THE ROLE OF MASS-TRANSFER IN THE ELECTROLYTIC REDUCTION OF HEXAVALENT CHROMIUM AT GAS EVOLVING ROTATING CYLINDER ELECTRODES

The rate of electrolytic reduction of hexavalent chromium from acidic solution at a hydrogen-evolving rotating cylinder lead cathode was studied under conditions of different current densities, Cr6+ concentrations and rotation speeds. The rate of the reaction was found to follow a first order rate equation. The specific reaction rate constant was found to increase with increasing rotation speed until a limiting value was reached with further increase in rotation speed. Mechanistic study of the reaction has shown that at relatively low rotation speeds the reduction of Cr6+ is partially diffusion controlled, at higher speeds the reaction becomes chemically controlled. The limiting specific reaction rate constant was related to the operating current density by the equation K = 0.044i1.385. The current efficiency of Cr6+-reduction was measured as a function of current density, initial Cr6+ concentration and rotation speed. Possible practical applications are discussed.