Cellulose
Cellulose is a long-chain polysaccharide that can be extracted from biomass in the form of cellulose nanofibers (CNF) and has enormous potential in water treatment due to its high surface area, renewability, chemical inertness, sustainability, and availability.
Nitro-oxidation
Nitro-oxidation is a powerful one-step approach to simultaneously extract CNF from cellulose and also functionalize it with carboxyl groups. The proposed mechanism is shown above. Through the addition of HNO3, raw biomass is subject to delignification and fibrillation; these are the processes by which lignin and hemicellulose, which constitute the network within which cellulose is found, are removed. HNO2 is then produced from the reaction between HNO3 and NaNO3, which liberates the NO+ ions in presence of excess acid. The NO+ attacks the primary hydroxyl group at C6 to form an aldehyde group that is subsequently carboxylated in the presence of H+. The end product is carboxylated cellulose nanofibers.
Mechanism of Removal
Nitro-oxidized primarily serves as an adsorbent, due to its abundance of carboxyl groups, which lend the surface a negative charge effective at binding oppositely charged heavy metals like Cu(II), Cd(II), Pb(II), etc. However, this may not be the only mechanism, as chemical precipitation can also play a huge role.