Unraveling mechanisms behind reduced nitrate leaching with graphite nanomaterials addition with fertilizers in soil column experiments.

Overuse or mistimed application of nitrogen fertilizer can cause nitrate contamination in groundwater and surrounding surface waters. Previous greenhouse studies have explored the use of graphene nanomaterials, including graphite nano additive (GNA), to reduce nitrate leaching in an agricultural soil while growing lettuce crops. To investigate the mechanism of GNA addition in suppressing nitrate leaching, we conducted soil column experiments using native agricultural soils under saturated or unsaturated flow conditions to simulate varied irrigation. We investigated the effects of temperature (4 °C compared with 20 °C) on microbial activity and dose effect of GNA was also explored (165 mg/kg soil and 1650 mg/kg soil) for biotic soil column experiments whereas a single temperature condition (20 °C) and GNA dose (165 mg/kg soil) was employed for abiotic (autoclaved) soil column experiments. Results showed GNA addition had minimal effects on nitrate leaching in saturated flow soil columns due to short hydraulic residence times (∼3.5 h). In comparison, longer residence times (∼3 d) in unsaturated soil columns reduced nitrate leaching by 25-31% relative to control soil columns without GNA addition. Furthermore, nitrate retention in the soil column was found to be suppressed at 4 °C compared with 20 °C, suggesting a bio-mediated mechanism for GNA addition to reduce nitrate leaching. In addition, the soil dissolved organic matter was found to be associated with nitrate leaching, where less nitrate leaching occurring when higher dissolved organic carbon (DOC) was measured in leachate water. Following studies of adding soil-derived organic carbon (SOC) resulted in greater nitrogen retention in the unsaturated soil columns only when GNA was present. Overall, the results suggest that GNA-amended soil reduces nitrate loss through increased N immobilization in the microbial biomass or loss of N in gaseous phase through enhanced nitrification and denitrification process.

Coremoval of Energetics and Oxyanions via the In Situ Coupling of Catalytic and Enzymatic Destructions: A Solution to Ammunition Wastewater Treatment.

Ammunition wastewater contains toxic nitrated explosives like RDX and oxyanions like nitrate and perchlorate. Its treatment is challenged by low efficiency due to contaminant recalcitrance and high cost due to multiple processes needed for separately removing different contaminant types. This paper reports a H2-based low-energy strategy featuring the treatment of explosives via catalytic denitration followed by microbial mineralization coupled with oxyanion reduction. After a nitrate- and perchlorate-reducing biofilm incapable of RDX biodegradation was coated with palladium nanoparticles (Pd0NPs), RDX was rapidly denitrated with a specific catalytic activity of 8.7 gcat-1 min-1, while biological reductions of nitrate and perchlorate remained efficient. In the subsequent 30-day continuous test, >99% of RDX, nitrate, and perchlorate were coremoved, and their effluent concentrations were below their respective regulation levels. Detected intermediates and shallow metagenome analysis suggest that the intermediates after Pd-catalytic denitration of RDX ultimately were enzymatically utilized by the nitrate- and perchlorate-reducing bacteria as additional electron donor sources.

Reductive destruction of multiple nitrated energetics over palladium nanoparticles in the H2-based membrane catalyst-film reactor (MCfR).

Nitrated energetics are widespread contaminants due to their improper disposal from ammunition facilities. Different classes of nitrated energetics commonly co-exist in ammunition wastewater, but co-removal of the classes has hardly been documented. In this study, we evaluated the catalytic destruction of three types of energetics using palladium (Pd0) nano-catalysts deposited on H2-transfer membranes in membrane catalyst-film reactors (MCfRs). This work documented nitro-reduction of 2,4,6-trinitrotoluene (TNT), as well as, for the first time, denitration of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and pentaerythritol tetranitrate (PETN) over Pd0 at ambient temperature. The catalyst-specific activity was 20- to 90-fold higher than reported for other catalyst systems. Nitrite (NO2-) released from RDX and PETN also was catalytically reduced to dinitrogen gas (N2). Continuous treatment of a synthetic wastewater containing TNT, RDX, and PETN (5 mg/L each) for more than 20 hydraulic retention times yielded removals higher than 96% for all three energetics. Furthermore, the concentrations of NO2- and NH4+ were below the detection limit due to subsequent NO2- reduction with > 99% selectivity to N2. Thus, the MCfR provides a promising strategy for sustainable catalytic removal of co-existing energetics in ammunition wastewater.

