Both to avert the formation of large aggregates. Upon

Both the products (silica and lignin) were
separated from paddy straw powder by alkali dissolution, followed by precipitation
using an acid. Lignin and silica were dissolved by reacting with sodium
hydroxide (NaOH, strong base) to form water soluble sodium silicate (Eq. I) as
depicted in the following reaction. A schematic separation process of silica and
lignin from agro-residue waste is outlined in Fig. 1. The filtered Na2SiO3
was tan coloured liquid
while the residue left behind was light brown in colour. The obtained sodium
silicate was then neutralized slowly with sulphuric acid solution to pH 6.5,
where silica was precipitated (Eq. II). This process of slow titration was
performed under mild stirring to allow proper diffusion of sodium sulfate (Na2SO4)
and to avert the formation of large aggregates. Upon reaching pH 6.5, silica
was gradually precipitated after 18 h of incubation. The resultant silica
precipitates were washed frequently with ultrapure H2O until no Na2SO4
was detected in the wash and the recovered silica was dried in an oven at
50°C overnight. The dried silica appeared as a fine pale white powder at an
overall yield of 9.26%. Simultaneously, lignin was also precipitated by further
lowering down the pH of the sodium silicate solution to pH 4.0 with sulphuric
acid and the precipitates thus obtained were washed and dried. The dried lignin
appeared as a fine brown powder with an overall yield of 2.30%. Minu
et al. (2012) have reported the extraction of silica and lignin from the black
liquor generated during the production of bio-ethanol from agro-residues. Lu
and 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 been
developed for nano-silica and lignin recovery from paddy straw.