# Aminated Covalent Organic Polymers for Anionic Dye Adsorption in Aqueous Systems
## 3 Results and Discussion
### Adsorption Isotherm  
To evaluate the adsorption capacity of the prepared ACOP, anionic dye adsorption was performed as a function of initial dye concentration, and the results were fitted using the [PERSON] (Equation (2)) and Freundlich (Equation (3)) isotherm models (Figure 6). As summarized in Table 2, the calculated Langmuir \(q_{\rm m}\) values were 351.9 and 227.9 mg/g for MO and OG, respectively. The \(q_{\rm m}\) value of ACOP was 1.5 times higher for MO than for OG. The lower adsorption efficiency of OG on ACOP than on MO may be caused by the ionization of OG in water as OG\({}^{2-}\), accompanied by the release of two Na\({}^{+}\) from two sulfonyl groups (Table 1). The two Na\({}^{+}\) ions might interfere with the adsorption of OG on ACOP because of the lower \(q_{\rm m}\) value relative to that of MO. Moreover, the hydrophobic methyl groups (\(-\)CH\({}_{3}\)) of MO make it possible to enhance the adsorption efficiency of ACOP more than the hydrophilic groups of OG because of the hydrophobic interaction (\(\pi\)-\(\pi\) interaction), despite the presence of divalent OG in water. [PERSON] et al. reported a similar tendency for the adsorption of orange-type dye on MgAl-layered double hydroxide (LDH) [57]. The adsorption of orange-type dyes, such as orange II and OG, which can release Na\({}^{+}\) ions in aqueous systems, tends to lower the adsorption capacity compared with the adsorption of MO because of the co-adsorption of Na\({}^{+}\) and dye molecules. Figure 5: Dye adsorption experiment results of MO and OG on ACOP as a function of contact time and amount of adsorbent (initial dye concentration: 100 mg/L, initial adsorbent concentration: 0.25–1.0 g/L, and contact time: 180 min). Figure 6: Adsorption isotherms of MO and OG fitted using the Langmuir (red line) and Freundlich (blue line) isotherm models (initial MO concentration: 200–350 mg/L, initial OG concentration: 80–350 mg/L, initial adsorbent concentration: 2.0 g/L, and contact time: 180 min). Furthermore, the separation factor (R\({}_{L}\); dimensionless constant) was calculated using the following equation (Equation (4)) [58]. \[R_{L}=1/(1+a_{L}C_{0}) \tag{4}\]  
where \(C_{0}\) is the highest initial adsorbate concentration (mg/L), and \(a_{\rm L}\) is the Langmuir constant (L/mg). The calculated \(R_{\rm L}\) value implies the isotherm, and it can be interpreted as unfavorable (\(R_{\rm L}>1\)), linear (\(R_{\rm L}=1\)), favorable (\(0<R_{\rm L}<1\)), or irreversible (\(R_{\rm L}=0\)). The \(R_{\rm L}\) values of ACOP for MO and OG were determined as 0.0342 and 0.0543, respectively, indicating that the anionic dye adsorption on ACOP was a favorable reaction. Additionally, the isotherm results were fitted using the Freundlich isotherm model, which represents a heterogeneous adsorption system. The n values, which indicate the presence or absence of favorable adsorption conditions, were determined as 3.753 and 3.072 for MO and OG, respectively. These values indicated that the adsorption of anionic dyes on ACOP was favored. The correlation coefficients (\(R^{2}\)) obtained using the Langmuir and Freundlich isotherm models indicated that dye adsorption on the ACOP was interpreted better by the Freundlich isotherm model than the Langmuir isotherm model, which is consistent with previous reports related to multilayer adsorption on adsorbent surfaces [19; 59]. The obtained \(q_{\rm m}\) value was compared to those obtained in other studies related to the adsorption of MB or OG on various adsorbents (Table 3). The \(q_{\rm m}\) values for anionic dyes are in the range of a few tens to a few hundred mg/g, with substantial deviation according to the adsorbent materials. In general, nanosized adsorbents showed superior adsorption performance compared with conventional adsorbent materials. In particular, porous polymers, including COP, COF, and MOF, showed comparably higher adsorption performance than other types of adsorbent materials due to their high surface area and tunable functionality.