H31N-01
Compound Specific Isotope Analysis of the aqueous photodegradation of substituted chlorobenzenes

Wednesday, 16 December 2015: 08:00
3024 (Moscone West)
Elodie Passeport1, Ning Zhang2, Langping Wu2, Hartmut Herrmann3, Barbara Sherwood Lollar4 and Hans Richnow2, (1)University of Toronto, Civil Engineering and Chemical Engineering and Applied Chemistry, Toronto, ON, Canada, (2)Helmholtz Centre for Environmental Research UFZ Leipzig, Leipzig, Germany, (3)Leibniz Institute for Tropospheric Research, Leipzig, Germany, (4)University of Toronto, Toronto, ON, Canada
Abstract:
In surface water, environmental contaminants can undergo a range of natural transformation processes mediated by 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 objective of this study was to use CSIA to better understand the reaction mechanisms of substituted chlorobenzenes during their aqueous photodegradation. We conducted laboratory experiments using a temperature-controlled photoreactor, and focused on: dichlorobenzene (DCB) isomers, i.e., 1,2-DCB, 1,3-DCB, and 1,4-DCB; 3- and 4-chloromethylbenzene (3- and 4-CMB); and 3- and 4-nitrochlorobenzene (3- and 4-NCB).

The results showed that experiments conducted in the absence of light and those focusing on direct photodegradation (i.e., when molecules directly absorb light) showed no significant carbon isotope fractionation. During indirect photodegradation, involving reactions with OH radicals, the CMBs reacted the fastest (first-order reaction rate constant, kCH3 > 1.2 h-1), whereas NCB reacted the slowest (kNO2 < 0.02 h-1). The most pronounced isotope fractionation was observed for the NCBs (up to enrichment factors of -4.8±0.5‰) whereas the least significant were for the CMBs (≤ -1.0±0.1‰). A Hammett plot of these results suggests that reaction rates increase 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.

Previous Abstract | Next Abstract >>