Effect Of Redox Conditions Research Articles

Microelectrode-Based Diffusivity Measurements of Hydrogen in Molten 2LiF-BeF2

Detailed knowledge of the mass transport behavior of hydrogen isotopes in molten 2LiF-BeF2 salt (FLiBe) is essential to the design of new nuclear fusion technologies that make use of this molten salt to breed tritium to fuel the fusion reaction. However, typical studies of hydrogen transport using macroelectrodes are limited to solely measuring the permeability of an analyte—a property defined as the product of concentration squared and diffusivity. This challenge necessitates the development of microelectrodes capable of dynamically measuring diffusion coefficients when concentrations are variable. Unlike macroelectrodes, microelectrodes can measure diffusivity and concentration independently due to a steady-state current proportional to the product of diffusivity and concentration captured in addition to the Cottrellian current response.This steady-state current is established due to a continuously expanding diffusion layer from the electrode that cannot expand indefinitely within finite experimental setups. Therefore, COMSOL Multiphysics simulations mirroring experimental conditions were performed to validate the analytical mass transport model based on the solution of Fick’s Second Law in radial coordinates. These simulations employed a 2D axisymmetric model to examine the conditions under which the infinite bulk solution approximation is valid, confirming that our electrode design would yield true steady-state current responses indicative of radial diffusion.Platinum microelectrodes developed in this study served a dual purpose: they not only addressed previous limitations in high-temperature molten salt experimentation but also enabled dynamic measurements of both the diffusion coefficient and concentration of hydrogen in molten FLiBe introduced by lithium hydride. Square wave voltammetry and chronoamperometry were performed on a three-electrode cell using a platinum working microelectrode, a tungsten counter electrode, and a Ni/Ni2+ thermodynamic reference electrode. The successful measurement of hydrogen diffusivity in these experiments not only offers essential insights into molten FLiBe behavior but also sets the stage for future investigations into the effects of redox conditions, impurities, and equilibration time on tritium within nuclear fusion reactors.

Effects of Redox Conditions on Geochemical and Metallogenic Zoning: An Example of Mesozoic Magmatic Belts in Eastern Yakutia

Effects of Redox Conditions on Geochemical and Metallogenic Zoning: An Example of Mesozoic Magmatic Belts in Eastern Yakutia

The solubility of Cu, Ag and Au in magmatic sulfur-bearing fluids as a function of oxygen fugacity

Magma-derived fluids containing chlorine and sulfur are critical for the transport of ore metals to porphyry ore-forming environments. However, experimental data on the speciation and the solubility of ore metals in these fluids at conditions relevant to the arc magmatism are scarce. In particular, the effect of redox conditions on ore metal speciation and solubilities in sulfur-bearing fluids has not yet been experimentally constrained. We performed experiments to determine the effect of oxygen fugacity (fO2) on the solubility of Cu, Ag and Au in high-temperature, low- density, low-salinity fluids and hypersaline brine. The experiments were conducted at T = 900 °C, P = 2000 bar varying fO2 in 7 steps between 0.5 log units below the Ni-NiO buffer (NNO − 0.5) to NNO + 2.5. A prototype rapid-quench Molybdenum-Hafnium Carbide (MHC) externally heated pressure vessel assembly was used, which was fitted with a Shaw membrane for precise control of fO2. The fluid phase was sampled as synthetic fluid inclusions (SFI) by in situ fracturing of quartz chips during the experiments. As capsule material, Au97Ag2Cu1 alloy was used, which imposed activities of 0.962, 0.0082 and 0.0097 for Au, Ag and Cu, respectively. The apparent solubility of Cu and Ag at the imposed metal activities increases by a factor of 7 in the H2O-NaCl-KCl-S low-salinity fluid with fO2 increasing from NNO − 0.5 to NNO + 2.5. The addition of 0.198 m HCl increases the overall solubility of Cu by a factor of 1.2–2.4. The apparent solubility of Au decreases by a factor of 9 as fO2 changes from NNO − 0.5 to NNO + 2.5. The relationship between the logarithms of the apparent Cu and Ag solubilities and fO2 is linear and the slope of the fitted line corresponds to 1+ oxidation state of these metals indicating that they are dominantly complexed by ligands that are S-free (e.g., chloride). Thermodynamic model calculations indicate that the dominant species for Cu and Ag are NaCuCl2 and NaAgCl2, respectively. For Au, the dominant species is predicted to be NaAu(HS)2 at low and intermediate fO2 conditions, and AuCl and NaAuCl2 at oxidizing conditions. The measured gold solubilities at intermediate fO2 do not indicate significant Au complexation with S species containing S in intermediate oxidation states. Considering previous studies on silicate melts and our experimental data for volatiles, we conclude that Cu and Ag likely have constant fluid/melt partition coefficients in the typical fO2 range of arc magmatism because they dissolve in the same oxidation state in the fluid and the melt (1+) and are dominantly chloride complexed in the fluid.

