Fe/P concentration ratio in Mozhaisk reservoir deposits as an indicator of phosphate sorption

A relationship is established between within-year variations in Fe_tot/P_tot ratio in pore solution and the concentration of organic P compounds in solid phase. The passage of the FeOOH-P system in the silts of the Mozhaisk Reservoir from the aerobic conditions to aerobic is generally accompanied by dissolution of FeOOH and desorption of phosphates. The change from anaerobic conditions to aerobic during the spring and autumn seasons exhibits hysteresis in oxidation of Fe compounds in silts, such that variations in Fe/P ratio in the solid phase of silt do not agree with those in the pore solution.     

Winter methane dynamics beneath ice and in snow in a temperate poor fen

The influence of winter on methane (CH₄) stored in pore water and emitted through snow was investigated in a temperate poor fen in New Hampshire over two winters. Methane accumulated beneath ice layers (1 cm) deposited by freezing rain, resulting in snow-pore air mixing ratios as high as 140 ppmv during the first winter and 600 ppmv during the second. An early winter snow crust of 300 kg m^?3 caused no discontinuity in a linear mixing ratio profile and therefore was not observed to retard snowpack emissions. Methane concentration-depth profiles in pore water steepened and concentrations increased by as much as 400 μM at the 10 and 20 cm depths as the ice cover formed. This suggests that the peat-ice cover plays an important part in CH₄ build-up in pore water by limiting the transport of gases between the peat and the atmosphere. Pore water concentrations gradually declined through late winter. The seasonality of dissolved CH₄ in pore water over two winters and one summer showed an average annual amplitude of 1.3 gCH₄m^?2 (25–75cm depth range), with a winter maximum of 4.7gCH₄m^?2. Emissions during the winter with average snowfall accounted for a larger percentage (9.2% in 1993–1994) of total annual emission than the winter with below-average snowfall and warmer air temperature (2% in 1994–1995). Emissions averaged 56 and 26mg m^?2 day^?1 during the first and second winter (December, January and February), respectively.     

High sulphate concentrations in squeezed Boom Clay pore water: evidence of oxidation of clay cores

Pore water has been extracted from Boom Clay by mechanical squeezing. Clay cores were obtained from various boreholes, all drilled at the SCK·CEN domain (Mol, Belgium).In contrast to pore water collected from piezometers, high sulphate concentrations are measured in the squeezed pore water. The lowest sulphate concentrations (<60 mg/l) were measured in pore waters squeezed immediately after drilling. Higher sulphate concentrations were often measured in the pore water when the clay cores were preserved for some time (generally <500 mg/l SO₄²⁻, but sometimes up to 20,000 mg/l SO₄²⁻). Nevertheless, a relation between preservation time and sulphate content could not be retrieved. However, major ion concentrations were obviously correlated with the sulphate content in the squeezed waters. The observed evolution in chemical composition were explained by water–rock interactions considering the pyrite oxidation and the subsequent ion exchange and mineral dissolution reactions.     

Chemical evidence of water movements in the deepest part of Lake Leman (Lake Geneva)

Regular surveys of bottom water chemistry (SiO₂, O₂, Fe, Mn) have been carried from 1978 to 1986 in the deepest 30 m of Lake Léman (max. depth 309 m) including interface waters sampled with a Jenkins Mortimer corer. When compared to normal chemical gradients near bottom, i.e. O₂ decrease and SiO₂ increase, three types of anomalies (lens, interface, and behaviour) have been observed on O₂ and SiO₂, the most sensitive chemical species. These anomalies were found throughout the year, in several stations of the deepest part of the lake and even along the slope of the lake basin. Major anomalies (O₂ + 3 to 10 mg ·l^–1; SiO₂ -1 to 2 mg·l^–1) were generally found at the sediment water interface and may extend 10–20 m above the sediment and last 10 weeks. Others marked lens anomalies could be observed for 3 to 4 months. Several mechanisms are probably responsible for this injection of surface waters along the lake slope (accumulation of turbid water on lake banks after severe windstorms; river density currents due to temperature and/or turbidity difference with lake waters). These water-inputs do not represent important volumes ( 1% total lake volume) but, when occuring at the interface, they ensure a sufficient oxygen level to prevent diffusion of phosphate and ammonia from pore waters when winter lake overturns do not reach bottom layers (from 1972 to 1980). Complete overturns, as observed in 1980/81, are connected with major interface anomalies (bottom O₂ moves up from 2 to 10 mg·l^–1) occuring before surface mixing reaches the deepest layers.     

