Electrochemical regeneration of hexavalent chromium from aqueous solutions in a gas sparged parallel plate reactor

In this study anodic oxidation of Cr₂(SO₄)₃ was carried out in an air-sparged divided parallel plate cell. Variables studied were current density, Cr₂(SO₄)₃ concentration, and superficial air velocity. The rate constant of Cr₂(SO₄)₃ oxidation was found to increase with increasing current density and Cr₂(SO₄)₃ concentration. The effect of air sparging was found to depend on Cr₂(SO₄)₃ concentrations, at high Cr₂(SO₄)₃ concentration (> 0.1 M) air sparging does not affect the rate constant of the reaction denoting that the reaction is charge transfer controlled. As Cr₂(SO₄)₃ concentration decreases below 0.1 M the reaction becomes under mixed diffusion and chemical control and the rate constant increases with increasing air superficial velocity, the lower Cr₂(SO₄)₃ concentration the higher the contribution of diffusion to the reaction rate. The current efficiency of the process ranged from 20 to 85% depending on current density and Cr₂(SO₄)₃ concentration. Electrical energy consumption which ranged from 1.8 to 14.4 kW h/kg of Cr⁶⁺ was found to increase with increasing current density and decreases with increasing Cr₂(SO₄)₃ concentration. Air sparging was found to decrease electrical energy consumption in the case of dilute solutions << 0.1 M Cr₂(SO₄)₃.