Simulated Groundwater Conditions Research Articles

Role of Al substitution in the reduction of ferrihydrite by Shewanella oneidensis MR-1.

Substitution of aluminum under natural environmental conditions has been proven to inhibit the transformation of weakly crystalline iron (oxyhydr)-oxides towards well crystalline iron oxides, thereby enhancing their long-term stability. However, exploration on the role of aluminum substitution in bacteria-mediated iron oxides transformation is relatively lacking, especially in the anaerobic underground condition where iron (oxyhydr)-oxides are easy to reduced. In this study, we selected four different levels of substitution aluminum prevalent in iron oxides under natural conditions, which are 0mol%, 10mol%, 20mol%, and 30mol% (mol Al/mol (Al + Fe)) respectively. With the presence of Shewanella oneidensis MR-1, we conducted a 15-day anaerobic microcosm experiment in simulated groundwater conditions. The experiment data suggested that aluminum substitution result in a decrease in bio-reduction rate constants of ferrihydrite from 0.24 in 0mol% Al to 0.17 in 30mol% Al. Besides, when containing substituted aluminum, secondary minerals produced by biological reduction of ferrihydrite changed from magnetite to akaganeite. These results were attributed to the surface coverage of Al during the reduction process, which affects the contact between S. oneidensis MR-1 and the unexposed Fe(III), thus inhibiting the further reduction of ferrihydrite. Since iron (oxyhydr)-oxides exhibit a strong affinity on multiple kinds of pollutants, results in this study may contribute to predicting the migration and preservation of contaminants in groundwater systems.

Aerobic and Anaerobic Biodegradation of 1,2-Dibromoethane by a Microbial Consortium under Simulated Groundwater Conditions.

This study was conducted to explore the potential for 1,2-Dibromoethane (EDB) biodegradation by an acclimated microbial consortium under simulated dynamic groundwater conditions. The enriched EDB-degrading consortium consisted of anaerobic bacteria Desulfovibrio, facultative anaerobe Chromobacterium, and other potential EDB degraders. The results showed that the biodegradation efficiency of EDB was more than 61% at 15 °C, and the EDB biodegradation can be best described by the apparent pseudo-first-order kinetics. EDB biodegradation occurred at a relatively broad range of initial dissolved oxygen (DO) from 1.2 to 5.1 mg/L, indicating that the microbial consortium had a strong ability to adapt. The addition of 40 mg/L of rhamnolipid and 0.3 mM of sodium lactate increased the biodegradation. A two-phase biodegradation scheme was proposed for the EDB biodegradation in this study: an aerobic biodegradation to carbon dioxide and an anaerobic biodegradation via a two-electron transfer pathway of dihaloelimination. To our knowledge, this is the first study that reported EDB biodegradation by an acclimated consortium under both aerobic and anaerobic conditions, a dynamic DO condition often encountered during enhanced biodegradation of EDB in the field.

A three chamber bioelectrochemical system appropriate for in-situ remediation of nitrate-contaminated groundwater and its reaction mechanisms

A novel laboratory experiment of three chamber bioelectrochemical (surface water-sediment-groundwater, SSG) system was established in this study, which combined a sediment microbial fuel cell (SMFC) reactor and biofilm electrode reactor (BER) and was self-driven. Simulated groundwater was firstly used to explore the reaction mechanisms of this system. The simulated groundwater conditions were static and the surface water and the groundwater systems were isolated. The results showed that the SMFC continuously supplied a stable voltage of 622 mV ± 20 mV, driving the BER and the related nitrate removal process. Compared to the control systems, the SSG system had higher nitrate removal with a denitrification rate of 3.87 mg N/(L·h). In addition, the sediment organic matter in the SMFC reactor decreased by 66.2%. Based on the electrochemical analysis and microbial community analysis, the SMFC reactor and BER worked synergistically to enhance the performance of both reactors in this system. The presence of microorganisms accelerated the electron transfer efficiency throughout the system, and the microcurrent helped a more fixed community structure to develop and stimulated the growth of denitrifying bacteria. The dominant genera detected in the mature biofilm samples were all microorganisms common in soil and groundwater, indicating that this system may be environmentally friendly. The nitrate removal efficiency for actual groundwater was higher than that for the simulated groundwater, indicating that the elements in the actual groundwater promote the nitrate removal efficiency. These results indicate that the SSG system has the potential for in-situ nitrate bioremediation, with minimal maintenance and health risk.

