Agro-residue is a key energy deriving agent whichplays an essential role in sustainable energy generation. Agriculture, in particular,by producing many wastes such as paddy straw, corn cob and wheat straw is performingcritical function in meeting the rising energy demand in an economical manner(Chandra et al., 2012). Rice (Oryzasativa) is an important agricultural crop and is the chief staple food inthe world (Liu et al., 2013). O.
sativa was initially cultivated around8000 years ago and India is the second prime producer of itaccounting for about more than 20% of total rice production globally. Although riceis the main agriculture cropafter wheat, but it also generates huge amount of agriculture waste in the formof straw accounting around more than 100 million tons annually in India.Globally, paddy straw represents the main crop residue with a yearly productionof more than 700 x 106 Mg (Croce et al., 2016). Roughly, one kg ofgrain harvesting from paddy is accompanied by the production of nearly 1.5 kgof paddy straw (Maiorella, 1985).
Overthe years, direct use of rice straw has presented few drawbacks as it is a marginalfeed in comparison to other crop residues. In addition, paddy stubbles generated after harvesting of grains is burntin situ, which is a regular disposal methodfollowed 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). Currently, numerous other straw management practicessuch as land filling and open field flaming are followed. These methods cause highenergy waste, air pollution and huge dumping space occupancy owing 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 deposited in plants primarily as phytoliths,which consist of amorphous hydrated silica. Silicon dioxide in a chemically combined form is ubiquitous in nature.
Itenters in paddy via their root system in a soluble form, probably as amonosilicic acid, which undergoes biomineralization to form a lignocellulose andSiO2 connected network (Patel and Karera, 1991). Silicapredominantly forms inorganic linkages, and some fraction of the SiO2is covalently bonded to various other organic compounds. Covalent bound SiO2cannot be dissolved in high pH solution and can tolerate hightemperatures. Silica in rice plant is mainly located in the hard epidermis (externallayer) as well as in the empty fissures of epidermal cells (Sun et al.
, 2001). Harvestingsilicon based materials such as SiO2 and its nano form have been thekey research area in the recent past because of their widespread functions inauto industry, information technology, fine chemistry and material science(Beall, 1994). Nano-silica plays significant role in SiO2 based materials likeresins, catalysts and biological membranes (Corma et al., 1997).
Silica based nanoparticlesalso found their place in making batteries apart from their extensive use in biologyand medicine (Ahmad et al., 2016). Likewise, lignin from biomass also has promisingapplications in bio-plastics,composites, carbon fibers, adsorbents and 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 reduce the operating cost andto protect the environment, a sustainable method to prepare nano-silica andlignin 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 regular washing with ultrapure H2O to get rid ofsoluble impurities, controlled heating to eliminate metal substances and the successivepurification 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 differenttechniques including X-ray powder diffraction, fourier-transform infraredspectroscopy, thermo-gravimetric analysis, scanning and transmission electronmicroscopy, and energy dispersive X-ray spectroscopy (EDS). Through the presentmethodology, highly pure nano-silica and lignin have been separatedsuccessfully from paddy straw.