Iron Hydroxide Precipitation Research Articles

Chemical precipitation within pyritic waste rock

Pyritic waste rock piles generate acid rock drainage (ARD). If the chemical precipitation of metals could be induced within waste rock piles by the weathering products of phosphate and carbonate minerals, seepage metal loads–and treatment costs–could be reduced. Seventeen 70-L drums were filled with refractory ore, supposedly non-reactive waste rock, freshly broken low-pyrite waste rock, ‘old’ low-pyrite rock (weathered four years or longer), freshly broken high-pyrite rock or old high-pyrite rock. Control drums were set aside for each category. Phosphate mining wastes, containing both phosphate and calcium carbonates, were added to the remainder. The drums were weathered for 2.7 years, stored for 4.5 years indoors and re-exposed outdoors for 1.3 years. Discharge was collected and analysed. In the first phase of the experiment, acid generation in the waste rock drums containing natural phosphate rock (NPR) was reduced by 85%. In storage, acidity continued to be generated on the waste rock surfaces but was reduced even further in the second phase of the experiment when the rocks were returned outdoors. Although the mechanisms causing this are not fully understood, carbonate neutralization, along with ferric phosphate and iron hydroxide precipitation on the mineral surfaces, as well as microbial biofilm formation, are likely contributory factors. The results suggest that the standard practice of estimating acid generation potential of base-metal waste rock should be reconsidered.

Densification of iron(III) sludge in neutralization

Acid mine drainage (AMD), of which iron is a substantial component, is a potential by-product in the mining industry. Conventional neutralization is a common approach to treat AMD, although it creates a major disposal problem due to the generation of voluminous sludge. Sludge recirculation improves solid density by slowing down the rate of neutralization and allowing the growth of precipitates, while existing solids act as seed particles by providing necessary surface area for precipitation. The mechanisms of iron sludge densification are not fully understood, mainly because of the complex nature of iron chemistry, and the variety of amorphous, polymeric oxides that could be formed. In this work, the effects of alkaline reagents, flocculant addition, and dosing sequence, on the precipitation of iron (III) hydroxide and densification of the recycled sludge were investigated. Slowly dissolving lime (Ca(OH) 2) was found to be more effective than caustic (NaOH) in producing sludge with higher solid contents. Polymers addition created stronger aggregates that could withstand shearing without significant size reduction, but the overall sludge density was lower than those produced without flocculant. Conditioning the sludge at pH between 3.5 and 4.5 by adding fresh lime in a specific dosing manner appeared to be conducive to the growth of large agglomerates. The final sludge solid content of ∼15 wt.% was considerably higher than others produced under different conditions. The plate-like structures of precipitates generated with more recycles in this instance, possibly helped ease the release of entrapped water between solids during shearing, thus producing sludge with higher solid density.

Internal deterioration of concrete by the oxidation of pyrrhotitic aggregates

This paper presents research results on the causes of a severe concrete deterioration, which occurred in many building foundations approximately 2 years after construction. Concrete samples were investigated with X-ray diffraction (XRD) analysis, a scanning electron microscope (SEM) and a petrographic examination performed with a stereomicroscope. It was found that the early cracking of concrete stemmed from the oxidation of the pyrrhotite found in the anorthosite aggregates used to produce the concrete. The oxidation process led to the precipitation of iron hydroxides having a higher volume than the original pyrrhotite does. The presence of micas (biotite) close to the pyrrhotite seemed to promote and accelerate the oxidation process.

Buffering Mechanisms in Acidic Mining Lakes – A Model-Based Analysis

Buffering mechanisms of acidic mining lakes were studied on an acidic open cast lignite mining lake (Plessa 111) in Lusatia, eastern Germany. The investigations were based upon laboratory experiments, hydrochemical modelling and mineralogical structure determination. The experiments were carried out as titrations of the lake water. The modelling was done using the hydrogeochemical simulator PHREEQC. Three different phase models were applied:(1) an iron- and aluminium hydroxide precipitation model, (2) an iron- and aluminium hydroxysulphate precipitation model and (3) an iron hydroxide and aluminium hydroxysulphate precipitation model with sorption on iron hydroxide. As a result of this study four different buffers in four successive sections of the titration curve could be detected in acidic mining lake water:(1) hydrogen sulphate buffering, (2) iron buffering, (3) aluminium buffering and (4) an additional buffering (solid phase buffer) which is based on ion exchange and mineral transformation. Mineralogical analysis showed that iron precipitates as schwertmannite, an iron hydroxysulphate. Aluminium precipitates are also hydroxysulphates. These results allow formulation of process-oriented description of the buffering processes in acidic mine waters. This could lead to better reliability in lake water quality and pH-development predictions of acidic mining lakes.

