Abstract
TEMPO (2,2,6,6-tetramethylpiperidine-1-oxylradical)-mediated oxidation nanofibers (TOCNF), as a biocompatible and bioactive material, have opened up a new application of nanocellulose for the removal of water contaminants. This development demands extremely sensitive and accurate methods to understand the surface interactions between water pollutants and TOCNF. In this report, we investigated the adsorption of metal ions on TOCNF surfaces using experimental techniques atthe nano and molecular scales with Cu(ii) as the target pollutant in both aqueous and dry forms. Imaging with in situ atomic force microscopy (AFM), together with a study of the physiochemical properties of TOCNF caused by adsorption with Cu(ii) in liquid, were conducted using the PeakForce Quantitative NanoMechanics (PF-QNM) mode at the nano scale. The average adhesion force between the tip and the target single TOCNF almost tripled after adsorption with Cu(ii) from 50 pN to 140 pN. The stiffness of the TOCNF was also enhanced because the Cu(ii) bound to the carboxylate groups and hardened the fiber. AFM topography, scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) mapping and X-ray photoelectron spectroscopy (XPS) indicated that the TOCNF were covered by copper nanolayers and/or nanoparticles after adsorption. The changes in the molecular structure caused by the adsorption were demonstrated by Raman and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). This methodology will be of great assistance to gain qualitative and quantitative information on the adsorption process and interaction between charged entities in aqueous medium.
Highlights
We focused on using TEMPO-oxidized cellulose nanofibers (TOCNF) with a high oxidization degree of carboxylate groups (1.2 mmol g−1)
The micrographs of the TOCNF characterized by SEM and atomic force microscopy (AFM) revealed some differences at the micro and nano scales
The in situ adsorption process by AFM is described as follows: (1) the sample was mounted on the scanner; the sample and the tip were covered by drops of degassed Milli-Q water; the morphology, adhesion force and stiffness were first measured in MQ water; (2) MQ water was removed and 100 mg L−1 CuSO4 solution was added for 5 minutes to ensure adsorption; (3) the CuSO4 solution was Nanoscale exchanged and rinsed thrice with MQ water; (4) the sample was scanned on the same fiber in fresh MQ water as described in step (1)
Summary
Nanocellulose and cellulosic substrates used as contaminant adsorbents have attracted increased attention for their environmental engineering applications in recent years.[1,2] Owing to the hierarchical structure and tailorable adsorption behavior via subsequent surface chemical modification[2,3] with carboxylic, sulfate and phosphate groups,[4] nanocellulose is a biocompatible and a bioactive material[5] and shows excellent potential as a promising carrier material for the immobilization of water pollutants such as dyes,[6,7] pesticides,[8] bacteria and TEMPO (2,2,6,6-tetramethylpiperidine-1-oxylradical)-mediated oxidation nanofibers (TOCNF), a new bio-based nano-material prepared from abundantly available wood biomass by the position-selective catalytic oxidation of C6 primary hydroxyls,[14] have opened up a new application of nanocellulose for the removal of water contaminants. Imaging with in situ atomic force microscopy (AFM), together with a study of the physiochemical properties of TOCNF caused by adsorption with Cu(II) in liquid, were conducted using the PeakForce Quantitative NanoMechanics (PF-QNM) mode at the nano scale.
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