pH dependant competitive N-de-ethylation and degradation of Rhodamine B photocatalyzed by cationic surfactant stabilized ZnS nanoparticles

The purpose of the present study was to determine the role of solution pH on the photodegradation behaviour of Rhodamine B (RhB), an industrially important dye. The potential application of such a study lies in the optimization of the reaction parameters for the treatment of waste water containing such dyes. In this work, ZnS nanoparticles (NPs) have been synthesized using cetyltrimethylammonium bromide (CTAB) as a stabilizing agent. The ZnS NPs were characterized by UV-visible spectroscopy, powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The photocatalytic activity of the semiconductor was evaluated by studying the degradation of RhB at various solution pH using 254 nm ultra violet (UV) light. At pH 4, the absorption peak of RhB was blue shifted by 57 nm within 30 min, suggesting a 4-fold N-de-ethylation. However, at pH 11, the blue shift in the absorption band was only about 20 nm during 30 min of irradiation. Besides, the band was found to decrease with increase in irradiation time indicating the degradation of the dye. The mechanistic investigations suggested that the hydroxyl radicals generated in the solution were responsible for both the de-ethylation and degradation of RhB in the presence of the ZnS NPs