Nanocellulose-based composite beads as the amphoteric adsorbent for effective adsorption of cationic and anionic dyes

Treating water pollution, especially dye-contaminated wastewater, in an economical and efficient manner remains a challenge as the surface charge diversity of contaminants. This work designed a nanocellulose-based amphoteric adsorbent based on 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCNs) with in situ polymerization of diallyl dimethyl ammonium chloride (PDMDAAC). To explore the adsorption capacity of the amphoteric adsorbent (TOCN/PDMDAAC) for negatively and positively charged contaminants, methyl orange (MO, an anionic dye) and methylene blue (MB, a cationic dye) were employed as adsorbates. The effect of adsorbent on the adsorption performance at different pH, initial concentration of dye, contact time and ionic strength were investigated by batch treatment method. Equilibrium isotherms, kinetics and thermodynamics of dyes onto TOCN/PDMDAAC were examined, which implied that the adsorption behavior was monolayer and the adsorption process was spontaneous endothermic. The adsorption mechanisms are hydrogen bonding and electrostatic interactions between the dye and the active site on the adsorbent. The adsorption behavior of the binary blended dye system of MO and MB has also been investigated, and the addition of the other dye of higher concentration showed a synergistic adsorption behavior. According to the Langmuir model, at pH 7, 298 K, the equilibrium adsorption of MO and MB onto TOCN/PDMDAAC was as high as 404.86 mg g-1 and 520.83 mg g-1, respectively. These results indicated that TOCN/PDMDAAC was an effective amphoteric adsorbent, which offered a promising strategy for the adsorption of multicomponent dyes in an eco-friendly way.