Adsorption of volatile n-alkanes and polar organic solvents by water hyacinth (Eichhornia crassipes) root biomass: Thermodynamics, mechanism and effect of chemical treatment

Abstract

The use of lignocellulosic materials as biosorbents has both environmental and economic benefits. They are biodegradable, safe to use and most importantly renewable and less costly. The objectives of this present study were to study the effect of treatment on the performance of water hyacinth root biomass as a biosobent for n-hydrocarbons and polar organic solvents. Water hyacinth, a lignocellulosic material was chemically treated by two methods, nitric acid and organic solvent. The effect of the treatment was studied by Fourier Transform Infra-red (FT-IR) spectroscopy, Thermogravimetric analysis, X-ray diffraction (XRD), scanning electron microscopy and other analyses. FT-IR absorption bands were obtained at 3421, 2855, 1457 and 1035 cm-1 (O-H stretch, C-H vibration, C-H asymmetric deformation, and C-O stretch, respectively) and 1508, 1541 and 1559 cm-1 (all aromatic skeletal vibrations characteristic of lignin), as well as a C=O carboxylate stretch vibrational band at 1654 cm-1. The chemical composition and FT-IR analyses showed that treatment resulted in a decrease in the amount of lignin in the biomass. X-ray diffraction and scanning electron microscopy confirm the root biomass to be amorphous and not to have a strongly structured surface. The XRD revealed that treatment increased crystallinity. Thermogravimetric analysis studies displayed that the treated biomass is more thermally stable than the untreated biomass. The surface area of the biomass increased upon treatment. Thermodynamic parameters for the adsorption of n-alkanes hexane to nonane on ground dried water hyacinth (Eichhornia crassipes) root biomass were studied between 40oC and 70oC column temperature using gas solid chromatography, before and after treatment of the root biomass with mineral acid and organic solvent. The enthalpy of adsorption (Ha) values of 20.42KJ.mol-1 for hexane to -44.44KJ.mol-1 for nonane were obtained for the adsorption of the n-alkane series on untreated water hyacinth root biomass. Mineral acid and organic solvent treatments led to greater adsorption bond strength, with Ha values of -34.01 to -36.33 kJK-1mol-1 respectively after mineral acid treatment, and - 40.98 to -46.7 kJ.mol-1 after organic solvent treatment. Factors affecting the adsorbentadsorbate interaction are discussed. The adsorption properties of dried water hyacinth root biomass towards four polar solvents (dichloromethane, ethylacetate, diethylether, and acetone) were studied by inverse gas chromatography between 400C and 700C. The enthalpy of adsorption (Hads) values obtained for the polar probes range from -51.234 for acetone, an amphoteric probe to -74.658 kJ.mol-1 for dichloromethane an acidic probe for untreated biomass. Mineral and solvent treatments led to reduction in the values of enthalpy of adsorption due to removal of high energy sites on the surface of water hyacinth root biomass. The Lewis acidity, Ka, and Lewis basity Kb parameters were calculated from the net retention volumes, Vn of the probes. The b a K K ratios for the untreated, acid-treated and organic solvent treated biomass were found to be 9.2175, 6.7425 and 6.5733 respectively, indicating that all the surfaces of the untreated, mineral acid and organic solvent treated water hyacinth root biomass are basic in nature.