Impact of millimetric to micrometric inter-electrode distances: Is there a way to maximize the organic pollutant degradation yield and minimize the cathode scaling and chlorate formation during wastewater treatment?

The influence of a range of inter-electrode distances from 50 µm to 1 mm has been investigated for the first time, in order to assess to possibility to maximize the degradation and mineralization efficiency, while minimizing the cathode scaling and the inorganic by-products formation. Tylosin has been selected as representative pharmaceutical pollutant in wastewater, while the influence of Ca²⁺, HCO₃⁻/CO₃²⁻ and Cl⁻ was carried out. Advanced electro-oxidation with boron-doped diamond (BDD) anode and stainless-steel cathode was implemented to treat this synthetic effluent in a scalable filter-press reactor operated in a recirculated batch flow-by mode.

The first interesting feature is that cathodic OH⁻ and anodic H⁺ formations were not counterbalanced at short micro-distances (50 µm), meaning that electro-precipitation (until 50 % of CaCO₃ precipitation) and degradation/mineralization (until 100 %) could still occur at such range of distance. Secondly, the gain of mass transfer at the shorter distance (50 µm) couldn't counteract the higher energy needed to achieve similar degradation efficiency compared to the distances of 500 µm and 1 mm. Lastly, too high distances such as 1 mm suffer from lower mass transfer compared to sub-millimetric distances. Thus, an intermediate distance of 500 µm led to better performance in terms of tylosin degradation (100 % of degradation, 39 % of mineralization), while minimizing electro-precipitation (26 % of CaCO₃ cathodic precipitation) and unwanted inorganic chlorinated by-products formation (ClO₃⁻ = 2.8 mg L⁻¹). This was obtained at a current density of 0.1 mA cm⁻², leading to lower energy requirement (0.018 kWh g-tylosin⁻¹). In these low-current conditions the formation of perchlorate could be avoided.