THE ELEMENTAL COMPOSITION OF SOME MARINE PHYTOPLANKTON1

We analyzed the cellular content of C, N, P, S, K, Mg, Ca, Sr, Fe, Mn, Zn, Cu, Co, Cd, and Mo in 15 marine eukaryotic phytoplankton species in culture representing the major marine phyla. All the organisms were grown under identical culture conditions, in a medium designed to allow rapid growth while minimizing precipitation of iron hydroxide. The cellular concentrations of all metals, phosphorus, and sulfur were determined by high‐resolution inductively coupled plasma mass spectrometry (HR‐ICPMS) and those of carbon and nitrogen by a carbon hydrogen nitrogen analyzer. Accuracy of the HR‐ICPMS method was validated by comparison with data obtained with 55Fe radioactive tracer and by a planktonic reference material. The cellular quotas (normalized to P) of trace metals and major cations in the biomass varied by a factor of about 20 among species (except for Cd, which varied over two orders of magnitude) compared with factors of 5 to 10 for major nutrients. Green algae had generally higher C, N, Fe, Zn, and Cu quotas and lower S, K, Ca, Sr, Mn, Co, and Cd quotas than coccolithophores and diatoms. Co and Cd quotas were also lower in diatoms than in coccolithophores. Although trace element quotas are influenced by a variety of growth conditions, a comparison of our results with published data suggests that the measured compositions reflect chiefly the intrinsic (i.e. genetically encoded) trace element physiology of the individual species. Published field data on the composition of the planktonic biomass fall within the range of laboratory values and are generally close to the approximate extended Redfield formula given by the average stoichiometry of our model species (excluding the hard parts): image While clearly this elemental stoichiometry varies between species and, potentially, in response to changes in the chemistry of seawater, it provides a basis for examining how phytoplankton influence the relative distributions of the ensemble of major and trace elements in the ocean.

Studies on the transfer of heavy metals between sedimentary phases with a multi-chamber device: Combined effects of salinity and redox variation

FSrstner, U., Ahlf, W. and Calmano, W., 1989. Studies on the transfer of heavy metals between sedimentary phases with a multi-chamber device: combined effects of salinity and redox variation. Mar. Chem., 28: 145-158. Mobilization and transfer of heavy metals to major sediment components after changes of typical estuarine conditions (salinity, redox potential/pH) have been studied in a newly developed experimental device, consisting of a central chamber connected with six external chambers and separated by membranes of 0.45-~m pore diameter. Algal cell walls (Scenedesmus quadricauda), bentonite, Al-oxide, Mn-oxide, quartz powder and goethite were used as model sediment components. Significant metal enrichment was found on algal cell walls, and on freshly precipitated iron hydroxide. Oxidation of anoxic mud from Hamburg harbour causes lowering of pH and remobilization of cadmium and zinc. The experiments on the effect of seawater on sediment components and dredged mud indicate the dominant role of organic substrates in the binding of metals such as cadmium and copper, suggesting that even at relatively small percentages of organic substrates these materials are primarily involved in metabolic processes and thus may constitute the major carriers by which metals are transferred within the food chain. These results demonstrate that it is particularly problematic to disperse waste materials in ecologically productive, high-energy environments such as estuaries.

Fe2O3 Particles Coated with Silica Using TEOS

Abstract Fine particles of SiO2-coated Fe2O3 were prepared by depositing silica on an iron(III) hydroxide precipitate using tetraethoxysilane (TEOS). The silica deposition proceeded under mild conditions at 20 °C; crystallization hardly occurred and silica loading was controlled by the amount of TEOS charged, whereas the precipitate crystallized to a mixture of FeO(OH) and α-Fe2O3 under hydrothermal conditions. The specific surface area of the SiO2-coated Fe2O3 increased with silica loading, and exceeded 280 m2 g−1 even after calcination at 500 °C. The silicate species deposited on the precipitate of iron(III) hydroxide prevented the primary particles from agglomerating during calcination, resulting in the high surface area and hard reducibility of the SiO2-Fe2O3.

