Photocatalytic degradation of iron-cyanocomplexes by TiO2 based catalysts

The removal of iron-cyanocomplexes in industrial effluents is a difficult process, due to the resistance of these compounds to conventional treatments for cyanide wastewater detoxification. The mechanism of both the homogeneous photolysis of these compounds and their heterogeneous photocatalytic oxidation with Degussa P25 TiO2 and silica-supported TiO2 photocatalysts has been investigated. The activities of the tested catalysts for complexed cyanide degradation were found to be different from those observed for free cyanide photo-oxidation. The best activity was found for the photocatalyst synthesized by supporting 20 wt.% of TiO2 on SBA-15 silica as compared with the commercial catalyst Degussa P25 and the other supported catalysts tested. On the basis of detected intermediate species, a mechanism for iron-cyanocomplexes photodegradation is suggested. The influence of the textural properties of the support and titania loading on the process is discussed. The results point out that the high activity observed, when SBA-15 is used as support of TiO2, seems to be related to the microporosity of the material acting as molecular sieve, which avoids the deactivation of the semiconductor. The porous structure of the SBA-15 material limits the access of the iron-cyanocomplexes to the TiO2 particles whereas the free cyanides homogeneously released can reach the semiconductor surface, being subsequently oxidized to cyanate.