Calcite Lattice Research Articles

Relationship between δ18O and minor element composition of Terebratalia transversa

ABSTRACTWith their extensive fossil record and shells of stable low-Mg calcite, rhynchonelliform brachiopods are attractive sources of climate information via seawater temperature proxies such as stable oxygen isotope composition. In Terebratalia transversa (Sowerby) there is a progression towards oxygen isotope equilibrium in the calcite of the innermost secondary layer. This study confirms the lack of any vital effects influencing oxygen isotope composition of T. transversa, even in specialised areas of the innermost secondary layer. Calcite Mg/Ca ratio is another potential seawater temperature proxy, that has the advantage of not being influenced by salinity. Mg concentrations measured by electron microprobe analyses indicate that there is no concomitant decrease in Mg concentration towards the inner secondary layer, associated with the progressive shift towards oxygen isotope equilibrium. Mg distribution is heterogeneous throughout the shell and correlates with that of sulphur, which may be a proxy for organic components, suggesting that some of the Mg may not be in the calcite lattice. It is essential therefore, to determine the chemical environment of the magnesium ions to avoid any erroneous temperature extrapolations in brachiopods or any other calcite biomineral.

Biomimetic synthesis of calcite films by a polymer-induced liquid-precursor (PILP) process: 1. Influence and incorporation of magnesium

Magnesium-bearing calcium carbonate films have been synthesized via a polymer-induced liquid-precursor (PILP) mineralization process. A variety of morphological features of biominerals can be mimicked with this PILP process; therefore, our group has proposed that this crystallization system can be used as an effective in vitro model system for examining mechanistic issues related to biomineralization. Here, the effect of the Mg 2+/Ca 2+ ratio on the rate of transformation of the amorphous precursor films was investigated using polarized optical microscopy (POM), and the final crystalline structure and composition were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), inductively coupled plasma spectroscopy (ICP) and energy dispersive spectroscopy (EDS). The entrapment of high levels of magnesium in the deposited precursor films had a pronounced inhibitory effect on the amorphous to crystalline transformation, and furthermore, influenced the polycrystalline nature of the film. The magnesium content incorporated within the calcite lattice (8–26%) resembles the range found in biologically formed high magnesium-bearing calcite, while much lower levels were formed via the conventional solution crystallization process. The formation of non-equilibrium morphologies and similar compositions of magnesium-bearing calcite via the PILP process further supports our hypothesis that the PILP process may play a fundamental role in the formation of calcitic biominerals in nature. In the realm of biomimetic engineering, the PILP process may also establish itself as a new method to produce thin ceramic films with variable compositions under ambient conditions.

Arsenic incorporation in natural calcite lattice: Evidence from electron spin echo spectroscopy

Quaternary travertines of the Middle Pecora Valley (Tuscany, Italy) contain up to 257 ppm arsenic. Such a content is environmentally relevant, but low enough to make the exact chemical speciation of arsenic difficult by applying conventional investigation techniques. The task was addressed by use of the Electron Spin Echo (ESE) spectroscopy, taking advantage from the modulation by the arsenic nucleus of the decay spectrum of the paramagnetic ion Mn(II), occurring as replacement of Ca in the calcite lattice. Interpretation of the spectra suggests that arsenic occurs in the calcite lattice in the position of C, through the substitution CO 2− 3 ⇔ AsO 3− 3. This mechanism of arsenic incorporation by calcite may be an effective limit of arsenic mobility under conditions where immobilization through sorption by iron and/or manganese oxyhydroxides is not operating.

Formation of star-shaped calcite crystals with Mg 2+ inorganic mineralizer without organic template

Star-shaped calcite crystals with 3 ¯ m symmetry were obtained in the mixed solvent of ethanol and H 2O (4:1 vol%) using Mg 2+ as grow mineralizer without any organic template under the solvothermal condition. The crystals branched to the six directions perpendicular to the c-axis. In the process, Mg 2+ takes an important influence on such novel morphology via entering the crystal lattice of calcite to absorb the special plane and change the general growth habit. The aqueous solvent is favorable to form aragonite, while the presence of alcohol promotes the formation of calcite, the thermodynamically stable phase. The products were characterized by the techniques of XRD, SEM, SAED, IR and ICP. The formation process was also primarily studied.