PH dependence of iron photoreduction in a rocky mountain stream affected by acid mine drainage

AbstractThe redox speciation of dissolved iron and the transport of iron in acidic, metal‐enriched streams is controlled by precipitation and dissolution of iron hydroxides, by photoreduction of dissolved ferric iron and hydrous iron oxides, and by oxidation of the resulting dissolved ferrous iron. We examined the pH dependence of these processes in an acidic mine‐drainage stream, St Kevin Gulch, Colorado, by experimentally increasing the pH of the stream from about 4·0 to 6·5 and following the downstream changes in iron species. We used a solute transport model with variable flow to evaluate biogeochemical processes controlling downstream transport. We found that at pH 6·4 there was a rapid and large initial loss of ferrous iron concurrent with the precipitation of aluminium hydroxide. Below this reach, ferrous iron was conservative during the morning but there was a net downstream loss of ferrous iron around noon and in the afternoon. Calculation of net oxidation rates shows that the noontime loss rate was generally much faster than rates for the ferrous iron oxidation at pH 6 predicted by Singer and Stumm (1970. Science 167: 1121). The maintenance of ferrous iron concentrations in the morning is explained by the photoreduction of photoreactive ferric species, which are then depleted by noon. Copyright © 2001 John Wiley & Sons, Ltd.

Hydrogeochemistry of seepage water collected within the Youngcheon diversion tunnel, Korea: source and evolution of SO4‐rich groundwater in sedimentary terrain

AbstractIn the Youngcheon Diversion Tunnel area, South Korea, 46 samples of tunnel seepage water (TSW) and borehole groundwater were collected from areas with sedimentary rocks (mainly sandstone and shale) and were examined for hydrogeochemical characteristics. The measured SO4 concentrations range widely from 7·7 to 942·0 mg/l, and exceed the Korean Drinking Water Standard (200 mg/l) in about half the samples. The TDS (total dissolved solid) content generally is high (171–1461 mg/l) from more shale‐rich formations and also reflects varying degrees of water–rock interaction. The water is classified into three groups: Ca SO4 type (61% of the samples collected), Ca SO4 HCO3 type (15%) and Ca HCO3 type (24%). The Ca HCO3 type water (mean concentrations=369 mg/l Ca, 148 mg/l HCO3 and 23 mg/l SO4) reflected the simple reaction between CO2‐recharged water and calcite, whereas the more SO4‐rich nature of Ca SO4 type water (mean concentrations=153 mg/l Ca, 66 mg/l HCO3 and 416 mg/l SO4) reflected the oxidation of pyrite in sedimentary rocks and fracture zones. Pyrite oxidation resulted in precipitation of amorphous iron hydroxide locally within the tunnel as well as in high concentrations of Ca (mean 153 mg/l) and Na (mean 49 mg/l) for TSW, and is associated with calcite dissolution resulting in pH buffering. The pyrite oxidation required for the formation of Ca SO4 type water was enhanced by the diffusion of oxygenated air through the fractures related to the tunnel's construction. The subsequent outgassing of CO2 into the tunnel resulted in precipitation of iron‐bearing carbonate. Copyright © 2001 John Wiley & Sons, Ltd.

REMOVAL OF IRON FROM AQUEOUS SOLUTIONS BY ION EXCHANGE WITH NA-Y ZEOLITE

Iron removal from aqueous solutions by batch ion exchange with a solid Na-Y zeolite has been studied. The pH of the solution was monitored continuously during the ion exchange process and the impact of pH on iron hydroxide precipitation and zeolite structural stability is discussed. In the case of the Fe(II)/Na-Y exchange system, the pH of the iron solutions was low enough to prevent the oxidation of Fe(II) and subsequent hydroxide deposition. The Al and Si contents in the solution were negligible, indicating maintenance of structural integrity, while scanning electron microscopic analysis did not reveal any structural breakdown. The ion exchange equilibrium isotherm, constructed at 291 ± 2 K and a total solution positive charge concentration of 0.1 equiv. dm−3, exhibited a sigmoidal shape and a maximum exchange of 74% of the indigenous sodium content; maximum exchange was essentially independent of exchange temperature. An increase in the initial Fe(II) concentration, in the range 0.005–0.05 mol dm−3, lowered the removal efficiency, but the external Fe(II) was preferred to the indigenous sodium over the entire concentration range. A maximum Fe(II) recovery of 84% from the maximally exchanged zeolite was achieved using 2 mol dm−3 solutions of NaCl as regenerant while the regenerated Na-Y delivered 68% of the original Fe(II) exchange capacity. The Fe(II) recovery was lower from samples exchanged under reflux conditions, while drying the loaded zeolite at 383 K also suppressed the degree of recovery. The treatment of Fe(III) solutions with Na-Y was not feasible due to the acidity associated with the zeolite/salt slurries, which promotes excessive hydroxide deposition and structural disintegration of the zeolite.

