Jing Ma

Clément Trellu
email: clement.trellu@univ-eiffel.fr

Nihal Oturan

Stéphane Raffy

Mehmet A Oturan
email: mehmet.oturan@univ-eiffel.fr

Porous Magnéli phase obtained from 3D printing for efficient anodic oxidation process

Keywords: 3D printing, electro-oxidation, sub-stoichiometric titanium oxide, hydroxyl radical, mass transport, paracetamol

3D printing was used for the first time for synthesis of a 3D TiOx electrode applied to the removal of organic compounds from water by electrooxidation process. After characterization of the synthetized material, effectiveness and reaction mechanisms of the electrode for water treatment was assessed under galvanostatic mode through (i) using probe molecules (terephthalic acid and oxalic acid) and paracetamol (PCT) as model compound (ii) monitoring the fate of degradation by-products and mineralization yields, and (iii) quenching experiments using ethanol to clarify oxidation mechanism.

Results emphasized that the suitable porous structure of the 3D TiOx promoted both direct electron transfer and hydroxyl radical-mediated oxidation at the electrode surface.

Compared to Ti/TiOx and BDD (boron-doped diamond) plates, 3D TiOx anode achieved faster degradation of PCT (3.3-and 1.7-times enhancement, respectively), lower accumulation of degradation by-products and higher mineralization yield (75% vs 15% and 66%, for Ti/TiOx and BDD anodes, respectively, after 2-h treatment, under similar operating conditions). This study highlights the potential of 3D printing for designing electrode materials with suitable porous structure able to favor mass transport conditions and consequently enhance the efficiency of electrochemical advanced oxidation processes.

Introduction

Anodic oxidation (AO) (or electrooxidation) is a widely applied electrochemical advanced oxidation process (EAOP) based on in-situ generation of reactive species, mostly hydroxyl radicals (  OH) [START_REF] Martínez-Huitle | Single and Coupled Electrochemical Processes and Reactors for the Abatement of Organic Water Pollutants: A Critical Review[END_REF][START_REF] You | Monolithic Porous Magnéli-phase Ti4O7 for Electro-oxidation Treatment of Industrial Wastewater[END_REF][START_REF] Oturan | Outstanding performances of the BDD film anode in electro-Fenton process: Applications and comparative performance[END_REF][START_REF] Fan | Sun, 3D Printing of Porous Nitrogen-Doped Ti3C2 MXene Scaffolds for High-Performance Sodium-Ion Hybrid Capacitors[END_REF][START_REF] Nienhauser | Boron-doped diamond electrodes degrade short-and long-chain per-and polyfluorinated alkyl substances in real industrial wastewaters[END_REF][START_REF] Garcia-Segura | Applied photoelectrocatalysis on the degradation of organic pollutants in wastewaters[END_REF][START_REF] Martínez-Huitle | A critical review over the electrochemical disinfection of bacteria in synthetic and real wastewaters using a boron-doped diamond anode[END_REF][START_REF] Nidheesh | An overview on the removal of synthetic dyes from water by electrochemical advanced oxidation processes[END_REF][START_REF] Du | Nanostructured electrodes for electrocatalytic advanced oxidation processes: From materials preparation to mechanisms understanding and wastewater treatment applications[END_REF]. Formation of a large amount of  OH, in cooperation with direct electron transfer (DET) and mediated oxidation (thanks to the reactive species generated from the inorganic matrix), can accomplish high efficiency in terms of degradation and mineralization rate of organic pollutants in water [START_REF] Martínez-Huitle | Single and Coupled Electrochemical Processes and Reactors for the Abatement of Organic Water Pollutants: A Critical Review[END_REF][START_REF] Zaky | Mechanism of p -Substituted Phenol Oxidation at a Ti4O7 Reactive Electrochemical Membrane[END_REF][START_REF] Trellu | Electrochemical technologies for the treatment of pesticides[END_REF][START_REF] Brito | Removal of antibiotic rifampicin from aqueous media by advanced electrochemical oxidation: Role of electrode materials, electrolytes and real water matrices[END_REF][START_REF] Trellu | Comparative study on the removal of humic acids from drinking water by anodic oxidation and electro-Fenton processes: Mineralization efficiency and modelling[END_REF][START_REF] Mousset | Advanced electrocatalytic pre-treatment to improve the biodegradability of real wastewater from the electronics industry -A detailed investigation study[END_REF][START_REF] Hao | Electrochemical treatment of wastewater contaminated by organics and containing chlorides: Effect of operative parameters on the abatement of organics and the generation of chlorinated by-products[END_REF][START_REF] Souza | Electrochemically-based hybrid oxidative technologies for the treatment of micropollutants in drinking water[END_REF][START_REF] Brillas | Electro-fenton process and related electrochemical technologies based on fenton's reaction chemistry[END_REF][START_REF] Panizza | Anodic oxidation of 2-naphthol at boron-doped diamond electrodes[END_REF][START_REF] Martínez-Huitle | Conductive-synthetic diamond materials in meeting the sustainable development goals[END_REF].

