## 3 Results and Discussion
### Textural and Morphological Characteristics
#### 3.3.2 SEM Investigation  
Figure 6a,d illustrates the SEM micrographs of C1 and C4 clays, showing larger pore fractions with occurrence of palygorskite fibers as laths and bundles around the pore mouths. This pattern can probably be linked with the observed high surface areas and high porosity characters of these samples. On the other hand, the image of C2 clay (Figure 6b) indicates the palygorskite fibers, which are believed to be overgrown on montmorillonite beds and radiated into the pore spaces. The pores seem to be blocked by palygorskite fibers, which may be linked with the lower surface area and pore volume values of C2 clay derived from N\({}_{2}\) adsorption data, (Table 3), as compared with the other clays. On the other hand, aggregated large particles of contained minerals were observed in the obtained images of C3 and C5 clays, (Figure 6c,e). It is well known that the clay surface is covered by hydroxyl groups in aqueous solution [33; 34; 35; 36], which are not necessarily equivalent. Some of them may act as an acid, some may act as a base and the rest may be amphoteric with different isoelectric points. Each acid strong enough to react with a solution of a given pH dissociates a proton (Clay-O\({}^{-}\) + H\({}^{+}\)), producing a negative charge. On the other hand, each base strong enough to react with the solution associates with a proton from the solution (Clay-OH\({}_{2}\)\({}^{+}\)), yielding a positive charge. Figures S3-S7 show the potentiometric titration curves of clays in the study. In all cases, curve 1 represents a blank titration curve of 25 mL HCl (0.0042 mol L\({}^{-1}\)) + 25 mL KCl (0.02 mol L\({}^{-1}\)) against 0.042 mol L\({}^{-1}\) KOH. Curve 2, however, is obtained using the same HCl+ KCl solution in which 0.1 g of clay sample is soaked overnight upon titration against the same solution of KOH. The total amounts of acid (Clay-O\({}^{-}\)) on the surface are denoted as \(\Delta\Gamma_{\text{O}^{-}}\) at the same time, while as the total amounts of base (Clay-OH\({}_{2}\)\({}^{+}\)) on the surface are denoted as \(\Gamma_{\text{H}}\)\({}^{+}\). In addition, the pH of the solution at zero charge, i.e., in presence of equal amounts of the negative and positive charges, is called the point of zero charge (P.Z.C) (Table 3). Figure 5: Pore size distribution curves for in the study clays. the potentiometric titration curves in Figure S5 ensure the presence of both acidic and basic hydroxyl groups on the surface of C3 clay. For the C4 and C5 clay samples (Figures S6 and S7), the titration curve completely coincides with the blank curve in the pH range from 7.0 to 11.0, which may refer to the absence of acidic properties of these clays--i.e., they have only basic properties, which seems most likely due to the presence a large amount of calcite in these clays [16]. Figure 7 displays the variation of the logarithm of the obtained amounts of acids (\(\Delta\Gamma_{\mathrm{O}}\)\({}^{-}\)) and of bases (\(\Delta\Gamma_{\mathrm{H}}\)\({}^{+}\)) with pH values. The \(\Delta\Gamma_{\mathrm{H}}\)\({}^{+}\) and \(\Delta\Gamma_{\mathrm{O}}\)\({}^{-}\) parameters were estimated from the difference between titration curves 1 and 2 in Figures S3-S7, while the obtained values of point zero charge are depicted in Table 3. As shown in Figure 7a, in the case of C1 clay, the amounts of acids (log \(\Delta\Gamma_{\mathrm{O}}\)\({}^{-}\)) increases gradually with an increasing pH up to pH \(\approx\) 10. However, for C2 clay, log \(\Delta\Gamma_{\mathrm{O}}\)\({}^{-}\) increases sharply up to pH = 5 and then increases gradually up to pH = 10, which may suggest that the surface of C2 clay may be covered with strong acidic hydroxyl groups. Figure 7b, displays the acid-base properties of C3 clay. The amounts of acids (log \(\Delta\Gamma_{\mathrm{O}}\)\({}^{-}\)) increase sharply with increasing the pH up to pH = 9.5. However, log \(\Delta\Gamma_{\mathrm{H}}\)\({}^{+}\) remains unchanged in the pH range between 6 and 7 and then decreases sharply up to pH = 8.8.