On the Mechanism and Quantitative Toxicity Evolution of Catalytic TAML and NewTAML Hydrogen Peroxide Oxidative Destruction of Micropollutants in River Water and Municipal Wastewater
thesisposted on 13.04.2020, 15:33 by Yogesh SomasundarYogesh Somasundar
Anthropogenic activities over the centuries have led to depletion of Earth’s natural resources and damage to ecology at alarming magnitudes and accelerating rates. Contamination of freshwater is a major component of the ecological injuries which is the most economically accessible point for transformative cleanup to support the survival of mankind and other species. In addition to preventive efforts to conserve water bodies, the world is turning towards replenishing and reusing water through wastewater treatment. Unfortunately, conventional wastewater treatments are not designed to remove micropollutants (MPs) – substances that produce undesired effects, including endocrine disruption, in organisms at low doses (or low concentrations in water for aquatic life), typically parts per trillion (ng/L) – low parts per billion (μg/L) concentrations. Advanced oxidation processes that are capable of removing MPs are very expensive or work at acidic conditions which make them unsuitable for economical wastewater treatment applications. This thesis is an effort towards advancing water purification and reclamation, including real wastewater studies, utilizing sustainably designed catalysts, viz., TAMLs and NewTAMLs, that mimic the chemistries of oxidizing enzymes. Tetraamido macrocyclic ligand (TAML) activators are mechanistically faithful, small molecule replicas of peroxidase and cytochrome P450 enzymes. They are capable of oxidizing numerous MPs in conjunction with hydrogen peroxide applications conducted at nM catalyst concentrations. Over 25 years TAMLs have been iteratively improved to surpass TAMLs with NewTAMLs as by far and away the best peroxidase mimics that are non-halogenated, exhibit superior reactivities and longer lifetimes. A tunable kill switch introduced in NewTAMLs helps to control the lifetimes of the catalysts, including allowing quicker or much slower degradations.
Propranolol is heavily prescribed β blocker drug and persistent MP. It is used herein as a model micropollutant for characterizing the reactivities of various TAMLs and NewTAMLs. At 11.2 ppm hydrogen peroxide, 100nM of the more reactive TAMLs and NewTAMLs are capable of oxidizing 15 ppb propranolol to mineralization in 5 and 30 minutes in buffered water and river water, respectively. The best performing NewTAML with a partially muted kill switch (work is proceeding in the IGS to turn it off completely) provided similar performances to the previous best TAML, but at 1/10th the concentration (NewTAML 10 nM, TAML 100 nM). A set of 38 MPs were evaluated simultaneously in real wastewaters of Tucson, AZ with 5 treatments of NewTAML/H2O2 followed temporally for 6 hours. 4 treatments of ozone viz., 2, 4, 6, and 8 ppm were analyzed after 72 h of ozone dosage (not effective contact time) to compare the NewTAML and ozone treatments. Detailed kinetic analyses of the 38 MPs revealed that (i) most of the reaction is completed within the first 30-60 min for NewTAMLs after which the catalyst is inactivated. (ii) The best performing 200 nM NewTAML/22.4 ppm H2O2 outperformed across the board compared to 2 ppm ozone, the current standard dose used in Neugut Plant in Dübendorf Switzerland. From the performance of NewTAML/H2O2 described in this work, it can be projected that 70 nM of NewTAML and 15 ppm of H2O2 can effectively treat 22,500 tonnes of wastewater, the daily amount produced by 150,000 Europeans.
Digging deeper into the kinetic and mechanistic aspects of propranolol oxidation helped us to discover and characterize substrate inhibition in TAMLs using UV-vis, fluorescence, MS, NMR and DFT calculations. Additionally, propranolol-TAML associates were isolated for the very first time. It was determined that substrate inhibition is not a significant factor at ppt-ppb of substrates which is the typically identified concentrations in wastewaters. Oxidation products were identified, isolated where possible and kinetically characterized at each step of oxidation process leading to minerals. Kinetic rate constants at each step of the oxidation was utilized to build theoretical simulation profiles which agreed with experimental composition profiles. Mass (%-composition of intermediates) and toxicity profiles for the overall degradation helped provide a comprehensive view of the sustainable oxidation process. Redox potentials of various TAMLs and NewTAMLs have been correlated to their reactivities and substrate interactions using Linear Free Energy Relationships (LFER). To conserve time for rapid evaluation of decontamination of various colorless MPs, an alternative kinetic model based on parallel or competitive reactions has been developed to follow the reaction progress of these colorless MPs using UV-Vis spectroscopy with phenol and propranolol as the model colorless substrates. In totality, this thesis serves to (i) advance the mechanistic understanding of TAML catalyzed oxidations, (ii) provide a holistic approach to decontamination processes – mass/toxicity versus time profiles should be considered together, to provide more complete information on potential environmental effects of decontamination procedures, (iii) provide extensive data supporting the applicability of NewTAML/ H2O2 as a viable wastewater treatment technology with experiments on complex real waters including lab water, river water and wastewater.
- Doctor of Philosophy (PhD)