Modelling the photochemical generation kinetics of 2-methyl-4-chlorophenol, an intermediate of the herbicide MCPA (2-methyl-4-chlorophenoxyacetic acid) in surface waters

Abstract

2-Methyl-4-chlorophenol (MCP) is the main transformation intermediate of the herbicide MCPA in surface waters and it is more toxic than its parent compound. MCP is produced from MCPA by both direct photolysis and ^•OH reaction. The latter process has higher yield of MCP from MCPA: 0.5 vs. 0.3 for the direct photolysis. Our model results show that the formation rate of MCP would be higher in waters that contain low organic matter and high nitrate and nitrite. Such conditions are favourable to MCPA direct photolysis and ^•OH-induced transformation, which are both inhibited by organic matter, while ^•OH formation is enhanced by nitrate and nitrite. Good agreement is obtained between model predictions and field data of MCPA transformation in the Rhône river delta (Southern France). The field data also suggest that MCP undergoes slightly faster transformation than MCPA in that environment.

Keywords:

• environmental photochemistry
• sensitized transformation
• chromophoric dissolved organic matter
• photodegradation of contaminants
• degradation by-products

Acknowledgements

Financial support by PNRA – Progetto Antartide is gratefully acknowledged. The Ph.D. grant of Elisa De Laurentiis is provided by Progetto Lagrange – Fondazione CRT, Torino, Italy. Davide Vione acknowledges financial support from Università di Torino-EU Accelerating Grants, project TO_Call2_2012_0047 (Impact of radiation on the dynamics of dissolved organic matter in aquatic ecosystems-DOMNAMICS).