Mechanism of deep removal of iron impurities from zinc sulfate solutions via ultrasonic-enhanced goethite process

This study reports an ultrasound-enhanced goethite process for deep removal of iron impurities from a zinc sulfate solution. This method eliminates the reliance on external seed crystals by using the ultrasonic cavitation effect to directly induce the formation of α-iron(III) oxide-hydroxide (a-FeOOH). Single-factor experiments were conducted to optimize process parameters. The Fe removal efficiency reached 99.72 % under optimal conditions: ultrasonic power = 160 W, pH = 3.0, reaction time = 80 min, and hydrogen peroxide (H₂O₂) dosage = 12 mL. The resulting goethite slag exhibited an iron content of 30.72 % and a moisture content of 39.21 %, showing a 3.92 % increase in the iron content and an 11.05 % decrease in the moisture content compared with the traditional process. Multiscale characterization and density functional theory (DFT) calculations were used to explore the mechanism. DFT calculations confirmed that α-FeOOH formed bidentate [adsorption energy (ΔE) = - 6.161] and monodentate (ΔE = - 3.223) ligand adsorption configurations with SO₄^2- under conventional and ultrasonic conditions. The monodentate configuration exhibited low thermodynamic stability and was highly prone to desorption, facilitating goethite formation. Electron paramagnetic resonance analysis revealed that ultrasound promoted the cleavage of H₂O₂ to generate abundant ·OH radicals, accelerating the oxidation kinetics of Fe²⁺. Furthermore, ultrasound disrupted iron hydroxide [Fe(OH)₃] colloidal encapsulation, inhibited byproduct formation, and guided the directional hydrolysis of Fe³⁺ into highly crystalline goethite. This technology provided a theoretical foundation and technical support for achieving efficient and clean iron removal in hydrometallurgical zinc smelting processes.