Marine polysaccharide-based hydrogels for critical materials selective removal and recovery: A review

Critical materials (rare earth elements and lithium) are crucial components in the fabrication of advanced technologies. Local source from mining, especially in the United States, is very limited that recovery of these materials from secondary sources such as those in aqueous matrix like industrial effluents and brine is becoming imperative. Recovery of critical materials from aqueous matrix using commercialized adsorbents by solid-phase extraction is efficient but a costly operation. One approach taken to reduce the overall cost is to utilize marine polysaccharide-based biopolymeric hydrogels as an economical alternative to the costly commercialized adsorbents. Unlike its counterparts, the application of marine polysaccharide-based biopolymeric hydrogels to resource recovery presents a complex adsorption chemistry that can be accounted for by the fabrication strategies implemented. The lack of collective foundational understanding of different removal mechanisms presented by these class of adsorbents may hinder creative strategies to fabricate ion-selective hydrogels. This review highlights the different methodologies to synthesize hydrogel system of the three most abundant marine polysaccharide-based hydrogels namely chitosan, alginates, and carrageenan, their adsorption/desorption performance, and proposed adsorption/desorption mechanisms in rare earth elements recovery and direct lithium extraction from an aqueous matrix. This work also scrutinized the recovery efficiency of recently reported hydrogel materials for critical materials recovery and proposed key questions to serve as inception in the design conceptualization of hydrogel materials suitable for this application.