3.4.2. Adsorption Kinetics

As shown in Figure 4g~i, the adsorption process of pBC-Polym-0.04 was very fast in the initial stage (≤30 min), involving 76% of the total capacity. This was presumably due to the physical adsorption. In this step, the negative charged Cr(VI) was adhered to positive charged adsorbent by electrostatic attraction. Additionally, the fast adsorption process was related to the high affinity to aqueous solution of the pBC-Polym-*x*, which promised effective mass transfer. The subsequently accumulative adsorption of Cr(VI) ultimately reached the equilibrium within 3 h. The pseudo-first and pseudo-second order models were adopted to analyze the adsorption process. As seen in Table 3, the pseudo-second-order model with the correlation coefficient (*R*2) > 0.99 indicated that the Cr(VI) adsorption was mainly controlled by chemical sorption. The adsorption process was deduced as the electrostatic attraction and redox-chelation. The Cr(VI) adsorption was initiated by electrostatic interaction. The Cr(VI) binding on surface of adsorbent was reduced to Cr(III) and chelated by quinoid imine group - the oxidation product of benzenoid amine structure [37,46]. The elaborate verification of adsorption mechanism was stated in Supplementary Section S7.


**Table 3.** The kinetics parameter of Cr(VI) adsorption on pBC-Polym-0.04 and BC-Polym-0.08.

Furthermore, the Weber–Morris intraparticle diffusion model was used to describe the adsorption process (Figure 4j). The whole adsorption can be divided into three regions: (1) Cr(VI) diffusion in aqueous solution onto the exterior surface of adsorbent; (2) Cr(VI) permeation into the inner section of adsorbent and Cr(VI) diffusion into the internal polymers; (3) chemical sorption and reaching equilibrium. Clearly, the surface diffusion of pBC-Polym-0.04 completed within 1 minute. From 0~1 min, Cr(VI) was instantly captured by the poly Schiff base deposited on the surface of aerogel. In the second procedure, faster permeation and mass transfer of pBC-Polym-0.04 was observed. For BC-Polym-0.04, ascribed to the weaker affinity toward solution, the permeation and adsorption of inner polymers lasted from 5~120 min. The kid listed in Table 3 manifested the high adsorption rate of pBC-Polym-0.04.
