## 3 Results
### Sorption Studies  
The sorption studies of ibuprofen and ketoprofen on 4 VP copolymers and poly(St-co-DVB) were performed in the following two modes: static and dynamic. Firstly, static experiments were made, where 50 mg of each sorbent was mixed with 20 mL of drug solution. The concentration of IBU and KETO in the water solution was 20 mg/L. It is the maximum concentration that can be operated since the solubility of IBU in water is 21.0 mg/L [38], and KETO is 21.3 mg/L [39]. The effect of solution pH was investigated to \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sample** & \(T_{2\%}\) [\({}^{\circ}\)C] & \(T_{50\%}\) [\({}^{\circ}\)C] & \(T_{max1}\) [\({}^{\circ}\)C] & \(\Delta\)\(m_{1}\) [\%] & \(T_{max2}\) [\({}^{\circ}\)C] & \(\Delta\)\(m_{2}\) [\%] & \(R_{m}\) [\%] \\ \hline poly(4 VP-co-14 DMB) & 316 & 335 & 334 & 66.3 & 460 & 28.8 & 4.9 \\ poly(4 VP-co-TRIM) & 301 & 322 & 320 & 70.7 & 469 & 25.8 & 3.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters characterizing the thermal stability of 4 VP copolymers. Figure 6: TG and DTG curves and DSC thermograms for 4 VP copolymers. achieve the maximum sorption of target drugs. The efficiency of sorption was expressed as the percentage of drug amount adsorbed on the sorbent. As can be seen from Figure 7, the drug sorption is strongly pH-dependent. For both studied drugs, the increase in pH value causes a drop in drug sorption. In pH 11, more than 97% of drugs remain in the water solution, regardless of the sorbent used. On the other hand, in pH 3, 100% sorption of ibuprofen and ketoprofen was obtained on poly(4 VP-_co_-14 DMB) and almost 100% on poly(4 VP-_co_-TRIM), while a significant amount of both drugs remained in the solution contacted with poly(St-_co_-DVB). These results indicate that the sorption of ibuprofen and ketoprofen on 4 VP sorbents is the most effective in an acidic environment with pH 3. Ibuprofen and ketoprofen are carboxylic acids; their pK\({}_{\text{a}}\) values are 5.30 [38] and 4.45 [39], respectively, so at pH 3, these compounds are protonated. While the sample pH is increased to neutral and then to basic, the carboxylic group is deprotonated, and drug molecules are not able to interact with the sorbent by means of hydrogen bonding. Additionally, the target drugs are aromatic compounds; thus, the \(\pi\)-\(\pi\) interactions may also be responsible for their sorption on all the discussed copolymers. However, when the chemical nature of copolymers will be considered together with the efficiency of sorption, it can be stated that the main role in the sorption mechanism plays hydrogen bonding. Hydrophobic sorbent poly(St-_co_-DVB) may interact with target molecules mainly through \(\pi\)-\(\pi\) interactions. Even in pH 3, the sorption efficiency on this copolymer was much lower than on 4 VP copolymers. In their crosslinked networks, besides aromatic rings, the nitrogen atom in the pyridine ring and carbonyl groups are present and can interact by hydrogen bonding. It was also earlier reported that the extraction of acidic NSAIDs from the copolymers containing 2-vinylpyridine is based on hydrogen bonding [17; 18]. The sorption results obtained in pH 3 indicated that the sorption capacity of 4 VP copolymers in an acidic pH was not exceeded. Therefore, the next experiment was made to assess the 4 VP copolymer's sorption capacity towards ibuprofen and ketoprofen in pH 3. In this case, 24 mg of sorbent was mixed with 50 mL of drug solution with a concentration of 20 mg/L. The calculated sorption capacity of poly(4 VP-_co_-TRIM) for ketoprofen is 39.50 mg/g and for ibuprofen 90.81 mg/g, whereas the sorption capacity of poly(4 VP-_co_-14 DMB) for ketoprofen is 40.13 mg/g and for ibuprofen 75.36 mg/g. These values seem to be very high, and, therefore, 4 VP copolymers can be considered the efficient sorbents for ibuprofen and ketoprofen removal from water samples, even with neutral pH. Sorption properties of 4 VP microspheres in dynamic conditions were tested towards ibuprofen and ketoprofen using the SPE technique.