Advanced oxidation kinetics of aqueous tri alkyl phosphate flame retardants and plasticizers

• Published in: Environmental science & technology 2009-04, Vol.43 (8), p.2937-2942
• Main Authors: Watts, Michael J., Linden, Karl G.
• Format: Article
• Language: English
• ISSN: 0013-936X; 1520-5851

Tri alkyl phosphate esters are a class of anthropogenic organics commonly found in surface waters of Europe and North America, due to their frequent application as flame retardants, plasticizers, and solvents. Four tri alkyl phosphate esters were evaluated to determine second-order rates of reaction with ultraviolet- and ozone-generated •OH in water. In competition with nitrobenzene in UV irradiated hydrogen peroxide solutions tris(2-butoxyethyl) phosphate (TBEP) was fastest to react with •OH ( k OH,TBEP =1.03×10 10 M -1 s -1 ), followed sequentially by tributyl phosphate (TBP), tris(2-chloroethyl) phosphate (TCEP), and tris(2-chloro iso propyl) phosphate (TCPP) ( k OH,TBEP =6.40×10 9 , k OH,TBEP =5.60×10 8 , & k OH,TBEP =1.98×10 M -1 s -1 ). A two-stage process was used to test the validity of the determined k OH for TBEP and the fastest reacting halogenated alkyl phosphate, TCEP. First, •OH oxidation of TCEP and TBEP, in competition with nitrobenzene, was measured in ozonated hydrogen peroxide solutions. Applying multiple regression analysis, it was determined that the UV-H 2 O 2 and O 3 -H 2 O 2 data sets were statistically identical for each compound. The subsequent validated k OH were used to predict TCEP and TBEP photodegradation in neutral pH, model surface water after chemical oxidant addition and UV irradiation (up to 1000 mJ/cm 2 ). The insignificant difference, between the predicted TBEP and TCEP photodegradation and a best-fit of the first-order exponential decay function to the observed TBEP and TCEP concentrations with increasing UV fluence, was further evidence of the validity of the determined k OH . TBEP oxidation rates were similar in the surface waters tested. Substantial TCEP oxidation in the model surface water required a significant increase in H 2 O 2 .