Agro-residue is one of the key energy derivingagents which can play a significant role in eco-friendly energy generation.
Agriculture, in particular, by producing many wastes such as paddy straw, corn coband wheat straw is playing promising function in meeting the rising energydemand in a cost-effective manner (Chandra et al., 2012). Rice (Oryza sativa) is one of the mostimportant agricultural crop and is the leading staple food in the world (Liu etal.
, 2013). Rice was initially cultivated around 8000 years ago and India isthe second prime producer of it accounting for nearly 20% ofworld rice production. Though rice is the second major cereal crop after wheat but in addition it generateshuge amount of agriculture waste in the form of straw accounting around more than100 million tons annually in India. Globally, paddy straw represents the majorcrop leftover with an annual production of 731 x 106 Mg (Croce etal., 2016). To an estimate, one kilogram of grain harvesting from paddy isaccompanied by the production of about 1.5 kilogram of rice straw (Maiorella,1985).
Overthe years, direct use of paddy straw has presented some limitations as it is a subsidiaryfeed in comparison to other cereal straw. Moreover, paddy stubbles generated after harvesting of grains is burnt in situ, which is a regular managementpractice followed in all the countries. During the past few years, insane humanactivities such as burning of paddy residues have caused an enormous increasein the atmospheric concentration of greenhouse gases (Lohan et al.
, 2018; Abrahamet al., 2017; Chen et al., 2012). At present, some other paddy straw disposal methodssuch as land filling and open field flaming are followed. These methods cause highenergy waste, air pollution and huge landfill space occupancy due to their lowbulk density.
Paddystraw is a bulky and tough biomass containing high silica (SiO2) depositsin addition to lignin and cellulosic components (Chiew and Cheong, 2011; Sun etal., 2001). SiO2 is accumulated in plants mainly in the form ofphytoliths, which consist primarily of amorphous hydrated silica. Silicon dioxide in a chemically combinedform is ubiquitous in nature. It enters in paddy via their root system in asoluble form, probably as a monosilicic acid, which undergoes biomineralizationto form a lignocellulose and SiO2 connected network (Patel andKarera, 1991). Silica predominantly forms inorganic linkages, and some fractionof the SiO2 is covalently bonded to various other organic compounds.
Covalent bound SiO2 cannot be dissolved in high pH solution and can toleratehigh 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 etal., 2001). Harvesting silicon based materials such as SiO2 and itsnano form have been the key research area in the recent past because of theirwidespread functions in auto industry, information technology, fine chemistryand material science (Beall, 1994). Nano-silica plays an important role in silicabased materials such as catalysts, resins and biological membranes (Corma etal., 1997). Silica based nanomaterials have also found their place in makingbatteries in addition to its extensive use in biology and medicine (Ahmad etal.
, 2016). Likewise, lignin from biomass also has promising applications in bio-plastics, composites, carbon fibers, adsorbentsand dispersants (Norgren and Edlund, 2014). Concurrentseparation of silica and lignin from paddy straw is of great economical and environmentalimportance. Therefore, to addressthis alarming issue, our research group has been investigating novel methods toexplore 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 reducethe operating cost, an alternative and sustainable route to prepare nano-silicaand lignin molecules would be of great interest. Few studies have reported theextraction of silica from rice husk also (Battegazzore et al., 2014; Carmona etal., 2013; Zhang et al., 2010). Inthe present study, an efficient method has been developed to extract amorphoussilica in nano form and lignin from paddy straw in high purity.
Herein, acombination of frequent washing with ultrapure H2O to get rid ofsoluble substances, controlled heating to eliminate organic and metalimpurities and the successive purification of nano-silica and lignin by syntheticroute was investigated. Thereafter, a slow gelation cum drying process was followedto extract nano-silica and lignin. Obtained nano-silica and lignin were characterizedextensively using various techniques including X-ray powder diffraction, fourier-transforminfrared spectroscopy, thermo-gravimetric analysis, scanning and transmissionelectron microscopy, and energy dispersive X-ray spectroscopy (EDS). Throughthe present methodology, pure nano-silica and lignin have been recoveredsuccessfully from paddy straw.