Water–rock interaction induced by contaminated groundwater in a karst aquifer, Greece

The karst system of SW Trifilia is composed of a thick sequence of carbonate sediments, which have experienced two types of dolomitization and dedolomitization processes and comprise an extended aquifer. The application of fertilizers in the region have not only caused the degradation of the groundwater quality but also induced hydrochemical changes exerting major control on dolomitization processes. Factor analysis indicates high correlation coefficient between NH 4 + , NO 3 − , Ca2+ and Mg2+, which can be attributed to cation-exchange processes involving clay minerals. The application of a conservative mixing model showed that the calculated groundwater types indicate a cation-exchange process between NH 4 + , derived from fertilizers, and between Ca2+ and Mg2+. Mg2+ released from smectite interlayers, exchanged for NH 4 + in the groundwater and favor a dolomitization process through the partial replacement of Ca2+ in the lattice of calcite (dedolomite) contained in precursor dolomites. This recent stage dolomitization occurred near the water level and within the phreatic zone only and had not influenced the whole karst massif; it also resulted in low Mg/Ca values found in the zone characterized by intensive application of nitrogen-based fertilizers and the absence of overlying impermeable strata.

Depolarization in percussion drilling of Solenhofen limestone

In a series of measurements of electromagnetic radiation (EMR) from percussion-drilled Solenhofen limestone, nonoscillating signals (associated with electric depolarization) preceded by short oscillating signals (associated with crack propagation) were obtained. Depolarization is therefore assumed to be caused by stress relaxation around a propagating crack. Using the parameters of EMR signals, the polarization generating displacements of the Ca2+ ions in the calcite lattice caused by drilling were calculated.

Incorporation of Inorganic Anions in Calcite

AbstractA number of analytical methods and techniques (X‐ray diffraction, FT‐IR and electron paramagnetic resonance spectroscopy, ionic chromatography, scanning electron microscopy, etc.) have been used to study the mode and sites of incorporation of some common inorganic anions (SO42−, NO3− and Cl−) in the calcite crystal lattice. Calcite, the thermodynamically stable calcium carbonate polymorph under standard conditions, was prepared by spontaneous precipitation from calcium hydroxide and carbonic acid solutions. The particular co‐anion was added into the carbonic acid solution in the form of a calcium salt before mixing the reactants. The system prepared in such a way is particularly suitable for investigating the mode of foreign anion incorporation into the crystal lattice of calcite. Apart from the constituent ions and the products of water protolysis, this system contains only the co‐anion examined. In order to study the local environment in the calcite crystal lattice by EPR spectroscopy, Mn2+ was used as a paramagnetic substitute for Ca2+. The experiments were performed at 25 °C and the samples for analyses were taken after a predetermined ageing time of 20 min. It was found that the co‐anions, when added in the concentrations examined, did not affect the morphology of the calcite crystals but were incorporated into the calcite structure. The largest distortion of the calcite structure was obtained when sulfate ions were added. The measured parameters of the calcite unit cell are given, the results obtained are discussed and a model that explains the possible mode and sites of sulfate incorporation is presented. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

A conceptual model for near-surface kinetic controls on the trace-element and stable isotope composition of abiogenic calcite crystals

The surface of a crystal in equilibrium with solute-bearing fluid generally has a composition that differs from that of the bulk crystal. If the crystal is growing, the surface composition may be “captured” by the newly formed lattice to a degree that depends upon the growth rate and the mobility of atoms in the near-surface region: rapid growth promotes this growth “entrapment,” high near-surface mobility works against it. Natural calcites may be particularly susceptible to this kind of kinetic disequilibrium, because their precipitation rates from aqueous solution can be relatively high even at near-ambient temperatures, where ion mobility in the critical near-surface region may be limited.Existing laboratory data on trace-element uptake as a function of calcite growth rate are examined here in the context of recent discoveries concerning the structure, chemistry and kinetics of the near-surface region of calcite crystals. Recent demonstrations that ions can be mobile in the outermost few nanometers of the calcite lattice even at room temperature have the greatest potential to affect growth entrapment. The model of Watson and Liang (1995)—which quantifies entrapment efficiency in terms of growth rate, diffusivity and surface-layer thickness—is modified to include a depth-dependent diffusivity and possible depletion (as well as enrichment) of some elements in the near-surface region. With these changes, the model is shown to be qualitatively consistent with the body of experimental data on trace element uptake during calcite precipitation.This apparent success of the model invites application to stable isotopes. Constraining data are few, but available information on oxygen isotope fractionation can be used to show that growth entrapment at ambient temperatures may (depending on model assumptions) produce deviations from calcite/H2O equilibrium of up to several ‰. The preferred choice of 18O/16O for the surface layer is lighter than the lattice equilibrium value, and leads to a reduction in 18O/16O of crystals grown at higher growth rates, mimicking “vital effects.”

