Ni-EDTA Containing Wastewater Treatment Using Electrochemical Advanced Oxidation Processes

Abstract

The treatment of electroless plating wastewaters containing ethylenediamine tetraacetic acid complexed Ni (i.e. Ni-EDTA) has become a major problem due to the high stability and low biodegradability. Electrochemical advanced oxidation processes (EAOP) have been widely applied in removing persistent compounds by generation of highly oxidative reactive species such as hydroxyl radicals. We investigate the feasibility of treatment of Ni-EDTA containing wastewaters via EAOP employing a range of anodes and operating conditions. Both bismuth and nickel doped lead oxide anodes prepared via a co-electrodeposition process exhibit excellent electrochemical degradation of Ni-EDTA complexes and facilitate concomitant removal of Ni from solution via deposition of elemental Ni on the cathode surface. The effects of Bi/Ni doping ratio and electrodeposition current density on the electrode properties were thoroughly investigated. Our results show that minor deterioration in Ni-EDTA degradation performance was observed with minimal lead leakage. Compared with pure PbO2 and Bi-doped PbO2 electrodes, Ni doping increased the oxygen evolution potential as well as the reactive site concentration and reduced the electron transfer resistance thereby resulting in superior Ni-EDTA degradation performance. To increase the extent of Ni removal, we investigated the efficacy of five cathode materials. Our results show that carbon felt provides the best Ni removal efficiency as a result of its high surface area and low interference from side reactions such as the hydrogen evolution reaction. The Ni accumulated on the carbon felt surface was successfully recovered either as a nickel salt solution by acid leaching or as high purity NiO by calcinating the Ni-loaded carbon felt cathode. Our results also show that the performance of the regenerated carbon felt after acid leaching is comparable to that of the fresh cathode. We also developed a computational fluid dynamics (CFD) model to investigate flow behaviour and its impact on the Ni-EDTA in a flow-through reactor. The CFD model was used to investigate the effect of electrode aperture size and shape on Ni-EDTA degradation with the end goal of optimising electrode design. Additionally, we have developed a mathematical kinetic model that satisfactorily describes Ni-EDTA removal, Ni recovery, and TOC removal over a range of Ni and EDTA concentrations and provides a good description of the oxidation of various EDTA degradation intermediates