*3.5. Desorption*

The desorption of MB and CR from CNF, GnP and CNF–GnP by ethanol, acetonitrile, acetone and 400 mM NaCl was investigated. The desorption of dye adhered onto CNF, GnP and CNF–GnP was not effective using acetonitrile, acetone and NaCl. Upon immersion in acetonitrile, acetone or 400 mM NaCl for 1 h, only 23.6%, 17.2% or 28.3% of pre-adsorbed MB in CNF–GnP hybrid (100 mg L−<sup>1</sup> MB, 20 mL MB solution, 16 h) was desorbed, respectively. Upon immersion in acetone or 400 mM NaCl, only 6.0% or 20.1% of pre-adsorbed MB in GnP was desorbed. Concerning CNF, only 36.6% MB was desorbed by acetone. Although 400 mM NaCl could desorb 88.6% MB under the same conditions, the pure CNF aerogel could not remain intact after immersion for 1 h. Strong ionic conditions destroyed the hydrogen bonding network in pure CNF. Dyes adhered to CNF, GnP and CNF–GnP were desorbed rapidly by ethanol (Figure 8). Desorption of MB and CR adhered to CNF–GnP by ethanol was relatively more effective. After 1h of immersion, 42.4% of the MB and 51.0% of the CR were desorbed from CNF–GnP by ethanol. Finally, after four rounds of desorption, 79.2% of the MB and 78.3% of the CR were desorbed. Anhydrous ethanol is a protic solvent that contains polarized oxyhydrogen bonds which ionize to form alkoxyl negative ions and protons (hydrogen ions). It can provide lone pair electron interaction with MB (cationic dye) molecules. Concurrently, anhydrous ethanol also can provide protons with CR (anionic dye) molecules to form hydrogen bonds. The rapid desorption of both MB and CR demonstrate that the CNF–GnP hybrid aerogel could be easily regenerated for repeated dye removal applications.

**Figure 8.** Cyclic desorption of MB (**a**) and CR (**b**) with CNF–GnP, CNF, GnP by ethanol.

## **4. Conclusions**

Hybrid aerogels containing TEMPO oxidized cellulose nanofibrils (CNFs) and carbon nanomaterials (carbon nanotubes (CNTs) or graphene nanoplates (GnPs)), were designed and synthesized by freeze-drying to remove organic dyes from single and binary systems. When the CNF to GnP mass ratio was 3:1, the hybrid aerogel exhibited the most effective adsorption of both methylene blue (MB) and Congo red (CR) among all the hybrid systems tested. The final adsorption capacities of CNF–GnP 3:1 aerogels for MB and CR reached 1166.1 mg g−<sup>1</sup> and 507.1 mg g−<sup>1</sup> at initial dye concentrations of 500 mg L−<sup>1</sup> and 2000 mg L<sup>−</sup>1, respectively. The CNFs enhanced the dispersion of carbon nanomaterials in an aqueous environment. The hybrid aerogels were mechanically strong and exhibited water-activated shape recovery. Seen in a single dye adsorption system, the adsorption ability measurements demonstrate that the CNF–GnP 3:1 aerogel possessed the adsorption capacity and adsorption rate close to CNF aerogels in cationic MB solutions. The adsorption capacity and adsorption rate of CNF–GnP aerogels was more efficient than the CNF aerogel in an anionic CR solution. Dye adsorptions to CNF–GnP followed a pseudo-second-order adsorption kinetic and the existence of CNF did not affect the adsorption kinetics of GnP. The adsorption followed a monolayer

Langmuir isotherm. Concerning a binary system, the CNF–GnP aerogel removed cationic MB as well as anionic CR at a higher total dye adsorption capacity than pristine CNF or GnP. Moreover, 79.2% and 78.3% of the MB and CR were desorbed from CNF–GnP by using ethanol as the desorption agent, suggesting the reusability of this hybrid material. Results of this study indicate that CNF–GnP show promise as high-potential adsorbents for organic dye removal.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/10/1/169/s1, Figure S1: Conductometric titration curves of CNF (a) with HCl added and (b) without HCl added., Figure S2: Optical micrographs of (a) CNF/CNT 3:1 after 5 min ultrasonic treatment; (b) CNF/CNT 3:1 after 30 min ultrasonic treatment; and (c) CNF/GnP 3:1 after 5 min of ultrasonic treatment; (d) CNF/GnP 3:1 after 30 min ultrasonic treatment, Table S1: Chemical structure, molecular weight, maximum absorption wavelength and electrical property of methylene blue and Congo red. Table S2: Group statistics of the final adsorption of MB. Table S3: Independent sample tests of the final adsorption of MB onto CNF–GnP 3:1 and CNF–CNT 3:1. Table S4: Group statistics of the final adsorption of CR. Table S5: Independent sample tests of the final adsorption of CR onto CNF–GnP 3:1 and CNF–CNT 3:1. Movie S1: Shape recovery of CNF aerogel in water after compression. Movie S2: Shape recovery of CNF–CNT aerogel in water after compression. Movie S3: Shape recovery of CNF–GnP aerogel in water after compression in water.

**Author Contributions:** Conceptualization, J.G.; methodology, A.B.D.; investigation, Z.Y. and W.J.; data curation, Z.Y.; writing—original draft preparation, Z.Y.; writing—review and editing, J.G. and A.B.D.; supervision, C.H.; funding acquisition, C.H. and J.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Guangzhou Science and Technology Bureau, grant number 201904010308, the Guangdong Government Science and Technology, grant number 2017B020238003 and Bureau of Guangdong Forestry, grant number 2017-LYBZ-012.

**Conflicts of Interest:** The authors declare no conflict of interest.
