Improved recovery of cadmium from aqueous medium by alginate composite beads filled by bentonite and phosphate washing sludge

Abstract

Alginate (AL)-based composite beads filled with bentonite (BE) and phosphate washing sludge (PS) have been prepared and used as low-cost adsorbents for removal of cadmium (Cd) from aqueous solution. Such composite beads were prepared by solvent mixing technique followed by a crosslinking reaction. The obtained humid beads were dried using two approaches, namely air drying (AD) and freeze-drying (FD) and then separately used as adsorbents. All the materials investigated in this study were analyzed in terms of their structural, textural, morphological, thermal properties as well as their chemical composition and size distribution. The adsorption capacities of the as-prepared freeze-dried and air-dried composite beads toward Cd ions were evaluated in batch experiments. To optimize the adsorption conditions, the effect of adsorbent dosage, equilibrium time, Cd concentration, initial pH and temperature was evaluated and compared. It was found that the adsorption capacities of BE- and PS-filled composite beads are higher than those determined with BE and PS. The performed adsorption studies showed that the adsorption equilibrium was reached within 180 min. The pseudo-second order model can be effectively used to evaluate the adsorption kinetics and the adsorption isotherms can also be well described by the Langmuir isotherm model with a maximum adsorption capacity of 54.27 and 53.24 mg/g for PS- and BE-filled composites, respectively. Moreover, Cadmium recovery was enhanced with temperature increase and the thermodynamics studies revealed that the adsorption processes were spontaneous and endothermic in nature.