Sorption of Nickel Ions on Iron(III) Hydroxide Freshly Precipitated from a Solution of Iron(II) Sulfate. Part 1. Mechanism and Efficiency of the Sorption Process

The paper studies the sorption properties of iron(III) hydroxide obtained by precipitation from a solution of iron(II) sulfate in relation to nickel ions. The study was carried out at room temperature on a model solution of sodium sulfate (400 mg/L), which imitated polluted natural and waste water. It is shown that the removal of nickel ions from a model solution by a precipitate of freshly precipitated iron(III) hydroxide at pH 7 and 8 is described with satisfactory accuracy by the classical Freundlich, Langmuir, and Dubinin–Radushkevich adsorption isotherms. The values of free energy of adsorption calculated using this last equation do not exceed 8 kJ/mol, which indicates the physical nature of adsorption and excludes ion-exchange interaction of nickel ions with iron(III) hydroxide. The most complete removal of nickel ions from solution occurs during sorption with iron(III) hydroxide at pH 8. The sorption capacity of iron(III) hydroxide for nickel ions, both at pH 7 and at pH 8, is almost an order of magnitude greater than the similar value for many mineral, carbon, and coal sorbents. A comparison of the efficiency of sorption purification of a model solution using iron(III) hydroxide precipitates obtained by precipitation from solutions of iron(II) sulfate and iron(III) chloride showed that the deepest removal of nickel ions is achieved using iron(III) hydroxide precipitates obtained from a FeCl₃ solution.