Degradation of surfactants by hydroxyl radicals photogenerated from hydroxoiron(iii) complexes

Abstract

The Fe(III)-photoinduced oxidation of anionic lauryl sulfate (LS⁻) and cationic cetyltrimethylammonium (CTA⁺) surfactants has been investigated in aqueous solution. Competition experiments using 2-propanol showed that the initial rate of disappearance is proportional to the concentration of the photogenerated HO˙ radicals scavenged by the surfactants (the degradation of lauryl sulfate involves attack by HO˙ only) and no direct photoinduced charge-transfer reaction occurs between the Fe(III) species and the surfactant ions. Ageing of the Fe(III) solution did not significantly influence the efficiency of photodegradation in air-saturated systems. Conversion of Fe(III) to Fe(II) in aerated solution reached a steady-state level of ca. 50% after 2 h irradiation. In nitrogen-saturated systems, the rate of surfactant oxidation decreased due to the total reduction of Fe(III). Addition of H₂O₂ doubled the quantum yield of the disappearance of both detergents as a result of the photo-Fenton reaction. The photoinduced oxidation of both surfactants was most efficient in acidic solutions of pH 2–3, without H₂O₂, and for the photo-Fenton system; the quantum yields are Φ_NaLS = 0.011, Φ_CTAB = 0.012 without H₂O₂, and Φ_NaLS = 0.024, Φ_CTAB = 0.027 in the photo-Fenton system with irradiation at 366 nm. For the disappearance of 4 × 10⁻⁴ M detergent, due to the first oxidation step, 4 h of irradiation (at pH 2.6) is sufficient, whereas 100% mineralization of the total organic carbon content requires prolonged photolysis for at least 10 h. The formation of carbon dioxide dramatically accelerated after a 2 h induction period (1 h in the photo-Fenton system), indicating the cleavage of the long hydrocarbon chains to shorter intermediates in the first stage of the mechanism. The following step is total mineralization of these smaller compounds, which were identified as mostly hydroxy acids via GC-MS.