Study of Nitrogen-Modified Titanium Dioxide as an Adsorbent for Azo Dyes

Abstract

The removal of two azo dyes from aqueous solution was investigated using a highly active nitrogen-modified titanium dioxide (TiO2/N). This photocatalyst was prepared by pressure modification of a commercial sample of anatase, TiO2 (supplied by Zaklady Chemiczne Police SA, Poland). The adsorption of two azo dyes (Reactive Red 198 — a monoazo dye and Direct Green 99 — a polyazo dye) onto the unmodified and nitrogen-modified TiO2 surface was determined. Adsorption onto unmodified TiO2 led to an isotherm which could be described by the Freundlich model, whereas that onto the nitrogen-modified TiO2 surface was best described by the Langmuir model. The adsorption isotherms of Direct Green 99 onto TiO2/N from aqueous solutions at various pH values were compared with those onto unmodified TiO2 during dye removal under UV irradiation. Such a comparison is important since it provides information as to how solutes interact with adsorbents. Zeta potential measurements of TiO2 provided an explanation for the observed behaviour in terms of the surface charge. The IR spectrum of the photocatalyst obtained exhibited new absorption bands at 1430–1440 cm−1 which were attributed to the bending vibrations of NH+4 and at 1535 cm−1 associated with NH2 groups or NO2 and NO groups.