Nanopolysaccharides for adsorption of heavy metal ions from water

Sammanfattning: With population expansion and industrialization, heavy metal has become one of the biggest and most toxic water pollutants, which is a serious problem for human society today. The aim of this work is to explore the potential of nanopolysaccharides including nanocellulose and nanochitin to remove metal ions from contaminated water. The above nano-polysaccharides are of interest in water purification technologies due to their high surface area, high mechanical properties, and versatile surface chemistry. Silver, copper and iron are the main metal ions targeted in the study, due to their abundance in industrial effluents. The first study shows that the effect of pH conditions, surface charge and nature of surface functional groups on native nanocellulose and nanochitin on silver ion adsorption. The highest Ag+ ion removal efficiency observed for cellulose nanocrystals (CNC), was 64 % followed by chitin nanocrystals (ChNC, 37%) and cellulose nanofibers (CNF, 27%). 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 adsorption. This study showed that the adsorption performance of cellulose nanofibers was inferior to cellulose nanocrystals and to enhance the adsorption capacities of nanofibers, surface functionalization of nanofibers was explored in the following study.In the second study, cellulose and chitin nanofibers functionalized with carboxylate entities have been prepared by chemically modifying the industrial residues, by TEMPO mediated oxidation followed by mechanical disintegration. The increase in copper adsorption on the nanofibers correlated both with the pH and carboxylate content and reached maximum values of 135 mg g-1 and 55 mg g-1 for highly oxidized cellulose and chitin nanofibers, respectively. Furthermore, the metal ions could be easily recovered from the contaminated nanofibers through a washing procedure in acidic water. The adsorption capacity of oxidized cellulose nanofibers for other metal ions, such as Nickel (II), Chromium (III) and Zinc (II), was also demonstrated. In the third study, the adsorption selectivity and removal efficiency of metal ions (Ag+, Cu2+ and Fe3+) from model water and industrial effluents by adsorption onto native and enzymatically phosphorylated nanocelluloses were studied. Phosphorylation significantly improved the functionality and sorption behaviour of nanocellulose. The removal efficiency is considered as being driven by the nature and density of functional group on the nanocellulose surface. Generally, when the mixture of metal ions are present in water the metal ion selectivity was in the order Ag+ > Fe3+ > Cu2+, irrespective of the surface functionality of nanocellulose. Nanocelluloses showed efficient adsorption in case of industrial effluent from mirror making industry and above 99% removal of Cu2+ and Fe3+ by phosphorylated nanocellulose was observed. The work has demonstrated that nano-polysaccharides, especially after surface modification are highly promising biosorbents for capturing heavy metal ions from water and of great industrial relevance and may enable next-generation water purification technologies.