Investigations on different positions of electrodes and their effects on the distribution of Cr at the water sediment interface

In most mining areas, significant concentrations of metals such as Pb, Cu, Cr, Zn, Cd, Fe, Mn, Co, Ni, Hg, Ar, halogenated organic compounds and radionuclides are found. Of those, Cr is one of the well-known heavy metals that forms toxic species. It is necessary to study the mobilization and accumulation of Cr at the sediment water interface in an electric field at varying different positions and conditions of the electrode arrangement. The tests were carried out with a natural sediment containing heavy metals from the river Weisse Elster (Germany). The electrokinetic experiments have been performed in columns filled with sediment using electrodes made of conductive polymers (polyethylene with carbon black) at a maximum current density of 0.5 mA/cm². The experimental results suggest that the mobilization and accumulation of Cr highly depends on chemical factors for e.g., pH value, redox potential, respectively redox status and the content of Fe, A1 and organic matter in the soil or sediment. The sorption of Cr (III, VI) is very high in the pH range > 4.5. As expected, a high mobilization of Cr (III, VI) was seen in the case of the experiments with the anode at the sediment, because the pH value was lower than mentioned above. On the opposite, the best conditions for the Cr (III, VI) immobilization is high pH values (cathode at the sediment).     

Electron transfer at the mineral/water interface: Selenium reduction by ferrous iron sorbed on clay

The mobility and availability of the toxic metalloid selenium in the environment are largely controlled by sorption and redox reactions, which may proceed at temporal scales similar to that of subsurface water movement under saturated or unsaturated conditions. Since such waters are often anaerobic and rich in Fe²⁺, we investigated the long-term (?1 month) kinetics of selenite sorption to montmorillonite in the presence of Fe²⁺ under anoxic conditions. A synthetic montmorillonite was used to eliminate the influence of structural Fe. In the absence of aqueous Fe²⁺, selenite was sorbed as outer-sphere sorption complex, covering only part of the positive edge sites, as verified by a structure-based MUSIC model and Se K-edge XAS (X-ray absorption spectroscopy). When selenite was added to montmorillonite previously equilibrated with Fe²⁺ solution however, slow reduction of Se and formation of a solid phase was observed with Se K-edge XANES (X-ray absorption near-edge spectroscopy) and EXAFS (extended X-ray absorption fine-structure) spectroscopy. Iterative transformation factor analysis of XANES and EXAFS spectra suggested that only one Se reaction product formed, which was identified as nano-particulate Se(0). Even after one month, only 75% of the initially sorbed Se(IV) was reduced to this solid species. Mössbauer spectrometry revealed that before and after addition and reduction of Se, 5% of total sorbed Fe occurred as Fe(III) species on edge sites of montmorillonite (≈2 mmol kg⁻¹). The only change observed after addition of Se was the formation of a new Fe(II) species (15%) attributed to the formation of an outer-sphere Fe(II)-Se sorption complex. The combined Mössbauer and XAS results hence clearly suggest that the Se and Fe redox reactions are not directly coupled. Based on the results of a companion paper, we hypothesize that the electrons produced in the absence of Se by oxidation of sorbed Fe(II) are stored, for example by formation of surface H₂ species, and are then available for the later Se(IV) reduction. The slow reaction rate indicates a diffusion controlled process. Homogeneous precipitation of an iron selenite was thermodynamically predicted and experimentally observed only in the absence of clay. Interestingly, half of Fe was oxidized in this precipitate (Mössbauer). Since DFT calculations predicted the oxidation of Fe at the water-FeSe solid interface only and not in the bulk phase, we derived an average particle size of this precipitate which does not exceed 2 nm. A comparison with the Mössbauer and XAS spectra of the clay samples demonstrates that such homogenous precipitation can be excluded as a mechanism for the observed slow Se reduction, emphasizing the role of abiotic, heterogeneous precipitation and reduction for the removal of Se from subsurface waters.     

Temporal behavior of manganese and iron in a sandy coastal sediment exposed to water column anoxia

