Reductive Efficiencies Of Nitroaromatic Compounds In Groundwater By Zero Valent Iron

Many treatment technologies for groundwater pollution caused by organic compounds have been developed, including in situ chemical oxidation, biosparging, in-well aeration, multi-phase extraction, permeable reactive barriers (PRB), etc. The PRB technology has been applied widely to engineering in the developed countries because of its lower cost and convenience for management, however, it is studied primarily in China. As a type of reactive media of permeable barriers, zero-valent iron (ZVI) has been widely used due to its low cost, availability and excellent reductive efficiency. We used commercial reductive iron powder as reductant in this study, investigated systematically the reductive characteristics and mechanisms of nitroaromatic compounds (NACs) by Fe0 system, analyzed the kinetics of reductive reaction, in view of the higher content of ferrous and manganese ions in groundwater in the north-east of China, examined the effects of ferrous and manganese ions on reductive reaction. As is helpful for the application of PRB technology using Fe0 as media.The specific surface area of commercial iron powder is 3.5697m2/g, the reductive characteristics of nitrobenzene by Fe0 was studied in this experiment. The reductive products of reaction were determined using UV scanning and GC/MS method. The experiment results showed that the major reductive product was aniline. The total mass balance of reductive process was calculated, an d it showed that there existed volatilization of nitrobenzene and adsorption of aniline during the reductive process, which resulted in the mass loss. The effects of pH on reduction were examined at different pH value of 3.0, 5.0, 7.0, 9.0, 12.0, respectively, and the change of pH value during the reaction was also determined. The results showed that lower pH value was favorable for the reductive reaction, and the higher reductive rate was obtained at lower pH value. The pH value of solution increased over time when the pH value was lower than 7.0, however, the variation of pH was not observed apparently when the pH value was higher than 7.0. The effects of initial nitrobenzene concentration on reduction were examined at different initial concentrations (0.127~3.248mmol/L). The reduction of nitrobenzene was fitted with pseudo-first-order kinetic model, and the results indicated that the reduction of nitrobenzene was well fitted to pseudo-first-order kinetic model at different pH values and initial concentrations. The formation of aniline followed zero order reaction at different pH values.The effects of ferrous and manganese ions on reduction were firstly studied in this paper. Nitrobenzene was selected as the pollutant, and the Fe2+ and Mn2+ ions of different concentrations were added into the Fe0 system. According to the GC/MS results, the reactive rate was enhanced due to the addition of Fe2+ ions, however, the reactive pathway was not changed, and there existed no significant influence on the reduction of nitrobenzene due to the addition of Mn2+ ions. Fe2+ ions were not the main reductant, and it had a buffering capacity in the Fe0 system. The reduction of nitrobenzene caused by ferrous hydroxide deposits was examined, the results showed that the reductive reaction only occurred when the Fe2+ ions coexisted with Fe(OH)2. The effects of dissolved oxygen (DO) on reduction were also examined, the results showed that the presence of DO in solution lowered the reactive rate, resulting in the increase of pH value. However, the reductive rate of nitrobenzene was increased after the addition of Fe2+ ions, the final reductive ratio of nitrobenzene depended on the amount of Fe2+ ions in solution. The corrosive products were determined using X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods, the results showed that lepidocrocite was formed on the surface of iron powder, and the iron corrosion coating had a stratified structure under the presence of Fe2+ ions, the inner layer mainly consisted of magnetite with negligible lepidocrocite. The addition of Fe2+ ions accelerated the reductive rate of lepidocrocite to Fe3O4 by Fe0.Six monosubstituted nitrobenzenes (NBs) were selected, according to a method based on all band ultraviolet scanning and HPLC, the reductive products of NACs were determined using internal standard gas chromatography, the results indicated that the reductive products of each monosubstituted NB were the corresponding aniline. The reduction of NACs was examined at different initial concentrations, the results showed that the NACs concentration decreased over time and the corresponding concentration of reductive products aniline increased over time, the reduction process was well fitted with pseudo-first-order kinetic model. The relative rate constant krel of each NAC was compared in the reference of 4-ClNB, the results showed that there existed a linear relationship between logKrel and Eh1'(X-NB), and the reactive rate of o-substituted compounds was the fastest, which could be explained by o-substituted effect. The competition experiments with binary mixtures of 4-ClNB and each one of the other NACs were performed, the results showed that there existed apparent competition, and the competition was quantitatively compared by definition of competition coefficient Qc. The order of Qc correlated well with its Eh1', and there existed no correlation with logKow.