Type I vs Type II photodegradation of pollutants

Abstract

[EN] Rose Bengal (RB) is a widely used photocatalyst due to its high quantum yield of singlet oxygen (O-1(2)) formation. Hence, when RB has been employed for wastewater remediation, the observed photodegradation has been attributed to reaction between the pollutants and the O-1(2) formed (Type II mechanism). However, RB could also react, in principle, via electron transfer (Type I mechanism). Herein, competition between Type I vs Type II oxidation has been investigated for RB in the photodegradation of emerging pollutants such as diclofenac (DCF) and acetaminophen (ACP). In parallel, the photocatalyst perinaphthenone (PN) has also been evaluated for comparison. The degree of removal achieved for both pollutants in aerated/deaerated aqueous solutions irrespective of the employed photocatalyst does not support the involvement of O-1(2) as the main species responsible for removal of the pollutants. Photophysical experiments showed that the triplet excited states of RB and PN are efficiently quenched by both DCF and ACP. Moreover, O-1(2) emission was also quenched by DCF and ACP. Thus the contribution of Type I versus Type II in the photodegradation has been evaluated from the experimentally determined rate constants. Nevertheless, at the upper limit for the typical concentration of emerging pollutants (10(-5) M) photodegradation proceeds mainly via Type I mechanism.