## 3 Results and Discussion
### Adsorption Equilibrium and Kinetics of SiO\({}_{2}\) and the rGO/SiO\({}_{2}\) Composites  
Herein, to investigate the removal ability of SiO\({}_{2}\) and rGO/SiO\({}_{2}\), we used NR adsorption (pH 5) at 298 K. The parameters of the pseudo-first-order kinetic and pseudo-second-order kinetic models are listed in Tables 1 and 2, respectively. The correlation coefficients of the pseudo-second-order model are all higher than 0.920, and the pseudo-second-order model agrees well with the experimental values. Therefore, the adsorption of NR on SiO\({}_{2}\) and rGO/SiO\({}_{2}\) composites was more consistent with the pseudo-second-order kinetic model. Figure 6: Water adsorption/desorption isotherms of the (**a**) SiO\({}_{2}\), (**b**) SG 0.30, and (**c**) SG 1.00 samples at 298 K.  
Figure 7: Raman spectra of GO, rGO, SG 0.30, and SG 1.00. Figure 8 shows the effect of the ratio of GO and SiO\({}_{2}\) on the adsorption rate of NR (C\({}_{0}\) = 10 mg/L). For the samples at different ratios, the adsorption equilibrium with NR could be achieved within nearly 120 min, and no significant change was observed from 120 to 360 min. As the amount of rGO increased, the adsorption efficiency and adsorption capacity first increased and then decreased. Clearly, compared with the other samples, SG 0.30 was the first sample to reach adsorption equilibrium (30 min), and displayed the highest experimental adsorption amount (12.853 mg g\({}^{-1}\)). The reason the adsorption rate of SG 0.30 was faster than that of SG 0.50 and SG 1.00 might be that rGO and silica formed a better synergistic interaction to form more \"core-shell\" structures in SG 0.30. As the amount of rGO increased, excess rGO began to agglomerate, hindering the adsorption of NR and leading to a decrease in the adsorption rate. Moreover, the reason the adsorption capacity of SG 0.05 was lower than that of SiO\({}_{2}\) might be the absence of a \"core-shell\" structure and the poor affinity of rGO toward organic contaminants [34]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**10 mg/L NR**} \\ \hline  & \(\mathbf{K_{1}}\) (g mg\({}^{-1}\) min\({}^{-1}\)) & \(\mathbf{Q_{e(theo)}}\) (mg g\({}^{-1}\)) & \(\mathbf{Q_{e(exp)}}\) (mg g\({}^{-1}\)) & \(\mathbf{R^{2}}\) \\ \hline SiO\({}_{2}\) & 0.00183 & 3.08774 & 6.853 & 0.88963 \\ SG 0.05 & 0.06663 & 0.54623 & 2.564 & 0.88130 \\ SG 0.10 & 0.01538 & 0.94818 & 6.233 & 0.92896 \\ SG 0.20 & 0.04993 & 5.59599 & 11.415 & 0.96972 \\ SG 0.30 & 0.25666 & 23.32906 & 12.853 & 0.96010 \\ SG 0.50 & 0.04295 & 17.44791 & 12.754 & 0.99298 \\ SG 1.00 & 0.05905 & 11.42164 & 12.555 & 0.84625 \\ \hline  & \multicolumn{4}{c}{25 mg/L NR} \\ \hline SiO\({}_{2}\) & 0.00093 & 6.74796 & 9.505 & 0.98911 \\ SG 0.30 & 0.03375 & 29.60994 & 36.030 & 0.98066 \\ \hline  & \multicolumn{4}{c}{50 mg/L NR} \\ \hline SiO\({}_{2}\) & 0.00256 & 27.66644 & 49.263 & 0.84640 \\ SG 0.30 & 0.00761 & 49.14524 & 66.635 & 0.97839 \\ \hline \end{tabular}
\end{table} 
Table 1: Pseudo-first-order kinetic parameters for the adsorption of neutral red at various initial concentrations. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**10 mg/L NR**} \\ \hline  & \(\mathbf{K_{2}}\) (g mg\({}^{-1}\) min\({}^{-1}\)) & \(\mathbf{Q_{e(theo)}}\) (mg g\({}^{-1}\)) & \(\mathbf{Q_{e(exp)}}\) (mg g\({}^{-1}\)) & \(\mathbf{R^{2}}\) \\ \hline SiO\({}_{2}\) & 0.00217 & 7.024 & 6.853 & 0.98982 \\ SG 0.05 & 0.37591 & 2.573 & 2.564 & 0.99989 \\ SG 0.10 & 0.10984 & 6.286 & 6.233 & 0.99993 \\ SG 0.20 & 0.01827 & 11.834 & 11.415 & 0.99985 \\ SG 0.30 & 0.03387 & 13.441 & 12.853 & 0.99829 \\ SG 0.50 & 0.00333 & 14.401 & 12.754 & 0.99255 \\ SG 1.00 & 0.00629 & 14.489 & 12.555 & 0.99234 \\ \hline  & \multicolumn{4}{c}{25 mg/L NR} \\ \hline SiO\({}_{2}\) & 0.00015 & 11.051 & 9.505 & 0.92476 \\ SG 0.30 & 0.00151 & 39.541 & 36.030 & 0.99604 \\ \hline  & \multicolumn{4}{c}{50 mg/L NR} \\ \hline SiO\({}_{2}\) & 0.00026 & 50.839 & 49.263 & 0.99934 \\ SG 0.30 & 0.00026 & 72.993 & 66.635 & 0.99945 \\ \hline \end{tabular}
\end{table}