Azo compound degradation kinetics and halonitromethane formation kinetics during chlorination

Area of Science:

• Environmental Chemistry
• Water Treatment Chemistry
• Organic Contaminant Analysis

Background:

• Azo compounds are widely used dyes and can transform into harmful halonitromethanes (HNMs) during water chlorination.
• Halonitromethanes, such as chloropicrin (TCNM), are genotoxic and raise concerns for drinking water safety.
• The influence of environmental factors like pH on HNM formation from azo compounds is not fully understood.

Purpose of the Study:

• To investigate the impact of pH on the chlorination kinetics of three azo compounds: Methyl Orange (MO), Acid Orange II (AO), and Acid Red 1 (AR 1).
• To elucidate the formation pathways and kinetics of trichloronitromethane (TCNM) from azo dye precursors.
• To assess the potential risks associated with azo compounds as precursors for genotoxic HNMs in drinking water.

Main Methods:

• Kinetic experiments were conducted to measure the reaction rates of MO, AO, and AR 1 with chlorine across a pH range of 6.3 to 9.0.
• A two-compartment first-order model was employed to analyze the formation kinetics of TCNM.
• The speciation of chlorine and azo compounds was considered in relation to pH-dependent reaction rates.

Main Results:

• Apparent second-order rate constants for MO, AO, and AR 1 varied significantly with pH, ranging from 16.7 to 99.3 M⁻¹s⁻¹.
• The formation of TCNM from azo compounds involves both fast- and slow-reacting precursors, with precursor ratios dependent on the specific azo compound and pH.
• The fraction of fast-reacting TCNM precursors showed distinct pH-dependent trends for MO, AO, and AR 1, with maximum predicted molar yields of TCNM ranging from 1.5% to 2.5%.

Conclusions:

• Azo compounds are significant precursors for the formation of genotoxic halonitromethanes during water chlorination.
• pH is a critical factor influencing the rate and pathway of TCNM formation, with precursor-specific responses.
• The findings highlight the potential risks posed by azo compounds in drinking water treatment and underscore the need for careful management.