The active species in drinking water chlorination: the case for Cl2O
A. LYNN ROBERTS
Chlorination has been the gold much more important role in the fortified with a small excess of Cl2 gassjust one route to FACsthe standard for disinfecting municichlorination of p-xylene than did reactivities of the resulting species pal water supplies since World HOCl, ” says Roberts, referring to toward a given molecular intruder War II, but chlorine’s success in a 1988 ES&T article (Environ. Sci. could be reliably calculated. But killing bacterial and viral threats Technol. DOI 10.1021/ that is just not the case. Digging has a downside. (Editor’s note: es00174a009). The neglect of Cl2O in the literature remains somethrough the literature, Sivey unsee this May 1, 2010 issue’s Feathing of a mystery to Roberts and earthed Cl2O, a species long ture by LaKind et al. for more inknown to be present in FAC, but her team. Reinhard, who is at formation on chlorine use for Stanford University, reports water disinfection (Environ. he is “happy to see our work Sci. Technol. DOI 10.1021/ ([performed] more than 20 es903241k)). Disinfection years ago) confirmed and byproducts of very dilute validated.” Reinhard agrees chemical contaminants are with Roberts’s questioning of an unfortunate consequence the widespread practice of of chlorine-based disinfeccomputing an “apparent” tion of drinking water. Free rate constant for FAC soluavailable chlorine (FAC) sotions, as well as her call for lutions are often treated as a caution in interpreting curblack box, with the active rent k values for HOCl. chlorinating agent near neuHowever, he adds that the tral pH usually modeled as a Postulated chlorination mechanism for the common conclusions “pertain mainly dilute solution of hypochlor- herbicide dimethenamid in drinking water disinfection. to relatively slow-reacting ous acid (HOCl). John Sivey, Roberts and co-workers implicate Cl2O as an important active species (in addition to HOCl and Cl2), with ClO substrates and less to fastCorey McCullough, and as the leaving group X-. reacting haloform precurLynn Roberts of the Johns sors, which were the focus of Hopkins University report in most of the chlorination rethis issue of ES&T (Environ. at such low concentrations that search done over the past 30 Sci. Technol. DOI 10.1021/ most researchers neglect itsor years.” Roberts is reluctant to es9038903) that chlorine solutions perhaps are not even aware of it. concur with this assessment withare not nearly that simple, with “If you wish to minimize disinout further investigation. chlorine monoxide (Cl2O) implifection byproducts, it helps to cated as an important active chloThe chlorination of organic mihave a thorough understanding of rinating agent under typical cropollutants in drinking water the fundamental reactions indrinking water and wastewater can confound toxicologists, given volved,” says Roberts. Sivey retreatment conditions. that one chemical released into calls the informal Zeroth Law of Drinking water supplies contain the environment might be repackKinetics; namely, “know thy reaca range of micropollutants, such aged as several others through tants.” as the herbicide dimethenamid, multiple possible chlorinations. The researchers added dimethe3.6 million kg of which was The transformation of dimethenanamid (g5 µM) to 25-mL FAC sospread on U.S. farmland in 2002. mid to chlorodimethenamid in lutions (g100 µM), carefully “Michelle Hladik, a previous Ph.D. chlorinated drinking water illusvarying factors, such as concenstudent in my group,” says Robtrates this point: the focus should tration, pH, buffer, chloride conerts, “found that in every case in not be on the parent contaminant centration, and temperature, to which dimethenamid was present alone. “Sometimes these transforfollow chlorination within a 1-h in source waters that subsemation products from organic window. The low concentration of quently underwent chlorination, contaminants are less toxic than O relative to HOCl was found Cl 2 its disappearance from the source the parent compounds, and to be offset by a much higher rate water coincided with the appearsometimes they are more toxic,” of reaction. “Their arguments are ance of chlorodimethenamid in says Susan Richardson of the U.S. convincing,” says Anett Georgi of the finished water.” Sivey and EPA. “As a result, it’s important to the Helmholtz Centre for EnvironMcCullough sought to characterunderstand the transformations mental Research (Germany). ize the chlorination reaction in that can occur upon chlorination, “Cl2O should be considered as a the lab, “but the model in which and the mechanisms and kinetics potential chlorinating agent for HOCl served as the sole active are an integral part of this.” organic compounds under typical constituent of FAC didn’t fit the —STEVEN C. POWELL conditions of drinking water and data,” Roberts explains. wastewater treatment with FAC.” Given the historical importance “Martin Reinhard and Evangeof municipal chlorination, the calos Voudrias elegantly demonsual observer might think that if a strated that Cl2 and Cl2O played a well-defined aqueous solution is
10.1021/es100800t
2010 American Chemical Society
Published on Web 03/22/2010
May 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3203
