Aveneu Park, Starling, Australia

Agro-residue huge amount of agriculture waste in the form

Agro-residue is a key energy deriving agent which
plays an essential role in sustainable energy generation. Agriculture, in particular,
by producing many wastes such as paddy straw, corn cob and wheat straw is performing
critical function in meeting the rising energy demand in an economical manner
(Chandra et al., 2012). Rice (Oryza
sativa) is an important agricultural crop and is the chief staple food in
the world (Liu et al., 2013). O. sativa was initially cultivated around
8000 years ago and India is the second prime producer of it
accounting for about more than 20% of total rice production globally. Although rice
is the main agriculture crop
after wheat, but it also generates huge amount of agriculture waste in the form
of straw accounting around more than 100 million tons annually in India.
Globally, paddy straw represents the main crop residue with a yearly production
of more than 700 x 106 Mg (Croce et al., 2016). Roughly, one kg of
grain harvesting from paddy is accompanied by the production of nearly 1.5 kg
of paddy straw (Maiorella, 1985).

            Over
the years, direct use of rice straw has presented few drawbacks as it is a marginal
feed in comparison to other crop residues. 
In addition, paddy stubbles generated after harvesting of grains is burnt
in situ, which is a regular disposal method
followed in all the countries. During the past few years, insane human
activities such as burning of paddy residues have caused an enormous increase
in the atmospheric concentration of greenhouse gases (Lohan et al., 2018; Abraham
et al., 2017; Chen et al., 2012). Currently, numerous other straw management practices
such as land filling and open field flaming are followed. These methods cause high
energy waste, air pollution and huge dumping space occupancy owing to their low
bulk density.

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            Paddy
straw is a bulky and tough biomass containing high silica (SiO2) deposits
in addition to lignin and cellulosic components (Chiew and Cheong, 2011; Sun et
al., 2001). SiO2 is deposited in plants primarily as phytoliths,
which consist of amorphous hydrated silica. Silicon dioxide in a chemically combined form is ubiquitous in nature. It
enters in paddy via their root system in a soluble form, probably as a
monosilicic acid, which undergoes biomineralization to form a lignocellulose and
SiO2 connected network (Patel and Karera, 1991). Silica
predominantly forms inorganic linkages, and some fraction of the SiO2
is covalently bonded to various other organic compounds. Covalent bound SiO2
cannot be dissolved in high pH solution and can tolerate high
temperatures. Silica in rice plant is mainly located in the hard epidermis (external
layer) as well as in the empty fissures of epidermal cells (Sun et al., 2001). Harvesting
silicon based materials such as SiO2 and its nano form have been the
key research area in the recent past because of their widespread functions in
auto industry, information technology, fine chemistry and material science
(Beall, 1994). Nano-silica plays significant role in SiO2 based materials like
resins, catalysts and biological membranes (Corma et al., 1997). Silica based nanoparticles
also found their place in making batteries apart from their extensive use in biology
and medicine (Ahmad et al., 2016). Likewise, lignin from biomass also has promising
applications in bio-plastics,
composites, carbon fibers, adsorbents and dispersants (Norgren and Edlund,
2014).

            Concurrent
separation of silica and lignin from paddy straw is of great economical and environmental
importance. Therefore, to address
this alarming issue, our research group has been investigating novel methods to
explore more economical ways to make full use of paddy straw (Purohit et al.,
2017; Manisha and Yadav, 2017). Hence, in order to reduce the operating cost and
to protect the environment, a sustainable method to prepare nano-silica and
lignin molecules would be of great interest. Few studies have reported the
extraction of silica from rice husk also (Battegazzore et al., 2014; Carmona et
al., 2013; Zhang et al., 2010).

            In
the present study, an efficient method has been developed to extract amorphous
silica in nano form and lignin from paddy straw in high purity. Herein, a
combination of regular washing with ultrapure H2O to get rid of
soluble impurities, controlled heating to eliminate metal substances and the successive
purification of nano-silica and lignin by synthetic route was explored. Further,
a slow gelation and drying was done to extract pure nano-silica and lignin. Thereafter,
obtained lignin and nano-silica were characterized thoroughly using different
techniques including X-ray powder diffraction, fourier-transform infrared
spectroscopy, thermo-gravimetric analysis, scanning and transmission electron
microscopy, and energy dispersive X-ray spectroscopy (EDS). Through the present
methodology, highly pure nano-silica and lignin have been separated
successfully from paddy straw.

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