The nature of the anode material is a crucial parameter for the AO process. Magnéli phases (TinO2n-1, 3<n<10) have been identified as potentially favorable materials owing to their high reactivity for  OH generation, good electrical conductivity and lower production cost compared to standard materials based on boron-doped diamond (BDD) anodes [START_REF] Chaplin | The Prospect of Electrochemical Technologies Advancing Worldwide Water Treatment[END_REF][START_REF] Nayak | Fabrication and characterization of porous, conductive, monolithic Ti4O7 electrodes[END_REF][START_REF] Trellu | Electro-oxidation of organic pollutants by reactive electrochemical membranes[END_REF][START_REF] Trellu | Integration of sub-stoichiometric titanium oxide reactive electrochemical membrane as anode in the electro-Fenton process[END_REF]. The efficiency of AO can also be limited by the transport of molecules from bulk solution to electrode surface [START_REF] Trellu | Electrochemical technologies for the treatment of pesticides[END_REF]. Recent research has been focused on the development of porous TinO2n-1 electrodes in order to improve (i) the electro-active surface area and (ii) mass transport conditions, particularly in flow-through configuration [START_REF] Mousset | Interest of micro-reactors for the implementation of advanced electrocatalytic oxidation with boron-doped diamond anode for wastewater treatment[END_REF][START_REF] Radjenovic | Facing the Challenge of Poly-And Perfluoroalkyl Substances in Water: Is Electrochemical Oxidation the Answer?[END_REF]. Therefore, it is important to be able to control the porous structure of the material [START_REF] Trellu | Electro-oxidation of organic pollutants by reactive electrochemical membranes[END_REF]. The development of fabrication methods to customize electrodes characteristics would be noteworthy for an efficient AO process. Porous monolithic TinO2n-1 electrodes are generally fabricated from (carbo)thermal reduction of TiO2 or anodization-cathodization method (blue TiO2) [START_REF] Nayak | Fabrication and characterization of porous, conductive, monolithic Ti4O7 electrodes[END_REF][START_REF] Trellu | Mineralization of organic pollutants by anodic oxidation using reactive electrochemical membrane synthesized from carbothermal reduction of TiO2[END_REF][START_REF] Zhang | Degradation of 2,4-dichlorophenoxyacetic acid by a novel photoelectrocatalysis/photoelectro-Fenton process using Blue-TiO2 nanotube arrays as the anode[END_REF][START_REF] Xu | Effective degradation of aqueous carbamazepine on a novel blue-colored TiO2 nanotube arrays membrane filter anode[END_REF][START_REF] Cai | Extremely efficient electrochemical degradation of organic pollutants with co-generation of hydroxyl and sulfate radicals on Blue-TiO2 nanotubes anode[END_REF]. However, the morphology (porous structure) of these electrodes is less controllable and can be greatly influenced by parameters concerning TiO2 precursor and reduction process (e.g., temperature, treatment time) [START_REF] Trellu | Electro-oxidation of organic pollutants by reactive electrochemical membranes[END_REF]. Thus, a great challenge for material synthesis is related to the development of methods able to precisely control both chemical composition and porous structure of the material.

Three-dimensional (3D) printing (additive manufacturing) is a technique which fabricates materials by depositing layers upon layers [START_REF] Ngo | Additive manufacturing (3D printing): A review of materials, methods, applications and challenges[END_REF]. 3D printing has been developed since 1986 and applied in several different fields (aerospace, engineering, material science, chemistry, etc.) [START_REF] Ngo | Additive manufacturing (3D printing): A review of materials, methods, applications and challenges[END_REF]. This technique allows shaping diverse precursors into materials with specific morphology in a large range of scales [START_REF] Ngo | Additive manufacturing (3D printing): A review of materials, methods, applications and challenges[END_REF][START_REF] Browne | 3D Printing for Electrochemical Energy Applications[END_REF][START_REF] Tofail | Additive manufacturing: scientific and technological challenges, market uptake and opportunities[END_REF]. Most importantly, 3D printing makes the customization of material morphology more controllable, including shape, thickness and porous structure [START_REF] Fan | Sun, 3D Printing of Porous Nitrogen-Doped Ti3C2 MXene Scaffolds for High-Performance Sodium-Ion Hybrid Capacitors[END_REF][START_REF] Zhang | 3D printing of structured electrodes for rechargeable batteries[END_REF][START_REF] Lu | 3D printing well organized porous iron-nickel/polyaniline nanocages multiscale supercapacitor[END_REF][START_REF] Ruiz-Morales | Three dimensional printing of components and functional devices for energy and environmental applications[END_REF][START_REF] Ambrosi | 3D-printing technologies for electrochemical applications[END_REF][START_REF] Foo | Three-Dimensional Printed Electrode and Its Novel Applications in Electronic Devices[END_REF]. Based on the success acquired in other fields, the AO process may also benefit from 3D printing for fabrication of electrode material with suitable characteristics. The key research question investigated in this study was to assess whether 3D printing might be suitable for optimization of the porous structure of anodes used for the removal of organic compounds from water by anodic oxidation, particularly for improving mass transport conditions.

