# Natural Clays as Potential Amino Acids Carriers for Animal Nutrition Application
## 3 Results
### Effect of Methionine Content in the Initial Solution  
The effect of initial methionine content is reported in Figure 4a. It was evident that all the carriers were able to capture the target molecule; however, in different extents and, apparently, via different mechanisms. VEG was able to adsorb larger amounts of methionine, but, at higher methionine content, a plateau is reached. On the other side, ADS and BIO, which evidenced a fully overlapped linear behavior, were able to capture lower amounts of methionine, on average 1/3 of VEG, but no site saturation was apparently present. To deeper comprehend differences in the intercalation/adsorption characteristics and the occurring mechanisms when the different sorbents are used, adsorption isotherms measurements were analyzed. The methionine-clay interaction was interpreted according  
Figure 3: FTIR subtraction spectra (ADS + MET) – (ADS) at different pH. Pure methionine spectrum is also reported as reference.  
to both Langmuir and Freundlich models [24; 25]. It is well known, indeed, that the Langmuir model describes a more homogeneous distribution of adsorption sites energy, where equal active sites can be occupied only once, while the Freundlich model is able to describe complex and heterogeneous surfaces.  
Figure 4: _Cont._  
For all the systems, the Freundlich model provided the best agreement with the experimental data (Figure 4b). Moreover, considering fitting results, it appears that for the MET-VEG system, higher values of both the adsorption capacity (K \(\cong\) 0.068 for VEG vs. K \(\cong\) 0.023 for ADS or BIO) and the heterogeneity factor (n \(\cong\) 1.9 for VEG vs. n \(\cong\) 1.5 for ADS or BIO) were found. As a result, saturation over VEG occurred faster than over ADS or BioKi; moreover, VEG capture appeared also to occur only above a threshold concentration of about 1.7 g/L (Figure 4c).  
To obtain information on the strength of interaction between methionine and carriers, three selected samples (two for MET-VEG and one for MET-ADS), after capture, were washed once and twice, by subsequent resuspensions of the powders in 50 mL of demineralized water for 10 min at 25 \({}^{\circ}\)C, stirring rate at 600 rpm. After solid-liquid separation as usual, the methionine in the supernatant solution was analyzed by COD. VEG released almost up to 55% of the captured methionine, while ADS released less than 10%, even though the washing conditions were the same as applied to VEG (Figure 4a, open triangles). Such methionine was probably weakly bounded on the surface and/or not intercalated in the montmorillonite interlayer. Conversely, no release was found in the case of the MET-ADS sample (Figure 4, open squares) and MET-BIO one (Figure 5, open circles, not visible being overlapped), pointing out a stronger MET-ADS interaction.  
These tests can give only preliminary indications; however, it can be concluded that methionine was more strongly captured by ADS than by VEG, and this behavior may be due to the different nature and morphology of the constituent mineral substrate, i.e., zeolite-type and montmorillonite-type, respectively.  
Further investigation on interaction strength in MET-ADS samples was carried out by combined thermal analysis (TG) and FTIR at increasing temperature, and data is discussed in Section 3.5.  
Figure 4: Comparison between (**a**) capture capabilities (open symbols: MET–ADS and MET–VEG washed once and twice); (**b**) adsorption isotherms (lines: Freundlich model); (**c**) isotherms in the 0–0.25 g/L range of methionine equilibrium concentration.  
Moreover, considering the release of methionine by washing observed for the MET-VEG system, this carrier was considered unsuitable for the target application; therefore, it was not further considered in the experimentation, except for comparison.