Photocatalytic activity of nanoparticles: the development of the standardized measurement for physiological conditions.

Recently a new International Standard for testing nanomaterial photocatalytic activity under physiological conditions was issued by Technical Committee 229 (Nanotechnologies) of the International Organization for Standardization (ISO 20814:2019 Nanotechnologies-Testing the photocatalytic activity of nanoparticles for NADH oxidation). The document offers a robust, high throughput photocatalytic assay using a bio-compatible indicator nicotinamide amide dinucleotide (NAD) and provides a screening tool to gauge nanomaterial potency for phototoxicity. This paper describes the measurement principles behind this assay, the scope of the standard and its validation through an interlaboratory comparison study using a traceable standard reference material (SRM 1898).

H2-Based Membrane Catalyst-Film Reactor (H2-MCfR) Loaded with Palladium for Removing Oxidized Contaminants in Water.

Scalable applications of precious-metal catalysts for water treatment face obstacles in H2-transfer efficiency and catalyst stability during continuous operation. Here, we introduce a H2-based membrane catalyst-film reactor (H2-MCfR), which enables in situ reduction and immobilization of a film of heterogeneous Pd0 catalysts that are stably anchored on the exterior of a nonporous H2-transfer membrane under ambient conditions. In situ immobilization had >95% yield of Pd0 in controllable forms, from isolated single atoms to moderately agglomerated nanoparticles (averaging 3-4 nm). A series of batch tests documented rapid Pd-catalyzed reduction of a wide spectrum of oxyanions (nonmetal and metal) and organics (e.g., industrial raw materials, solvents, refrigerants, and explosives) at room temperature, owing to accurately controlled H2 supply on demand. Reduction kinetics and selectivity were readily controlled through the Pd0 loading on the membranes, H2 pressure, and pH. A 45-day continuous treatment of trichloroethene (TCE)-contaminated water documented removal fluxes up to 120 mg-TCE/m2/d with over 90% selectivity to ethane and minimal (<1.5%) catalyst leaching or deactivation. The results support that the H2-MCfR is a potentially sustainable and reliable catalytic platform for reducing oxidized water contaminants: simple synthesis of an active and versatile catalyst that has long-term stability during continuous operation.

Long-Term Continuous Co-reduction of 1,1,1-Trichloroethane and Trichloroethene over Palladium Nanoparticles Spontaneously Deposited on H2-Transfer Membranes.

1,1,1-Trichloroethane (1,1,1-TCA) and trichloroethene (TCE) are common recalcitrant contaminants that coexist in groundwater. H2-induced reduction over precious-metal catalysts has proven advantageous, but its application to long-term continuous treatment has been limited due to poor H2-transfer efficiency and catalyst loss. Furthermore, catalytic reductions of aqueous 1,1,1-TCA alone or concomitant with TCE catalytic co-reductions are unstudied. Here, we investigated 1,1,1-TCA and TCE co-reduction using palladium nanoparticle (PdNP) catalysts spontaneously deposited on H2-transfer membranes that allow efficient H2 supply on demand in a bubble-free form. The catalytic activities for 1,1,1-TCA and TCE reductions reached 9.9 and 11 L/g-Pd/min, values significantly greater than that reported for other immobilized-PdNP systems. During 90 day continuous operation, removals were up to 95% for 1,1,1-TCA and 99% for TCE. The highest steady-state removal fluxes were 1.5 g/m2/day for 1,1,1-TCA and 1.7 g/m2/day for TCE. The major product was nontoxic ethane (94% selectivity). Only 4% of the originally deposited PdNPs were lost over 90 days of continuous operation. Documenting long-term continuous Pd-catalyzed dechlorination at high surface loading with minimal loss of the catalyst mass or activity, this work expands understanding of and provides a foundation for sustainable catalytic removal of co-existing chlorinated solvents.

Evaluating performance, degradation, and release behavior of a nanoform pigmented coating after natural and accelerated weathering.

