## 3 Results and Discussion  
### Physico-Chemical Properties of PACs  
The porous nature of the samples was investigated by N\({}_{2}\) physisorption at \(-\)196 \({}^{\circ}\)C. All of them displayed very high surface area values, of 823 and 932 m\({}^{2}\)/g for the commercial C1-AC and C2-AC, respectively, up to 1199 m\({}^{2}\)/g for SCGs-AC, prepared by spent coffee grounds treated with KOH and then pyrolyzed at high temperature. In particular, the latter sample showed a 30% higher BET surface area with respect to the commercial active carbons used for comparison. The cumulative pore volume of the three sample was comparable and very high, in the range 0.491-0.523 cm\({}^{3}\)/g. According to the IUPAC classification [59], the mean pore diameter was borderline between micro- and mesopore regions for SCGs-AC (1.7 nm) and typical of mesoporous materials for C1-AC and C2-AC (2.5 nm and 2.1 nm, respectively). It is well established that the adsorption characteristics of activated carbons derive from the nature of the precursor, production method, and experimental conditions during the preparation. Thus, they are recognized as versatile adsorbents due to their high specific surface area, porosity, high adsorption capacities, and surface properties [42]. In particular, the pH-PZC values of the activated carbons here considered, calculated using the solid addition method reported by [PERSON] et al. [57], ranged in the interval 8.8-10.5 (Table 2). Activated carbons are amphoteric materials in which surface charges depend on the pH of the solution. As an example, SCGs-AC sample displayed a pH-PZC of 9.5, which gave rise to a negative charge on the surface of the activated carbon for pH values higher than the pH-PZC, with an electrostatic interaction between the negatively charged surface of the AC and a cationic molecule. As the pH of the system decreases, the number of positive charges on the surface increases, and the surface sites on the activated carbon do not tend to absorb the cations of dye due to electrostatic repulsion. An inverted mechanism occurs in the presence of an anionic probe molecule, with stronger electrostatic interactions at acidic pH values and repulsive interactions in strong alkaline conditions when the pH is higher than the pH-PZC of the material. All the adsorption determinations here reported have been carried out at pH about 6, thus reducing the negative charge amount on the AC surface in the order C2-AC \(>\) SCGs-AC \(>\) C1-AC. ESEM-EDX was applied to observe the morphology and pore size of PACs and to determine their composition. Figure 1a,b illustrate 1600X and 400X magnified ESEM micrographs of SCGs-AC sample. The surface was smooth, full of cavities and characterized by partially flattened cylindrical pores. In the pores, traces of foreign elements (i.e., Mg, Al, Si or S, \(\leq\) 1% _w/w_) could be observed. The morphology is quite similar to that reported by [PERSON] [38], where the pores observed (\(\mu\)m range) act as channels for the microporous network. The textural features of the commercial samples, observed from micrographs in Figure 1c-f, were significantly different from those obtained for the SCGs-AC sample. These images showed the extent of inhomogeneity of the surfaces of C1-AC and C2-AC, which displayed a more compact and less porous structure compared to the adsorbent generated from spent coffee grounds. By comparing the ESEM-EDX elemental composition of the three samples (Table 2), it emerged that SCGs-AC contained a much higher amount of oxygen with respect to the commercial materials