Experimental and theoretic mechanism of successive adsorption of oppositely charged Cu(II) and Cr(VI) onto amine-rich cellulose adsorbent

A novel layer-by-layer adsorption was proposed and used for adsorption of Cu(II) and Cr(VI) on the pre-bleached sawdust cellulose coated with polyethylenimine (PSC-PEI). It was found that PSC-PEI loaded with Cu(II) cations was favorable for Cr(VI) anions adsorption. The maximum adsorption capacities estimated by Langmuir model for Cu(II) and Cr(VI) were 80.0 and 93.5 mg/g, respectively. The kinetic regression results, as fitted by the pseudo-second order model, revealed that the rate constant (k2) for Cr(VI) at 0.07 g/mg/min is significantly greater than that observed for Cu(II) at 0.02 g/mg/min, indicating that PSC-PEI exhibited a stronger affinity towards Cr(VI). The first-layer adsorption mechanism for Cu(II) involved the formation of copper-amine complex. Zeta potential and XPS results revealed that the second-layer adsorption of Cr(VI) mainly involved electrostatic attraction and redox reaction. The simulated results for dynamic column test showed good agreement between the theoretical values and the experimental values. The mass transfer mechanism indicated that Cu(II) adsorption was dependent on external mass transfer process, while the internal mass transfer is the rate-determining step for Cr(VI) adsorption. The saturated adsorbent was regenerated by washing with 5% NaOH and 5% H2SO4 solutions and the adsorption ability of more than 70% was sustained after three cycles of regeneration. This study demonstrated that the oppositely charged Cu(II) cations and Cr(VI) anions could be effectively removed by amine-rich cellulose adsorbent from wastewater through layer-by-layer adsorption.