The adsorption of palmytic acid on rice husk ash chemically modified with Al(III) ion using the sol-gel technique.

Silica-incorporated aluminum (RHA-Al) was synthesized from rice husk ash (RHA) using the sol-gel technique. RHA-Al was calcined at 500 degrees C for 5 h to yield RHA-Al(C). The ratio of silica to alumina was found to be 4:1. The BET analysis of RHA-Al(C) showed an increase in total pore volume and specific surface area compared to RHA-Al. SEM and XRD showed that RHA-Al and RHA-Al(C) were composed of microcrystals and the surface of both samples had a porous structure. Adsorption studies of palmytic acid on RHA-Al and RHA-Al(C) at 30, 40, and 50 degrees C conformed to the Langmuir isotherm. The equilibrium parameter, R, revealed that both are good adsorbents for palmytic acid. The Gibbs free energy of adsorption, DeltaG(ads)(0), was determined to be between -21.0 and -26.0 kJ mol(-1). DeltaH(ads)(0) and DeltaS(ads)(0) for RHA-Al were found to be 26.2 kJ mol(-1) and 158 J mol(-1), respectively. Corresponding values for RHA-Al(C) were 31.7 kJ mol(-1) and 178 J mol(-1). The adsorption of fatty acid on RHA-Al and RHA-Al(C) was an endothermic process, which occurred spontaneously. An FTIR study on the adsorbed material was used to determine the possible adsorbed complex on the surface of the adsorbent.