Distribution and reactivity of O2-reducing components in sediments from a layered aquifer

The redox status of subsurface aqueous systems is controlled by the reactivity of solid redox-sensitive species and by the inflow of such species dissolved in groundwater. The reactivity toward molecular oxygen (O2) of solid reductants present in three particle size fractions of sediments from a pristine aquifer was characterized during 54 days. The stoichiometric relationships between carbon dioxide (CO2) production and O2 consumption was used in combination with sulfate production to discriminate between the contributions of sedimentary organic matter (0-87%), pyrite (6-100%), and siderite (0-43%) as the dominant reductants. The observed simultaneous oxidation of these reductants indicates that they are reactive on the same time scales. The measured reduction capacity 18-84 micromol O2/g) ranged from 8 to 42% of the total reduction capacity present as pyrite and organic carbon in the total sediment fraction (<2 mm). Fine fractions (<63 microm) were 10-250 times more reactive than their corresponding total fractions. Oxygen consumption rates decreased continuously during carbonate buffered conditions, due to a decreasing reactivity of reductants. Acidification accelerated pyrite oxidation but impeded SOM respiration. Our findings indicate that the geological history of aquifer sediments affects the amounts of organic matter, pyrite and siderite present, while environmental conditions, such as pH and microbial activity, are important in controlling the reactivity of these reductants. These controls should be considered when assessing the natural reduction activity of aquifer sediments in either natural or polluted systems.     

Sorption of the herbicide dichlobenil and the metabolite 2,6-dichlorobenzamide on soils and aquifer sediments

The worldwide used herbicide dichlobenil (2,6-dichlorobenzonitrile) has resulted in widespread presence of its metabolite 2,6-dichlorobenzamide (BAM) in pore- and groundwater. To evaluate the transport of these compounds we studied the sorption of dichlobenil and BAM in 22 sediment samples of clayey till, sand, and limestone including sediments exhibiting varying oxidation states. Dichlobenil sorbed to all investigated sediments, with a high sorption in topsoils (Kd = 7.4-17.4 L kg(-1)) and clayey till sediments (Kd = 2.7-126 L kg(-1)). The sorption of the polar metabolite BAM was much lower than the sorption of dichlobenil but followed the same tendency with the highest sorption in the topsoils (Kd = 0.24-0.66 L kg(-1)) and in the clayey till sediments (Kd = 0.10-0.93 L kg(-1)). The sorption of both compounds was significantly higher (2-47 times) in the unoxidized (reduced) clayey till than in the weathered (oxidized) clayey till. Such a difference in sorption capacity could neither be explained by a higher organic carbon content, sorption to clay minerals, differences in clay mineralogy, nor by blocking of reactive surface sites on clay minerals by iron oxides. However, by removing an average of 81% of the organic carbon from the reduced clayey till with H2O2, the sorption decreased on average 50%. Therefore, most of the sorption capacity in the reduced clayey till was related to organic carbon, which indicates that sorption processes are affected by changes in organic compound composition due to weathering.     

In situ chemical reduction of aquifer sediments: enhancement of reactive iron phases and TCE dechlorination

In situ chemical reduction of aquifer sediments is currently being used for chromate and TCE remediation by forming a permeable reactive barrier. The chemical and physical processes that occur during abiotic reduction of natural sediments during flow by sodium dithionite were investigated. In different aquifer sediments, 10-22% of amorphous and crystalline FeIII-oxides were dissolved/reduced, which produced primarily adsorbed FeII, and some siderite. Sediment oxidation showed predominantly one FeII phase, with a second phase being oxidized more slowly. The sediment reduction rate (3.3 h batch half-life) was chemically controlled (58 kJ mol(-1)), with some additional diffusion control during reduction in sediment columns (8.0 h half-life). It was necessary to maintain neutral to high pH to maintain reduction efficiency and prevent iron mobilization, as reduction generated H+. Sequential extractions on reduced sediment showed that adsorbed ferrous iron controlled TCE reactivity. The mass and rate of field-scale reduction of aquifer sediments were generally predicted with laboratory data using a single reduction reaction.     

