PHOTOCATALYTIC REDUCTION OF THE ENVIRONMENTAL-POLLUTANT CR(VI) OVER A CADMIUM-SULFIDE POWDER UNDER VISIBLE-LIGHT ILLUMINATION

The photocatalytic reduction of the pollutant Cr(VI) over CdS in different crystalline states under visible light illumination was investigated. The photocatalytic activity of heat-treated (HT) and non-heat-treated (NHT) CdS was systematically compared. The Cr(VI), contained in an aqueous solution of 200 mug/ml, was almost totally eliminated within 30 min when NHT CdS was used as catalyst and displayed a much higher activity than that of HT CdS. The addition of a hole-scavenger such as ethanol, which acted optimally at 10-20%, remarkably improved the activity of HT CdS. The macroscopic difference in photocatalytic activity between NHT and HT CdS was microscopically explained by the differences in their morphological structures, surface states, and light responses which were determined by means of X-ray diffraction, static adsorption and UV-visible spectra. The adsorbance and adsorption state of Cr(VI) was determined on the CdS surface and its photoreducibility was proven by X-ray photoelectron spectroscopy and temperature-programmed desorption. The photoreduction rate of pollutant Cr(VI), either during experiments indoors or during experiments in sunlight was remarkably improved. 99.3% of the Cr(VI) was reduced from a solution of 200 mug/ml concentration during 1 h of sunlight irradiation. Applications are given for the photocatalytic elimination of Cr(VI) in waste water under sunlight irradiation.