3. Results and discussion
Dispersed cellulose fibers were initially isolated from printed
paper wastes via pretreatment with sodium hydroxide solution
(12 wt%) for the removal of residual lignin and ink particles, as
well as maceration of tightly packed cellulose bundles. Well dispersed
cellulose fibers were subsequently dissolved in an ionic
liquid, 1-allyl-3-methylimidozoium chloride (AMIMCl) to form a
homogeneous cellulose solution. In this study, a cellulose solution
of 4 wt% was used since cellulose solution of higher concentration
would be very viscous and hence difficult to be extruded directly
into the coagulation bath. Besides, cellulose solution of too low
concentration was found to be undesirable due to the formation of
cellulose beads which would collapse readily upon drying.
Through combined gravity force and appropriately applied pressure,
uniform droplets of cellulose solution were being extruded
through syringe needle nozzles of varying diameters. Larger but
uniform droplet sizes were obtained with increased diameter of
syringe needle nozzles under constant applied pressure. Spherical
cellulose beads were formed instantly as droplets of cellulose were
being introduced into the coagulation bath. The formation of cellulose
beads was governed by the relative rates of diffusion of nonsolvent
and cellulose molecules into the gelling zone [11]. Syringe
needles of different nozzle diameters were used in order to prepare
cellulose beads of mean diameters which varied within the
range of 0.4–2.2 mm (Fig. 1). The mean diameters of cellulose
beads as prepared by using syringe needle nozzles of diameter
0.50 mm, 0.80 mm and 1.20 mm were 0.4170.03 mm,
0.8370.11 mm, and 2.1470.08 mm, respectively. The mean masses of cellulose beads were observed to increase with their
mean diameters at 4.72, 9.30, and 27.8 mg/per bead, respectively
(Fig. 1(d)).
Fig. 2 shows the cross-sectional SEM micrographs of cellulose
beads with different mean diameters. These micrographs showed
the presence of 3D nanofibrillar networks which constituted the
highly porous microstructure of cellulose beads. Intermolecular
hydrogen bonds formed within the crystalline regions of cellulose
fibers could have served as physical cross-linkers during the coagulation
process which led to the formation of 3D nanofibrillar
network microstructure [12]. Cellulose beads were highly porous
in nature as evidenced by their substantially high specific surface
areas which ranged between 107 m2
/g and 498 m2
/g depending
on their mean sizes. The specific surface areas of cellulose beads
were observed to vary inversely proportional to their mean diameters
of 0.4170.03 mm, 0.8370.11 mm, and 2.1470.08 mm at
498 m2
/g, 324 m2
/g, and 107 m2
/g, respectively (Fig. 3). The lower
specific surface area of larger cellulose beads could be attributed to
higher packing density of their 3D nanofibrillar networks microstructure.
Being non-toxic, biocompatible, biodegradable, abundance
and cheap, the potential utility of cellulose beads with optimized
microstructure as drug delivery carriers or absorbent
materials is therefore envisaged.