*3.5. Adsorption Isotherm and Thermodynamics of Dencichine on the RCMs and RPMs*

The adsorption isotherms of dencichine on the RCMs and RPMs at (298 K) with dencichine concentrations of 80, 100, 120, 140, 160 and 180 μg/mL are shown in Figure 7a. As can be seen in Figure 7, the adsorption process of dencichine on RCMs and RPMs was obviously affected by the initial concentration. The dencichine adsorption amount for RCMs and RPMs increased with increasing dencichine concentration.

**Figure 7.** Adsorption thermodynamics of dencichine on RCMs and RPMs. (**a**) Adsorption isotherms; (**b**) Langmuir isotherm model; (**c**) Freundlich isotherm model.

In order to analyze the adsorption mechanism, fitting Langmuir Freundlich isotherm models to the experimental data (Figure 7b,c) is helpful and allows further understanding of the adsorption mechanism. The equations of these two models are as follows [54–56].

Langmuir isotherm equation:

$$\frac{1}{\mathcal{Q}\_{\text{e}}} = \frac{1}{\mathcal{Q}\_{\text{m}}} + \frac{1}{\mathcal{K}\_{\text{3}}\mathcal{Q}\_{\text{m}}} \times \frac{1}{\mathcal{C}\_{\text{e}}} \tag{9}$$

Freundlich isotherm equation:

$$
\ln \text{Q}\_{\text{e}} = \ln \text{k}\_{4} + \frac{1}{\text{n}} \ln \text{C}\_{\text{e}} \tag{10}
$$

where Ce represents the concentration of dencichine at equilibrium (mg/mL), Qe represents the dencichine adsorption amount for the RCMs and RPMs at equilibrium (mg/g), 1/n is the dimensionless Freundlich constant, Qm represents the saturation adsorption capacities of monolayer coverage (mg/g), K3 represents the Langmuir constant (mL/mg) and K4 represent the Freundlich constant (mg/mL).

The Freundlich and Langmuir isotherms for the adsorption of dencichine on the RCMs and RPMs are represented in Figure 7b,c, and the fitting data is shown in Table 2. From the adsorption isotherms data, it is observed that the correlation coefficient (R2) for the adsorption of dencichine on the RPMs adsorption has a higher value for the Freundlich equation (0.9976) than the Langmuir (0.9968), indicating that the Freundlich model is more suitable for the adsorption process of dencichine on RPMs. Overall, 0 < 1/n < 1 indicates that the adsorption process easily occurs and has excellent adsorption capacity, 1/n (0.7988) in the Freundlich equation can be seen as a reflection of the easy adsorption behavior. The Langmuir model (0.9570) fits the adsorption data less than the Freundlich model (0.9842) for the adsorption of dencichine on the RCMs. In total, 1/n (2.062) in the Freundlich equation can be seen as a reflection of the adsorption behavior. These results indicated that

the adsorption of dencichine on the RCMs was multilayer adsorption. According to the prediction of the Langmuir isothermal model, the maximum adsorption capacity of RPMs for dencichine at 25 ◦C is 85.11 mg/g. These results further proved the application prospect of RCMs in the separation of dencichine.


**Table 2.** Parameters of Langmuir adsorption model and Freundlich adsorption model.

To understand the effect of temperature on the adsorption amount of dencichine on the RCMs and RPMs, the results are discussed for the different temperatures, and the results are represented in Figure 8. As shown in this figure, the adsorption process of dencichine on the RCMs and RPMs is affected by temperature. At low temperatures, the dencichine adsorption capacities for the RCMs and RPMs increased with increasing temperature. The contact probability of the RCMs of dencichine increases with the increase in temperature. The RCMs and RPMs have temperature sensitivity because of the hydrogen bonding interaction with the dencichine molecules, and the hydrogen bond is destroyed gradually with the increase in temperature. With the increase in temperature, the adsorption capacity of the RCMs decreased, indicating that high temperature is not conducive to the progress of the adsorption process. This phenomenon proves that chemisorption is dominant in the adsorption process.

**Figure 8.** Adsorption thermodynamics of dencichine on RCMs and RPMs.
