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Mercury contamination in drinking water is a worldwide problem due to its severely harming effects on the human body. A nanostructured natural bioadsorbent, carboxycellulose nanofiber extracted from raw moringa plant using the nitro-oxidation method (termed NOCNF), capable of effectively remediating this problem has been demonstrated. Nitro-oxidation is a simple approach that can extract carboxylated nanocellulose directly from raw biomass. In this study, the produced NOCNF contained a large density of carboxylate groups on the cellulose surface (0.97 mmol/g), capable of removing Hg2+ ions by simultaneous electrostatic-interactions and mineralization processes. Using the Langmuir analysis, the adsorption results indicated that the highest Hg2+ removal capacity of this NOCNF was 257.07 mg/g, which is higher than most of the reported values. The interactions between Hg2+ and NOCNF were further characterized by Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), transmission electron microscopy (TEM) with electron diffraction and wide-angle X-ray diffraction (WAXD) methods, suggesting the existence of two distinct removal mechanisms: predominant adsorption at low Hg2+concentrations (< 250 ppm) and predominant mineralization at high Hg2+ concentrations (> 1000 ppm). The applications of NOCNF were illustrated in both suspension form, as an adsorbent/coagulant, and dry powder form using filtration column. The results indicated that NOCNF in suspension exhibited a higher maximum removal efficiency of 81.6 % as compared to the dry state of 74.3 %. This work demonstrated the feasibility of extracting nanostructured adsorbents from biomass feedstocks to tackle the Hg2+ contamination problem in drinking water.

Two different chemical methods, TEMPO-oxidation and nitro-oxidation, were used to extract carboxyl cellulose nanofibers (CNFs) from non-wood biomass sources (i.e., jute, soft and hard spinifex grasses). The combined TEMPO-oxidation and homogenization approach was very efficient to produce CNFs from the cellulose component of biomass; however, the nitro-oxidation method was also found to be effective to extract CNFs directly from raw biomass even without mechanical treatment. The effect of these two methods on the resulting cross-section dimensions of CNFs was investigated by solution small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The UV-Vis spectroscopic data from 0.1 wt% TEMPO-oxidized nanofiber (TOCNF) and nitro-oxidized nanofiber (NOCNF) suspensions showed that TOCNF had the highest transparency (> 95%) because of better dispersion, resulted from the highest carboxylate content (1.2 mmol/g). The consistent scattering and microscopic results indicated that TOCNFs from jute and spinifex grasses possessed rectangular cross-sections, while NOCNFs exhibited near square cross-sections. This study revealed that different oxidation methods can result in different degrees of biomass exfoliation and different CNF morphology.

Carboxycellulose nanofibers were extracted from untreated jute using a simple nitro-oxidation method based on nitric acid/sodium nitrite. The characteristics of nitro-oxidized carboxycellulose nanofibers (NOCNF) with low crystallinity (35%), high carboxylate content (1.15 mmol/g) and high surface charge (− 70 mV) made them an excellent substrate for Pb(II) ion removal from water. For example, a low concentration of NOCNF suspension (0.23 wt%) could remove a wide range of Pb(II) ions ranging from 50 to 5000 ppm in a short time-interval ( 1000 ppm). Evidence of nanoscale lead hydroxide crystallization, induced by the lead(II)-NOCNF aggregated scaffold, was confirmed by FTIR, UV–visible spectroscopy, SEM/EDS, WAXD and TEM measurements.

Carboxylated cellulose nanofibers (CNFs), having an average width of 7 nm and thickness of 1.5 nm, were produced by TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation method. The fiber cross-sectional dimensions were determined using small-angle X-ray scattering (SAXS), transmission electron microscopy and atomic force microscopy techniques, where the rheological properties under different concentration and ionic strength were also investigated. The formation of hydrogel was evidenced by increasing the CNF concentration or ionic strength of the solvent (water), while the gel structure in ion-induced CNF hydrogels was found to be relatively inhomogeneous. The gelation behavior was closely related to the segmental aggregation of charged CNF, which could be quantitatively characterized by the correlation length (ξ) from the low-angle scattering profile and the scattering invariant (Q) in SAXS.