Conventional wisdom has it that HOCl serves as the principal ?active? chlorinating agent in solutions of free available chlorine (FAC), widely used to disinfect drinking water and wastewater. Likewise, HOBr is almost invariably believed to represent ?the? active brominating agent. Although other species (such as Cl2O, Cl2, BrCl, BrOCl, and Br2O) are known to occur as minor constituents of FAC, they are generally believed to be present at concentrations so low as to be negligible. This overlooks the fact that minor constituents can dominate reaction rates if their inherent reactivity outweighs their lesser abundance. Results recently published by the PI?s group demonstrate that several ?overlooked? halogenating agents may force revision of this paradigm. The PI hypothesizes they could play critical roles in the rate-limiting steps for ?slow? formation of the disinfection byproducts (DBPs) that predominate in distribution systems. The speciation of Cl2O, Cl2, BrCl, Br2O, and BrOCl differs from that of HOCl/OCl- or HOBr/OBr-. Their existence could have profound implications on our understanding of the fundamental processes that govern the rates of DBP generation. For example, [Cl2O] will be proportional to [HOCl]2. To the extent that Cl transfer from Cl2O to an organic compound factors into the rate of DBP formation, the kinetics will not be first order in [HOCl] (and, hence, in [FAC] at constant pH). This calls into question the utility of ?apparent? rate coefficients and intrinsic second-order rate coefficients previously reported in the literature. A dependence on [Cl2O] implies that chlorination rates of compounds that themselves lack acid-base speciation should exhibit a (-2)nd order dependence on [H+] at pH above the pKa of HOCl. Finally, the existence of BrCl implies that rates of bromination will increase with [Cl-], and the high reactivity of BrOCl implies that rates of bromination will increase with [FAC] at constant [Br-]. These results could force reconsideration of water treatment practices: the PI?s calculations indicate that the chloride content of FeCl3 commonly added as a coagulant could increase chlorination rates more than tenfold.
The research team intends to examine the kinetics of reaction(s) of an array of organic compounds, with an emphasis on species hypothesized to play a role in the rate of ?slow? DBP generation. They will carefully model the influence of solution conditions on the rate of parent compound loss and reaction product formation, including important DBPs. The suite of target compounds will also include partially halogenated species whose further halogenation has been proposed to represent rate-limiting steps in DBP formation. Finally, undergraduate and high school researchers, working in collaboration with a PhD student, will examine the influence of solution conditions ? especially [FAC] and [Cl-] as a function of pH ? on the rate of DBP formation on chlorination (or bromination) of natural organic matter. The results could assist in formulating improved recommendations for strategies to reduce DBP generation, thereby protecting public health.
Beyond the advancement of scientific discovery, graduate and undergraduate training, and public health aspects of this work, the project will enable K-12 outreach activities, and will broaden the participation of underrepresented groups. Specifically, this project will allow the PI to offer research internships (summer plus ensuing academic year) to students from Baltimore Polytechnic Institute, Baltimore?s magnet science, math, and engineering public high school. Such interns will work closely with undergraduate and PhD student participants, fostering in all participants an appreciation of diverse learning perspectives. Finally, the PI will continue her practice of engaging 5th-7th graders in environmental research by mentoring the research projects required as part of the FIRST Lego League (FLL) annual robotics challenge. Recent projects have included experiments at Johns Hopkins University related to carbon sequestration and research on self-composting toilets.