High-efficient decomplexation of Cu-EDTA and Cu removal by high-frequency non-thermal plasma oxidation/alkaline precipitation

Abstract Organic-heavy metal complexes, with strongly chemical stability and high solubility in water, are difficult to eliminate by traditional chemical precipitation. An efficient strategy, i.e., high-frequency non-thermal plasma (NTP) oxidation/alkaline precipitation, was employed to decompose organic-heavy metal complexes and precipitate free metal ions. Cu-ethylene diamine tetraacetic acid (Cu-EDTA) was chosen as the model. The experimental results showed that Cu-EDTA removal efficiency reached 98.8% at 6 kV after 10 min NTP treatment with air as the carrier gas. Relatively greater removal performance of Cu-EDTA was observed at a lower initial concentration and acidic conditions. Oxygen feeding favored Cu-EDTA decomplexation. Excessive Cu2+ could promote Cu-EDTA decomplexation, whereas excessive EDTA disfavored this process. ·OH and O2·- generated in the NTP system exhibited significant roles in the Cu-EDTA decomposition process, as analyzed by radical quenching tests. Cu-EDTA decomplexation process was analyzed by spectroscopic and chromatographic techniques. Some small organic substances and inorganic anions, including crotonic acid, acrylic acid, lactic acid, ethylene glycol, acetamide, acetic acid and NO3-, were detected as the main intermediate byproducts. The residual toxicity after NTP treatment was evaluated via toxicity prediction. Furthermore, Cu(II) removal and precipitation processes were explored by X-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy.