## 3 Results
### Effect of Adsorption Parameters  
To evaluate the contact time effect on the adsorption capacity and removal efficiency, measurements were performed with contact times between 0 and 60 min using adsorbent dosage, initial oil concentration, temperature, and a sample trembling speed of 22 \({}^{\circ}\)C, 3 mg adsorbent, 100 ppm, and 350 rpm. As shown in Figure 2, the removal efficiency of the emulsified oil by rGO-Fe\({}_{2}\)O\({}_{3}\) was very rapid in the first several minutes, with 95% of the equilibrium capacity achieved in 10 min. This initial rapid adsorption was due to the abundance of empty binding sites on the rGO-Fe\({}_{2}\)O\({}_{3}\) surface. As the rGO-Fe\({}_{2}\)O\({}_{3}\) surface became saturated with the adsorbed oil molecules, the adsorption rate slowed, approaching  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Emulsion Concentration, ppm**} \\ \cline{2-5}  & **25** & **50** & **100** & **200** \\ \hline Zeta Potential (mV) & \(-29.2\) & \(-27.4\) & \(-40.6\) & \(-34.4\) \\ Droplet size, Population 1 (nm) & 50 & 91 & 142 & 106 \\ Droplet size, Population 2 (nm) & 142 & 295 & 4800 & 712 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Zeta potential of particle size for various diesel emulsion concentrations.  
Figure 1: **(a,b)** SEM of images of rGO-Fe\({}_{2}\)O\({}_{3}\)-NC, **(c)** XRD pattern of GO and rGO-Fe\({}_{2}\)O\({}_{3}\)-NC, and **(d)** N\({}_{2}\) adsorption–desorption isotherms and the surface area, pore volume, and average pore size.  
an equilibrium. In addition, the repulsion between the rGO-Fe\({}_{2}\)O\({}_{3}\) and the emulsified oil molecules could occur due to the increased electrostatic (repulsive) force as the active sites on the rGO-Fe\({}_{2}\)O\({}_{3}\) surface became saturated with dissolved oil molecules [48].  
Batch measurements were performed to study the influence of the rGO-Fe\({}_{2}\)O\({}_{3}\) dose and the initial oil concentration on the adsorption capacity and removal efficiency. The dosage varied between 1 and 20 mg, and the initial oil concentrations were between 25 and 200 ppm. All other parameters, including temperature, agitation speed, and contact time, were constant at 30 min, 350 rpm, and 22 \(\pm\) 1 \({}^{\circ}\)C. Figure 2c,d show the adsorption capacity and removal efficiency dependence on the rGO-Fe\({}_{2}\)O\({}_{3}\) nanocomposite dose and the initial oil concentration. The adsorption capacity rose with the initial oil concentration due to the increased mass transfer driving force caused by the higher concentration gradient between the bulk solution and the rGO-Fe\({}_{2}\)O\({}_{3}\) surface at a higher initial oil concentration [49].  
Furthermore, the adsorption capacity declined with rGO-Fe\({}_{2}\)O\({}_{3}\) doses of up to 5 mg but remained constant for dosages of more than 5 mg, possibly because of the adsorbent saturation with oil. Therefore, 1 mg of rGO-Fe\({}_{2}\)O\({}_{3}\) was chosen as the standard adsorbent dosage for the remaining experiments. Meanwhile, the removal efficiency of emulsified oil dropped with increasing C\({}_{0}\), while the capacity increased with C\({}_{0}\). Thus, the maximum removal efficiency occurred at lower initial concentrations due to the limited vacant adsorption sites.