Effects of pH on dechlorination of trichloroethylene by zero-valent iron

The surface normalized reaction rate constants ( k sa) of trichloroethylene (TCE) and zero-valent iron (ZVI) were quantified in batch reactors at pH values between 1.7 and 10. The k sa of TCE linearly decreased from 0.044 to 0.009 l/h m 2 between pH 3.8 and 8.0, whereas the k sa at pH 1.7 was more than an order higher than that at pH 3.8. The degradation of TCE was not observed at pH values of 9 and 10. The k sa of iron corrosion linearly decreased from 0.092 to 0.018 l/h m 2 between pH 4.9 and 9.8, whereas it is significantly higher at pH 1.7 and 3.8. The k sa of TCE was 30–300 times higher than those reported in literature. The difference can be attributed to the pH effects and precipitation of iron hydroxide. The k sa of TCE degradation and iron corrosion at a head space of 6 and 10 ml were about twice of those at zero head space. The effect was attributed to the formation of hydrogen bubbles on ZVI, which hindered the transport the TCE between the solution and reaction sites on ZVI. The optimal TCE degradation rate was achieved at a pH of 4.9. This suggests that lowering solution pH might not expedite the degradation rate of TCE by ZVI as it also caused faster disappearance of ZVI, and hence decreased the ZVI surface concentration.

Chemical evolution of coal mine drainage in a non-acid producing environment, Wasatch Plateau, Utah, USA

The causes and problems of coal mine drainage, particularly acid mine drainage, in the Eastern and Interior Coal Provinces of the United States are well documented. West of the Mississippi River, where coal mines account for about 45% of total US coal production and where acid mine drainage is rare, the chemical evolution of coal mine drainage is less well documented and understood. In this investigation, we have used solute and isotopic compositions of non-evolved inflow groundwater and evolved mine discharge water to quantify the chemical evolution of mine discharge water in a western underground coal mine. Water enters the mine from fractures and roof bolt holes, which intercept groundwater in the overlying rock. Carbon-14, and 3H data indicate that these waters recharged between 12,000 and 19,500 years ago. The TDS and solute compositions of roof drip waters are spatially zoned and TDS concentrations range from about 300 to 550 mg l −1. After the water encounters minerals and other substances in the mine, the chemical differences between various mine regions become more pronounced and the TDS of mine drainage water increases to about 850 mg l −1. The TDS of mine drainage is related to water–rock ratios. Mine drainage issuing from the older mined areas, where water–rock ratios are low, has the greatest TDS. Geochemical and isotopic mass balance calculations were performed to quantify chemical reactions in the mine, and to identify sources contributing to the TDS of mine drainage. Chemical reaction pathways evaluated include pyrite oxidation, dissolution of native and rock dust gypsum, dissolution of calcite and dolomite, precipitation of calcite, ion exchange, precipitation of iron hydroxide, and organic decomposition of mining machine emulsion fluid. Solute and isotopic mass transfer reaction calculations demonstrate that the oxidation of pyrite triggers a series of cascading in-mine chemical reactions that are the primary cause of the elevated TDS of mine drainage relative to the TDS of roof-drip water. Pyrite oxidation does not result in acid drainage because of the buffering effect of abundant carbonate minerals. Dissolution of gypsum, both native and gypsum dust previously used as rock dust, is also a significant contributor of SO 4 2−. Ion exchange of Ca 2+ on the sodium zeolite analcime, which occurs in the coal, accounts for an increase in Na + concentrations. Oxidation of fugitive longwall emulsion fluid produces abundant CO 2(g) some of which indirectly affect the TDS of mine drainage.

