Environ. Sci. Technol. 1999, 33, 367-368
Response to Comment on “Chlorination and Formation of Organoiodinated Compounds: The Important Role of Ammonia” SIR: We appreciate the opportunity to respond to the comments of Hatch regarding our paper (1). We are pleased that a specialist of halogen and halides chemistry and related analytical methods can help us discuss the mechanism which could account for our observations. Hatch suggests that hypoiodous acid (HOI) is the probable intermediate responsible for the formation of iodoform from the chloramination of natural waters in the presence of iodide ions and gives valuable arguments to support this proposal. In the case of chlorination of iodide-containing water in the presence of ammonia, the reactions proposed between inorganic chloramines and iodide ions or organic chloramines and iodine (reactions 1 and 2 of the comments) and the literature cited can indeed account for the observed iodoform. We found that iodoform was also produced in the presence of glycine or methylamine and iodide ions, for chlorine doses below 1 mol/mol for glycine and below 2 mol/mol for methylamine. The reactions of the N-chloroamino acid and the N-chloromethylamine with iodide ions to form HOI are expected. In his work, Hatch (2) presents substantial evidence that both iodide and iodine are oxidized to HOI by Nchlorosuccinimide. Considering the chlorination of iodide containing waters in the absence of ammonia or for chlorine doses above the breakpoint, Nagy et al. (3) showed that the overall reaction in pure water was rapid (kHOCl/I- ) 4.3 × 108 M-1 s-1) and involved a complex mechanism with many intermediates. The high concentration of HOCl relative to iodide under our experimental conditions may allow iodide ions oxidation not only into HOI but also into iodate ions and therefore prevent iodoform formation. A maximum of 215 µg I2/L is expected in the absence of NH4+ (Table 2). Moreover HOCl is a much stronger oxidant than HOI. It follows that the chlorination of the organic matter is favored compared with the reaction of HOI with organics. Oxidation of Water with Iodine. The rate of production of hypoiodous acid (HOI) from iodine (I2) (reaction 3 in the comments) is very rapid (kf ) 2.8 s-1; kb ) 4.4 × 1012 M-2 s-1; (4)). The overall equilibrium occurs via a complicated mechanism involving the probable intermediates I2OH- and H2OI+ (4-6). At pH 8, iodine is present principally as HOI under our experimental conditions. However, using iodine did not allow any significant production of iodoform for initial doses below 1 mg I2 L-1. It seems therefore that a minimal I2 dose is required for a significant iodoform production. The total amount of hypoiodous acid produced by the chloramine/I- system may be sufficient for this “iodine/HOI demand” considering the occurrence of induced chain reactions. Therefore HOI produced from the reaction of the 0.2 mg/L initial concentration of iodides with excess of monochloramine may be reduced into I- due to the “iodine demand”, and the resulting I- reoxidized to HOI until a sufficient amount is available to allow iodoform production. Here we understand the essential role played by the monochloramines for HOI regeneration. The production of iodoform during the chloramination of the water containing iodide ions was improved in the 10.1021/es982012e CCC: $18.00 Published on Web 12/05/1998
1999 American Chemical Society
presence of I2 under our experimental conditions (1). It seems that the Figures 5 and 6 in our article (1) require some further explanations. Iodine was introduced in the water after dilution of a commercial iodine solution containing iodide ions. For each initial concentration of iodine, the concentration of iodide ions was adjusted to the ratio 1.99 mg I-/mg total iodine as I2. For each dose of I2 in Figure 5 (second line of the abscissa) we allowed for the concentration of I- present simultaneously to I2 (this concentration is indicated in the first line of abscissa). It means that the dashed bars represent the iodoform produced from solutions containing both the concentration of I2 indicated on the second line of the abscissa plus I- in concentration mentioned on the first line of the abscissa. For example, a treatment of the water with 100 µg/L I2 corresponds to the simultaneous addition of 200 µg I-/L. Similarly in Figure 6 for the experiments with (I2 + I-) ) 200 µg/L, the concentration of I- was approximately twice the concentration of I2, i.e., the concentration of each species was respectively 130 µg I-/L and 70 µg I2/L. In Figure 5, in each case we have 50% more total “I” in the I2 + I- solution (dashed bars) than in just the I- solutions (white bars). It means that the highest production of CHI3 observed for 100 µg I2/L results from a total “I” of 300 µg/L, i.e., 100 µg/L of I2 plus 200 µg/L of I-. This total 300 µg/L “I” is 50% more “I” than in the 200 µg/L I- alone. For this example we can notice that we obtain 50% more CHI3 from I2 + I(300 µg/L I) than from I- ) 200 µg/L which strengthens the scheme suggested by Hatch in his comments (reactions 1 and 2). For I2 ) 50 µg/L (total “I” ) 150 µg/L) CHI3 is half of the production from I2 ) 100 µg/L (total “I” ) 300 µg/L). However, assuming this scheme, the production of CHI3 observed from 50 and 100 µg I-/L seems underestimated. Figure 6 poses also some problems. In these experiments, keeping the units of concentration in µg/L (whether I- or I2) as we did should lead to the same HOI (reactions 1 and 2 of the comments). As a consequence, the same CHI3 production should be obtained in the presence of I- and I2 with (I- + I2) ) 200 µg/L as when 200 µg I-/L is used. Moreover if chain reactions occur as predicted above, the I2/chloramine system is rapidly equivalent to the I-/chloramine one. The enhanced formation of CHI3 when I2 is present is unexpected with regards to the scheme proposed by Hatch. This enhancement would tend to confirm the data presented in Figure 5 for I) 100 µg/L and (I- + I2) ) 150 µg/L and reinforce our statement “iodine in solution induces a greater CHI3 formation during chloramination than equal content of iodide ions”. Nonetheless, remember that the raw water of Figure 6 (RWb) was different from Figure 5 (RWa), and in such complex medium one parameter may escape our argument. Obviously further experiments are necessary to clarify these results relative to I2 and chloramines. Except for this latter reservation, considering that a minimum iodine dose is required for the production of iodoform in the natural waters, the cyclic reactions induced between excess chloramine (from the chlorination of ammonium ions) and iodide ions with HOI regeneration are attractive and reinforce the assumption that HOI is the iodination agent responsible for the iodoform production. However, the mechanism may involve more intermediates than suggested. We would really like to thank Gary L. Hatch for his contribution in the better understanding of the role of chloramines in the enhancement of iodoform formation during chlorination of waters containing iodides. VOL. 33, NO. 2, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Literature Cited (1) Karpel Vel Leitner, N.; Vessella, J.; Dore, M.; Legube, B. Environ. Sci. Technol. 1998, 32, 1680-1685. (2) Hatch, G. L. Anal. Chem. 1984, 56, 2238-2241. (3) Nagy, J. C.; Kumar, K.; Margerum, D. W. Inorg. Chem. 1988, 27, 2773-2780. (4) Eigen, M.; Kustin K. J. Am. Chem. Soc. 1962, 84, 1355-1361.
Nathalie Karpel Vel Leitner,* Bernard Legube, and Johanne Vessella Laboratoire de Chimie de l’Eau et de l’Environnement UPRES A 6008, Ecole Supe´rieure d’Inge´nieurs de Poitiers 40, avenue du Recteur Pineau 86022 Poitiers Cedex France
(5) Palmer, D. A.; van Eldik R. Inorg. Chem. 1986, 25, 928-931. (6) Lengyel, I.; Epstein, I. R.; Kustin, K. Inorg. Chem. 1993, 32, 5880-5882.
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