## 3 Results
### Adsorption Mechanism  
The solution pH is an important element in the mechanism investigations since it influences the dominant adsorbate species and the surface charge of the solid. In this study, the influence of pH on adsorption efficiency of CV dye was studied by fixing stirring time at 240 min and an initial dye concentration of 30 mg/L by using 1 g/L DSD. Results show that pH was found to have little effect on the CV dye removal by DSD adsorbent, which is illustrated in Figure 4b. This observation is consistent with findings of other researchers for the removal of cationic dyes using different kinds of soils [36; 37].  
At pH ranging from 3 to 11, the adsorption capacity increases slightly with increasing pH values. The basic condition (pH = 11) was more favorable for the CV dye adsorption with a maximum uptake of 27.19 mg/g. The slight improvement of adsorption efficiency can be explained by the weak role of electrostatic attraction between the negative surface charges of DSD material and the positive charge of CV\({}^{+}\) dye at pH above pH\({}_{\text{PZC}}\). The hydroxyl groups on the solid surface are ionized (such as Al-OH and Si-OH) at high pH values and give it a negative charge [31; 38].  
Figure 3: N\({}_{2}\) adsorption/desorption isotherm of DSD material at 77 K and its textural characteristics.  
In contrast, the DSD adsorbent reacts as a positive surface at pH levels lower than pH\({}_{\text{PZC}}\) and does not favor the attraction forces with CV\({}^{+}\) adsorbate. Additionally, the competition between the H\({}^{+}\) protons and the C\({}_{25}\)H\({}_{30}\)N\({}_{3}\)\({}^{+}\) dye can occurred at acid pH, which slightly decreases the CV removal efficiency [39].  
The effect of ionic strength on the adsorption capacity (_qe_, mg/g) was investigated further to better understand the role of electrostatic attraction in the dye removal process. As shown in Figure 4c, the adsorption efficiency was not significantly influenced by the variation of ionic strength in the range 0 M to 1.0 M. This finding suggests that electrostatic attraction  
Figure 4: (**a**) pH\({}_{\text{PZC}}\) of DSD sample; effect of (**b**) initial pH value and (**c**) ionic strength on the adsorption capacity of DSD adsorbent to crystal violet dye.  
attraction forces play a minor part in the adsorption process, which was confirmed previously in the pH effect study presented in Figure 4b. Hence, the electrostatic attraction is a secondary mechanism in removing CV dye by DSD adsorbent.  
The cationic dye removal onto clay is mostly occurred through cation exchange between, i.e., CV\({}^{+}\) and the clay's exchangeable cations (sodium, calcium, potassium, etc.) [40]. The cations released leave the clay surface with negative charge. Hence, the cationic dye molecules are attracted by the negatively charged surface of clay [41]. Other adsorption mechanisms can take place, such as pore filling or simply adsorption [38; 42].  
This study found that the mesopore volume of DSD powder was dominant, and it was found to be about 94.4% of the total pore volume. Therefore, the adsorption of dye molecules onto the meso-pores network through pore-filling process is very expected. In addition, the distribution of pore size was predominantly located at around 1.278, 3.658, and 6.074 nm (Figure 5). Thus, the pore filling of CV dye molecules with a molecular size of 1.28 nm and 1.34 nm can be an adequate mechanism during the adsorption process onto the DSD adsorbent. The existence of the intraparticle diffusion mechanism and their steps will be further discussed in the next section (stirring time and kinetic study).