Chromium Remediation or Release? Effect of Iron(II) Sulfate Addition on Chromium(VI) Leaching from Columns of Chromite Ore Processing Residue

摘要

Chromite ore processing residue (COPR), derived from theudso-called high lime processing of chromite ore, containsudhigh levels of Cr(III) and Cr(VI) and has a pH between 11 andud12. Ferrous sulfate, which is used for remediation ofudCr(VI) contamination in wastewater and soils via reductionudto Cr(III) and subsequent precipitation of iron(III)/chromium-ud(III) hydroxide, has also been proposed for remediationudof Cr(VI) in COPR. Instead, however, addition of FeSO4 toudthe infiltrating solution in column experiments with COPRudgreatly increased leaching of Cr(VI). Leached Cr(VI) increasedudfrom 3.8 to 12.3 mmol kg-1 COPR in 25 pore volumesudwith 20 mM FeSO4, reaching solution concentrations asudhigh as 1.6 mM. Fe(II) was ineffective in reducing Cr(VI) toudCr(III) because it precipitated when it entered the columnuddue to the high pH of COPR, while Cr(VI) in solutionudwas transported away with the infiltrating solution. Theudlarge increase in leaching of Cr(VI) upon infiltration of sulfate,udeither as FeSO4 or Na2SO4, was caused by anion exchangeudof sulfate for chromate in the layered double hydroxideudmineral hydrocalumite, a process for which scanning electronudmicroscopy with energy-dispersive X-ray microanalysisudprovided direct evidence.