Formation of Cyanogen Chloride from the Reaction of Monochloramine with Formaldehyde

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• 作者：Erik J. Pedersen ; III ; Edward T. Urbansky ; Benito J. Mariñ ; as ; and Dale W. Margerum
• 刊名：Environmental Science & Technology
• 出版年：1999
• 出版时间：December 1, 1999
• 年：1999
• 卷：33
• 期：23
• 页码：4239 - 4249
• 全文大小：201K
• 年卷期：v.33,no.23(December 1, 1999)
• ISSN：1520-5851

文摘

Methanediol dehydrates to give formaldehyde, whichreacts rapidly and reversibly with monochloramine toform N-chloroaminomethanol. Under drinking waterconditions, N-chloroaminomethanol undergoes a relativelyslow decomposition that eventually leads to the formationof cyanogen chloride (ClCN) in apparently stoichiometricamounts. The following reaction sequence is proposed:CH₂(OH)₂ mages/entities/lrarr2.gif"> CH₂O + H₂O; CH₂O + NH₂Cl mages/entities/lrarr2.gif"> CH₂(OH)NHCl;CH₂(OH)NHCl mages/entities/rarr.gif"> CH₂NCl + H₂O; CH₂NCl mages/entities/rarr.gif"> HCl + HCN;CN^- + NH₂Cl + H⁺ mages/entities/rarr.gif"> ClCN + NH₃. These reactions werestudied at 25.0 mages/entities/deg.gif">C and an ionic strength of 0.10 M (NaClO₄).Stopped-flow photometry was used to monitor rapid,reversible reactions, and photometry was used to studyrelatively slow decomposition reactions. Equilibrium and rateconstants for the addition of formaldehyde to monochloramine were (6.6 ± 1.5) × 10⁵ M^-1 and (2.8 ± 0.1) × 10⁴ M^-1s^-1, respectively. The dehydration of N-chloroaminomethanolwas catalyzed by both H⁺ and OH^-, with respective rateconstants of 277 ± 7 and 26.9 ± 5.6 M^-1 s^-1. Undercharacteristic drinking water conditions, the decay ofN-chloroaminomethanol is the rate-limiting step. N-Chloromethanimine, formed by the dehydration of N-chloroaminomethanol, had a decomposition rate constant of(6.65 ± 0.06) × 10^-4 s^-1. At the relatively high methanediolconcentrations used in this study, the intermediaryN-chlorodimethanolamine was formed by the rapid andreversible reaction of N-chloroaminomethanol withformaldehyde. N-Chlorodimethanolamine then decayedrelatively slowly. The following reaction sequence isproposed: CH₂(OH)NHCl + CH₂O mages/entities/lrarr2.gif"> {CH₂(OH)}₂NCl;{CH₂(OH)}₂NCl mages/entities/rarr.gif"> CH₂NCl + CH₂O + H₂O. The equilibriumand rate constants for the addition of formaldehyde toN-chloroaminomethanol were (9.5 ± 2.5) × 10⁴ M^-1 and(3.6 ± 0.1) × 10³ M^-1 s^-1, respectively. The decompositionof N-chlorodimethanolamine was catalyzed by OH^-, witha rate constant of 19.2 ± 3.7 M^-1 s^-1. N-Chlorodimethanolamine would not be present under typical drinking watertreatment conditions.