Kinetics and isotherm study on adsorption of chromium on nano crystalline iron oxide/hydroxide: linear and nonlinear analysis of isotherm and kinetic parameters

Nano crystalline iron oxide was used as an adsorbent for removal of chromium from aqueous solutions. Linear and non-linear equations were applied to model the kinetic and isotherm data. Non-linear analysis included error analysis using the Solver addin of Microsoft Excel and Origin. Error analysis methods fitted the data better than Origin for determination of isotherm and kinetic parameters. Non-linear analysis suggests adsorption of chromium on nano crystalline iron oxide followed Langmuir isotherm. The maximum adsorption capacity of the adsorbent was found to be 11.18 mg/g. The system followed a pseudo second order model on the basis of linear and non-linear data. The Langmuir constant and partition coefficient methods were used to determine the thermodynamic parameters. Both methods suggested that the adsorption of chromium by nanocrystalline iron oxide is spontaneous and exothermic (−47.26 kJ/mol). The free energy values varied from −24.23 to −27.22 kJ/mol, and the process of removal progressed with a decrease in entropy (0.9256 kJ/mol). Thermodynamic parameters predicted by the linear Langmuir constant method suggested that the system is endothermic (9.65 kJ/mol), spontaneous, and occurs with an increase in entropy (0.1142 kJ/mol).