The Henry's constant of monochloramine

Area of Science:

• Environmental Chemistry
• Water Treatment Science
• Chemical Engineering

Background:

• Monochloramine is a key disinfectant in drinking water and a byproduct of wastewater chlorination.
• Its gas-phase partitioning and volatilization behavior remain understudied.
• Preliminary data suggest significant monochloramine loss to the atmosphere from open or aerated solutions.

Purpose of the Study:

• To investigate the gas-aqueous phase partitioning of monochloramine.
• To determine the Henry's law constant (K_H) for monochloramine across a range of temperatures.
• To assess the significance of volatilization as a monochloramine loss mechanism in various water systems.

Main Methods:

• Monochloramine solutions were exposed to atmospheric conditions and active aeration.
• Loss rate constants were measured for sealed and unsealed/aerated solutions.
• Henry's law constants were determined using an equilibrium headspace technique at five temperatures (11–32 °C).

Main Results:

• Monochloramine loss rates were significantly higher in open and aerated systems compared to sealed systems.
• Measured Henry's law constants ranged from 8 × 10^-3 to 4 × 10^-2, classifying monochloramine as semi-volatile.
• At 20 °C, K_H was determined to be 1.7 × 10^-2, considerably higher than ammonia.

Conclusions:

• Volatilization is a substantial monochloramine loss pathway in open systems.
• Henry's law constant values indicate that volatilization is relevant in environments like rivers, swimming pools, and cooling towers.
• Understanding these partitioning dynamics is crucial for effective water disinfection and management.