Removal of nickel(II) ions during water purification with ferrous sulfate. Part 1. Mechanism and efficiency of the process

The purification of natural water and wastewater from nickel ions is critically important for both environmental protection and human health due to their high toxicity. This study aimed to investigate the removal of nickel ions from contaminated aqueous solutions using the coagulant FeSO4. The results demonstrate that the removal of nickel ions via an iron(III) hydroxide precipitate, formed during coagulation at pH 7 and pH 8, can be accurately described by classical adsorption isotherms, including the Langmuir, Freundlich and Dubinin? Radushkevich models. The calculated free energy of adsorption, based on the Dubinin?Radushkevich equation, does not exceed 8 kJ mol^{-1} , indicating the physical nature of the adsorption process and ruling out ion-exchange interactions between nickel ions and iron(III) hydroxide. The sorption capacity of the resulting iron(III) hydroxide precipitate for nickel ions at pH 8 is 0.727 mg (mg Fe)-1 of added Fe to the solution. At pH 7, the sorption capacity depends on the initial coagulant concentration and ranges from 0.105 to 0.730 mg (mg Fe)^{-1} . A comparison between the coagulants Fe \mathrm{SO_4} and the previously studied Fe \mathrm{Cl_3} reveals that FeSO4 is more effective for nickel ion removal when the initial iron ion concentration is below 70 mg L^{-1} . However, at higher initial concentrations of iron, Fe \mathrm{Cl_3} demonstrates greater efficacy.