Sulfidation of ZVI/AC composite leads to highly corrosion-resistant nanoremediation particles with extended life-time

Nanoscale zero-valent iron (nZVI) is a powerful reductant for many water pollutants. The lifetime of nZVI in aqueous environments is one of its limitations. Sulfidation of the nZVI surface by reduced sulfur species is known to significantly modify the particle properties. In the present study we examined various post-synthesis sulfidation methods applied on Carbo-Iron, a composite material where iron nanostructures are embedded in colloidal activated carbon (AC) particles. In such cases, where ZVI is surrounded by carbon, sulfidation largely inhibits the anaerobic corrosion of ZVI in water whereas its dechlorination activity was slightly increased. Even at a very low molar S/Fe ratio of 0.004 a strong decrease of the corrosion rate by a factor of 65 was achieved, while concurrently dechlorination of tetrachloroethene (PCE) was accelerated by a factor of three compared to the untreated particles. As a consequence, over 98% of the reduction equivalents of the sulfidated ZVI were utilized for the reduction of the target contaminant (33 mg L−1 PCE) under simulated groundwater conditions. In a long-term experiment over 160 days the extended life-time and the preservation of the reduction capacity of the embedded ZVI were confirmed. Reasons for the modified reaction behavior of Carbo-Iron after sulfidation compared to previously studied nZVI are discussed. We hypothesize that the structure of the carbon-embedded iron is decisive for the observed reaction behavior. In addition to reaction rates, the product pattern is vastly different compared to that of sulfidated nZVI. The triple combination of ZVI, AC and sulfur makes the composite particle very suitable for practical in-situ applications.

Alteration of compacted GMZ bentonite by infiltration of alkaline solution

AbstractConcepts for geological disposal of high-level radioactive waste usually include bentonite buffer materials. Numerous studies have been performed with most usingWyoming bentonite. Gaomiaozi (GMZ) bentonite has been selected as a potential buffer/backfill material for the deep geological repository of high-level radioactive waste in China. In this context, the highly alkaline environment induced by cementitious materials in the repository is likely to alter montmorillonite, the main clay mineral in GMZ bentonite. This alteration may result in deterioration of the physical and/or chemical properties of the buffer material. To acquire quantitative data which would allow us to assess the dissolution of montmorillonite and changes in the diffusivity of hydroxide ions as well as their effects on the swelling pressure and permeability of the compacted GMZ bentonite, an experimental study was conducted under highly alkaline (NaOH solutions with various pH values were used), simulated groundwater conditions. The GMZ bentonite also contains cristobalite which may also have been dissolved. The microstructure of the compacted bentonite samples after the experiments was determined by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Energy dispersive spectroscopy (EDX) was carried out to identify mineralogical changes. At pH >13, the permeability of specimens increased significantly; the swelling potential decreased with increasing pH. Furthermore, the pore volume and pore size of GMZ bentonite changed when exposed to alkaline solution, resulting in an increase in porosity and permeability. The main alteration mechanisms of compacted GMZ bentonite undergoing infiltration by highly alkaline solution are likely to be dissolution and modifications in terms of the microstructure and mineralogy.

Surface passivation limited UO2 oxidative dissolution in the presence of FeS.

Iron sulfide minerals produced during in situ bioremediation of U can serve as an oxygen scavenger to retard uraninite (UO2) oxidation upon oxygen intrusion. Under persistent oxygen supply, however, iron sulfides become oxidized and depleted, giving rise to elevated dissolved oxygen (DO) levels and remobilization of U(IV). The present study investigated the mechanism that regulates UO2 oxidative dissolution rate in a flow-through system when oxygen breakthrough occurred as a function of mackinawite (FeS) and carbonate concentrations. The formation and evolution of surface layers on UO2 were characterized using XAS and XPS. During FeS inhibition period, the continuous supply of carbonate and calcium in the influent effectively complexed and removed oxidized U(VI) to preserve an intermediate U4O9 surface. When the FeS became depleted by oxidization, a transient, rapid dissolution of UO2 was observed along with DO breakthrough in the reactor. This rate was greater than during the preceding FeS inhibition period and control experiments in the absence of FeS. With increasing DO, the rate slowed and the rate-limiting step shifted from surface oxidation to U(VI) detachment as U(VI) passivation layers developed. In contrast, increasing the carbonate concentrations facilitated detachment of surface-associated U(VI) complexes and impeded the formation of U(VI) passivation layer. This study demonstrates the critical role of U(VI) surface layer formation versus U(VI) detachment in controlling UO2 oxidative dissolution rate during periods of variable oxygen presence under simulated groundwater conditions.

