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

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.