Electrokinetically enhanced reduction of chromium(VI) by aqueous iron(II) in contaminated clays

A well-controlled experimental analysis was performed to examine the influence of electrokinetics on the in-situ reduction of hexavalent chromium, [Cr(VI)] using aqueous [Fe(II)]. Stoichiometric analysis and empirical correlations of the measured chromium and iron concentrations along with the corresponding redox potentials were compared with a known thermodynamic relationship called the Nernst equation, which provided the basis for the mathematical examination.; [Cr(VI)] was reduced to the less mobile and toxic trivalent chromium, [Cr(III)], species using [Fe(II)]. The [Fe(II)] solution was electrokinetically injected into the soil under anoxic conditions; helping to decrease the redox potential near anode and keep more [Fe(II)] available in solution. The application of electrokinetics has contributed to the success of the redox reactions by providing the "driving force" necessary for the delivery of [Fe(II)] and also to effectively lower the energy necessary for the redox reactions to occur.; The results showed a low final soil pH (<3); indicating that more [Cr(III)] was available than [Cr(VI)]. The average redox potentials measured within the soil were compatible with the theoretical Nernstian response. There was a measured redox potential shift of 0.37 V upward from the linear fit of the Nernst relationship. This was attributed to the over-potential created in the soil by electrokinetics; which in effect, caused the soil matrix to behave as a capacitor. The results showed better agreement with the Nernst behavior at lower pH values (2--3) than at higher ones (4--6). The deviation away from the Nernst fit at higher pH values may have been due to the effects of sorption, the ongoing reactions between other competing species within the soil medium, and as a result, the current density achieved in the soil-water system.; The kinetics data also demonstrated the benefit of electrokinetics. These results agreed well with reported information on the kinetics of [Cr(VI)] reduction in aqueous systems. Approximately a 0.67-order dependence on the molar concentration of [Fe(II)] and some dependence on pH were observed. In summary, the results showed that known thermodynamic and kinetics approaches might successfully be used to predict the effectiveness of the electrokinetically enhanced reduction of [Cr(VI)] in contaminated soilwater systems.