*3.1. Pretreatment of Bacteria Cellulose*

The overview of pathway to fabricate the porous bacteria cellulose/poly Schiff base was described in Scheme 1. Bacteria cellulose was saturated with the sodium silicate solution (alkaline). Then, HCl acid reacted with sodium silicate to produce SiO2 aggregation. The formation of SiO2 could enlarge the inner space of bacteria cellulose and disassemble the large bacteria cellulose fiber bundles into much thinner fibers. The SiO2 was removed by NaOH solution and the pure modified bacteria cellulose was obtained by water rinse and subsequent freeze-drying. After the pretreatment, the bacteria cellulose was still macro-scale monolith. As shown in Figure 1a,b, the smooth surface of pristine bacteria cellulose consisted of compact fibers. In contrast, the pretreated bacteria cellulose had many macroscopic holes (Figure 1e), which should be generated by the SiO2 formation. The photograph and high-magnification images of bacteria cellulose/SiO2 composites can verify this (Figure 1c,d). More interestingly, the original bacteria cellulose fibers with diameter 400 nm was decreased to 30 nm (Figure 1f). The small nanofibers were most possibly free from the aggregated cellulose fibers by alkaline treatment. Moreover, the SiO2 formation within the gap or pores in the aggregated fiber bundles would also produce small nanofibers.

**Scheme 1.** The pathway of porous bacteria cellulose/poly Schiff base fabrication.

**Figure 1.** The photograph and of SEM images of bacteria cellulose (**a**,**b**), bacteria cellulose/SiO2 (**c**,**d**) and pBC (**e**,**f**); the photograph of adsorbent with ultra-low density (**g**) and the content of poly Schiff base in pBC-Polym-0.08 and BC-Polym-0.08 with various reaction time (**h**).

XRD was used to validate the variation of bacteria cellulose before and after pretreatment. As seen in Figure 2a, the raw bacteria cellulose exhibited characteristic of typical cellulose I by diffraction peaks at 14.3◦ and 22.6◦ [30–32]. Noticeably, after the pretreatment, the peak intensity at 14.3◦ and 22.6◦ attenuated obviously, which indicated the decrease of crystallinity. On the other hand, the relative intensity of 16.4◦ increases. Moreover, a small peak located at 20.0◦ appeared, indexed as the (110) of cellulose II. It inferred that the structure of semi-crystalline region and amorphous region was altered via pretreatment. This can be explained by the separation of small cellulose nanofibers from original fiber bundles, which inevitably exposed new facet. It must be mentioned that in the raw cellulose the H bonding (by –OH) pushed the formation of cellulose crystal. The decrease of crystallinity of bacteria cellulose demonstrated the breaking of H bonding, which signified that the –OH is free from the crystal. This is conducive to increasing its affinity toward aqueous solution, which can promote the mass transfer within the body of bacteria cellulose.

**Figure 2.** (**a**) XRD patterns of raw bacteria cellulose and pretreated bacteria cellulose. (**b**) FTIR spectra of pBC, pBC-Polym-0.02, pBC-Polym-0.04 and pBC-Polym-0.08 and (**c**) magnified graph in the range of 1750 to 1450 cm<sup>−</sup>1.