A porous Magnéli phase electrode was successfully synthesized for the first time using a 3D printing technique. Porous structure of the material was optimized for application in stirred-tank reactor. This electrode was then used as anode in the AO process to remove organic pollutants from water. Experiments were performed in synthetic solutions. First, the chemical composition and morphology of the synthetized electrode were characterized. Then, terephthalic acid (TA) and oxalic acid (OA) were used as probe molecules for assessing the electrochemical reactivity of this electrode. Finally, it was applied for degradation and mineralization of a model pollutant (paracetamol (PCT)).

Different quenchers were used for highlighting the key reaction mechanisms. Aromatic degradation by-products formed during AO process were identified and quantified. The effectiveness of this new electrode was assessed by comparison with a Ti/TiOx plate anode for highlighting the role of the 3D structure. The comparison was also made with a BDD anode since it is considered as gold standard material for such applications.

Materials and methods

Chemicals

Paracetamol (98%), hydroquinone (≥99%), phosphoric acid (85-88%), formic acid (≥96%) and 2-hydroxyterephthalic acid (97%) were obtained from Sigma-Aldrich.

Terephthalic acid (≥99%) was purchased from Acros Organics. Sodium hydroxide (≥98%) and oxalic acid (≥97%) were obtained from Fluka. Ethanol (≥97%) was obtained from Chimie-Plus Laboratoires. All chemicals were used as received.

Ultrapure water (R>18.2 MΩ, obtained from Smart2Pure, Molsheim) was used for eluents preparation of HPLC and UHPLC.

Electrode synthesis

The 3D printed porous electrodes (hereafter referred as 3D TiOx) were synthesized using an ExOne binder jetting 3D printer by Saint-Gobain Research Provence, CREE.

An ExOne Innovent Enhanced Recoater was used for printing the green body from a TiOx powder mixture (40 wt% of Ti5O9 and 60 wt% of Ti6O11) and aqueous binder (BA005, ExOne). The TiOx powder contained fine and coarse particles with a ratio of 20:80. Then the green body was heated up to 375 °C (at an increasing rate of 5 °C min -1 ) and held for 1 h under air to remove the binder. After replacing the air atmosphere with argon, the body was further heated up to 1500 °C (at a rate of 5 °C min -1 ) and held for 4 h. Cooling down was performed at 5 °C min -1 .

Physico-chemical characterization

Porous structure of 3D TiOx was observed by a digital optical microscope (KEYENCE, VHX-7000) and scanning electron microscopy (SEM, TM3030 and TM-1000, HITACHI). Electrode surface roughness was also characterized by optical microscopy (KEYENCE, VHX-7000). X-Ray diffraction (XRD) (Bruker D8 ENDEAVOR, Cu Kα radiation source, λ=1.5418 Å at 40 kV and 40 mA) was used to study the Magnéli phase composition of 3D TiOx. Mercury porosimetry was performed to analyze the distribution of pores smaller than 345 µm. Pores with size larger than 345 µm were counted and presented as pores per inch (ppi). The void volume of 3D TiOx was calculated by subtracting solid volume of the printed skeleton (using the density of 4.33 g cm -3 obtained by Helium pycnometry) from bulk volume of the electrode. The porosity (void fraction) was then obtained from the ratio between void volume and bulk volume of 3D TiOx.

Electrochemical characterization

Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were conducted on a Metrohm Autolab PGSTAT to study oxygen evolution potential (OEP) and electroactive surface area (EASA) of 3D printed TiOx, respectively. An Ag/AgCl (3 M KCl) electrode was used as reference electrode, 100 mM NaClO4 was used as supporting electrolyte and a stainless-steel electrode served as counter electrode.

LSV tests were performed from open circuit potential to 3.0 V vs Ag/AgCl/3 M KCl at a scan rate of 0.01 V s -1 . Raw data were corrected to compensate ohmic drop [START_REF] Krstajic | Cathodic behaviour of RuO2-doped Ni/Co3O4 electrodes in alkaline solutions: Hydrogen evolution[END_REF].