Experimental study of europium (III) coprecipitation with calcite

Partitioning of Eu(III) in calcite, CaCO 3, was evaluated with the aim of collecting data on partition coefficients and to enhance understanding of the incorporation mechanisms. This information will aid in the interpretation of geological processes from rare Earth element (REE) data and in the use of Eu(III) as a chemical analogue for the trivalent actinides, particularly Am(III) and Cm(III). Coprecipitation experiments were carried out by the constant addition method at 25°C and P CO2 = 1 atm. Eu(III) was strongly partitioned from the solution into calcite. For dilute solid solutions (X Eu < 0.001), Eu partition coefficients were estimated to be 770 ± 290 and found to be independent of calcite precipitation rate in the range of 0.02 to 2.7 nmol mg −1 min −1. This could be explained by the approximately equal values of the Eu partition and adsorption coefficients. Several solid solution models were tested. A vacancy model for Eu 2(CO 3) 3-CaCO 3 is consistent with the experimental results and constraints on geometry for Eu fit in the calcite lattice. For low Eu content, vacancy density is independent of Eu concentration in the solid so logarithm of the ion activity product, log (Eu) 2(CO 3 2−) 3, depends linearly on log X Eu 2. The fit of the data to such a model is good evidence that Eu(III) is taken up as a true solid solution, not simply by physical trapping. A model using EuOHCO 3-CaCO 3 is also consistent with the uptake stoichiometry, but EuOH 2+ substitution for Ca 2+ would be expected to distort the calcite structure more than is compatible with such a high K D. Several other models, including EuNa(CO 3) 2-CaCO 3, were abandoned because their stoichiometric relationships did not fit the experimental data.

Aqueous cadmium uptake by calcite: a stirred flow-through reactor study

Uptake of cadmium ions from solution by a natural Mg-containing calcite was investigated in stirred flow-through reactor experiments. Input NaCl solutions were pre-equilibrated with calcite (pH 8.0) or not (pH 6.0), prior to being spiked with CdCl 2. For water residence times in the reactor less than 0.5 h, irreversible uptake of Cd by diffusion into the bulk crystal had a minor effect on the measured cadmium breakthrough curves, hence allowing us to quantify “fast” Cd 2+ adsorption. At equal aqueous activities of Cd 2+, adsorption was systematically lower for the pre-equilibrated input solutions. The effect of variable solution composition on Cd 2+ adsorption was reproduced by a Ca 2+-Cd 2+ cation exchange model and by a surface complexation model for the calcite-aqueous solution interface. For the range of experimental conditions tested, the latter model predicted binding of aqueous Ca 2+ and Cd 2+ to the same population of carbonate surface sites. Under these circumstances, both adsorption models were equivalent. Desorption released 80 to 100% of sorbed cadmium, confirming that fast uptake of Cd 2+ was mainly due to binding at surface sites. Slow, irreversible cadmium uptake by the solid phase was measured in flow-through reactor experiments with water residence times exceeding 0.7 h. The process exhibited first-order kinetics with respect to the concentration of adsorbed Cd 2+, with a linear rate constant at 25°C of 0.03 h −1. Assuming that diffusion into the calcite lattice was the mechanism of slow uptake, a Cd 2+ solid-state diffusion coefficient of 8.5×10 −21 cm 2 s −1 was calculated. Adsorbed Cd 2+ had a pronounced effect on the dissolution kinetics of calcite. At maximum Cd 2+ surface coverage (∼10 −5 mol m −2), the calcite dissolution rate was 75% slower than measured under initially cadmium-free conditions. Upon desorption of cadmium, the dissolution rate increased again but remained below its initial value. Thus, the calcite surface structure and reactivity retained a memory of the adsorbed Cd 2+ cations after their removal.

