One-step rapid and complete removal of Ni(II)-EDTA by O₃/H₂O₂/Fe⁰ process: The role of high-valent Ni species

The removal of complexed Ni from wastewater typically requires advanced oxidation processes for decomplexation, followed by chemical precipitation or adsorption steps. Simplifying this process while meeting stringent discharge standards remains challenging. Here, we developed a one-step O₃/H₂O₂/Fe⁰ system for the rapid and complete removal of Ni(II)-EDTA. In this system, Ni(II)-EDTA undergoes decomplexation and the resulting Ni²⁺ can be subsequently removed. Through this two-stage process, the concentration of total Ni can be decreased to below 0.1 mg/L at pH₀ 4.0–8.0. The O₃/H₂O₂/Fe⁰ system generated multiple reactive oxidizing species, i.e., HO^·, O₂^·−, ¹O₂, and Fe(IV). The probe method suggested Fe⁰ enhanced the production of ¹O₂ and Fe(IV), indicating their significant roles in this system. In the decomplexation stage, combined experimental evidence, including chemiluminescence, revealed the formation of Ni(IV) species. It progressively degraded into smaller chelated Ni(IV) intermediates and ultimately converted to Ni²⁺ and Ni(II/III) hydroxides. In the second stage, the Ni(II/III) hydroxides formed in the first stage catalyzed the oxidative removal of the Ni²⁺, resulting in final products composed of Ni(II/III) hydroxides. The H₂O₂ could favor the generation of Ni(II/III) hydroxides in the first stage, enhancing the subsequential removal of the Ni²⁺. These results can lead to a promising strategy for the removal and recovery of complex Ni from industrial wastewater.