Influence of dissolved organic matter on the photodegradation and volatilization kinetics of chlorpyrifos in coastal waters

• Photolysis rate of chlorpyrifos in saline waters is enhanced by high inputs of DOM
• Saline waters: photolysis and volatilization can be major transformation processes
• Medium pressure Hg lamps are more efficient than other UV irradiation sources
• The main degradation product of chlorpyrifos was 3,5,6-trichloro-2-pyridinol

Irradiation by a medium-pressure mercury (Hg) lamp was used to study the photodegradation of chlorpyrifos (O,O-diethyl-O-3,5,6-trichloropyridin-2-yl phosphorothioate) in salt, deionized water (unbuffered), and a natural saline water. The UV irradiation produces 3,5,6-trichloro-2-pyridinol (TCP) with accelerated photodegradation kinetics of the parent compound. The results show that chlorpyrifos photolysis followed pseudo-first-order kinetics in the presence and absence of salt with no significant difference between the photodegradation rates in saline waters and unbuffered deionized water. Addition of Suwannee River natural organic matter (SRNOM) to mimic the mixing of freshwaters with seawaters significantly changed the photodegradation rate constant of chlorpyrifos in saline waters. The influence of hydrolysis and volatilization was also evaluated. While photolysis was found to be the main transformation process in unbuffered deionized water, both volatilization and photolysis can be important pathways of chlorpyrifos loss in natural saline waters and under aerated conditions.

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