Electron efficiency of zero-valent iron for groundwater remediation and wastewater treatment

A rapid zero-valent iron (Fe-0) target reduction reaction in groundwater remediation and wastewater treatment is commonly pursued. However, the economic importance of chemical consumption efficiency was ignored. In this study, we introduced a new economy-based factor of electron efficiency (EE) defined as the percentage of electrons utilized in a target reduction over an entire consumption at a given time interval. A measurement strategy was established and performed in batch experiments using different types of Fe-0 materials and one substrate (Cr-VI or nitrobenzene). Fe-0 materials free of surface oxides were prepared and used for EE measurement. We obtained EE values within half-life times of initial concentration of substrate, which were affected by pH, Fe-0 dosage, coexisting organic material, and Fe-0 type. Then we compared these values with the associated reaction rate constants (k). The results showed that the organic ligands or nano-sized Fe-0 were able to increase both k and EE. Even though lowering the pH or lifting the Fe-0 dosage dramatically accelerated the reactions, no resulting benefits were observed - the EE was even reduced around sevenfold. The results implied that accelerated reaction rates were acquired at the expense of chemical consumption. Thus, the EE factor must be intelligently balanced against the reaction rate to assess Fe-0-based groundwater remediation and wastewater treatment. (C) 2012 Elsevier B.V. All rights reserved.