Reviews - Mechanistic Decay - Chemical Studies - Unknown Intermediate - Empirical Demand

- Pipe Walls - Metal Reactions - Microbial Reactions - Other

(See also: Nitrification in Distribution Systems)


Major Reports & Review Papers on Chloramine Models
Citation Notes Abstract
Wahman Simulator
on-line model

Model implementation from Jafvert & Valentine (Environ. Sci. Technol., 1992, 26 (3), pp 577-586) and Vikesland et al. (Water Res., 2001, 35 (7), pp 1766-1776). The provided application generates two side-by-side breakpoint curves (A and B) for comparison purposes with user defined conditions. Because several simulations are conducted to generate a breakpoint curve, simulation updates may take approximately a minute to complete.


Mechanistic Decay Models
Citation Notes Abstract
Wahman, D. G. and G. E. Speitel (2012). "Relative Importance of Nitrite Oxidation by Hypochlorous Acid under Chloramination Conditions." Environmental Science & Technology 46(11): 6056-6064.
  Nitrification can occur in water distribution systems where chloramines are used as the disinfectant. The resulting product, nitrite, can be oxidized by monochloramine and hypochlorous acid (HOCl), potentially leading to rapid monochloramine loss. This research characterizes the importance of the HOCl reaction, which has typically been ignored because of HOCl's low concentration. Also, the general acid-assisted rate constants for carbonic acid and bicarbonate ion were estimated for the monochloramine reaction. The nitrite oxidation reactions were incorporated into a widely accepted chloramine autodecomposition model, providing a comprehensive model that was implemented in AQUASIM. Batch kinetic experiments were conducted to evaluate the significance of the HOCl reaction and to estimate carbonate buffer rate constants for the monochloramine reaction. The experimental data and model simulations indicated that HOCl may be responsible for up to 60% of the nitrite oxidation, and that the relative importance of the HOCl reaction for typical chloramination conditions peaks between pH 7.5 and 8.5, generally increasing with (1) decreasing nitrite concentration, (2) increasing chlorine to nitrogen mass ratio, and (3) decreasing monochloramine concentration. Therefore, nitrite's reaction with HOCl may be important during chloramination and should be included in water quality models to simulate nitrite and monochloramine's fate.
Vikesland, P. J., K. Ozekin, et al. (2001). "Monochloramine decay in model and distribution system waters." Water Research 35(7): 1766-1776.
updates MCA 2nd order rcn for carbonates Chloramines have long been used to provide a disinfecting residual in distribution systems where it is difficult to maintain a free chlorine residual or where disinfection by-product (DBP) formation is of concern. While chloramines are generally considered less reactive than free chlorine, they are inherently unstable even in the absence of reactive substances. These reactions, often referred to as "auto-decomposition", always occur and hence define the maximum stability of monochloramine in water. The effect of additional reactive material must be measured relative to this basic loss process. A thorough understanding of the auto-decomposition reactions is fundamental to the development of mechanisms that account for reactions with additional substances and to the ultimate formation of DBPs. A kinetic model describing auto-decomposition was recently developed. This model is based on studies of isolated individual reactions and on observations of the reactive ammonia-chlorine system as a whole. The work presented here validates and extends this model for use in waters typical of those encountered in distribution systems and under realistic chloramination conditions. The effect of carbonate and temperature on auto-decomposition is discussed. The influence of bromide and nitrite at representative monochloramine concentrations is also examined, and additional reactions to account for their influence on monochloramine decay are presented to demonstrate the ability of the model to incorporate inorganic demand pathways that occur parallel to auto-decomposition.
Jafvert, C. T. and R. L. Valentine (1992). "Reaction Scheme for the Chlorination of Ammoniacal Water." Environmental Science & Technology 26(3): 577-586.
Basic formation & decomp rate expressions A kinetic model of the reacting aqueous chlorine-ammonia system is proposed which describes equally well the rapid "breakpoint" oxidation of ammonia, where the applied chlorine dose (Cl2) to ammonia-nitrogen molar ratio (Cl/N) is greater than approximately 1.6; the slow oxidation of ammonia in aqueous chloramine solutions (Cl/N < 1); and the transition region of 1 < Cl/N < 1.6, where rapid initial decay results in chloramine species residuals. Calculated time-dependent concentrations of the chlorine species, determined by numerical solution of the rate expressions, compare favorably to measured values, determined during experiments performed over ranges of initial pH (6-8) and Cl/N (0.25-2.0) conditions. The experimentally measured species include free chlorine (HOCl + OCl-), monochloramine (NH2Cl), and dichloramine (NHCl2). In addition, the model appropriately considers the catalysis of certain key reactions by several commonly encountered inorganics, such as bicarbonate and phosphate species.
Wei, I. W.-T. (1972). Chlorine-Ammonia Breakpoint Reactions: Kinetics and Mechanism. Applied Science. Cambridge, Harvard University. PhD: 161.    