Influence of Silicate Anions on the Morphology of Calcite Crystals

Calcite crystals with interesting morphologies were synthesized using various oligomers of silicate in solution. The crystals were grown by adding an appropriate amount of sodium silicate to a supersaturated calcium bicarbonate solution at room temperature. The morphology of nucleated calcite crystals changed significantly with the concentrations of silicate anion in solution. At lower levels, trapezoidal shaped crystals exhibiting indented and terraced structures were formed. At a Ca2+/silicate ion ratio of 1:1, oriented crystals with pseudohexagonal shapes were nucleated in solution. A higher concentration of sodium silicate produced polycrystalline calcite crystal aggregates. Powder XRD data indicates significant expansion of the calcite lattice because of the incorporation of silicate anions. We hypothesize that the observed sequential changes in morphology of calcite crystals with added silicate anion concentration may be due to the incorporation of monomeric, oligomeric, and polymeric forms of silicate anion species into the calcite lattice.

Molecular Dynamics Simulations of the Growth Inhibiting Effect of Fe2+, Mg2+, Cd2+, and Sr2+ on Calcite Crystal Growth

Molecular dynamics simulations were employed to investigate the initial stages of growth of calcium, magnesium, iron, cadmium, and strontium carbonates at two experimentally observed calcite steps. The calculated energies suggest that in a solution containing all five cations MgCO3, FeCO3, and SrCO3 grow onto the steps in preference to CdCO3 and especially CaCO3. Initial incorporation of all impurity ions is more exothermic at the more open, obtuse step than at the acute step edge (by 44−86 kJ mol-1), although growth next to an existing CaCO3 unit occurs preferentially at the acute step. Growth of full rows of impurity carbonates is highly exothermic for the first row at −25 to −155 kJ mol-1 per MCO3 depending on M (M = Mg, Fe, Cd, Sr), but is much less so for a subsequent row (−5 to −80 kJ mol-1 per MCO3), due to increasing mismatch between the rows of MCO3 at the surface and the underlying calcite lattice, especially for the large strontium ion. Calcite growth, which is a slightly endothermic process in its pure form (on average +1.8 to +35 kJ mol-1 per CaCO3), is severely hindered by the presence of impurity ions at the step edges, when the average enthalpies of the CaCO3 growth process increase to +15 to +75 kJ mol-1 per CaCO3, depending on the type of cation decorating the step. The results of these molecular dynamics simulations therefore suggest that growth of impurity carbonates at the calcite steps is initially an energetically very favorable process, hence competing effectively with growth of pure calcite. Subsequent incorporation of impurities, however, becomes more and more endothermic, until finally the presence of steps decorated with rows of impurities inhibits further crystal growth, in agreement with experimental evidence, which shows for example that the presence of magnesium and iron ions inhibits calcite growth.

Rare earth element geochemistry of hydrothermal deposits and Calyptogena shell from the Iheya Ridge vent field, Okinawa Trough.

Although the rare earth element (REE) geochemistry of hydrothermal systems have intensively been investigated at mid-oceanic ridges, studies are relatively few at the converging plate zones like the Okinawa Trough. The “chimney” deposits obtained by using submersible “SHINKAI 2000” from the Iheya Ridge site, Okinawa Trough are largely composed of massive carbonates and silicates with minor amounts of sulfides, and appear black and white depending upon their composition. The concentrations of REEs are 2–3 orders of magnitude higher in the sulfide-rich black portion than in the white portion. Both deposits show a pronounced positive Eu anomaly and the light REE enrichment which are characteristic of the high-temperature fluids venting from sediment-free oceanic ridges. Whilst the Okinawa Trough REE data are unique in the hydrothermal vent systems in having a flat chondrite-normalized pattern for the REEs heavier than Tb. The Eu-anomaly is greater in the white deposit than in the black deposit, suggesting that Eu2+ is more mobile (resistant to scavenging by sulfides) than stricktly trivalent REEs in the hydrothermal solution and may be incorporated into the former by substitution of Sr2+ in the calcite lattice. The Calyptogena shell shows the REE pattern typical for seawater without any positive Eu anomaly, suggesting that the influence of hydrothermal solution is negligible. Although the light REE concentrations are comparable, the heavy REEs are significantly higher in the shell than in the white deposit. This suggests that the concentrations of the heavy REEs in the carbonate-forming hydrothermal solution may be lower than those of ambient seawater. Thus, the hydrothermal system in the Okinawa Trough may not be a source but a sink for dissolved REEs in seawater.

