Diffusion-charge transfer characterization of a rotating cylinder electrode reactor used for the complete electrocatalytic removal of nitrate from water

Groundwater nitrate contamination is an emerging threat in stressed regions under intensive farming although, lately, efforts to valorize such residues are highly encouraged. Here, electrochemical nitrate removal has been investigated as a versatile strategy for this purpose, using a reactor equipped with a cheap central Fe-based rotating cylinder electrode (RCE) as cathode and six concentric Ti|IrO2 plates as anodes. The study of the effect of Ecath and rotational speed (ω) on NO3− electroreduction from a synthetic aqueous solution with high conductivity revealed the feasibility of complete nitrate removal, which only required 100–120 min at Ecath = −1.80 V vs Hg|Hg2SO4|sat. K2SO4 within the ω-range of 100–500 rpm. The concentration decays agreed perfectly with a first-order kinetics. NH3 was accumulated as main product, being partly volatilized due to the quick solution alkalization, whereas NO2− was not found. Linear sweep voltammetries demonstrated the high electrocatalytic activity of carbon steel RCE as compared to inactive stainless steel. Koutecky-Levich analysis showed that the reduction process with carbon steel at Ecath from −1.80 V involved 8 electrons. The participation of H radical in the reduction mechanism was ascertained by electron paramagnetic resonance. The mass transport and charge transfer of the RCE reactor were characterized under turbulent flow by means of the dimensionless Damköhler (Da) number, as well as from the Sherwood-Reynolds-Schmidt (Sh-Re-Sc) analysis. A mixed regime with a prevalence of mass transport control was determined at Ecath from −1.8 V. The Sh = 0.70Re0.46Sc0.356 correlation obtained for this reactor may serve to guide the scale-up of electrochemical NO3− removal as more electrocatalytic cathode materials are developed. Successful NO3− elimination from solutions with low conductivity that mimicked groundwater is finally reported.