3.4.1. Adsorption Isotherm

Effect of initial concentration of Cr(VI) (100~500 mg L<sup>−</sup>1) on the adsorption was investigated. As shown in Figure 4a, the adsorption capacity of the series of pBC-Polym-*x* rapidly increased with the rise of Cr(VI) concentration. Then, the adsorption process tended to be saturated. Similar trend was found for the samples without pretreatment (Figure 4b). However, the adsorption performance is much better for pBC-Polym-*x*. This can be ascribed to the high polymer loading and the high affinity to aqueous solution of pBC. On the other hand, the saturated capacity of pBC-Polym-0.04 was very close to that of pBC-Polym-0.08. The possible reason was that the thicker polymer coating on the nanofibers (pBC-Polym-0.08) decreased the utilization efficiency of the active materials.

**Figure 4.** (**a**) Cr(VI) adsorption isotherms of pBC-Polym-0.02, pBC-Polym-0.04 and pBC-Polym-0.08, and (**b**) BC-Polym-0.02, BC-Polym-0.04 and BC-Polym-0.08. Their corresponding fitted Langmuir (**c**,**d**) and Freundlich isotherms (**e**,**f**). Effect of contact time on Cr(VI) adsorption (**g**) and kinetics modeling: (**h**) pseudo-first-order kinetic plots, (**i**) pseudo-second-order kinetic plots and (**j**) plots of intra particle model.

Furthermore, the experimental data (Figure 4c~f) were simulated by Langmuir and Freundlich models. The Langmuir and Freundlich equations are shown below:

$$\frac{\mathbf{C}\_{\varepsilon}}{q\_{\varepsilon}} = \frac{\mathbf{C}\_{\varepsilon}}{q\_{m}} + \frac{1}{bq\_{m}}\tag{5}$$

$$
\log q\_{\varepsilon} = \log K\_f + \frac{1}{n} \log \mathbb{C}\_{\varepsilon} \tag{6}
$$

where *Ce* is the Cr(VI) concentration at equilibrium, *qe* is the adsorption capacity at equilibrium, *qm* is the maximum adsorption capacity at saturation, *b* is the isotherm constant for Langmuir model, *Kf* and *n* are the constants of isotherm equation. The parameters were listed in Table 1.

**Table 1.** Isotherms parameter of Cr(VI) adsorption on pBC-Polym-x and BC-Polym-x samples.


Based on the calculation, adsorption behavior can be better described by the Langmuir model. This demonstrated a homogenous monolayer adsorption on poly Schiff base-coated cellulose nanofibers. The maximum adsorption capacity of pBC-Polym-0.04 was 321.5 mg g<sup>−</sup>1. Remarkably, the adsorption capacity in this research was higher than most of the macroscopic adsorbents in previous studies (Table 2).


**Table 2.** Comparison pBC-Polym-0.04 with other macroscopic adsorbents.
