# Activated Carbons as Effective Adsorbents of Non-Steroidal Anti-Inflammatory Drugs
## 3 Results  
### Adsorption Equilibrium  
Figure 1 compares the adsorption isotherms of naproxen sodium, ibuprofen sodium, and diclofenac sodium on the GAC activated carbon at 15, 25, and 35 \({}^{\circ}\)C. Under the tested experimental conditions, the largest adsorption amount was noticed for NPX and the smallest for IBP. The reasons for this phenomenon are the different solubilities and molecular structures of the tested compounds. Substances that are, in general, less soluble in water are more hydrophobic, i.e., they have a better affinity for the lyophilic surface of the used adsorbent [8; 21]. Among the tested adsorbates, ibuprofen sodium has the highest solubility in water, and diclofenac sodium has the lowest solubility in water. This means that IBP has the worst affinity for the lyophilic surface of GAC [6]. At the same time, D, despite its most hydrophobic character, has the largest particle size among the adsorbed substances selected for testing, which may hinder its adsorption to the microporous spaces of the adsorbent. Figure 1: \(Cont.\)Figure 1: The adsorption isotherms (lines represent the fitted GL equation) at 15 (**a**), 25 (**b**), and 35 \({}^{\circ}\)C (**c**) and the adsorption capacities (**d**) for ibuprofen sodium, diclofenac sodium, and naproxen sodium on activated carbon GAC. The equilibrium data were analyzed using the GL isotherm. All related parameters are presented in Table 2. In systems with ibuprofen sodium, the heterogeneity parameter m is equal to 1, after which the GL isotherm is simplified to the T equation. For the D/GAC_25 \({}^{\circ}\)C system, the full GL isotherm equation was the best for its analysis. For the remaining systems, the heterogeneity parameter n is equal to 1, after which the GL isotherm reduces to the GF equation. Therefore, it can be concluded that medium heterogeneity effects were observed in the studied experimental systems. The adjustment values of the adsorption capacity are comparable with the experimental values. The quality of the fitting is very good, which is confirmed by the low value of standard deviations (SD(a) ranging from 0.033 to 0.088) and high determination coefficient values (R\({}^{2}\) ranging from 0.909 to 0.985). Figure 2 compares the adsorption isotherms of ibuprofen sodium, diclofenac sodium, and naproxen sodium on GAC activated carbon at 15 \({}^{\circ}\)C, 25 \({}^{\circ}\)C, and 35 \({}^{\circ}\)C. Under the given experimental conditions, a clear impact of temperature on the amount of adsorption of these compounds was observed. It was shown that higher temperatures favor the sorption of IBP, D, and NPX, which indicates the endothermic nature of this process. It is also apparent that, in the case of sorption from solutions, the system is a three-component system, where an adsorbate, a solvent, and an adsorbent can be distinguished, which interact with each other. In the experimental systems tested, the adsorption mechanism is based mainly on dispersion interactions between \(\pi\) electrons from the aromatic rings of diclofenac sodium and \(\pi\) electrons from the graphene layers of GAC activated carbon. Additionally, hydrogen bonds can also be distinguished between solvent and adsorbate molecules, adsorbate/solvent molecules, and the surface groups. Interestingly, the strength of these forces changes with temperature. At higher temperatures, increased dehydration of both the molecules of sodium salts of drugs and the activated carbon grains occurs. This makes the adsorbate-water aggregates more flat, which allows them to penetrate the micropores of the adsorbent more easily. Moreover, there is also an increase in the dipole moment, which significantly intensifies the interactions between activated carbon and ibuprofen sodium, diclofenac sodium, or naproxen sodium [8; 32; 33].