Fabrication of composites with ultra-low chitosan loadings and the adsorption mechanism for lead ions.

Through a facile impregnation-precipitation strategy, chitosan was dispersed on bentonite to prepare an organic/inorganic hybrid composite for Pb2+ adsorption. The strong promotion effect of a small amount of highly dispersed chitosan on the Pb2+ adsorption capacity of clay minerals was unveiled. With a chitosan loading of 0.4 wt%, the experimental adsorption capacity reached 261.3 mg/g. The good dispersion of chitosan played a crucial role in the high capacity. The large proportion of mesopores in the adsorbent facilitated mass transfer, and thereby adsorption equilibrium states could be achieved within 15 s. The adsorption isotherms were consistent with the Freundlich expression. The Pb2+ adsorption capacity was suppressed with the addition of 150 ppm Ca2+ and almost eliminated in the presence of 150 ppm Mg2+. The adsorption enthalpy change was measured to be - 28.6 kJ/mol and Gibbs free energy change was in the range of - 18.4 to - 16.7 kJ/mol, indicating that this adsorption process was exothermic and spontaneous. The FTIR and XPS results demonstrated that the amino groups on chitosan could bond with Pb2+, and contributed to the high adsorption capacity. DFT calculation results showed that the amino and hydroxyl groups in adjacent chitosan units could be tri-coordinated with Pb2+, and the energy of system was greatly decreased due to the coordination interaction.