An organic-based hybrid hydrogels: its preperation method, structural investigation, and lanthanum uptake

Abstract

Rare earth elements (REEs) have been found broad applications in superconductive materials, catalysts, magnetics, hydrogen storage materials, and rechargeable batteries. By integrating REEs into different fields, their waste has caused severe environmental and health problems due to their contamination with groundwater sources and soil. Therefore, separating and recycling these elements from water sources and soil has become an urgent issue. The traditional methods for sequestration of the REEs from aqueous solutions are ion exchange, precipitation, solvent extraction, and adsorption. Adsorption is a promising technology for recovering REEs due to its facile application, economical, non-toxicity, and reusability. There are many adsorbent materials, such as carbon-based nanostructures, cyclodextrin, silica-based materials, organic and inorganic-based polymers etc., utilized for this approach. In recent years, researchers have focused on developing new hydrogel materials with highly porous morphology and cross-linked three-dimensional polymer networks, proven to be excellent sorbents for metal ions. However, their application for removing the REEs is very limited. In this study, we developed and characterized a shungite-based poly(vinyl alcohol) (PVA) hybrid hydrogel as a potential sorbent material for removing REEs. As an inorganic filler, shungite, a natural carbon-based mineral, has been integrated into the PVA-based hydrogel. The shungite-PVA hydrogel was synthesized using a freezing-thawing method using boric acid as a crosslinking agent. The surface morphology was investigated by scanning electron microscope (SEM) (Fig.1), and SEM images revealed homogenous pores on the surface of pure PVA hydrogel. After introducing shungite as an inorganic filler, most pores filled with shungite. In this study, the synthesized shungite-PVA hydrogels were used to sequestrate lanthanum (La3+) ions from aqueous solutions. The shungite-PVA hydrogels exhibited a high affinity towards La3+ ions with 39.1 mg/g uptake ratio for the contact time, and 51.4 mg/g maximum capacity. To investigate the kinetics of La3+ ion adsorption onto Shungite- PVA hydrogels, the non-linear forms of conventional kinetic models were used, namely pseudofirst and pseudo-second order rate laws. The plotted data was well-fitted to pseudo-second order with R2=0.999. Furthermore, the adsorption capacity and affinity of shungite-PVA hydrogel towards La3+ were evaluated by adsorption isotherms viz. Langmuir and Freundlich. Based on R2 values, the Langmuir model fits better with the experimental data and showed the adsorption of La3+ is deemed as monolayer adsorption.