responses, however, 500 mL min -1 (with 29 an average residence time of 1.09 min obtained in the residence time distribution 30 analysis) allowed to obtain a better performance due to the high mass transport to and 31 from the electrodes. As expected, polarization was also found to play an important role, 32 since for cathode-to-anode flow direction, lower H2O2 concentrations were determined 33 when compared with anode-to-cathode flow arrangement, indicating that part of the H2O2 34 produced in the cathode could be destroyed at the anode. A fluorescence study of 35 hydroxyl radical production on the other hand, showed that higher yields were obtained 36 using an anode-to-cathode flow direction (up to 3.88 µM), when compared with 37 experiments carried out using a cathode-to-anode flow direction (3.11 µM). The removal 38 of a commercial formulation of the antibiotic amoxicillin (AMX) was evaluated in terms of 39 total organic carbon, achieving up to 57.9 % and 38.63% of the pollutant mineralization 40 using synthetic and real sanitary wastewater spiked, respectively. Finally, the efficiency 41

Introduction 44
Electrochemical advanced oxidation processes (EAOPs) constitute an attractive 45 approach to treat wastewater effluents contaminated with persistent pollutants. These parallel to the anode and cathode surfaces (Zhou et al. 2017). In these systems, 63 convection becomes negligible near the electrode-solution interface and therefore, in 64 order to reduce the mass transport limitations of the conventional flow-by reactors, it is 65 important to incorporate fluid mixer promotors such as mechanical agitators or pumps. 66 In this way, flow-through electro-Fenton reactors stand out as an attractive option to 67 overcome these weaknesses since the solution flows through porous anode and cathode 68 electrodes, increasing mass transport events that result in improved electrochemical 69 conversion, current efficiency and reduced energy consumption (Zhou et al. 2017

Electro-Fenton Reactor 156
The treatment was carried out using a three-compartment cylindrical reactor (horizontally 157 oriented) made of Nylamid. The central compartment with a 141 cm 3 volume, and two 158 identical sections were coupled on the up-and low-ends of the middle section using 159 stainless steel screws (Fig. 1a). The experimental set-up consisted of the reactor 160 equipped with a power source, a peristaltic pump, a recirculation tank and an oxygen 161 concentrator (Fig. 1b). Carbon cloth circular pieces (28 cm 2 effective area, 0.6 mm 162 thickness, 0.5 Ω in 2 electrical resistivity) served as electrical contacts for cylindrical 163 carbon felt electrodes (6 cm diameter, 2.35 cm length), positioned between the 164 compartments ( Fig. 1c and 1d). For the electro-Fenton assays, while in the compartment 165 next to the cathode a cation exchange resin containing Fe(II) was placed, the same 166 amount of a Na + -activated cation exchange resin was located in the compartment next 167 to the anode (Fig. 1e). As can be seen in Fig. 1a, the reactor was fed in such a way that

C1
C2 C3 (2) (1) (3) distribution function (E(t)) and the average residence time (µ) after the pulse tracer input 192 was obtained using Eq. (4) and (5) In these equations, C(t) corresponds to the tracer concentration evolution at the reactor Here, Yj is the response variable, Xi is the independent variable and βo, and βi,  Table 2. Once the conditions that maximize the H2O2 electrogeneration 210 were determined, the percentage of current efficiency (ϕ) for the production of H2O2 was 211 computed using Eq. (7), where n is the number of transferred electrons (2) to produce 212 H2O2 via oxygen reduction, F is the Faraday constant (96 485 C mol -1 ), CH2O2 is the H2O2 213 concentration (in g L -1 ), V is the volume of the solution (in L), MH2O2 is the molecular 214 weight of H2O2 (34.01 g mol -1 ), I is the applied current (in A) and t is the treatment time 215 The removal of AMX on the other hand, was determined from the TOC assessment. 243 While spectrophotometric tests were carried out using a DR600 HACH apparatus and

290
The coefficient R 2 , is defined as the ratio of the explained to the total variation and is a 291 measure of the degree of fitting. For a good fit, R 2 should be at least 0.80 (Fu et al. 2007). 292 The obtained R 2 (of 0.9871 for H2O2 generation and 0.9891 for energetic consumption) 293 can be seen to be high for both responses, reflecting that the model describes 294 reasonably well the process performance. This was also confirmed by the adjusted 295 determination coefficients (adj-R 2 ) characterized by the values of 0.9614 and 0.9674 for 296 H2O2 production and energetic consumption, respectively. Analysis of variance (ANOVA) 297 of the results was also performed to evaluate the significancy of the models (Table 1- and energetic consumption indicate that as expected, the highest yields of H2O2 300 production and energetic consumption are reached when applying the maximum current 301 and treatment time ( Fig. 1-S). 302 where Pi is the percentage contribution of each independent factor i, and where bi 318 represents the estimation of the main effect of the factor i.  but also that the cathode-to-anode configuration allows to obtain the higher yields. Fig.  411 6b on the other hand, shows the mineralization efficiency of the cathode-to-anode 412 configured electro-Fenton process for real sanitary wastewater, which was previously 413 contaminated with AMX as described in the experimental section. Using this complex 414 effluent matrix, a 38.6% of mineralization was achieved after 30 min of reaction. The 415 lower effectiveness of the process using this solution is probably related to the presence 416 of soluble organic matter that not only inhibits the electro-generation of H2O2, but also 417 promotes competitive reactions. As it can be noted from the experimental data, the quasi-418 steady state was reached after 3 mean hydraulic residence times. 419 It is interesting to compare the AMX mineralization data obtained in this study with 420 reports using electro-Fenton process. In this way, Kadji  •: electro-Fenton process, anode-to-cathode flow arrangement with oxygen saturation, 434 blue ▲ electrochemical oxidation process, cathode-to-anode flow set-up with oxygen 435 saturation, green ▼: adsorption (without current). Using real sanitary wastewater: empty 436 black ■ electro-Fenton process, cathode-to-anode flow arrangement with oxygen 437 saturation. 300 mA, pH 7, 500 mL min -1 recirculation flow rate. 438

439
The concentration of the NO3was also determined as evidence of the degradation of 440 the antibiotic by means of the electro-Fenton process arranged in the cathode-to-anode 441 configuration. Fig. 7a shows the NO3concentration on time, and it can be noted that the 442 ion concentration increases with increasing treatment time up to a concentration of 3.6 443 mg L -1 . The presence of NO3is explained by the mineralization reaction of AMX (Eq. 444 12), which produces NH4 + that in turn, when interacting with • OH radicals, generates NO2 -445 (Eq. 13) and eventually NO3 -(Eq. 14). 446  The performance of a cylindrical flow-through electro-Fenton reactor using carbon felt 477 electrodes was evaluated. An experimental design methodology was applied to evaluate 478 the effects of four independent variables and to determine the best experimental 479 conditions. Among the studied factors current and treatment time were the most 480 meaningful parameters. The best operational parameters were found to be: current of 481 300 mA during 30 min, 500 mL min -1 of recirculation flow rate, with the electrodes placed 482 in the middle of the reactor and at cathode-to-anode flow direction achieving a H2O2 483 concentration of 16.1 mg L -1 and energy consumption of 3.21 kWh m -3 . Under these 484 conditions, the system allows to obtain a • OH concentration of 3.11 µM, a AMX 485 mineralization efficiency of 57.9% and a fecal coliforms inactivation of 90%. The 486 proposed reactor appears to be a promising modular technology that can be used as 487 tertiary treatment to remove contaminants of emerging concern and to the disinfection of 488