Adsorption of volatile polar organic solvents on water hyacinth ( Eichhornia crassipes  ) root biomass: thermodynamic parameters and mechanism

The adsorbent properties of dried water hyacinth root biomass towards four polar solvents (dichloromethane, ethyl acetate, diethyl ether and acetone) were studied by inverse gas chromatography between 40 and 70 °C. The enthalpy of adsorption values obtained for the adsorption of the four solvents on untreated root biomass range from -51.234 kJ mol^-1 for acetone, an amphoteric solvent, to -74.658 kJ mol^-1 for dichloromethane, an acidic solvent. Mineral acid and organic solvent treatment led to reduction in the values of the enthalpy of adsorption for all four solvents. The Lewis acidity parameters calculated from the enthalpy of adsorption values were 0.408, 0.267 and 0.356, while the corresponding Lewis basicity parameters were 3.76, 1.80 and 2.34, respectively, for untreated, mineral acid-treated and organic solvent-treated water hyacinth root biomass. The Lewis basicity parameter-to-Lewis acidity parameter ratios for the untreated, acid-treated and organic solvent-treated biomass were found to be 9.22, 6.74 and 6.57, respectively, indicating (a) that all the surfaces of the untreated, mineral acid-treated and organic solvent-treated water hyacinth root biomass are basic in nature and (b) that for all volatile polar solvents studied, the adsorption interaction involves the lowest unoccupied molecular orbital of the solvent as the electron acceptor and the highest occupied molecular orbital of the water hyacinth root biomass surface adsorbent site as the electron donor.

Keywords
Biosorbent; Enthalpy of adsorption; Gas–solid chromatography; Inverse gas chromatography; Lewis acid–base properties