Apport de la cathodoluminescence à la connaissance de la spéciation du manganèse dans les carbonates pélagiques

Mn contents analysis of pelagic carbonates from the Upper Jurassic of the Vocontian trough (Châteauneuf-d'Oze section, Hautes-Alpes) allowed the definition of six third-order eustatic sequences that agree with the chronostratigraphic chart [17]. Cathodoluminescence intensity enables a semi-quantitative approach of Mn trapped in the calcite lattice; thus, CL-results show that most of Mn is found in the pelagic limestones as Mn2+. The Mn content variations observed in the low stands (low Mn contents) and in the high-stand systems tract samples (high Mn contents) reflect the sea water Mn content variations, which appear to be linked to the hydrothermal activity, especially by the ocean ridge. In the Vocontian realm, third-order sequences of the Upper Jurassic seem to be controlled by tectono-eustatism.

The effects of biofilms on chemical processes in surficial sediments

1. The objectives of the present work were: (a) to evaluate the effects of the development and the presence of a photosynthetically active algal biofilm on chemical fluxes and processes at the sediment–water interface; (b) to measure the effects of the biofilm on chemical concentration gradients in the bulk sediment; and (c) to monitor pH and dissolved oxygen concentration in the biofilm, and through the sediment–water interface using microelectrodes.2. Two experiments were performed over a period of 8 weeks using a recirculating fluvarium channel containing river sediments with an exposed surface of 0.2 m2 and 20 dm3 of overlying solution. The first experiment was in darkness with minimal effects of a photosynthetically active biofilm. The second experiment in natural light produced a complex photosynthetic biofilm involving a succession of diatoms, green algae and cyanobacteria.3. The solution overlying the biofilm was monitored continuously for dissolved calcium, silicon, phosphorus, alkalinity and oxygen, as well as conductivity, temperature and pH. The surface of the sediment was also monitored for biological and physical changes as the biofilm developed. The overlying solution was analysed over a period of 48 h at 2‐h intervals to examine the effects of a well‐developed algal biofilm. At the end of the 48 h, pH and oxygen microelectrodes were used to measure gradients above and through the biofilm, and porewaters were analysed from sediments which had been longitudinally sectioned at a maximum depth resolution of 0.5 mm.4. Biofilm development had a large influence on the composition of the overlying solution and the development of vertical concentration gradients of solutes in the porewater. Once a diatom community was established, the concentration of dissolved silicon was low (&lt; 40 μm), with all the silicon diffusing from the underlying sediment being consumed in the biofilm (≈ 0.026 μmol m−2 s−1 at the end of the experiment).5. The concentrations of calcium, alkalinity and phosphorus in digested sediment increased near the sediment–water interface. X‐ray diffraction analysis showed that calcite was formed at the surface. Estimation of the fluxes of alkalinity and calcium in the overlying solution were consistent with calcite formation during daylight and possible dissolution in darkness. The maximum precipitation flux of calcium was 0.87 μmol m−2 s−1 and the maximum net release flux was 0.89 μmol m−2 s−1.6. The net loss of soluble reactive phosphorus from the overlying solution measured over the intensive sampling period of 48 h is consistent with a coprecipitation mechanism and a surface density of phosphorus included in the lattice of calcite of 0.097 μmol m−2. Processes in the biofilm rather than diffusion from the sediment porewater control chemical fluxes of calcium, alkalinity and phosphorus from the sediment to the overlying water.

Otoconin-90, the mammalian otoconial matrix protein, contains two domains of homology to secretory phospholipase A2.