Interaction of kappa-carrageenan with nickel, cobalt, and iron hydroxides.

In a previous paper, it was shown that the functional polysaccharide kappa-carrageenan acts as an efficient stabilizer to prevent the precipitation of iron oxides and hydroxides up to very high pH values (Jones, et al. Colloid Polym. Sci. 2000, 278, 491-501). Here, this process was investigated for its application to synthesize stable cobalt and nickel oxide particles. Nickel hydroxide nanoparticles were well stabilized for long periods, while the hydroxide of cobalt(II), although initially stable at pH 13, progressively showed some complex structure rearrangement, invoked by alkaline oxidation of the primary particles. The final product of this oxidation/self-assembly process are spherical, fluffy superstructures of cobalt(III) oxide platelets with an overall diameter of 12 microns, in coexistence with 65-75% of free kappa-carrageenan.

Weathering of rocks and neogenesis of minerals associated with lichen activity

The weathering action of lichens on rocks and the biogeophysical and biogeochemical alteration of rock-forming minerals, their influence in dissolution and precipitation reactions and in bioformation of new minerals are reviewed. The intimate adhesion of lichen thalli to the rock surface and the hyphal penetration in less coherent areas of the rock cause a physical disaggregation and fragmentation of the mineral surface. Chemical weathering is essentially due to the excretion of organic acids. Depending on the nature of the minerals, etching patterns and decomposition features are formed by biosolubilisation processes. The presence of calcium, magnesium, manganese and copper oxalate crystals at the rock–lichen interface and in the lichen thalli suggests that oxalic acid, secreted by the mycobiont, is one of the most active agents of chemical alteration. Transformation of minerals to siliceous relicts, precipitation of iron oxides and hydroxides and formation of alumino-silicates are presumably related to the activity of organic acids in complexing and removing elements from the substrate. The involvement of the so-called `lichen acids', a group of mainly polyphenolic compounds, as bioweathering agents has been only recently well documented. Present and further progresses in the study of the lichen–rock relationship rely on application of modern instrumental and analytical techniques. The relevance of lichen weathering to the biodeterioration of stoneworks of artistic value is considered.

Systematic Study of Iron Control Chemicals Used During Well Stimulation

Summary A new type of experimental procedure was developed to study iron control chemicals in acidizing treatments (citric acid, nitrilo-triacetic acid, and acetic acid) at temperatures from 25 to 95°C. This procedure gives plots of iron in solution versus equilibrium pH, while previous work has generally studied iron concentration in completely spent acid solutions. Hydrochloric acid concentrations of 7.5 to 28 wt %, sodium chloride concentrations of 0 and 5 wt%, and iron (III) concentrations from 1000 to 10 000 mg/kg were examined with varying concentrations of commonly used iron control chemicals. A systematic study of this kind has not previously been reported in the literature. The following new results were found: Iron (III) hydroxide precipitated from spent acid at much lower values of pH than are generally believed in the literature. The general belief is that iron (III) hydroxide precipitation begins at a pH of about 2.2 and is complete at a pH of 3.3. From experimental results, precipitation begins at pH values of about 1 and is nearly complete by pH 2 at 25°C. As the temperature was increased, iron (III) became significantly less soluble in the spent acid. This means that iron (III) hydroxide precipitation is potentially more damaging to the formation than generally believed. Time-dependent iron precipitation can occur in the absence of iron control chemicals. Time-dependent precipitation of ferric/calcium complexes with nitrilotriacetic acid (NTA) and citric acid was found. In partially spent acid, precipitation of previously unreported complexes containing an iron/calcium/chelating agent in a 1/1/1 mole ratio was observed. This behavior is important because of potential formation damage during the acidizing treatment itself, or during shut-in of the well after stimulation. Synergism with citric acid/acetic acid mixtures was found. These two iron control chemicals were able to prevent iron precipitation better in combination than individually.

Groundwater chemistry and fracture mineralogy in the basement granitic rock in the Tono uranium mine area, Gifu Prefecture, Japan. Groundwater composition, Eh evolution analysis by fracture filling minerals.

