Adsorption isotherms

3.1. Comparison of the adsorption isotherms of different adsorbents

The adsorption on the surface of adsorbent is the initial step preceding the photocatalytic degradation step. It is an interaction between the molecules to be degraded and the surface of adsorbent. It is principally these molecules in the adsorbed state onto TiO2 that would determine the initial speed of photocatalysis (Amalric et al., 1996; Robert et al., 2000;

Guillard et al., 2003).

To understand well the processes of adsorption as well as their modelling, it is necessary to carry out the isotherms of adsorption. These experiments allow to show the differences between the adsorbents.

To determine the capacities of adsorption of various adsorbents, the isotherms of adsorption at 25 °C were realized. The study was carried out using four adsorbents, TiO2

media AC media, TiO2/ AC tissue and the original granular AC using phenol as a pollutant reference model. The experimental conditions are discussed in the previous chapter II. As previously mentioned for the original granular AC, the mass was equal to 0.5 g and for the tissue, the total mass was 0.8 g corresponding to 0.5 g of AC. The calculation is then made by two different ways for AC media and AC / TiO2 tissue: the classic calculation which used for the total mass of media and the other calculation by taking into account only the mass of AC.

The adsorption isotherms of phenol using granular AC, AC media, TiO2/ AC tissue and TiO2

media are represented in Fig. IV-7.

123

Fig. IV-7. Adsorption isotherms of phenol onto original granular AC, AC media, TiO2/ AC tissue and TiO2 media at 25 °C.

It was noticed that the adsorption capacity of original granular AC was about 10-20%

higher than that of AC media on the plateau when comparing the same AC weights. It may be explained on the assumption that some sites of AC in the media were not in contact with the aqueous solution owing to the binding with the fibres used for fabricating it. These fibres recover partially the grains of AC and hinder some of its sites of adsorption.

A decrease of the capacity of adsorption of the TiO2/AC tissue is observed as compared to the AC media. The same trend is even observed when the isotherms are calculated by taking into account only the mass of AC, the isotherms remains lower than that of original granular AC. This phenomenon seems to be due to the decrease of the available number of sites of adsorption due to the blocking of the access to some of them by TiO2/ SiO2

mixture, deposited in the form of aqueous suspension on the fibres. These results were well supported by SEM and EDX analyses discussed earlier. In these results, SEM and EDX analyses confirmed the presence of Ti and Si blocking some of the adsorption sites of the AC in the TiO2/ AC tissue (Figs. IV-2 and IV-5).

124 Figure IV-7 also indicated that among all studied adsorbents, TiO2 media has the lowest adsorption capacity. This can be partially explained on the basis of the much lower SBET (75 m².g^-1) of TiO2 media support as compared to that of the original granular AC (1065 m².g^-1).

3.2. Comparison of the Langmuir and Freundlich models parameters

The Langmuir and Freundlich models of adsorption are tested for comparison and used to obtain the parameters of phenol adsorption on different media. The isotherm equations were detailed in chapter I.

The Langmuir model has two linearization procedures. As seen in the preceding chapter I, the first version is more accurate at low concentrations while the second is rather applied at high concentrations. The first linearization is used with TiO2 media, while the second is applied with all other adsorbents used

The parameters of Langmuir and Freundlich models as well as the correlation coefficients indicating the agreement between the experimental and the calculated data by these two models are summarized in Table IV-3. The adsorption constant of Langmuir (KL) is higher for the original granular AC than for all studied adsorbents, indicating a more favourable adsorption of the phenol onto the original granular AC than onto the others adsorbents. It was noticed that TiO2 media exhibits the lower KL value. For all the tissues, the presence of fibres and/or TiO2/SiO2 induced an apparent decrease in the affinity of these materials to phenol as they are themselves adsorbents with worst affinity. Finally, for adsorbents composed with several materials like these tissues, KL is a global value where the negative impact of the worst adsorbent has an important influence.

For TiO2/AC tissue, the value of qmax calculated from Langmuir model support the hypothesis of the unfavourable role that played by the binder on the phenol adsorption. The qmaxcalculated for TiO2/ AC tissue is lower than that of original granular AC; this means that the sites of adsorption of TiO2/ AC tissue are partially masked by the binder.

Based on the correlation coefficients, the equilibrium data was slightly better fitted in the Langmuir adsorption isotherm than the Freundlich equation (Table IV-3).

125 Table IV-3

Parameter constants of Langmuir and Freundlich models for phenol adsorption Parameters of Langmuir model