Electro-oxidation of a commercial herbicide using a three-dimensional layered Ti4O7 porous anode

SILVA BARNI, M. F.; DOUMIC, L.; PROCACCINI, R.; AYUDE, A.; ROMEO, H.

Santa Fé

Congreso; X Congreso Argentino de Ingeniería Química (X CAIQ). Congreso Internacional ?100 años de Ingeniería Química en Argentina y LatinoAmérica"; 2019

Asociación Argentina de Ingenieros Químicos (AAIQ)

Electrochemical advanced oxidation processes (EAOP) are positioned as emerging technologies for water treatment since they are characterized by a high efficiency and low cost, likewise constituting a set of green procedures. Electro-oxidation (EO), also known as anodic oxidation, is the most common EAOP. Sub-stoichiometric titanium oxides (particularly Ti4O7) constitute attractive building blocks to be used as anodes for the EO of pollutants due to their high electrical conductivity, chemical stability, high oxygen evolution potential and low manufacturing cost. Even though Ti4O7 electrodes have been recently considered to be used as anodes for the electrochemical degradation of organic pollutants, until now, only electrodes with a plate-like configuration have been used, with little attention given to three-dimensional (3D) porous geometries, which could clearly offer advantages over their 2D non-porous counterparts. The aim of this work was to evaluate the use of 3D Ti4O7 porous electrodes (90% porosity) in EO processes. The tested platforms are characterized by a layered architecture, which ensures, on the one hand, a higher exposed surface area respect to non-porous anodes and, on the other hand, allows recirculating of the treating solution under a flow-through configuration. The EO of a comercial herbicide (bentazone) was conducted in a three-electrode electrochemical cell. The effect of both the reactor configuration (flow-through vs. Traditional batch) and electrode´s porosity on the oxidation efficiency were investigated.Porous Ti4O7 electrodes with a laminar morphology were obtained by using a cryogenic technique based on the directional freezing of TiO2 dispersions (Massazza et al., 2015). Briefly, TiO2 nanoparticle dispersions (5% vol) were poured in plastic molds and then vertically immersed into liquid nitrogen (-196 °C) at a constant advancing ice front rate. Different immersion velocities (1, 5, 10 and 15 mm/min) were used in order to produce porous monoliths characterized by a layer-like continuous structure which extended along the whole material, with different interlayer distances. According to the processing conditions, layer-to-layer spacings of about 10, 5, 3 and 2 µm were obtained at 1, 5, 10 and 15 mm/min immersion rates, respectively. Once obtained, the layered TiO2 structures were reduced at high temperature to obtain the electrically conducting phase (Ti4O7), according to the procedure described by Massazza et al., (2015). EO of bentazone was carried out under both batch and flow-through modes in a three-electrode cell under stirring (500 rpm) at 25-27°C. For the flow-through configuration, a peristaltic pump at a flow rate of 10 mL/min was used to continuously recirculate the targeting solution through the layered electrode architectures. The tested anode (monolithic Ti4O7), was immersed into 50 mL of bentazone solution (85 ppm), and a platinum wire was employed as the cathode. The EO experiments were conducted at galvanostatic condition (1 mA), using a saturated Calomel electrode (SCE) as the reference electrode. Samples were collected periodically and analyzed in terms of total organic carbon (TOC) and bentazone concentration. EO experiments showed that the removal of bentazone in the traditional batch mode was similar for electrodes obtained under immersion velocities of 1, 5 and 10 mm/min; 31% of the initial bentazone being removed after 6h of treatment, in which only 12% of TOC was consumed. For the electrode processed at 15 mm/min, 16% of bentazone removal was achieved. Electrodes prepared at higher immersion velocities lead to shorter interlayer spacings and, correspondingly, higher volumetric surface areas.Strikingly, electrodes obtained at 15 mm/min, regardless of their higher surface area with respect to their counterparts, evidenced the lowest bentazone degradation, which may be ascribed to mass transfer limitations arising from a more closed and intricate structure. These results were clearly mirrored in the flow-through experiments. In the flow-through mode, the lower the immersion velocity employed in the preparation of the electrode, the higher the oxidation extent reached. That is, best results were observed for electrodes immersed at 1 mm/min, attaining 85% and 47% of bentazone and TOC consumption, respectively. Even though the electrode obtained at 1mm/min was characterized by the lowest volumetric surface area, the larger interlayer spacing probably facilitate the flow through the electrode, enhancing this way the mass transfer.It is worth mentioning that electrodes obtained at 15 mm/min could not be employed under the flow-through mode since the solution was not able to go through its porous structure.This work demonstrates that the use of 3D layered Ti4O7 anodes under a flow-through mode is a promising approach for the enhanced EO of organic pollutants. Our results reveal that the regulation of the electrode interlayer spacing is fundamental to increase the EO efficiency, providing a readily accessible porous structure to facilitate the liquid flow through the electrode architecture while minimizing simultaneously the pressure drop.