The mineralogy of fracture system and the chemistry of groundwater have been studied in deep boreholes drilled in granitic rock to understand the chemical evolution of groundwater and redox conditions within the deep underground which preserved the Tono uranium deposit. Geological studies revealed that fracture system within the granitic rock can be classified into intact zones, moderately fractured zones and intensely fractured zones, using the degree of fracturing. Fracture fillings within the fractured zones consist mainly of clay minerals such as montmorillonite. Hydrogen and oxygen isotopic data show that young, shallow groundwater has been traveled into a depth of at least approximately 186 m. The precipitation of iron hydroxide provides an evidence to support the idea that the oxidizing surface water has traveled down to a depth of approximately 130 m from the surface. Groundwater is rich in Na+, Ca2+ and HCO3- near the surface and rich in Na+ and HCO3- in depth. Mineral saturation indices suggest that the dissolution of plagioclase and calcite is more significant in the shallower part than in the deeper part of the groundwater system. Na+ concentration of the groundwaters increases with depth and Ca2+, Mg2+ concentrations decrease at a depth of 200 m and over. Below this depth, the increase in Na+ concentrations is nearly equivalent to the decrease in combined Ca2+ and Mg2+ concentrations, suggesting that a Na-(Ca2+, Mg2+) ion exchange reaction occurs between the montmorillonite and groundwater. The chemical evolution of groundwater in the granitic rock is controlled mainly by the dissolution of plagioclase and calcite, and by the ion exchange reaction between montmorillonite in the fractured zones and groundwater. In-situ Eh-value of groundwater in the fractured zones is approximately 0 mV at a depth of approximately 180 m. This value is on the boundary between the Fe(OH)3, and Fe2+ stability field on the Eh-pH diagram for the Fe-S-H-O system. The redox conditions of deep groundwaters in the fracture system are probably controlled by Fe2+ and Fe3+ in the granitic rocks. On the other hand, the amount of pyrite in the granite is limited. Pyrite crystals that maintain their original form are observed in the intact zones, whereas the pyrite with dissolution texture is found in the fractured zones. This can be explained that the minerals in the fractured zones have reacted with oxidized groundwater over a long period of time. There is a possibility that the redox condition of groundwater is controlled by aqueous reduced sulfur species and pyrite, and that the Eh value of groundwaters may be around -300 mV.

Contaminant source and mobilization in industrial wastes in Dalälven river drainage area, Sweden

From a summary of mineralogical and geochemical studies, this paper presents patterns of contaminant source, mobilization, and retention in three types of industrial wastes deposited in the Dalälven river drainage area in central Sweden. These industrial wastes contain significant amounts of sulfides and have been subjected to atmospheric oxidation for several tens to hundreds of years. They are the major sources of metals and non-metal contaminants discharged into the Dalälven river. The collected data are of considerable value toward the remedial management of this kind of industrial wastes. The mill-tailings initially contain carbonate minerals which neutralize the acidity and maintain the leachate pH at near neutral values at the bottom of the oxidation zone (about 40 cm deep beneath the surface). The acid neutralization results in a great deal of precipitation of ferric iron hydroxides, leading to the formation of Fe-precipitated layers - a potential barrier to metal movement and O2 diffusion downward through the tailings. The mine waste dump is composed of coarse-grained rock fragments and has been covered by crushed limestone since 1985 in an attempt to neutralize the acid leachate. However, limestone cover has much less buffering effect than expected. As a result, the underlying soil and ground water contains high levels of SO4, Fe, and other metals, and have low pH values. The sulfuric acid industry wastes consist of 70-80% iron oxides which are porous in texture. The porous texture provides a large surface area for the adsorption of SO4. During roasting, sphalerite acquired alteration rim which is composed of Zn-oxides. The mineralogical properties of the SAI wastes play an important role in the formation of hydrated sulfates of Fe and Zn. Due to the occurrence of these highly soluble sulfates, in situ flushing is a practical remedial approach for the SAI wastes, and a cleansing project is carrying out in Falun.

