Coupled diffusion and oxidation of ferrous iron in soils. II. A model of the diffusion and reaction of O2, Fe2+, H+ and HCO3− in soils and a sensitivity analysis of the model

Kinetic equations are developed for a system in which a column of reduced soil is exposed to oxygen at one end. The equations are combined in a simulation model in which they are solved by finite-difference methods. The model predicts the consequent diffusion of oxygen into the column; the diffusion of ferrous iron towards the oxidation zone; the rate of formation and concentration profile of the ferric hydroxide formed; and the diffusion by acid-base transfer of the acidity produced in the oxidation reaction. A sensitivity analysis of the model, in which runs were made for a wide range of input parameters, showed that for most combinations of parameters, in water-saturated soil, substantial amounts of iron are transferred towards the air-exposed surface, leading to a well-defined zone of ferric hydroxide accumulation. The profile of total iron in this zone is often banded. The pH in the zone falls by at least two units. A small amount of air-filled pore space increases the depth of the oxidation front dramatically. The model indicates that coupled iron oxidation and diffusion reactions, which are very widespread in natural soils, may be understood quantitatively.     

旱作褐土中氧化铁的厌氧还原与光合型亚铁氧化特征

土壤中铁的氧还过程与碳氮转化及自净能力关系密切,已还原亚铁的氧化受土壤性质的影响。采用室内恒温培养试验研究了旱作褐土中铁还原氧化过程、及其与水溶性碳、NO3-、SO42-的关系。结果表明旱作褐土中铁氧化物在厌氧光照条件下可先被还原后被再次氧化,其再氧化量介于1.46~3.00 mg g-1之间,平均2.09 mg g-1;再氧化速率常数介于0.23~0.80 d-1之间,平均0.48 d-1。再氧化量与土壤无定形铁、水溶性硫酸盐含量、阳离子交换量显著负相关,与土壤总氮、总磷显著正相关;再氧化速率常数与土壤有机碳显著负相关,与黏粒含量极显著正相关。厌氧光照培养可使旱作褐土水溶性无机碳平均降低52.74%,水溶性NO3-降低92.15%,水溶性SO42-增加55.38%。研究结果为深入理解旱作土壤潜在的微生物铁循环转化方式提供理论支持。     

Nitrite stability influenced by iron compounds

The decomposition of nitrite was studied in the presence of (1) different amounts of ferrous iron and (2) an amorphous and a crystalline (haematite) iron product at different pH and Eh conditions. It was found that ferrous iron positively influenced the nitrite decomposition. Even at pH 6, where self-decomposition is excluded, some nitrite was decomposed. It was shown that at all studied pH values the second order decomposition rate increased as the amount of ferrous iron increased. From the calculation of the activation energy it was found that the dependence of the rate constant on temperature increased when the medium was more acid, or when the amount of Fe²⁺ increased at the same pH. The nitrite half-life was longest at pH 6, 25°C and 200 mg Fe²⁺ l^?1; it was shortest at pH 4, 30°C and 800 mg Fe²⁺ l^?1. The experiments with Fe²⁺ derived from solid iron compounds showed that all conditions favouring a high amount of ferrous iron in solution, such as low redox potential, low pH, amorphous or less crystalline material, enhanced nitrite decomposition.     

Visualisation of oxidizing power of rice roots and of possible participation of bacteria in iron deposition

The oxidizing power of rice roots was observed in narrow transparent root boxes containing different media. Plants precultivated in nutrient solution were embedded in semisolid agar medium to observe oxidation of ferrous iron cations and leuco methylene blue as well as solubilization of ferrous sulfide. In the presence of ferrous sulfate reddish brown coloration due to formation of ferric oxide/hydroxide was observed around the roots and on the root surface during one day of incubation. When agar medium blackened by ferrous sulfide was used, the root zone became transparent. Within a few hours leuco methylene blue was oxidized to methylene blue on and near the roots. Furthermore, seedlings were grown in agar medium containing ferrous sulfide inoculated with soil filtrate. Besides diffuse ferric iron precipitation, iron was also deposited on spherically shaped structures in the rhizosphere and near the agar surface as well as in slimy layers appearing on the root surface. The spherical structures and slimy layers were obviously bacterial colonies extending with time. As the roots grew old, parts of them turned black. In the rhizosphere, black spots occurred resembling colonies of sulfate-reducing bacteria. Rice was also grown in sand supplemented with nutrients and iron sulfide. While root growth was straight in agar, it was twisted in the sand medium. Again, heavy ferric iron deposition occurred on the root surface. On older root parts the lateral roots became blackish. The results suggest participation of bacteria in ferric iron deposition in the rhizosphere of rice.     