The influence of bottom water anoxia on manganese (Mn), iron (Fe), and sulfur (S) biogeochemistry was examined in defaunated sandy sediment from Kærby Fed, Denmark, under controlled laboratory incubations. The initial narrow peaks and steep gradients in solid Mn(IV) and Fe(III) as well as porewater Mn²⁺ and Fe²⁺ observed in the upper 2–5 cm of the sediment indicate rapid metal reduction-oxidation cycles under oxic conditions in the overlying water. The fe zones were generally displaced about 0.5 cm downward compared with the Mn zones due to differences in reactivity. Mn(IV) was reduced and gradually disappeared first (within 10 d) when the sediment was exposed to anoxia followed by reduction and disappearance of Fe(III) (day 7 to 18). The associated loss of Mn²⁺ to the overlying water was most rapid during the first 15 d, whereas the Fe²⁺ efflux initiated around day 10, and after a few days with modest rates the efflux peaked around day 20. A considerable portion of the total Mn (26%) and Fe (23%) inventory initially present in the sediment was lost by efflux after about 1 mo of anoxia. The ability of the sediment to retain upward diffusion of H₂S gradually disappeared in a temporal pattern closely related to the changes in pool size of the reactive Mn and Fe present. The total metal pool in Kærby Fed sediment prevented H₂S release to the overlying water for at least a month of anoxia. It is speculated that external supplies from the overlying water allows a rapid refuelling of surface Mn and Fe oxides in the field when oxic conditions returns between periods of anoxia.     

Size distribution of iron and manganese species in freshwaters

A recently established technique for size fractionation of particulate matter in freshwaters, based on low volume filtration through Nuclepore filters in the field, has been applied to the study of Fe and Mn species in 3 freshwater lakes and a stream feeding one of the lakes. The technique has also been used in a series of laboratory experiments to provide further insight into the process of particle formation. The results are complemented with scanning electron microscope examination of the particles.The raw data are transformed into mass size distributions, which are generally unimodal, with the major part of the mass confined to a single log unit size range. These size distributions are compared and contrasted with a theoretical model of particle behaviour in a lake. The results suggest that particulate Fe and Mn behave according to particle theory. The observed size distributions represent a balance between hydraulic input/output, aggregation, and gravitational settling, as well as chemical precipitation and dissolution processes. Overall particle aggregation is shown to be a slow process in the waters examined, with a time constant of the order of days. There is evidence for a fairly stable particle regime in the 0.05–0.4 μm size range.Deviations from the unimodal distribution in the epilimnion of the lakes indicate the association of Fe, but not Mn, with phytoplankton. In the waters examined Mn was most frequently found in solution, while Fe was predominantly particulate. These findings are interpreted in terms of the differing redox behaviour of the metals.     

On the stress distribution at the bonding interface between rock and bolt

Rock bolt is a major reinforcement technique for roadways in coal mines in China. Generally, separation of the bonding interface between the rock mass and the bolt may lead to collapse of the bolted rock mass due to stress concentration. In order to establish the stress concentration mechanism on the bonding interface, the distribution functions for shear stress and longitudinal force on the interface are derived using Mindlin's model, and a failure criterion for the interface is proposed. In addition, influencing factors for the stress distribution mode are identified. Both analytical study and numerical simulations by ANSYS have shown that, as the elastic modulus of rock increases, both shear stress and the longitudinal force-concentrating zone move towards the outer end of bolt and decrease gradually from the free surface to the rock mass body. Also, there is an optimizing cement thickness which results in relatively uniform distributions of shear stress and longitudinal force on the interface. It is valuable to investigate the bolt invalidity mechanism as well as reinforcement safety estimation in underground rock engineering design.     

High-resolution profiles of iron, manganese, cobalt, cadmium, copper and zinc in the pore water of estuarine sediment

The remobilization of iron, manganese, cobalt, cadmium, copper and zinc in the pore water of estuarine sediment cores at Yingkou was assessed using diffusive equilibrium in thin films and diffusive gradients in thin films techniques. A relatively anoxic system (+33.7 to ?224.1 mV) in the sediment cores might cause the reductive release of iron, manganese and cobalt into pore water from the estuarine sediment. High-average concentrations of iron (47.85 μg ml^?1) and manganese (3.81 μg ml^?1) were observed using diffusive equilibrium in thin films on the sediment core, but the concentration of cobalt (18.02 ng ml^?1) was relatively low. A strong correlation between iron and cobalt was observed based on the vertical profiles of the metals. Manganese and iron were more readily released from the solid phase to the solution. The peak cobalt, copper and zinc concentrations were observed in the upper layer (2–4 cm) measured using diffusive gradients in thin films. However, the peak iron, manganese and cobalt concentrations were located in the deeper layer (≥7 cm). In addition, the concentration profiles measured using diffusive gradients in thin films of cobalt, copper and zinc were independent of the iron, manganese and cobalt distribution with respect to depth.     

Dissolved iodine flux from estuarine sediments and implications for the enrichment of iodine at the sediment water interface

During the anaerobic decomposition of organic matter in sediments iodine is released into solution. Three techniques have been applied to independently estimate the resulting flux of soluble I from the sediments to the overlying water of Mud Bay, Georgetown, South Carolina. Flux estimates (summer) range between ~ 5 and 41 μmol/m²/day. The estimates predicted from either the pore water I concentration gradient across the sediment-water interface or the dissolved I production rate are higher than the apparent flux measured directly at the same site. This suggests that I which is released to the pore water under the anoxic conditions below the sediment surface reacts with a sedimentary component at or near the sediment water interface and is lost from solution.     

