based materials has been proven to be very effective in the removal of heavy metals
(Zhu et al. 2020; Khoathane et al. 2015). On the other hand, amination of surface
hydroxyls of nanocellulose via grafting and quaternation with various alkyl chains
(methylamine, butylamine, and hexylamine) by mechanical homogenization could
introduce a positive charge at the surface and hence be used for the removal of
anionic pollutants (Wang et al. 2016; Yue et al. 2019).
5 Spectrum of Neoteric NC-Based Materials
and Exploratory Endeavors for Environmental
Applications
For environmental applications, nanocellulose may be employed as filtration membranes, adsorbent, flocculent, catalyst, and so on (Fig. 4.4a). In fact, many of the
exceptional properties of NC, such as high porosity, high surface area, and tunable
surface chemistries, have increased the application of NC in water treatment aimed at
removing pollutants (Derami et al. 2020; Xu et al. 2018a; Varghese et al. 2019).
Among others, the prominent materials that have captured immense research impetus over the recent decades for pollutant capturing/mitigation and other allied
applications are the hydrogels – the three-dimensional colloidal gels of cross-linked
polymers that can absorb and retain water and aqueous substances (Wichterle and
Lim 1960; Klein and Poverenov 2020; Nam et al. 2018; Sinha and Chakma 2019).
Although many hydrogels with demonstrable economic importance have been
synthesized from polyethylene glycol (PEG), polyvinyl alcohol (PVA), and polyvinyl pyrrolidone (PVP), environmental concerns have driven the use of natural
polymers, such as nanocellulose. CNCs- or CNFs-based hydrogels offer a variety
of practical advantages, including biodegradability, nontoxicity, high water content,
and biocompatibility. In fact, the mechanical strength of cellulose hydrogels during
the swollen state is considered to be problematic, but this has been improved with
composite fabrication, cross-linking, and other approaches. For example, Xu et al.
(2019) produced TEMPO-oxidized individualized cellulose nanofibrils cross-linked
physically and chemically, having a compressive strength of 450 kPa and a compressive strain of 90% without cracking. The process for the production of hydrogels
based on CNCs includes either physical or chemical stabilization of an aqueous
solution of a certain concentration. Depending on the shape, manufacturing process,
type of cross-linking, and application, CNC-based hydrogels are classified as traditional bulk hydrogels and injectable hydrogels (Shojaeiarani et al. 2019). Pertinently,
dual cross-linked hydrogels appear to be very efficient in solving the mechanical
strength problems associated with nanocellulose-based hydrogels. The method uses
both physical and chemical cross-linking approaches, and the resulting hydrogels
with their highly porous structure, similarity to the natural extracellular matrix
(ECM), the capability of enclosing cells within their matrix, and the ability to access
deep-seated areas merit special mention, both in biomedical and environmental
applications.
4 Harnessing the Sustainable Bioresource, Cellulose at the Nanoscale for. . .
75
(Zhu et al. 2020; Khoathane et al. 2015). On the other hand, amination of surface
hydroxyls of nanocellulose via grafting and quaternation with various alkyl chains
(methylamine, butylamine, and hexylamine) by mechanical homogenization could
introduce a positive charge at the surface and hence be used for the removal of
anionic pollutants (Wang et al. 2016; Yue et al. 2019).
5 Spectrum of Neoteric NC-Based Materials
and Exploratory Endeavors for Environmental
Applications
For environmental applications, nanocellulose may be employed as filtration membranes, adsorbent, flocculent, catalyst, and so on (Fig. 4.4a). In fact, many of the
exceptional properties of NC, such as high porosity, high surface area, and tunable
surface chemistries, have increased the application of NC in water treatment aimed at
removing pollutants (Derami et al. 2020; Xu et al. 2018a; Varghese et al. 2019).
Among others, the prominent materials that have captured immense research impetus over the recent decades for pollutant capturing/mitigation and other allied
applications are the hydrogels – the three-dimensional colloidal gels of cross-linked
polymers that can absorb and retain water and aqueous substances (Wichterle and
Lim 1960; Klein and Poverenov 2020; Nam et al. 2018; Sinha and Chakma 2019).
Although many hydrogels with demonstrable economic importance have been
synthesized from polyethylene glycol (PEG), polyvinyl alcohol (PVA), and polyvinyl pyrrolidone (PVP), environmental concerns have driven the use of natural
polymers, such as nanocellulose. CNCs- or CNFs-based hydrogels offer a variety
of practical advantages, including biodegradability, nontoxicity, high water content,
and biocompatibility. In fact, the mechanical strength of cellulose hydrogels during
the swollen state is considered to be problematic, but this has been improved with
composite fabrication, cross-linking, and other approaches. For example, Xu et al.
(2019) produced TEMPO-oxidized individualized cellulose nanofibrils cross-linked
physically and chemically, having a compressive strength of 450 kPa and a compressive strain of 90% without cracking. The process for the production of hydrogels
based on CNCs includes either physical or chemical stabilization of an aqueous
solution of a certain concentration. Depending on the shape, manufacturing process,
type of cross-linking, and application, CNC-based hydrogels are classified as traditional bulk hydrogels and injectable hydrogels (Shojaeiarani et al. 2019). Pertinently,
dual cross-linked hydrogels appear to be very efficient in solving the mechanical
strength problems associated with nanocellulose-based hydrogels. The method uses
both physical and chemical cross-linking approaches, and the resulting hydrogels
with their highly porous structure, similarity to the natural extracellular matrix
(ECM), the capability of enclosing cells within their matrix, and the ability to access
deep-seated areas merit special mention, both in biomedical and environmental
applications.
4 Harnessing the Sustainable Bioresource, Cellulose at the Nanoscale for. . .
75