## 3 Results and Discussion
### Adsorption of Various Dyes on CPP200  
The adsorption capacities of CPP200 toward MB, AYR and NR and their diverse electrical properties were evaluated. Since MB, AYR and NR possess different molecular weights, using units of mmol/g is more suitable for describing and comparing the adsorption capacities of various organic dyes (Figure 8, Table S4). The adsorption capacities of dyes by CPP200 followed the order of AYR (0.0131 mmol/g) \(<\) NR (0.1353 mmol/g) \(<\) MB (0.1597 mmol/g). It is well known that MB is a cationic dye, while AYR is an anionic dye and NR is a neutral dye. The great difference of adsorption capacities could be attributed to the distinct electrostatic interactions between adsorbate and adsorbent. Therefore, CPP-derived BCs exhibited high adsorption selectivity for cationic dyes.  
FT-IR analyses indicated that CPP200 possessed abundant oxygen-containing functional groups such as carboxyl, carbonyl and phenolic species. Thus, the surfaces of BCs were negative charged, facilitating their electrostatic attraction toward cationic dyes, and the adsorption capacity of MB by CPP200 was relatively higher than that for the other tested dyes. Conversely, the electrostatic repulsion between anionic AYR and CPP200 was attributed to the lower adsorption capacity observed.  
CPP200 post-adsorptions of various organic dyes were analyzed using FT-IR (Figure 9). Following MB adsorption, the FT-IR spectra of CPP200-MB showed weakened bands, suggesting that the oxygen-containing functional groups on the surface of CPP200 were involved in the adsorption process. The oxygen-containing functional groups, such as hydroxyl (-OH), epoxy (-O-), carbonyl (-C=O) and carboxyl (-COOH), could interact with organic substances by forming hydrogen bonds [53]. The hydrogen bonds that were formed between BCs and MB also contributed significantly to the adsorption [54]. Obviously, the peak at 1601.8 cm\({}^{-1}\) that corresponded to the stretching vibration of -C=O red-shifted to 1595 cm\({}^{-1}\), and became more pronounced due to hydrogen bonding interactions. In addition, MB could form hydrogen bonds with -COOH and -OH of CPP200, leading to peak broadening and distortions in the wave number range of 3100-3435 cm\({}^{-1}\). After adsorption of MB, several new peaks appeared at 1373 cm\({}^{-1}\), 1323 cm\({}^{-1}\), 1215 cm\({}^{-1}\), 882 cm\({}^{-1}\), 790 cm\({}^{-1}\) and 668 cm\({}^{-1}\). The two peaks at 1322.8 cm\({}^{-1}\) and 1372.8 cm\({}^{-1}\) may be attributed to C-N stretching vibrations [55; 56]. The new peak at 882 cm\({}^{-1}\) corresponds to vibrations of C-S bonds, while the new peaks at 789.8 cm\({}^{-1}\) and 668 cm\({}^{-1}\) correspond to C-H out-plane bending vibrations for the aromatic rings of MB molecules. The shift of the FT-IR peaks as well as the appearance of new peaks confirmed that MB was successfully adsorbed onto CPP200. For the adsorption of NR by CPP200, the peaks at 1602 cm\({}^{-1}\) and 1225 cm\({}^{-1}\) red-shifted to 1512 cm\({}^{-1}\) and 1198 cm\({}^{-1}\), respectively. Newly-appearing C-N stretching vibrations (1315 cm\({}^{-1}\)) and N=C stretching vibrations (1624 cm\({}^{-1}\)) could be observed in the spectra. Furthermore, we observed that there was basically no change in the spectra of CPP200 both pre- and post-adsorption of AYR. The relatively lower adsorption capacity of CPP200 for AYR may be attributed to electrostatic repulsions between the adsorbate and adsorbent.  
Figure 8: Adsorption capacity of CPP200 toward different dyes in aqueous solutions (dye concentration of 50 mg/L, adsorbent dosage of 5.0 mg and contact time of 90 min, T = 25 \({}^{\circ}\)C).