Efficacy of cellulose nanocrystals fabricated from Crotalaria juncea as novel adsorbent for removal of cationic dyes

Porous cellulose nanocrystals (CNCs) have been fabricated by acid hydrolysis of pre-treated Crotalaria juncea bast fibers, a major agricultural waste. The objective of this study is to explore the efficacy of cellulose nanocrystals obtained from C. juncea (CJ) as adsorbent for the removal of dyes. Response surface methodology has been employed to model statistically and optimize the process variables for the removal of methylene blue (MB) using Design Expert Software. The CNC obtained has also been characterized using BET, FTIR, and SEM. The BET surface area is found to be 7.5 m², with pore sizes ranging from 2 to 8 nm. The adsorption of methylene blue by CNC has been observed to be most improved at neutral pH, lower MB concentration and higher loading (4 g/l). The equilibrium is achieved within 1 h of contact and maximum MB removal of 69% was achieved. In order to improve the adsorption efficiency, surface modification with 2,2,6,6-Tetramethylpiperidinyloxy (TEMPO) was carried out to both pre-treated CJ and CNC. The surface modification had a negative impact on adsorption efficiency of MB while better adsorption efficiency was imparted towards malachite green (MG). An increase in adsorption efficiency from 62% to 92% was observed for TEMPO modified CNC. The variations in the adsorption performance of CNC and TEMPO CNC towards the two cationic dyes are due to varying charge density, surface area and shape of both the adsorbate and the adsorbent, with stearic hindrance being the major factor in regulating adsorption in case of unmodified CNC, and electronic nature dominating in case of TEMPO CNC.