## 3 Results and Discussion
The \(pK_{u}\) value of uronic acids in the alginate chains is around 3-4 [32; 33], meaning that in ultrapure water, the carboxylic acid groups are mainly deprotonated and interact with calcium ions to form the hydrogel network [34; 35]. In an acidic medium, some of the Ca\({}^{2+}\) ions in the -COO-Ca-OOC- groups of the hydrogel network are replaced by H\({}^{+}\) to form -COOH groups. The Ca-ALG network is partially undone, increasing the mobility of the polymeric segments and their interaction with the solvent. The SI was even greater in NaCl because of the replacement of Ca\({}^{2+}\) with Na\({}^{+}\), loosening the hydrogel network [36]. The loss of water, at 60 \({}^{\circ}\)C, in wt% with respect to the swollen sample, is shown in Figure S5 (Supplementary Material). The maximum weight loss was around 95 wt% for the three samples, reaching equilibrium mass in 3 h. Thus, the presence of polypyrrole inthe hydrogel did not importantly affect the loss of water behavior. Finally, the extraction efficiency of the hydrogel beads was determined for the extraction of atrazine (ATZ), caffeine (CAF), and progesterone (PRO) in aqueous medium by applying the SPE technique. Although SPE is a well-established technique, low-cost commercial phases, such as octadecylsilane (C18), have several disadvantages, including poor selectivity that leads to co-extraction of interferents, and difficulty in extracting polar compounds from aqueous matrices, since they mostly have hydrophobic character [12]. Polymeric sorbents, such as Oasis HLB and Strata X, have better performance for polar compounds but have cost limitations. To overcome these problems, several new materials have recently been developed to reach high sorption capacities, selectiveness, stability, longer lifetimes, and low costs [37; 38; 39]. Especially for aqueous samples, hydrogel-based materials are very promising extraction techniques because of their structure formed by a highly hydrophilic polymeric network and their ability to be easily modified or functionalized. These materials, both naturally occurring and synthetic, are capable of absorbing and retaining large amounts of water without dissolving [11; 40], enabling interactions between analytes from aqueous samples and the polymeric hydrogel network. In this study, all hydrogels were able to extract the three analytes in the investigated media. The chromatogram after water extraction is shown in Figure S5 (Supplementary Material). The peak area of each analyte after extraction is shown in Figure 4. As observed in Figure 4, neat alginate (ALG) beads were able to extract all compounds, indicating that the polysaccharide contributes to the extraction process, acting as an effective extraction phase, as previously reported [11]. The addition of polypyrrole to the alginate hydrogel did not affect the extraction of CAF or ATZ from water, indicating that these compounds are being majorly extracted by the alginate. CAF (log(K\({}_{\text{ow}}\)) = 0.07) and ATZ (log(K\({}_{\text{ow}}\)) = 2.61) are expected to interact better with alginate. However, it is clear that the presence of polypyrrole  
Figure 3: Swelling index (in wt%) of ALG, PPy-ALG and ALG@PPy hydrogels disks in (**left**) ultrapure water, (**center**) HCl 0.1 mol L\({}^{-1}\) and (**right**) NaCl 0.1 mol L\({}^{-1}\) (replicates = 3). in the hydrogel beads increased the peak area of PRO (log(K\({}_{\text{ow}}\)) = 3.87). The extraction of PRO from NaCl aqueous solutions followed the same trend. Although the extraction of PRO from HCl aqueous solution resulted in higher peak areas, statistical differences were not observed when comparing the three samples. The results demonstrated that polypyrrole particles contribute to an increase in the hydrophobic character of the composite.