Adsorption of Cd(II) and Cr(VI) by a Complex Humic Sorbent: Kinetic and Redox Characteristics of the Process

A study is performed of the maximum saturation of humic acids with Fe(III) ions when obtaining a complex humic sorbent. The complex sorbent HA_modFe retains 16.5 mmol/g of the functional groups of humic acids out of a total of 25 mmol/g that can participate in exchange reactions with metal ions. The maximum adsorption of Cd(II) by the sorbent is observed after 5 h of sorbate–sorbent contact and is 76 ± 2 mg/g. Extending the period of contact to 24 h results in the desorption of Cd(II) and the sorption exchange capacity of the sorbent drops to 55 ± 2 mg/g. The maximum adsorption of Cr(VI) is observed after 250 h of sorbate–sorbent contact. Having Cd(II) in the solution does not affect the sorption of Cr(VI) by the complex sorbent, which is 61 ± 1 mg/g. The rate of the adsorption of the considered metal ions is described by an equation of the pseudo-second order and is accompanied by mixed processes of diffusion. The redox potential of the falls 77 mV in the sorbent + Cr(VI) system and 38 mV in the sorbent + Cr(V) + Cd(II) system with a 250 h period of contact.