The effects of redox conditions on arsenic re-release from excavated marine sedimentary rock with naturally suppressed arsenic release.

Massive quantities of marine sedimentary rock are excavated from urban coastal areas. The excavated rock often releases arsenic with concurrent oxidation of framboidal pyrite, but the arsenic release is naturally suppressed with subsequent atmospheric exposure. The present study evaluated the re-release of arsenic from excavated rock in which arsenic release has been naturally suppressed by the atmospheric exposure in the presence of sulfate ions under various redox conditions using the biological reduction method. The atmospheric exposure and subsequent batch leaching test revealed that the amount of arsenic release that was naturally suppressed corresponded to 1.2% of the total arsenic content. The sequential extraction analysis also showed that the arsenic in the exposed rock was altered to insoluble phases. We observed a re-release of 6.0-18.2% of the total arsenic content under reductive conditions (< + 70mV of Eh), exceeding the amount of arsenic that was naturally suppressed, even in the presence of sulfate ions. The correlation in the amount of arsenic and iron re-released demonstrates that arsenic re-release under reductive conditions is mainly regulated by the iron dissolution up to 10mgkg-1 even in the presence of sulfate ion. Further reduction and dissolution of iron did not cause further increase in the arsenic re-release. Therefore, excavated marine sedimentary rock should be reused under redox conditions in which iron is not reduced. Otherwise, treatments such as chemical immobilization should be performed.

Effects of redox conditions and temperature on the degradation of Sphagnumn-alkanes

Leaf wax n-alkanes in peat deposits are important proxies for paleoenvironmental reconstructions. The n-alkane compositions of Sphagnum species, which are important peat forming plants, are commonly dominated by the C23 and C25 homologs. However, questions remain about the stability of Sphagnum-derived n-alkanes to early diagenetic alterations. In this study, a 13-month litterbag decomposition study using Sphagnum palustre collected from the Dajiuhu peatland, central China, was conducted under controlled laboratory conditions to assess the influences of redox (aerobic vs anaerobic) conditions and environmental temperatures (15 °C vs 25 °C) on the chain-length distributions and carbon isotopic compositions of Sphagnum n-alkanes. Results clearly reveal the preferential degradation under aerobic conditions of mid-chain n-alkanes relative to their long-chain homologs (C29 and C31), resulting in the increase of the relative proportions of long-chain n-alkanes. The change in n-alkane distribution is accompanied by an increase in the δ13C values of the mid-chain n-alkanes, whereas those of the long-chain n-alkanes remain nearly constant. In contrast, neither chain-length distributions nor n-alkane δ13C values were much affected under anaerobic degradation conditions. Statistical analysis reveals that redox conditions play significant roles in the degradation of Sphagnum n-alkanes, whereas the influence of air temperature is not significant. These findings will improve applications of n-alkane paleoenvironmental proxies to reconstruct the types of plants and the depositional conditions that contributed to peat sequences, particularly under settings with fluctuating water levels that affect redox conditions.