Pigments with nanoscale dimensions are added to exterior coatings to achieve desirable color and gloss properties. The present study compared the performance, degradation, and release behavior of an acrylic coating that was pigmented by a nanoform of Cu-phthalocyanine after both natural (i.e., outdoor) and accelerated weathering. Samples were weathered outdoors in three geographically distinct locations across the United States (Arizona, Colorado, Maryland) continuously for 15 months. Identically prepared samples were also artificially weathered under accelerated conditions (increased ultraviolet (UV) light intensity and elevated temperatures) for three months, in one-month increments. After exposure, both sets of samples were characterized with color, gloss, and infrared spectroscopy measurements, and selectively with surface roughness measurements. Results indicated that UV-driven coating oxidation was the principal degradation pathway for both natural and accelerated weathering samples, with accelerated weathering leading to an increased rate of oxidation without altering the fundamental degradation pathway. The inclusion of the nanoform pigment reduced the rate of coating oxidation, via UV absorption by the pigment, leading to improved coating integrity compared to non-pigmented samples. Release measurements collected during natural weathering studies indicated there was never a period of weathering, in any location, that led to copper material release above background copper measurements. Lab-based release experiments performed on samples weathered naturally and under accelerated conditions found that the release of degraded coating material after each type of exposure was diminished by the inclusion of the nanoform pigment. Release measurements also indicated that the nanoform pigment remained embedded within the coating and did not release after weathering.

Quantifying temporal and geographic variation in sunscreen and mineralogic titanium-containing nanoparticles in three recreational rivers.

Detection of metal nanoparticles (NPs) in the environment is an analytical challenge of interest due to increasing use of nanomaterials in consumer and industrial products. Detecting NPs associated with human activities is affected by both the magnitude and variation in background concentrations of natural NPs. In this work, we investigated the potential release of titanium dioxide (TiO2) NPs from sunscreen in three recreational rivers, with a time-intensive sampling regime on one river, in order to determine the range and variability of natural, background titania (Ti). Conventional ICP analysis for total metal concentrations, single particle ICP-MS for NP concentrations, and electron microscopy aided in assessing mineralogical morphology and composition. Oxybenzone, a widely-used organic sunscreen, was measured and used as a surrogate for the intensity of recreational activity in the water. Statistically significant increases in Ti concentrations were observed in Clear Creek, CO during one recreation period, but the significance of other instances of recreation-associated Ti increases was unclear, in part due to storm impacts on the natural suspended sediment load of the stream. A comparison of three recreational rivers showed increases in both Ti mass concentrations and NP sizes occur during recreation in both Clear Creek, CO and the Salt River, AZ, but no detectable changes in the Truckee River, NV. However, size distributions were variable in background samples, which make the significance of differences observed during recreation unclear. These results underline that the release of engineered nanoparticles to a natural system cannot be detected without a well-defined background, including measures of its variability during the study period.

The Nature and Oxidative Reactivity of Urban Magnetic Nanoparticle Dust Provide New Insights into Potential Neurotoxicity Studies.

The recent discovery of magnetic nanoparticles (NPs) in human brain tissue has raised concerns regarding their source and neurotoxicity. As previous studies have suggested that magnetite in urban dust may be the source, we collected urban magnetic dust and thoroughly characterized the nature of ambient urban magnetic dust particles prior to investigating their neurotoxic potential. In addition to magnetite, magnetic dust contained an abundance (∼40%) of elemental iron (Fe0). The coexistence of magnetite and elemental iron was found in magnetic dust particles of inhalable (<10 µm) and nanoscale (<200 nm) size ranges with these particles small enough to enter the human brain via the respiratory tract and olfactory bulbs. The magnetic dust also contained nonferrous water-soluble metals (particularly Cu) that can induce formation of reactive oxygen species (ROS). Previous studies used engineered pure-magnetite for in vitro ROS studies. However, while magnetite was present in all magnetic dust particles collected, engineered pure-magnetite was relatively unreactive and contributed minimally to the generation of ROS. We fill a critical knowledge gap between exposure to heterogeneous ambient iron-particles and in vitro experiments with engineered versus ambient, incidental iron-bearing nanoscale minerals. Our work points to the need to further investigate the presence and properties of magnetic NPs in respirable dust with respect to their potential role in neurodegeneration.

The complex puzzle of dietary silver nanoparticles, mucus and microbiota in the gut.

