Die gleichmäßige Konzentrationsverteilung der Proben deutet auf eine homogene Ausfällung von Fe und Mn in der Ausfällungszone hin. Die unvollständige Oxidation von Fe(II) wird durch den Einbau von Fe(II) in Kaicit erklärt, was bedeutet, dass das eingeschlossene Fe(II) vor Sauerstoff teilweise wird.
1 INTRODUCTION 1 1.1 Iron and manganese removal from ground water 1
PRECIPITATES FROM AN IN SITU GROUND WATER
Extraction by 5M HCl (Fetot, Fe(ll), Mntot) 32
Adsorption of cations on carbonate minerals 49
Preparation of the calcite suspension 55
Fe(ll) oxygenation kinetics in calcite suspensions 59
Products of oxygenation of Fe(ll) in calcite suspensions 59
Homogeneous and heterogeneous oxygenation kinetics of Fe(ll) in presence and in absence of calcite 76
Repeated adsorption and oxygenation of Fe(ll) in calcite
Analysis of the Fe oxides produced after repeated addition
Heterogeneous oxygenation kinetics of Fe(ll) 85
PRESENCE OF GOETHITE 91
Adsorption and oxidation of Fe(ll) on goethite 94
Effect of the major ions HC03", Ca2+ and Mg2+ 101
Heterogeneous oxidation of Fe(ll): Comparison of the
Fe(ll) removal from ground water by heterogeneous
INTRODUCTION
Iron and manganese removal from ground
- The "iron calamity"
- Iron and manganese in ground water
- Manganese oxides
- Removal of iron and manganese
In the natural aquatic systems, microbially mediated reduction of the Fe and Mn oxides can take place in the absence of oxygen and nitrate. In Menznau, the pumping of groundwater possibly draws reduced groundwater with greater Fe(ll).
H CHARACTERIZATION OF IRON AND MANGANESE
Introduction
In most previous studies on in situ delay and demanganation, optimum operating conditions were investigated with little emphasis on precipitation products (Hallberg and Martinell, 1976; Boochs et. In the present study we will focus on the deposits formed in an in situ.
La Neuveville site
At a distance of 7 m from the supply well there are five injection wells (satellite wells), where air-saturated water is periodically injected to regenerate the oxidation zone (oxygen = 8 mg/L; iron . and manganese < 0.2 mg/L ). To ensure a complete removal of iron and manganese, the treatment plant is designed to have overlap.
Materials and Methods
- Aquifer sample selection
- Analytical methods
- Results and Discussion
- Grain size distribution of the aquifer material
- Extraction by 5M HCl (Fetot, Fe(ll), Mntot)
- Reductive dissolution experiments
- Identification of Fe-oxides by XRD
- Comparison of the results from the different
- Accumulation of iron and manganese in the
- Conclusions
Correlation between the iron oxide concentrations of Fe(lll)(TiEDTA) in the sieved samples and their respective specific surface area, which was calculated based on the radius of the particles and the density p = 2.6 g/cm3. In the sieved samples, only small amounts of manganese were extracted by reductive reagents, indicating that most of the manganese was present (Mntot, Figure 2-4).
SUSPENSIONS*
Introduction
- Adsorption of cations on carbonate minerals
- Interaction of Fe(ll) with CaC03
- Mineralogy of Fe(ll)/Ca carbonate
- Ferrous iron oxygenation: kinetics and product
- Deferrisation for drinking water production
- Preparation of the calcite suspension
- Analytical methods
- Spectroscopies
- Sorption experiments
- Desorption of Fe(ll) with ligand
- Dissolution kinetics with C02
- Fe(ll) oxygenation kinetics in calcite suspensions
- Products of oxygenation of Fe(ll) in calcite suspensions
In this study, Fe(ll) (0.114 M) was deposited in supersaturated CaCO3 solutions with calcite and other natural calcareous sediments. There, Veizer (1983) indicates an average of 6.8 mol per mil Fe(ll) in marine calcite, which is similar in composition to the. The oxygenation of Fe(ll) in calcite systems was studied by Loeppert, who found that the rate increases with surface area.
However, in this work the oxidation of the 0.01 M initial Fe(ll) concentration produced so many protons that the pH varied. Loeppert and co-workers also characterized the Fe oxides formed by the oxidation of Fe(II) in the presence of calcite; they mainly identified. Can the oxidation of Fe(ll) be accelerated in the presence of calcite, like other surfaces, especially Fe(ll) oxides.
Particle size distribution measurements showed no noticeable changes in particle size over the 8 months.
