Photodegradation of surfactants IX: The photocatalysed oxidation of polyoxyethylene alkyl ether homologues at TiO 2-water interfaces

Abstract

The photocatalysed mineralization of the non-ionic surfactants polyoxyethylene alkyl ethers (C n E m , n = 10, 12, 14, 16 and 18 and m = 5, 6, 7, and 8) has been investigated in an UV-illuminated heterogeneous TiO 2 suspension under aerated conditions. The temporal course of the photo-oxidation of these surfactants was monitored by a dye extraction spectroscopic procedure. The formation of peroxide and carbonyl intermediates and their ultimate mineralization of CO 2 were also examined. As the ethoxyl chain length of C 12E m homologues increases, the degradation rate which occurs via pseudo-first-order kinetics decreases in the order C 12E 5 > C 12E 6 > C 12E 7 > C 12E 8. The C n E 8 homologues ( n = 10, 12, 14, 16 and 18) exhibit the same degration tendencies irrespective of the alkyl chain length. In the competitive photodegration of the polyethoxyl moiety and the long alkyl chain, the ethoxyl chain is more easily cleaved than the alkyl group. Complete mineralization of the surfactants was evidenced by monitoring the stoichiometric evolution of CO 2 originating mostly from the ethoxyl moiety of C 12E m after irradiation for 20 h. The initial photo-oxidation occurs sequentially one by one from the terminal position of the ethoxyl chain. The participation of ·OH radicals in the photo-oxidation process was determined using 5,5-dimethyl-1-pyrroline-1-oxide spin-trapping electron spin resonance measurements.