Reduction of hexa-valent chromium with zero-valent iron: Batch kinetic studies and rate model

Reduction of hexa-valent chromium in aqueous solution by zero-valent iron in the form of a thin plate over a pH range of 3–5.5 has been investigated. The rate of removal of Cr(VI) declines sharply at a longer time indicating passivation of the surface. A kinetic model that takes into account the direct reduction, the passivation process and the resultant decline of the ‘active’ surface area of reaction is proposed. The model parameters have been estimated by global optimization at each of the pH values. The reduction reaction conforms to 2/3-order kinetics with respect to the Cr(VI) concentration; the passivation reaction is linear in the Cr(VI) concentration. A good match of the experimental and theoretically computed concentrations as function of time was observed. The EDX of the reacted surface showed existence of a mixed oxide layer on it. A reacted plate, after cleaning of the surface, regained its original activity.