Reaction of benzophenone UV filters in the presence of aqueous chlorine: Kinetics and chloroform formation

The transformation of two benzophenone UV filters (Oxybenzone and Dioxybenzone) was examined over the pH range 6–11 in the presence of excess aqueous chlorine. Under these conditions, both UV filters were rapidly transformed by aqueous chlorine just above circumneutral pH while transformation rates were significantly lower near the extremes of the pH range investigated. Observed first-order rate coefficients (kobs) were obtained at each pH for aqueous chlorine concentrations ranging from 10 to 75 μM. The kobs were used to determine the apparent second-order rate coefficient (kapp) at each pH investigated as well as determine the reaction order of aqueous chlorine with each UV filter. The reaction of aqueous chlorine with either UV filter was found to be an overall second-order reaction, first-order with respect to each reactant. Assuming elemental stoichiometry described the reaction between aqueous chlorine and each UV filter, models were developed to determine intrinsic rate coefficients (kint) from the kapp as a function of pH for both UV filters. The rate coefficients for the reaction of HOCl with 3-methoxyphenol moieties of oxybenzone (OXY) and dioxybenzone (DiOXY) were k1,OxY = 306 ± 81 M−1s−1 and k1,DiOxY = 154 ± 76 M−1s−1, respectively. The kint for the reaction of aqueous chlorine with the 3-methoxyphenolate forms were orders of magnitude greater than the un-ionized species, k2,OxY = 1.03(±0.52) × 106 M−1s−1 and k2_1,DiOxY = 4.14(±0.68) × 105 M−1s−1. Also, kint for the reaction of aqueous chlorine with the DiOXY ortho-substituted phenolate moiety was k2_2,DiOxY = 2.17(±0.30) × 103 M−1s−1. Finally, chloroform formation potential for OXY and DiOXY was assessed over the pH range 6–10. While chloroform formation decreased as pH increased for OXY, chloroform formation increased as pH increased from 6 to 10 for DiOXY. Ultimate molar yields of chloroform per mole of UV filter were pH dependent; however, chloroform to UV filter molar yields at pH 8 were 0.221 CHCl3/OXY and 0.212 CHCl3/DiOXY.