USE OF TARTARIC ACID AND ISOPROPYL ALCOHOL IN THE REDUCTION OF HEXAVALENT CHROMIUM AND APPLICATION TO CHROMIUM CONTAMINATED SOILS AND CHROMITE ORE PROCESSING RESIDUE

Abstract

Although chromium (Cr) is a naturally occurring metal, in the oxidation state +(VI), it is a health concern when present in soils and natural waters due to its solubility and toxicity. Tartaric acid and isopropyl alcohol were evaluated for reduction of Cr(VI) at environmentally relevant pH values, in the presence of soils, and from chromite ore processing residue (COPR). Soil samples were taken from profiles located in delineations of five soil mapping units in Maryland, USA: Christiana-Russett Complex, Askecksy, Annapolis, Jackland, and Ingleside. In solution, the rate of reduction of Cr(VI) by the tartaric acid-Cr-isopropyl alcohol complex was lowered from 0.128 to 0.011 h-1 as pH was raised from 3.0 to 5.0; however, in the presence of the Russett and Jackland soils, the rates of reduction were 0.037 and 0.020 h-1, respectively despite pH values of 5.3 and 5.0. In addition to Cr(VI) reduction, 97.6 and 89.9 µM Mn(II), and 427 and 67.6 µM Fe(II) were solubilized from the Russett and Jackland soils, respectively. Adding soluble Mn2+ and Fe3+ to the five soils with tartaric acid and isopropyl alcohol enhanced reduction of Cr(VI) in all soils, with the addition of Mn2+ enhancing reduction by an additional 0.27 mM Cr(VI) in the Jackland soil and to 1.46 mM in the Downer soil. Furthermore, the addition of tartaric acid and isopropyl alcohol to Mn-oxide coated sand (1.8x10-1 µmol Mn/mg) showed reductive dissolution of Mn(III,/IV)(hydr)oxides, and the resulting Mn(II) enhanced reduction to 1.24 mM (62%) of Cr(VI). When applied to COPR, tartaric acid-Cr-isopropyl alcohol or tartaric acid-Cr-Mn complexes reduced 0.3 mM (30%) Cr(VI), although when COPR was mixed with the Atsion, Collington, or Russett soils, pH values remained below 5.0 and 0.84 mM (84%) of the Cr(VI) was reduced. This work showed that a tartaric acid and isopropyl alcohol solution reductively dissolves Mn(III,/IV)(hydr)oxides from soils, and the resulting Mn(II) enhances reduction of Cr(VI), which can be potentially applied to the reduction of COPR-derived Cr(VI) in a soil remediation strategy.