Both the products (silica and lignin) wereseparated from paddy straw powder by alkali dissolution, followed by precipitationusing an acid. Lignin and silica were dissolved by reacting with sodiumhydroxide (NaOH, strong base) to form water soluble sodium silicate (Eq. I) asdepicted in the following reaction.
A schematic separation process of silica andlignin from agro-residue waste is outlined in Fig. 1. The filtered Na2SiO3was tan coloured liquidwhile the residue left behind was light brown in colour.
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The obtained sodiumsilicate was then neutralized slowly with sulphuric acid solution to pH 6.5,where silica was precipitated (Eq. II). This process of slow titration wasperformed under mild stirring to allow proper diffusion of sodium sulfate (Na2SO4)and to avert the formation of large aggregates. Upon reaching pH 6.
5, silicawas gradually precipitated after 18 h of incubation. The resultant silicaprecipitates were washed frequently with ultrapure H2O until no Na2SO4was detected in the wash and the recovered silica was dried in an oven at50°C overnight. The dried silica appeared as a fine pale white powder at anoverall yield of 9.26%.
Simultaneously, lignin was also precipitated by furtherlowering down the pH of the sodium silicate solution to pH 4.0 with sulphuricacid and the precipitates thus obtained were washed and dried. The dried ligninappeared as a fine brown powder with an overall yield of 2.30%. Minuet al. (2012) have reported the extraction of silica and lignin from the blackliquor generated during the production of bio-ethanol from agro-residues. Luand Hsieh, (2012) has reported the separation of nano-silica from rice residue.Synthesis of nano-silica from semi-burned paddy straw ash has also been documented(Zaky and co-workers, 2008).
However, in this study a direct method has beendeveloped for nano-silica and lignin recovery from paddy straw.