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

This study evaluates the ability of Fe^II-oxalate complexes for the generation of OH 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. The significant inhibition of 4-CP degradation in the presence of H₂O₂ and OH 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. 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.