A new method for determining ferrous iron in paddy soils

Introduction

To make clear the chemical behavior of free iron compounds in paddy soils, it is necessary to find an accurate and rapid method for determining ferrous iron in waterlogged paddy soils. Several methods^1,2,3) have been proposed for this purpose, most of which use dilute acids, such as sulfuric and hydrochloric acid, as extracting reagents. According to the writers' experience, however, these methods seem to be inadequate. The reason is that the acid solutions extract not only ferrous iron, but also ferric iron and reducing substances from soils, and the latter two react immediately in the extract to produce ferrous iron, thus we obtain larger value for ferrous iron than the one actually existing in soils. The writers have found that acetate buffer of pH 2.8 is a very suitable extractant for ferrous iron, and they have been able to establish a new method for the determination of ferrous iron in soils using this buffer. The experimental details will be given in this paper.     

Stability behavior of soil colloidal suspensions in relation to sequential reduction of soils

The cause of the decrease in the Fe²⁺ concentration of the soil solution in the later period of soil waterlogging was investigated. After 7-d incubation of the soil solutions separated from previously waterlogged soils (PWdS), a greyish precipitate (PPT) was observed in the soil solutions. The color of the PPT became reddish brown after separation from the solutions and freeze-drying. The PPT observed in 14-d-PWdS contained 352.6 g Fe kg^-1, 62.5 g C kg^-1, 22.6 g P kg^-1, 11.3 g Si kg^-1, 9.9 g N kg^-1, 0.7 g Al kg^-1 and a trace amount of Mn. However, Ca, Mg, K, and Na could not be detected. It was concluded that the separated PPT was dominated by amorphous ferric hydroxide based on the chemical analysis, broad IR absorption band at 585 cm^-1 and exothermic peak at 301°C. The data of chemical analysis and the characteristic IR bands of the PPT suggested that organic substances and presumably aluminosilicate anion were adsorbed onto the freshly-formed ferric hydroxide. The dominant phase of the greyish PPT in the reductive soil solution was considered to be ferrous PPT and was assumed to consist mainly of carbonate and/or hydroxide, and concomitantly of phosphate. The formation of the ferrous PPT in the soil solution in the later period of soil waterlogging was considered to (i) cause the decrease of concentration of Fe²⁺ ion and of other divalent cations such as Ca²⁺ due to the re-adsorption of Ca²⁺ on soil clays through the cation exchange reaction with Fe²⁺ ion, and consequently (ii) enhance the dispersion of the soil colloidal suspension.     

Sorption and oxidation of manganous ions and reduction of manganese oxide by cell suspensions of a manganese oxidizing bacterium

Cells of a soil Arthrobacter sp., and the manganese oxide they formed rapidly adsorbed manganous ions (Mn²⁺) from aqueous solutions. These ions could be desorbed by copper ions. There was no evidence that the adsorbed manganous ions were rapidly oxidized by non biological reactions. In gently stirred mixtures of cells and manganous ions the maximum rate of oxidation occurred at pH 6.5. The rate was very sensitive to small changes in pH below about pH 5.7 and above about pH 7.5. No oxidation occurred at pH 5.4 or at pH 7.9. Manganous ion concentrations between 0.5 mM and 6 mM had little effect on the maximum rate of oxidation. Higher concentrations became progressively inhibitory and 40 mM was completely inhibitory. The concentration for half the maximum rate was about 0.1 mM. Within limits the rate of oxidation increased in direct proportion to the numbers of bacteria in suspension. In deep vessels, static suspensions of the bacteria reduced bacterial manganese oxide and the rate of reduction was increased greatly by small additions of methylene blue. Methylene blue also partially inhibited the oxidation of manganese by cell suspensions in shallow vessels. These results are discussed in relation to manganese transformations in soil.     

Adsorption of chloride and nitrate by variable charge soils in relation to the electric charge of the soil

A cell consisting of a chloride-selective electrode and a nitrate-selective electrode was directly put in the soil suspension to determine the concentration ratio NO₃^?/Cl^? for studying the adsorption of these two ions by three soil samples from variable charge soils. It was found that such factors as the iron oxide content of the soil, the pH of the suspension, the concentration of the respective anion, the kind of accompanying cations, and the dielectric constant of solvent etc. can all affect the amounts and the ratio of the two anions adsorbed. The adsorption was chiefly caused by coulombic force, but another mechanism, presumably a covalent force between the anion and the metal atom on the surface of soil particles, may also be involved, at least for chloride ions.     