Porous Media-Induced Aggregation of Protein-Stabilized Gold Nanoparticles

Gold-nanoparticles (AuNPs) are employed for cancer treatment, drug delivery, chemical analyses, and many other uses. As AuNP manufacture increases, it is imperative that we understand the environmental fate of these nanomaterials. We investigated the transport and stability of AuNPs under simulated groundwater conditions. Batch experiments indicated that 16 nm AuNPs stabilized with bovine serum albumin (BSA-cit-AuNPs) was slightly more stable under high ionic strength conditions than citrate-functionalized AuNPs (cit-AuNPs) of the same core size. Both types of AuNPs were injected into glass bead-packed columns and subjected to transport with varying NaCl and CaCl2 concentrations. BSA-cit-AuNPs deposited less than cit-AuNPs in the presence of increasing concentrations of CaCl2, but the opposite trend was observed in the presence of increasing concentrations of NaCl. This finding differed from the results obtained in the batch studies. Calculated attachment efficiencies (α) failed to reflect the observed experimental column data, with α at maximum only approaching 0.1 even though a majority of the AuNPs were retained in the column. Colloid filtration theory fails to predict and explain this discrepancy. We conclude that media induced nanoparticle aggregation is responsible for the inconsistency.

Effect of groundwater chemistry on the swelling behavior of a Ca-bentonite for deep geological repository

The swelling properties of buffer material for high level radioactive waste repository in a near-field environment are of particular importance for achieving the low permeability sealing function. In this study, the free swelling behavior of a potential buffer material Zhisin clay is evaluated under simulated groundwater conditions such as immersion in NaCl, CaCl2, and Na2SO4 solutions at various concentrations. Experimental results indicate that Zhisin clay, being a Ca-bentonite, exhibits reduced swelling strain in salt solutions. The amount of decrease in swelling strain upon saline water intrusion is affected by both the type and concentration of electrolyte. At the same concentration, the swelling strains in CaCl2 solution are lower than those in NaCl solution due to the quasi-crystals formed in the presence of calcium ions. Also, the swells in Na2SO4 solution are found to be lower than those in NaCl solution. This is attributed to the precipitation of CaSO4, which acts as binding agent and results in aggregation of clay particles.

Oxidative dissolution of UO2 in a simulated groundwater containing synthetic nanocrystalline mackinawite

The long-term success of in situ reductive immobilization of uranium (U) depends on the stability of U(IV) precipitates (e.g., uraninite) in the presence of natural oxidants, such as oxygen, Fe(III) hydroxides, and nitrite. Field and laboratory studies have implicated iron sulfide minerals as redox buffers or oxidant scavengers that may slow oxidation of reduced U(IV) solid phases. Yet, the inhibition mechanism(s) and reaction rates of uraninite (UO2) oxidative dissolution by oxic species such as oxygen in FeS-bearing systems remain largely unresolved. To address this knowledge gap, abiotic batch experiments were conducted with synthetic UO2 in the presence and absence of synthetic mackinawite (FeS) under simulated groundwater conditions of pH=7, PO2=0.02atm, and PCO2=0.05atm. The kinetic profiles of dissolved uranium indicate that FeS inhibited UO2 dissolution for about 51h by effectively scavenging oxygen and keeping dissolved oxygen (DO) low. During this time period, oxidation of structural Fe(II) and S(-II) of FeS were found to control the DO levels, leading to the formation of iron oxyhydroxides and elemental sulfur, respectively, as verified by X-ray diffraction (XRD), Mössbauer, and X-ray absorption spectroscopy (XAS). After FeS was depleted due to oxidation, DO levels increased and UO2 oxidative dissolution occurred at an initial rate of rm=1.2±0.4×10−8molg−1s−1, higher than rm=5.4±0.3×10−9molg−1s−1 in the control experiment where FeS was absent. XAS analysis confirmed that soluble U(VI)-carbonato complexes were adsorbed by iron oxyhydroxides (i.e., nanogoethite and lepidocrocite) formed from FeS oxidation, which provided a sink for U(VI) retention. This work reveals that both the oxygen scavenging by FeS and the adsorption of U(VI) to FeS oxidation products may be important in U reductive immobilization systems subject to redox cycling events.

