Experimental and Theoretical Investigation of the Effect of Cellulose Nanowhiskers on the Pb(II) Adsorption by Superabsorbent Hydrogel Nanocomposites

Cellulose nanowhiskers (CNWs) were incorporated into a superabsorbent hydrogel of starch grafted with poly(acrylic acid) (ST-g-PAAc) to obtain adsorbent materials with superior performance for Pb(II) removal from water. Herein, various experimental and theoretical analyses were conducted to highlight and elucidate the contribution of CNWs to the adsorption process. The presence of 10 wt% CNWs in the hydrogel led to an approximately 12% increase in the adsorption capacity of Pb(II) compared to the hydrogel without CNWs. Indeed, 25 mg of composite hydrogel required a short contact time (60 min) to achieve high adsorption capacities (935.8 mg/g). It was demonstrated that CNWs in the composite matrix also mitigate the effects of temperature and competition with other ions, enhancing the stability, selectivity, and efficiency of the Pb(II) adsorption. Density functional theory (DFT) calculations revealed that the hydroxyl groups of CNWs play a crucial role by providing additional binding energies (30 kcal/mol) for Pb(II) ions, favoring the spontaneity and kinetics of the adsorption process. Kinetic and isothermal investigations revealed that the adsorption process on the CNW-containing hydrogel involves chemisorption and intra-particle diffusion, indicating multiple steps during the adsorption of Pb(II) ions. Furthermore, the CNW-containing hydrogel demonstrated excellent reusability, with only an 8% loss in adsorption capacity after six consecutive reuses. This characteristic makes the composite hydrogel highly attractive for practical applications in real-world scenarios. In summary, the experimental and theoretical data collected in this study confirm the superior adsorption performance of the composite hydrogel due to the presence of CNWs.