School of Civil and Environmental Engineering

Ph.D. Thesis Defense Announcement

Organic Contaminants Destruction Using UV/Free Chlorine Process: Mechanisms and Modeling

By

Weiqiu Zhang

Advisor:

Dr. John Crittenden (CEE)

Committee Members:

Dr. Yongsheng Chen (CEE), Dr. Ching-Hua Huang (CEE), Dr. Sotira Yiacoumi (CEE), Dr. Donggang Yao (MSE)

Date & Time: December 6th, 2019 at 10:00 AM

Location: Capstone Building, Room338C, 828 West Peachtree St

Advanced oxidation processes (AOPs) are effective technologies to oxidize recalcitrant organic contaminants in aqueous
phase. The UV/free chlorine process is a promising AOP because of generating various reactive radicals at room temperature
and pressure. These electrophilic radicals eventually mineralize organic contaminants into CO2 and H2O. Understanding the
degradation mechanisms is critical to design the UV/free chlorine process with the lowest energy consumption and greatest
toxicity reduction. Many researches have conducted experiments to shed the light on the degradation of some selected
organic compounds. However, these experimental studies are very time consuming and expensive. With respect to developing
kinetic models that can simulate the oxidation mechanisms, most studies invoked the simplified pseudo steady state
assumption because the mechanistically complex radicals-initiated chain reactions. Accordingly, conducting experiments and
developing simplified kinetic models would be impossible to fully elucidate the oxidation mechanisms of all organic
contaminants that may be found in aqueous phase.
To overcome the above-mentioned challenges, we developed a first-principles based kinetic model to simulate/predict the
oxidation mechanisms of various organic compounds in the UV/free chlorine process. First, we collected photolysis and
chemical reactions regarding the target organic compounds oxidation from literature. Second, we developed a rate constants
estimator to predict the rarely reported rate constants (e.g. organic compounds react with radicals) using group contribution
method or fitting experimental data through genetic algorithm. Third, we developed a stiff ordinary differential equations
solver using Gear's algorithm to predict the time-dependent concentration profiles of various target organic
compounds. Our prediction results agreed with experimental data under various operational and water matrix conditions.
After verifying our kinetic model: (1) we developed quantitative structure activity relationships using Hammett constants of
organic compounds and our predicted rate constants; (2) we determined that chlorine monoxide radicals was the dominant
radicals to oxidize organic contaminants; (3) we optimized the operational conditions (i.e. UV intensity and free chlorine
dosage) that resulted in the lowest energy consumption. Furthermore, based on the predefined reaction rules, we successfully
implemented graph theory to develop a computerized pathway generator specific for the UV/free chlorine process. The
pathway generator can automatically predict all possible reactions and byproducts/intermediates (e.g. degradation of
trichloroethylene generated 6,608 reactions). Therefore, our fundamental understanding about the detailed degradation
mechanisms can be significantly improved. However, we have noticed that it is difficult to estimate the rate constants of all
possible involved reactions at current stage, because we have very limited experimental data. Consequently, future work will
mainly focus on developing new methods (e.g. quantum chemistry) to estimate the rate constants of all possible involved
reactions. Then we can predict the time-dependent concentration profiles of byproducts and toxicity profiles of the system.
Finally, we investigated the disinfection byproducts formation potentials (DBPFPs) in the UV/free chlorine process. We
determined the controlling factor for the design of this process was organic contaminant destruction rather than DBPFPs
reduction. Overall, our study can be used to design the most cost-effective UV/free chlorine process for practical application