## 4 Discussion
### Carriers and Release of Methionine  
The synthesis procedure applied to prepare Met-ADS and the Met-BIO samples has demonstrated to be effective in adsorbing the organic molecule onto the carriers. However, considering the different phase composition and structure of the clays, different kinds of  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & **Sample Condition** & \multicolumn{2}{c}{**Percentage Recovered (\%)**} \\ \hline
**Timepoint** & **Sample** & & **Luminal** & **Basolateral** \\ \hline \multirow{4}{*}{0 h} & \multirow{2}{*}{CTR} & KRB buffer pH 7 (no & \multirow{2}{*}{-} & \multirow{2}{*}{-} \\  & & Met) & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\ \hline \multirow{4}{*}{3 h} & \multirow{2}{*}{Met-ADS} & KRB buffer pH 7 (no & \multirow{2}{*}{-} & \multirow{2}{*}{-} \\  & & Met) & & \\  & & & & \\  & & & & \\  & & & & \\ \hline \multirow{4}{*}{3 h} & \multirow{2}{*}{Met-ADS} & KRB buffer pH 3 (no & \multirow{2}{*}{-} & \multirow{2}{*}{-} \\  & & Met) & & \\  & & & & \\  & & & & \\ \hline \multirow{4}{*}{3 h} & \multirow{2}{*}{Met-ADS} & KRB buffer pH 3 (no & \multirow{2}{*}{-} & \multirow{2}{*}{-} \\  & & Met) & & \\  & & & & \\ \hline \end{tabular}  
* Met = Methionine; ADS = Adsorbene; KRB = Kreb’s Ringer Buffer.  
\end{table}
Table 3: Methionine percentage investigated in the Using chamber with porcine mucosa. Results are presented as a percentage of the luminally applied dose recovered (%) at the beginning of the experiment (0 h) and after 3 h of incubation.  
interaction are present in ADS and BIO. In the case of BIO, a pure zeolitic material, MET is trapped inside the zeolitic cages; while in the case of ADS, a mixture of montmorillonite and zeolite-type materials, Met interacts with the carrier via different mechanisms. Indeed, in ADS, the expendable montmorillonite component can capture Met via both interlayer intercalation, without ions exchange, and surface adsorption, while the zeolitic component behaves as BIO, i.e., via a trapping mechanism ([PERSON] et al. 2021, accepted on 14 June).  
From the results in Table 1, it is evident that Met is strongly bonded to the carrier. This behavior can be explained considering the nature of the carrier. The ADS is a complex mixture of different mineral clays and zeolites, such as Chabazite (PDF card 01-085-1046), Phillipsite (PDF card 01-072-4634), Bentonite (PDF card 00-003-0015), and Illite (PDF card 01-075-0948). The BIO is a fully zeolite-based material (i.e., Chabazite (PDF card 01-085-1046), Phillipsite (PDF card 01-072-4634), Illite (PDF card 01-075-0948), Hydrosodalite (PDF card 01-073-5304), and Albite (PDF card 19-1184) as found by XRD analysis of the powders. A strong Met trapping by the carrier is present; this is essentially due to the zeolites structural cages, which are able to trap and strongly interact with organic molecules in general. Indeed, the same amount of Met was etched in near- neutral or harsher conditions (i.e., strong acid or base conditions), where a larger release was expected. Therefore, it can be concluded that only Met weakly bounded at the surface of the carrier is released, while no effect of water, high or low pH is exerted on trapped Met.