Effect of Ionic Charge on Adsorption from Aqueous Solutions

In an aqueous suspension, nanotubular titanates tend to develop a negative zeta potential due to the dissociation of titanic acid (e.g. H2Ti3O7 = H1 + HTi3O7~).41 This phenomenon greatly affects the ability of charged molecules to adsorb onto the surface of nanotubes. The affinity of species in the cationic form is much better than that of anionic species, indicating the major role of electrostatic interactions between the adsorbent and the adsorbate during adsorption from aqueous solution.

Figure 3.8 shows adsorption isotherms for two organic dyes from aqueous solution onto the surface of titanate nanotubes and TiO2 nanoparticles (P-25, Degussa). Methylene Blue (MB) is a cationic dye, whereas Eriochrome Black T (EBT) is an anionic one. The adsorption isotherms showed Langmuir-type adsorption, according to:

t asKLC

where as corresponds to the amount of dye forming a monolayer on the surface of the adsorbent, and KL is a constant of adsorption. When the concentration of MB in the solution exceeds 0.05mmoldm"3, the surface of titanate nanotubes is saturated with adsorbed MB molecules. The amount of adsorbed MB on the surface of nanotubes can be estimated as ca. 8mmol(MB) mol(TiO2)_1.

Eriochrome Black
Figure 3.8 Isotherm for a) Methylene Blue (MB) and b) Eriochrome Black T (EBT) adsorption onto the surface of ( ■ ) titanate nanotubes and (•) spheroid P-25 TiO2 nanoparticles from aqueous suspension at 25 °C. Structures are indicated for c) cationic MB and d) anionic EBT.

By contrast, the as value for MB on the surface of P-25 nanoparticles is almost two orders of magnitude smaller, at ca. 0.6 mmol(MB) mol(TiO2)_1. The values of the adsorption constant, KL, and the saturation concentration, as, for nanotubes and nanoparticles are estimated by fitting data from Figure 3.8 a to Equation (3.6) and are shown in Table 3.2. Such a large difference in as values is difficult to explain by the difference in the values of specific surface area alone.

Figure 3.8b shows the isotherm of Eriochrome Black T (EBT) adsorption from an aqueous solution on the surface of nanotubes and nanoparticles. From Equation (3.6), the amounts of dye in monolayer as on the surface of nanotubes can be estimated (see Table 3.2). Despite the fact that the specific surface area of nanoparticles is almost a fifth that of nanotubes, the amount of EBT adsorbed onto the surface of the nanoparticles is greater than that adsorbed onto the nanotubes.

In aqueous solution, both dyes experience electrolytic dissociation. In the case of MB this leads to the formation of a cationic form of the dye over a wide pH range, whereas with EBT, an anionic form of the dye is produced (see Figure 3.8c and d). At the same time, titanate nanotubes and TiO2 nanoparticles (P-25) suspended in aqueous media develop a zeta potential, which is negative for nanotubes41 and positive for nanoparticles [the isoelectric point is 6.5 (ref. 42) in a pH range of 4 to 5]. Adsorption is preferred where the sign of the charge of dye and adsorbent is different, rather than where both have same sign. This indicates that electrostatic interactions dominate during the adsorption of charged dyes from aqueous media.

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