Hundreds of consumer and commercial products containing silver nanoparticles (AgNPs) are currently used in food, personal-care products, pharmaceutical, and many other applications. Human exposure to AgNPs includes oral intake, inhalation, and dermal contact. The aim of this review was to focus on oral intake, intentional and incidental of AgNPs where well-known antimicrobial characteristics that might affect the microbiome and mucus in the gastrointestinal tract (GIT). This critical review summarizes what is known regarding the impacts of AgNPs on gut homeostasis. It is fundamental to understand the forms of AgNPs and their physicochemical characterization before and during digestion. For example, lab-synthesized AgNPs differ from "real" ingestable AgNPs used as food additives and dietary supplements. Similarly, the gut environment alters the chemical and physical state of Ag that is ingested as AgNPs. Emerging research on in vitro and in vivo rodent and human indicated complex multi-directional relationships among AgNPs, the intestinal microbiota, and the epithelial mucus. It may be necessary to go beyond today's descriptive approach to a modeling-based ecosystem approach that might quantitatively integrate spatio-temporal interactions among microbial groups, host factors (e.g., mucus), and environmental factors, including lifestyle-based stressors. It is suggested that future research (1) utilize more representative AgNPs, focus on microbe/mucus interactions, (2) assess the effects of environmental stressors for longer and longitudinal conditions, and (3) be integrated using quantitative modeling.

Yttrium Residues in MWCNT Enable Assessment of MWCNT Removal during Wastewater Treatment.

Many analytical techniques have limited sensitivity to quantify multi-walled carbon nanotubes (MWCNTs) at environmentally relevant exposure concentrations in wastewaters. We found that trace metals (e.g., Y, Co, Fe) used in MWCNT synthesis correlated with MWCNT concentrations. Because of low background yttrium (Y) concentrations in wastewater, Y was used to track MWCNT removal by wastewater biomass. Transmission electron microscopy (TEM) imaging and dissolution studies indicated that the residual trace metals were strongly embedded within the MWCNTs. For our specific MWCNT, Y concentration in MWCNTs was 76 µg g-1, and single particle mode inductively coupled plasma mass spectrometry (spICP-MS) was shown viable to detect Y-associated MWCNTs. The detection limit of the specific MWCNTs was 0.82 µg L-1 using Y as a surrogate, compared with >100 µg L-1 for other techniques applied for MWCNT quantification in wastewater biomass. MWCNT removal at wastewater treatment plants (WWTPs) was assessed by dosing MWCNTs (100 µg L-1) in water containing a range of biomass concentrations obtained from wastewater return activated sludge (RAS) collected from a local WWTP. Using high volume to surface area reactors (to limit artifacts of MWCNT loss due to adsorption to vessel walls) and adding 5 g L-1 of total suspended solids (TSS) of RAS (3-h mixing) reduced the MWCNT concentrations from 100 µg L-1 to 2 µg L-1. The results provide an environmentally relevant insight into the fate of MWCNTs across their end of life cycle and aid in regulatory permits that require estimates of engineered nanomaterial removal at WWTPs upon accidental release into sewers from manufacturing facilities.

Copper release and transformation following natural weathering of nano-enabled pressure-treated lumber.

Commercially available lumber, pressure-treated with micronized copper azole (MCA), has largely replaced other inorganic biocides for residential wood treatment in the USA, yet little is known about how different outdoor environmental conditions impact the release of ionic, nano-scale, or larger (micron-scale) copper from this product. Therefore, we weathered pressure treated lumber for 18 months in five different climates across the continental United States. Copper release was quantified every month and local weather conditions were recorded continuously to determine the extent to which local climate regulated the release of copper from this nano-enabled product during its use phase. Two distinct release trends were observed: In cooler, wetter climates release occurred primarily during the first few months of weathering, as the result of copper leaching from surface/near-surface areas. In warmer, drier climates, less copper was initially released due to limited precipitation. However, as the wood dried and cracked, the exposed copper-bearing surface area increased, leading to increased copper release later in the product lifetime. Single-particle-ICP-MS results from laboratory prepared MCA-wood leachate solutions indicated that a) the predominant form of released copper passed through a filter smaller than 0.45 micrometers and b) released particles were largely resistant to dissolution over the course of 6 wks. Toxicity Characteristic Leaching Procedure (TCLP) testing was conducted on nonweathered and weathered MCA-wood samples to simulate landfill conditions during their end-of-life (EoL) phase and revealed that MCA wood released <10% of initially embedded copper. Findings from this study provide data necessary to complete a more comprehensive evaluation of the environmental and human health impacts introduced through release of copper from pressure treated lumber utilizing life cycle assessment (LCA).

High-throughput analysis of photocatalytic reactivity of differing TiO2 formulations using 96-well microplate reactors.

