Toward mechanically robust and highly recyclable adsorbents using 3D printed scaffolds: A case study of encapsulated carrageenan hydrogel

Biopolymeric hydrogels have emerged as promising materials for water treatment; however, they exhibit limitations in terms of mechanical robustness and durability. To address these shortcomings, we implemented several strategies, such as (i) incorporation of graphene oxide (GO) to expand the range of molecular interactions within the hydrogels, (ii) increasing the degree of carrageenan biopolymer crosslinking by elevating the temperature of the ionic crosslinking bath, thereby enhancing the mechanical robustness of the hydrogels, and lastly, (iii) encapsulation of the nanocomposite hydrogels within 3D printed scaffolds to enhance the hydrogel durability. This study introduces the first adsorbent based on encapsulated hydrogels for water treatment, demonstrating a remarkable increase in its reusability, surpassing previous reports by at least 400 %. Through the optimization of the 3D printed scaffold design, we achieved a 140 % increase in the mass of encapsulated hydrogel, and engineered the available surface area to enhance both the durability and the environmental performance of the hydrogels. The addition of GO increased the adsorption capacity to 166.1 mg/g and the storage modulus at 10 Hz to 12.48 kPa, representing a 55 % and 305 % enhancement compared to the neat hydrogel, respectively. Moreover, the higher degree of ionic crosslinking further increased the storage modulus of the hydrogel by 261 %. Increasing the degree of crosslinking resulted in a lower hydrogel swelling ratio, improved chemical stability, and increased the reusability of the hydrogel beads. Hydrogel encapsulation significantly increased the chemical stability and reusability of the adsorbents. More than 90 % of the initial mass of the encapsulated hydrogel remained intact after 20 regeneration cycles. The reported results present a promising avenue toward the industrial-scale application of sustainable and green hydrogels for water treatment.