The interaction of metal ions at the manganese dioxide-solution interface

The adsorption of metal ions was measured and alkalimetric titrations were performed on a synthetic sample of hydrous manganese dioxide. The principal characteristics of this phase, its stoichiometry, X-ray diffraction pattern and surface area, resemble closely naturally occurring manganese minerals.Both sets of experiments indicate that the affinity of the metals for the surface followed the order: Mg < Ca < Sr < Ba < Ni < Zn < Mn <-Co, and that the interaction can be characterized by its pH dependence. A comparison of the amount of metal absorbed with the amount of acid released by the surface showed that the interaction of metals with the hydrous manganese dioxide surface involves (1) the separation of a proton from the covalent bond at the surface, and (2) the association of a solute cation with this site. Mg < Ca < Sr < Ba < Ni < Zn < Mn <-Co, The relative degree of the bond strength is reflected by the specific adsorption potentials, which are determined from the amount of metal that is absorbed by the surface, in the absence of any electrostatic attraction, at the pH of zero point of charge.     

The control of cathodoluminescence in dolomite by iron and manganese

Variations in the cathodoluminescent properties of carbonates are usually attributed to differing proportions of manganese (Mn²⁺) as the most important activator, and iron (Fe²⁺) as the main inhibitor of luminescence. Interactions between manganese and iron concentrations and the luminescent properties of dolomite are demonstrated by petrographic and chemical analyses of 86 samples of dolomite representing a range of depositional environments and ages (Cambrian to Cretaceous) and a wide geographical distribution (North America and Europe). Iron and manganese are positively correlated in the dolomites, with the former showing a greater range of variation. Very small amounts of manganese are sufficient to activate the luminescence and as little as 100 ppm Mn²⁺ is present in highly luminescing samples. The intensity of luminescence is not proportional to the manganese concentration. Iron begins to quench luminescence as its concentration reaches 10,000 ppm. Above that level, luminescence is rapidly lost and total extinction occurs among samples containing more than 15,000 ppm Fe²⁺, regardless of the manganese concentration.     

Adsorption of monocarboxylates at the water/goethite interface: The importance of hydrogen bonding

The adsorption of monocarboxylates (acetate, benzoate, and cyclohexanecarboxylate) at the water/goethite interface was studied as a function of pH and ionic strength by means of quantitative adsorption measurements and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. ATR-FTIR spectra were obtained of suspensions prepared in both H₂O and D₂O. In order to identify the number of predominating surface complexes and to improve the resolution of overlapping peaks the ATR-FTIR spectra were subjected to a 2D correlation spectroscopic analysis. The adsorption envelopes of acetate, benzoate, and cyclohexanecarboxylate are similar and depend strongly on pH and ionic strength, but the pH dependence is also correlated to the slightly different pK_a values of the monocarboxylic acids. At the molecular level, the ATR-FTIR spectroscopic results reveal two surface complexes: one solvent-surface hydration-separated ion pair and one surface hydration-shared ion pair. The former predominates at circumneutral pH values while the latter forms mainly in the acidic pH range. We find no evidence for direct inner-sphere coordination between the carboxylic oxygens and the Fe(III) ions present at the surface. The identification of surface hydration-shared ion pairs emphasizes the importance of comparatively strong ionic hydrogen-bonding interactions for adsorption processes at the water/goethite interface.     

Adsorption of natural dissolved organic matter at the oxide/water interface

Natural organic matter is readily adsorbed by alumina and kaolinite in the pH range of natural waters. Adsorption occurs by complex formation between surface hydroxyls and the acidic functional groups of the organic matter. Oxides with relatively acidic surface hydroxyls, e.g. silica, do not react strongly with the organic matter. Under conditions typical for natural waters, almost complete surface coverage by adsorbed organic matter may be expected for alumina, hydrous iron oxides and the edge sites of aluminosilicates. Potentiometric titration and electrophoresis indicate that most of the acidic functional groups of the adsorbed organic matter are neutralized by protons from solution. The organic coating is expected to have a great influence on subsequent adsorption of inorganic cations and anions.     

