Removal of hexavalent chromium from industrial wastewater by electrocoagulation: A comprehensive comparison of aluminium and iron electrodes

The present paper deals with the treatment of an industrial wastewater containing Cr (VI) by electrocoagulation using Al or Fe electrodes, in a discontinuous system with forced recirculation of the solution. Treatment of the waste impurities is achieved by electrocoagulation whereas Cr (VI) removal requires its reduction to Cr (III) prior to precipitation in the form of neutral hydroxide. The speciation of the various trivalent species, namely Cr (III), Fe (III) and Al (III) was investigated to determine the optimal pH range for treatment of Cr-containing wastes by using Al or Fe electrodes. With aluminium electrodes, the presence of Cr (VI) does not affect the COD abatement. Moreover Cr (VI) species was observed to be faster removed from the wastewater with higher current densities. However it was shown that the occurrence of the direct electrochemical reduction of Cr (VI) at the Al cathode surface, does not suffice to predict the experimental reduction rates. With Fe electrodes, COD abatement by electrocoagulation is retarded by the presence of Cr (VI), whereas the rate of iron dissolution is unaffected by chromate in the wastewater. The observed difference in the COD abatement rates suggests that the removal of generated Cr (III) by precipitation might be assisted by the presence of coagulant Fe(OH)3. Removal of Cr (VI) from the wastewater in the cell could be successfully modelled taking into account the chemical reaction with electrogenerated Fe (II), its side oxidation by air oxygen, and the electrochemical reduction of Cr (VI) at the iron cathode.