Electrochemical decomplexing and oxidation of organic (chelating) additives in effluents from surface treatment and metal finishing

The electrochemical decomplexing and oxidation of two frequently used complexing agents in surface treatment and metal finishing-EDTA (ethylenediaminetetraacetic acid) and NTA (nitrilotriacetic acid)-and of organic non-complexing additives used in nickel-plating baths were the subject of this study. Using a Ti-RuO2 electrode, a partial indirect oxidation by in-situ electrochemical generation of chlorine compounds could be achieved for EDTA and NTA. At a boron-doped diamond (BDD) electrode however, complete decomplexing and high COD (Chemical Oxygen Demand) and TOC (Total Organic Carbon) (up to 95%) removal occurred at an average current density of 2 A dm(-2). It is shown that direct electrochemical oxidation at a BDD electrode resulted in lower energy consumption and higher treatment rates than indirect oxidation at a Ti-RuO2 electrode. Decomplexing at the BDD electrode occurred at high current efficiencies ranging from 71% to 95% with decomplexing rates in the order of 3.13 mmol (Ah)(-1) and 5.02 mmol (Ah)(-1) for EDTA and NTA respectively. COD removal rates obtained were 0.090 g (Ah)(-1) for EDTA, 0.100g (Ah)(-1) for NTA and 0.205g (Ah)(-1) for the nickel-plating additives. Electrochemical decomplexing and oxidation of common chelating agents can render the subsequent metal precipitation and biological wastewater treatment of surface treatment and metal finishing effluents more efficient. (C) 2003 Society of Chemical Industry.