Focused Chemical Studies - Non-metals
Citation Notes Abstract
McKay, G., B. Sjelin, et al. (2013). "Kinetic study of the reactions between chloramine disinfectants and hydrogen peroxide: Temperature dependence and reaction mechanism." Chemosphere 92(11): 1417-1422.
  The temperature-dependent kinetics for the reaction between hydrogen peroxide and chloramine water disinfectants (NH2Cl, NHCl2, and NCl3) have been determined using stopped flow-UV/Vis spectrophotometry. Rate constants for the mono- and dichloramine-peroxide reaction were on the order of 10-2 M-1 s(-1) and 10(-5) M-1 s(-1), respectively. The reaction of trichloramine with peroxide was negligibly slow compared to its thermal and photolytically-induced decomposition. Arrhenius expressions of ln(KH202-NH2Cl) = (17.3 +/- 1.5)-(51 500 +/- 3700)/RT and ln(k(H202-NHCl2)) = (18.2 +/- 1.9)-(75800 +/- 5100)/RT were obtained for the mono- and dichloramine peroxide reaction over the temperature ranges 11.4-37.9 and 35.0-55.0 degrees C, respectively. Both monochloramine and hydrogen peroxide were first-order in the rate-limiting kinetic step and concomitant measurements made using a chloride ion selective electrode showed that the chloride was produced quantitatively. These data will aid water utilities in predicting chloramine concentrations (and thus disinfection potential) throughout the water distribution system.
Rayson, M. S., M. Altarawneh, et al. (2010). "Theoretical Study of the Ammonia-Hypochlorous Acid Reaction Mechanism." Journal of Physical Chemistry A 114(7): 2597-2606.
  A mechanism for the oxidation of ammonia by hypochlorous acid to form nitrogen gas has been developed at the B3LYP/6-31G(d,p) level of theory using the Gaussian 03 software package. The formation of NH(2)Cl, NHCl(2), and NCl(3) was studied in the gas phase, with explicit water molecules included to examine the transition state energy in aqueous Solution. The inclusion of explicit water molecules in the transition state dramatically reduced the reaction barrier in reactions involving transfer of a hydrogen atom between molecules, effects that were not taken into account through use of a solvation model alone. Three mechanisms were identified for the decomposition of chloramine species to form N(2), involving the combination of two chloramine species to form hydrazine, dichlorohydrazine and tetrachlorohydrazine intermediates. The highest barrier in each pathway was found to be the formation of the hydrazine derivative.
Andres, J., M. Canle, et al. (2001). "A B3LYP/6-31G** study on the chlorination of ammonia by hypochlorous acid." Chemical Physics Letters 342(3-4): 405-410.   B3LYP/6-31G** calculations were performed on the chlorination of NH3 by HOCl, considering explicit participation of zero, one, two, three and four water molecules. Detailed analysis of the free energy profiles shows the mechanism is water-assisted. Cl transfer from the O of HOCl to the N of NH3 is accompanied by proton transfers along a chain of hydrogen-bonded water molecules, Preferential stabilisation of the transition structure arises from cooperative fluctuations involving the hydrogen-bonded chain of water molecules. Calculations show solvent reorganization associated with proton translocations is coupled with Cl transfer in the vicinity of the transition structure.
Jafvert, C. T. and R. L. Valentine (1987). "DICHLORAMINE DECOMPOSITION IN THE PRESENCE OF EXCESS AMMONIA." Water Research 21(8): 967-973.    
Morris, J. C. and R. A. Isaac (1981). A Critical Review of Kinetic and Thermodynamic Constants for the Aqueous Chlorine-Ammonia System. Water Chlorination : Environmental Impact and Health Effects. Volume 4, Book 1, 49-62.
Valentine, R. L. and R. E. Selleck (1983). Effect of Bromide and Nitrite on the Degradation of Monochloramine. Water Chlorination : Environmental Impact and Health Effects. Volume 4, Book 1, 125-137    
Weil, I. and J. C. Morris (1949). "EQUILIBRIUM STUDIES ON N-CHLORO COMPOUNDS .2. THE BASE STRENGTH OF N-CHLORO DIALKYLAMINES AND OF MONOCHLORAMINE." Journal of the American Chemical Society 71(9): 3123-3126.    

