Thermodynamic parameters for the adsorption of volatile n-alkane hydrocarbons on water hyacinth (Eichhornia crassipes) root biomass: Effect of organic solvent and mineral acid treatment

Abstract

The use of lignocellulosic fibres as biosorbents has both environmental and economic benefits. They are biodegradable, safe to use and most importantly renewable and less costly than synthetic fibres. The objectives of the present work were to study the performance of lignocellulosic fibres from water hyacinth (Eichhornia crassipes), a pervasive fresh water aquatic weed widely distributed in the tropics, as a biosorbent for n-alkane hydrocarbons, n-hexane to n-nonane. The free energy (ΔG_a), enthalpy (ΔH_a) and entropy (ΔS_a) for the  adsorption of  n-alkanes hexane to nonane on ground dried water hyacinth (E. crassipes) root biomass were studied between 40 and 70°C column temperature using inverse gas chromatography, before and after treatment of the root biomass with mineral acid and organic solvent. The free energy of adsorption (ΔG_a), enthalpy of adsorption (ΔH_a) and entropy of adsorption (ΔS_a) values of -20.42 to -35.992 kJ.mol^-1, -21.18 to -33.704 kJK^-1mol^-1 and +0.0023 to +0.0041 kJ.mol^-1 respectively, were obtained for the adsorption of the n-alkane series on untreated water hyacinth root biomass. Mineral acid and organic solvent treatment leads to greater adsorption bond strength, with ΔG_a and ΔH_a values of -36.08 to -38.52 kJ.mol^-1 and -34.01 to -36.33 kJK-1mol^-1, respectively, after mineral acid treatment, and -40.98 to -46.7 kJ.mol^-1 and -38.35 to -43.7 kJK^-1mol^-1, respectively, after organic solvent treatment. The results show that mineral acid and organic solvent treated water hyacinth root biomass has potential as biosorbent for the adsorption of volatile n-alkane hydrocarbons. Factors affecting the adsorbent-adsorbate interaction are discussed. 

Key words: Water hyacinth, inverse gas chromatography, adsorption free energy, adsorption enthalpy, hydrocarbons.