## 4 Cu\({}^{2+}\) Adsorption Capacity of Volcanic Glass  
The elemental composition of volcanic glass mainly consists of aluminosilicates, similar to clay minerals and zeolites, although its structure is amorphous. Previous research has established that an increase in temperature should cause an expansion of the amorphous structure; however, this thermal treatment can also produce dehydroxylation of the -OH groups, which are involved in the process of Cu adsorption [46], as indicated by the following reaction:  
\[-\mathrm{Si-OH+Cu^{2+}\leftrightarrow Si-O-Cu^{+}+H^{+}\ or-Al-OH+Cu^{2+} \leftrightarrow Al-O-Cu^{+}+H^{+}}\]  
The isotherm adsorption data (Figure 3) show how Cu adsorption decreases when volcanic glass is activated with more concentrated H\({}_{2}\)SO\({}_{4}\) solutions, following the trend H\({}_{2}\)SO\({}_{4}\) (0.0 M) \(>\) H\({}_{2}\)SO\({}_{4}\) (0.2 M) \(>\) H\({}_{2}\)SO\({}_{4}\) (0.4 M) \(>\) H\({}_{2}\)SO\({}_{4}\) (0.6 M) due to modification of the point of zero charge of the volcanic glass [17; 47]. These data are in agreement with the literature, since the heavy metal adsorption takes place in a narrow pH range [47; 48; 49]. In the case of adsorption of Cu\({}^{2+}\) species, it has been reported that Cu adsorption increases at higher pH values [24; 47]. Under these activation conditions, the leaching of volcanic glass must be discarded, since it requires stronger conditions [50; 51]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Sample** & **O 1\(s\)** & **Si 2\(p\)** & **Al 2\(p\)** & **Mg 2\(p\)** & **Ca 2\(p\)** & **Na 1\(s\)** & **K 1\(s\)** & **Cu 2\(p\)** \\ \hline
**Volcanic Glass** & 61.5 & 19.8 & 3.7 & 2.8 & 1.6 & 0.7 & 0.7 & – \\
**Cu-Volcanic Glass** & 62.1 & 19.9 & 3.8 & 2.2 & 0.9 & 0.7 & 0.7 & 0.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Atomic concentration on the surface (%). Figure 2: C 1s core level spectrum and O 1s core level spectrum of Cu-volcanic glass. The isotherm data were fitted to the Langmuir model (Table 3). These data reveal that volcanic glass without previous acid treatment reaches the \(q_{max}\) maximum value (4.32\(\cdot\)10\({}^{-4}\) mol g\({}^{-1}\)); however, the \(q_{max}\) value decreases accordingly as the H\({}_{2}\)SO\({}_{4}\) concentration used in the prior activation increases. The strength of the adsorbate-adsorbent interaction, defined by the \(K_{L}\) value, shows how volcanic glass without acid activation exhibits a stronger interaction. Since volcanic glass without previous acid activation reached maximum adsorption capacity, this material was recycled to evaluate its behavior in liquid paraffin separation. Prior to this study, volcanic glass was characterized after the adsorption process to show the Cu adsorbed. The elemental bulk composition of Cu-volcanic glass was evaluated from XRFS analysis (Table 1). These data indicate that the amount of Cu (in the form of CuO) after the adsorption process was 2.2%, while the amounts of other elements were close to that shown for raw volcanic glass, which discards the leaching of these elements along the adsorption process or along the acid activation. In the same way, the chemical composition on its surface was determined by XPS, reaching a Cu content of 0.6% (Table 2). The Cu \(2p\) core level spectrum displays a main peak at about 932.8 eV, which is ascribed to Cu\({}^{2+}\) species in the form of CuO, while the other contribution, located at about 935.2 eV, is assigned to the shake-up satellite, which is typical of bivalent cations [52]. From the atomic concentrations, a slight decrease of the Mg and Ca species on its surface can be observed, which could indicate partial leaching of these species or deposition of the Cu species in these zones. Finally, the Cu content on its surface was also confirmed by TEM (Figure 4). This micrograph shows how Cu\({}^{2+}\) species are distributed on the surface in some parts of volcanic glass; in other parts, it is not present.