# Enhanced Adsorption of Methyl Orange by Mongolian Montmorillonite after Aluminum Pillaring
## 4 Discussion
### MO Adsorption of Modified MMt  
After modification, the maximum adsorption ability of P-MMt towards MO is improved significantly, compared to that of the precursor MMt. The hydrophilic property of MMt results in it being less attracted to non-polar organic compounds, performing poorly in MO adsorption [10; 48; 49]. In addition, the highly negatively charged surface of MMt may hinder the MO (anionic dye) moving to MMt by repulsion. After Al-pillaring, the maximum MO adsorption ability increased from 0.25 mg/g (MMt) to 6.23 mg/g (P-MMt-C400). It has been reported that strong Lewis acidity may arise from the coordinately unsaturated aluminum centers on the aluminum pillars [48; 49]; thus, the Lewis acidity sites from the aluminum coordination in Al-pillared MMt (e.g., P-MMt-C400) can provide adsorption sites to attract MO onto their surface.  
If we consider that the MO molecules were horizontally adsorbed into the porous structure of the adsorbent, the cross-section of the MO molecule is 0.67 \(\times\) 0.38 nm (Table 2). In order to adsorb the MO in the horizontal orientation, the size of the pore should be at least 0.67 nm. In addition, it has been reported that pollutant molecules can be adsorbed into pores with a diameter 1.3-1.8 times that of the pollutant [50]. If the maximum value (1.8 times) is taken, the minimum pore size adsorbing the MO in the horizontal orientation is ~1.2 nm [6]. However, according to Figure 4, the increase in the surface area of Al-pillared MMt is mainly attributed to micropores ranging from 0.4 to 1.2 nm. Therefore, even under these conditions, the higher surface area of Al-pillared MMt (P-MMt and P-MMt-C400) could not provide more available adsorption sites to adsorb more MO even though it has a larger surface area due to the smaller size of its micropores. In contrast, our results indicate that the improvement of the MO adsorption ability of Al-pillared MMt is due to the property of the Keggin Al\({}_{13}\) complex and the increase in the surface area contributed by micropores larger than 1.2 nm. It is therefore safe to conclude that the MO is adsorbed onto the surface of the Keggin Al\({}_{13}\) complex which is in the interlayer, edge, and bared surface of MMt and its modified products, and this absorption can be associated with the contribution from the increase in meso- and macro-pores, and even micropores larger than 1.2 nm after modification.  
The kinetics fitting results (Table 4) show that the adsorption kinetic of P-MMt-C400 is better fitted to the PSO, indicating that the MO adsorption processes of these modified MMts are supposed to be dominated by chemisorption. Further interpretation of the kinetic data shows that the interaction between P-MMt-C400 and MO is electrostatic. According to the isotherm fittings (Table 5), three adsorption isotherm models (Langmuir, Freundlich, and Redlich-Peterson) are perfect fits for the data of MO adsorption by P-MMt-C400. Recent studies have reported that either the Langmuir model or the Freundlich model fits the organic dye contaminant adsorption on modified clay [6; 8; 51]. The difference of R\({}^{2}\) between the PSO and PFO model fittings is not significant in this study. MO probably interacts with the modified MMt by mixed interaction modes, as reflected by the low value of the Freundlich 1/n parameter, which is strongly related to the energetic heterogeneity of the adsorption processes. Future study is therefore desired in order to fully understand the reaction mechanism.  
In this study, the Redlich-Peterson model was better fitted to the MO adsorption isotherm of P-MMt-C40 than the Langmuir and Freundlich models. The \(\beta\) value of the Redlich-Peterson model for MO adsorption by P-MMt-C400 is 0.76, which is not close to 1, indicating that the isotherms are close to the Freundlich model, and suggesting that the adsorption process of P-MMt-C400 is heterogeneous multilayer adsorption.