Laboratory batch and column tests were conducted to examine the reduction pathways and kinetics of Nnitrosodimethylamine (NDMA) by iron (Fe) and nickelenhanced iron (Ni/Fe). A decrease in NDMA concentration and increases in dimethylamine (DMA) and ammonium were observed in both Fe and Ni/Fe columns. In the Fe column, the transformation process of NDMA appeared to follow pseudo-first-order kinetics with respect to NDMA, with an average half-life of 13(2 h. A small amount of nickel (0.25%) plated onto the iron greatly enhanced NDMA transformation rates. At early time the NDMA half-life in the Ni/Fe column was 2 min but as time progressed the halflife increased to 4 min, and departures from first-order kinetics were observed. The mass balances of carbon in DMA and nitrogen in DMA and ammonium improved over time and reached 100% and 90%, respectively, after NDMA had passed through the column for more than 50 pore volumes (PV). No 1,1-dimethylhydrazine, nitrous oxide, or methane were detected. Based on the electrochemical properties of NDMA, the transformation mechanism of NDMA with Fe and Ni/Fe is postulated to be catalytic hydrogenation, resulting in N-N bond breakdown to form DMA and ammonium as final products. Nickel, being a much stronger catalyst than Fe for catalytic hydrogenation, resulted in a much faster reduction rate of NDMA. Of several methods tested, flushing the Ni/Fe column with 0.01 N sulfuric acid proved to be the most effective in restoring the Ni/Fe activity. The rapid transformation rate on Ni/Fe and the formation of nontoxic products indicate that this material may be applicable for treating NDMA contaminated water, both in-situ and above ground.
To elucidate the reduction mechanism of N-nitrosodimethylamine (NDMA) by granular iron, various electrochemical experiments using a mercury electrode were conducted. The studies included differential pulse voltammetry and exhaustive potentiostatic electrolysis. The results of the NDMA electroreduction experiments were compared with the results obtained in the column and batch experiments of Part 1 of this study. The results show that (1) electroreduction of NDMA occurs at potentials more negative than -1.3 V and this potential cannot be achieved under the conditions of the column and batch experiments and (2) different reduction products of NDMA were observed in the electrochemical tests relative to the column and batch tests. That is, dimethylamine (DMA) and nitrous oxide were formed in the electrochemical reduction experiments, whereas ammonia and DMA were produced in the column and batch experiments. The difference in product formation and more importantly the fact that the iron cannot reach the potentials required for electroreduction indicate that the reduction of NDMA on iron cannot take place by direct electron transfer. The process of catalytic hydrogenation was found to be consistent with all experimental observations and is proposed as the alternative mechanism.
Pentaerythritol tetranitrate (PETN), a nitrate ester, is used primarily as an explosive. It is of environmental concern, posing a threat to aquatic organisms with an estimated EC50 five times greater than that of RDX. This study evaluated the kinetics and products of PETN degradation in the presence of granular iron. PETN transformation in columns containing 100% granular iron and 30% iron mixed with 70% silica sand followed pseudo first-order kinetics, with average half-lives of 0.26 and 1.58 min, respectively. The reduction pathway was proposed to be sequential denitration, in which PETN was stepwise reduced to pentaerythritol with the formation of pentaerythritol trinitrate and pentaerythritol dinitrate as intermediates. The intermediate of pentaerythritol mononitrate was not detected; however, the nearly 100% nitrogen mass recovery supported complete denitration. Nitrite was released in each denitration step and was subsequently reduced to ammonium by iron. Nitrate was not detected during the experiment suggesting that hydrolysis was not involved in PETN degradation. Batch experiments showed that when solid-phase PETN is present, dissolution is the rate-limiting factorfor PETN mass removal. Using 50% methanol as a cosolvent PETN solubility was enhanced and thus the removal efficiency was improved. The results demonstrate excellent potential of using iron to remediate PETN-contaminated water.
Degradation of dissolved chlorinated solvents using granular iron is an established in situ technology. This paper reports on investigations into mixing iron and bentonite with contaminated soil for in situ containment and degradation of dense nonaqueous phase liquid source zones. In the laboratory, hypovials containing soil, water, bentonite, iron, and free-phase trichloroethene (TCE) were assembled. Periodic measurement of TCE, chloride, and degradation products showed progressive degradation of TCE to nondetectable levels. Subsequently, a demonstration was conducted at Canadian Forces Base Borden near Alliston, Ontario, Canada, where, in 1991, a portion of the surficial aquifer was isolated and free-phase tetrachloroethene (PCE) was introduced. Using a drill rig equipped with large-diameter mixing blades, three mixed zones were prepared containing 0%, 5%, and 10% granular iron by volume. The bentonite was added to serve as a lubricant to facilitate injection of the iron and to isolate the contaminated zone. Analysis of core samples showed reasonably uniform distributions of iron through the mixed zones. Monitoring over a 13-month period following installation showed, relative to the control, a decline in PCE concentrations to virtually nondetectable values. Reaction rates in the laboratory tests were similar to those reported in the literature, while the rate in the field test was substantially lower. The lower rate may be a consequence of mass transfer limitations under the static conditions of the field test. Results indicate that mixing iron and bentonite into source zones may be an effective means of source-zone remediation, with the particular advantage of being relatively immune to effects of geologic heterogeneity.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.