Quantifying differences in the impact of variable chemistry on equilibrium Uranium(VI) adsorption properties of aquifer sediments

Uranium adsorption-desorption on sediment samples collected from the Hanford 300-Area, Richland, WA varied extensively over a range of field-relevant chemical conditions, complicating assessment of possible differences in equilibrium adsorption properties. Adsorption equilibrium was achieved in 500-1000 h although dissolved uranium concentrations increased over thousands of hours owing to changes in aqueous chemical composition driven by sediment-water reactions. A nonelectrostatic surface complexation reaction, >SOH + UO?2? + 2CO?2? = >SOUO?(CO?HCO?)2?, provided the best fit to experimental data for each sediment sample resulting in a range of conditional equilibrium constants (logK(c)) from 21.49 to 21.76. Potential differences in uranium adsorption properties could be assessed in plots based on the generalized mass-action expressions yielding linear trends displaced vertically by differences in logK(c) values. Using this approach, logK(c) values for seven sediment samples were not significantly different. However, a significant difference in adsorption properties between one sediment sample and the fines (< 0.063 mm) of another could be demonstrated despite the fines requiring a different reaction stoichiometry. Estimates of logK(c) uncertainty were improved by capturing all data points within experimental errors. The mass-action expression plots demonstrate that applying models outside the range of conditions used in model calibration greatly increases potential errors.     

Coupled effect of chemotaxis and growth on microbial distributions in organic-amended aquifer sediments: observations from laboratory and field studies

The inter-relationship of growth and chemotactic response exhibited bytwo common soil-inhabiting bacteria was investigated to determine its impact on bacterial migration. Filter-chambers were used to simulate aquifer sediments characterized by vertical gradients of organic contaminants in both artificial groundwater flow systems in the laboratory and within the screened intervals of observation wells in a sandy aquifer. A labile model contaminant (acetate) was added to the top compartments of the three-part chambers, whereas bacteria with a demonstrated propensity to grow on and chemotactically respond to acetate were introduced to the lower compartments. The motility and chemotactic response of Pseudomonas putida F1 resulted in 40 to 110% greater abundances in the upper compartments and concomitant 22 to 70% depletions in the lower compartments relative to the nonchemotactic controls over 2 days. Bacteria were in greatest abundance within the sand plug that separated the upper and lower compartments where sharp acetate gradients induced a strong chemotactic response. This observation was consistent with predictions from a mathematical model. In agreement with the laboratory results, the down-well filter-chamber incubations with Pseudomonas stutzeri in the aquifer indicated that 91% fewer bacteria resided in the lower compartment than the control experiment without acetate at 15 h. The combination of chemotaxis and growth greatly accelerated the migration of bacteria toward and subsequent abundance at the higher acetate concentration.     

Geochemical cycling of arsenic in a coastal aquifer

Biogeochemically modified pore waters from subterranean estuaries, defined as the mixing zone between freshwater and saltwater in a coastal aquifer, are transported to coastal waters through submarine groundwater discharge (SGD). SGD has been shown to impact coastal and perhaps global trace metal budgets. The focus of this study was to investigate the biogeochemical processes that control arsenic cycling in subterranean estuaries. Total dissolved As, as well as a suite of other trace metals and nutrients, were measured in a series of wells and sediment cores at the head of Waquoit Bay, MA. Dissolved As ranged from below detection to 9.5 microg/kg, and was associated with plumes of dissolved Fe, Mn, and P in the groundwater. Sedimentary As, ranging from 360 to 7500 microg/kg, was highly correlated with sedimentary Fe, Mn, and P. In addition, amorphous Fe (hydr)oxides were more efficient scavengers of dissolved As than the more crystalline forms of solid-phase Fe. Given that dissolved As in the surface bay water was lower than within the subterranean estuary, our results indicate that the distribution and type of Fe and Mn (hydr)oxides in coastal aquifers exert a major influence on the biogeochemical cycling of As in subterranean estuaries and, ultimately, the fate of groundwater-derived As in marine systems influenced by SGD.     

