Saturated fatty acid adsorption by acidified rice hull ash

Abstract

AbstractRice 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 (C8, C10, C12, C14, C16, and C18) was carried out on RHA‐A1 and RHA‐A14 at 32±1°C. The adsorption data conformed to the Langmuir isotherm. The specific surface area of RHA‐A1 was 183.84 m2 g−1 while that of RHA‐A14 was 174.67 m2 g−1. The specific pore volume of RHA‐A1 was 0.216 cm3 g−1 while that of RHA‐A14 was 0.234 cm3 g−1. The acid‐treated ash, RHA‐A14 (qm=0.43±0.03 mmol g−1 where qm 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 (qm=0.25±0.03 mmol g−1). The free energy of adsorption, ΔG°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 ΔG°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 ΔG°ads values showed that the process of adsorption took place through physisorption on both RHA.