## 3 Results and Discussion
### Adsorption measurements
#### 3.2.2 Optimization for Maximal Dye Removal  
To achieve an optimum condition for maximal dye removal on the MCC, the influences of the MCC dosage, initial dye concentration, and initial pH were investigated in turn by univariate analysis, and the results are shown in Figure 5.  
Whist the effect of the temperature on the dye removal with time was shown in Figure S2. Figure 4: Dye removal as a function of the contact time on different types of adsorbent (0.6 g adsorbent dosage, pH 5.5, 100 mg L\({}^{-1}\) initial RR120 concentration at room temperature). Figure 5: _Cont._  
The effect of the MCC dosage on the level of the RR120 dye removal with time is shown in Figure 5a for an initial dye concentration of 100 mg L\({}^{-1}\) and pH 5.5. For each level of dosage, the dye removal was relatively fast in the first 60 min of contact time, and then the rate decreased gradually to approach equilibrium at a dosage level of 0.3, 0.6, 0.9, and 1.2 g, respectively. The maximum dye removal was around 80% at a dosage level of 0.3 g and a contact time of 420 min, and increasing the dosage level to 0.6-1.2 g gave 100% dye removal but decreasing contact times of 300, 120, and 90 min at 0.6, 0.9, and 1.2 g, respectively. Increasing the adsorbent dosage increased the amount of available active sites and so gave a higher removal rate. Since complete dye removal was obtained with a MCC dosage of from 0.6 g upwards, then 0.6 g was selected for further investigation. The dye removal as a function of the contact time at different levels of initial dye concentration from 10 to 100 mg L\({}^{-1}\) are shown in Figure 5b. Increasing the initial dye concentration leads to more contact time being required to reach equilibrium adsorption due to the limit of available active sites for dye binding. However, complete dye removal was achieved at each level of dye concentration. Further increasing the initial dye concentration to 300 mg L\({}^{-1}\) or 500 mg L\({}^{-1}\) required a much longer contact time (around 8 h) to completely remove the dye from the water (lower part of the right corner of Figure 5b). Normally, the full coverage of dye molecules on the active sites of the adsorbents was found when increasing the initial dye concentration. However, this was not found in this work, which infers that the adsorbents used in this work have plenty of active sites on their surface. Considering the relationship between the equilibrium adsorption capacity and initial dye concentration (middle part in Figure 5b), it showed a linear positive slope, reflecting that the capacity increased with the increasing initial dye concentration. More dye molecules transferred from the higher concentration in bulk to the lower concentration on surface of the adsorbents [26]. Consequently, more dyes occupied on the active sites of the adsorbents [27; 28; 29]. However, for further investigation, an initial dye concentration of 100 mg L\({}^{-1}\) was selected. From Figure 3, changes in the pH can change the sign of charges and their magnitude on the surface of the dyes and MCC, leading to a change in the adsorption capacity. However, it was observed from Figure 5c that the MCC achieves complete dye removal in both acidic and basic conditions but with different required contact time intervals. A shorter contact time is required at a lower pH, because the electrostatic attraction between the negative surface charges of the dye and the positive surface charges of the MCC are contributed. The amino groups (-NH\({}_{2}\)) of the MCC are protonated to (-NH\({}_{3}\)+) in the acidic solution [24], whereas they are deprotonated to the (-NH\({}^{-}\)) group at a basic condition [25]. However, the net surface charge of the MCC was still positive in a basic condition, while the surface charge of the dye was negative over the entire pH range. The difference in the magnitude of the zeta potential between the dye and MCC at an acidic condition is greater, and so the dye was adsorbed on the MCC faster at a lower pH [27; 30; 31; 32].