Calculation of EASA was based on the measurement of the capacitance of the material using the method described by Chaplin et al. [START_REF] Nayak | Fabrication and characterization of porous, conductive, monolithic Ti4O7 electrodes[END_REF]. CV tests were performed on 3D TiOx electrode from 0.75 to 1.17 V vs Ag/AgCl (3 M KCl) at scan rates from 0.006 to 0.010 V s -1 . The double-layer capacitance of the 3D TiOx electrode was than calculated from Eq (1):

𝐼 -𝐼 2 = 𝐶 𝑣 1.
where Ia and Ic are anodic and cathodic current plateau obtained from CV scans, Cdl is the double-layer capacitance of the electrode and v is the scan rate (V s -1 ). EASA was then calculated by taking into consideration the specific capacitance of this kind of metal oxide (60 µF cm -2 ) [START_REF] Bockris | The Electrocatalysis of Oxygen Evolution on Perovskites[END_REF].

A commercial BDD electrode (Diachem) obtained from Condias (Germany) and a Ti/TiOx plate electrode synthesized from plasma spraying by Saint-Gobain Research Provence, CREE, were used in this study to compare with 3D TiOx electrode. The synthesis method and characterization details of Ti/TiOx plate are provided in our previous study (reference [START_REF] Ma | Development of Ti/TiOx foams for removal of organic pollutants from water: Influence of porous structure of Ti substrate[END_REF]).

Experimental

Batch experiments were conducted using an undivided cell maintained under continuous agitation. 3D TiOx electrode was used as anode while carbon felt was used as cathode.

Both electrodes had similar shape with a geometric surface area of 23.6 cm 2 . They were set in parallel in a solution of 300 mL. The ratio between the surface of the electrode and the volume of the solution (Selectrode/Vsolution) was 7.9 m -1 . The inter-electrode gap was 2.5 cm. All experiments were conducted in galvanostatic mode under mass transport limitation (j>jlim with jlim calculated from Eq. ( 2)) in order to assess whether the 3D printed TiOx electrode presents a more favorable porous structure for mass transport enhancement. Therefore, the current density was set at 5 mA cm -2 for all experiments. Samples were taken at different time intervals for analysis.

𝑗 = 𝑛𝐹𝑘 𝐶 2.
where jlim is the limiting current density (A m -2 ), F is the Faraday constant, km is the mass transport coefficient (m s -1 ) as calculated in previous studies using the same configuration [START_REF] Ma | Investigating the reactivity of TiOx and BDD anodes for electro-oxidation of organic pollutants by experimental and modeling approaches[END_REF], C is the concentration of target molecule (mol m -3 ) and n is the number of electrons involved in oxidation of the target molecule to CO2.

Paracetamol (PCT) (C0 = 0.1 mM, initial TOC = 9.6 mg L -1 ) was selected as model pollutant to study the performance of 3D TiOx. Terephthalic acid (TA) (C0 = 0.1 mM, TOC0 = 9.6 mg L -1 ) was chosen as a suitable probe molecule for  OH-mediated oxidation as it was reported in previous studies [START_REF] Jing | Mechanistic Study of the Validity of Using Hydroxyl Radical Probes to Characterize Electrochemical Advanced Oxidation Processes[END_REF]. Similarly, oxalic acid (OA) (C0 = 0.4 mM, TOC0 = 9.6 mg L -1 ) was chosen as probe molecule for DET due to its low reaction rate with  OH [START_REF] Shih | Electrochemical degradation of oxalic acid over highly reactive nano-textured Γ-and Α-MnO2/carbon electrode fabricated by KMnO4 reduction on loofah sponge-derived active carbon[END_REF]. In these conditions, the limiting current density was 0.825 mA cm -2 and 0.220 mA cm -2 for oxidation of TA and OA, respectively.

Quenching experiments were also conducted to verify the contribution of  OH-mediated oxidation and DET to PCT degradation. Ethanol was chosen as quencher because of its high reaction rate constant with  OH [START_REF] Wu | Single-Atom Cobalt Incorporated in a 2D Graphene Oxide Membrane for Catalytic Pollutant Degradation[END_REF]. Ethanol was added at a concentration of 0.1 M.

Reproducibility of the 3D printed TiOx electrodes were tested by repeating paracetamol degradation experiments using two 3D TiOx electrodes synthesized from the same method.

The data obtained from degradation experiments were plotted as -ln(C/C0) versus reaction time to study how the data fit with the pseudo-first order kinetic model.

Analytical methods

PCT and its main by-products were analyzed by a Dionex UltiMate 3000 UHPLC system with an UltiMate 3000 RS pump (at flow rate of 0.1 mL min -1 ), an UltiMate 3000 diode array detector (λ = 243 nm) and a Hypersil GOLD™ column (100×2.1 mm, 1.9 µm), which was set in an oven at 40 °C. A mixture of methanol (15%) and phosphate buffer (85%) (5 mM Na2HPO4 and 7.5 mM H3PO4) was used as eluent.

