Surface interactions of humic acid - anion doped titania binary system

Abstract

Humic acids are natural weak polyelectrolytes which play active role in binding mechanism of organic molecules that results in the transportation of contaminants in soils and aquifers. Humic acids constitute the major fraction of natural organic matter that should be removed during water treatment. Besides conventional treatment methods, application of advanced oxidation processes more specifically photocatalysis has gained much attention in recent decades. Titanium dioxide (TiO2) is universally recognized as a standard photocatalyst. Since photocatalysis occurs through a surface oriented mechanism, the adsorptive properties of TiO2 specimens deserve special attention. Moreover, nowadays visible light activated TiO2 has been the subject of numerous investigations. The understanding of the surface interactions prevailing between the anion doped oxide surface and humic subfractions is important for the determination of the role of humic substances during photocatalysis. The aim of this study was to investigate surface interactions between different molecular size fractionations of humic acid and TiO2 specimens, namely bare TiO2 and anion doped TiO2. In this study, two different commercial TiO2 specimens (Degussa P-25 and Hombikat UV-100) were used. Adsorption experiments were conducted with humic acid solutions having diverse molecular size fractions (0.45 μm filtered fraction, 100 kDa fraction and 30 kDa fraction). Moreover, bare TiO2 and anion doped TiO2 (N-doped TiO2, S-doped TiO2, C-doped TiO2 and N-S co-doped TiO2) specimens in the range of 0.1-1.0 mg mL-1 were used as adsorbents. Adsorption properties of humic acid and its molecular size fractions onto bare TiO2 and anion doped TiO2 specimens were evaluated and compared in terms of UV-vis spectroscopic parameters i.e. color forming moieties (Color436) and UV absorbing centers (UV365, UV280 and UV254) and fluorescence spectroscopy in emission and synchronous scan modes as well as dissolved organic carbon. Adsorption isotherms were evaluated in terms of the adsorption isotherm types (Types C, L and S). Adsorption isotherms were modeled to Freundlich and Langmuir adsorption isotherm models by evaluating the results of specified UV-vis parameters. The effect of doping on two different TiO2 specimens (Degussa P-25 and Hombikat UV-100) could be evaluated through the role of decreasing molecular size of humic acid. The most significant effect could be attributed to the surface properties of TiO2 specimens present in aqueous solutions of humic acid. The results based on varying molecular size fractions of humic acid displayed remarkable differences with respect to the morphological character of TiO2 specimens.