Desorption kinetics of phenanthrene in aquifer material lacks hysteresis

Desorption experiments were carried out in flow through columns following long-term sorption batch experiments (up to 1010 days at 20 degrees C; Rügner, H.; Kleineidam, S.; Grathwohl, P. Long-term sorption kinetics of phenanthrene in aquifer materials. Environ. Sci. Technol. 1999, 33, 1645-1651) to elucidate sorption/desorption hysteresis phenomena of phenanthrene in aquifer materials. Most of the sorbents employed in this study (homogeneous lithocomponents separated from aquifer sediments or fresh rock fragments) showed highly nonlinear sorption isotherms because of coal particles embedded inside the grains. Because sorption capacities were high, sorption equilibrium was not reached in most of the sorbents during the initial sorptive uptake experiments lasting up to 1010 days. Desorption was studied up to 90 days at 20 degrees C. The temperature was raised after that stepwise from originally 20 to 30, 40, 50, and finally to 70 degrees C for selected samples to estimate activation energies of desorption. A numerical intraparticle pore diffusion model was used to fit sorptive uptake data and subsequently for pure forward prediction of the release rates in the desorption column experiments. Desorption was initially fast followed by extended tailing which in other studies is fitted by using multirate first-order models. Our results demonstrate that the retarded intraparticle pore diffusion model can predict the desorption rates with a single diffusion rate constant obtained independently from the long-term batch sorption experiment. No evidence for hysteresis was found, suggesting that many hysteresis phenomena reported earlier are experimental artifacts resulting from nonequilibrium effects and "nonphysical" models. The different temperature steps allowed one to additionally calculate activation energies of desorption (45-59 kJ mol(-1)), which were in reasonably good agreement with results from earlier studies for a retarded pore diffusion process. In addition, equilibrium sorption isotherms were determined at 20 and 40 degrees C to compare sorption and desorption enthalpies. Both were in good agreement, confirming that desorption was not significantly different from sorption.     

Oxymyoglobin Oxidation as Affected by Oxidation Products of Phosphatidylcholine Liposomes

Supernatants (105,000 ×g) of oxidized phosphatidylcholine (PC) liposomes contained higher levels of thiobarbituric acid-reactive substances, and caused greater metmyoglobin formation when combined with oxymyoglobin than those of controls (C) or PC plus tocopherol (PCT) (P<0.05). Products of PC liposome oxidation crossed dialysis membranes (mol wt cut-off = 500 Dalton) and accelerated the oxidation of oxymyoglobin more than C or PCT (P<0.05). The addition of known oxidation products of oleic acid and linoleic acid, fatty acids typical of PC, accelerated oxymyoglobin oxidation relative to controls (P<0.05). Results demonstrated that oxidation products of PC were prooxidative towards oxymyoglobin.     

Lipid Oxidation. Part 3. Oxidation of Egg Phosphatidylethanolamine

Phosphatidylamine isolated from egg yolk phospholipids was autoxidized at 60 °C. Oxidation products were separated by thin layer chromatography on silica gel and by column chromatography on DEAE-cellulose. The fractions were characterized by UV and IR spectra and by chemical analysis. Polyunsaturated fatty acids were destroyed during the oxidation, the content of primary amino groups decreased and imine derivatives appeared.     

中式香肠中脂肪氧化对蛋白质氧化的影响

以中式香肠为研究对象,通过对加工、储藏过程中包装、温度、光照以及抗氧化剂的不同处理,建立中式香肠的脂肪氧化模型,结合对其蛋白质氧化的跟踪检测,分析了中式香肠中脂肪氧化与蛋白质氧化之间的相关性。结果表明:对于脂肪氧化的抑制作用,有效抑制了蛋白质氧化形成羰基类物质,且与脂肪氧化初级和次级产物均表现较好的相关性;脂肪氧化对于蛋白质游离硫醇基流失的促进作用是由脂肪氧化次级产物挥发性醛类物质引起的。     

烤肉中脂类氧化及抗氧化研究

肉类食品中脂类的氧化会产生许多不利的影响,如使食品风味发生变化以及产生有害人体健康的物质等。本文综述了烤肉中脂类的氧化,分析了影响脂类氧化的因素,并总结了抗氧化的方法。