*3.1. E*ff*ect of Adsorbent Structure*

Figure 1 shows the uptake of MB or CR after 16 h with constant agitation on different nanosorbents. The initial MB and CR concentrations were 500 mg L−<sup>1</sup> and 2000 mg L−1, respectively. Pure CNF adsorbed the highest amount of cationic MB per unit mass (1207.2 mg g<sup>−</sup>1) due to the large quantity of negatively charged carboxyl groups present on its surface. However, the amount of CR adsorbed on pure CNF was low (175.5 mg g<sup>−</sup>1) possibly due to Coulombic repulsions between negatively charged CNF and anionic CR. Pure GnP powder (1491.7 mg g<sup>−</sup>1) was able to adsorb the highest amount of CR followed by pure CNT powder (777.9 mg g<sup>−</sup>1). The ability of GnP to remove MB was also superior to CNT, which can be attributed to the larger surface area of GnP with more sites available for adsorption. Compared to nanocellulose, carbon nanomaterials could adsorb both cationic and anionic dyes mainly though π–π interaction [37]. Among the hybrid aerogels, CNF–GnP aerogels were generally better than CNF–CNT aerogels in the ability to remove both types of dyes from water at the same CNF to carbon

nanomaterial ratio. Moreover, the adsorption capacity of MB onto CNF–GnP 3:1 (1166.1 mg g<sup>−</sup>1) was very close to that of pure CNF. As the quantity of carbon nanomaterials in the hybrid increased, the MB uptake gradually decreased because CNF was displaced by nano adsorbents with lower affinity for MB. Interestingly, among all hybrid aerogels, CNF–GnP 3:1 also exhibited the highest CR uptake (507.1 mg g<sup>−</sup>1), almost two times greater than pure CNF. Similar results were obtained for CNF–CNT hybrids, with CNF–CNT 3:1 having superior MB and CR uptakes than other CNF–CNT combinations. This may suggest that high CNF concentrations can reduce carbon nanomaterial aggregation and improve dispersion quality to promote more effective contact between the solute and the sorbent. *t*-test results indicated the adsorption of MB onto CNF–GnP 3:1 and CNF–CNT 3:1 had no significant difference (*p* > 0.01), but CNF–GnP 3:1 was superior to CNF–CNT 3:1 in the adsorption of CR (*p* < 0.01) (Tables S2–S5). Based on these observations, hybrid aerogels comprising a CNF to carbon nanomaterial ratio of 3:1 were selected for further dispersion, mechanics, morphology, and chemical structure analysis. The CNF–CNT 3:1 and CNF–GnP 3:1 sorbents are reported henceforth as CNF–CNT and CNF–GnP, respectively.

**Figure 1.** The final adsorption of MB and CR onto CNF–CNT aerogels with the mass ratio of 1:0, 3:1, 1:1, 1:3 and 0:1 (**a**), CNF–GnP aerogels with the mass ratio of 1:0, 3:1, 1:1, 1:3 and 0:1 (**b**). Initial MB concentration was 500 mg L<sup>−</sup>1. Initial CR concentration was 2000 mg L−1.

The dispersion states of CNT and GnP aqueous suspensions in the presence of CNFs were investigated by optical microscopy after probe sonication for 5 min and 30 min (Figure S2). After 5 min of ultrasonic treatment, the average particle sizes of CNTs and GnPs were 8.8 ± 13.1 μm (Figure S2a) and 6.6 ± 11.0 μm (Figure S2c), respectively. After 30 min of ultrasonic treatment, large particle aggregations disappeared in each case (Figure S2b,d). The average particle sizes of CNTs and GnPs were 2.1 ± 0.5 μm and 3.1 ± 0.9 μm, respectively. The hydrophobicity of carbon nanomaterials and their tendency to readily form aggregates by hexagonal packing of individual particles with high van der Waals binding energy can greatly reduce the surface area available for adsorption. Ultrasonication provided sufficient energy to break the aggregates of carbon nanomaterials. While TEMPO-oxidized CNFs had negatively charged surface carboxyls, the counterions on the surface of the CNFs induced dipoles in the sp2 carbon lattice of the carbon nanomaterials. Then, the charges on the CNFs induced electrostatic stabilization between CNF and CNT/GnP that prevented the carbon nanomaterials from reaggregation [26]. Van der Waals interactions also may occur between CNF and CNT/GnP. CNTs contained few hydroxyl groups and were able to form hydrogen bonds with CNFs. The homogeneous CNF/CNT and CNF/GnP suspensions remained stable for at least 12 h, which was long enough for the preparation of the hybrid aerogels.

Pure CNF aerogel was white with a porous external structure and turned black with the incorporation of carbon nanomaterials (Figure 2, inset). The aerogels formed inside centrifuge tubes during freeze-drying remained intact as cylindrical blocks and could be easily cut into slices using a

sharp blade with no apparent deformation. The compression stress-strain curves of the three aerogels are shown in Figure 2. All curves exhibited "slow slope type" before the strain reached 80%. The initial strength of CNF and CNF–GnP aerogels was similar and higher than that of CNF–CNT. After the strains reached 80%, the curves were "steep type". Inflection points were observed when the strains were approximately 80%. Under the strains of 80%, the compression strengths of CNF, CNF–CNT and CNF–GnP were 0.064, 0.014 and 0.036 MPa, respectively. When immersed in water after compression testing, CNF, CNF–CNT and CNF–GnP aerogels exhibited a water activated shape recovery property (Supplementary Movies S1–S3). The aerogels absorbed water and restored the deformation. Most of the water could be easily squeezed out with tweezers and the compact aerogels could reabsorb water and return to their original size and shape. The fact that the aerogel cylinders were easily squeezed to ~10% of their length and quickly regained the same dimensions with a complete recovery, indicate that the aerogels were mechanically strong, and their open structure allowed the liquid solution to rapidly and freely flow in and out.

