Thermodynamics and adsorption isotherms for the biosorption of Cr(VI), Ni(II) and Cd(II) onto maize cob

Abstract

The use of maize (Zea mays) cob for the biosorption of Cr(VI), Ni(II) and Cd(II) is reported. Biosorption experiments were carried out using batch process with 8g weight of adsorbent and variable adsorbate concentrations. Variations in the concentration of the different adsorbates during the adsorption process were monitored by the use of Shimadzu AA650 Double Beam Atomic Absorption/Flame spectrophotometer. Thermodynamic quantities such as ΔH and ΔS were determined in which ΔH varied from 1,271.21Jmol^-1 to 1,466.59Jmol^-1 at 20mg/L adsorbate concentration and 1,276.20 to 1,872.42Jmol^-1 at 60mg/L suggesting increasing endothermicity of the adsorption process. The adsorption process was found to be spontaneous, with spontaneity increasing with increase in adsorbate concentration and temperature. Also, the ΔG values suggested high affinity of Cr over the other adsorbates. Equilibrium data were tested using Freundlich and Langmuir adsorption isotherms. The values of the numeric constants n and K_F from Freundlich plots and the linearity of their plots suggested a chemisorption process closely defined by Freundlich isotherm.

Keywords: Adsorbate, Adsorbent, Adsorption isotherms, Maize cob, Thermodynamics