Nanocellulose extracted from wood pulps using TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation and sulfuric acid hydrolysis methods was characterized by small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS) techniques. The dimensions of this nanocellulose (TEMPO-oxidized cellulose nanofiber (TOCN) and sulfuric acid hydrolyzed cellulose nanocrystal (SACN)) revealed by the different scattering methods were compared with those characterized by transmission electron microscopy (TEM). The SANS and SAXS data were analyzed using a parallelepiped-based form factor. The width and thickness of the nanocellulose cross section were ∼8 and ∼2 nm for TOCN and ∼20 and ∼3 nm for SACN, respectively, where the fitting results from SANS and SAXS profiles were consistent with each other. DLS was carried out under both the V mode with the polarizer and analyzer parallel to each other and the H mode having them perpendicular to each other. Using rotational and translational diffusion coefficients obtained under the H mode yielded a nanocellulose length qualitatively consistent with that observed by TEM, whereas the length derived by the translational diffusion coefficient under the V mode appeared to be overestimated.

Mechanistic behavior and flow properties of cellulose nanofibers (CNFs) in aqueous systems can be described by the crowding factor and the concept of contact points, which are functions of the aspect ratio and concentration of CNF in the suspension. In this study, CNFs with a range of aspect ratio and surface charge density (380–1360 μmol/g) were used to demonstrate this methodology. It was shown that the critical networking point of the CNF suspension, determined by rheological measurements, was consistent with the gel crowding factor, which was 16. Correlated to the crowding factor, both viscosity and modulus of the systems were found to decrease by increasing the charge density of CNF, which also affected the flocculation behavior. Interestingly, an anomalous rheological behavior was observed near the overlap concentration (0.05 wt %) of CNF, at which the crowding factor was below the gel crowding factor, and the storage modulus (G′) decreased dramatically at a given frequency threshold. This behavior is discussed in relation to the breakup of the entangled flocs and network in the suspension. The analysis of the mechanistic behavior of CNF aqueous suspensions by the crowding factor provides useful insight for fabricating high-performance nanocellulose-based materials.

Nanocelluloses, in the form of carboxycellulose nanofibers, with low crystallinity (CI ∼ 50%), high surface charge (−68 mV), and hydrophilicity (static contact angle 38°), were prepared from an untreated (raw) Australian spinifex grass using a nitro-oxidation method employing nitric acid and sodium nitrite. The resulting nanofibers (NOCNF) were found to be an effective medium to remove Cd2+ ions (cadmium­(II)) from water. For example, a low concentration of NOCNF suspension (0.20 wt %) could remove Cd2+ ions over a large concentration range (50–5000 ppm) in a relatively short time period (≤5 min). The results showed that at low Cd2+ concentrations (below 500 ppm), the remediation mechanism was dominated by interactions between carboxylate groups on the NOCNF surface and Cd2+ ions, which also acted as a cross-linking agent to gel the NOCNF suspension. At high Cd2+ concentrations (above 1000 ppm), the remediation mechanism was dominated by the mineralization process of forming Cd­(OH)2 nanocrystals, which was verified by TEM and WAXD. Based on the Langmuir isotherm model, the maximum Cd2+ removal capacity of NOCNF was around 2550 mg/g, significantly higher than those of any adsorbents reported in the literature. NOCNF exhibited the highest removal efficiency of 84%, when the Cd2+ concentration was 250 ppm. This study demonstrated a simple pathway to convert underutilized biomass into valuable absorbent nanomaterials that can effectively remove cadmium­(II) ions from water.

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