Adsorption and photocatalysis activity of TiO2/bentonite composites

This paper explores the impact of the TiO2 nature and concentration immobilized onto bentonite clay on the adsorption and the photocatalytic degradation of methyl orange and salicylic acid at pH 3 and 10. Commercial P25 TiO2 and nanoparticles prepared via the sol-gel method have been deposited onto the clay. We paid particularly close attention to the quantitative determination of the TiO2 content deposited onto the clay from the analysis of the XRD patterns using a Rietveld-based fitting method. The TiO2 particles play the major role on the photochemical activity and on the formation of the hydroxyl radicals. The constant rate of degradation increases linearly with the titania content immobilized onto the clay. The pollutant removal proceeds more favorably with bentonites containing mixed rutile and anatase crystallographic phases TiO2 (P25) than with pure anatase sol-gel titanium dioxide. The photodegradation performances are also affected by the adsorption capacity of the clay. When the adsorption of the pollutant remains weak (at basic pH), the bentonite plays no role in the degradation activity. At the same TiO2 content, the photocatalytic efficiencies of the unsupported and immobilized titania are equivalent. Conversely, at acidic pH the contaminant adsorption becomes significant and all the composites produce the complete degradation of the pollutants. The immobilization of the titania leads to a reduction of the time necessary to remove the contaminants through the enrichment by adsorption of the pollutant onto the clay. The fastest photodegradation is achieved with the composite containing the smallest amount of titania (57 wt.%) but with the largest uptake of pollutant adsorbed.