Spatial distribution of phosphorus forms in sediments in the Gulf of Gdańsk (southern Baltic Sea)

The paper analyses the concentrations of total phosphorus and its forms in sediments from the Gulf of Gdańsk on the basis of studies conducted at 25 sampling sites in 2001–05. The phosphorus speciation analysis was performed by sequential extraction. The extensive spatial variability of P_tot concentrations and speciation was found to be dependent on the physicochemical properties of the sediments, the oxygen conditions in the water and sediments, and the depth of the water column above the sediment surface. In the coastal zone, the sedimentation of riverine suspended matter and the sorption and chemisorption processes exert a considerable influence on P speciation. Over 70% of variation of total phosphorus concentration in sediments in the Gulf of Gdańsk could be explained by changes of proportion of fine fraction of sediments (grain size <0.0625 mm). Maximum P_tot concentrations were recorded in clays and silts in the deep water, stratified part of the Gulf of Gdańsk. In the coastal zone, where sandy sediments are dominant, phosphorus concentrations were much lower; this was due to the considerable dynamics of the bottom water and intensive sea floor transport. P_tot concentrations in the Gulf of Gdańsk sediments ranged from 1.75 to 957.17 μmol g⁻¹ d.w. Of all the forms of phosphorus, the highest concentrations were found for organic phosphorus (Org-P). Of its inorganic forms, the highest concentrations were of phosphorus bound to clay minerals and aluminium oxides (NaOH-P), the lowest ones were of loosely bound phosphorus (NaCl-P). On the basis of determinations of total phosphorus concentrations in sediments of a given type and the available data on the seabed areas covered by particular sediment types in the Gulf of Gdańsk, the mass of total phosphorus in the surficial sediment layer (0–2 cm) was estimated at ca. 15.6×10³ tonnes.     

太湖草源性"湖泛"水域沉积物营养盐释放估算

于太湖草源性"湖泛"暴发期,采集柱状沉积物并应用peeper被动采样装置获得"湖泛"区原位沉积物间隙水.泥水样品分析表明:"湖泛"发生水域表层(0~7 cm)沉积物的含水率、孔隙度和有机质含量均明显高于对照区,其中有机质含量更是对照区样品的4倍左右,沉水植物残体促使表层沉积物物化性质改变的作用明显;"湖泛"发生水域表层沉积物间隙水中铵态氮(NH+4-N)、溶解性反应磷(SRP)及Fe2+含量远高于未发生区,植物残体降解对沉积物厌氧环境的营造显著.运用分子扩散模型对沉积物释放通量估算:"湖泛"发生区沉积物NH+4-N、SRP和Fe2+的释放速率分别是对照区的49.8、15.3和123.1倍.研究认为,草源性"湖泛"水体氮、磷等营养物含量升高的主要原因是沉积物的释放,而"湖泛"所营造的厌氧环境是氮、磷释放急剧增加的主要驱动因素.     

Studying chemical exchange across the water-bed interface in Golubaya Bay,the Black Sea

The results of first application of automated bottom station for studying chemical exchange across the water-bed interface (IO RAS lander) were obtained for the case of sediments in the northwestern Black Sea near Gelendzhik (Golubaya Bay). The lander is equipped with bottom chambers, sensors of environmental characteristics, and samplers, allowing fluxes of chemical elements through the water-bed interface to be determined directly. Bottom sediments were represented by organogenous silts containing hydrogen sulfide. The high activity of diagenetic processes resulted in a higher consumption of O₂ by sediments (the flux from the bottom water was 130 mM/(m²/day), high fluxes of biogenic elements (P, Si,) and metals (Mn, Fe). At the same time, fluxes across the water-bed interface were evaluated based on the difference between concentrations of these elements in the near-bed and silt water (Fick’s law). The obtained data demonstrate the priority of direct measurements of fluxes (lander) over the calculation method. The first application of the lander suggests the need for methodological improvements both of engineering character and in chemical-analytical support.     