The ability to sense orientation relative to gravity requires dense particles, called otoconia, which are localized in the vestibular macular organs. In mammals, otoconia are composed of proteins (otoconins) and calcium carbonate crystals in a calcite lattice. Little is known about the mechanisms that regulate otoconial biosynthesis. To begin to elucidate these mechanisms, we have partially sequenced and cloned the major protein component of murine otoconia, otoconin-90 (OC90). The amino acid sequence identified an orphan chimeric human cDNA. Because of its similarity to secretory phospholipase A2 (sPLA2), this gene was referred to as PLA2-like (PLA2L) and enabled the identification of human Oc90. Partial murine cDNA and genomic clones were isolated and shown to be specifically expressed in the developing mouse otocyst. The mature mouse OC90 is composed of 453 residues and contains two domains homologous to sPLA2. The cloning of Oc90 will allow an examination of the role of this protein in otoconial biosynthesis and in diseases that affect the vestibular system.

Conditions of meteoric calcite formation along a Variscan fault and their possible relation to climatic evolution during the Jurassic–Cretaceous

Two calcite cements, filling karst cavities and replacing Lower Carboniferous limestones at the Variscan Front Thrust, were precipitated after mid‐Jurassic Cimmerian uplift and subsequent erosion but before late Cretaceous strike‐slip movement. The first calcite (stage A) is nonferroan and crystals are coated by hematite and/or goethite. These minerals also occur as inclusions along growth zones. The calcite lattice contains &lt; 0·07 mol.% Fe, but Mn concentrations can be as high as 0·72 mol.% in bright yellow luminescent zones. Primary, originally one‐phase, all‐liquid, aqueous inclusions have a final melting temperature between −0·2° and +0·2 °C, indicating a meteoric origin of the ambient water. The δ13C and δ18O values of the calcites are between −7·3‰ and −6·3‰, −7·8‰ and −5·5‰ on the Vienna PeeDee Belemnite (VPDB) scale, respectively. The second calcite (stage B) consists of ferroan (0·13–0·84 mol.% Fe) blocky crystals with Mn concentrations between 0·34 and 0·87 mol.%. Primary, single‐phase aqueous fluid inclusions indicate precipitation from a meteoric fluid below 50 °C . The δ13C values of stage B calcites vary between −7·3‰ and −2·1‰ VPDB and the δ18O values between −7·9‰ and −7·2‰ VPDB. A precipitation temperature below 50 °C for the stage A calcites and the presence of iron oxide/hydroxide inclusions in the crystals indicate near‐surface precipitation conditions. Within this setting, the geochemistry of the nonferroan stage A calcites reflects precipitation under oxic to suboxic conditions. The ferroan stage B calcites precipitated in a reducing environment. The evolution from the stage A to stage B calcites and the associated geochemical changes are interpreted to be related to the change from semiarid to humid conditions in western Europe during late Jurassic–Cretaceous times. A change in humidity can explain the evolution of groundwater from oxic/suboxic to reducing conditions during calcite precipitation. The typically higher δ13C values of the stage B compared to the stage A calcites can be explained by a smaller contribution of carbon derived from soil‐zone processes than from carbonate dissolution in the groundwater under humid conditions. The small shift to lower δ18O between stage A and B calcites may be caused by a higher precipitation temperature or a decrease in the δ18O value of the meteoric water. This decrease could have been caused by a change in the source of the air masses or by an increase in the amount of rainfall during the early mid‐Cretaceous. Although the latter interpretation is preferred, it cannot be proven.

Glacial northeast Atlantic surface water PCO2: Productivity and deep-water formation

The carbon isotopic composition of bound organic material within the crystal lattice of planktonic foraminiferal calcite is a valuable proxy for the isotopic composition of primary photosynthate in surface waters. This proxy can be used to estimate p co 2 in the surface waters of the northeast Atlantic during the last glacial period, if combined with data on the carbon isotopic composition of foraminiferal calcite. Our first estimates of surface water p co 2 at the location of Biogeochemical Ocean Flux Studies Core 5K (50 °41.3′N, 21 °51.9′W) indicate that this region was probably a more important sink for CO 2 during the glacial than during the Holocene. This increased sink could not have been caused by local high primary productivity, because glacial primary productivity as estimated from benthic foraminiferal data was extremely low at 5K. We thus conclude that the enhanced sink at these latitudes resulted from increased convection of surface water to intermediate or greater depths, i.e. from the southward shift of an important centre of deep-intermediate water formation, as confirmed by computer simulation.