Redox effects on relative permeability in Fe-rich clay bearing sandstones

Abstract The presence of iron (Fe)-rich clay minerals makes the hydrocarbon-bearing sandstone reservoirs prone to the Fe oxidation-reduction (redox) reactions. At depths relevant to hydrocarbon recovery applications, oxygen is scarce and Fe2+ is the most common oxidation state of Fe on the grain surface and in the free liquid. Fe2+ is referred to as the reduced state. At surface conditions, where oxygen is abundant, Fe2+ in clays can oxidize to Fe3+ which is referred to as the oxidized state. The change in the redox state of structural Fe can alter the surface wettability of the clays as well as the arrangement of the clay layers and the pore morphology. Consequently, the distribution of fluid phases and their flow properties in the pore space may be influenced. In this study, the effects of redox conditions on rock-fluid interactions in sandstone samples with Fe-rich clays were investigated by a series of steady-state coreflooding experiments. Two types of sandstone core material were tested; each contained different types of Fe-rich clays, such as smectite, vermiculite and chlorite. For corefloods at reduced conditions, a reducing agent (erythorbic acid) was added in the brine. For corefloods at oxidized conditions, the brine was prepared at room conditions and contained oxygen. Corefloods started with an oxidized-brine/oil co-injection sequence followed by a reduced-brine/oil co-injection sequence. Effects of the redox conditions were analyzed by comparing the relative permeability and total mobility of the fluid phases obtained at oxidized and reduced conditions. Both core types were responsive to the changes in redox conditions; the samples with vermiculite were relatively more sensitive than the samples with other Fe-rich clays, such as chlorite. This suggested that the initial condition of the core could be important, and measurements taken in oxidized core samples may not be representative of reservoir conditions where the rock is in reduced state. Using a reducing agent should always be considered for coreflooding studies, especially when the Fe-rich clays are present in the core samples.

Effect of redox conditions on the structure and solubility of sulfur- and selenium-AFm phases

In the context of safe radioactive waste disposal, the incorporation of selenium and sulfur, a major competitor for Se, in AFm phases was investigated by synthesizing and characterizing Se(VI)-, Se(IV)-, S(VI)-, S(IV)-, S(II)-AFm phases to account for a range of possible redox conditions in a cement-based repository for low and intermediate level radioactive waste.The X-ray spectra revealed crystalline AFm phases with trigonal or pseudo-trigonal structure. Different interlayer distances (hkl 00l) were observed for the phases described in the hexagonal lattice, whereas no shift of the (hkl 110) diffraction peak was detected. The type of the interlayer anion seems to control the crystal symmetry of the AFm phase. The amount of water in the interlayer depends on the interlayer anion, relative humidity and temperature of exposure. The AFm phases show similar solubility products suggesting that high sulfur concentration in the cement pore solution can decrease Se binding by AFm phases.

Redox transformation of soil minerals and arsenic in arsenic-contaminated soil under cycling redox conditions.

Changes in the saturation degree of aquifers control the geochemical reactions of redox-sensitive elements such as iron (Fe), sulfur (S), and arsenic (As). In this study, the effects of redox conditions and the presence of Fe and S on the behavior of As in a soil environment were investigated by observation in a batch experimental system. Arsenic was stable on Fe(III) solid surface in an oxidizing environment but was easily released into the aqueous phase following the reductive dissolution of Fe during an anoxic period. The alternating redox cycles led to a change in the concentrations of Fe, S, and As in both the aqueous and solid phases. The composition of Fe minerals changed to a less crystalline phase while that of solid phase As changed to a more reduced phase in both the As-contaminated natural soil and FeS-amended soil batch systems. This tendency was more prominent in the batch containing higher amounts of total Fe and S. These results show that a redox cycle can increase the possibility of As contamination of groundwater during dissolution and reprecipitation of Fe minerals and simultaneous microbial reduction of S and/or As species.

Effect of redox conditions on the evolution of magmas of Changbaishan Tianchi volcano, China–North Korea

In this paper we use data on composition of melt, fluid, and crystal inclusions in minerals to reconstruct the compositions and formation conditions of the parental melts for the trachybasalts, trachybasaltic andesites, comendites, and pantellerites of Changbaishan Tianchi (Paektusan) volcano on the border of China and North Korea. The melts crystallized within broad temperature and pressure ranges of 1200–700 °C and 3100–1000 bar. The redox conditions expressed as ΔNNO are +0.9 to +1.4 log units for the basalt melts, −1.6 to −2.1 log units for the trachybasaltic andesite melts, −2.6 to −1.9 log units for the comendite melts, and −1.6 to −3.2 log units for the pantellerite melts. All melts are rich in alkalis and FeO. The sulfur contents of the mafic melts vary from 0.06 to 0.13 wt%. The pantellerite melts are sulfur-free, but the groundmass of the pantellerites contain pyrite crystals. Mafic melts have a range in water contents from 0.1–1.0 wt%. The H2O contents of the comendite melts vary from 3.7 to 4.8 wt%, and the pantellerite melts contain 0.2–0.6 wt% H2O. It has been revealed that the magma crystallization was accompanied by changes in oxygen fugacity caused by the decompression of the magma chamber beneath Changbaishan Tianchi volcano. Such decompression resulted in open-system crystal fractionation during magma degassing. We propose that the degassing is responsible for variations in water and sulfur contents in the melts. As the pressure decreases, the gas phases become progressively enriched in H2O and S, and the S oxidation state is changed from S2− to S4+. Sulfur degassing in the form of SO2 at decompression and spinel precipitation during open-system magmatic differentiation resulted in melt reduction.