Identification of by-products was performed by comparison with the retention time obtained from standard solutions. Quantification was obtained from external calibration.

Calculation of mineralization current efficiency

The mineralization of PCT refers to its complete conversion into carbon dioxide and the general mineralization reaction can be expressed as follow (eq. ( 1)) [START_REF] Trellu | Mineralization of organic pollutants by anodic oxidation using reactive electrochemical membrane synthesized from carbothermal reduction of TiO2[END_REF][START_REF] Hien | Comparison of homogeneous and heterogeneous electrochemical advanced oxidation processes for treatment of textile industry wastewater[END_REF][START_REF] Garcia-Segura | Mineralization of the recalcitrant oxalic and oxamic acids by electrochemical advanced oxidation processes using a boron-doped diamond anode[END_REF]:

𝐶 𝐻 𝑁𝑂 + 14𝐻 𝑂 → 8𝐶𝑂 + 𝑁𝐻 + 33𝐻 + 34𝑒 3.
In this study, the mineralization yield of PCT was reported as TOC removal and the mineralization current efficiency can be therefore calculated using eq. ( 2):

𝑚𝑖𝑛𝑒𝑟𝑎𝑙𝑖𝑧𝑎𝑡𝑖𝑜𝑛 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 (%) = 𝑛 𝐹 𝑉 (𝑇𝑂𝐶 -𝑇𝑂𝐶 ) 12000 𝑚 𝐼 𝑡 × 100 4.

where n is the number of electrons involved in the mineralization reaction, F is Faraday constant, V is the solution volume, TOC0 is the initial TOC, TOCt is the TOC at time t, 12000 (mg mol -1 ) is a conversion factor, m is the number of carbon atoms, I is the current intensity (A) and t is the reaction time (s).

Results and discussion

3.1 Physicochemical characterization of 3D TiOx electrode The pore size distribution smaller than 345 µm was depicted in Table 1. Pore size of 2-10 µm accounted the majority of pores smaller than 345 µm. The total porosity of the material was calculated as 84.3%. The porous structure of 3D TiOx is at millimeter scale.

As reported in a previous study [START_REF] Ma | Development of Ti/TiOx foams for removal of organic pollutants from water: Influence of porous structure of Ti substrate[END_REF], the coarse roughness of electrode at millimeter scale is able to improve mass transport conditions in stirred-tank reactor. The effective roughness factor was calculated following the methodology described by Ma et al.

(2022). The effective roughness factor takes only into consideration the electrode roughness that is not averaged within the diffusion field. With a diffusion layer of 30 µm, effective roughness factor of 3D TiOx was 5.2; this value was 5 times greater than BDD (1.07) and Ti/TiOx plate (1.002) thanks to its porous structure at millimeter scale.

Electrochemical characterization of 3D TiOx electrode

The EASA of 3D TiOx was obtained from the measurement of the capacitance of the electrode (Fig. 3). The value obtained was 146 cm 2 , thus corresponding to a roughness factor of 60. This roughness factor is much higher than the effective roughness factor previously calculated since it takes into consideration also the electrode roughness at micrometer scale. For comparison, the EASA of Ti/TiOx plate electrode was 2200 cm 2 (with roughness factor of 93). The higher EASA of the Ti/TiOx plate electrode might be ascribed to (i) the use of plasma spraying technique that promotes electrode roughness at micrometer and sub-micrometer scale, (ii) the lack of interconnectivity of some pores in the 3D TiOx electrode. The former reason might be related to the removal of the aqueous binder during the first step of thermal treatment of the 3D TiOx electrode. LSV was performed in order to measure the OEP of the electrodes. Electrodes are usually divided in two classes, including (i) active electrodes with low overpotential for oxygen evolution reaction and (ii) non-active electrodes with high overpotential for oxygen evolution reaction [START_REF] Panizza | Direct and Mediated Anodic Oxidation of Organic Pollutants[END_REF]. Non-active electrodes are usually the most suitable electrodes for the formation of  OH. Results from LSV are presented in Fig. 4.

