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by the pseudo-second order equation than by the first order one, which is confirmed by the R 2 and S yx values in Table 4. The kinetic constant k 2 decreases from salicylic acid > diclofe- nac > ibuprofen > acetaminophen. Furthermore, the k 2 corresponding to the adsorption of acetaminophen is an order of magnitude lower than the k 2 determined for the adsorp- tion of the other pharmaceuticals here considered. On the other hand, the fitted q e for the adsorption of salicylic acid is an order of magnitude lower than for the rest of drugs. Kinetic results on the adsorption of the considered pharma- ceuticals from ultrapure water are given in Fig. 2. As for the adsorption from the secondary effluent, fittings to the pseudo- second order equation are better than those to the pseudo-first order one; salicylic acid was the one showing the lowest adsorbed equilibrium concentration ( q e ), which was an order of magnitude lower than for the rest of pharmaceuticals; and the adsorption of acetaminophen was the slowest one with a k 2 an order of magnitude lower than for the rest of drugs. From ultrapure water, the kinetic constant k 2 also decreases from sal- icylic acid > diclofenac > ibuprofen > acetaminophen. When comparing the adsorption kinetics of each pharmaceutical from wastewater (Fig. 1) with its adsorption from ultrapure water (Fig. 2), differences are not especially relevant as confirmed by the pseudo-second order kinetic parameters in Table 4. Equilib- rium was attained quite quickly from either the STP secondary effluent or ultrapure water, for all the pharmaceuticals equilib- rium being attained within 200 min. In any case, under the experimental conditions here used, the kinetic constant k 2 was slightly higher (diclofenac, salicylic acid, ibuprofen) or equal (acetaminophen) for the adsorption from the secondary STP effluent than from ultrapure water. On the other hand, the adsorbed concentration in the equilibrium ( q e ) was slightly higher (diclofenac, ibuprofen) or equivalent (salicylic acid, acet- aminophen) from ultrapure than from the secondary STP effluent. The equilibrium experimental data on the adsorption of diclofenac, ibuprofen, salicylic acid and acetaminophen from the secondary STP effluent and from ultrapure water are shown in Figs. 3 and 4, respectively, together with fittings to the isotherm models considered. Parameters determined from these fittings are depicted in Table 5. The equilibrium isotherms obtained for the adsorption of the pharmaceuticals from the STP effluent (Fig. 3) show some differences regarding the adsorption capacity and shape of isotherm. The adsorption capacity decreases from diclofenac > ibuprofen acetaminophen > salicylic acid. These capacity differences must be related to the drugs properties, among which S w and log K ow seem to have been especially determinant (Calisto et al., 2015). With respect to the isotherm, fittings to the Sips model are the most accurate for diclofenac, ibuprofen and acetaminophen. However, for salicylic acid, ambiguous fit- tings to the Sips isotherm were obtained, while both the Lang- muir and, especially, the Freundlich isotherm models fitted experimental results. These observations are confirmed by parameters in Table 5, which make evident differences between the capacity and the isotherm model fittings. As shown in Fig. 4, the adsorption equilibrium of pharmaceuticals from ultrapure water mostly resembles their adsorption from the STP secondary effluent. The equilibrium capacity also decreases from diclofenac > ibuprofen acetaminophen > salicylic acid and the Sips isotherm model
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