Modeling batch equilibrium and kinetics of copper removal by crab shell

The adsorption characteristics of copper from aqueous solutions on crab shell were determined by batch tests. The uptake equilibrium and kinetics were affected by the pH of the sorption system. In the pH range of 3 to 6, the extent of copper removal was found to increase with increasing pH. A Langmuir-Freundlich model with pH-dependent parameters and an extended Langmuir-Freundlich model with pH-independent parameters were found to account very well for the measured constant pH equilibrium isotherms. Four existing rate models (second-order reversible reaction, second-order irreversible reaction, pseudofirst-order, and Elovich) were evaluated in simulating transient sorption profiles measured over a pH range of 3 to 6. The rate coefficients of the four models exhibited a linear dependence on the pH of the sorption system. Given the mathematical simplicity of the four rate models and their apparent success in accounting for the experimental observations throughout the whole time course of sorption, any one of the models can be used as a means for predicting the transient behavior of the copper-crab shell sorption system with reasonable accuracy.