The rapid development of photocatalysts for water decontamination benefits from availability of sensitive platforms for screening photocatalytic reactivity. The standard approach typically involves quantifying the degradation of a single dye compound in a slurry system in individual beakers, which requires tedious photocatalyst separation and long operation time. We present a simple and efficient method for assessing the photocatalytic activity of different photocatalyst nanomaterials that eliminates the solid separation process. The 96-well microplate method demonstrated an improved applicability as a high-throughput screening method for photocatalytic reaction mechanisms using a wide range of chemical substrates (i.e., methyl orange, methylene blue, terephthalic acid, and ß-nicotinamide adenine dinucleotide coenzyme) and photocatalyst concentrations (1-100 mg/L). By employing photocatalysts at lower concentrations compared to the slurry system, rapid screening was accomplished through direct spectrophotometric or spectrofluorometric measurements. The mass-normalized rate constants of dye degradation were used to determine the photocatalytic reactivity of three commercial TiO2 nanomaterials, which followed an order of SRM TiO2 1898 ≈ Degussa TiO2 P90 > Food-grade TiO2 E171. The extent of hydroxyl radical involvement in methyl orange degradation was estimated to be ∼74% by using radical scavengers in the microplate reactor. Given the utilization of low-concentration photocatalyst, this protocol may be used for evaluating photocatalytic reactivity and oxidative stress caused by photocatalyst exposure in an aquatic environment. We further evaluated photocatalytic reaction kinetics with respect to energetic and photonic efficiency. The method could greatly facilitate comparisons across different laboratories when quantifying photocatalytic reactivity and efficiency, which would aid in standardizing bench-scale photocatalysis testing.

Four release tests exhibit variable silver stability from nanoparticle-modified reverse osmosis membranes.

Modification of polyamide reverse osmosis (RO) membranes with silver nanoparticles (AgNP) may effectively control biofouling. While silver leaching tests are usually performed during membrane development, the lack of common testing protocols limits cross-comparison among different labs. We compare four release tests to quantify the release of dissolved and nanoparticulate silver from polyamide RO membranes prepared through in situ surface functionalization: (1) batch immersion, (2) dead-end filtration, (3) cross-flow filtration, and (4) low-pressure water jetting. For the first time, we demonstrated the possibility of AgNP release to membrane-treated water through direct AgNP detachment. When using Nanopure water as an extraction solution, water jetting resulted in the fastest silver mass release, while dead-end filtration caused the slowest release based on the initial release kinetic data. Dead-end filtration exhibited silver mass release an order of magnitude lower than the other three tests. Although cross-flow filtration may best represent the RO membrane operation, quantifying silver release suffers from poor mass balance due to the adsorption of dissolved silver by various reactor components and large volumes of water usage. A commonly applied batch immersion method was low cost and easily performed, but may not induce hydraulic shear sufficient for AgNP detachment from a RO membrane. The information on silver release behavior may depend on the specific test, which is important for assessing antimicrobial efficacy and service life of the nanoparticle-functionalized membranes.

Rapid Mobilization of Noncrystalline U(IV) Coupled with FeS Oxidation.

The reactivity of disordered, noncrystalline U(IV) species remains poorly characterized despite their prevalence in biostimulated sediments. Because of the lack of crystalline structure, noncrystalline U(IV) may be susceptible to oxidative mobilization under oxic conditions. The present study investigated the mechanism and rate of oxidation of biogenic noncrystalline U(IV) by dissolved oxygen (DO) in the presence of mackinawite (FeS). Previously recognized as an effective reductant and oxygen scavenger, nanoparticulate FeS was evaluated for its role in influencing U release in a flow-through system as a function of pH and carbonate concentration. The results demonstrated that noncrystalline U(IV) was more susceptible to oxidation than uraninite (UO2) in the presence of dissolved carbonate. A rapid release of U occurred immediately after FeS addition without exhibiting a temporary inhibition stage, as was observed during the oxidation of UO2, although FeS still kept DO levels low. X-ray photoelectron spectroscopy (XPS) characterized a transient surface Fe(III) species during the initial FeS oxidation, which was likely responsible for oxidizing noncrystalline U(IV) in addition to oxygen. In the absence of carbonate, however, the release of dissolved U was significantly hindered as a result of U adsorption by FeS oxidation products. This study illustrates the strong interactions between iron sulfide and U(IV) species during redox transformation and implies the lability of biogenic noncrystalline U(IV) species in the subsurface environment when subjected to redox cycling events.