Unknown Intermedates
Citation Notes Abstract

Leung, S. W. and R. L. Valentine (1994). "An Unidentified Chloramine Decomposition Product - II. A Proposed Formation Mechanism." Water Research 28(6): 1485-1495.

  This study proposed an empirical rate expression for the formation of an unidentified product generated in chloramine decomposition; the rate expression was incorporated into an existing model describing the fate and speciation in the chlorine-ammonia system. Specific rate constants were estimated based on experimental data and trial-and-errors computing technique. A simple mechanism was proposed for formation of the unidentified product. This mechanism hypothesized that an intermediate species was formed in the reactions of chloramine decomposition. The intermediate species was assumed reacting with nitrite to form the unidentified product, it also might react with HOCl to form other end-product(s). Concentration of the intermediate species and nitrite formed were estimated by pseudo steady-state assumption in the chlorine-ammonia system.
Leung, S. W. and R. L. Valentine (1994). "An Unidentified Chloramine Decomposition Product - I. Chemistry and Characteristics." Water Research 28(6): 1475-1483.   This paper reports on a study of the chemistry and structural characteristics of an unidentified product found in organic free aqueous solution in the chloramination disinfection of drinking water. Mass balance on decomposing chloramine solutions were made using spectrophotometric, titrimetric and ion chromatographic methods in the presence and absence of added nitrite. The concentration of the unidentified product was determined from the difference of the measured and predicted chloramine spectra. Nitrite is believed to have an important role in the formation of the unidentified product. Added nitrite can dramatically increase the concentration of this product, and it is believed that nitrite may be involved in its formation during slow chloramine decomposition. Nitrite is not expected to be stable in the presence of chloramines as predicted from prior studies. Nitrite was observed, however, in rapidly decomposing solutions of dichloramine. Nitrate, chloride and nitrite were observed in photolysis of the unidentified product. The unidentified product contains both nitrogen and chlorine, and has an estimated molar absorptivity of 5000 M(-1) cm(-1) from the mass balance of chloride if one chlorine per molecule is assumed. Nitrate was also observed along with the unidentified product in slow chloramine decomposition at near or above neutral pH; however, the unidentified product was not detected in chloramine solutions at pH 3.5, but nitrite was observed. The formation of the unidentified product is not believed to be acid or base catalyzed, but is proportional to the total oxidant lost in slow monochloramine decomposition. Monochloramine is not likely involved in the rate-limiting step in the formation of the unidentified product.



Empirical Decay and Organic Demand
Citation Notes Abstract

Fisher, I., G. Kastl, and A. Sathasivan. 2012. A suitable model of combined effects of temperature and initial condition on chlorine bulk decay in water distribution systems. Water Research 46:3293-3303.

fitting & testing 2 site decay models for bulk solution reaction Maintaining a chlorine residual is a major disinfection goal in many water distribution systems. A suitable general model of chlorine decay in the transported bulk water is an essential component for efficiently modelling chlorine concentration in distribution systems. The two-reactant model meets basic suitability criteria, including accurate prediction of chlorine residual over hundreds of hours, commencing with chlorine concentration 0-4 mg/L. This model was augmented with an equation that increases the decay coefficients with temperature according to Arrhenius theory. The augmented model was calibrated against decay-test data sets to obtain a single invariant set of parameters for each water. Model estimates of chlorine residuals over time closely matched decay-test data, over the usual operating ranges of initial chlorine concentration (1-4 mg/L) and temperature (3.5-28 degrees C). When the augmented model was fitted to partial data sets, it also predicted the data reserved for validation very well, suggesting that this model can accurately predict the combined effect of initial chlorine concentration and temperature on chlorine bulk decay in distribution systems, using a single set of invariant parameters for a given source water.