The mobility and plant uptake of gallium and indium, two emerging contaminants associated with electronic waste and other sources

Gallium (Ga) and indium (In) are increasingly susceptible to soil contamination via disposal of electronic equipment. Chemically similar to aluminium (Al), these elements may be mobile and bioavailable under acidic conditions. We sought to determine extent and nature of Ga and In mobility in the soil – plant system and thus their potential to enter the food chain. Batch sorption experiments on a high fertility silt loam (pH 5.95, CEC 22 meq 100 g−1) showed strong retention of both elements to the soil matrix, with mean distribution coefficient (KD) values of 408 and 2021 L kg−1 for Ga and In respectively. KD increased with concentration, which we attributed to precipitation of excess ions as insoluble hydroxides. KD decreased with increased pH as Ga/In(OH)2+ and Ga/In(OH)2+ transitioned to Ga/In(OH)4-. Movement into the aboveground portions of perennial ryegrass (Lolium perenne L.) was low, with bioaccumulation factors of 0.0037 for Ga and 0.0002 for In; foliar concentrations peaked at 11.6 mg kg−1 and 0.015 mg kg−1 respectively. The mobility of Ga and In in the soil - plant system is low compared to other common trace element contaminants such as cadmium, copper, and zinc. Therefore, Ga and In are likely to accumulate in soils and soil ingestion, either directly, via inhaled dust, or dust attached to food, will be the largest pathway into the food chain. Future work should focus on the effect of redox conditions on Ga and In, as well as uptake into acidophilic plants such as Camellia spp., which accumulate Al.

Impact of Redox Condition on Fractionation and Bioaccessibility of Arsenic in Arsenic-Contaminated Soils Remediated by Iron Amendments: A Long-Term Experiment

Iron-bearing amendments, such as iron grit, are proved to be effective amendments for the remediation of arsenic- (As-) contaminated soils. In present study, the effect of redox condition on As fractions in As-contaminated soils remediated by iron grit was investigated, and the bioaccessibility of As in soils under anoxic condition was evaluated. Results showed that the labile fractions of As in soils decreased significantly after the addition of iron grit, while the unlabile fractions of As increased rapidly, and the bioaccessibility of As was negligible after 180 d incubation. More labile fractions of As in iron-amended soils were transformed into less mobilizable or unlabile fractions with the contact time. Correspondingly, the bioaccessibility of As in iron-amended soils under the aerobic condition was lower than that under the anoxic condition after 180 d incubation. The redistribution of loosely adsorbed fraction of As in soils occurred under the anoxic condition, which is likely ascribed to the reduction of As(V) to As(III) and the reductive dissolution of Fe-(hydr)oxides. The stabilization processes of As in iron-amended soils under the anoxic and aerobic conditions were characterized by two stages. The increase of crystallization of Fe oxides, decomposition of organic matter, molecular diffusion, and the occlusion within Fe-(hydr)oxides cocontrolled the transformation of As fractions and the stabilization process of As in iron-amended soils under different redox conditions. In terms of As bioaccessibility, the stabilization process of As in iron-amended soils was shortened under the aerobic condition in comparison with the anoxic condition.

Effect of redox conditions and copper ions on pyrite depression by sodium metabisulfite and sodium sulfite in seawater

Effect of redox conditions and copper ions on pyrite depression by sodium metabisulfite and sodium sulfite in seawater

Biodegradation of pharmaceuticals and endocrine disruptors with oxygen, nitrate, manganese (IV), iron (III) and sulfate as electron acceptors