For optimal comparison, OEPs were all measured at the value of current density of 1.5 mA cm -2 . This value of the current was estimated using the tangent method. The 3D TiOx presented an OEP of 3.06 V vs Ag/AgCl, while the OEPs of Ti/TiOx plate and BDD were 2.78 and 2.04 V vs Ag/AgCl, respectively. The OEP of 3D TiOx was sufficient for  OH generation (2.17 V vs Ag/AgCl/3 M KCl) [START_REF] Kapałka | The importance of electrode material in environmental electrochemistry: Formation and reactivity of free hydroxyl radicals on boron-doped diamond electrodes[END_REF] and comparable to that of other Magnéli phase electrodes reported in the literature [START_REF] Trellu | Electro-oxidation of organic pollutants by reactive electrochemical membranes[END_REF][START_REF] Lin | Development of macroporous Magnéli phase Ti4O7 ceramic materials: As an efficient anode for mineralization of polyand per fluoroalkyl substances[END_REF]. The 3D porous structure of the material might also influence the behavior of the OER, as reported in previous studies [START_REF] Asim | Nanostructured 3D-porous graphene hydrogel based Ti/Sb-SnO2-Gr electrode with enhanced electrocatalytic activity[END_REF][START_REF] Zhou | Electrochemical oxidation of pyrrole, pyrazole and tetrazole using a TiO2 nanotubes based SnO2-Sb/3D highly ordered macro-porous PbO2 electrode[END_REF]. However, it is not possible to give clear conclusions on the influence of this parameter for our study. short and results in the accumulation of  OH only in a thin layer at the anode surface (in the range 0.1 -1 µm) [START_REF] Kapałka | The importance of electrode material in environmental electrochemistry: Formation and reactivity of free hydroxyl radicals on boron-doped diamond electrodes[END_REF]. The electrochemical behavior of BDD might promote the formation of  OH (and/or hinder further evolution of  OH in wasting reactions) that would increase the thickness of this reaction zone. However, 3D TiOx electrode achieved a degradation kinetic rate of 0.020 ± 0.001 min -1 , which was 2.5 times higher than that of Ti/TiOx plate and 1.3 times higher than that of BDD plate (0.016 ± 0.001 min -1 ). After 90 min of electrolysis, 3D TiOx reached a TA degradation rate of 88%, which was higher than both BDD (77%) and Ti/TiOx plate (56%) electrodes. These results emphasized that the suitable porous structure of 3D TiOx is able to strongly improve the reactivity of the electrode for  OH-mediated oxidation of organics thanks to mass transport enhancement. Such optimization of the morphology of the electrode even cm 2 , Selectrode/Vsolution = 7.9 m -1 , current density = 5 mA cm -2 .

Degradation of paracetamol in the absence and presence of quenchers

Electrodes under study were then applied for the removal of PCT as model pollutant.

Fig. 6(a) shows PCT degradation kinetic obtained with 3D TiOx, BDD plate and Ti/TiOx plate. 3D TiOx achieved 97% of PCT degradation after 90 min of treatment, which is higher than that obtained with BDD (83%) and Ti/TiOx plate (60%). The degradation kinetic obtained with 3D TiOx was respectively 1.7 and 3.3 times higher than with the BDD plate and Ti/TiOx plate electrodes. This is consistent with the previous subsection ( § 3.3), which provides evidences that the electrochemical reactivity of 3D TiOx for both OH-mediated oxidation and DET was greatly improved compared to the BDD and Ti/TiOx plate electrodes. The results obtained in this study were also compared with recent studies from the literature using TiOx electrodes (Table SI 1). However, it is difficult to compare these results since different parameters were used, including nature and concentration of pollutants, electrode surface area, solution volume.

Ethanol (100 mM) was then used as quencher of  OH in order to assess the degradation of PCT through DET (Fig. 6(a)). Compared to previous results obtained without  OH quenchers, the degradation kinetic of PCT was divided by 1.7, 2.2 and 1.9 for 3D TiOx, BDD plate and Ti/TiOx plate anodes. These results are consistent with the slower degradation kinetics obtained using OA as probe molecule for DET compared to those obtained with TA as probe molecule for  OH. The degradation kinetic through  OH-mediated oxidation was faster since the accumulation of  OH within a thin layer (≈ 1 µm) at the vicinity of the anode surface increases the reaction zone compared to DET that occurs only at the anode surface.

Fate of degradation by-products

Degradation of the mother molecule might lead to the formation of toxic by-products which, should also be removed from the solution. Therefore, the fate of primarily aromatic by-products and mineralization yields were also monitored.

Hydroquinone (HQ) and benzoquinone (BQ) were identified as the main aromatic by-products of PCT degradation in this study. It is consistent with previous studies related to the degradation pathway of PCT [START_REF] Sirés | Electrochemical Degradation of Paracetamol from Water by Catalytic Action of Fe 2+ , Cu 2+ , and UVA Light on Electrogenerated Hydrogen Peroxide[END_REF]. Fig. 7(a) shows the evolution of HQ and BQ during PCT degradation by AO using 3D TiOx anode. Fig. 7(b) shows the fraction of HQ and BQ in the solution compared to the amount of PCT degraded. In the absence of ethanol, very low accumulation of HQ and BQ was observed for both 3D TiOx and BDD, with a slight advantage for 3D TiOx. After 2 h of treatment, the sum of HQ and BQ was representing less than 1.5% of the amount of PCT degraded with 3D TiOx and BDD anodes (Fig. 7b). By comparison, higher concentrations of HQ and BQ were measured in the solution during the degradation of PCT by Ti/TiOx plate anode.