Reactions with Pipe Walls
Citation Notes Abstract
Westbrook, A. and F. A. Digiano (2009). "Rate of chloramine decay at pipe surfaces." Journal American Water Works Association 101(7): 59-70   .Water utilities must ensure that adequate residual concentrations of secondary disinfectants persist throughout the distribution system to prevent bacterial regrowth. To predict disinfectant concentrations, a utility must use a combination of modeling and water quality field sampling, which is expensive. The need for sampling can be reduced if a utility has a more robust water quality modeling capacity. This study extends the understanding of the rate of chloramine decay in the distribution system, enabling utilities to make better predictions of chloramine residuals and, perhaps, to reduce the amount of costly sampling required. This research demonstrates that a simple, inexpensive new lab method, the pipe section reactor, can be used to systematically study the rate of disinfectant decay in various pipe materials and water quality conditions. In addition, this study provides kinetic models that can be used to predict chloramine decay rates in all distribution systems.



Reactions with Metals
Citation Notes Abstract
Zhang, Y. Y. and Y. P. Lin (2013). "Release of Pb(II) from the reduction of Pb(IV) corrosion product PbO2 induced by bromide-catalyzed monochloramine decomposition." Environmental Science & Technology 47: 10931-10938. surface-specific Pb reduction rate  
Lin, Y. P. and R. L. Valentine (2008). "Release of Pb(II) from Monochloramine-Mediated Reduction of Lead Oxide (PbO2)." Environmental Science & Technology 42(24): 9137-9143.   A contributing factor causing the sudden release of excessive lead into drinking water is believed to involve the change in redox conditions occurring when monochloramine (NH2Cl) replaces free chlorine as a disinfectant. Studies suggest that NH2Cl cannot effectively oxidize Pb(II) to form PbO2, a Pb(IV) mineral scale formed from the oxidation of metallic lead and Pb(II) species by free chlorine. Unexpectedly, we observed that NH2Cl is actually capable of reducing PbO2 to form Pb(II). We systematically investigated this reaction by varying important water chemistry factors such as solution pH, total carbonate concentration, and the Cl/N molar ratio to control chloramine speciation and its rate of decomposition via a complex set of autodecomposition reactions. The amount of Pb(II) formed was found to be proportional to the amount of NH2Cl that autodecomposed regardless of the rate of this reaction. This implies that the rate of Pb(II) release is proportional to the absolute rate of NH2Cl decomposition. It is proposed that the species responsible for the reduction of PbO2 is likely a reactive intermediate produced during the decay of NH2Cl. This finding is the first to report that NH2Cl can act as a reductant.


Microbial Reactions
Citation Notes Abstract
Maestre, J. P., D. G. Wahman, et al. (2013). "Monochloramine cometabolism by Nitrosomonas europaea under drinking water conditions." Water Research 47(13): 4701-4709.   Chloramine is widely used in United States drinking water systems as a secondary disinfectant, which may promote the growth of nitrifying bacteria because ammonia is present. At the onset of nitrification, both nitrifying bacteria and their products exert a monochloramine demand, decreasing the residual disinfectant concentration in water distribution systems. This work investigated another potentially significant mechanism for residual disinfectant loss: monochloramine cometabolism by ammonia-oxidizing bacteria (AOB). Monochloramine cometabolism was studied with the pure culture AOB Nitrosomonas europaea (ATCC 19718) in batch kinetic experiments under drinking water conditions. Three batch reactors were used in each experiment: a positive control to estimate the ammonia kinetic parameters, a negative control to account for abiotic reactions, and a cometabolism reactor to estimate the cometabolism kinetic constants. Kinetic parameters were estimated in AQUASIM with a simultaneous fit to all experimental data. The cometabolism reactors showed a more rapid monochloramine decay than in the negative controls, demonstrating that cometabolism occurs. Cometabolism kinetics were best described by a pseudo first order model with a reductant term to account for ammonia availability. Monochloramine cometabolism kinetics were similar to those of ammonia metabolism, and monochloramine cometabolism was a significant loss mechanism (30-60% of the observed monochloramine decay). These results suggest that monochloramine cometabolism should occur in practice and may be a significant contribution to monochloramine decay during nitrification episodes in drinking water distribution systems.


Citation Notes Abstract