Immobilization of selenate by iron in aqueous solution under anoxic conditions and the influence of uranyl

In proposed high level radioactive waste repositories a large part of the spent nuclear fuel (SNF) canisters are commonly composed of iron. Selenium is present in spent nuclear fuel as a long lived fission product. This study investigates the influence of iron on the uptake of dissolved selenium in the form of selenate and the effect of the presence of dissolved uranyl on the above interaction of selenate. The iron oxide, and selenium speciation on the surfaces was investigated by Raman spectroscopy. X-ray Absorption Spectroscopy was used to determine the oxidation state of the selenium and uranium on the surfaces. Under the simulated groundwater conditions (10 mM NaCl, 2 mM NaHCO 3, <0.1 ppm O 2) the immobilized selenate was found to be reduced to oxidation states close to zero or lower and uranyl was found to be largely reduced to U(IV). The near simultaneous reduction of uranyl was found to greatly enhance the rate of selenate reduction. These findings suggest that the presence of uranyl being reduced by an iron surface could substantially enhance the rate of reduction of selenate under anoxic conditions relevant for a repository.

Removal of chlorophenols from groundwater by chitosan sorption

The equilibrium and kinetics of chlorophenol (CP) sorption by chitosan, poly D-glucosamine, were studied under simulated groundwater conditions. Lower temperature, from 25°C to 15°C and then 5°C, markedly decreased the adsorption rates by a factor of 30–53% and 7–22%. Comparison between two types of chitosan, flakes and highly swollen beads, demonstrated that the maximum pentachlorophenol (PCP) uptake capacities in Langmuir and Freundlich models depend on the specific surface area of the particle. Low temperature (5°C) significantly increased the PCP uptake capacity in comparison to higher temperatures (15°C and 25°C). PCP uptake capacity was halved at pH levels higher than 6.5, and NaCl concentrations greater than 1% blocked PCP sorption almost completely. Of five kinds of chlorophenols, i.e. 2,4,6-trichlorophenol (2,4,6-TCP), 3,4-dichlorophenol (3,4-DCP), 2,3-dichlorophenol (2,3-DCP), 2,6-dichlorophenol (2,6-DCP), 3-monochlorophenol (3-MCP), TCP had the maximum sorption efficiency on flake-type chitosan, followed by DCPs, and finally MCP (the three kinds of DCP, with the same elemental compositions, achieved similar sorption performances).

Removal of chlorophenols from groundwater by chitosan sorption

The equilibrium and kinetics of chlorophenol (CP) sorption by chitosan, poly D-glucosamine, were studied under simulated groundwater conditions. Lower temperature, from 25°C to 15°C and then 5°C, markedly decreased the adsorption rates by a factor of 30–53% and 7–22%. Comparison between two types of chitosan, flakes and highly swollen beads, demonstrated that the maximum pentachlorophenol (PCP) uptake capacities in Langmuir and Freundlich models depend on the specific surface area of the particle. Low temperature (5°C) significantly increased the PCP uptake capacity in comparison to higher temperatures (15°C and 25°C). PCP uptake capacity was halved at pH levels higher than 6.5, and NaCl concentrations greater than 1% blocked PCP sorption almost completely. Of five kinds of chlorophenols, i.e. 2,4,6-trichlorophenol (2,4,6-TCP), 3,4-dichlorophenol (3,4-DCP), 2,3-dichlorophenol (2,3-DCP), 2,6-dichlorophenol (2,6-DCP), 3-monochlorophenol (3-MCP), TCP had the maximum sorption efficiency on flake-type chitosan, followed by DCPs, and finally MCP (the three kinds of DCP, with the same elemental compositions, achieved similar sorption performances).

Dissolution of montmorillonite in compacted bentonite by highly alkaline aqueous solutions and diffusivity of hydroxide ions

