Strong nanostructured film and effective lead (II) removal by nitro-oxidized cellulose nanofibrils from banana rachis

Banana rachis is an abundant source of cellulose from agricultural waste. Fine-diameter cellulose nanofibrils (CNFs) should have a high capacity for heavy metal removal from wastewater based on a specific surface area. With nitro-oxidation and simple mechanical blending, the banana rachis resulted in individualized cellulose nanofibrils (NOCNFs) of length = 1231 nm, diameter = 5.26 nm, and negatively charged carboxylic groups at the C6 position with degrees of oxidation (DO) from 0.455 to 1.466 mmol/g. The NOCNFs were employed to remove Pb (II) from water and prepare films to investigate the mechanical performance. Increased Pb (II) removal efficiency was observed correlating to the increase in DO (0.455–1.466 mmol/g) and NOCNF concentration in support of a chemisorption mechanism. The NOCNFs showed a maximum adsorption capacity of 1667 mg/g at DO = 0.455 mmol/g, which is very high compared to adsorbents from literature and with the potential to increase at a DO of 1.466 mmol/g. NOCNF films showed impressive mechanical properties in support of high intrinsic NOCNF strength even at high DO: the tensile strength = 254 MPa, Young modulus = 10.7 GPa, and strain to failure = 5.7%. The nitro-oxidation also preserved the long-range cellulose order in the NOCNFs even at high DO with cellulose I crystalline index = 99.999%, essential to the remarkable Pb (II) removal capacity and mechanical performance. The study shows the potential of banana rachis from waste as a sustainable material for water purification and the possibilities of controlled banana rachis CNFs functionalization.