Aminopolycarboxylic acids modified oxygen reduction by zero valent iron: Proton-coupled electron transfer, role of iron ion and reactive oxidant generation

Abstract

The oxygen reduction reaction (ORR) activated by Fe⁰ in the presence of three aminopolycarboxylic acids (CAs), i.e. nitrilotriacetic acid (NTA), ethylenediamine-N,N'-disuccinic acid (EDDS) and ethylenediaminetetraacetic acid (EDTA), for the degradation of sulfamethazine (SMT) was investigated. At optimum conditions, Fe⁰/EDDS/O₂, Fe⁰/EDTA/O₂ and Fe⁰/NTA/O₂ systems presented SMT removal of 58.2%, 75.3% and 93.8%, respectively, being much higher than that in the Fe⁰/O₂ system (1.36%). The generation of surface-bound Fe²⁺ (Fe²⁺) and dissolved iron ion was enhanced by CAs. ORR through a two-electron transfer pathway was mainly responsible for H₂O₂ generation in NTA and EDTA systems, while a single-electron ORR was the major source for producing H₂O₂ in EDDS system. •OH produced by the homogeneous reaction of Fe²⁺ and H₂O₂ was the main species for SMT degradation. Fe⁰/EDDS/O₂ produced more ¹O₂ than Fe⁰/EDTA/O₂ and Fe⁰/NTA/O₂; however, the radical contributed negligibly to SMT removal. The caging effect of CAs might be a major factor influencing the reaction rate of Fe²⁺ and O₂. CAs provided protons to accelerate the electron transfer, the production of Fe²⁺ and thus the contaminant removal. This study is of great significance for revealing ORR mechanisms in the Fe⁰-chelate system.

Keywords: Aminopolycarboxylic acids; Fe(0); H(2)O(2); Oxygen reduction reaction; Proton-coupled electron transfer.