# Natural Clays as Potential Amino Acids Carriers for Animal Nutrition Application
## 3 Results
### Effect of Intercalation Operating Parameters: Contact Time and pH Solution  
Considering that pH is a fundamental parameter which can influence the uptake, the pH values of solutions containing pure methionine in the concentration range 0.0201-0.2000 M were first analyzed. All the measured pH values were between 5.22 and 5.61; therefore, it \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Mineral Clay** & **Phase Composition (XRD)** & **Surface Area** & **Pore Volume** & **Pore Diameter** & **Particle** \\  & **[m\({}^{2}\)/g]** & **[cm\({}^{3}\)/g]** & **[nm]** & **Dimensions [\(\mu\)m]** \\ \hline
**Adsorbene** & Chabazite (PDF card 01-085-1046), & & & & \\  & Phillipsite (PDF card 01-072-4634), & & & & \\  & Bentonite (PDF card 00-003-0015), & & & & \\  & Illite (PDF card 01-075-0948) & & & & \\  & Chabazite (PDF card 01-085-1046), & & & & \\  & Phillipsite (PDF card 01-072-4634), & & & & \\  & Illite (PDF card 01-075-0948), & & & & \\  & Hydroosodalite (PDF card & & & \\  & 01-073-5304), Albite (PDF card & & & & \\  & 19-1184) & & & & \\
**Veegum K** & Montmorillonite & 50 & 0.11 & 9 & 2, 5, 10 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Phase composition and morphological characteristics of the considered mineral clays. can be assumed that pH is constant (the average pH value is 5.4) within the methionine concentration range experimented in this study (Supporting S4). In order to set the best conditions for the intercalation reaction on the Adsorbene substrate, two operating parameters were considered, i.e., contact time and pH of the methionine solution. Therefore, for an experimental campaign considering two sets of intercalation tests, both at fixed methionine concentration (0.2 M), one was performed at different contact time, namely in the range 10-360 min, while the second was performed at pH range 1-8. Results are reported in Figure 2a,b. As shown in Figure 2a, where captured methionine is reported as a function of contact time, a very fast reaction occurred; the equilibrium was reached in 15 min which corresponded to a constant methionine capture of 0.18 g\({}_{\text{MET}}\)/g\({}_{\text{ADS}}\). Accordingly, to achieve the equilibrium conditions, all the experiments performed in this work were performed at 30 min of contact time. Similarly, the influence of pH on methionine capture is plotted in Figure 2b. No pH effect was observed; indeed, a methionine capture of 0.18 g\({}_{\text{MET}}\)/g\({}_{\text{ADS}}\), corresponding to the equilibrium and only depending on contact time, was observed. This result can be traced back to the reported buffer capacity of both the amino acids [35] and mineral clays [36]. The samples MET-ADS contacted at different pH were also analyzed by FTIR spectroscopy, which has been proven to be a useful technique in investigating structure and conformation of organic moieties in clay composites [26; 37]. Pristine solid methionine is present as the zwitterionic form. Assignments of the main IR bands were determined from comparison with literature data. The diagnostic bands due to the \(-\)NH\({}_{3}\)\({}^{+}\) group are assigned as follows: NH stretching modes at 2725 and 2615 cm\({}^{-1}\), with a component at 2100 cm\({}^{-1}\) ca. ; NH deformation mode at 1580 cm\({}^{-1}\) with weaker components at 1515 and 1655 cm\({}^{-1}\) (shoulder). Correspondingly, the COO\({}^{-}\) group is characterized by two bands at 1610-15 asymmetric stretching and 1415 cm\({}^{-1}\) symmetric stretching. These features are mixed extensively with \(-\)NH\({}_{3}\)\({}^{+}\) bending vibrations that fall at similar frequencies [38; 39; 40]. The side chain of methionine molecules is reported to show two sharp characteristic bands due to CH deformation of the S-CH\({}_{3}\) group at 1445 and 1340 cm\({}^{-1}\). Other deformation modes of CH in the side appear as weak and sharp bands below 1350 cm\({}^{-1}\). The C-S stretching mode is reported at lower frequencies, showing two bands at 718 and 684 cm\({}^{-1}\)[41].