Adsorption of Cd ions from aqueous solutions by iron modified pomegranate peel carbons: kinetic and thermodynamic studies

The experimental conditions for preparation of pomegranate peel carbon and Fe(III) modified pomegranate peel carbon were studied. The effects of main experimental parameters on carbon preparation such as carbonization time, carbonization temperature and Fe(III) impregnation ratio in pomegranate peel were investigated. The prepared carbons in various conditions were characterized by consideration of the production yield, ash content, iodine number, pH of zero point charge and their ability for adsorption of methylene blue. After preparation of carbons, their efficiency for removal of Cd²⁺ species from aqueous solution was investigated. The effect of experimental parameters such as Cd²⁺ initial concentration, pH of solution and contact time was studied by batch adsorption experiments. The fitting of experimental data in thermodynamic isotherms matched the linear results with Langmuir and Freundlich isotherms. The adsorption capacity for Cd²⁺ species on Fe(III) modified pomegranate peel carbon was 22.72 mg/g and the adsorption kinetic presented the pseudo-second-order kinetic model.     

On the adsorption of n-alkylammonium chlorides at fluorite/solution interface

Experimental adsorption isotherms of alkylammonium chlorides (with 10, 12 and 14 carbon atoms) on fluorite are given. For the monolayer region two straight segments have been obtained. They are interpreted, on the basis of the Frumkin-Fowler model of adsorption on heterogeneous surfaces, as being due to the adsorption on two different domains with different potential energies. The model has been used to obtain the normal interaction energy adsorbate-adsorbent. Also, chain-chain interaction has been discussed on the basis of the theory of Cases et al., based on the Frumkin-Fowler model discussed above. The changes in the thermodynamic quantities enthalpy and entropy related to the process were calculated using the Clausius-Clapeyron equation. Isosteric heat of adsorption has been found to be negative for 0 < θ < 0.7 (where θ is the surface coverage by the collector ions), with an abrupt change of ?6.5 kcal/mol around θ = 0.1. The global change in entropy is only positive for θ < 0.1, while it is negative for 0.1 < θ < 0.7.     

Competitive sorption of Ni and Zn at the aluminum oxide/water interface: an XAFS study

Trace metals (e.g. Ni, Zn) leached from industrial and agricultural processes are often simultaneously present in contaminated soils and sediments. Their mobility, bioavailability, and ecotoxicity are affected by sorption and cosorption at mineral/solution interfaces. Cosorption of trace metals has been investigated at the macroscopic level, but there is not a clear understanding of the molecular-scale cosorption processes due to lack of spectroscopic information. In this study, Ni and Zn cosorption to aluminum oxides (γ-Al₂O₃) in binary-sorbate systems were compared to their sorption in single-sorbate systems as a function of pH using both macroscopic batch experiments and synchrotron-based X-ray absorption fine structure spectroscopy. At pH 6.0, Ni and Zn were sorbed as inner-sphere surface complexes and competed for the limited number of reactive sites on γ-Al₂O₃. In binary-sorbate systems, Ni had no effect on Zn sorption, owning to its lower affinity for the metal oxide surface. In contrast, Zn had a higher affinity for the metal oxide surface and reduced Ni sorption. At pH 7.5, Ni and Zn were sorbed as mixed-metal surface precipitates, including Ni–Al layered double hydroxides (LDHs), Zn–Al LDHs, and likely Ni–Zn–Al layered triple/ternary hydroxides. Additionally, at pH 7.5, Ni and Zn do not exhibit competitive sorption effects in the binary system. Taken together, these results indicated that pH critically influenced the reaction products, and provides a crucial scientific basis to understand the potential mobility, bioavailability, and ecotoxicity of Ni and Zn in natural and contaminated geochemical environments.

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平原河流水沙界面生源物质迁移转化过程及水环境调控的研究进展

生源物质作为影响水质的重要因素,其在河流中的迁移转换一直倍受关注。水沙界面物质交换过程受众多环境和水动力因素的影响,过程极其复杂,不仅影响着河流生源要素的通量演变规律,还关系到河流水环境问题。为探究水沙界面生源物质迁移转化过程及作用机制,结合国内外研究最新进展,对泥沙颗粒与生源物质的微界面作用、水沙界面对生源物质的迁移转化作用、水流条件对泥沙吸附解吸生源物质作用机理进行总结和概述,总结阐述了河流水质模型和闸坝泵条件下的水环境调控模型,最后指出平原河流水沙运动对生源物质输运作用机理及水环境调控。针对上述已有的研究成果和存在不足,对今后研究方向提出展望。     

不平衡输沙含沙量垂线分布研究

采用理论推导和数值计算的方法,进一步研究了不平衡输沙条件下的含沙量垂线分布,首次提出了非饱和泥沙通量的概念及其表达式,推导了不平衡输沙含沙量垂线分布的理论解,包括不平衡输沙含沙量垂线分布指数公式和幂函数公式,以及相应的底部含沙量确定方法。同时利用差分法求解剖面二维扩散方程,得到了不同参数的不平衡输沙含沙量垂线分布。其结果除少数误差稍大外,两种途径得出的含沙量垂线分布彼此符合良好。