The surface area of bio-char was estimated by nitrogen
adsorption at 77 K using an automatic adsorption instrument
(Quanta chrome Corporation, Autosorb 1C) that operates on the
static volumetric technique. Before gas adsorption measurements,
bio-char was degassed in the vacuum chamber at 200 C for 4 h to
remove the adsorbed moisture or other impurities bounded to the
surface.