Application of biomimetic high valence state iron complexes to contaminant oxidation

Abstract

Catalysed oxidation processes are of interest in water/wastewater treatment for recalcitrant pollutant degradation. Inspired by natural oxidative enzymes, iron complexes that can be activated to a high valence state are recognized as potential potent oxidation catalysts though there are still significant gaps with regard to their application in ambient aqueous environments. Investigating iron complexes with such potential and studying their application to degradation of organic contaminants pushing them further in that direction is thus important to harness this potential in real technologies. In this thesis, two iron complexes studied are Fe-TAML (TAML = tetraamido macrocylic ligand) and Fe-tpena (tpena– = N,N,N'-tris(2-pyridylmethyl)ethylenediamine-N'-acetate). To enhance feasibility, Fe-TAML is immobilized by graphite and used to catalyze rhodamine B (RhB) degradation by both hydrogen peroxide and electrolysis. RhB can be efficiently degraded by both activation methods in the presence of Fe-TAML/Graphite. The graphite support not only largely reduces Fe-TAML concentration in the liquid phase, it also facilitates RhB degradation in different ways: on one hand, it enriches RhB concentration at the liquid-solid interface which results as an immediate degradation upon peroxide addition; on the other hand, it enhances electron transfer and thus enables Fe(IV)TAML generation which is hardly detected in homogenous condition. In comparison, electrochemical activation enables Fe(IV)-TAML to degrade RhB at neutral pH rather than the basic pH used for peroxide activation. To better understand the behaviour of Fe-tpena in aqueous solution, this thesis examines a range of methods for its homogeneous activation including electrolysis, hydrogen peroxide, and hypochlorite; all of which have been found to show promise for contaminant degradation. At potentials higher than 1.3V (vs NHE), Fe-tpena can be activated to Fe(IV)-tpena at its natural pH of ~3; the resulting Fe(IV)-tpena solution is relatively stable and has shown oxidative activity toward various substances including RhB and formate. Activated by peroxide at circumneutral pH, Fe-tpena can catalyze RhB or formate degradation through generating both HO• and Fe(IV)-tpena. While HO• leads to significant destruction of the Fe-tpena complex, activation by hypochlorite leads to more extensive Fe(IV)-tpena generation and greater stability of the Fe-tpena complex.