Eco-friendly aqueous fabrication of cellulose/ZIF-8 biocomposite beads for Pb2+ elimination: process optimization and adsorptive efficiency assessment

Abstract

A green, scalable, and fully aqueous synthesis of ZIF-8 was developed, achieving a 90% yield of phase-pure nanocrystals (average size ≈ 110 nm) without organic solvents or high-energy input. By employing citric acid as a Zn²⁺ modulator and triethylamine as a deprotonating/nucleation agent, premature precipitation was effectively suppressed, resulting in highly crystalline ZIF-8 with a Brunauer–Emmett–Teller (BET) surface area of 498 m²/g and a micropore volume of 0.62 cm³/g. These nanocrystals were subsequently encapsulated (1 wt.%) within barley-straw-derived cellulose beads cross-linked with sodium alginate, pectin, and CaCl₂, yielding mechanically robust, spherical biocomposite beads (2–3 mm diameter). Despite polymer encapsulation reducing the accessible surface area to 2.33 m²/g and shifting porosity from micro- to mesoporous (average pore diameter 20.92 nm), the beads exhibited an exceptional Pb²⁺ adsorption capacity of 319 mg/g (batch mode, pH 5, 328 K, adsorbent dosage 0.5 g/L), outperforming most reported cellulose- and MOF-based adsorbents. Adsorption followed pseudo-second-order kinetics and the Freundlich isotherm, confirming chemisorption on heterogeneous sites. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses verified coordination of Pb²⁺ with carboxylate, hydroxy, and imidazolate nitrogen groups. In fixed-bed column tests, a total uptake of 268.65 mg/g was achieved, while the beads retained over 95.9% removal efficiency after five regeneration cycles with 0.01 M HNO₃. Structural, textural, and chemical stability of the regenerated beads after five adsorption–desorption cycles was further confirmed by BET surface area analysis, field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and FTIR. This work presents a sustainable, industrially viable route for producing high-performance, regenerable MOF–biopolymer hybrid adsorbents for heavy metal remediation.

Graphical abstract