Alteration of microbially precipitated iron oxides and hydroxides

Iron oxide and hydroxides can be precipitated from solution with both Fe 21 and Fe 31 31 ratio of the microbial precipitate is determined by both the initial environ- ment and subsequent diagenesis. To evaluate the thermal aspects of diagenesis, biological precipitates, either largely Fe 21 or equally divided between Fe 21 and Fe 31 states, were heated at 80 8C for 12 weeks, under various redox conditions and compared to samples maintained under the same conditions at 4 8C. Mossbauer spectroscopy showed the iron oxide and hydroxides precipitated as Fe 21 to be more stable than that as Fe 31 . Only under air at 80 8C are the ferrous minerals altered to hematite, while the more labile ferric minerals are altered to Fe(OH)2 at 4 8C and to hematite at 80 8C. In contrast, chemically precipitated Fe compounds, when incubated with the consortium, only form Fe 31 com- pounds, mainly fine-grained hematite. When no microbes are present, goethite is formed during diagenesis. Fe speciation in sediments may reflect a combination of microbial me- diation that causes the initial precipitation of iron oxides and hydroxides and the subsequent conditions of the diagenetic processes characteristic of that particular depositional environment.

Diagenesis in oxidising and locally reducing conditions — an example from the Triassic Skagerrak Formation, Denmark

Diagenetic changes in red and white parts of the Skagerrak Formation (Triassic) from onshore wells in Denmark were analysed by scanning electron microscope, electron microprobe, and petrographic microscope in reflected and transmitted light. The diagenetic sequences of the red and white parts of the Skagerrak Formation are in many ways the same: early infiltration clays, followed by precipitation of caliche calcite, dolomite, mixed-layer illite/smectite, quartz, feldspar, kaolin minerals, illite, chlorite and anhydrite with increasing burial. Differences between the red and white parts of the Skagerrak Formation comprise oxidation stage in detrital minerals (oxidation of glauconite in the red part and reduction of biotite in the white part), the presence of eogenetic authigenic minerals (pyrite and poikilotopic calcite in the white part, hematite and red coatings in the red part), the amount of mesogenetic authigenic kaolin minerals (larger in the white part), the chemistry of mesogenetic authigenic chlorite (Mg-rich in the red part and Fe-rich in the white part), and the preservation stage of detrital opaque minerals (magnetite, hematite, chromite, ilmenite show none or minor alteration in the red part, whereas they have been almost completely altered in the white part). As the differences between red and white areas are found both in early eogenetic and in mesogenetic authigenic minerals, different oxidising conditions must have prevailed immediately after the deposition and continued into burial. Oxidising conditions were dominant in the arid Triassic climate and the widespread red colour of the Skagerrak Formation was probably formed during early diagenesis due to precipitation of iron oxides or hydroxides. Locally reducing areas were formed, probably by oxidation of organic material. Precipitation of oxidised iron minerals was thus prevented, and the original white colour was preserved. Reducing conditions prevailed until the reducing agent was exhausted, then an outer rim of the reduction spot might locally have been secondarily oxidised. These changing redox conditions explain the occurrence of hematized authigenic pyrite and crandallite group minerals in red areas.

Reduction of aromatic nitrocompounds with hydrazine hydrate in the presence of an iron oxide hydroxide catalyst: II. Activity, X-ray diffraction and Mössbauer study of the iron oxide hydroxide catalyst

The catalytic activities of several iron oxides and oxide hydroxides have been tested in the reduction of substituted nitroarenes using hydrazine hydrate as reducing agent. An iron oxide catalyst, prepared by rapid precipitation of iron hydroxides from solutions of iron(III)chloride, showed the highest activity mainly due to its large BET surface area compared to other iron oxides. Mössbauer spectroscopy and X-ray powder diffraction showed this catalyst to be ferrihydrite, a poorly ordered iron oxide hydroxide. During in situ experiments, in the absence of a nitrocompound, the complete transformation of ferrihydrite to Fe 3O 4 (magnetite) was observed. After addition of 4-nitrotoluene the catalyst was reoxidized to γ-Fe 2O 3 (maghemite). The same experiment carried out in the presence of 4-nitrotoluene showed that after filtration and drying the catalyst still remained ferrihydrite. Thus, reduction of the catalyst by hydrazine hydrate and subsequent reoxidation by 4-nitrotoluene seems to occur only at the surface of the catalyst. No deactivation was observed after eight times recycling of the catalyst. Carrying out the reduction at 328 K, one could observe an increase in particle size. Increasing the reaction temperature from 328 to 373 K caused a successive increase in particle size of the ferrihydrite catalyst as well as the formation of α-Fe 2O 3 (hematite) which led to a significant decrease in activity. During the filtration procedure in air, the reduced catalyst showed pyrophoric behaviour that led to the formation of α-Fe 2O 3 (hematite).