All-biomass cellulose/casein adsorbent fabricated via the “green solvent system” of ionic liquid for the efficient removal of Cu(II), Cd(II) and Pb(II)

The development of all-biomass adsorbents capable of effectively removing metal ions from wastewater is urgently required in accordance with the trend toward sustainable development and the requirements for environmental protection. Herein, a strategy was proposed to achieve convenient preparation of cellulose/casein composite hydrogel bio-adsorbents via adopting ionic liquid as a corporate solvent. The favorable solubility of ionic liquid and its negligible vapor pressure enabled the bio-adsorbents to obtain a honeycomb-shaped surface structure while recycling ionic liquid in the material preparation process through simple vacuum evaporation technology. The well-designed bio-adsorbents exhibited a rapid adsorption rate (≤ 90 min), and possessed remarkably high adsorption capacities of 232.6 mg g−1 for Cu(II), 270.3 mg g−1 for Cd(II), and 306.6 mg g−1 for Pb(II), respectively. Besides, kinetic analysis displayed that the kinetic data was well described by pseudo-second-order model, while the equilibrium data was well fitted by Langmuir isotherm model. Furthermore, thermodynamic analysis evidenced that the removal of metal ions by bio-adsorbents was endothermic and spontaneous with a high value of ΔH (> 40 kJ mol−1). Further combined with the results of FT-IR and XPS analysis, it can be concluded that the binding of the bio-adsorbents to the metal ions was primarily achieved through chemisorption and monolayer adsorption, and both cellulose and casein contributed to the high adsorption capacity of adsorbent through chelation. Lastly, the bio-adsorbent can effectively treat simulated wastewater with low concentrations of metal ions (2–8.5 mg L−1), and displayed a satisfactory performance of removing heavy metals in actual industrial effluent.