In the presence of ethanol, much higher concentrations of HQ were observed during the treatment. The predominance of HQ compared to BQ might be ascribed to the fast reduction of BQ to HQ at the cathode. Such higher accumulation of by-products can be explained by the recalcitrance of these compounds for further oxidation by DET. For example, BQ is well known for its recalcitrance to oxidation by DET [START_REF] Jing | Mechanistic Study of the Validity of Using Hydroxyl Radical Probes to Characterize Electrochemical Advanced Oxidation Processes[END_REF]. These results highlight that  OH-mediated oxidation is also a key parameter for further degradation of by-products.

Therefore, the better behavior of 3D TiOx and BDD for  OH-mediated oxidation (section 3.3) might explain the lower accumulation of aromatic by-products observed during experiments without quenchers. These results are also consistent with mineralization yields (i.e., TOC removal rate) obtained after two hours of treatment (Fig. 8). In absence of quenchers, the effectiveness of the electrodes followed the same order as for the accumulation of aromatic by-products: 3D TiOx > BDD >> Ti/TiOx plate. 3D TiOx achieved 75% of TOC removal after 2 h of treatment (Fig. 8)). It corresponds to a current efficiency of 8.7%. By comparison, Ti/TiOx plate anode achieved a five times lower mineralization efficiency, corresponding to a current efficiency of only 1.6%. The mineralization yield achieved with 3D TiOx was even higher than that obtained with BDD (66%). BDD is usually considered as a gold standard for such application, particularly for achieving complete combustion of organic compounds [START_REF] Martínez-Huitle | Single and Coupled Electrochemical Processes and Reactors for the Abatement of Organic Water Pollutants: A Critical Review[END_REF][START_REF] Oturan | Outstanding performances of the BDD film anode in electro-Fenton process: Applications and comparative performance[END_REF][START_REF] Garcia-Segura | Mineralization of the recalcitrant oxalic and oxamic acids by electrochemical advanced oxidation processes using a boron-doped diamond anode[END_REF][START_REF] De Oliveira Silva | Electrochemical treatment of soil-washing effluent with boron-doped diamond electrodes: A review[END_REF][START_REF] Bansal | Development of nano boron-doped diamond electrodes for environmental applications[END_REF][START_REF] Brillas | Mineralization of paracetamol in aqueous medium by anodic oxidation with a boron-doped diamond electrode[END_REF].

Therefore, the results obtained here highlight the great promises of 3D TiOx for further development in removal of organic pollutants.

It is also interesting to notice that the addition of ethanol as quencher of  OH had a bigger adverse effect on the accumulation of by-products for 3D TiOx compared to BDD anode. The addition of 100 mM of ethanol might not ensure for 100% quenching of  OH. Therefore, this behavior might be ascribed to the more favorable reactivity of BDD for the accumulation of a great amount of  OH at the electrode surface; since it is more difficult to fully quench a higher local concentration of  OH. Thus, the presence of a higher residual concentration of  OH might explain the lower accumulation of aromatic by-products during quenching experiments with BDD anode. These results further highlight that the effectiveness of the 3D TiOx is tightly related to its suitable porous structure for mass transport enhancement, rather than to a superior reactivity.

Besides, the nature of the water matrix might also influence effectiveness of the process for degradation of target compounds, as it has been widely reported in literature. Further studies would be required for application to each specific water/wastewater.

Reproducibility of the performance of 3D TiOx electrode

Two different 3D TiOx electrodes obtained from the same synthesis method were compared for paracetamol degradation (Fig. 9) in order to assess the reproducibility of the performances of this 3D TiOx electrode. Very similar degradation kinetic was obtained (less than 7% of difference), thus suggesting that 3D printing is a reliable and robust method for synthesis of TiOx electrodes with reproducible performance.

Conclusions

The development of 3D printing techniques offers new possibilities for the synthesis of electrode materials with suitable properties for removal of organic compounds from water by the electrooxidation process. Particularly, electrode morphology must promote mass transport enhancement of organic pollutants from bulk to the electrode surface. In this study, 3D printing enabled the synthesis of a TiOx material with an appropriate coarse roughness allowing the increase of effective roughness factor. Such morphology allowed for improving the effectiveness of the process for application in stirred-tank reactor in which mass transport is limited by the diffusion layer at the electrode surface.

Results emphasized that 3D TiOx allowed (i) 2.8 times enhancement of degradation kinetics of OA (probe molecule for DET) compared to both BDD plate and Ti/TiOx plate anodes, (ii) 2.5 and 1.3 times enhancement of degradation kinetics of TA (probe molecule for  OH-mediated oxidation) compared to Ti/TiOx plate and BDD plate anodes, respectively, (iii) 3.3 and 1.7 times enhancement of degradation kinetics of PCT compared to Ti/TiOx plate and BDD plate anodes, respectively, (iv) lower accumulation of toxic aromatic by-products such as HQ and BQ, and, (v) higher mineralization yield than Ti/TiOx plate and BDD plate electrodes.

