Hexavalent Chromium Remediation by Redox-Sorption Processes.

Chromium (Cr) is a common metal contaminant due to its extensive use in industrial activities such as metal production, leather tanning, and wood treatment.  The fate and toxicity of Cr depend upon its speciation.  Hexavalent chromium [Cr(VI)], predominantly found in groundwater, is highly mobile and a known carcinogen.  Trivalent chromium [Cr(III)], however, is an essential micronutrient for humans and fairly immobile.  Therefore, reduction of Cr(VI) to Cr(III) followed by immobilization of Cr(III) on sorptive particle surfaces may be a viable groundwater remediation strategy.  The objective of this study was to characterize Cr(VI) dynamics during the reduction-adsorption treatment.  Varying amounts (0-10 mM) of Cr(VI) were reduced to Cr(III) using sodium dithionite (0-15 mM).  Two types of iron oxide prepared from ferric chloride and ferrous chloride were used as adsorbents for the Cr and then removed by filtration.  Cr concentration was measured after each step in the treatment using UV spectroscopy.  Preliminary results show that sodium dithionite is effective in reducing Cr(VI) to Cr(III).  Both iron oxides are capable of adsorbing Cr(III) and Cr(VI), with the oxide derived from ferric chloride being more effective in terms of both maximum Cr adsorption and strength of adsorption.  The concentration of dithionite (9 mM) resulting in the largest removal of Cr from solution was that which reduced approximately 65% of the original Cr(VI) to Cr(III). This allowed for adsorption on both positive and negative sites on the oxides. However, Cr(VI) is only weakly adsorbed by both oxides and should not be considered immobilized. Overall, it is concluded that reduction of Cr(VI) by sodium dithionite followed by adsorption onto iron oxide does remove Cr from solution and could be used as a groundwater remediation technique.