Recovery of Cr(VI) and removal of cationic metals from chromium slag using a modified bipolar membrane system

A bipolar membrane electrodialysis (BMED) system equipped with a ‘back-to-back’ chromium slag compartment was fabricated to simultaneously recover hexavalent chromium (Cr(VI)) as Na2CrO4 and remove trivalent chromium (Cr(III)), Fe3+, Mg2+ and Ca2+ from chromium slag. Desorption experiments showed that the solution pH had a significant effect on the heavy-metal desorption efficiency from chromium slag. The BMED system can effectively recover more than 70.6% of Cr(VI) and simultaneously remove more than 99.9% of Fe3+, 87.8% of Mg2+, 26.2% of Ca2+ and 23.1% of Cr(III). When the number of ‘back-to-back’ chromium slag compartment modules was increased from one to two and three, the current efficiency increased from 11.2% to 17.2% and to 39.1%, respectively, while the amount of recovered heavy metals increased by factors of two and three, respectively. An analysis of the binding states showed that different chromium states were interchanging, which is beneficial to the Cr(VI) recovery and the Cr(III) removal. The experimental results indicated that the BMED system is as an effective technique for the simultaneous recovery of Cr(VI) and removal of Cr(III) from chromium slag and other types of solid waste contaminated with heavy metals.