Hyper-Cross-linked Cellulose Nanofibrils with Spontaneous and Reversible Adsorption of Aromatic Pollutants from Water as a Valid Alternative to Fossil-Based Adsorbents

Abstract

In this work, a novel high surface area adsorbent based on cellulose and inspired by hyper-cross-linked polymers was designed. Cellulose nanofibrils (CNF) were functionalized with poly(vinylbenzyl chloride) and hyper-cross-linked through Friedel-Crafts alkylation, yielding a micro/mesoporous material characterized by a specific surface area of 409 m²/g, microporous fraction of 50%, and biobased content of about 70 wt %. The functionalized CNF, tested for the adsorption of 2,4-dichlorophenol (DCP) from water at 298 K, were able to remove 90% of the pollutant from a 62.5 mg/L DCP solution and adsorb 284 mg/g at a higher concentration (1000 mg/L). Thermodynamic studies demonstrated the multilayer adsorption of the hyper-cross-linked CNF, the exothermic nature of the process, and its spontaneity. The hyper-cross-linked cellulose nanofibrils were reusable with efficiency above 98% in 5 subsequent cycles. The adsorption performance was stable across varying pH levels, and interference from natural organic matter (e.g., humic acids) was minimal (<10%). This work marked a promising step toward more sustainable sorbent materials by demonstrating the potential of cellulose nanofibrils as functional scaffolds. The strategy could be extended to waste-derived cellulose sources and biobased aromatic compounds, paving the way for fully renewable porous adsorbents.

Keywords: adsorption; aromatic pollutants; cellulose; hyper-cross-linking; regeneration.

1

Scheme of the carboxylation process (a), PVBC grafting (b), and hyper-cross-linking reaction (c).

2

FTIR spectra of CNF, CNF_2h, CNF_4h, CNF_24h, CNF_48h, and CA (a), magnification of (a) showing the 2000–1500 cm^–1 region of CNF and carboxylated CNF spectra (b).

3

TGA traces in a nitrogen atmosphere (a), and 5 and 50 wt % degradation temperatures (b) of CA, CNF, CNF_2h, CNF_4h, and CNF_24h.

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TEM images of CNF (a, e), CNF_2h (b, f), CNF_4h (c, g), and CNF_24h (d, h).

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SEM images of CNF (a–c), CNF_2h (d–f), CNF_4h (g–i), and CNF_24h (l–n).

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CNF-PVBC SEM (a, b) and TEM (c, d) images, ATR-FTIR spectra of PVBC and CNF-PVBC (e), ¹³C CP MAS NMR spectra of CNF-PVBC, PVBC, and CNF (f), and TGA traces of PVBC and CNF-PVBC in a nitrogen atmosphere (g).

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Photographic image (a), ATR-FTIR spectrum (b), TGA trace in a nitrogen atmosphere (c), SEM (d, e), and TEM (f, g) images of xCNF-PVBC.

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CNF and xCNF-PVBC nitrogen adsorption/desorption isotherms at 77 K (adsorption in full symbols, desorption in empty symbols) (a) and NLDFT pore size distribution (b, c).

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Experimental points (dots) and fitting curves of DCP equilibrium adsorption by xCNF-PVBC at 298, 308, 318, and 328 K (a) and adsorption kinetics of DCP from a 62.5 mg/L solution by xCNF-PVBC at 298 K (b); percentage of DCP adsorbed at 298, 308, 318, and 328 K by xCNF-PVBC versus the initial concentration C ₀ (c); thermodynamic parameters of DCP adsorption by xCNF-PVBC: enthalpy (d), entropy (e), and Gibbs free energy changes (f).

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Schematic representation of possible xCNF-PVBC adsorption mechanisms (a) and FTIR spectra of xCNF-PVBC and xCNF-PVBC after DCP adsorption (b).

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Adsorption efficiency of DCP (C ₀ = 62.5 mg/L) by xCNF-PVBC in repeated adsorption/regeneration cycles (a); effect of pH (b) and HA (c) on the adsorption of DCP.