Chromium removal capability, water resistance and mechanical behavior of foams based on cellulose nanofibrils with citric acid

Cellulose is a natural polysaccharide with beneficial adsorbent characteristics and a correct ecologically alternative material to design new adsorbents for water remediation contaminated with pollutants, such as metal ions that are toxic and nocive against biological systems. In this contribution, we confected foams based on cellulose nanofibrils (CNFs) with citric acid, an organic tricarboxylic acid from biosources, via a freeze-drying approach. Also, we investigated the effect of citric acid content and heat finishing treatment on the activation of crosslinking reactions in the foams. The increase of citric acid content causes slight enlargement of pore size and reduction of the activation energy for the thermal decomposition of amorphous cellulose in the foams. The heat treatment significantly affects only the cellulose releasing rate in water. The mechanical behavior of the CNF foams is changed after wetting. The CNF foams presented a two-step mechanism for the chromium adsorption kinetic, involving a pseudo-second-order and intraparticle diffusion in neutral pH via reversible physisorption. Besides the metal adsorption capacity, the foams presented suitable water resistance to be applied as adsorbant in aqueous media, releasing low cellulose content via Fickian diffusion with an initial burst effect.