Biodegradation of pharmaceuticals and endocrine disrupting compounds was examined in long term batch experiments for a period of two and a half years to obtain more insight into the effects of redox conditions. A mix including lipid lowering agents (e.g. clofibric acid, gemfibrozil), analgesics (e.g. diclofenac, naproxen), beta blockers (e.g. atenolol, propranolol), X-ray contrast media (e.g. diatrizoic acid, iomeprol) as well as the antiepileptic carbamazepine and endocrine disruptors (e.g. bisphenol A, 17α-ethinylestradiol) was analyzed in batch tests in the presence of oxygen, nitrate, manganese (IV), iron (III), and sulfate. Out of the 23 selected substances, 14 showed a degradation of >50% of their initial concentrations under aerobic conditions. The beta blockers propranolol and atenolol and the analgesics pentoxifylline and naproxen showed a removal of >50% under anaerobic conditions. In particular naproxen proved to be degradable with oxygen and under most anaerobic conditions, i.e. with manganese (IV), iron (III), or sulfate. The natural estrogens estriol, estrone and 17β-estradiol showed complete biodegradation under aerobic and nitrate-reducing conditions, with a temporary increase of estrone during transformation of estriol and 17β-estradiol. Transformation of 17β-estradiol under Fe(III)-reducing conditions resulted in an increase of estriol as well. Concentrations of clofibric acid, carbamazepine, iopamidol and diatrizoic acid, known for their recalcitrance in the environment, remained unchanged.

Effect of redox conditions on iron metal phase segregation during experimental high-temperature centrifuge modeling of the origin of the Moon’s core

The possible origin of the Moon’s metallic core at the precipitation of iron–sulfide phases during the partial melting of ultramafic material under various redox conditions was experimentally modeled by partially melting the model system olivine (85 wt %) + ferrobasalt (10 wt %) + metallic phase Fe95S5 (wt %) in a high-temperature centrifuge at 1430–1450°C. The oxygen fugacity fO2 was determined from the composition of the quenched experimental silicate melts (glasses). A decrease in fO2 is proved to be favorable for the segregation of iron–sulfide melt from the silicate matrix. The metallic phase is most effectively segregated in the form of melt droplets, and these droplets are accumulated in the lower portions of the samples under strongly reduced conditions, at fO2 ∼ 4.5–5.5 orders of magnitude lower than the iron–wustite buffer.

The effect of redox conditions and bioirrigation on nitrogen isotope fractionation in marine sediments

Nitrogen isotopic signatures of sources and sinks of fixed nitrogen (N) can be used to constrain marine nitrogen budgets. However, the reported fractionation during benthic N2 production varies substantially. To assess the range and mechanisms responsible for such observations, we conducted a model study to evaluate the extent to which nitrification, denitrification, and anaerobic ammonium oxidation contribute to the isotopic composition of in situ N2 production. Different hydrodynamic regimes were taken into account, ranging from bioirrigation to diffusion-dominated transport. The benthic redox conditions were found to control the N isotope effect, which under reducing conditions is driven by fractionation during nitrification and anaerobic ammonium oxidation and under oxidizing conditions by fractionation during denitrification. Environmental parameters, such as the mineralization rate, the bioirrigation intensity, and chemical composition of the overlying water affect the benthic redox zonation and therefore also the benthic N isotope effect. The N isotope effect of benthic N2 production was computed for a wide range of bioirrigation intensities and mineralization rates, and found to be approximately −3‰ for commonly encountered conditions. This value is similar to previous estimates of the global N isotope effect of benthic N2 production, and further constrains the relative importance of water column vs. benthic N2 production.

Gross nitrous oxide production drives net nitrous oxide fluxes across a salt marsh landscape.

Sea level rise will change inundation regimes in salt marshes, altering redox dynamics that control nitrification - a potential source of the potent greenhouse gas, nitrous oxide (N2 O) - and denitrification, a major nitrogen (N) loss pathway in coastal ecosystems and both a source and sink of N2 O. Measurements of net N2 O fluxes alone yield little insight into the different effects of redox conditions on N2 O production and consumption. We used insitu measurements of gross N2 O fluxes across a salt marsh elevation gradient to determine how soil N2 O emissions in coastal ecosystems may respond to future sea level rise. Soil redox declined as marsh elevation decreased, with lower soil nitrate and higher ferrous iron in the low marsh compared to the mid and high marshes (P<0.001 for both). In addition, soil oxygen concentrations were lower in the low and mid-marshes relative to the high marsh (P<0.001). Net N2 O fluxes differed significantly among marsh zones (P=0.009), averaging 9.8±5.4μgNm(-2) h(-1) , -2.2±0.9μgNm(-2) h(-1) , and 0.67±0.57μgNm(-2) h(-1) in the low, mid, and high marshes, respectively. Both net N2 O release and uptake were observed in the low and high marshes, but the mid-marsh was consistently a net N2 O sink. Gross N2 O production was highest in the low marsh and lowest in the mid-marsh (P=0.02), whereas gross N2 O consumption did not differ among marsh zones. Thus, variability in gross N2 O production rates drove the differences in net N2 O flux among marsh zones. Our results suggest that future studies should focus on elucidating controls on the processes producing, rather than consuming, N2 O in salt marshes to improve our predictions of changes in net N2 O fluxes caused by future sea level rise.

