Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Dr. Ajay K. Ray

2nd Supervisor

Dr. George Nakhla

Joint Supervisor

3rd Supervisor

Dr. Virender K. Sharma

Joint Supervisor

Abstract

Recently, the tetraoxy high-valent iron(VI), known as ferrate(VI) (Fe(VI); FeVIO42), received a great attention as a water-treatment chemical, because of its unique oxidation, disinfection, and coagulation properties. Though Fe(VI) has shown remarkable efficiency in oxidizing several pollutants in water, it has sluggish reactivity with some emerging organic contaminants, especially at basic pH conditions. Thus, the main objective of this PhD thesis was to activate or catalyze Fe(VI) oxidation reactions, at mild alkaline pH conditions, to enhance the oxidative transformation of organic pollutants and reduce the required dosage of Fe(VI) and contact time.

The activation of Fe(VI) by adding simple acids (HCl, HNO3, and CH3COOH) to the Fe(VI)-contaminant mixed solution in deionized water under slightly alkaline pH conditions was demonstrated for the first time. Acid activation of Fe(VI) resulted in increased oxidative transformation of caffeine (psychostimulant, CAF), acesulfame potassium (artificial sweetener), and atenolol (β-blocker) by ~30% within seconds to minutes (versus minutes to hours with non-activated Fe(VI)). A possible reason for the augmentation of the oxidative transformation of organics may be the increasing formation of reactive intermediate species, FeV/FeIV, in the Fe(VI)-contaminant-acid mixture.

Further experiments demonstrated that acid-activated Fe(VI) oxidizes CAF in water at three times lower molar ratio of Fe(VI) to CAF than oxidative transformation observed by non-activated Fe(VI) (8.0 versus 25.0). CAF oxidation by acid-activated Fe(VI) was not negatively affected by anions such as Cl-, HCO3-, and SO42-, and cations such as Na+ and Mg2+. Natural organic matter (NOM) and secondary effluent (SE) wastewater organics decreased the efficiency of CAF transformation. However, acid-activated Fe(VI) could mineralize other organics present in both NOM and SE as indicated by the dissolved organic carbon removal. Comparatively, no mineralization was seen without activation of Fe(VI). Four oxidized products of CAF were identified by the liquid chromatography high resolution mass spectrometry technique. The reaction pathways of the oxidation of CAF by activated Fe(VI) have been proposed.

Moreover, the employment of simple solid silica gel (SiO2) to remarkably enhance the oxidative transformation of CAF by Fe(VI) in water at mild alkaline pH conditions has also been demonstrated for the first time. Complete removal of CAF was achieved at Fe(VI) to CAF molar ratio of 6.0 in the presence of 4 g SiO2/L. In the presence of SiO2 gel, Na+, Mg2+, Ca2+, Cl-, HCO3-, and SO42- ions had no significant effect on the removal of CAF by Fe(VI). NOM decreased the removal of CAF by Fe(VI) in the presence of SiO2 gel. However, the influence of NOM on removal efficiency of CAF by Fe(VI) diminished by increasing the amount of SiO2 gel in water. Identification of three products of CAF by Fe(VI)-SiO2 system suggests a reaction pathway initiated by the attack on the C4=C5 double bond of CAF, which is supported by the frontier molecular orbitals calculations of CAF.

The findings of this work may spur further research on the impact of different activation methods and solid materials on the oxidation of a range of pollutants by Fe(VI).

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