Biosorption of Cd²⁺ by untreated dried powder of duckweed Lemna aequinoctialis

Abstract

The duckweed Lemna aequinoctialis was used as a biosorbent material for Cd²⁺ adsorption in this study. Influencing factors of Cd²⁺ adsorption by L. aequinoctialis in aqueous solution were investigated and the process of the Cd²⁺ biosorption was optimized. The results of single-factor experiments suggested that all the factors studied except temperature had significant effects on the removal efficiency of Cd²⁺ by L. aequinoctialis. Based on the results of single-factor experiments, optimization of the Cd²⁺ biosorption was performed by varying four independent parameters using the central composite design under response surface methodology. The optimal conditions for the maximum removal of Cd²⁺ were as follows: grain size of 150–200 mesh, stirring speed of 75 rpm, Cd²⁺ initial concentration of 40 mg/L, and sorbent concentration of 8 g dry matter/L. The maximum removal efficiency of 83.5% was obtained, which was in consistence with the predicted value of 83.6%. This process followed pseudo-second-order kinetics and the experimental data fitted well to Langmuir and Freundlich isotherm models. The maximum capacity of duckweed to adsorb Cd²⁺ was 33.0 mg/g, demonstrating that untreated dry powder of L. aequinoctialis represents a promising biosorbent for Cd²⁺ removal. Fourier transform infrared spectroscopy analysis indicated that the –OH groups of carbohydrate compounds and the –NH₂ groups of amide compounds may be the main groups involved in the adsorption of Cd²⁺ by L. aequinoctialis.

Keywords:

• Biosorption
• Cadmium
• Duckweed
• Lemna aequinoctialis
• Response surface methodology

Acknowledgments

This work was supported by the National Sci-tech Pillar R&D Program of China (No. 2011BAD22B03), Innovative Programs of the Chinese Academy of Sciences (No. KSCX2-EW-J-22 and No. KSCX2-EW-G-1-1), and “Western Light” talent cultivation program of CAS (Y2C5021100).