## 3 Results and Discussion
### Adsorption Performances of DE and DE-LDH
#### 3.2.4 Effect of Contact Time  
The adsorption dependency on the contact time of the adsorbate and adsorbent was studied by varying the contact time to find out the equilibrium time required for the adsorption. The removal efficiency of CR with different concentrations 50, 100 and 250 mg/L for DE and DE-LDH was performed as a function of the contact time, adjusting this from 5 to 180 min (Figure 5d). The adsorption rate of dye on DE-LDH was rapid in the beginning due to the availability of freely accessible active sites with larger surface areas, and attained saturation after 30 min as there were no dye molecules present to adsorb on to DE-LDH. On the other hand, DE showed slow adsorption kinetics and the removal efficiency decreased with the increase in contact time adjusted from 10 to 180 min for all the dye concentrations. This decrease was attributed to the very weak adsorption on DE surfaces due to the smaller surface area and weakly adsorbed dye molecules which could easily be detached from DE as the contact time increased. Thus, it is evident that even by increasing the contact time it is difficult for dye molecules to adsorb on DE surfaces. Obviously, DE-LDH showed enhanced CR adsorption at higher concentrations, which required only 30 min when compared to DE. Figure 6a-c shows photographs of the supernatant of CR at various concentrations (50, 100 and 250 mg/L) before and after adsorption on DE and DE-LDH, and its corresponding UV-visible spectra. This confirms DE-LDH could absorb higher amounts of CR compared to DE.
Langmuir isotherm was higher than that of the Freundlich isotherm. Therefore, the adsorption of CR occurred in the monolayer on the surface of DE-LDH and its maximum adsorption capacity was found to be 305.8 mg/g. The separation factor (\(\rm{R_{L}}\)) values were calculated using Equation (S2) (Supplementary Materials) and the obtained values are between zero and one (Table 2), revealing that the adsorption process is favorable. On the other hand, high \(\rm{K_{L}}\) values depict an admirable adsorption process and these values showed a higher affinity between CR and DE-LDH surfaces, but a lower affinity between DE and CR. Moreover, the adsorption coefficient and \(\rm{K_{F}}\) values of adsorbents depict that the amount of CR adsorbed on DE-LDH (42.1 mg/g) is higher than that of DE (0.54 mg/g). In addition, the non-linear plot \(\rm{C_{e}}\) vs. \(\rm{q_{e}}\) showed that the Freundlich isotherm is finely fitted with the experimental value for DE, and the Langmuir isotherm is finely fitted with the experimental value for DE-LDH (Figure 7c,f).  
Figure 6: _Cont._  
Figure 6: Supernatant of CR before and after adsorption on DE and DE-LDH with respect to contact time and corresponding UV-visible spectra (**a**) 50 mg/L (**b**) 100 mg/L and (**c**) 250 mg/L (adsorbent dosage: 40 mg, pH: 7, volume: 25 mL, temperature: 30 \({}^{\circ}\)C).  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Langmuir Constants**} & \multicolumn{4}{c}{**Freundlich Constants**} \\ \hline \multirow{2}{*}{**Adsorbents**} & **q\({}_{\text{m}}\) (mg/g)** & **K\({}_{\text{L}}\) (L/mg)** & **R\({}_{\text{L}}\) (\(\pm\)0.05)** & **R\({}^{2}\)** & **1/n (\(\pm\)0.1)** & **K\({}_{\text{F}}\) (mg/g)** & **R\({}^{2}\)** \\  & **(\(\pm\)2)** & **(\(\pm\)0.05)** & **R\({}_{\text{L}}\) (\(\pm\)0.05)** & **R\({}^{2}\)** & **1/n (\(\pm\)0.1)** & **K\({}_{\text{F}}\) (\(\pm\)1)** & **R\({}^{2}\)** \\ \hline DE & 23.2 & 0.002 & 0.89 _to_ 0.29 & 0.650 & 0.489 & 0.54 & 0.939 \\ DE-LDH & 305.8 & 0.049 & 0.28 _to_ 0.01 & 0.985 & 0.336 & 42.1 & 0.926 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Adsorption isotherm parameters for adsorption of CR onto DE and DE-LDH.  
Figure 7: Adsorption isotherm plots of CR on DE and DE-LDH.