The suitable porous structure of 3D TiOx (with higher effective roughness factor) promotes both DET and  OH-mediated oxidation and allows for a more efficient use of current for the removal of organic compounds by AO under mass transport limitation.

Continuous development of 3D-printing technique (e.g., improvement of the resolution) might continue to offer new possibilities for development of electrode materials with precise control of the porous structure and therefore more efficient removal of organic pollutants from water/wastewater. Full-scale application also still depends on further study of influence of the real wastewater matrix, design of suitable large-scale reactor, and assessment of electrode lifetime.

  TA and its by-product (2-hydroxyterephthalic acid (HTA)) were analyzed by a HITACHI Elite LaChrom HPLC system. The HPLC system included a fluorescence detector set at Ex/Em = 315/435 nm (L-2480) (for HTA analysis), an UV detector set at λ = 254 nm (L-2400) (for TA analysis), a pump with flow rate of 0.8 mL min -1 and a LiChroCART Purospher® STAR RP-18e (5 µm) column set at 40 °C. Chromatograms obtained from HPLC analysis for TA, PCT and their by-products are provided in supporting information (Fig.SI-1and Fig.SI-2). Evolution of the total organic carbon (TOC) of treated solutions was measured by a TOC analyzer (SHIMADZU, TOC-L) in order to assess the mineralization yield of OA and PCT.

Fig. 1 -Fig. 2 -

 12 Fig. 1 -XRD pattern for 3D TiOx electrode

Fig. 3 -

 3 Fig. 3 -Cyclic voltammetry on 3D TiOx at different scan rates (a) and plot and linear regression of (Ia-Ic)/2 versus scan rate (b). Reference electrode: Ag/AgCl/3 M KCl, supporting electrolyte: 0.1 M NaClO4 and scan rate: 6-10 mV s -1 )

Fig. 4 -

 4 Fig. 4 -LSV curves obtained with 3D TiOx anode compared to BDD and Ti/TiOx plate anodes (reference electrode: Ag/AgCl/3 M KCl, supporting electrolyte: 100 mM NaClO4, Selectrode/Vsolution was 7.9 m -1 , scan rate: 0.01 V s -1 )

Fig. 5 -

 5 Fig. 5 -Degradation kinetic of OA by AO using 3D TiOx anode compared to BDD and Ti/TiOx plate anodes (a) and corresponding pseudo first order rate constants (b); degradation kinetic of TA by AO using 3D TiOx compared to BDD and Ti/TiOx plate anodes (c) and corresponding pseudo first order rate constants (d); generation of HTA during degradation of TA by 3D TiOx, BDD and Ti/TiOx plates anodes (e). Experimental conditions: initial CTA = 0.1 mM, initial COA = 0.4 mM, geometric surface area = 23.6

Fig. 6 -

 6 Fig. 6 -Degradation kinetic of PCT by 3D TiOx in comparison with BDD and Ti/TiOx plate electrodes with and without using ethanol as quencher (a) and corresponding pseudo first order rate constants (b). Experimental conditions: initial CPCT = 0.1 mM, Cethanol /CPCT = 1000, geometric surface area = 23.6 cm 2 , Selectrode/Vsolution = 7.9 m -1 , current density = 5 mA cm -2 .

Fig. 7 -Fig. 8 -

 78 Fig. 7 -Evolution of HQ and BQ (a) and total amount of HQ and BQ during the PCT degradation by AO using 3D TiOx anode in comparison with BDD and Ti/TiOx plate anodes (b) in absence and presence of ethanol. Experimental conditions: initial CPCT = 0.1 mM, Cethanol /CPCT = 1000, geometric surface area = 23.6 cm 2 , Selectrode/Vsolution = 7.9 m -1 , current density = 5 mA cm -2 .

Fig. 9 -

 9 Fig. 9 -Degradation kinetics of PCT by two different 3D TiOx synthesized from the same synthesis method. Experimental conditions: initial CPCT = 0.1 mM, geometric surface area = 23.6 cm 2 , Selectrode/Vsolution = 7.9 m -1 , current density = 5 mA cm -2 .

Table 1 -

 1 Pore size distribution of 3D TiOx electrode

	Pore size	0 -2 µm	2 -10 µm	10 -20 µm	20 -100 µm 100 -345µm
	Vol %	0.8	31.4	3.7	4.6	5.5

Acknowledgement Jing Ma acknowledges China Scholarship Council for the financial support for her PhD work (201906970041). Authors acknowledge Saint-Gobain Research Provence for providing TiOx electrodes.

List of abbreviations