Removal Mechanism of Hexavalent Chromium by Ferrous Oxalate: Role of Ferrous Iron and Oxalate

Hexavalent chromium (Cr(VI) contamination in water and wastewater can be treated by reduction of Cr (VI) to Cr(III) with reductants. In this study, the practicality of using FeOx crystals obtained from wastewater was evaluated by studying their reaction with Cr(VI). Cr(VI) was completely removed within 1 to 8 hours of reaction with FeOx, depending on the initial concentration up to 1.9 mM tested. After complete removal of Cr(VI) by FeOx, 22%-38% of the original concentration of Cr(VI) was converted to Cr(III) in the solution, indicating the remaining 62%-78% of the Cr(VI) was associated with the FeOx surface. Cr(VI) was removed by FeOx via reduction to Cr(III) and adsorption/precipitation on the FeOx surface. At pH 3, Cr(VI) was completely removed by FeOx within 20 minutes. Around 20% of the initially added Cr(VI) remained in the solution as Cr(III), while approximately 80% was removed from the solution. At pH 7, roughly 30% of the Cr(VI) was reduced to Cr(III) and the remaining 40% appeared to be in the solid phase after 2-hour reaction. At pH 10, approximately 40% of the Cr(VI) was removed, but Cr(III) was not observed in the solution. The Cr(VI) removal by FeOx is mainly achieved by the reduction of Cr(VI) to Cr(III) under acidic to neutral conditions, and by the adsorption of Cr(VI) onto the solid phase under alkaline conditions. The reduction of Cr(VI) occurred mainly through the reaction with ferrous iron of FeOx, which was enhanced by oxalate of FeOx, forming a soluble Fe(III)-oxalate complex with Fe(III) and reducing the passivation of the FeOx surface. Our study indicates that FeOx is effective at removing Cr(VI) at various concentrations and pH levels.