Rudolf Giovanoli

Transformation of phosphorus species in settling seston and during early sediment diagenesis

Sequential P extraction was combined with electron microscop and X-ray spectroscopy to characterise various P species and to study their transformation in settling seston and in recent sediment. During early diagenesis most of the particulate P formed in the water was redissolved. No net transformation into species that would resist dissolution was observed.It was shown that• the phosphorus (P) content and the P flux of settling particles varied seasonally over one order of magnitude• particles became enriched with reductant soluble P (BD-P) while settling through the hypolimnion• changes in BD-P were highly significantly correlated with changes in reductant soluble iron (BD-Fe)• bacteria oxidising Fe and Mn seemed to be mainly responsible for this increase in P concentration• other fractions including organic P did not change during sedimentation• most of the organic P and of the Fe bound P and 70% of TP was released from the sediment during early diagenesis• the sediment surface did not act as a trap for P migrating upwards from deeper sediment layers• CaCO3 sedimentation contributed little to P sedimentation but significantly to the permanent burial of P.

Early diagenetic influences on iron transformations in a freshwater lake sediment

Abstract Iron transformations in a calcium carbonate rich fresh-water sediment were studied by analyzing the relevant constituents of both interstitial water and solid matter. Analysis of interstitial water shows that the observed redox sequence NO − 3 NH + 4 , MnO 2 Mn(II) , FeOOH/Fe(II), SO 2− 4 S(−II) is roughly in agreement with that predicted by the Gibbs Free Energy for the corresponding reactions. In contrast to marine sediments, these redox transitions occur in the uppermost sediments, i.e., at depths of 0–4 cm. Deeper in the sedimentary sequence, the depth profile for dissolved iron exhibits a steady non-linear increase up to 400 μmol dm−3. In this anoxic zone, according to thermodynamic predictions, iron (II)-minerals such as iron sulfide, siderite, and vivianite should precipitate while Fe(III) oxides should be completely dissolved. However, microscopic analysis showed that Fe(III) oxides were present throughout the studied sediment. Furthermore, scanning electron microscope/energy dispersive spectroscopy analysis suggests the presence of iron sulfide could be verified but not that of siderite or vivianite. These observations indicate kinetic control of iron transformations. We have investigated the importance of kinetic control of iron distribution in anoxic sediments using a diagenetic model for dissolved iron(II). A rough estimate of time scales for dissolution and precipitation rates was made by imposing limiting boundary conditions. Using the calculated rate constant, we established that more than 1000 years would be required for the complete dissolution of Fe(III) oxides, which is agreement with our observations and experimental data from the literature. Calculated precipitation rates of Fe(II) for a given mineral phase such as siderite yield a maximum value of 3 μg(FeCO3) g−1(dry sediment) yr−1. Such low rates would explain the absence of siderite and vivianite. Finally, it can be inferred from the Mn T Fe T ratio in the sediments that this ratio depends on the redox conditions of the sediment-water interface at the time of deposition. Thus, this ratio can be used as “paleo-redox indicator” in lacustrine sediments.

Kinetics and mechanism of the dehydration of γ-FeOOH

Abstract The dehydration of γ-FeOOH to γ-Fe 2 O 3 in vacuo has been investigated by thermoanalysis. Results have been checked by electron microscopy and diffraction and by x-ray diffraction. Authors find that formal kinetics are not conclusive. Electron micrographs, however, show directly that random nucleation occurs, producing perfectly oriented but disordered γ-Fe 2 O 3 crystals of about 70 A size. The results can be accomodated with an atomistic model of lattice collapse.

Thermogravimetry of manganese dioxides

Abstract A number of manganese dioxides have been synthesized and investigated by X-ray diffraction, electron microscopy and thermogravimetry under flowing oxygen. The TG curves show, (a) water loss, (b) release of OH-water, and (c) oxygen release. The water loss is negligible for fully crystalline β-MnO 2 and the significant main step is due to the transformation to α-Mn 2 O 3 . However, γ- and e-MnO 2 exhibit a remarkable mass loss long before the main step occurs. Phyllomanganates also show high water loss and the nucleation of new phases. Comparison of the various water loss curves reveals that three types of water are present. (i) Adsorbed (loosely bound) molecular water which desorbs from 25 to 105°C. (ii) Interlayer water which in phyllomanganates is still molecular but more tightly bound. It is released at temperatures overlapping with those of adsorption water release but extending up to 150–250°C. (iii) The condensation of OH-groups in γ- and e-MnO 2 leads to the release of much more tightly bound water (which we call OH-water) from 105 to 500°C and even higher. The release of OH-water is accompanied by the nucleation of β-MnO 2 . Battery active material has some adsorbed molecular water but the significant feature is the presence of OH groups and an equivalent amount of Mn 3+ ions in the lattice. Vacancies are a characteristic feature. Thermogravimetry in combination with X-ray diffraction and electron microscopy is shown to be a valuable means of characterising MnO 2 .

Biogeochemistry of iron in an acidic lake.

