Adsorption behavior of heavy metal ions from aqueous medium on nanocellulose

Abstract: The aim of this work was to explore the potential of nanocellulose, nanocellulose derivatives and nanochitin to remove metal ions from contaminated water. The above nano-polysaccharides were of interest in water purification technologies due to their high surface area and versatile surface chemistry. Silver, copper and iron are the primary metal ions targeted in the study, due to their abundance in industrial effluents. The first part of the study explored the potential of native nanocellulose and nanochitin isolated from bioresidues in removing silver ions from contaminated water. The highest Ag(I) removal for cellulose nanocrystals (CNC) was 34.4 mg/g, corresponding to 64 % removal ratio (CNC > ChNC > CNF). Wavelength dispersive X-ray analysis (WDX) and X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of silver ions on the surface of the nanocellulose and nanochitin after sorption. This study showed that the sorption performance is pH dependent and adsorption by cellulose nanocrystalswas superior to cellulose nanofibers. The second part of the work focused on evaluating the surface adsorption enhancements after nanocellulose surface modifications viz. enzymatically phosphorylation and TEMPO-mediated oxidation. Both surface modifications dramatically improved the functionality and sorption capacity; a ten fold increase in Cu(II) adsorption was observed for TEMPO-mediated oxidized CNF compared to native CNF. Generally, when the mixture of metal ions were present in water the metal ion selectivity was in the order Ag(I) > Fe(III) > Cu(II), irrespective of the surface functionality of nanocellulose. Phosphorylated nanocelluloses demonstrated the capacity to reduce Cu(II) and Fe(III) concentrations in the effluent from mirror making industry to the level that meets WHO drinking water requirements. The increase in Cu(II) adsorption on TEMPO-mediated oxidized cellulose nanofibers (TOCNF) correlated both with the pH and carboxylate content and reached maximum values of 135 mg/g for highly oxidized cellulose. Furthermore, the Cu(II) could be easily recovered from the contaminated nanofibers through a washing procedure with acidic water. The adsorption capacity of TOCNF for other metal ions, such as Ni (II), Cr (III) and Zn (II), was also demonstrated. The third part of the work aimed at gaining deeper understanding of the Cu(II) sorption behavior onto TOCNF. The carboxylate groups introduced by TEMPO- oxidation on nanocellulose surface provided negatively charged sorption sites for Cu(II) ions. The metal sorption had fast kinetics (te < 20s) and increase in temperature lead to a mild decrease in Cu(II) sorption capacity. The equilibrium sorption data fitted well with Langmuir isotherms. Furthermore SEM analysis showed copper element-containing nanoparticles with a rather narrow size distribution on TOCNF, which opens up a new and a promising possibility of converting the TOCNF after Cu (II) adsorption into a variety of value-added products. TOCNF coupled with the adsorbed copper exhibited superhydrophilicity and decreased the filtration time for the TOCNF suspension after copper sorption. A linear correlation between Δ [H+] and the corresponding Δ [Cu(II)] in the solution during Cu(II) sorption was found and discussed. This work has demonstrated that nano-polysaccharides, particularly nanocellulose are highly promising biosorbents for scavenging metal ions from water and of great industrial relevance and may enable next-generation of water purification technologies.