Sunlight Photolysis
Professor Rosario-Ortiz, CU - Roberto Rodriguez, UT Health

Research Approach

Task 1
Lab-scale evaluations of the production of reactive intermediates from water samples.

This initial task will focus on the fundamental study of the capacity of photochemical processes to result in inactivation of pathogens and modification to DOM that would impact DBP formation during subsequent chemical disinfection. Quarterly water samples from the two field sites will be collected for one year (the field sites will be located in Colorado and Puerto Rico). Initially, the samples will be characterized for the formation of ROS, which are important in the inactivation of pathogens. The formation of triplet state DOM will also be assessed as it is not clear whether this reactant has a significant effect on pathogen inactivation. A solar simulator will be used and the reactive intermediates will be quantified using chemical probes. Excited triplet state DOM formation will be estimated by measuring the degradation of 2,4,6-trimethylphenol, a probe that has been reported to efficiently react with such species (Canonica, Hellrung et al. 2006). The formation of HO• radicals, a highly reactive species influencing the indirect degradation of organic contaminants, will be measured by use of benzene as a probe while monitoring the reaction product (phenol). Singlet oxygen (1O2) will be monitored using furfuryl alcohol as a probe. Each chemical probe will be quantified (either degradation of formation of a byproduct) using methods developed in HPLC. The PI’s lab has had extensive experience conducting this type of analysis in DOM samples from surface and wastewater effluents (Dong and Rosario-Ortiz 2012; Lee, Glover et al. 2013; Mostafa and Rosario-Ortiz 2013). For each of these species, a steady-state concentration and quantum yield will be calculated. The PI has also worked on the development of empirical models to relate steady state  concentrations  of  these  reactive intermediates  to  easily  measurable  properties such as UV-Vis absorbance. Figure 4 shows an example of the ability to predict 1O2 concentrations based on optical parameters of the DOM. It is expected that this task will also allow the development of an empirical model relating the formation of ROS to spectral parameters (such as E2/E3, which represents the ratio of the absorbance of the DOM at 254 over 365 nm).


The previous analysis will be performed on each sample collected during the initial year from both sites. It is expected that differences in DOM composition and concentration will have an impact on yields or steady state concentrations of these reactive intermediates. The lab work will also evaluate  the  temperature dependence  of  the  formation  of  these  reactive intermediates to correspond to the temperature fluctuations that would be observed mostly in the Colorado site (temperatures in Puerto Rico are fairly stable during the year) and will establish also the dependence on the specific solar energy flux. This first task will be accomplished during the first year of the project. A second component to this task will be to evaluate how exposure to light would impact the reactivity of DOM to chlorine and the subsequent formation of DBPs. To evaluate this, quarterly samples will be taken from both sites and exposed to simulated sunlight for different times, including exposures in the order of twice as much as the retention times observed in the specific ponds. These samples will then be chlorinated using the uniform condition tests (UFC) (Summers, Hooper et al. 1996) and DBP yields will be obtained for THMs, HAAs and HANs.

Task 2
Lab-scale evaluations of the inactivation of pathogens via photochemical processes.

After the initial evaluation of the efficiency for the formation of reactive intermediates (Task 1), the work will focus on the inactivation of pathogens. Water samples from the two sites will be spiked with either E. coli or enterococci at various concentrations. These microbes have been chosen based on EPA’s recommendation for their use as indicator bacteria for fecal contamination in wastewater, physiological differences (E. coli is gram-negative while enterococci is gram-positive), and their relative ease of use. Samples will be exposed to simulated sunlight over a length of time that will be determined based on the degree of inactivation observed in order to reach at least 2-log inactivation. In order to investigate the role of photosensitizers on bacterial inactivation compared to direct irradiation, exposure experiments will be conducted in the presence and absence of external photosensitizers (i.e., DOM from the study sites). Filtered and unfiltered samples will be measured as well in order to understand the role of particles in these processes. These experiments will be conducted in a stirred batch system under direct irradiance as they are commonly carried out. Quantification of the inactivation achieved should allow for the estimation of fractional contribution of direct vs. indirect (as well as particle-associated vs suspended) photo inactivation at various depths. This task will last one year and will be conducted during the second year of the project.

Task 3
Field scale evaluation of efficacy of photochemical processes for pathogen inactivation.

After the initial evaluation of the efficiency for the formation of reactive intermediates (Task 1) and their effect on inactivation of pathogens (Task 2), the team will conduct a field-scale evaluation of this process. As stated before, two sites (one in CO and one in PR) will be evaluated for a total of six months (see support letters). The field sites will allow the evaluation of the effect of photochemical processes on the inactivation of key microorganisms and also examine the effect on DBP precursors. This task will allow the team to develop design criteria for the effective use of these ponds for different small systems.