Standardization of the reducing power of zero-valent iron using iodine

Abstract

Because iron-based materials that are used for the permeable reactive barrier systems come in various shapes, sizes, and with various surface properties depending on the manufacturing sources, their reductive powers vary in a wide spectrum. A new experimental procedure to evaluate the reductive power of iron material was developed in this study. Tri-iodide (I3 −) was used as the representative oxidizing agent that reacts with zero-valent iron (ZVI). Three iron-based materials (two scraps, two powders) and four chlorinated chemicals [perchloroethene (PCE), trichloroethene (TCE), 1,1,1-trichloroethane (TCA), and pentachlorophenol (PCP)] were used in this study. Redox reactions were conducted in glass vials containing aqueous solutions of chlorinated compounds or tri-iodide with known masses of iron material. After a predetermined reaction time each vial was opened and the solution was analyzed for the concentration of reduced compound. The apparent rate contant (ki obs ) of iodine reduction reaction with ZVIs was found to be proportional to that (kc obs ) of chlorinated contaminant. The surface area-normalized reduction rate constants (kc nor ) for contaminants and tri-iodide (ki nor ) were also proportional to each other. The ratio of rate constants, Knor (= kc nor /ki nor ) was estimated for each contaminant; 3.29 × 10−7, 5.86 × 10−7, 6.70 × 10−7, and 7.87 × 10−10 M, for PCE, TCE, TCA, and PCP, respectively. The results of this study suggest that the reductive power of ZVI materials can be standardized using tri-iodide, and thus, can provide a good reference for the quantitative assessment of the reactivity of metallic reducing agents of environmental interest including ZVIs.