Application of chitosan-citric acid nanoparticles for removal of chromium (VI)

•Preparation of surface-functionalized chitosan nanoparticles (CS–CA).•Successfully introduce a large number of surface functional groups on the CS–CA.•Adsorptive properties of the CS–CA nanoparticles improved by grafting modification.•Adsorption isotherm, kinetic and thermodynamic models were studied.•The adsorbent which adsorbed Cr (VI) can be recovered completely. In the present study, CS–CA nanoparticle was prepared for forming a new amide linkage, by grafting the amino groups of CS in the presence of carboxylic groups of CA that acts as cross-linking agent. The as-prepared CS–CA nanoparticle samples were characterized by use of dynamic light scattering (DLS), scanning electron microscopy (SEM), Fourier-transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques, which showed that the cross-linking agent preserved during the chemical modifications. The adsorption capacity of the CS–CA nanoparticles for the removal of Cr (VI) in aqueous solution was studied. The adsorption equilibrium data taken at the optimized condition, i.e., 25°C and pH of 3, were analyzed with the Langmuir, Freundlich and Redlich–Peterson isotherm models. The kinetics of Cr (VI) adsorption on CS–CA nanoparticles obtained at different initial concentrations were also analyzed using the pseudo-second-order model.