The photodegradation of 1,3,6,8-tetrabromocarbazole in n-hexane and in solid-mediated aqueous system: Kinetics and transformation mechanisms

The kinetics and mechanisms of photochemical transformation of 1,3,6,8-tetrabromocarbazole (1368-BCZ) in n-hexane and pre-coated on solid particles in aqueous solution are investigated. Under simulated sunlight irradiation, 1368-BCZ dissolved in n-hexane rapidly photodegraded, obeying the pseudo first-order kinetics model. In the aqueous system, particles of seven different solids with coated 1368-BCZ were suspended during the reaction. The photodegradation kinetics of 1368-BCZ on three pure particles (silica gel, Al2O3, and Kaolin) fitted with the "double-logarithmic" model. At pH = 7.0 and T = 15 degrees C, 84% of 1368-BCZ with initial coating concentration of 100 mu g g(-1) silica gel disappeared in 1 h. The products and intermediates identified suggested that debromination and hydroxylation were the two main photochemical transformation pathways of 1368-BCZ in both n-hexane and aqueous systems. The proposed reaction pathways were supported by the results of quantum chemical calculation. However, several products including 1-monobromocarbazole (1-BCZ), 3-monobromocarbazole (3-BCZ), dibromocarbazole-1 (di-BCZ-1), and 3,6-dibromocarbazole (36-BCZ) were found during the reaction in n-hexane but not detected in the aqueous systems. This work provides useful information in the assessment of environmental fate of polyhalogenated carbazoles.