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.

Cellulose particles in micro and nano scales has shown excellent potential to adsorb water pollutants such as dyes, pesticides, bacteria and virus, and a wide range of heavy metal ions, including Ag(Ⅰ), U(Ⅱ), Fe(Ⅲ), Cu(Ⅱ), Ni(Ⅱ), Cr(Ⅲ) and Zn(Ⅱ) 1, 2. However, mechanisms of adsorption and desorption the contaminants to/from cellulose micro or nano particles are largely unknown. The aim of the study was to develop fundamental understanding about the interaction and adsorption behavior of silver ions on cellulose surfaces using colloidal probe and spectroscopy techniques. Force interactions between three types of cellulose microspheres viz. native cellulose microspheres (CM), sulfate cellulose microspheres (SCM), phosphate cellulose microspheres (PCM) and silica surface in AgNO3 solution were studied with atomic force microscopy (AFM). The AFM results were further elaborated by extensive spectroscopy investigations.

The aim of this study was to develop a fundamental understanding of the adsorption behavior of metal ions on cellulose surfaces using experimental techniques supported by computational modeling, taking Ag(I) as an example. Force interactions among three types of cellulose microspheres (native cellulose and its derivatives with sulfate and phosphate groups) and the silica surface in AgNO3 solution were studied with atomic force microscopy (AFM) using the colloidal probe technique. The adhesion force between phosphate cellulose microspheres (PCM) and the silica surface in the aqueous AgNO3 medium increased significantly with increasing pH while the adhesion force slightly decreased for sulfate cellulose microspheres (SCM), and no clear adhesion force was observed for native cellulose microspheres (CM). The stronger adhesion enhancement for the PCM system is mainly attributed to the electrostatic attraction between Ag(I) and the negative silica surface. The observed force trends were in good agreement with the measured zeta potentials. The scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) analyses confirmed the presence of silver on the surface of cellulose microspheres after adsorption. This study showed that PCM with a high content of phosphate groups exhibited a larger amount of adsorbed Ag(I) than CM and SCM and possible clustering of Ag(I) to nanoparticles. The presence of the phosphate group and a wavenumber shift of the P−OH vibration caused by the adsorption of silver ions on the phosphate groups were further confirmed with computational studies using density functional theory (DFT), which gives support to the above findings regarding the adsorption and clustering of Ag(I) on the cellulose surface decorated with phosphate groups as well as IR spectra.

Force interactions between native cellulose microsphere (CM), sulfate cellulose microsphere (SCM), phosphate cellulose microsphere (PCM) and glass substrate under the effect of silver ions in the model solution were investigated using atomic force microscopy (AFM). The special interest was the influence of Ag+ ions and pH on the interaction between the probe and glass substrate. The probe was functionalized with above three types of cellulose microspheres by gluing method using contact mode with AFM. Functionalized probe and glass substrate were immersed in aqueous Ag+ solutions (50mg/L) at various pH value (4.25, 5.07, 5.62 and 6.63). Zeta potential studies showed negatively charged surfaces for CM, SCM, PCM and silica powders in model Ag+ solutions. The analysis revealed a decrease of adhesion force for CM and SCM system but an increase of adhesion force for PCM system with the increase of pH. The possible mechanism of the interaction under the effects of Ag+ ions, pH, and functional group contents as well as the shape of the cellulose microspheres was discussed. The SEM and SEM-EDS analysis confirmed the presence of silver ions on the surface of the cellulose microspheres after adsorption and force measurements.