Salt effect on sensitized photooxidations. A kinetic approch to environmental decomposition of marine contaminants

Abstract

The salt effect on the kinetics of singlet molecular oxygen [O₂(¹Δ_g)]-mediated photooxidations of sea water contaminants was investigated. Two families of photooxidizable compounds were employed in the study: anthracene derivatives and phenols. The presence of salt (NaCl in H₂O and LiCl in MeCN, in both cases in the range 0-0.45 M) produces changes in the photooxidation rate. For solvent-polarity-dependent reactions, this behavior can be predicted, by knowing the solvent-polarity dependence of the rate constant for chemical reaction of the substrates with O₂(¹Δ g) in non-saline solutions (k_r). For the cases of photooxidations possessing solvent-polarity-independent or scantily-dependent k_r values, the photooxidation rates decrease as the salt content in the solution increases, mainly due to a predominance of the physical quenching pathway. In addition, the quantum yield for O₂(1 Δ_g) generation (Φ_Δ) was determined in a series of saline solutions, in the range of 0-0.45 M in water and MeCN solutions, in the presence of NaCl and LiCl respectively. The Δ values are independent, within the experimental error on the salt content.