Results and discussion
- Adsorption kinetics
- Fe(ll) sorption capacity on calcite
- Incorporation of Fe(ll) into calcite
- Homogeneous and heterogeneous oxygenation
This is reflected in the spread of the data and in the large error. In Figures 3-5, our results (24 hours) are shown together with data from Zachara et al. 1991): the logarithm of the dissolved metal concentration. mol/L) is plotted against the logarithm of the adsorbed concentration. To understand the kinetics of the sorption process over several days, desorption experiments were performed with ferrozine for batch.
Assuming spherical particles, the width of the dissolved layer can be estimated from dissolved calcium and medium grain size. concentrations are drawn against the concentration of released calcium. Dissolved Fe(ll) versus dissolved Ca2+ after different equilibration times 24h ((}), 48h (O), 1 week(A, ) This secondary X-axis, Ar, represents the depth of the layer. In this geological process the movement of Ca2+ Mg2+ however, it is thought to include a Ca2+ site.
Decrease of Fe(ll) (initial concentration 1x10"5 M) as a function of time after addition of oxygen (7 mg/L) to the calcite suspension (1 g/L), which had been equilibrated for 1.
1.5 LÏ-2.0
Adsorption versus oxygenation kinetics of Fe(ll)
In Figure 3-11 the adsorption kinetics of Fe(ll) is compared with its oxygenation kinetics for two different CaC03 concentrations (1 and 10 g/L). It is 10 times faster at 10 g/L than at 1 g/L, which is further evidence for increased oxygenation by calcite. Comparison of adsorption kinetics (filled symbols) and oxidation kinetics (open symbols), with Fe(ll) =1»10+5 M at pH 7.0, adsorption in controlled N2/CO2 atmosphere; oxidation with 6.2 mg/oxygen in solution.
- Repeated adsorption and oxygenation of Fe(ll) in
- Analysis of the Fe oxides produced after
- Heterogeneous oxygenation kinetics of Fe(ll)
- Repeated addition and oxygenation of Fe(ll) in calcite suspensions
- Summary
The increase in oxygenation kinetics in the presence of calcite can be explained by the formation of a more reactive adsorbed Fe(ll) species. V1, approximately 7 times the initial rate constant. the following 12th and 25th cycles, there is a progressive increase of a) Change of Fe(ll) oxygenation kinetics in the presence of calcite after several cycles of Fe(ll) addition and oxygenation (pH 7.0; 1 g/L calcite; 2.5 mg/L O2). Concentrations of iron species determined by reductive and acid decomposition of Fe oxide products.
However, acceleration of the oxygenation kinetics is not found when Fe(ll) is first equilibrated with CaC03 a few hours before oxygenation (15 hours). Other effects, which may affect the calcite surface, cannot be excluded, especially considering the instability of the CaC03 surface. The transport rate of the Fe(ll) in the calcite at the surface of the.
When Fe(ll) is added to an aerated suspension of calcite, acceleration of the oxidation is observed, which depends on the surface.
4 ADSORPTION AND
PRESENCE OF GOETHITE
Introduction
- Ion adsorption on goethite
- Adsorption and oxidation of Fe(ll) on goethite
- Fe(ll) adsorption kinetics
- Sorption reversibility
- Effect of the major ions HC03", Ca2+ and Mg2+
In an aqueous environment, water binds to imperfectly coordinated iron centers on the surface of Fe-oxides. Near the surface, the ions in the solution redistribute to replace the surface. They depend on the pH, the saturation of surface bonds and the concentration of the electrolyte background.
Despite the large range of the experimental Fe(ll) loadings, a similar concentration of Fe(ll) surface is reported. Consequently, this species is responsible for the rapid reduction of nitro-aromatic compounds and of the uranyl ion. However, desorption can be slower than adsorption and possibly a fraction of the adsorbed metal ion remains associated with the Fe oxide (Cornell and Schwertmann, 1996; Coughlin and Stone, 1995).
The possible effect of the main ions on the Fe(ll) adsorption and oxygenation was tested in the present study.
Materials and methods
- Analytical and experimental methods
- Analytical methods
- Sorption experiments
- Fe(ll) oxygenation kinetics
All experiments with Fe(ll) were performed in an anoxic glove box containing a palladium catalyst to remove oxygen (Coy Laboratory. Products Inc., Michigan). Fe(ll) was measured photometrically by the Ferrozine method at 562. Uvikon spectrophotometer, Bio-Tek Kontron Instruments). Adsorption kinetics were performed similarly: in a 250- or 500-ml batch containing goethite (5.25 m2/L), FeS04 was added to a final.
At the end of the adsorption experiments, the batch containing the goethite suspension and the Fe(ll) was acidified with HCl (1M) to 3.5 <. After addition of Fe(ll) to the bubbled solution, 3-4 ml of sample was taken after certain reaction times, quenched in acetic acid/sodium acetate buffer (2M, pH 4.5). Ferrozine was then added to bind the remaining Fe(ll) and the samples were filtered into a buffer (acetic acid/acetate) and measured by spectrophotometry.
