Amino-functionalized nanocrystalline cellulose as an adsorbent for anionic dyes

In this present work, amino-functionalized nanocrystalline cellulose (ANCC) was prepared by a process involving, (1) extraction of nanocrystalline cellulose (NCC) from fully bleached hardwood kraft pulp by sulfuric acid hydrolysis, (2) sodium periodate oxidation of NCC to yield the corresponding C-2/C-3 dialdehyde nanocellulose (DANC) and (3) grafting with ethylenediamine to obtain ANCC through a reductive amination treatment. Properties of DANC and ANCC were characterized by conductometric titration, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction and atomic force microscopy. It was found that the primary amine groups of ANCC (0.77–1.28 mmol g⁻¹) increased with the increase of ethylenediamine dosage. The successful grafting was further evidenced by Kaiser test and FT-IR analysis. Zeta potential measurements showed that ANCCs were amphoteric, and their isoelectric points were between pH of 7–8. Chemical modifications of the cellulose nanowhiskers reduced the crystallinity but the initial cellulose I polymorph was retained. The cross-sectional dimension of nanowhiskers was slightly decreased from about 5–10 to 3–8 nm after the oxidation, and a better dispersibility was observed. ANCC sample was then applied as an adsorbent to remove anionic dyes in aqueous solutions. It demonstrated the maximum removal efficiency at acidic conditions. The acid red GR adsorption on ANCC fitted well with the Langmuir model, with a maximum theoretical adsorption capacity of 555.6 mg g⁻¹. The adsorption of congo red 4BS, acid red GR and reactive light yellow K-4G followed pseudo second order kinetics, indicating a chemisorption nature.