Effect of Chemical Modification of the Coal Fly Ash onto Adsorption of Chromium(III) Ions in the Presence of Cobalt(II) Ions in a Single and Binary System

The purpose of the research was to determine the effect of chemical modification of waste coal fly ash (FA) with solutions of HClO4, KOH, sodium diethyldithiocarbamate (NaDDTC), and ammonium formate (HCOONH4) on the adsorption of chromium(III) ions in the presence of cobalt(II) in a single and binary system. In the study, the morphology of ash grains before and after modification was determined using scanning electron microscopy (SEM-EDS), mineral composition using X-ray diffraction (XRD), and chemical properties of sorbents using Fourier transform infrared spectroscopy (FT-IR). Evaluation of the sorption properties of the tested materials was determined using 8 models of adsorption isotherms. The adsorption of Cr(III) and Co(II) ions in a one-component system with FA, FA-NaDDTC, and FA-KOH is well described by the Langmuir and Redlich-Peterson models. The highest value of the maximum sorption capacity was obtained for FA-KOH (235 mg·g−1 and 65 mg·g−1 for Cr(III) and Co(II) ions, respectively). Studies have shown that FA-KOH has a higher selectivity for Cr(III) ions, which is related to the size of the radius of the hydrated metal ion and the value of the first equilibrium constant of the hydrolysis reaction. The obtained adsorption kinetic data were well expressed by the pseudo-second-order model (R2 = 0.998) but showed a very poor fit to the pseudo-first-order model (R2 < 0.680). The Elovich model and the intraparticle sorption model were also analyzed, which showed that the sorption process of Cr(III) and Co(II) ions is controlled by the diffusive boundary layer and diffusion inside the pores.