Adsorption, kinetic and thermodynamic studies for the biosorption of cadmium onto microalgae Parachlorella sp.

In this study, the biosorption of cadmium ions from aqueous solutions by different microalgae i.e., Parachlorella sp., Spirulina sp., Scenedesmus sp., and Nannochloropsis sp. was examined. The sorption capacity of Parachlorella sp. at 30 °C and pH of 7 was determined to be 90.72 mg/g that was shown to be 1.5 to 3 times higher than the sorption capacity of the other sorbents studied. The two characterization techniques employed, namely, scanning electron microscopy and Fourier-transform infrared spectroscopy confirmed the high potential of Parachlorella sp. in the biosorption of cadmium ions. Therefore, this microalga was studied in more details. In doing so, the effect of temperature, pH, and the initial concentration of cadmium ions on the uptake amount was investigated. The maximum experimental uptake occurred at 96.20 mg/g at pH of 7 and 35 °C, which is higher than the uptake of the other biosorbents reported in the literature. Some relevant thermodynamic parameters were also calculated. The negative value of the Gibbs free energy (−30.81 to −25.13 kJ/mol) and the positive values of the enthalpy change (+77.01 to +91.86 kJ/mol) and also the entropy change (+349.01 to +393.71 J/mol K) indicate the spontaneity, endothermicity, and desirability of the process of interest. In addition, the effect of mixing speed on the adsorption rate was investigated in a bioreactor. The rate of adsorption and the maximum uptake of cadmium ions at different mixing speeds were measured. The increase of the mixing speed to 250 rpm or higher decreased the uptake by less than half.