## 3 Results and Discussion
In agreement, larger adsorption concentrations at the equilibrium were also determined for ibuprofen onto GPP20 than onto the polymeric resin. When comparing the adsorption of acetaminophen and ibuprofen, larger \(q_{\mathrm{e}}\) were determined for acetaminophen than for ibuprofen onto the activated carbon GPP20, while the opposite was observed in the case of Sepabeads SP207. Since pH effects strongly affect the adsorption process in general and the adsorption capacity in particular, this difference must be related to the different pKa values of the pharmaceuticals (Table 2). According to these values, acetaminophen is neutral at the pH of ultrapure water (pH = 6.9), while ibuprofen is negatively charged. Therefore, in the case of acetaminophen, the main factor conditioning its binding is the occurrence of non-electrostatic interactions such as hydrogen-bonds and Van deer Waals-forces. Differently, the adsorption of ibuprofen is affected by electrostatic interactions. In fact, the point of zero charge (PZC) of GPP20 and Sepabeads SP207 were determined to be 7.4 and 8.6, respectively. Hence, while the surface of GPP20 at the working pH is mostly uncharged, that of the polymeric resin is positively charged, which explains its higher affinity and capacity for ibuprofen (negative charged form) than for acetaminophen (neutral form). In waste waster (Figures 7 and 8), the adsorption equilibrium isotherms remained mostly the same as those in ultrapure water, with the above observations being also applicable. Yet, at the pH of waste water (7.8), acetaminophen and ibuprofen are neutral and negatively charged, respectively. On the other hand, the surface of the GPP20 is mostly neutral, and that of Sepabeads SP207, positively charged. With respect to the isotherm modeling, as may be seen in Figures 5-8, the best fittings were to the Langmuir-Freundlich model. This is confirmed by \(R^{2}\) and \(S_{\mathrm{xy}}\) values in Table 5, which make evident the differences between fittings to the different isotherm models. Figure 6: Equilibrium results on the adsorptive removal of ibuprofen from ultrapure water by adsorption onto (**a**) the activated carbon GPP20 and (**b**) the polymeric resin Sepabeads SP207. Experimental results are shown together with fittings to the Freundlich, to the Langmuir and to the Langmuir-Freundlich isotherm models. Error bars stand for the standard deviation of three experimental replications. Note: for a better visualization of fittings, the scale of axis Y has been adjusted to results. As may be seen in Table 5, according to fittings to the Langmuir-Freunlich isotherm, the maximum adsorption capacity (\(Q_{\mathrm{m}}\)) values obtained for acetaminophen onto GPP20 were two orders of magnitude larger than onto Sepabeads SP207. Then, for each adsorbent, equivalent \(Q_{\mathrm{m}}\) were obtained for the adsorption of acetaminophen from ultrapure and from waste water. In the case of ibuprofen, the Langmuir-Freundlich fitted \(Q_{\mathrm{m}}\) values were one order of magnitude larger onto GPP20 than onto Sepabeads SP207. Also, for each adsorbent, similar \(Q_{\mathrm{m}}\) were determined for the adsorption of ibuprofen from ultrapure and from waste water. These results are in agreement with those by [PERSON] et al. [29], who also found that the adsorption capacity of activated carbon was larger than that of several polymeric resins in the removal of pharmaceuticals (acetaminophen and malidixic acid) from water. Figure 8: Equilibrium results on the adsorptive removal of ibuprofen from waste water by adsorption onto (**a**) the activated carbon GPP20 and (**b**) the polymeric resin Sepabeads SP207. Experimental results are shown together with fittings to the Freundlich, to the Langmuir and to the Langmuir-Freundlich isotherm models. Error bars stand for the standard deviation of three experimental replications. Note: for a better visualization of fittings, the scale of axis Y has been adjusted to results.