# Adsorption Separation of Various Polar Dyes in Water by Oil Sludge-Based Porous Carbon
## 3 Results
### The Adsorption Properties of OSC for Organic Dyes
This leads to the diffusion of physically absorbed dye molecules, which have weak binding forces, resulting in desorption after prolonged adsorption. Figure 6c shows the effect of varying OSC700 dosages on the adsorption efficiency of MG, MB, and CV. As shown, increasing the OSC700 dosage continuously improves the removal efficiency, while the adsorption amount gradually decreases. Notably, when the OSC700 dosage was 0.2 g, the adsorption of MG by OSC700 increased most rapidly. Subsequently, as the OSC700 dosage increased, the removal efficiency of MB and CV tended to stabilize, whereas the removal efficiency of MG increased almost linearly with the amount of adsorbent, indicating that the adsorption capacity of OSC700 for MG had not yet reached saturation, whereas the adsorption of MB and CV had already reached saturation. The adsorption capacity of all three dyes decreased, with MG showing the most significant decline. This is primarily attributed to the substantial increase in MG adsorption sites on the OSC700 surface. Although the amount of OSC700 was increased, the number of surface-active sites did not increase significantly, resulting in minimal changes to the adsorption capacity of MB and CV. This highlights the importance of optimizing the amount of OSC700 used during adsorption to achieve high removal efficiency while ensuring efficient utilization of the adsorbent material. Figure 6d illustrates the effect of different temperatures on the adsorption efficiency of OSC700 for MG, MB, and CV. As shown, in the temperature range of 25-55 \({}^{\circ}\)C, the removal rate of OSC700 for MG, which has the minimum polarity, remains above 96%. In contrast, the highest removal rates for CV and MB are 87.7% and 75.2%, respectively, both exhibiting a decreasing trend. This indicates that the adsorption process of OSC700 for MG, MB, and CV is spontaneous, exothermic, and entropy-increasing, as confirmed by the adsorption thermodynamic model shown in Figure 8f. The consistently high removal efficiency of MG throughout the temperature range indicates that OSC700 has a strong affinity for MG, likely due to the favorable compatibility between the adsorbent's surface properties and the MG molecular structure. As the temperature rises, the removal efficiency of MB and CV decreases, potentially due to weakened adsorption or more favorable interactions at lower temperatures. The spontaneous and exothermic nature of the adsorption process, accompanied by an increase in entropy, indicates that adsorption not only releases heat but also leads to a more disordered state, characteristic of physical adsorption [52]. Figure 6e,f illustrate the impact of various pH levels on the adsorption efficiency of OSC700 for MG, MB, and CV. As shown, within the pH range of 2.0-10.0, the removal rate of OSC700 for MG, which has the minimum polarity, remains consistently above 97%, with a stable adsorption capacity of 9.9 mg/g. This indicates that OSC700 is effective in removing MG from water across a range of environmental pH conditions. Although protonation of the adsorbent's surface functional groups at lower pH values may reduce its ability to adsorb cationic dyes, the sustained high removal efficiency suggests that alternative forces, such as hydrophobic interactions or hydrogen bonds, play a significant role in the effective adsorption of dyes. The maximum removal rates of MB and CV are 80.9% and 91.5%, respectively, with corresponding maximum adsorption capacities of 9.1 mg/g and 8.0 mg/g. Under acidic conditions, the high concentration of H\({}^{+}\) in the aqueous solution protonates the functional groups (such as -COOH) on the OSC700 surface, reaching a maximum at pH 6.0. As a result, the adsorption capacity of OSC700 for MB and CV decreases due to electrostatic repulsion, hindering the adsorption process. At pH-6, where the surface charge of OSC700 approaches the isoelectric point (i.e., the point of zero surface charge), the functional groups on OSC700 may be in a neutral or less protonated state.