Surface characterization of untreated and solvent-extracted rice straw

Surface characteristics of untreated and solvent extracted rice straw and intact rice plant stem and sheath tissue have been investigated using reflectance Fourier transform infrared spectroscopy (FTIR-ATR), electrokinetic analysis (EKA), and scanning electron microscopy (SEM). Klason lignin and Kappa numbers were determined according to TAPPI standard methods. Sohxlet extractions were performed with hexanes, ethyl acetate, chloroform, ethanol–toluene and benzene–ethanol azeotropes. The FTIR reflectance spectra showed differences between the untreated sheath and stem tissues. It was found that the functional groups of the stem interior tissues were masked by another substance, which is proposed to be potassium oxalate monohydrate. This mineral salt, readily removed by water-washing, was affected by the solvent extractions with varying results. The extracted amounts ranged from 1 to 7%. While lipid removal was detectable by the FTIR reflectance method, the streaming potential method showed only slight differences between the curves representing the zeta potential (mV) as a function of pH of the untreated versus extracted samples. Reduced values of the iso-electric points were observed for the extracted samples as compared to the untreated rice straw, indicative of more acidic surface characteristics.