# Aminated Covalent Organic Polymers for Anionic Dye Adsorption in Aqueous Systems
## 3 Results and Discussion  
### Characterization of Prepared ACOP  
The chemical structure of the prepared ACOP was characterized using FT-IR, XPS, and SEM (Figures 1 and 2). In the FT-IR spectra, the characteristic vibrations attributed to the primary amine from NH\({}_{2}\) stretching and deformation vibrations in melamine were observed at 3467, 3416, and 1625 cm\({}^{-1}\), respectively [46]. Moreover, the vibrations caused by the carbonyl groups, namely C-H stretching and C=O stretching, in terephthalaldehyde were observed at 2865 and 1686 cm\({}^{-1}\), respectively [47]. After ACOP preparation, the aforementioned characteristic vibrations of melamine and terephthalaldehyde disappeared or decreased considerably owing to the Schiff base reaction, which is known to occur between the primary amine group in melamine and the carbonyl group in terephthalaldehyde through dehydration (Scheme 1) [32]. The vibrations on ACOP at 1341, 1541, and 1472 cm\({}^{-1}\) are attributed to the C-N symmetric stretching and C=N stretching vibrations in the triazine ring, and this supported the successful polymerization of ACOP [48,49]. Additionally, newly developed vibrations were observed at 3404 and 1146 cm\({}^{-1}\) for secondary amine groups such as animal groups (HN-C-NH) [50]. For detailed characterization of the carbon and nitrogen in the prepared ACOP, XPS spectra of the material were obtained. As displayed in Figure 1, the C 1 s spectra of the material were deconvoluted into five peaks. The peaks at 284.8 eV (C-C) and 285.3 eV (C=C) were found to correspond to the carbon atoms of the benzene ring. The peaks at 287.8 eV (C-N) and 293.4 eV (\(\pi\)-\(\pi^{\alpha}\) transition in the triazine ring) were attributed to the triazine ring. The peak at 286.3 eV (C-N) was attributed to the carbon bonded to the electronegative nitrogen atoms in linkages. Taken together, these results confirmed successful ACOP polymerization through a Schiff base reaction between melamine and terephthalaldehyde. Moreover, the  
Figure 1: FT-IR spectra of terephthalaldehyde, melamine, and ACOP (left). XPS spectra on C 1 s (right top) and N 1 s (right bottom) of ACOP. Figure 2: SEM images of prepared ACOP. deconvoluted N 1 s spectra contained three peaks at 399.1 eV, 400.4 eV, and 405.2 eV, which were attributed to C-N=C in the triazine ring, amine (C-NH-C) moieties, and \(\pi\) excitation in the triazine ring, respectively [31]. In SEM images (Figure 2), tens of nanoscale ACOP particles were found to be aggregated with intraparticle pores, representing the typical morphology of chain-structure polymers [51]. The detail-specific surface area (SSA) and pore structure of the prepared ACOP were evaluated using its N\({}_{2}\) adsorption-desorption hysteresis loop (Figure 3). According to this hysteresis loop, the prepared ACOP exhibits the characteristics of a typical type II material, which are either nonporous or microporous [52]. The SSA was determined to be 528.07 m\({}^{2}\)/g, which is consistent with the results reported in the literature [53]. In addition, the detail pore volume and pore size, calculated using the Barrett-Joyner-Halenda (BJH) method, were 0.41 cm\({}^{3}\)/g and 88.71 A, respectively. Based on the results of this pore analysis, the prepared ACOP can be utilized as a size-selective adsorbent. Surface charge is one of the essential factors in the adsorption of charged pollutants on various materials [54,55]. To investigate the surface charge of the prepared ACOP as a function of pH, its zeta potential was recorded in the pH range of 4-10. As shown in Figure 4, the surface charge of the prepared ACOP decreased gradually from +21.85 mV at pH 4 to \(-\)16.23 mV at pH 10. Moreover, the point of zero charge (pH\({}_{\text{P2C}}\)) of the ACOP was found to be at pH 8.26; at this point, the ACOP possesses a neutral charge. Below pH\({}_{\text{P2C}}\), the surface charge of the ACOP was positive, meaning that the amine groups in the ACOP were protonated and promoted to adsorb negatively charged pollutants.