SELENOCYANATE (SeCN-) CONTAMINATED WASTEWATER TREATMENT USING TiO2 PHOTOCATALYSIS: SeCN- COMPLEX DESTRUCTION, INTERMEDIATES FORMATION, AND REMOVAL OF SELENIUM SPECIES

摘要

Removal of selenocyanate (SeCN-) complex from synthetic wastewater was investigated using TiO2 based photocatalytic degradation (PCD) process. Using only UV-light showed no change in SeCN- however adding TiO2 to the same reactor system caused near complete disappearance of SeCN-. Also selenocyanate disappearance was noted at both acidic and basic pH values and over a wide concentration range. At pH 4 a gradual buildup of cyanide species along with disappearance of SeCN-, and formation of selenite (SeO32-) and selenate (SeO42-) supported SeCN- complex destruction. The selenite results showed an initial increase followed by a gradual decrease. Similar was noted at pH 6 and pH 10. However at pH 12 a larger buildup of selenite was explained based on reduced selenite adsorption on to TiO2 surface and consequently selenite's reduced oxidation to selenate. In any case the respective results showed that noted selenocyanate disappearance occurs because of SeCN- complex destruction. Furthermore removal of produced selenite and selenate via reduction route, using EDTA (ethylenediaminetetraacetic acid) as hole scavenger indicated that adding EDTA at the beginning of experiment inhibits selenocyanate degradation. Furthermore addition of EDTA at 120 min reaction time yielded undesirable re-appearance of selenocyanate in aqueous phase that indirectly indicated that selenocyanate degradation transpires at TiO2 surface. Also out of all experiments with EDTA added at 180, 300, and 380 min reaction time, it was for the 380 min experiment for which the selenate amount showed near complete selenium recovery indicating that most SeCN- complex has been destroyed followed by oxidation of released selenium to selenite and then to selenate; the released selenate also showed near complete removal at reaction time higher than 380 min (after addition of EDTA) that was initiated by the reduction route to elemental selenium.