Results and Discussion
- Adsorption kinetics
- Langmuir isotherm
- Desorption
- Fe(ll) oxygenation
- Heterogeneous oxidation of Fe(ll): Comparison of
The reversibility of the Fe(ll) adsorption was tested by acidifying the Fe(ll)-bearing goethite suspensions to pH «4. In the presence of Ca2+ and Mg2+ the recovery was slightly faster both at pH 7 and 8. The desorption properties of pH 7 and 8 show a. A possible explanation is that only a fraction, a, of the initial sites is regenerated, while the other one becomes less reactive.
Schematic model of the catalytic cycle: partial (a) regeneration of the reactive surface sites on the goethite (>FeOH) or calcite surface (>CaCO3). This corresponds to the parallel formation of less reactive sites, which leads to a decrease in the heterogeneous oxidation.
- uM [>FeOH]
- uM [>CaC03]
Goethite has a five times greater adsorption capacity than calcite with respect to the surface area (mol/m2). First-order representation of the oxygenation kinetics of Fe(ll) (10 jaM Fe(ll), 2.6 mg/L oxygen) at pH 7 (saturated CaC03 solution) under varying conditions: No solid (O);. For calcite, the rate constant for adsorption is ten times faster than the rate constant for oxygenation, while it is only a factor of two.
Doubling the oxygen concentration results in a rate coefficient on the order of the rate constant for adsorption. However, under high oxidant concentrations, adsorption can become limiting, to a greater extent for goethite than for calcite. With calcite, one unit of calcium carbonate is dissolved in place; The calcite surface, in this way, retains the protons produced by hydrolysis, after the oxidation of Fe(ll) to Fe(llll) (Figure 3-12).
The adsorption isotherm (Langmuir) yields a maximum adsorption capacity for Fe(ll) on goethite at pH 7 of 4.8 fxmol/m2 (2.9 places/nm2).
- Fe(ll) removal from ground water by
In addition to the greater adsorption capacity of Fe(ll) on goethite, the oxygenation of Fe(ll) in the presence of goethite is approximately two orders of magnitude faster than on calcite or in homogeneous systems (Table 4-2). Even small increases in goethite in a solid matrix of carbonate result in a noticeable change of the system with respect to the Fe(ll) oxygenation kinetics. In a field system it has been observed that the injection of small amounts of oxygen leads to a stoichiometric oxygenation of Fe(ll) (von Gunten et al., 2002).
As a result of this study, heterogeneous oxidation and precipitation is the proposed mechanism for Fe(ll) removal under in situ groundwater. Heterogeneous oxidation-precipitation occurs in two steps: First, Fe(ll) is adsorbed on a natural surface of the groundwater matrix (quartz, .alumina, clay, calcite, etc.), leading to the adsorbed Fe(ll) species. .
CONCLUSIONS AND OUTLOOK
Manganese removal
However, it also shows that the conditions for chemical oxidations of manganese were not satisfied.
In situ deferrisation and risk of clogging
Based on our findings, the dimensions of the precipitation zone can be optimized in relation to the expected volume of the precipitation if all the. In situ groundwater treatment is an applied subject, which includes aspects of hydrology, mineralogy, microbiology, engineering sciences. More fundamental hydrological aspects can be introduced for a better understanding of possible changes occurring in the aquifer as a result of the precipitation of iron and.
Transformation of the solid may be accompanied by a changing reactivity of its surface.
APPENDIX
Zeitschrift der America Water Work Association. lösliches Mangan von oxidbeschichteten Filtermedien: Sorptionsrate und 1995) Mögliche Anwendungen der In-situ-Behandlung. von eisen- und manganhaltigem Grundwasser. bei der Wasserversorgung aus Grund- und Oberflächenwasser;. 1989) Wirkung von Ionenstärke und Ionen. 1974) Schätzung der Anzahl der Flächen. Calcitschicht, die am Ca-45Ca-Isotopenaustausch mit der Lösung beteiligt ist.
Iron metabolism in anoxic environments at near neutral pH. 1992) Chemistry of the solid-water interface. The effect of ferric hydroxide on the oxidation of ferrous ions in neutral solutions. 1980) Acceleration of the oxidation of Fe2+ ions by Fe(lll)-oxyhydroxides. Surface complexation modeling of carbonate effects on the adsorption of Cr(VI), Pb(ll) and U(VI) on goethite.
1991) Primary products of iron(ll) oxygenation at an oxic-anoxic boundary: nucleation, aggregation and aging.
