Recovery of trivalent and hexavalent chromium from chromium slag using a bipolar membrane system combined with oxidation.

Chromium slag (CS) with large quantities of multivalent Cr species (III and VI) generated during chromium salt production is hazardous to nature and living organisms. Furthermore, CS discharge leads to considerable resource wastage. Herein, a bipolar membrane electrodialysis (BMED) system was employed along with hydrogen peroxide (HO) oxidation for simultaneously recovering Cr(III) and Cr(VI) from CS in the form of NaCrO. A bipolar membrane was used to produce OH under a direct electric field, providing an alkaline environment for the oxidative conversion of Cr(III) to Cr(VI) in the presence of HO, followed by the recovery of Cr(III) and Cr(VI) as NaCrO. The effect of HOcontent on Cr(III) oxidation and that of the current density on chromium recovery, current efficiency and specific energy consumption were investigated. Moreover, the morphology of chromium in CS before and after the BMED treatment was analysed. The HO content affected the Cr oxidation rate from Cr(III) to Cr(VI). The current density affected chromium removal, current efficiency and specific energy consumption. At a current density of 2 mA/cm, the total chromium recovery exceeded 67% and the remaining chromium was mainly in the residual state (RES). When the number of CS compartments increased, the current efficiency was enhanced and the specific energy consumption decreased. Binding state analysis show that Cr(III) and different species of Cr(VI) could be transformed into exchangeable Cr(VI) after HO oxidation and BMED treatment. After the treatment, 92% of the remaining chromium in CS was in the RES. Thus, the employed method can effectively recover chromium from CS and other chromium-contaminated solid waste.