## 3 Results and Discussion
### Modeling of Sorption Isotherms--Influence of pH
It has been shown that the efficiency of drug removal can be strongly influenced by the presence of different ionizable species of the drug by changing the process conditions, i.e., sorption can be positively influenced by an increase in pH as in this study or negatively influenced in the case of ciprofloxacin and multi-walled CNT (above pH 7) [37], and cefdini in the pH range of 4-10 using TiO\({}_{2}\) as a sorbent [45]. To investigate the sorption capacity and multilayer formation on heterogeneous surfaces, the parameters \(n\) and \(K_{\text{F}}\) were calculated from the logarithmic form of the Freundlich isotherm [46,47]. In the case of sorption of both pharmaceuticals to TiO\({}_{2}\) in suspension, the \(n\) values are higher than 1, indicating that this type of sorbent is favorable for the removal of IMT and CRZ at low concentrations [48,49]. The sorption of IMT on TiO\({}_{2}\)-GF mesh is described by \(n\) close to 1 (at pH 5 and 9), confirming the linearity of the isotherm and the constant affinity for sorption in the applied concentration range. At the surface of TiO\({}_{2}\)/CNT-GF, sorption was a cooperative process with the same \(n\) values for each pH showing the decrease in affinity of IMT sorption due to the filled active binding sites [50]. CRZ showed a constant sorption potential with higher sorbate concentration on both immobilized catalysts with \(n\) values close to unity. The sorption capacity coefficients \(K_{\text{F}}\) were highest for the suspension, confirming the highest affinity for sorption of IMT in this form. Other \(K_{\text{F}}\) values are relatively low (19.3-27.2 (\(\upmu\)g/g)(mL/\(\upmu\)g)\({}^{1/n}\)), indicating the mobility of pharmaceutical in the sorbent [51]. CRZ generally showed higher tendency to sorb to all TiO\({}_{2}\) forms. The Langmuir model in conjunction with the Freundlich isotherm also fitted the experimental data very well, as in the case of cefdini and TiO\({}_{2}\) suspension [45], indicating the complexity of sorption considered as a possible monolayer, and heterogenous  
Figure 2: pH influence on IMT and CRZ sorption (1—suspended TiO\({}_{2}\), 2—TiO\({}_{2}\)-GF, 3—TiO\({}_{2}\)/CNT-GF). multilayer process [45; 48; 52]. At pH 9, the highest values of maximum saturated monolayer sorption capacity \(q_{m}\) (except for immobilized TiO\({}_{2}\) at pH 7) of the three TiO\({}_{2}\) modifications were obtained, indicating a higher sorption capacity at the mentioned pH, which is in agreement with the determined \(K_{d}\) values. In general, the TiO\({}_{2}\) suspension showed the better capacity to sorb both pharmaceuticals, which was expected due to the higher number of active sites on the catalyst surface and easier mass transfer while immobilization of material by trapping active sites may decrease the surface activity [22]. Higher mass of TiO\({}_{2}\) does not necessarily bring higher sorption performance, rather the performance of the sorbent or catalyst plays a major role. The results are also consistent with the results of the SEM analysis (Figure 1), which shows the agglomeration occurred during the binding of TiO\({}_{2}\) and TiO\({}_{2}\)/CNT to GF meshes, resulting in a decrease in the active surface area of the sorbent. However, to avoid problems in sample reuse and filtration, it is better to immobilize only TiO\({}_{2}\) and not to use it in combination with nanotubes because the different active sites of the two materials may cause heterogeneous sorption [45].