Comparative Analysis of the Thermodynamics and Adsorption Isotherms for the Adsorption of Some Metal Ions from aqueous solution using Sawdust Material

Abstract

The experimental results of the thermodynamic and adsorption isotherms of best fit for the adsorption of Cr(VI), Ni(II),  Fe(II) and Cd(II) from aqueous solution onto Sawdust were studied. Adsorption experiments were carried out using batch process with a fixed (8g) weight of adsorbent and variable adsorbate concentrations. Changes in concentration of the different adsorbates during the adsorption process were monitored by the use of Shimadzu AA650 Double Beam Atomic Absorption/Flame spectrophotometer. Thermodynamic parameters such as ΔH and ΔS were calculated from Van’t Hoff plots which subsequently lead to the computation of ΔG. At high adsorbate concentrations, ΔH varied from 292, 754, 180 and 1,868Jmol^-1 respectively for Cr, Ni, Fe and Cd. While ΔS varied in the form 2.89, 10.04, 0.83 and 35.66Jmol^-1K^-1 respectively for the same set of adsorbates. Whereas at the highest temperature studied ΔG changes according to the order -666.45, -2,590.26, -96.69 and -10,006.20Jmol^-1respectively. This shows that the adsorption process is endothermic and spontaneous, with spontaneity increasing as the temperature increases. Equilibrium data were tested using Freundlich, Langmuir and Dubinin-Radushkevich adsorption isotherms. The adsorptions of Cr, Ni and Fe onto sawdust were found to be a favourable physical adsorption process which fitted well to the Freundlich adsorption isotherm. However, the adsorption of Cd having R_L value of 1.0000 is a linear process as suggested by Langmuir isotherm.