Nutrient dynamics in a lowland stream impacted by sewage effluent: Great Ouse, England

Nutrient and ancillary chemical changes in a stretch of the Great Ouse river near Brackley in Northamptonshire, UK, were measured on a seasonal basis over one year with an initial pilot study in the spring of 1994. River bed-sediments were characterized by their physical, adsorptive and chemical properties and a batch returned to the laboratory for experiments in a fluvarium channel. These experiments involved studies of the release and uptake of soluble reactive phosphorus to or from the overlying water in oxic conditions and release when the dissolved oxygen concentration was reduced to near zero. There was no impact of the point-sources on the concentration of nitrate in the river, a slight effect on the concentration of silicon during low-flow conditions in the summer and net uptake in the spring caused by the growth of diatoms. However there was a substantial impact on phosphorus concentrations, particularly during the summer sampling when the river was in low flow. The results for the winter showed little impact of point discharges because of the high dilution of the treated effluent. The bed-sediments at this time were found to be close to equilibrium with respect to the concentration of soluble reactive phosphorus in the overlying water. Both the fluvarium channel and field measurements obtained in the autumn are consistent with a lower net uptake of phosphorus and degradation of vegetation in the river. In the spring and summer visits, the phosphorus concentrations increased immediately downstream of the main point input and then decreased in concentration at the next downstream site. This effect was particularly noticeable in the summer and was consistent with a large uptake of phosphorus to the bed-sediment and associated vegetation. The contribution of the bed-sediment was estimated using a chemical model describing the uptake kinetics by the Elovich equation and also a parabolic equation. The stability of the waters with respect to calcite and calcium phosphate minerals was assessed in detail. Seasonal changes in the sediment composition were consistent with the deposition of calcite and coprecipitation of inorganic phosphate in the lattice of calcite, either abiotically, or in association with algal biofilms in the sediment. Good correlation between the total phosphorus and calcium contents of the sediments were evident, particularly at the sites furthest from the main sewage input. Measurements of the equilibrium phosphate concentrations of the surface sediments showed that they did not respond quickly to the higher concentrations of dissolved phosphorus found in the summer. It is also evident that the use of the equilibrium phosphate concentration to predict the magnitude of the release of soluble reactive phosphorus becomes less reliable as the solution concentration approaches the equilibrium phosphate concentration. Perturbations may arise because of changes in the surface micro-layer caused by a number of processes such as particle-size fractionation, biological activity or changes in the local redox conditions. However bearing these constraints in mind, with an equilibrium phosphate concentration of the sediments generally below 5 μmol dm −3, the release of phosphorus to the overlying water is not expected until the concentration is below this value. The results also show that phosphorus is not accumulating in the surface sediment and that much of the phosphorus in the sediment is not easily desorbed.

X-ray standing wave investigation of the surface structure of selenite anions adsorbed on calcite

The adsorption of selenite ions (SeO 2− 3) from a dilute aqueous solution onto a freshly-cleaved calcite (101̄4) surface was studied with the X-ray standing wave (XSW) technique. The complex ion SeO 2− 3 is found to selectively adsorb at the CO 2− 3 site via ionic exchange, forming a two-dimensional solid-solution of the form Ca(SeO 3) x (CO 3) 1 − x at the interface. The calcite (101̄4), (0006) and (112̄0) Bragg reflections were used to triangulate the Se position with respect to the calcite lattice. The local surface structure at the SeO 2− 3 adsorbate site, derived from the XSW results, is consistent with a model in which the base of the SeO 2− 3 trigonal pyramid aligns with (and replaces) the CO 2− 3 equilateral triangular group. The SeO 2− 3 adsorption saturated at a coverage of 0.02 monolayers. Under identical chemical conditions, selenate (SeO 2− 4) adsorption was inhibited.