Nitrogen conversion from ammonia to trichloronitromethane: Potential risk during UV/chlorine process.

Area of Science:

• Environmental Chemistry
• Water Treatment Technologies
• Organic Chemistry

Background:

• Ammonia presence in water treated by UV/chlorine can lead to monochloramine formation.
• Monochloramine photolysis generates nitrite (NO2-) and reactive nitrogen species (RNS), influencing disinfection byproduct (DBP) formation.
• Trichloronitromethane (TCNM) is a concerning DBP whose formation pathways require further investigation.

Purpose of the Study:

• To evaluate the formation of trichloronitromethane (TCNM) during UV/chlorine treatment of ammonia-containing water.
• To investigate the influence of operational parameters (pH, N:Cl ratio) on TCNM formation.
• To assess the genotoxicity of TCNM and elucidate its formation mechanism.

Main Methods:

• Utilized model organic compounds and natural organic matter in ammonia-containing water.
• Applied UV/chlorine treatment and analyzed TCNM formation.
• Correlated TCNM formation with Hammett constants of precursors.
• Performed computational chemistry (Gaussian program) to study reaction mechanisms.

Main Results:

• TCNM formation was enhanced by increasing N:Cl molar ratio and pH.
• Nitrite (NO2-) and RNS yields from monochloramine photolysis increased under optimized conditions.
• TCNM formation showed a linear correlation with Hammett constants of model precursors.
• TCNM significantly increased the genotoxicity of disinfection byproducts (DBPs).
• Computational analysis suggested electrophilic substitution of •NO2 at ortho/para positions of phenol.

Conclusions:

• Ammonia significantly impacts TCNM formation in UV/chlorine treated water, primarily through enhanced NO2- and RNS production.
• Operational parameters like pH and N:Cl ratio are critical for controlling TCNM formation.
• TCNM contributes to the overall genotoxicity of DBPs, highlighting its health implications.
• The study provides insights into the reaction mechanism of TCNM formation, involving electrophilic attack by reactive nitrogen species.