Substituted chlorobenzenes present in surface water can be eliminated via natural biodegradation, adsorption, and photodegradation. Understanding these processes can help develop more effective remediation strategies. One method that can be used, Compound Specific Isotope Analysis (CSIA), is based on the faster reaction rates of molecules containing light (e.g., 12C) versus heavy (e.g., 13C) isotopes. While CSIA is widely used to study the fate and removal of groundwater contaminants, to date, its application to more complex environments such as surface waters is very limited. The objectives of this study were: (i) to determine stable carbon enrichment factors (εC) for direct photolysis and reaction with hydroxyl radicals of substituted chlorobenzenes; (ii) to compare εC values with those for other environmental processes; (iii) to propose reaction mechanisms for aqueous photodegradation of substituted chlorobenzenes based on kinetics, isotope data, and Hammett relationship. We conducted laboratory experiments using a temperature-controlled photoreactor, with chloro- (DCBs), methyl- (CMBs), and nitro-substituted chlorobenzenes (NCBs). The results showed that dark and direct photolysis experiments showed no significant degradation and carbon isotope fractionation. During indirect photodegradation, involving reactions with OH radicals, the CMBs reacted the fastest (pseudo first-order reaction rate constant, kCH3 &gt; 1.2 h-1), whereas NCBs reacted the slowest (kNO2 &lt; 0.02 h-1). The most pronounced isotope fractionation effects were observed for the NCBs (with εC values up to -4.8±0.5‰) whereas the least significant εC values were for the CMBs (≤ -1.0±0.1‰). A Hammett plot of these results suggests that reaction rates increased in concert with electron donating groups as present in DCBs and NCBs. These results are among the first successful attempts to apply CSIA to new environments such as surface waters, and transformation reactions such as photodegradation. They suggest that CSIA has the potential to identify chemical reaction mechanisms and distinguish between transformation processes.

Compound specific isotope analysis of aqueous photodegradation of substituted chlorobenzenes

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.