Source rock characteristics of Eocene bituminous shales and the effect of redox conditions on source rock potential, Nallıhan (Ankara) Central Anatolian Basin

abstractEocene bituminous shales within the Çamalan Formation of Paleocene-Eocene aged Kızılçay Group exposing around Tekirler (Nallıhan, Ankara), central Anatolia, were deposited under oxic and suboxic redox conditions and have Corg values in the range of 2.37 to 12.42 wt% (average 6.32 wt%). Samples are represented by Type-I kerogen and their maturity level is immature-early mature. Eocene bituminous shales are enriched in some major and trace elements, such as Ca, Mg, Si, Na, K, V, Ni, U, Th, P, S, Mo, Cu, Zn, Cr, Co, Pb, Au, Ag, As, Re, La, Ce, Pr, Nd, Li, Sm, and Y. Paleoredox indicators (e.g., V/(V+Ni), Ni/Co, U/Th, V/Sc, Mo/Mn) yield that Eocene bituminous shales were mostly deposited oxic–suboxic conditions.

The micro-scale investigation on the effect of redox condition on the release of the sediment phosphorus in Lake Hongfeng

选取贵州红枫湖为研究对象,在实验室条件下模拟了自然、好氧和厌氧条件下沉积物内源磷的释放过程,联合应用微电极技术和沉积物磷形态分析对沉积物水界面开展了微尺度观测与研究.结果表明,厌氧条件下红枫湖沉积物总磷含量显著降低,且主要是NaOH提取态磷(NaOH-P)和残渣态磷(rest-P)含量降低所致,厌氧条件下沉积物孔隙水中磷酸盐浓度明显升高,而好氧条件下沉积物孔隙水磷酸盐浓度显著降低,反映厌氧条件显著促进了红枫湖沉积物磷释放.厌氧条件下沉积物内部溶解氧浓度下降、硫还原活动增强可能是导致NaOH-P释放的主要原因.O₂浓度的降低加速了沉积物还原作用并产生大量H₂S,进而与二价铁离子形成硫化亚铁沉淀,最终导致NaOH-P(Fe-P)释放到孔隙水中.好氧条件向厌氧条件的转换可通过改变沉积物内部pH值分布和微生物活动促使rest-P释放:厌氧条件下,厌氧微生物不仅可以消耗硫酸根产生H₂S,导致pH值降低,还可消耗有机质,将有机磷转变为无机磷.上述研究结果表明,沉积物水界面氧化还原环境可影响沉积物氧渗透深度、pH值分布、微生物活动、硫循环以及有机质降解过程,进而控制沉积物磷的形态转化与释放.联合应用微电极技术和沉积物磷形态分析对湖泊沉积物水界面开展微尺度观测研究是揭示沉积物内源磷释放机制与控制因素的有效途径.;In this paper, Lake Hongfeng, located in Guizhou Province, was chosen to study the release process of the sediment phosphorus under the natural, aerobic and anaerobic conditions. Microscale observation of the physical and chemical properties of sediment-water interface by microelectrode technology was used, together with traditional P-speciation analysis, to explore the P-releasing mechanisms. The results showed that under anaerobic conditions, the total content of phosphorus in the sediment in Lake Hongfeng decreased significantly, which was mainly caused by the reduction of NaOH-P and rest-P, indicating that the aerobic condition promoted the phosphorus release from sediments. The concentration of phosphate in the pore water of sediment increased greatly under anaerobic condition, and the trend was contrary to that under aerobic condition. Under the anaerobic condition, concentration of dissolved oxygen decreased and sulfate reduction was enhanced, which released NaOH-P from the sediment. Decrease of O₂ accelerated the sediment reducing processes and produced a large amount of H₂S, which combined with ferrous icon to form the precipitation-ferrous sulfide, resulting in release of NaOH-P(Fe-P)into the pore water. The conversion of aerobic to anaerobic condition could promote the release of rest-P by changing the distribution of pH value in the sediment as well as microbial activities. Under the anaerobic condition, anaerobic microorganisms could not only contribute to the sulfate reduction and H₂S production, leading to pH decrease, but also consume organic matter and convert organic phosphorus into inorganic phosphorus. The redox environment of sediment-water interface could affect the penetration depth of sediment-oxygen, pH distribution, microbial activity, sulfur cycle as well as organic matter degradation process, which controlled the speciation transformation and release of sediment phosphorus. Microelectrode technology, together with traditional P-speciation analysis, is an effective way to reveal the release mechanisms of sediment phosphorus as well as the control factors.