A semiempirical mathematical model of the secondary pollution of water bodies by soluble iron and manganese forms

A semiempirical mathematical model of iron and manganese migration from bottom sediments into the water mass of water bodies has been proposed based on some basic regularities in the geochemistry of those elements. The entry of dissolved forms of iron and manganese under aeration conditions is assumed negligible. When dissolved-oxygen concentration is <0.5 mg/L, the elements start releasing from bottom sediments, their release rate reaching its maximum under anoxic conditions. The fluxes of dissolved iron and manganese (Me) from bottom sediments into the water mass (J _Me) are governed by the gradients of their concentrations in diffusion water sublayer adjacent to sediment surface and having an average thickness of h = 0.025 cm: \({J_{Me}} = - {D_{Me}}\frac{{{C_{Me\left( {ss} \right)}} - {C_{Me\left( w \right)}}}}{h}\) (D _Me ≈ 1 × 10^–9 m²/s is molecular diffusion coefficient of component Me in solution; C _Me(ss) and C _Me(w) ≈ 0 are Me concentrations on sediment surface, i.e., on the bottom boundary of the diffusion water sublayer, and in the water mass, i.e., on the upper boundary of the diffusion water sublayer). The value of depends on water saturation with dissolved oxygen (\({\eta _{{O_2}}}\)) in accordance with the empiric relationship \({C_{Me\left( {ss} \right)}} = \frac{{C_{_{Me\left( {ss} \right)}}^{\max }}}{{1 + k{\eta _{{O_2}}}}}\) (k is a constant factor equal to 300 for iron and 100 for manganese; C _Me(ss) ^max is the maximal concentration of Me on the bottom boundary of the diffusion water sublayer with C _Fe(ss) ^max ≈ 200 μM (11 mg/L), and C _Mn(ss) ^max ≈ 100 μM (5.5 mg/L).     

Tetrahedral occupancy of ferric iron in (Mg,Fe)O: Implications for point defects in the Earth's lower mantle

We investigated the concentration and site occupation of ferric iron (Fe³⁺) in (Mg,Fe)O to understand the influence of point defects on transport properties such as atomic diffusion, electrical conductivity and viscosity. We conducted Mössbauer spectroscopy of (Mg_0.8Fe_0.2)O single crystals synthesized at temperatures from 1673 to 2273 K and pressures from 5 to 15 GPa with Re–ReO₂ and Mo–MoO₂ oxygen fugacity buffers. The isomer shift of the Mössbauer spectra suggests that Fe³⁺ occupies mostly the tetrahedral site at reduced conditions and both the octahedral and tetrahedral sites at oxidized conditions. We formulate a thermodynamic model of point defect dissolution in (Mg,Fe)O which suggests that unassociated tetrahedral Fe³⁺ is more stable than unassociated octahedral Fe³⁺ at high-pressure and low oxygen fugacity due to the effect of configurational entropy. The pressure dependence of Fe³⁺ concentration indicates a change in the dominant site occupancy of Fe³⁺: (1) Fe³⁺ in the tetrahedral site, (2) Fe³⁺ in the octahedral site, and (3) defect clusters of Fe³⁺ and cation vacancy, in the order of increasing oxygen fugacity and decreasing pressure. This is in reasonable agreement with previously reported experiments on Fe³⁺ concentration, Mg–Fe interdiffusivity and electrical conductivity. We consider it plausible that (Mg,Fe)O accommodates Fe³⁺ in the tetrahedral site down to the lower mantle. Based on our results and available experimental data, we discuss the solubility competition between Fe³⁺ and protons (H⁺), and its implications for transport properties in the lower mantle.     

