Selective electrochemical removal of heavy metals from reverse osmosis concentrate using electrografted chelating...

Selective removal of dissolved heavy metals from reverse osmosis concentrate (ROC) remains a major challenge for inland desalination due to their high ionic strength and electrode fouling in electrochemical systems. Here, we demonstrate a one-step electrografting of ethylenediaminetetraacetic acid (EDTA) onto glassy carbon in aqueous phosphate electrolyte for simultaneous removal of Cd(II), Co(II), Cu(II), Ni(II), Pb(II), and Zn(II) at ppb levels. Cyclic voltammetry revealed an irreversible EDTA oxidation peak at 1.1 V vs Ag/AgCl, and passivation together with ferri/ferrocyanide probe measurements confirmed the formation of a surface-bound sub monolayer. Electron-number analysis indicated competing radical and carbocation pathways, with increased radical self-reaction at higher EDTA concentrations. Using glassy carbon plates, response surface optimization identified optimal electrografting conditions of 0.4 mM EDTA, 1.3 V, and 13 min, achieving 75% average removal after 5 h at -0.8 V and a 3.5-fold improvement relative to pristine electrodes. Extending cathodic polarization to 20 h increased removal to 92.6% at -1.4 V with cell energy consumption of 1.11 kWh m^-3. Anodic stripping responses increased with accumulation time, supporting a mechanism involving electromigration, interfacial chelation, and electroreduction followed by deposition. Regeneration by polarity reversal released 67-95% of deposited metals, with 95.4% release in 50 mM HNO₃ at +0.5 V over 5 h. In actual ROC, the electrode maintained high removal for four of six spiked metals, while Co(II) and Ni(II) removal decreased due to matrix-driven speciation and Ca/Mg scaling. These findings provide a scalable electrochemical strategy for selective heavy metal removal and recovery from inland desalination brines.