Agro-residue of paddy residues have caused an enormous increase

Agro-residue is one of the most important
energy driving agents which can play a significant role in eco-friendly energy generation.
Agriculture, in particular, by producing many wastes such as paddy straw, corn cob
and wheat straw is playing promising role in meeting the increasing energy
demand in a cost-effective and sustainable manner (Chandra et al., 2012). Rice
(Oryza sativa) is one of the most
important agricultural crop and is the second largest staple food in the world
(Liu et al., 2013). Rice was first cultivated around 8000 years ago and India
is the second prime producer of it accounting for 20% of world rice
production. Though paddy is the
world’s second largest cereal crop after wheat but it also generates huge
amount of agriculture waste in the form of straw accounting around 110 million
tons annually in India. Globally, paddy straw represents the major crop leftover
with an annual production of 731 x 106 Mg (Croce et al., 2016). To
an estimate, one kilogram of grain harvesting from paddy is accompanied by the
production of about 1.5 kilogram of rice straw (Maiorella, 1985).

            Over
the years, direct use of paddy straw has presented some limitations as it is a subsidiary
feed in comparison to other cereal straw. 
Moreover, paddy stubbles generated after harvesting of grains is burnt in situ, which is a regular management
practice 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). At present, some other paddy straw disposal methods
such as land filling and open field flaming are followed. These methods cause high
energy waste, air pollution and huge landfill space occupancy due 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 accumulated in plants mainly in the form of
phytoliths, which consist primarily 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 tough epidermis
(external layer) as well as in the space between the 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 an important role in silica
based materials such as catalysts, resins and biological membranes (Corma et
al., 1997). Silica based nanomaterials have also found their place in making
batteries in addition to its 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 protect the environment and reduce
the operating cost, an alternative and sustainable route 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 frequent washing with ultrapure H2O to get rid of
soluble substances, controlled heating to eliminate organic and metal
impurities and the successive purification of nano-silica and lignin by synthetic
route was investigated. Thereafter, a slow gelation cum drying process was followed
to extract nano-silica and lignin. Obtained nano-silica and lignin were characterized
extensively using various 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, pure nano-silica and lignin have been recovered
successfully from paddy straw.