Iron oxidation state in lower mantle mineral assemblages: I. Empirical relations derived from high-pressure experiments

The oxidation state of iron can significantly influence the physical and chemical properties of lower mantle minerals. To improve methods for estimation of Fe³⁺/∑Fe, synthetic assemblages of (Mg,Fe)(Si,Al)O₃ perovskite and (Mg,Fe)O ferropericlase were synthesised from oxide starting mixtures in Re or Fe capsules at 26 GPa and 1650-1850 °C using a multianvil press. (Mg,Fe)(Si,Al)O₃ majorite was also present in some of the run products. Both electron energy loss spectra (EELS) and Mössbauer spectra were measured for each run product, and a robust fitting method was developed for Mössbauer spectra using EELS results as a standard that enabled Fe³⁺/∑Fe of (Mg,Fe)(Si,Al)O₃ perovskite to be determined from Mössbauer spectra of multiphase assemblages. There is a close to linear variation between Fe³⁺/∑Fe and Al concentration in (Mg,Fe)(Si,Al)O₃ perovskite, independent of oxygen fugacity. The concentration of Fe³⁺ in (Mg,Fe)O increases with increasing iron concentration along curves of constant oxygen fugacity, where higher oxygen fugacity stabilises greater Fe³⁺ concentrations. Fe²⁺/Mg partition coefficients calculated from chemical composition data corrected for measured Fe³⁺/∑Fe showed values nearly identical within experimental error for all samples, and independent of Al concentration and oxygen fugacity. Simple empirical relations were derived to calculate Fe³⁺/∑Fe in (Mg,Fe)(Si,Al)O₃ perovskite and (Mg,Fe)O ferropericlase samples for which no Mössbauer or EELS data were available, and tested by applying them to calculation of Fe²⁺/Mg partition coefficients from literature data for (Mg,Fe)(Si,Al)O₃ perovskite-(Mg,Fe)O assemblages where only total iron concentrations had been measured. Results showed Fe²⁺/Mg partition coefficients that were equal to existing values within experimental error, hence confirming the validity of the empirical relations.     

Seasonal variation in sediment transport on the Russian River shelf,California

Near-bottom currents, light transmission and scattering, and bottom pressure were measured with GEOPROBE tripods and vector-averaging current meters during June 1979 to April 1980 on the central shelf 10 km west of the Russian River, California. The instruments were located on the mid-shelf mud belt composed of bimodal sandy clayey silts contributed principally by the Russian River. During the summer season of persistent northwesterly, upwelling-favorable winds, the average and maximum current speeds 5 m above the bottom were 11 and 31 cm s^?1, respectively. The mean (subtidal) flow at 5 m above bottom was poleward and slightly offshore at about 6 cm s^?1. The strongest wave-generated bottom currents were about 10 cm s^?1, but oscillatory velocities > 5 cm s^?1 were infrequent. Suspended-matter concentrations, derived from the optical data at 1.9 m above the bottom, ranged from 1 to 6 mg l^?1. The optical data show that the currents and waves were generally below threshold levels for sediment erosion through the summer. In contrast, during the autumn and, particularly, the winter months, the average and maximum concentrations of suspended matter increased substantially. The increases were primarily caused by larger waves from distant storms and short intervals of strong currents associated with local storms and, secondarily, by the large seasonal flow of the Russian River. Wind-driven and wave-generated bottom currents were as large as 37 and 45 cm s^?1, respectively, during local storms in December 1979 and February 1980. Suspended-matter concentrations averaged about 7 mg l^?1 during non-storm winter periods, but increased to nearly 150 mg l^?1 during a December storm. Estimates of suspended-matter flux near the bottom show that the local winter storms, which had a combined duration of about 12 days, could account for 30 to 50% of the total annual suspended-sediment transport at the mid-shelf site. Although intervals of large swell were at times superimposed on southward advective currents, the major sediment-transport events were caused by strong southerly winds that produced poleward bottom currents with a significant offshore component. The primary aspects of the distribution of modern sediments on this shelf are in good agreement with the observed poleward transport.     

