Novel green fabrication of stable hydrogel beads from industrial waste lignin for efficient Pb(ii) ion removal†

Abstract

The adsorption of water contaminants using renewable biopolymer hydrogel beads represents an environmentally friendly and economically viable solution for pollution control. Traditional methods for preparing these beads are often not green, are slow, and lack versatility in producing beads with specific biopolymer compositions. We utilized a novel, green and efficient approach using liquid nitrogen for the instantaneous cooling of solution droplets, facilitating the rapid formation of gelatin–lignin biopolymer beads. This process enables the production of beads from low-concentration solutions with various lignin-to-gelatin ratios, which is challenging with other techniques. The rapid cooling results in beads with a thin shell and a highly porous gel network, enhancing their stability under harsh chemical conditions. These beads exhibit superior lead (Pb(II)) ion adsorption performance, maintaining stability across 10 adsorption–regeneration cycles in both acidic and basic environments. The optimal bead composition of 0.45 lignin/0.55 gelatin achieved approximately 155 mg g⁻¹ Pb(II) ion adsorption. The optimal beads exhibited a 3.5-fold higher adsorption capacity than gelatin-only beads, which disintegrated after three cycles, highlighting lignin's crucial role in enhancing bead stability and adsorption performance. The results further demonstrated that a dosage of 0.1 g was sufficient to achieve 96% removal of Pb(II) from 50 mL of solution with an initial concentration of 100 mg L⁻¹. The long-term stability and mechanical integrity of the beads were further enhanced through amide and ester crosslinking of lignin and gelatin using EDC as a zero-length crosslinker. A concentration of 1 mM EDC improved the adsorption capacity by 32% compared to the non-crosslinked beads over 10 adsorption–regeneration cycles. The influence of fabrication methods and swelling–deswelling cycles on bead morphology was analyzed using advanced cryo-SEM imaging. This innovative approach offers a more efficient, sustainable, and versatile method for producing high-value hydrogel beads from industrial waste byproducts.