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MONITORING THE SPECIATION OF AQUEOUS FREE CHLORINE FROM PH 1-12 WITH RAMAN SPECTROSCOPY TO DETERMINE THE IDENTITY OF THE POTENT LOW-PH OXIDANT
Citation:
CHERNEY, D., S. E. DUIRK, J. C. TARR, AND T. W. COLLETTE. MONITORING THE SPECIATION OF AQUEOUS FREE CHLORINE FROM PH 1-12 WITH RAMAN SPECTROSCOPY TO DETERMINE THE IDENTITY OF THE POTENT LOW-PH OXIDANT. APPLIED SPECTROSCOPY. Society for Applied Spectroscopy, 60(7):764-772, (2006).
Impact/Purpose:
Conditions for treatment of DW vary widely. However, most all processes involve some form of conventional treatment (filtration, etc.), and some form of disinfection. Also, systems sometimes use various other treatments, including softening by the addition of a base. Treatment processes can have profound effects on the pesticides and toxics that occur in DW sources. For example, hydrophobic chemicals may be partially removed by conventional treatment, however, percent removal can vary significantly depending on conditions. On the other hand, conventional treatment generally has little or no effect on hydrophilic chemicals.
If pollutants are not removed by conventional treatment, they may be altered by other treatment processes. For example, disinfection can transform some chemicals via oxidation; however, little is known about the identity of products formed by this process. Limited information shows that disinfection can yield products that are more toxic than the parent. Also, some chemicals are transformed via base-catalyzed hydrolysis during the softening process. The nature and extent of transformations vary greatly depending on treatment conditions.
EPA Program Offices recognize that treatment often has a large effect on pesticides and toxics that occur in DW sources; and they have articulated a need to incorporate these effects into risk assessments. This task will provide regulators with methods, tools, and databases to forecast the fate of pesticides and toxics during DW treatment. The early task outputs will be chemical-specific information from bench-scale studies that simulate disinfection and softening. However, all task efforts will be focused on the long-range goal of providing predictive models for chemical removal and transformation that cross chemical class and treatment conditions. Early experiments will provide information to elucidate transformation mechanisms. Next, we will investigate effects of varying treatment conditions and chemical speciation. This strategy will lead to broadly applicable tools for forecasting fate for a wide range of chemicals. Finally, we envision that the output of our predictive fate tools will be used as input into models developed under the ORD Computational Toxicology Initiative. In this fashion, the final contaminants and concentrations predicted by our models to occur in finished DW can then be considered for toxic potential. This will provide Program Offices with an integrated system for risk assessment and management for the pesticides and toxics in drinking water.
Description:
The speciation of aqueous free chlorine above pH 5 is a well-understood equilibrium of H2O + HOCl (equilibrium) OCl- + H3O+ with a pKa of 7.5. However, the identity of another very potent oxidant present at low pH (below 5) has been attributed by some researchers to Cl2 (aq), and by others to H2OCl+. We have conducted a series of experiments designed to ascertain which of these two species is correct. First, using Raman spectroscopy, we found that an equilibrium of H2O + H2OCl+ (equilibrium) HOCl + H3O+ is unlikely because the "apparent pKa" increases monotonically from 1.25 to 2.11 as the analytical concentration is increased from 6.6 to 26.2 mM. Second, we found that significantly reducing the chloride ion concentration changed the Raman spectrum and also dramatically reduced the oxidation potency of the low-pH solution (as compared to solutions at the same pH that contained equimolar concentrations of Cl- and HOCl). The chloride ion concentration was not expected to impact an equilibrium of H2O + H2OCl+ (equilibrium) HOCl + H3O+, if it existed. These observations supported the following equilibrium as pH is decreased: Cl2 (aq) + 2H2O (equilibrium) HOCl + Cl- + H3O+. The concentration-based equilibrium constant was estimated to be approximately 2.56 x 10(-4) M2 in solutions whose ionic strengths were ~0.01 M. The oxidative potency of the species in low pH solutions was investigated by monitoring the oxidation of secondary alcohols to ketones. These and other results reported here argue strongly that Cl2 (aq) is the correct form of the potent low-pH oxidant in aqueous free-chlorine solutions.