In this paper, the fate of iron in Lake Cristallina, an acidic lake in the Alps of Switzerland, is discussed. A simple conceptual model is developed in order to explain the observed diel variation in dissolved iron(II) concentration. Biotite weathering provides reduced iron that is oxidized and subsequently precipitated in the lake. The amorphous Fe(III)hydroxide (FeOOH ⋅xH2O), found in the sediments of Lake Cristallina, is an Fe(II) oxidation product. This oxygenation reaction is most probably catalyzed by bacteria surfaces, as indicated by the relatively high estimated oxidation rate compared to the oxidation rate of the homogeneous oxidation of inorganic Fe(II) species at the ambient pH of Lake Cristallina (pH 5.4 at 4 °C) and by the scanning electron micrograph pictures. Under the influence of light, these amorphous iron(III)hydroxide phases are reductively dissolved. The net concentration of Fe(II) reflects the balance of the reductive dissolution and the oxidation/precipitation reactions and tends to parallel the light intensity, leading to a diurnal variation in the Fe(II) concentration. The rate of the photochemical reductive dissolution of Lake Cristallina iron(III)hydroxides is greatly enhanced in situ and in the laboratory by addition of oxalate to the lake water.

The composition of settling particles in Lake Zürich

Settling particles in the Lake Zürich were collected at different depths. Water samples and particles have been analyzed for heavy metals and for organic matter elements. Samples were investigated by transmission electron microscopy, scanning electron microscopy, energy dispersive X ray and X ray. The elemental composition of the settling particles was found to be almost constant. Special remarks are made on iron and phosphate and on manganese. Manganese oxidizing microorganisms were found near the bottom of the lake.

Über kristallines chrom(III)hydroxid II. Thermische Zersetzung

Abstract By thermolysis the crystalline chromium(III)hydroxidetrihydrate decomposes to the amorphous hydroxide which is, under an appropriate atmosphere, superficially oxidized until x &2sbnd;Cr 2 O 3 nucleates. The nucleation temperature of Cr 2 O 3 depends on the atmosphere , the heating rate, and other experimental conditions. This x &2sbnd;Cr 2 O 3 is superficially oxidized [probably to Cr IV ],but no other phases can be detected by X-ray methods even with molybdenum radiation. The amorphous phase present just before nucleation of x &2sbnd;Cr 2 O 3 only shows slightest traces of short range order but no defined crystalline phase from X-ray evidence. This collides with results of other authors but is explicable on careful inspection of the experimental conditions: in extremely thin sample layers, where the desorption of, e.g. , H 2 O is comparatively easy, the reaction path is different from that in thick samples, where water etc. cannot readily escape but is caught in a microporous system in which considerable vapour pressures can build up. This confirms that topochemical reactions may lead to products that are difficult to obtain in equilibrium systems. By decomposition of crystalline Cr(OH) 3 ·3H 2 O under water vapour a short range order develops up to 300°C that may be attributed to a particular lattice, e.g. , to trigonal CrOOH of extremely small crystalline dimensions. However, we believe that the concept of crystal lattice becomes meaningless at such small dimensions.

Determination of activation energy in moist and dry conditions for the pyrolysis of Zn5(OH)6(CO3)2

Abstract Systematic work shows that the reaction rate and decomposition rate of Zn5(OH)6(CO3)2 depend on various parameters. The influence of the experimental atmosphere and the partial pressure of water are very important. The decomposition rates in moist air, dry air and high vacuum were determined.

Transformation of akaganeite into goethite and hematite in the presence of Mn

The interaction of Mn and akaganéite in neutral to alkaline media has been investigated using X-ray powder diffraction and transmission electron microscopy. Akaganéite transformed into goethite and/or hematite, whereas Mn precipitated as hausmannite and birnessite at pH > 12 and as manganite at pH 7.5–8.5. Mn influenced the kinetics of the transformation of akaganéite: the rate-determining step, i.e., the dissolution of akaganéite, was retarded by adsorbed Mn species. Hematite formation was not suppressed. By catalyzing the air oxidation of adsorbed Mn(II), akaganéite promoted the formation of birnessite. Akaganéite did not retard recrystallization of the Mn phases. The incorporation of Mn in the structure of goethite formed in this system was negligible, and jacobsite (MnFe2O4) did not form. The formation of mixed Mn-Fe phases appeared to require a ratio of Mn2+: Fetotal > 0.02; this ratio was not achieved due to the oxidation of Mn2+ at the akaganéite surface.

High temperature phase segregation of a new host for Er3+ upconversion: Cs3Tl2Cl9

Cs3Tl2Cl9 is one of the few air stable low-phonon host lattices of interest to Er3+ upconversion. Solution growth at ambient temperature demonstrated that water and a number of other molecular liquids do not yield Er3+doped crystals. Optical and differential scanning calorimetry measurements revealed a phase segregation at 310 °C which is reversible in the presence of 1 atm Cl2 . Growth from molten salt solutions or gas phase deposition techniques is hence restricted to a deposition temperature of less than 300 °C. Preliminary results show that molten ZnCl2 may be used as a flux to obtain lanthanidedoped single crystals or epitaxial layers.