Adsorption of Cr(Ⅵ) on porous sodium alginate/polyethyleneimine hydrogel beads and its mechanistic study

The sodium alginate (SA) is a biomass material which is abundant and can be easily acquired. It is currently used by many scientific researchers in laboratory research to prepare adsorbents to remove metal ions from aqueous solutions. However, SA based adsorbents generally exist as hydrogels, which are low in specific surface areas, slow in adsorption rates and have low adsorption capacities. In this study, calcium carbonate and polyethyleneimine (PEI) were added to SA matrix, and glutaraldehyde was used as a crosslinking agent to prepare porous SA/PEI beads via freeze-drying. The adsorption characteristics of synthesized adsorbent for Cr(Ⅵ) in aqueous solution were studied. The adsorption behaviors of Cr(Ⅵ) ions were evaluated by varying the experimental conditions including pH values, initial metal ion concentrations, adsorption temperature and adsorption time. Adsorption kinetics and thermodynamic models were applied to analyze the adsorption process. Characterization methods, including FTIR, Zeta potential, SEM, and XPS were comprehensively used to analyze the synthesis mechanism of SA/PEI beads and the mechanism of Cr(Ⅵ) adsorption. The results show that the removal rate of Cr(Ⅵ) by SA/PEI beads is negatively related to the initial concentration; the adsorption process conforms to the pseudo-second-order kinetics and Langmuir isotherm adsorption model, and the adsorption reaction is a spontaneous endothermic process. When the temperature is 318.15 K and the pH value is 2, the Langmuir isotherm adsorption fitting shows that the maximum adsorption capacity is 262.83 mg/g. The adsorption mechanism of SA/PEI beads on Cr(Ⅵ) is mainly physical adsorption dominated by electrostatic interactions.