Cyclic resistance and liquefaction behavior of silt and sandy silt soils

The liquefaction behavior and cyclic resistance ratio (CRR) of reconstituted samples of non-plastic silt and sandy silts with 50% and 75% silt content are examined using constant-volume cyclic and monotonic ring shear tests along with bender element shear wave velocity (V_s) measurements. Liquefaction occurred at excess pore water pressure ratios (r_u) between 0.6 and 0.7 associated with cumulative cyclic shear strains (γ) of 4% to 7%, after which cyclic liquefaction ensued with very large shear strains and excess pore water pressure ratio (r_u>0.8). The cyclic ring shear tests demonstrate that cyclic resistance ratio of silt and sandy silts decreases with increasing void ratio, or with decreasing silt content at a certain void ratio. The results also show good agreement with those from cyclic direct simple shear tests on silts and sandy silts. A unique correlation is developed for estimating CRR of silts and sandy silts (with more than 50% silt content) from stress-normalized shear wave velocity measurements (V_s1) with negligible effect of silt content. The results indicate that the existing CRR–V_s1 correlations would underestimate the liquefaction resistance of silts and sandy silt soils.     

Iron compound occurrence forms in freshwater deposits: Analytical review

Data on the occurrence forms and concentrations of crystalline, amorphous, and organic Fe in the solid phase of silts in lakes and reservoirs, as well as data on concentrations of Fe forms in the pore solution of deposits are analyzed. The major mechanisms of Fe compound transformations are briefly considered.     

Measuring streambank erosion due to ground water seepage: correlation to bank pore water pressure,precipitation and stream stage

Limited information exists on one of the mechanisms governing sediment input to streams: streambank erosion by ground water seepage. The objective of this research was to demonstrate the importance of streambank composition and stratigraphy in controlling seepage flow and to quantify correlation of seepage flow/erosion with precipitation, stream stage and soil pore water pressure. The streambank site was located in Northern Mississippi in the Goodwin Creek watershed. Soil samples from layers on the streambank face suggested less than an order of magnitude difference in vertical hydraulic conductivity (K_s) with depth, but differences between lateral K_s of a concretion layer and the vertical K_s of the underlying layers contributed to the propensity for lateral flow. Goodwin Creek seeps were not similar to other seeps reported in the literature, in that eroded sediment originated from layers underneath the primary seepage layer. Subsurface flow and sediment load, quantified using 50 cm wide collection pans, were dependent on the type of seep: intermittent low‐flow (LF) seeps (flow rates typically less than 0·05 L min^?1), persistent high‐flow (HF) seeps (average flow rate of 0·39 L min^?1) and buried seeps, which eroded unconsolidated bank material from previous bank failures. The timing of LF seeps correlated to river stage and precipitation. The HF seeps at Goodwin Creek began after rainfall events resulted in the adjacent streambank reaching near saturation (i.e. soil pore water pressures greater than ?5 kPa). Seep discharge from HF seeps reached a maximum of 1·0 L min^?1 and sediment concentrations commonly approached 100 g L^?1. Buried seeps were intermittent but exhibited the most significant erosion rates (738 g min^?1) and sediment concentrations (989 g L^?1). In cases where perched water table conditions exist and persistent HF seeps occur, seepage erosion and bank collapse of streambank sediment may be significant. Copyright © 2007 John Wiley & Sons, Ltd.     

Seasonal and diurnal variation of electron and iron concentrations at the meteor heights above Arecibo

We report observations of seasonal and local time variation of the averaged electron and iron concentrations, as well as simultaneous measurements of the two species, above the Arecibo Observatory (18.35°N, 66.75°N), Puerto Rico. The average Fe profile between 21:00 and 24:00 LT has a single peak at about 85 km with the exception of the summer when an additional peak exists at about 95 km. The higher Fe peak in the summer is correlated with higher electron concentrations in this season. The three nights of simultaneous measurements of electron and iron concentrations show that narrow layers of Fe and electrons are well correlated. Comparison of the climatological and simultaneous Fe and electron data suggests that recombination of Fe⁺ plays an important role in determining the Fe profile in the upper part of the Fe layer. Above 93 km, the Fe concentration appears to increase after sunset if the electron concentration exceeds about 4000 electrons cm⁻³. The average rate of Fe production is about 0.1 atom cm⁻³ s⁻¹ for all seasons at 100 km in the early evening hours. A chemical model reveals that the concentration of Fe⁺ must be 50–80% of the total ionization over Arecibo for typical equinox conditions to explain the observed rate of Fe production. These high relative Fe⁺ concentrations are consistent with in situ observations that Fe⁺ is usually the dominant ion in sporadic E layers in the nighttime lower E region. This suggests that the source of Fe⁺ is provided by sporadic E layers descending over Arecibo after sunset. The Fe density between 80 and 85 km decreases during the night, for all seasons. This is attributed to the formation of stable molecular Fe species, such as FeOH, due to the increase in O₃ and decrease in atomic O and H during the night at these altitudes.     

