# Adsorption Separation of Various Polar Dyes in Water by Oil Sludge-Based Porous Carbon
## 4 Discussion
### Adsorption Mechanism of Organic Dyes by OSC  
As shown in Figure 9, the FTIR spectrum of OSC700 prior to adsorption exhibits absorption peaks at 3450 cm\({}^{-1}\), 2920 cm\({}^{-1}\), 2850 cm\({}^{-1}\), 1620 cm\({}^{-1}\), 987 cm\({}^{-1}\), 910 cm\({}^{-1}\), and 474 cm\({}^{-1}\). After adsorption of the three organic dyes, the wavenumber and peak intensity of the -OH peaks at 3420 cm\({}^{-1}\) decreased, indicating that the adsorption process consumed a portion of the phenolic hydroxyl groups, leading to the formation of hydrogen bonds between the porous carbon surface and the adsorbate. This evidence confirms that the phenolic hydroxyl groups on the OSC700 surface play a major role in the chemisorption of dye molecules. Furthermore, the significant decrease in the absorption peaks at 2920 cm\({}^{-1}\) and 2850 cm\({}^{-1}\), corresponding to saturated -CH groups, indicates a reduction in the amount  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \(\Delta\)**H (kJ/mol)** & \(\Delta\)**S** & \multicolumn{2}{c}{\(\Delta\)**G (kJ/mol)**} \\ \cline{3-5}  & **(J/(mol:K)** & **298 K** & **298 K** & **298 K** \\ \hline MG & \(-\)6189.0721 & 46.28820 & \(-\)20,045.2574 & \(-\)20,385.9097 & \(-\)20,868.9016 \\ MB & \(-\)14,373.3921 & 1.4651 & \(-\)14,027.344 & \(-\)14,171.545 & \(-\)14,174.080 \\ CV & \(-\)15,045.3788 & 3.9898 & \(-\)16,242.086 & \(-\)16,290.533 & \(-\)16,452.179 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Thermodynamic parameters of OSC700 on MG, MB, and CV.  
of these groups on the surface as the polarity of the adsorbent increases. The intensity of the aromatic C=C double bond peak at 1620 cm\({}^{-1}\) undergoes a shift and decrease after adsorption, indicating that the adsorption of the three organic dyes by OSC700 occurs on the benzene rings, specifically on the C network structure of the porous carbon surface. Notably, the peak intensity decreases most significantly after MG adsorption, followed by MB, and then CV.  
The most pronounced changes in the entire infrared spectrum occur in the hydroxyl and fingerprint regions (920-980 cm\({}^{-1}\)), where the peak intensity increases and shifts significantly after adsorption. This is primarily attributed to the presence of benzene rings in the three organic dye molecules, which interact with the porous carbon surface through \(\pi\)-\(\pi\) EDA interactions, forming \(\pi\)-\(\pi\) bonds. This interaction is inferred in the fingerprint region, despite not being explicitly evident. The absorption peak at 1000 cm\({}^{-1}\) in the infrared fingerprint region is attributed to the -Si-O-Si- bond in the material, and even a slight modification in the molecular structure leads to a significant shift in this absorption peak. The interaction between OSC700 and the dyes (MG, MB, and CV) through hydrogen bonding or electrostatic attraction induces changes in the material's molecular structure, resulting in alterations to the peak intensity of the -Si-O-Si-. The peak intensity reveals that the hydroxyl and fingerprint regions of CV, which has the highest polarity and lowest adsorption capacity, exhibit the smallest enhancement at 920-980 cm\({}^{-1}\). In contrast, MG, which has the weakest polarity and largest saturated adsorption capacity, shows the most significant enhancement. This observation is in agreement with the results presented in Figure 7.