Novel Cu(II)–EDTA Decomplexation by Discharge Plasma Oxidation and Coupled Cu Removal by Alkaline Precipitation: Underneath Mechanisms

Abstract

Strong complexation between heavy metals and organic complexing agents makes the heavy metals difficult to be removed by classical chemical precipitation. In this study, a novel decomplexation method was developed using discharge plasma oxidation, which was followed by alkaline precipitation to treat water containing heavy metal-organic complex, that is, Cu-ethylenediaminetetraacetic acid (Cu-EDTA). The decomplexation efficiency of Cu complex reached up to nearly 100% after 60 min's oxidation by discharge plasma, which was accompanied by 82.1% of total organic carbon removal and energy efficiency of 0.62 g kWh^-1. Presence of free Cu²⁺ favored Cu-EDTA decomplexation, whereas the presence of excessive EDTA depressed this process. Cu-EDTA decomplexation was mainly driven by the produced ¹O₂, O₂^•-, O₃, and •OH by discharge plasma. Cu-EDTA decomplexation process was characterized by UV-vis, ATR-FTIR, total organic carbon, and three-dimensional fluorescence diagnosis. The main intermediates including Cu-EDDA, Cu-IDA, Cu-NTA, small organic acids, NH₄⁺, and NO₃^- were identified, accompanied by Cu²⁺ releasing. The followed precipitation process removed 78.1% of Cu²⁺, and Cu-associated precipitates included CuCO₃, Cu₂CO₃(OH)₂, CuO, and Cu(OH)₂. A possible pathway of Cu complex decomplexation and Cu²⁺ removal in such a system was proposed.