Highly alkaline environments induced by cementitious materials in radioactive waste repositories are likely to alter montmorillonite, the main constituent of bentonite buffer materials. Over long time periods, the alteration may cause the physical and/or chemical properties of the buffer to deteriorate. For the purpose of acquiring quantitative data to determine the effect of alteration on the permeability of a bentonite buffer, dissolution rates of montmorillonite and diffusivity of hydroxide ions in compacted sand–bentonite mixture specimens have been measured under highly alkaline, simulated groundwater conditions. Dissolution of montmorillonite was described by the linear dependence on time, W( t)= W(0)− R A t, under the employed experimental conditions of pH 13 to 14 and temperatures of 90 to 170 °C, where W( t) denotes the density of montmorillonite (mg of montmorillonite/m 3 of sand–bentonite mixture), W(0) the initial density, R A (Mg m −3 s −1) the rate of density decrease and t (s) the time after the contact with the simulated groundwater. R A is a function of pH and temperature (K), and expressed as R A=(0.013±0.007)exp[−(3.7±0.2)×10 4/ RT for the “pH 14.0” simulated groundwater, where R is the gas constant. The diffusivity of hydroxide ions was obtained in through-diffusion experiments combined with a pore diffusion model. The experiments were performed under relatively low temperatures of 10 to 50 °C to minimize the effect of alteration of bentonite. The effective diffusivity of hydroxide ions was in the order of 10 −10 to 10 −11 m 2/s, and is in the same order as those of Cl −, I − and tritiated water.

Microbial Nitrate Processing in Shallow Groundwater in a Riparian Forest

We measured denitrification, immobilization, and respiration in microcosms that simulated groundwater conditions in a riparian forest in Rhode Island. Measured rates were compared with rates of NO−3 removal measured in a companion study using a groundwater monitoring well network and 10-mo injection of NO−3 and a bromide tracer to groundwater in the same riparian forest. Limiting factors for denitrification were assessed, and microbial and root biomass and potential net N mineralization and nitrification were compared in aquifer material exposed to the 10 mo of NO−3 dosing and unexposed control material. While there was significant variation in water table levels, dissolved oxygen, dissolved organic C, and total C within the aquifer in the riparian forest, there was little spatial variation in denitrification or respiration rates. Rates were higher in summer than in winter and were nearly always limited by a lack of organic C. Limited evidence suggested that immobilization was not a significant sink for NO−3. Measured groundwater denitrification rates were high enough to eliminate 27% of an incoming NO−3 concentration of 10 mg N L−1 over a riparian zone width of 60 m, with a travel time of 1600 d, and equaled 6.0 kg N ha−1 yr−1. The measured denitrification rates were much lower than rates of groundwater NO−3 removal directly measured in the companion hydrologic study at the same time (120 kg N ha−1 yr−1), suggesting that there is still considerable uncertainty about the mechanisms of NO−3 removal from groundwater in riparian forests.

Degradation of the Herbicide Bromoxynil in Batch Cultures Under Groundwater Conditions

Abstract The biological degradation of bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) under simulated groundwater conditions was investigated. The influence of aerobic and anaerobic conditions on the degradation rate was examined in batch cultures at 8[ddot]C during 32 days. The cultures consisted of 400 ml groundwater plus 2000 ml salt basal medium. Final bromoxynil concentration was I mg/1. Incubation was carried out with and without 100 mg/1 acetate added as carbon source. Dissolved organic carbon (DOC), oxygen, nitrate and sulphate, bacterial number (CFU), enzymatic hydrolysis of fluorescein-di-acetate (FDA), and bromoxynil concentration was estimated along the test. Only anaerobic, nitrate reducing conditions caused biodegradation of bromoxynil after 17-21 days. The addition of acetate delayed this process, although the complete degradation after 32 days in both tests amounted to 99%. In spite of a high bioactivity, no degradation of bromoxynil could be found under aerobic conditions within 32 days.

Influence of the Gas-Water Interface on Transport of Microorganisms through Unsaturated Porous Media

In this article, a new mechanism influencing the transport of microorganisms through unsaturated porous media is examined, and a new method for directly visualizing bacterial behavior within a porous medium under controlled chemical and flow conditions is introduced. Resting cells of hydrophilic and relatively hydrophobic bacterial strains isolated from groundwater were used as model microorganisms. The degree of hydrophobicity was determined by contact-angle measurements. Glass micromodels allowed the direct observation of bacterial behavior on a pore scale, and three types of sand columns with different gas saturations provided quantitative measurements of the observed phenomena on a porous medium scale. The reproducibility of each break-through curve was established in three to five repeated experiments. The data collected from the column experiments can be explained by phenomena directly observed in the micromodel experiments. The retention rate of bacteria is proportional to the gas saturation in porous media because of the preferential sorption of bacteria onto the gas-water interface over the solid-water interface. The degree of sorption is controlled mainly by cell surface hydrophobicity under the simulated groundwater conditions because of hydrophobic forces between the organisms and the interfaces. The sorption onto the gas-water interface is essentially irreversible because of capillary forces. This preferential and irreversible sorption at the gas-water interface strongly influences the movement and spatial distribution of microorganisms.