Removal of chromium (VI) from aqueous solutions by granular composites based on laponite and alginate ionotropically cross-linked by iron and zirconium ions

Abstract

The conducted research established the porous structure of the materials through low-temperature nitrogen adsorption. It was demonstrated that the nitrogen sorption isotherms for the investigated samples belong to Type IV with H4 hysteresis associated with bottle-like pores. The specific surface area of the granulated composite samples was determined, ranging from 99 to 111 m2/g. Additionally, it was shown that an increase in iron content in the samples leads to a reduction in micropore content. Active centers on the material's surface, primarily composed of hydroxyl groups, were investigated using infrared spectroscopy. The kinetics of chromium (VI) adsorption were explored, and it was established that the iron-to-zirconium ratio in the reinforcement solutions hardly affects the duration of establishing adsorption equilibrium, which is approximately 60 minutes. Pseudo-first-order and pseudo-second-order models were applied to describe the kinetics of the adsorption process. The dependence of the chromium (VI) removal efficiency on the solution pH was determined. It was demonstrated that the synthesized samples exhibited high adsorption in acidic conditions with a sharp decline when transitioning to neutral and alkaline environments. It was shown that the maximum sorption capacities of the synthesized granulated composite samples significantly depend on the ratio of iron and zirconium ions. One of the most efficient samples is that with 50% Fe and 50% Zr, exhibiting a maximum sorption capacity of 13.29 mol/g at pH 6. The use of Langmuir and Freundlich models allowed establishing the fundamental adsorption properties of the materials.