1999 Progress Report: Use of Differential Spectroscopy to Probe Reactions between Natural Organic Matter and Chlorinated OxidantsEPA Grant Number: R826645
Title: Use of Differential Spectroscopy to Probe Reactions between Natural Organic Matter and Chlorinated Oxidants
Investigators: Benjamin, Mark M. , Korshin, Gregory V.
Institution: University of Washington - Seattle
EPA Project Officer: Lasat, Mitch
Project Period: August 1, 1998 through July 31, 2001 (Extended to February 28, 2002)
Project Period Covered by this Report: August 1, 1998 through July 31, 1999
Project Amount: $374,401
RFA: Exploratory Research - Environmental Chemistry (1998) RFA Text | Recipients Lists
Research Category: Sustainability , Land and Waste Management , Air , Engineering and Environmental Chemistry
Objective:The project objectives are to gather information about changes in the absorbance of natural organic matter (NOM) when it is subjected to various chemical regimes (chlorination, pH adjustment, high temperature), and to use that information to evaluate reaction pathways for the formation of halogenated disinfection by-products (DBPs) that form when drinking water is chlorinated.
The experiments conducted during the first year of the project focused on the use of a stopped flow reactor (SFR) to investigate the reactions of a pure, model compound with chlorine under conditions representative of those extant in drinking water treatment systems. The model compound used in these studies was 2,4-dihydroxybenzoic acid (DHBA). This compound consists of a highly activated aromatic ring that reacts rapidly with Afree chlorine@ (HOCl and OCl&) and that, at high chlorine doses, is known to have a high yield of chloroform. In these ways, and in its strong ability to absorb ultraviolet (UV) light, it is similar to the much more complex molecules that are present in natural water sources.
In the past, the speed with which DHBA and free Cl react has precluded characterization of the reaction kinetics (they are characterized as Ainstantaneous@) or of the reaction pathway. Using the SFR, we have been able to propose reasonable reaction pathways and characterize the corresponding rate constants. Specifically, we focused on changes in two peaks in the absorbance spectrum of DHBA when it is mixed with HOCl at near neutral pH. One peak, at a wavelength of 300 nm, decreased by approximately 35 percent within 5 seconds when ~70 mM DHBA was mixed with 140 mM HOCl. Thereafter, the absorbance at that wavelength began again, for at least the next several minutes. By contrast, at a wavelength of 525 nm, the absorbance peak first grew and then decayed over the same time period.
The data were interpreted as signifying the appearance and subsequent destruction of one or more intermediate species, ultimately yielding a relatively stable product. Using the same concentration of DHBA but three different concentrations of HOCl, a set of three intermediate species and one final product was hypothesized, each of which undergoes a reaction with HOCl that is first order with respect to the concentration of the species itself and that of HOCl. The rate constants for these hypothesized reactions were evaluated.
This research project was undertaken based on the premise that a key to understanding the formation of chlorinated disinfection by-products (DBPs) in drinking water systems, and perhaps to interfering with that process, is characterizing the dominant reaction pathways, intermediates, and kinetics. In such systems, the reactive organic matter is NOM, which is a complex composite of many different types of molecules. Because the analytical approach we are using is novel, it was important to assess its technical merits and limitations in simpler model systems. The results to date support the use of our approach and give us confidence that it can be applied in real systems. Although the results to date are not in themselves of great practical significance, they represent an important step forward toward the accomplishment of our ultimate goal. Therefore, we believe that the prognosis is very good for success in achieving that goal in the second and third years of the project.