Paired electrochemical removal of nitrate and terbuthylazine pesticide from groundwater using mesh electrodes

Highlights: • Successful paired electro-oxidation/electrodenitrification of groundwater with Fe cathodes. • Pseudo-first-order kinetics for NO₃⁻ and terbuthylazine (TBZE) removals with BDD and RuO_2. • NH₄⁺ or N-volatiles as main N-products depending on Cl⁻ content, pH and applied I. • Groundwater with BDD/Fe: total TBZE disappearance, sufficient NO₃⁻ removal ( < WHO limit). • Initial reaction pathway for TBZE degradation: 10 heteroaromatics + oxalic and oxamic acids. -- Abstract: Groundwater is one of the main freshwater resources on Earth, but its contamination with NO₃⁻ and pesticides jeopardizes its viability as a source of drinking water. In this work, a detailed study of single electro-oxidation (EO) and electrodenitrification and paired EO/electrodenitrification processes has been undertaken with simulated and actual groundwater matrices containing 100 mg dm⁻³ NO₃⁻ and/or 5 mg dm⁻³ terbuthylazine pesticide. Galvanostatic electrolyses were made with 500 cm³ of solutions at pH 4.0-10.5 and 250-1000 mA in tank reactors with a RuO₂ or boron-doped diamond (BDD) anode and one or two Fe cathodes, all of them in the form of meshes. Most of NO₃⁻ removals agreed with a pseudo-first-order kinetics. In Cl⁻-free media, NH₄⁺ predominated as electroreduction product. In chloride media, a greater amount of N-volatiles was determined alongside a slower electrodenitrification, especially with RuO₂ due to the partial re-oxidation of electroreduction products like NH₄⁺ by active chlorine. The pesticide decays were also fitted to a pseudo-first order kinetics, and its presence led to a smaller release of N-volatiles. Overall, BDD always favored the pesticide degradation thanks to the action of BDD(${}^{•}$OH), whereas RuO₂ was preferred for electrodenitrification under some conditions. The EO/electrodenitrification of groundwater was successful once the matrix was softened to minimize its hardness. The NO₃⁻ concentration was reduced below the limit established by the WHO. Overall, the BDD/Fe cell was more suitable than the RuO₂/Fe cell because it accelerated the pesticide removal with a simultaneous high degree of NO₃⁻ electroreduction. However, it produced toxic chlorate and perchlorate. A final post-treatment with an anion exchange resin ensured a significant removal of both ions, thus increasing the viability of the electrochemical approach to treat this type of water. Chromatographic analyses revealed the formation of ten heteroaromatic products like desethyl-terbuthylazine and cyanuric acid, alongside oxalic and oxamic as final short-chain carboxylic acids.