Mechanistic Consideration of Zinc Ion Removal by Zero-Valent Iron

Mechanism of zinc iron removal by zero-valent iron was discussed through zinc removal responses to several operational conditions of a packed column reactor with zero-valent iron powder. The adsorption isotherm observed implied that a kind of chemisorption was responsible for zinc removal. Zinc removal by zero-valent iron was enhanced by dissolved oxygen and ferric ion addition. However, it was deteriorated under acidic pH. In addition, zinc adsorbed on zero-valent iron was eluted by a reducing agent such as citric acid, whereas the zinc was not eluted by diluted sulfuric acid. Consequently, the zinc removal mechanism by zero-valent iron was inferred to be as follows: Zero-valent iron was firstly corroded and oxidized into ferric ion by dissolved oxygen. The ferric ion was precipitated as iron hydroxide onto the surface of the zero-valent iron powder. Zinc ion was adsorbed on and/or coprecipitated with the iron hydroxide. The iron hydroxide was finally oxidized and transformed into iron oxides.     

MICROMORPHOLOGY OF SOME TROPICAL ALLUVIAL CLAY SOILS

The micromorphological properties of some marine and estuarine tropical clays from Holocene coastal plain soils of Surinam and Thailand have been explained against the background of sedimentation and initial soil formation. During geogenesis stratified sediments are formed, above which are sediments with slightly disturbed stratification, with or without matric faecal pellets. Pedogenesis during the sedimentation phase included development of channels in the sediment, formation of channel neostrians, biological homogenization, possibly pyrite accumulation, and partial oxidation and precipitation of iron as ferric hydroxide. During the brackish water phase pyrite accumulates in various amounts, mass illuviation may occur, part of the iron may oxidize and precipitate and part of the pyrite may be oxidized. During and after the swamp phase further oxidation and precipitation of iron as ferric hydroxide occurs, ferric hydroxide crystallizes into goethite. Pyrite becomes oxidized, resulting in the formation of jarosite, gypsum, silica, and ferric hydroxide.     

Evaluation of slag application to decrease methane emission from paddy soil and fate of iron

Abstract

Organic carbon in paddy soil is oxidized to carbon dioxide by reducing electron acceptors for a certain period after submerging. Methane production commences after the reduction of iron oxide which is the most important electron acceptor in the soil. We aimed to study the long-term suppression of the methane emission from the paddy soil by single application of iron slag. A revolving furnace slag (RFS; 248 g Fe kg^?1) was applied to the potted soil at the rate of 0 (control) or 20 ton ha^?1 in 2000. Rice plants were successively cultivated on the potted soils for 3 years without further application of the RFS. Methane emissions from the potted soils with rice plants were measured by the closed chamber method during these cultivation periods. Total flux of CH₄ emission from the pot applied with ,FS decreased by 5–30% compared with the control. The RFS supplied free iron oxide to the potted soil, and its iron acted as the oxidizing agent as evidenced by the increase in ferrous iron content in the soil. The amount of iron lost from leaching at the bottom of the pots was estimated as 54–59 kg Fe ha^?1 year^?1 at the percolation rate of 20 mm d^?1. Accordingly, half-life of the iron in the applied RFS was calculated as 42–46 years. Therefore, there is a possibility that the suppressing effect of RFS on CH₄ emission is sustained for a half-century, Contents of heavy metals (Cd, Cu, and Zn) in the brown rice harvested from the pot applied with RFS were not significantly different with those from the control pot.     

Myoglobin oxidation in a model system as affected by nonheme iron and iron chelating agents

A model system was used to study the effect of nonheme iron on myoglobin oxidation at pH 5.6 and pH 7.2 at 23 degrees C. The addition of ferrous iron significantly (p < 0.05) increased the rate of myoglobin oxidation in the absence of lipid, demonstrating that iron promoted myoglobin oxidation independent of the effect of lipid oxidation. The addition of the type II, iron chelating antioxidants sodium tripolyphosphate (at pH 7.2) or milk mineral (at pH 5.6) negated the effect of added iron, slowing oxidation of myoglobin. A clear concentration dependence was seen for iron-stimulated myoglobin oxidation, based on both spectral and visual evidence. Further investigation is needed to determine the possible role for nonheme ferrous iron on myoglobin oxidation in vivo or in meat.     

