The have exhibited a very narrow particle size

The surface morphology of final products silica
and lignin obtained from paddy straw were analyzed using scanning and transmission
electron microscopy (Fig. 5 and 6). The SEM micrographs of silica particle clearly
depicted their size as few micrometer agglomerates (Fig. 5a). Due to the
non-conductive nature of silica, the charges were quickly accumulated even
after gold sputtering on the powder surfaces showing visible artefacts and thus
limiting the resolution. TEM image of the sample dried from diluted silica
suspension on carbon grid surface showed the dispersed silica in circular shape
(Fig. 5b). Whereas SAED pattern indicate the amorphous nature of the silica
particles (inset, Fig. 5c) which agreed well with the XRD pattern. Herein, uniform
shape and very small size of silica nanoparticles are observed. At high
magnification, the SiO2 particles were seen in the size range of 15
– 20 nm (Fig. 5b). Earlier reports on silica nanoparticles derived from paddy
waste had diameters ranging from 30 nm to 170 nm (Lu and Hsieh, 2012; Liu et
al., 2011; Liou and Wu, 2010; Wattanasiriwech et al., 2010; Zaky et al., 2008).
However, the silica nanoparticles isolated in this study have exhibited a very
narrow particle size distribution that could be valuable for numerous future
applications.

            The
elemental composition of silica was confirmed by EDS (Fig. 5c). The isolated
silica powder analysis clearly indicated peaks of Si and O element without any
other impurities which is reasonably matched with FTIR spectra of SiO2.
Paddy straw has been reported to contain various other metals such as Fe, Al,
Ca, K, Mg (Wu and workers, 2009). However, the absence of these metals in the
present study has confirmed that extensive washing of the paddy straw with ultrapure
H2O before grinding was highly effective in removing them. Metal
impurities have also been reported to be carried away with the volatiles during
acid hydrolysis of paddy straw powder (Liou, 2004). Also, the strategic lengthening
of the hydrolysis steps and break at some significant points might enhanced the
efficacy of the present separation process.

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            The
SEM (Fig. 6a) and TEM (Fig. 6b) images show high resolution micrographs of lignin
isolated from paddy straw powder. SEM image of the lignin is clearly
defibrillated and particles are polyhedric in shape, with granules of varied
dimensions (Fig. 6a).  Cotana et al.,
(2014) also observed multi-shaped particles of lignin isolated from residual
biomass. The TEM image clearly show separated lignin particles at nano scale
(Fig. 6b). The isolated lignin was amorphous in nature as observed by SAED pattern
(inset, Fig. 6c). Fig. 6c clearly depicts the elemental composition of isolated
lignin as determined by using EDS. The composition of lignin varied from biomass
to biomass. The lignin powder was found to contain only C and O element without
any other impurities. The fine detail observed in the micrographs of lignin
agrees well with physical parameters determined by XRD analysis. Separation of
lignin from various agro waste residues has also been documented (Watkins et
al., 2015; Zhang et al., 2012).