## 3 Results and Discussion
### Adsorption Tests  
#### 3.2.1 Organic Dyes Removal  
Adsorption tests on organic dyes were performed as preliminary studies with the aim of evaluating the affinity between PACs and adsorbates with different ionic properties. Adsorption occurred in a two-stage process, with a steep initial phase until 5 min for MB (Figure 2a) and 20 min for EB and BB (Figure 2b,c), as a result of the high concentration gradient. The only exception was for BB with C2-AC, whose trend continuously increased over time, reaching a plateau after approximately 2 h of contact time. It is usually reported that, at the beginning of the adsorption process, the filling of PACs pores is predominant, resulting in a marked removal rate (Figure 2). Once the pores volume has been filled, chemisorption is the driving force, whose intensity is proportional to the number of active sites in PACs surfaces [42]. The adsorption trends were confirmed by the results of the kinetic modelling (Table 3). The processes followed the pseudo-second-order kinetic in which the adsorption was proportional to the gradient squared (Equation (3)). Similar results were observed by [PERSON] et al. [37], who performed adsorption tests on MB, and by [PERSON] et al. [35], who reviewed the use of coffee as adsorbent for dyes and heavy metals in wastewaters. All PACs showed higher adsorption capacities for ionic adsorbates (Figure 2) as MB and EB. In particular, C2-AC presented always an intermediate removal rate with respect to SCGs-AC and C1-AC. SCGs-AC and C2-AC was the best PAC for MB adsorption, achieving 100% removal in 5 min of contact time (Table 4). With regard to EB, the removal rate of SCGs-AC was lower than those of C1-AC and C2-AC. However, the results were comparable if 180 min of adsorption time were considered (Table 4). Since the kinetics of adsorption could be affected by the appropriate surface functional groups of the matrix to attract the adsorbate molecule, it was suggested that the higher amount of \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sample** & **Details** & **MB** & **EB** & **BB** & **Ph** & **CP** & **BPA** \\ \hline \multirow{3}{*}{SCGs-AC} & q\({}_{\text{e}}\) & 40 & 24.8 & 19.8 & 143 & 194 & 116 \\  & Kinetic & 3.13 & 0.223 & 0.005 & 0.815 & 0.266 & 0.007 \\  & constant & R\({}^{2}\) & 0.9989 & 0.9955 & 0.9905 & 1 & 1 & 0.9969 \\ \hline \multirow{3}{*}{C1-AC} & q\({}_{\text{e}}\) & 40 & 25 & 20 & 122 & 190 & 175 \\  & Kinetic & 1.04 & 0.727 & 8.33 & 0.338 & 0.278 & 0.014 \\  & constant & R\({}^{2}\) & 0.9998 & 0.9983 & 1 & 1 & 1 & 0.9998 \\ \hline \multirow{3}{*}{C2-AC} & q\({}_{\text{e}}\) & 40 & 25 & 20 & 151 & 196 & 200 \\  & Kinetic & 8.93 & 0.471 & 0.238 & 0.441 & 0.521 & 0.005 \\ \cline{1-1}  & constant & R\({}^{2}\) & 1 & 0.9992 & 0.9975 & 1 & 1 & 0.9997 \\ \hline \hline \end{tabular}  
* The reported data are related to the average of triplicate measurements; the standard deviations were always less than 5%; q\({}_{\text{e}}\) in mg/g and kinetic constant, calculated according to Equation (3), in g/mg/min.  
\end{table} 
Table 3: Details of PACs pseudo-second-order kinetic model (best fitting) for organic dyes and phenolic compounds.\({}^{1}\). oxygen species present in the SCGs-AC sample (as determined by ESEM-EDX, Table 2) could create a wider series of van der Waals interactions and hydrogen capable of attracting more rapidly MB. The results of the adsorption isotherms (Table 5) showed that the equilibrium could be satisfactorily described by the Langmuir model, indicating a monolayer coverage onto char particles for commercial PACs and for SCGs-AC only with MB, as already reported by [PERSON] et al. [61]. [PERSON] was the best fitting model for EB and BB with SCGs-AC. There are numerous reports on organic molecules adsorption in pores that are barely large enough to admit the adsorbate molecule and it has been pointed out that dyes and humic acids have a size (1.5-3.0 nm) that favors their adsorption in mesopores (i.e., pore diameter \(>\) 2 nm) [62].