Quantitative structure–activity relationship for the oxidation of aromatic organic contaminants in water by TAML/H2O2

Abstract

Tetra-amido macrocyclic ligand (TAML) activator is a functional analog of peroxidase enzymes, which activates hydrogen peroxide (H2O2) to form high valence iron-oxo complexes that selectively degrade persistent aromatic organic contaminants (ACs) in water. Here, we develop quantitative structure–activity relationship (QSAR) models based on measured pseudo first-order kinetic rate coefficients (kobs) of 29 ACs (e.g., phenols and pharmaceuticals) oxidized by TAML/H2O2 at neutral and basic pH values to gain mechanistic insight on the selectivity and pH dependence of TAML/H2O2 systems. These QSAR models infer that electron donating ability (EHOMO) is the most important AC characteristic for TAML/H2O2 oxidation, pointing to a rate-limiting single-electron transfer (SET) mechanism. Oxidation rates at pH 7 also depend on AC reactive indices such as fmin− and qH+, which respectively represent propensity for electrophilic attack and the most positive net atomic charge on hydrogen atoms. At pH 10, TAML/H2O2 is more reactive towards ACs with a lower hydrogen to carbon atoms ratio (#H:C), suggesting the significance of hydrogen atom abstraction. In addition, lnkobs of 14 monosubstituted phenols is negatively correlated with Hammett constants (σ) and exhibits similar sensitivity to substituent effects as horseradish peroxidase. Although accurately predicting degradation rates of specific ACs in complex wastewater matrices could be difficult, these QSAR models are statistically robust and help predict both relative degradability and reaction mechanism for TAML/H2O2-based treatment processes.