Saturated fatty acid adsorption by acidified rice hull ash

Rice hull ash (RHA) was treated with 1.0 M HNO3 (RHA-A1) and another batch was treated with 14.0 M HNO3 (RHA-A14). RHA-A1 and RHA-A14 had a pH of 6.58 and 6.13, respectively. Adsorption of saturated fatty acids (C-8' C-10, C-12, C-14, C-16, and C-18) was carried out on RHA-A1 and RHA-A14 at 32 +/- 1 degrees C, The adsorption data conformed to the Langmuir isotherm. The specific surface area of RHA-A1 was 183.84 m(2) g(-1) while that: of RHA-A14 was 174.67 m(2) g(-1). The specific pore volume of RHA-A1 was 0.216 cm(3) g(-1) while that of RHA-A14 was 0.234 cm(3) g(-1). The acid-treated ash, RHA-A14 (q(m) = 0.43 +/- 0.03 mmol g(-1) where q(m) is the amount of adsorbate adsorbed to form a monolayer coverage on the ash particles) showed a twofold increase in the adsorption of fatty acid per gram ash compared to RHA-A1 (q(m) = 0.25 +/- 0.03 mmol g(-1)). The free energy of adsorption, Delta G degrees(ads), was determined to be -7.06 +/- 0.10 and -6.75 +/- 0.11 kcal mol(-1) for RHA-A1 and RHA-A14, respectively. The reduced Delta G degrees(ads) values observed for RHA-A14 were attributed to the electrostatic repulsion of the hydrophobic chain of the fatty acid adsorbed on adjacent sites and brought into close proximity of each other. The Delta G degrees(ads) Values showed that the process of adsorption took place through physisorption on both RHA.