Removal of ferrous iron from field drainage waters by conifer bark

Iron ochre deposition in field drainage systems is produced as a result of chemical and microbiological oxidation of ferrous iron in soil solution to the insoluble ferric form. Conifer bark, from several plant species, will absorb ferrous iron from solution quickly and irreversibly, the bark changing colour from brown to blue in the process. This bark, unlike that from deciduous trees, contains only small quantities of soluble phenolic components so it does not create an environmental problem under field conditions. Trials carried out at a test farm, where bark was placed within nylon mesh sacks and incorporated into the drainage system, were very successful, and drainage pipes which had previously blocked twice yearly remained relatively free of ochre. The use of bark from conifers can offer an inexpensive method of ameliorating the problem of ochre formation without producing environmental pollution.     

Methane and water soluble iron production under controlled soil pH and redox conditions

Abstract

When a soil is flooded, iron (Fe) reduction and methane (CH₄) production occurred in sequence as predicted by thermodynamics. The dissolution and precipitation of Fe reflected both soil pH and soil redox potential (Eh). The objective of our experiment was to determine both CH₄ production and Fe reduction as measured by Fe in solution in a flooded paddy soil over a wide range of closely controlled pH and Eh conditions. The greatest release of CH₄ gas occurred at neutral soil pH in combination with low soil redox potential (‐250 mV). Production of CH₄ decreased when soil pH was lowered in combination with an increase in the soil redox potential above ‐250 mV. Highest concentration of ferrous‐iron (Fe²⁺) under reducing conditions occurred when soil pH was lowered. Thus Fe reduction influenced CH4 formation in the flooded paddy soil. Results indicated that CH₄ production was inhibited by the process of ferric‐iron (Fe³⁺) reduction.     

Coupled diffusion and oxidation of ferrous iron in soils: III. Further development of the model and experimental testing

Equations are developed to predict the distribution of Fe²⁺ between solid and solution phases in a reduced soil undergoing oxidation at different pHs, based on cation-exchange equilibria and electrical neutrality in the solid and solution. The equations satisfactorily explained experimental results. They are incorporated in the model of Fe²⁺ diffusion and oxidation developed in Part II, and the model is also extended to allow for O₂ consumption in processes other than Fe²⁺ oxidation. The resultant predictions are tested against measured profiles of Fe(II), Fe(III) and pH in cylinders of reduced soil exposed to O₂ at one end. When oxidation rate constants measured in stirred soil suspensions were used to run the model, the predicted rates of O₂ consumption were too great and the spread of the oxidation front too small. Satisfactory agreement was achieved for oxidation rate constant values about one-eighth of those measured in the stirred suspensions. The findings are consistent with the rate of Fe²⁺ oxidation in soil being controlled by access of O₂ to Fe²⁺ sorption sites, as suggested in Part I. The revised model allows a study of the effects of Fe²⁺ oxidation on the mobility of other cations in reduced soils, e.g. nutrient cations in the rice rhizosphere. Fez+ oxidation and the accompanying acidification may greatly impede cation mobility in reduced soils.     

THE OXIDATION OF IRON SULPHIDES IN SOILS IN RELATION TO THE FORMATION OF ACID SULPHATE SOILS, AND OF OCHRE DEPOSITS IN FIELD DRAINS

Aerating pyritic soils causes acidification and the forrnation of acid sulphate soils, or cat-clay. The Oxidation of pyrite in soils is associated with the deposition in tile drains of a form of ochre quite distinct from that formed by the action of filamentous iron bacteria. Pyrite-derived ochre results from the action of Thiobacillus ferrooxidans, which, below pH 3.5–4.0, catalyses the Oxidation of Fe²⁺ and pyrite. In soils less acid than c. pH 4, pyrite oxidizes relatively slowly by chemical reactions to Fe²⁺ and SO²₄^?. Under these conditions iron enters the drains as Fe²⁺ and is there oxidized by T. ferrooicidans and deposited as hydrated ferric oxide. Once the soil becomes acid enough for T. ferrooxidans to multiply, the rate at which pyrite oxidizes increases several-fold, and at c. pH 3 iron appears in the drainage water in the ferric form. Liming seems to decrease the rate of Oxidation.     

Oxymyoglobin oxidation and membranal lipid peroxidation initiated by iron redox cycle.

Oxymyoglobin is the main pigment in muscle tissues, responsible for the bright red color of fresh meat. Oxidation of the heme iron from the ferrous to the ferric metmyoglobin produces the brownish color that consumers find undesirable in fresh meat. The aim of this study was to elucidate the mechanism of oxymyoglobin oxidation in muscle tissues by using a model system containing oxymyoglobin and muscle membranes oxidized by an iron redox cycle. Oxidation of oxymyoglobin was determined from the decrease in absorption of the solution measured by a spectrophotometer at 582 nm. Lipid peroxidation was determined by accumulation of TBARS and conjugated dienes. The higher rates of oxidation of oxymyoglobin (20 microM) and lipid oxidation were achieved by using ferric iron and ascorbic acid at concentrations of 50 and 200 microM, respectively. Increasing the concentration of ascorbic acid to 2000 microM switched its effect to antioxidative. Increasing the concentration of oxymyoglobin from 20 to 80 microM inhibited lipid peroxidation by >90% and partially prevented oxymyoglobin oxidation.     