The solubility of iron sulphides in synthetic and natural waters at ambient temperature

A critical evaluation of literature values for the solubility products, K _sp ^NBS = [Fe²⁺][HS^–] Fe²⁺ HS^– (H _NBS ⁺ )^–1, of various iron sulphide phases results in consensus values for the pKs of 2.95 ± 0.1 for amorphous ferrous sulphide, 3.6 ± 0.2 for mackinawite, 4.4 ± 0.1 for greigite, 5.1 ± 0.1 for pyrrhotite, 5.25 ± 0.2 for troilite and 16.4 ± 1.2 for pyrite.Where the analogous ion activity products have been measured in anoxic freshwaters in which there is evidence for the presence of solid phase FeS, the values lie within the range of 2.6–3.22, indicating that amorphous iron sulphide is the controlling phase. The single value for a groundwater of 2.65 (2.98 considering carbonate complexation) agrees. In seawater four values range between 3.85 to 4.2, indicating that mackinawite or greigite may be the controlling phase. The single low value of 2.94 is in a situation where particularly high fluxes of Fe (II) and S (–II) may result in the preferential precipitation of amorphous iron sulphide. Formation of framboidal pyrite in these sulphidic environments may occur in micro-niches and does not appear to influence bulk concentrations. Calculations show that the formation of Fe₂S₂ species probably accounts for very little of the iron or sulphide in most natural waters. Previously reported stability constants for the formation of Fe (HS)₂ and (Fe (HS)₃)^– are shown to be suspect, and these species are also thought to be negligible in natural waters. In completely anoxic pore waters polysulphides also have a negligible effect on speciation, but in tidal sediments they may reach appreciable concentrations and lead to the direct formation of pyrite. Concentrations of iron and sulphide in pore waters can be controlled by the more soluble iron sulphide phase. The change in the IAP with depth within the sediment may reflect ageing of the solid phase or a greater flux of Fe (II) and S (–II) nearer the sediment surface. This possible kinetic influence on the value of IAPs has implications for their use in geochemical studies involving phase formation.     

Early diagenesis in sediments from the eastern equatorial Pacific,I. Pore water nutrient and carbonate results

Interstitial waters were extracted from cores at three locations in the eastern equatorial Pacific and analyzed for nutrients, dissolved carbonate species, Mn and Fe. From the depth variation in pore water chemistry, we infer that organic matter oxidation reactions occur with depth in the following sequence: O₂ reduction, NO₃^? and MnO₂ reduction, and then ferric iron reduction. From NO₃^? results we infer that O₂ is largely or totally consumed within the top few centimeters of sediment. NO₃^? is completely reduced at a sediment depth of 20 cm at a site near the crest of the East Pacific Rise, but is preserved at levels of 20–30 μmol/kg at 40 cm depth at a Guatemala Basin site.We have calculated the alkalinity for pore water samples assuming ions diffuse according to relative ionic diffusion coefficients, that the stoichiometry of organic matter oxidation reactions is that of “Redfield” organic matter, and that the pore waters are saturated throughout with respect to CaCO₃. The measured alkalinity increase is only about half of the predicted value. The difference is probably a result of either enhanced mixing of the pore water in the top few centimeters of sediments by biological or physical processes, or the occurrence of an inorganic reaction which consumes alkalinity.At depths of oxygen and nitrate reduction in the sediments, the ion concentration product of CaCO₃ is the same, within the analytical error, as the solubility product of Ingle et al. [34] at 1 atm and 4°C. This result indicates CaCO₃ resaturation on pressure change during coring. Where pore water Mn concentrations become measurable, the ion concentration product increases, indicating either supersaturation with respect to calcite or that another phase is controlling the carbonate solubility.     

