Electrochemical Removal of Ni–EDTA Mediated by Chlorine and Hydroxyl Radicals on BDD Anodes

As a highly reactive oxidative species, chlorine radicals (ECl•0 = 2.47 V vs standard hydrogen electrode, SHE) could react with carboxylic and amino group-containing organic matter efficiently. To this end, mechanistic insights into the reactive species (Cl• and •OH)-mediated Ni–EDTA decomposition were examined in depth. The used boron-doped diamond (BDD) anode could achieve remarkably enhanced decomposition of Ni–EDTA in NaCl electrolyte, indicated by the apparent reaction kinetics constant being 2.7 and 4 times that in the presence of Na2SO4 and NaClO4, respectively. The experimental results and theoretical simulations revealed that the chlorine evolution reaction (CER) was thermodynamically favorable on the BDD anode, which facilitated the electro-generation of Cl•. Competitive kinetics and quenching experiments proved that the second-order rate constant of Cl• with Ni–EDTA is ∼1.38 × 1010 M–1 s–1, which is higher than that of •OH with Ni–EDTA (∼2.68 × 109 M–1 s–1). Furthermore, the experimental results and density functional theory (DFT) calculations show that the molecular changes of Ni–EDTA are mediated by Cl• and •OH through the H-abstraction and electron-transfer pathway in EDTA4–. This study demonstrates positive implications in the electrochemical decomplexation of wastewater containing chlorine, which makes water treatment more effective, more economical, and much easier.