Equilibrium and kinetics of biosorption of cadmium(II) and copper(II) ions by wheat straw

Biosorption equilibrium and kinetics of Cd2+ and Cu2+ ions on wheat straw, Triticum aestivum, in an aqueous system were investigated. Among the models tested, namely the Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherms, the biosorption equilibrium for both Cd2+ and Cu2+ was best described by the Langmuir model. The Langmuir biosorption capacity for Cd2+ was about 27% higher than that for Cu2+. It was also found that biosorption of Cd2+ and Cu2+ by wheat straw followed second-order kinetics. The equilibrium amount of metal ions adsorbed onto the wheat straw increased with increasing of pH from 4.0 to 7.0, and the effect was more pronounced for Cd2+ than for Cu2+. The equilibrium adsorbed amount also increased with the initial concentration of the metal ions, as expected. On the other hand, an increase of temperature from 25 to 30 °C only enhanced the biosorption of Cd2+ and Cu2+ slightly. The apparent temperature independence and the strong pH dependence of the amount of metal ions adsorbed along with moderate mean free energies of biosorption (between 8.0 and 12.9 kJ mol−1) altogether indicate that biosorption of Cd2+ and Cu2+ by wheat straw might follow a chemisorption mechanism.