## 4 Cu\({}^{2+}\) Adsorption Capacity of Volcanic Glass
Energy-Dispersive X-ray spectroscopy (EDX) analysis confirms that the zones where copper is present correspond with the presence of carbon, and therefore its distribution on the surface depends on carbonate location. These data corroborate that Cu incorporation takes place from the intercalation of Cu with Ca from calcite in the pristine mineral. The data could be in agreement with \begin{table}
\begin{tabular}{c c c c} \hline Sample & \(q_{max}\times 10^{3}\) (mmol g\({}^{-1}\)) & \(K_{L}\) (L g\({}^{-1}\)) & \(R\) \\ \hline Volcanic Glass (H\({}_{2}\)SO\({}_{4}\) 0.0 M) & 4.32 \(\pm\) 0.27 & 2.5 \(\pm\) 0.9 & 0.96 \\ \hline Volcanic Glass (H\({}_{2}\)SO\({}_{4}\) 0.2 M) & 4.00 \(\pm\) 0.14 & 1.5 \(\pm\) 0.5 & 0.97 \\ \hline Volcanic Glass (H\({}_{2}\)SO\({}_{4}\) 0.4 M) & 2.34 \(\pm\) 0.07 & 1.2 \(\pm\) 0.2 & 0.96 \\ \hline Volcanic Glass (H\({}_{2}\)SO\({}_{4}\) 0.6 M) & 1.81 \(\pm\) 0.08 & 0.5 \(\pm\) 0.1 & 0.94 \\ \hline \end{tabular}
\end{table} 
Table 3: Equilibrium adsorption parameters according to the Langmuir model to Cu\({}^{2+}\) adsorption on volcanic glass. Figure 3: Cu\({}^{2+}\) adsorption in volcanic glass (VG) with acid treatment and activated with H\({}_{2}\)SO\({}_{4}\) (0.2 M, 0.4 M, and 0.6 M) solution. the adsorption data, since the incorporation of Cu\({}^{2+}\) species must take place by a cationic exchange of Mg\({}^{2+}\) or Ca\({}^{2+}\) by Cu\({}^{2+}\) species, which is in agreement with the atomic concentrations on the surface (Table 2). The reaction that takes place with the carbonate species should be:  
\[(\mathrm{Ca_{x}Mg_{y}})\mathrm{CO_{3}}+2\mathrm{Cu^{2+}}\rightarrow(\mathrm{Ca_{ x-1}Mg_{y-1}Cu_{2}})\;\mathrm{CO_{3}}+\mathrm{Mg^{2+}}+\mathrm{Ca^{2+}}\]  
The adsorption isotherm of the raw volcanic glass reveals that Cu\({}^{2+}\) species are adsorbed at low \(c_{eq}\) values (Figure 3). These data suggest a strong interaction between raw volcanic glass and the Cu\({}^{2+}\) species, as was reported from the \(K_{L}\) values (Table 3). The acid activation of volcanic glass causes a decrease in the adsorption capacity, probably due to a slight dissolution of the carbonate species by the acid solution. In these cases, Cu\({}^{2+}\) adsorption could be ascribed mainly to the interaction of the Cu\({}^{2+}\) species with the silanol groups. The strength of this interaction is lower, as indicated by the isotherm profiles and the \(K_{L}\) values. However, \(K_{L}\) values for this material are in line with those found in the literature for other materials: 0.024 L/mg for chitosan-based materials [53]; 8-10 L/mg, depending on the temperature, for functional Poly-Ethylene Terephthalate (PET) fibers [54]; 0.002-0.0006 L/mg for carbon nanotube-mullite composites [55]; 0.015-0.239 L/mg for multiadsorbent systems containing tea waste and dolomite [56]; and 0.098-0.26 for nanocomposites of ZnO with montmorillonite [57]. To sum up, volcanic glass is an efficient material for the adsorption of heavy metals such as Cu. The presence of minor impurities of carbonate species improves Cu adsorption capacity by stronger chemical interactions. These Cu adsorption values are below those of other adsorbents, such as zeolites and activated carbons, due to these materials display high specific surface area and/or microporosity. Nonetheless, volcanic glass displays quite interesting Cu adsorption values, taking into account that this material is inexpensive and highly available. Figure 4: TEM micrograph of Cu-volcanic glass.