The formation of cyanogen chloride in drinking water from a reaction pathway involving formaldehyde and monochloramine as initial reactants

Erik John Pedersen, Purdue University

Abstract

Stoichiometric amounts of cyanogen chloride (ClCN) are formed under typical drinking water conditions by a reaction pathway involving formaldehyde and monochloramine as initial reactants. Formaldehyde reacts rapidly and reversibly with monochloramine to yield N-chloroaminomethanol in the following manner: CH2O + NH2Cl [special characters omitted] CH2(OH)NHCl. This reaction was studied by stopped-flow photometry at 25°C and [special characters omitted] = 0.1 M (NaClO4). The equilibrium and second-order rate constants were found to be (6.6 ± 1.5) × 105 M −1 and (2.8 ± 0.1) × 104 M −1s−1, respectively. The reaction was assisted by HPO42−, with a third-order rate constant of (7.5 ± 0.6) × 105 M−2s −1. N-chloroaminomethanol undergoes a relatively slow decomposition, eventually producing ClCN, via the following pathway: CH2(OH)NHCl [special characters omitted] CH2NCl + H2O; CH2NCl [special characters omitted] HCN + HCl; CN− + NH2Cl [special characters omitted] ClCN. These reactions were investigated at 25°C and an ionic strength of 0.1 M (NaClO4) by UV-photometry. The dehydration of N-chloroaminomethanol was assisted by H+ and OH −; respective second-order rate constants were found to be 277 ± 7 M−1s−1 and 26.9 ± 5.6 M−1s−1. Under characteristic drinking water conditions, this reaction is the rate-limiting step. ClCN undergoes simultaneous base-assisted hydrolysis, according to the following reaction: ClCN + H2O [special characters omitted] HOCN + H+ + Cl−. ClCN formation, from the dehydration of N-chloroaminomethanol, and hydrolysis was studied under representative drinking water conditions: pH 8 and 25.0°C. The decay reactions of both N-chloroaminomethanol and ClCN were found to be assisted by NH3, with respective second-order rate constants of 1.08 ± 0.14 M−1s−1 and (9.55 ± 2.00) × 10−2 M −1s−1. An overall kinetic model was constructed, mainly from the work presented herein, and used to predict ClCN levels over a wide range of characteristic drinking water conditions. Simulations showed that variations in pH, monochloramine dosage, and initial formaldehyde concentration should significantly impact the amount of ClCN produced in chloraminated drinking water.

Degree

Ph.D.

Advisors

Jafvert, Purdue University.

Subject Area

Environmental engineering|Sanitation

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