## 5 Comparison Study  
Only a few studies have been concerned with treating oily water containing heavy crude oil via hydrogels as a coating for the membrane surface, as investigated in this work. Most research focuses on lighter oils, such as organic solvents and gasoline, due to the challenges associated with causing crude oil fouling, especially in long-term tests. This issue arises from the lack of a stable and robust hydration layer, which depends on the hydrogel coating method. This method must ensure strong interfacial adhesion between the hydrogel coating and the substrate, as well as robust cohesive strength within the hydrogel coating itself. Moreover, when the oil becomes heavier, the performance of the material reported by other studies cannot fit the emission/reuse requirement (the oil cannot be removed completely). A pioneering study on microfiltration membranes conducted 30 years ago highlighted the challenges associated with this approach [75]. The AM-NaA/PVDF membrane in the present study could achieve high oil removal efficiency and outstanding anti-oil-fouling properties within a very thin hydrogel layer, with a short reaction time and requiring fewer chemicals, offering a better cost-effectiveness balance. Table 4 compares the current work with other previous studies that utilized modified membranes for the separation of crude oil-in-water emulsions.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Porous Materials**} & \multirow{2}{*}{**Substrate Type**} & \multirow{2}{*}{**Fabrication Method**} & **Modification Conditions (h/\({}^{\circ}\)C)** & **Inlet Oil** & **Oil Removal (mg/L)** & **Flux L/m\({}^{2}\cdot\)h** & **Ref.** \\ \hline DMPA@SMA & PVDF & Grafting and co-blending & \multirow{2}{*}{9/60} & \multirow{2}{*}{1034.6} & \multirow{2}{*}{99.2} & \multirow{2}{*}{420} & \multirow{2}{*}{[36]} \\ \cline{1-1} \cline{6-6} PAAS@PVDF & PVDF & Grafting and blending & & & & & \\ \cline{1-1} \cline{6-6} SA-TiO hydrogel & PVDF-g-IL & Surface coating & \multirow{2}{*}{8/60} & \multirow{2}{*}{1:90} & \multirow{2}{*}{99} & \multirow{2}{*}{1658} & \multirow{2}{*}{[40]} \\ \cline{1-1} \cline{6-6} F127@SMA & PVDF & Grafting and co-blending & & & & & \\ \hline PVDF/PHEMA, & non-woven fabric & Vapor-induced phase co-deposited & \multirow{2}{*}{-/85} & \multirow{2}{*}{1:99} & \multirow{2}{*}{99} & \multirow{2}{*}{1056} & \multirow{2}{*}{[77]} \\ \cline{1-1} TA/AEPPS & & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 4: Performance comparison of the AM-NaA/PVDF membrane (fabricated at H-6) with recently developed membranes for crude oil-water emulsion separation.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Salt**} & \multirow{2}{*}{\begin{tabular}{c} **Concentration** \\ **(g/100 mL)** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Ionic Strength (\(I_{c}\))** \\ **(mol/m\({}^{3}\))** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Double-Layer** \\ **Thickness (\(k^{-1}\)) (m)** \\ \end{tabular} } \\ \hline \multirow{2}{*}{\(\text{CaCl}_{2}\)} & 0.1 & 27.03 & \(1.85\times 10^{-9}\) \\  & 0.5 & 135.15 & \(8.28\times 10^{-10}\) \\  & 1.0 & 270.3 & \(5.85\times 10^{-10}\) \\ \hline \multirow{2}{*}{MgCl\({}_{2}\)-6H\({}_{2}\)O} & 0.1 & 14.76 & \(2.51\times 10^{-9}\) \\  & 0.5 & 73.8 & \(1.12\times 10^{-9}\) \\  & 1.0 & 147.6 & \(7.92\times 10^{-10}\) \\ \hline \multirow{2}{*}{NaCl} & 0.1 & 17.1 & \(2.33\times 10^{-9}\) \\  & 0.5 & 85.6 & \(1.04\times 10^{-9}\) \\ \cline{1-1}  & 1.0 & 171 & \(7.36\times 10^{-10}\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Ionic strength and double-layer thickness for each salt at specified concentrations.