Fe(II)/Fe(III) ratio in silt pore solution in the Mozhaisk Reservoir

Relationships between within-year variations in Fe(II)/Fe(III) ratio in silt pore solution were obtained. The samples to be examined were taken from silts of the channel zone of the Krasnovidovo Pool, the Mozhaisk Reservoir. Specific features of the within-year redistribution of Fe forms in the pore solution of the sediment column were identified.     

Relevant Reducing Agents in Remediation Fe0/H2O Systems

Metallic iron (Fe⁰) is often reported as a reducing agent for environmental remediation. There is still controversy as to whether Fe⁰ plays any significant direct role in the process of contaminant reductive transformation. The view that Fe⁰ is mostly a generator of reducing agents (e.g. H, H₂ and Fe^II) and Fe oxyhydroxides has been either severely refuted or just tolerated. The tolerance is based on the simplification that, without Fe⁰, no secondary reducing agents could be available. Accordingly, Fe⁰ serves as the original source of electron donors (including H, H₂ and Fe^II). The objective of this communication is to refute the named simplification and establish that quantitative reduction results from secondary reducing agents. For this purpose, reports on aqueous contaminant removal by Al⁰, Fe⁰ and Zn⁰ are comparatively discussed. Results indicated that reduction may be quantitative in aqueous systems containing Fe⁰ and Zn⁰ while no significant reduction is observed in Al⁰/H₂O systems. Given that Al⁰ is a stronger reducing agent than Fe⁰ and Zn⁰, it is concluded that contaminant reduction in Fe⁰/H₂O systems results from synergic interactions between H/H₂ and Fe^II within porous Fe oxyhydroxides. This conclusion corroborates the operating mode of Fe⁰ bimetallics as H/H₂ producing systems for indirect contaminant reduction.     

The soil nitrogen dynamics in an inland salt marsh as affected by various experimental water levels

An 87‐day incubation experiment was conducted to investigate the effects of various water levels on the soil nitrogen content in an inland salt marsh of Xianghai, China. Soil samples were collected from depths in the range of 0 to 30 cm at 10‐cm intervals after 15, 54 and 87 days of incubation. The total nitrogen (TN), available nitrogen, nitrate nitrogen (NO₃^?‐N) and ammonium nitrogen (NH₄⁺‐N) contents and the nitrogen activation coefficient (NAC) were measured to investigate the soil nitrogen dynamics under three inundation treatments (?5, 0 and 5 cm). The results showed that TN was generally accumulated in all soil layers and exhibited a peak value in subsurface soils at the ?5‐cm inundation level at the end of the incubation period, whereas no obvious changes were observed in the TN contents in the bottom soils at the three inundation levels. Compared with the initial contents in the soils, the NO₃^?‐N and NH₄⁺‐N contents were significantly reduced in all soil layers at the 0‐cm and the 5‐cm inundation levels at the end of the incubation period. In general, the available nitrogen content was higher in the surface soils compared with the deeper soils at all inundation levels. The NAC values exhibited a pronounced change in the subsurface soils at all inundation levels, decreasing significantly from 0.17 to 0.04 at the 5‐cm inundation level; however, a higher mean NAC value (0.126) was found over the incubation period at the 0‐cm inundation level compared with the 5‐cm and the ?5‐cm inundation levels. Nitrogen was released from plants into the soil under all inundation treatments over the incubation period. The soil organic matter exerted positive loadings on the TN and NO₃^?‐N contents and negative loadings on the NH₄⁺‐N content at all inundation levels, and the water content generally exhibited a negative relationship with all forms of nitrogen. Additionally, the total phosphorus content exhibited a positive correlation with the TN and NO₃^?‐N contents. Copyright © 2013 John Wiley & Sons, Ltd.