Impact of the drying process on the efficiency of alginate beads for cadmium removal from water: Kinetic, isotherm and thermodynamic study

Abstract In this study, the improvement of cadmium ions retention from aqueous medium by alginate beads was investigated. Two drying methods were performed, namely, air-drying and freeze-drying. The physical and chemical properties of the beads were characterized by different techniques (SEM, BET, FTIR, TGA/DTG). The microscopic observations show that the freeze-dried beads (AL-FD) have a smoother surface compared to the air-dried beads (AL-AD); besides, they have practically conserved their initial size. The specific surface areas were found to be 0.039 m2/g and 56.83 m2/g for AL-AD and AL-FD, respectively. Furthermore, the influence of pH and adsorbent dosage, as well as the kinetic, equilibrium and thermodynamic studies, were carried out in order to elucidate the adsorption process. Results obtained showed that the kinetic sorption process follows the non-linear fitting of Avrami fractionary order model and the sorption rate decreases when decreasing the pH of the medium. The equilibrium data were well fitted by the Langmuir non-linear model for AL-AD, and described by both Langmuir and Freundlich equations in the case of AL-FD. The uptake of AL-FD was greater than AL-AD, with maximum adsorption capacities of 60.98 mg/g and 83.06 mg/g for AL-AD and AL-FD, respectively. Moreover, the computed thermodynamic parameters, i.e., Δ G ∘ , Δ H ∘ , and Δ S ∘ , suggested that the adsorption of Cd(II) onto alginate beads is an endothermic, spontaneous, and physical adsorption process.