Kinetic Modeling of the Anodic Degradation of Ni-EDTA Complexes: Insights into the Reaction Mechanism and Products

Abstract

In this study, an electrochemical advanced oxidation process (EAOP) was employed to effectively degrade complexes of nickel and ethylenediaminetetraacetic acid (EDTA) present in electroless nickel plating wastewaters. Our results show that Ni-EDTA complexes can be effectively degraded by an EAOP with degradation of the complexes occurring at/near the anode surface via interaction with hydroxyl radicals generated on water splitting. Our results further show that the rate of Ni-EDTA degradation is not a function of the rate of any particular chemical reaction but, rather, is controlled by the rate of transport of Ni-EDTA to the anode surface. The oxidation of EDTA to smaller noncomplexing entities releases Ni 2+ , which is subsequently deposited onto the cathode as Ni 0 . While complete Ni-EDTA removal and Ni recovery are achieved within 2 h, the overall TOC removal by EAOP is limited, with only 50% TOC removal achieved after 2 h of treatment. The low affinity of small molecular weight EDTA degradation products (such as formic acid, glycine, oxamic acid, and acetic acid) for the anode surface limits oxidation of these compounds and overall TOC removal by the anodic oxidation process. 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. The mathematical model developed here, when coupled with the hydrodynamics of the electrochemical cell using a computational fluid dynamics tool, can assist in both cell design and the selection of operating parameters such that the performance of the EAOP process for Ni-EDTA degradation and TOC removal is optimized.

Approaches to Enhancing Cathodic Nickel Recovery from Ni-EDTA Containing Synthetic Wastewaters

“…As reported in our previous work, the rate of Ni-EDTA degradation is mainly determined by the Ni-EDTA transport rate to the anode surface. 14,18 Furthermore, the rate of release of Ni 2+ is similar to the Ni-EDTA degradation rate since the concentration of Ni present in other complexed forms (i.e., Ni-EDDA, Ni-NTA, etc.) is minimal (see Figure S6).…”

“…In comparison, the electrochemical advanced oxidation process (EAOP), involving anodic oxidation and cathodic reduction processes in one system, is a viable option for simultaneous Ni-EDTA degradation and Ni recovery due to minimal formation of toxic species, technical flexibility, and affordable cost . In the electrochemical process, the breakdown of the Ni-EDTA complex and release of Ni 2+ are achieved via anodic oxidation of EDTA (either via direct electron transfer (DET) and/or • OH mediated oxidation processes) , with the released Ni 2+ electrochemically reduced at the cathode surface thereby enabling simultaneous Ni recovery . However, it should be recognized that the other reactions, particularly hydrogen evolution and oxygen reduction that may occur at the cathode surface, may compete with Ni 2+ reduction reactions (eqs –): normalO 2 + 2 normalH 2 normalO + 4 normale − = 4 normalO normalH − normalO 2 + 2 normalH + + 2 normale − = normalH 2 normalO 2 2 normalH 2 normalO + 2 normale − = 2 normalH 2 + 2 normalO normalH − Ni 2 + + 2 normale − = Ni ( <...…”

Electrochemical Removal of Metal–Organic Complexes in Metal Plating Wastewater: A Comparative Study of Cu-EDTA and Ni-EDTA Removal Mechanisms

“…The initial pH of 3.7 used here is close to the pH of real metal plating wastewaters (pH 3−4). 10,26 The pH value of the electrolyte during oxidizing electrolysis at 3.5 and 5.5 V gradually increased to ∼5.0. The pH value in the catholyte in reducing electrolysis experiments showed a rapid increase to ∼6.0 in the first 30 min and, subsequently, gradually increased to 7.5 after 2 h at −1.2 and −2 V (vs Ag/AgCl).…”

“…• OH). 26,31 The Cu/Ni removal rate constants for a range of EDTA concentrations are shown in Figure 5b,c, and the temporal variation of total Cu, Ni, and TOC concentrations is shown in Figures S15 and S16. In the case of copper, the k app, Cu values in the absence and presence of EDTA (at both 1 and 2 mM) showed a small difference for oxidizing and reducing electrolysis, with this result indicating that the presence of EDTA, at least at the concentrations investigated here, may affect the Cu(II) reduction rate to some extent (Figure 5b).…”

“…), which have limited Ni binding capacity. 26 The removal of Ni from the solution mainly occurs via electrodeposition on the cathode with the (1)…”