**Figure 2.** Stress–strain curves of CNF, CNF–CNT and CNF–GnP. The inset is a photograph of the freeze-dried aerogels. 1: CNF, 2: CNF–GnP and 3: CNF–CNT.

The morphologies of the CNF, CNT, GnP, CNF–CNT and CNF–GnP adsorbents have been characterized by SEM (Figure 3a–h). Pure CNF aerogel exhibited a three-dimensional structure with an intercalation of flat and folded sheets and contained pores of various shapes, which may be ascribed to the high suspension concentration (i.e., 6 mg mL−1) used before freeze-drying. Pristine CNT and GnP powders revealed the presence of bundles and stacked aggregates due to strong attractive forces between individual particles (Figure 3c,d). The combination between CNT and CNF affected the formation of the CNF sheet structure (Figure 3e). This is the reason that CNF–CNT aerogel produces debris after compression performance testing (Supplementary Movie 2). CNT dispersed better in the CNF matrix (Figure 3f). However, at higher magnification (Figure 3f, inset), some CNT aggregates could be observed still in the matrix. Since both CNFs and CNTs were anisotropic rods, and CNTs were much longer than CNFs, individual CNT partly uncovered by CNFs may re-associate with each other during the freeze-drying process. Thus, the CNT surface active sites available for dye adsorption were reduced. CNF–GnP formed porous aerogels with the main framework still being composed of CNF, while granular GnPs were evenly distributed in the CNFs matrix after ultrasonication and freeze-drying (Figure 3g,h). This can be attributed to the strong interactions between CNF and GnP, preventing GnP stacking and improving the hydrophilicity of GnPs [38]. The graphene platelets were well separated by rod-like CNFs. To contrast with the irregular and aggregated structure of CNT and GnP powders, the hybrid aerogels exhibited an open pore network that can facilitate fast molecular diffusion, hence promoting the accessibility of adsorption sites to relatively large dye molecules. Noteworthy, the morphology of CNF–GnP aerogels was quite different from the curly morphology of pure graphene aerogel in a previous study [39].

**Figure 3.** FE–SEM images of CNF aerogel (**a**,**b**), CNT (**c**), GnP (**d**), CNF–CNT aerogel (**e**,**f**) and CNF–GnP aerogel (**g**,**h**). The insets in (**c**,**d**,**f**,**h**) are FE–SEM images of CNT, GnP, CNF–CNT and CNF–GnP at a high magnification, respectively.

The FTIR spectra of CNF, CNF–CNT, CNF–GnP, CNT and GnP are shown in Figure 4. The CNF spectrum showed common cellulose peaks: broad hydroxyl stretching at 3360 cm−<sup>1</sup> and bending at 1610 cm<sup>−</sup>1, predominant C–O peaks at 1168, 1112, and 1062 cm−1, and a C–H stretching peak at 2900 cm<sup>−</sup>1, respectively. The small shoulder at 1712 cm−<sup>1</sup> was associated with the carbonyl stretching of the carboxylic acid, confirming C6 primary hydroxyl conversion to carboxyls from TEMPO oxidation [21]. The CNT and GnP spectra were nearly featureless. A small bump at 1570 cm−<sup>1</sup> was assigned to C=C groups in graphene. Bending vibrations of C–O–C at 1210 cm−<sup>1</sup> and C–O at 1038 cm<sup>−</sup>1, respectively, indicated epoxide or C–OH structure existing in CNT and graphene. These weak vibration peaks confirmed that the degree of oxidation in CNT and GnP were low. The cellulose characteristic peaks also were observed in the hybrid aerogels containing 25% CNT/GnP. The change of wavenumber for O–H in the hybrids indicated the existence of hydrogen bonding between CNT/GnP and CNF [40]. Since CNT and GnP only had few oxygen containing groups, van der Waals forces and, perhaps, hydrophobic interactions also contributed to the combination of the carbon nanomaterials and CNFs.

**Figure 4.** FTIR spectra of CNF (**a**), CNF–CNT (**b**), CNF–GnP (**c**), CNT (**d**) and GnP (**e**). To optimize the representation, the region of 2500–1800 cm−<sup>1</sup> is omitted.

Based on adsorption, dispersion, mechanics, morphology and IR results mentioned above, the best performance of the CNF–GnP 3:1 hybrid aerogel perhaps resulted from the plate structure and large surface area of GnPs (i.e., The specific surface areas of GnP and CNT were 750 and 60 m2, respectively). A sufficient amount of CNFs prevented GnPs from stacking and improved the hydrophilicity of GnPs. However, CNTs still tangled with each other in the CNF matrix. Dye molecules adsorbed to the GnP portion mainly through π–π and hydrophobic interactions, while cationic MB was able to adsorb to the negatively charged CNF portion by electrostatic interactions. Thus, CNF–GnP 3:1 aerogel was selected for future adsorption kinetics and isothermal modeling.
