Promoted electrokinetic treatment of Cr from chromite ore processing residue with rhamnolipid: Focusing on the reactions on electrolyte-residue interfaces

Abstract

Performance of rhamnolipid alone and coupled with citric acid (CA) on Cr removal from chromite ore processing residue (COPR) was explored in electrokinetic (EK) treatment. Reactions on electrolyte-residue interfaces were studied emphatically to investigate the transformation and movement of chromium (Cr). The results showed that the removal rate of Cr(Ⅵ) was promoted to 95.91% with rhamnolipid (in EK3) near the anode, compared to deionized water (69.62% in EK2). Ionic migration was the dominant mechanism in recovering Cr(Ⅵ), though the direction and magnitude of electroosmotic flow (EOF) affected Cr(Ⅵ) migration. Rhamnolipid significantly alleviated Cr(Ⅵ) adsorption on a residue by forming micelles to reduce solid-liquid tension. Oxygens in rhamnolipid played a role in electron donation reducing Cr(Ⅵ) to Cr(Ⅲ) at low pH. CA addition (in EK4) promoted Cr(Ⅵ) reduction, and the removal efficiency peaked at 97.51%. Besides, rhamnolipid and CA expedited Cr(Ⅲ) migration forming Cr(Ⅲ)-complexes, and the recovery values were reinforced at 26.76% and 19.36% in EK3 and EK4, respectively, comparing with EK1 (6.48%) and EK2 (12.41%); soluble species (Mg2+ and Al3+) dissolving and transfer stimulated by CA hindered Cr(Ⅲ) recovery. Enhancing agents used increased energy consumption, though, and higher energy utilization efficiency was achieved. The leaching toxicity of Cr(Ⅵ) and total Cr significantly dropped after EK treatment, especially with rhamnolipid.