## 3 Results  
### Dark Adsorption Experiments  
#### 3.1.1 Adsorption Isotherms  
The adsorption isotherms obtained for 2,4-DCF-TiO2/stainless steel system for the four pH values of the working solution are presented in Figure 1. To describe the types of interactions between the adsorbate (2,4-DCF) and the adsorbent (TiO2/stainless steel membrane), and to determine the maximum adsorption capacity of the adsorbent, [PERSON] (Equation (3)) and Freundlich (Equation (4)) isotherm models were used [27]. The parameters of the adsorption process are presented in Table 2. As can be seen, the pH value of the working solution influences the adsorption process, the adsorption capacity at pH 3 being about 13% higher than at pH 7. The slight decrease of the adsorption capacity at pH 2 could be attributed to the increase of the instability of the photocatalytic membrane in strong acidic conditions (i.e., formation of titanium hydroxy complexes with hydroxyl ions release) which leads to the modification of the properties of the aqueous environment in which the adsorption takes place. \[\frac{\mathrm{C_{e}}}{\mathrm{q_{e}}}=\frac{1}{\mathrm{K_{L}}}+\frac{\mathrm{ C_{e}}}{\mathrm{q_{m}}} \tag{3}\]\[\mathrm{q_{e}}=\mathrm{K_{F}}\times\mathrm{C_{e}^{1/n}} \tag{4}\]  
where \(\mathrm{q_{e}}\) is the adsorption capacity at equilibrium (mg 2,4-DCF/g of adsorbent), \(\mathrm{K_{L}}\) is the Langmuir adsorption constant (L/mg), \(\mathrm{K_{F}}\) is the Freundlich adsorption constant (mg1-n.L\({}^{n}\)/g) related to the energy of adsorption, \(\mathrm{C_{e}}\) is the equilibrium concentration of 2,4-DCF in the aqueous solution (mg/L), \(\mathrm{q_{m}}\) is the maximum adsorption capacity (mg 2,4-DCF/g of adsorbent) and 1/n is the heterogeneity factor (dimensionless). The value of parameter n in the Freundlich model provides information on the average adsorption energy. If n has values less than 1, the interactions between adsorbent and adsorbate are weak, and if n is greater than 1 the interactions are strong. At values of n equal to 1 it is assumed that all active centers are similar in terms of energy, and the experimental data fit better on the theoretical curves obtained with the Langmuir model. In this respect, as can be seen for all working conditions, the parameter n has values above 1 which suggests that there is a strong interaction between 2,4-DCF and the adsorbent, but the number of heterogeneous active centers is limited. Therefore, this is also the reason why the experimental data fit better on the Langmuir isotherm. It can also be seen that \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**pH**} & \multicolumn{4}{c}{**Langmuir**} & \multicolumn{2}{c}{**Freundlich**} \\ \cline{2-6}  & **K\({}_{\mathrm{L}}\) (L/mg)** & **q\({}_{\mathrm{mmx}}\) (mg/g)** & **R\({}^{2}\)** & **K\({}_{\mathrm{F}}\) (mg1-n.L\({}^{n}\)/g)** & **n** & **R\({}^{2}\)** \\ \hline
2 & 0.016 & 23.568 & 0.9984 & 0.87 & 1.61 & 0.9794 \\
3 & 0.018 & 24.876 & 0.9822 & 1.05 & 1.68 & 0.9865 \\
5 & 0.012 & 22.173 & 0.9784 & 0.62 & 1.56 & 0.9826 \\
7 & 0.007 & 21.598 & 0.9818 & 0.34 & 1.42 & 0.9727 \\ \hline \end{tabular}
\end{table} 
Table 2: Fitting parameters for 2,4-DCF adsorption on TiO\({}_{2}\)/stainless steel photocatalytic membrane. Figure 1: Adsorption isotherms of 2,4-DCF (expressed as OC) from aqueous solutions in different pH conditions: (**a**) nonlinear fitting of Langmuir isotherm; (**b**) non-linear fitting of [PERSON] isotherm; (**c**) linear fitting of Langmuir isotherm; (**d**) linear fitting of [PERSON] isotherm. the value of parameter n increases with the decrease of the pH of the working solution which indicates that the interactions between the adsorbent and chlorophenol are stronger in acidic medium. The analysis of the adsorption parameters obtained with Langmuir model provides a similar image to the one outlined in the analysis of the adsorption parameters obtained with the Freundlich model.