Critical evaluation of the 2D-CSIA scheme for distinguishing fuel oxygenate degradation reaction mechanisms.

Although the uniform initial hydroxylation of methyl tert-butyl ether (MTBE) and other oxygenates during aerobic biodegradation has already been proven by molecular tools, variations in carbon and hydrogen enrichment factors (ε(C) and ε(H)) have still been associated with different reaction mechanisms (McKelvie et al. Environ. Sci. Technol. 2009, 43, 2793-2799). Here, we present new laboratory-derived ε(C) and ε(H) data on the initial degradation mechanisms of MTBE, ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME) by chemical oxidation (permanganate, Fenton reagents), acid hydrolysis, and aerobic bacteria cultures (species of Aquincola, Methylibium, Gordonia, Mycobacterium, Pseudomonas, and Rhodococcus). Plotting of Δδ(2)H/ Δδ(13)C data from chemical oxidation and hydrolysis of ethers resulted in slopes (Λ values) of 22 ± 4 and between 6 and 12, respectively. With A. tertiaricarbonis L108, R. zopfii IFP 2005, and Gordonia sp. IFP 2009, ε(C) was low (<|-1|‰) and ε(H) was insignificant. Fractionation obtained with P. putida GPo1 was similar to acid hydrolysis and M. austroafricanum JOB5 and R. ruber DSM 7511 displayed Λ values previously only ascribed to anaerobic attack. The fractionation patterns rather correlate with the employment of different P450, AlkB, and other monooxygenases, likely catalyzing ether hydroxylation via different transition states. Our data questions the value of 2D-CSIA for a simple distinguishing of oxygenate biotransformation mechanisms, therefore caution and complementary tools are needed for proper interpretation of groundwater plumes at field sites.

Abiotic hydroxylamine nitrification involving manganese- and iron-bearing minerals.

Hydroxylamine (NH2OH) undergoes biotic and abiotic transformation processes in soil, producing nitrous oxide gas (N2O(g)). Little is known about the magnitude of the abiotic chemical processes in the global N cycle, and the role of abiotic nitrification is still neglected in most current nitrogen trace gas studies. The abiotic fate of NH2OH in soil systems is often focused on transition metals including manganese (Mn) and iron (Fe), and empirical correlations of nitrogen residual species including nitrite (NO2-), nitrate (NO3-), and N2O(g). In this study, abiotic NH2OH nitrification by well-characterized manganese (Mn)- and iron (Fe)-bearing minerals (pyrolusite, amorphous MnO2(s), goethite, amorphous FeOOH(s)) was investigated. A nitrogen mass balance analysis involving NH2OH, and the abiotic nitrification residuals, N2O(g), N2O(aq), NO2-, NO3-, was used, and specific reactions and mechanisms were investigated. Rapid and complete NH2OH nitrification occurred (4-5 h) in the presence of pyrolusite and amorphous MnO2(s), achieving a 95-96% mass balance of N byproducts. Conversely, NH2OH nitrification was considerably slower by amorphous FeOOH(s) (14.5%) and goethite (1.1%). Direct reactions between the Mn- and Fe-bearing mineral species and NO2- and NO3- were not detected. Brunauer-Emmett-Teller surface area and energy dispersive X-ray measurements for elemental composition were used to determine the specific concentrations of Mn and Fe. Despite similar specific concentrations of Mn and Fe in crystalline and amorphous minerals, the rate of NH2OH nitrification was much greater in the Mn-bearing minerals. Results underscore the intrinsically faster NH2OH nitrification by Mn minerals than Fe minerals.

Stabilization of potassium permanganate particles with manganese dioxide.

A new potassium permanganate reagent with slow-release properties was designed and tested for possible application in in situ chemical oxidation. For this purpose, MnO(2)-coated KMnO(4) particles (MCP) were prepared by partial reduction of solid KMnO(4) using the acid-catalyzed reaction with n-propanol or the comproportionation of Mn(VII) and Mn(II) in n-propanol as reaction medium. Column tests showed that, for MCP with a residual KMnO(4) fraction of 70wt%, the duration of permanganate release under flow-through conditions was prolonged by a factor of 10 compared to untreated KMnO(4). While KMnO(4) is too soluble to be used in reactive barriers, MCP could be introduced into the aquifer by filling of trenches or boreholes; this would allow a prolonged passive dosing of permanganate into the flowing groundwater. In addition, experiments were conducted in order to determine the oxidation capability of native KMnO(4) particles and MCP in CH(2)Cl(2), a representative non-polar non-aqueous phase liquid (NAPL). It may be possible to utilize the significantly higher reactivity of MCP under these conditions for the design of slow-release permanganate particles for NAPL source treatment.

Nano-sized magnetic iron oxides as catalysts for heterogeneous Fenton-like reactions-Influence of Fe(II)/Fe(III) ratio on catalytic performance.

Nano-sized Fe(II, III) oxides with various Fe(II)/Fe(III) ratios were characterized and tested as catalysts for the oxidative degradation of phenol via Fenton-like reactions at neutral pH. Under conditions typically applied for wet peroxide oxidation, Fe(II) in magnetite is oxidized to Fe(III), successively converting the mineral into maghemite. The residual Fe(II) content in the catalyst core is of only minor benefit for the catalytic activity in phenol oxidation, i.e. magnetite is not superior to maghemite. Achievable reaction rates for phenol degradation appeared to be rather low, e.g. phenol half-life of about 12 h when 3 g L(-1) magnetite and 5 g L(-1) H(2)O(2) were applied. Preceding surface-reduction of maghemite by NaBH(4), leading to an over-stoichiometric Fe(II) content compared to magnetite, only enhanced the non-productive decomposition of H(2)O(2) rather than the rate of phenol degradation. Reaction rates were shown to be relatively insensitive to catalyst concentration in the range of 1-10 g L(-1), probably resulting from a scavenging of reactive species by the catalyst surface, whereby particle agglomeration seems to play a key role. Degradation experiments with various structurally distinct compounds were carried out, indicating a similar selectivity of the heterogeneous Fenton-like system to that known for oxidation with ·OH.