Influence of iron sulfides on abiotic oxidation of UO2 by nitrite and dissolved oxygen in natural sediments.

Iron sulfide precipitates formed under sulfate reducing conditions may buffer U(IV) insoluble solid phases from reoxidation after oxidants re-enter the reducing zone. In this study, sediment column experiments were performed to quantify the effect of biogenic mackinawite on U(IV) stability in the presence of nitrite or dissolved oxygen (DO). Two columns, packed with sediment from an abandoned U contaminated mill tailings site near Rifle, CO, were biostimulated for 62 days with an electron donor (3 mM acetate) in the presence (BRS+) and absence (BRS−) of 7 mM sulfate. The bioreduced sediment was supplemented with synthetic uraninite (UO2(s)), sterilized by gamma-irradiation, and then subjected to a sequential oxidation by nitrite and DO. Biogenic iron sulfides produced in the BRS+ column, mostly as mackinawite, inhibited U(IV) reoxidation and mobilization by both nitrite and oxygen. Most of the influent nitrite (0.53 mM) exited the columns without oxidizing UO2, while a small amount of nitrite was consumed by iron sulfides precipitates. An additional 10-day supply of 0.25 mM DO influent resulted in the release of about 10% and 49% of total U in BRS+ and BRS­ columns, respectively. Influent DO was effectively consumed by biogenic iron sulfides in the BRS+ column, while DO and a large U spike were detected after only a brief period in the effluent in the BRS­ column.

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.

Nano-FeS inhibits UO2 reoxidation under varied oxic conditions.

Bioreductive in situ treatment of U-contaminated groundwater can convert soluble U(VI) species to immobile reduced U(IV) solid phases such as UO2(s) to contain U movement. Once active bioremediation is halted, UO2 may be subsequently reoxidized if oxidants such as oxygen enter the reducing zone. However, iron sulfide minerals that form during bioreduction may serve as electron sources or oxygen scavengers and inhibit UO2 reoxidation upon oxygen intrusion. In this study, flow-through reactor experiments examined the abiotic kinetics of UO2 oxidative dissolution in the presence of oxygen and nanoparticulate FeS as a function of pH, dissolved oxygen (DO) concentration, and FeS content. The UO2 dissolution rates in the presence of FeS were over 1 order of magnitude lower than those in the absence of FeS under otherwise comparable oxic conditions. FeS effectively scavenged DO and preferentially reacted with oxygen, contributing to a largely unreacted UO2 solid phase during an "inhibition period" as determined by X-ray absorption spectroscopy (XAS). The removal of DO by FeS was significant but incomplete during the inhibition period, resulting in surface-oxidation-limited dissolution and greater UO2 dissolution rate with increasing influent DO concentration and decreasing FeS content. Although the rate was independent of solution pH in the range of 6.1-8.1, the length of the inhibition period was shortened by substantial FeS dissolution at slightly acidic pH. The reducing capacity of FeS was greatest at basic pH where surface-mediated FeS oxidation dominated.

[Effects of tillage mode and nitrogen application rate on nitrogen use efficiency of wheat in a farming-pasture zone of North China].

A field experiment was conducted in a farming-pasture zone in Chifeng City of Inner Mongolia Autonomous Region, North China to investigate the effects of different tillage modes and nitrogen (N) application rates on the grain yield and nitrogen use efficiency (NUE) of winter wheat. The results showed that long term conservation tillage increased the wheat NUE by 3% -4%, and decreased the environmental pollution by fertilizer N. Conservation tillage promoted the N absorption by wheat, and increased the grain yield. When the N application rate increased from 120 kg hm-2 to 360 kg . hm-2, the NUE decreased from 36. 5% to 26% , fertilizer N loss increased by about 5% , i. e. , the corresponding N loss was increased from 60 kg hm-2 to 200 kg hm-2, and the environmental N pollution increased markedly. The wheat NUE of the residual N in last season was less affected by tillage mode, but more affected by the N application rate in last season, with an overall tendency of the higher the N application rate in last season, the lower the NUE and the more the fertilizer N loss. After two seasons' wheat planting, the proportion of the total nitrogen recovery by the wheat-soil system was about 44% -50%, among which, the residual N in soil occupied about 13% -18% of applied N.