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

The oxygen reduction reaction (ORR) activated by Fe 0 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, Fe 0 /EDDS/O 2 , Fe 0 /EDTA/O 2 and Fe 0 /NTA/O 2 systems presented SMT removal of 58.2%, 75.3% and 93.8%, respectively, being much higher than that in the Fe 0 /O 2 system (1.36%). The generation of surface-bound Fe 2+ ( Fe 2+ ) and dissolved iron ion was enhanced by CAs . ORR through a two-electron transfer pathway was mainly responsible for H 2 O 2 generation in NTA and EDTA systems, while a single-electron ORR was the major source for producing H 2 O 2 in EDDS system. •OH produced by the homogeneous reaction of Fe 2+ and H 2 O 2 was the main species for SMT degradation. Fe 0 /EDDS/O 2 produced more 1 O 2 than Fe 0 /EDTA/O 2 and Fe 0 /NTA/O 2 ; however, the radical contributed negligibly to SMT removal. The caging effect of CAs might be a major factor influencing the reaction rate of Fe 2+ and O 2 . CAs provided protons to accelerate the electron transfer , the production of Fe 2+ and thus the contaminant removal. This study is of great significance for revealing ORR mechanisms in the Fe 0 -chelate system.