Development Of Functionalized Cellulose Nanofiber Adsorbents for Selective Critical-Metal Recovery from Industrial Wastewater

Abstract

Critical-metal recovery from industrial wastewater requires adsorbent systems capable of combining high adsorption capacity with metal selectivity, regeneration stability, and reliable performance under chemically complex conditions. This quantitative experimental study developed and evaluated functionalized cellulose nanofiber (CNF) adsorbents for selective critical-metal adsorption and recovery from synthetic and industrial wastewater. Six CNF formulations were examined, including unmodified, carboxylate, aminated, phosphorylated, thiolate, and ligand-grafted CNFs, using 180 completed adsorption experiments and 540 replicate observations. Functionalization substantially improved material properties, increasing specific surface area from 74.8 m²/g for unmodified CNF to 186.7 m²/g for ligand-grafted CNF, while functional-group density increased from 0.31 to 2.36 mmol/g and maximum porosity reached 91.3%. Optimized functionalized CNFs achieved equilibrium adsorption capacities of 241.7 mg/g for Dy(III), 287.4 mg/g for Pt(IV), and 318.6 mg/g for Pd(II), representing improvements of approximately 106.7–229.5% relative to unmodified CNF across the investigated metals. Multifetal competition reduced adsorption capacity by 11.4–26.2%, while real industrial wastewater produced additional matrix-related losses of 8.3–22.9%. Functional-group density was strongly associated with adsorption capacity (r = .88), and ligand density was strongly related to selectivity (r = .84). The pseudo-second-order kinetic model provided the strongest kinetic fit (R² = .987), while the Langmuir model best represented equilibrium behavior (R² = .982). Desorption efficiencies ranged from 88.4% to 96.1%, overall metal recovery reached 79.1–94.3%, and enrichment factors ranged from 3.2 to 6.7. After eight regeneration cycles, functionalized CNFs retained an average of 91.4% of initial adsorption capacity, with thiolate CNF retaining 96.7% for Pd(II). Functionalization exerted a large statistical effect on adsorption capacity (partial η² = .583), while the multivariable model explained 87.4% of capacity variation. These findings demonstrated that appropriately functionalized CNFs provided a selective, regenerable, and quantitatively effective platform for recovering strategically important metals from complex industrial wastewater.