Simultaneous and efficient removal of organic Ni and Cu complexes from electroless plating effluent using integrated catalytic ozonation and chelating precipitation process in a continuous pilot-scale system

In the industrial electroless metal plating process, efficient removal of organic-Ni and -Cu complexes from electroless plating effluent (EPE) remains a huge challenge. In this study, a novel and feasible process of heterogeneous catalytic ozonation combined with heavy metal chelation is proposed for the effective removal of organic-Ni and -Cu complexes from EPE. The method was evaluated in a continuous pilot-scale system (CPSS). Results show that the adoption of synchronous bi-directional flow enhanced the removal of total Ni and Cu. Under optimal conditions, the total Ni and Cu in the effluent decreased to 0.1 and 0.3 mg L⁻¹, respectively, meeting the Chinese discharge standard requirements. The CPSS indicated that efficient removal of total Ni and Cu (>95%) was consistently achieved for 300 days using the proposed process. Mechanistic analysis demonstrated that ethylenediamine tetraacetic acid-Ni (EDTA-Ni) and citrate-Cu (CA-Cu) were the main complexes present in EPE. Furthermore, there were 10 transformation products and their related pathways were also identified. The decomplexation of EDTA-Ni was found to be difficult, with its final products principally existing in the complexed states, while CA-Cu was readily broken down, releasing Cu (II) that can be easily chelated. Overall, these findings provide valuable and novel insights into the removal of organic-Ni and -Cu, supporting the practical application of the proposed process in advanced EPE treatment and other industrial wastewater treatment systems.