The effect of redox conditions and adaptation time on organic micropollutant removal during river bank filtration: A laboratory-scale column study

This study investigated the redox dependent removal and adaptive behaviour of a mixture of 15 organic micropollutants (OMPs) in laboratory-scale soil columns fed with river water. Three separate pilot systems were used consisting of: (1) two columns, (2) ten columns and (3) twenty two columns to create oxic, suboxic (partial nitrate removal) and anoxic (complete nitrate removal). The pilot set-up has some unique features - it can simulate fairly long residence times (e.g., 45days using the 22 column system) and reduced conditions developed naturally within the system. Dimethoate, diuron, and metoprolol showed redox dependent removal behaviour with higher biodegradation rates in the oxic zone compared to the suboxic/anoxic zone. The redox dependent behaviour of these three OMPs could not be explained based on their physico-chemical properties (hydrophobicity, charge and molecular weight) or functional groups present in the molecular structure. OMPs that showed persistent behaviour in the oxic zone (atrazine, carbamazepine, hydrochlorothiazide and simazine) were also not removed under more reduced conditions. Adaptive behaviour was observed for five OMPs: dimethoate, chloridazon, lincomycin, sulfamethoxazole and phenazone. However, the adaptive behaviour could not be explained by the physico-chemical properties (hydrophobicity, charge and molecular weight) investigated in this study and only rough trends were observed with specific functional groups (e.g. ethers, sulphur, primary and secondary amines). Finally, the adaptive behaviour of OMPs was found to be an important factor that should be incorporated in predictive models for OMP removal during river bank filtration.

Effects of redox conditions on the control of arsenic mobility in shallow alluvial aquifers on the Venetian Plain (Italy)

The Venetian Plain is known for the occurrence of areas with high concentrations of arsenic in groundwater (greater than 400μg/L). The study area represents the typical residential, industrial and agricultural features of most Western countries and is devoid of hydrothermal, volcanic or anthropogenic sources of arsenic. The aim of the study is to model the arsenic mobilization and the water–rock interaction by a complete hydrogeochemical investigation (analyses of filtered and unfiltered groundwater sediment mineralogy and geochemistry). The groundwater arsenic contamination and redox conditions are highly variable. Groundwaters with oxidizing and strongly reducing potentials have much lower arsenic concentrations than do mildly reducing waters. The grain size of the aquifer sediments includes gravels, sands and silty-clays. A continuous range of organic material concentrations is observed (from zero to 40%). The amount of sedimentary organic matter is highly correlated with the arsenic content of the sediments (up to 300mg/kg), whereas no relationships are detectable between arsenic and other chemical parameters.The occurrence of arsenic minerals was observed as a peculiar feature under the scanning electron microscope. Arsenic and sulfur are the sole constituents of small tufts or thin crystals concentrated in small masses. These arsenic minerals were clearly observed in the peat sediments, in agreement with the geochemical modeling that requires very reducing conditions for their precipitation from the groundwater. The modeling suggests that, under oxidizing conditions, arsenic is adsorbed; moreover, a continuous decrease in the redox potential causes increasing desorption of arsenic. If the reducing conditions become more intense, the formation of As-S minerals would explain the lower concentration of arsenic measured in the strongly reducing groundwater. Even if As-sulfides are rare under low-temperature conditions, the anomalous abundance of reductants (organic matter) can locally stabilize As-S minerals, which can scavenge large quantities of groundwater arsenic.