Heterogeneous Photocatalysis For The Treatment Of Contaminants Of Emerging Concern In Water

The simultaneous degradation of five organic contaminants: 1,4 dioxane, n-nitrosodimethylamine, tris-2-chloroethyl phosphate, gemfibrozil, and 17β estradiol, was investigated using a 1 L batch water-jacketed UV photoreactor utilizing titanium dioxide (TiO2) nanoparticles (Degussa P-25) as a photocatalyst. The primary objectives of this research were: (1) to experimentally assess the feasibility of heterogeneous photocatalysis as a promising alternative for the degradation of organic compounds in water; and (2) to model the chemical reactions by the application of two different approaches based on adsorption – surface reactions (Langmuir–Hinshelwood) and its simplification to a first order rate reaction. These objectives were motivated by the lack of information regarding simultaneous degradation of organic compounds in different categories as found in real aqueous matrices, and generation of specific intermediates that could eventually represent a potential risk to the environment. Contaminants were chosen based on their occurrence in water sources, their representativeness of individual sub-categories, and their importance as part of the CCL3 as potential contaminants to be regulated. Contaminant degradation was evaluated over time, and the TiO2 concentration and solution pH were varied under constant UV irradiation, oxygen delivery rate, mixing gradient, and temperature. 

 Specific accomplishments of this study were: (1) reaction kinetics data were obtained from the UV/TiO2 experiments and showed the potential that this UV/TiO2 process has for effectively removing different types of organic compounds from water; (2) a good fit was obtained between photocatalytic reaction kinetics models and the contaminant data using pseudo first-order and Langmuir-Hinshelwood (L-H) models; (3) results of the analytical methods developed in this study were validated by measurements performed by a certified laboratory; (4) the reaction kinetic parameters obtained in this study were normalized to electrical energy per order, reactor volume and surface area of the photocatalyst in order to provide rate constants with wider applicability for scale-up to more complex systems; and (5) degradation intermediates from the oxidation process and from interaction among compounds were identified and possible pathways for their formation suggested. This research has provided a better understanding of the photocatalytic process for the removal of organic contaminants from complex aqueous matrices

Degradation mechanism of tris(2-chloroethyl) phosphate (TCEP) as an emerging contaminant in advanced oxidation processes: A DFT modelling approach

As a typical toxic organophosphate and emerging contaminant, tris(2-chloroethyl) phosphate (TCEP) is resistant to conventional water treatment processes. Studies on advanced oxidation processes (AOPs) to degrade TCEP have received increasing attention, but the detailed mechanism is not yet fully understood. This study investigated the mechanistic details of TCEP degradation promoted by ·OH using the density functional theory (DFT) method. Our results demonstrated that in the initial step, energy barriers of the hydrogen abstraction pathways were no more than 7 kcal/mol. Cleavage of the P-O or C-Cl bond was verified to be possible to occur, whilst the C-O or C-C cleavage had to overcome an energy barrier above 50 kcal/mol, which was too high for mild experimental conditions. The bond dissociation energy (BDE) combined with the distortion/interaction energy (DIE) analysis disclosed origin of the various reactivities of each site of TCEP. The systematic calculations on the transformation of products generated in the initial step showed remarkable exothermic property. The systematic calculations on the transformation of products generated in the initial step showed remarkable exothermic property. The novel information at molecular level provides insight on how these products are generated and offers valuable theoretical guidance to help develop more effective AOPs to degrade TCEP or other emerging environmental contaminant