The role of iron and the factors affecting off-color development of polyphenols

Iron deficiency affects over two billion people worldwide (Lotfi, M.; Venkatesh Mannar, M. G.; Merx, R. J.; Naber-van den Heuvel, P. Micronutrient Fortification of Foods: Current Practices, Research,and Opportunities; Micronutrient Initiative: Ottawa, Ontario, Canada, 1996). However, fortifying foods with highly bioavailable iron is technically challenging because of off-color and off-flavor development, catalytic degradation of vitamins, and oxidation of lipids. The role of highly bioavailable iron in the off-color development of foods and beverages is not well-understood. The goal of this research was to examine the interaction of iron with simple phenolics and polyphenols. Factors that may affect off-color development, such as pH, oxygen, temperature, and reducing and chelating agents, were evaluated as a model for food products. Our results demonstrated that the iron that reacts with the simple phenolic, catechol, to develop off-color must be in the oxidized state, and the iron is reduced in the presence of catechol. Because this is an oxidation/reduction reaction, the redox potential of all of the components is critical to the color development. Ferrous iron sources with low redox potentials and ferric iron sources with high redox potentials caused off-color development with catechol. Only polyphenols that contain ortho-hydroxyl groups cause off-color development with iron. All of the factors tested affect off-color development and redox potential of the system. Low pH, low oxygen, high temperature, and the presence of reducing and chelating agents inhibited off-color development. To confirm the model, foods that contained these polyphenols were evaluated for off-color development when iron was added. The foods tested reacted similarly to the models of polyphenols with iron. Off-color development was caused by oxidation-reduction interactions between ferric iron and polyphenols that contained ortho-dihydroxyl groups. Ferrous iron needed to be oxidized to participate in off-color development. In addition, methods identified in the models to prevent off-color development were effective in most of the food products examined. Using the ferrous form of iron and maintaining it in its reduced form by lowering pH, removing oxygen, and including reducing agents, it was possible to fortify foods with highly bioavailable iron.     

Vegetable oil stability at elevated temperatures in the presence of ferric stearate and ferrous octanoate

The thermoxidative stability of partially hydrogenated soybean oil (PHSBO) was examined after addition of ferric stearate and ferrous octanoate, and then heating the samples at 120, 160, 180, and 200 degrees C. In a second experiment, the effect of iron concentration (ferric stearate) on PHSBO stability was examined at 180 degrees C, and at concentrations of approximately 0.5 and 1.2 mg of added iron/kg PHSBO. Oil samples were heated continuously for 72 h and sampled every 12 h. The acid value, p-anisidine value, color, dielectric constant and the triacylglycerol polymer content of oil samples were compared to oil samples containing no added iron. Generally, the value of each oxidative index increased with (1) an increase in temperature, (2) an increase in heating time, and/or (3) an increase in iron. The results demonstrate that low concentrations of iron will substantially increase the rate of oxidation for vegetable oil samples heated to temperatures of 120 degrees C to 200 degrees C.     

Spontaneous formation of thiuram disulfides in solutions of iron(III) dithiocarbamates

Dithiocarbamates are used as pesticides and rubber additives. Dithiocarbamates are the reduced forms of thiuram disulfides and both of these groups of substances induce allergic contact dermatitis. The allergic cross-reactivity pattern between dithiocarbamates and thiurams is unclear. The aim of this study was to investigate why these cross-reactions occur sometimes but not always. HPLC-analysis of buffer solutions of iron(III) dithiocarbamates demonstrated that thiuram disulfides were formed spontaneously and rapidly in high yield. No such oxidation was observed in solutions of copper(II), zinc(II), or sodium dithiocarbamates. However, sodium diethyldithiocarbamate and zinc diethyldithiocarbamate were oxidized in buffer solution when ferric salt was added. The influence of different metal ions on the oxidation reaction is probably an explanation for the cross-reactivity patterns seen between dithiocarbamates and thiurams. These findings also show that careful handling is necessary in analytical and biological studies with solutions of iron(III) dithiocarbamates. Oxidation of dithiocarbamates in aqueous buffer at physiological pH has not been shown before.