Oxidation of aquatic pollutants by ferrous–oxalate complexes under dark aerobic conditions

Abstract

This study evaluates the ability of Fe-II-oxalate complexes for the generation of (OH)-O-center dot through oxygen reduction and the oxidative degradation of aquatic pollutants under dark aerobic conditions (i.e., with oxygen but without light). The degradation of 4-chlorophenol (4-CP) was rapid in the mixture of Fe2+ and oxalate prepared using ultrapure water, but was absent without either Fe2+ or oxalate. The formation of Fe-II-oxalate complexes enables two-electron reduction of oxygen to generate H2O2 and subsequent production of (OH)-O-center dot. The significant inhibition of 4-CP degradation in the presence of H2O2 and (OH)-O-center dot scavenger confirms such mechanisms. The degradation experiments with varying [Fe2+], [oxalate], and initial pH demonstrated that the degradation rate depends on [Fe-II(Ox)(2)(2-)], but the degree of degradation is primarily determined by [Fe-II(Ox)(2)(2-)] + [Fe-II(Ox)(0)]. Efficient degradation of diverse aquatic pollutants, especially phenolic pollutants, was observed in the Fe-II-oxalate complexes system, wherein the oxidation efficacy was primarily correlated with the reaction rate constant between pollutant and (OH)-O-center dot. The effect of various organic ligands (oxalate, citrate, EDTA, malonate, and acetate) on the degradation kinetics of 4-CP was investigated. The highest efficiency of oxalate for the oxidative degradation is attributed to its high capability to enhance the reducing power and low reactivity with (OH)-O-center dot. (C) 2014 Elsevier B.V. All rights reserved.