## 3 Results
Figure 2: Equilibrium results on the adsorption of paracetamol onto (**a**) _Synechocystis_ sp. ; (**b**) activated carbon. Equilibrium experimental results of the adsorbed paracetamol (\(q_{e}\), mg g\({}^{-1}\)) versus paracetamol concentration in the liquid phase (C\({}_{e}\), mg L\({}^{-1}\)) are represented together with fittings to the [PERSON] and Freundlich equilibrium isotherm models. Note: Error bars stand for standard deviation. were quite similar, the Freundich isotherm described experimental results slightly better, pointing to a certain surface heterogeneity as compared with activated carbon. Fitted parameters for the [PERSON] and Freundlich isotherms are shown in Table 1. In the case of Langmuir, \(Q_{max}\) indicates the maximun adsorption capacity of the adsorbent material corresponding to a monolayer coverage by the adsorbate (paracetamol, in this study); meanwhile, \(K_{L}\) is related to the binding energy, the higher the \(K_{L}\), the largest affinity of the adsorbent for the adsorbate. The obtained \(Q_{max}\) values indicated that the commercial activated carbon poses a five times larger adsorption capacity (278 mg g\({}^{-1}\)) than the microalgae biomass (53 mg g\({}^{-1}\)). Moreover, the \(K_{L}\) constant corresponding to the commercial activated carbon was one order of magnitude higher than that of _Synechocystis_ sp., pointing to the lower affinity of the latter for paracetamol. In the Freundlich isotherm model, \(K_{f}\) and \(N\) parameters are empirical constants related to the adsorption capacity of the adsorbent and the adsorption intensity, respectively. \(K_{f}\) high values indicate a high adsorption capacity and, in this study, the \(K_{f}\) corresponding to the activated carbon was significantly higher than that of microalgae. Thus, the activated carbon displayed a \(K_{f}\) of 40 [(mg L\({}^{1/N}\))/(g mg\({}^{1/N}\))] while the microalgae had a value of 0.6 [(mg L\({}^{1/N}\))/(g mg\({}^{1/N}\))]. In any case, \(N>1\) for both adsorbents, which points to favorable adsorption of paracetamol. Research carried out on the adsorption of paracetamol from water using alternative adsorbents is still scarce. Crop residues [24] and pyrolysed pulp mill sludge [25] have been used for the adsorption of paracetamol from water and Langmuir fittings of equilibrium results revealed \(Q_{max}\) values of 218 mg g\({}^{-1}\) and 12 mg g\({}^{-1}\), respectively. Therefore, the maximum capacity of crop residues is larger than that obtained in this work when using microalgae (\(Q_{max}\) = 53 mg g\({}^{-1}\)), but similar to that of the here used activated carbon (\(Q_{max}\) = 278 mg g\({}^{-1}\)). The Freundlich fitted constant (\(N\) value) determined for the adsorption equilibrium onto crop residues [24] was 1.3, which is lower than the value obtained for the activated carbon in this work but almost the same to that obtained for microalgae. On the other hand, pyrolyzed pulp mill sludge [25] showed a lower \(Q_{max}\) than both the microalgae and activated carbon used here. Regarding the adsorption of paracetamol from aqueous solution onto activated carbons, some studies have been carried out using materials from different companies [26]. Equilibrium results from these studies and the corresponding fittings provided \(Q_{max}\) between 163 and 323 mg g\({}^{-1}\), which comprises the capacity of the here used activated carbon. It must be here highlighted that the separation of microalgae biomass from treated water is a key issue for the practical utilization of microalgae in wastewater treatment. In this study, at the end of adsorption experiments, separation of microalgae biomass was carried out by filtration. However, this is not feasible at a real scale, at which, flocculation-coagulation would be used for the separation of microalgae from the liquid phase. As described elsewhere [27; 28] different flocculants may be used for this purpose.