Calcite Precipitation Rate Research Articles

Electrochemical removal of calcium and magnesium ions from aqueous solutions

The present study shows upon, electrochemical removal of Ca 2+ and Mg 2+ ions from aqueous solutions. Calcite and brucite precipitation occurs through a chemical process following the electrochemical generation of OH − ions in the vicinity of the electrode surface. The kinetics of the electrochemical calcite and brucite precipitation were examined by measuring the electric conductivity as a function of time using different voltages and concentrations. The electric current density initially augments the electrochemical precipitation of calcite, but at a sufficiently higher current density the precipitation rate tends to an asymptotic limit. Furthermore, the precipitation rate of calcite is limited by diffusion kinetics of HCO 3 − ions and does not vary with the Ca 2+ concentration. In contrast, the electrochemical precipitation of brucite correlates with current density, as there are no limitations in diffusion kinetics. With increasing Mg 2+ concentration the rate constant of brucite decreases as small Mg 2+ concentrations or high pH values promote the generation of MgOH + (aq.) species in the reaction zone, which are responsible for an enhanced precipitation rate. In addition the specific energy consumption required for 1 mmol/L Ca 2+ removal strongly increases with the applied voltage, whereas the energy consumption required for 1 mmol/L Mg 2+ removal is nearly independent of the applied voltage.

Inhibition of calcite growth by alginate

The kinetics of calcite precipitation in the presence of alginate was investigated using the constant composition technique. In the concentration range investigated (0.0002–0.005 g L −1), alginate inhibits calcite precipitation. The extent of inhibition increased with increased alginate concentration and decreased solution supersaturation. Alginate adsorption, derived from normalized calcite precipitation rates, is described satisfactorily by the Langmuir adsorption model. At lowest supersaturation, alginate adsorption onto calcite probably reaches its maximal uptake of 7.5E-4 g m −2, corresponding to surface coverage of one molecule for each 200–300 nm 2, depending on the molecular mass of alginate. This means that one alginate molecule can be bound over 100–150 Ca surface sites. Initially, on the surface of the inhibited calcite, XPS identified alginate but after further time in solution, when the system had recovered, XPS demonstrated that it disappeared from the surface, presumably buried under the newly formed calcite. The alginate affinity constant decreases with increasing supersaturation, evidence for incomplete adsorption. A simple model based on competition between growth and desorption effectively describes the observed change in the adsorption constant.

Comparison of rates of ureolysis between Sporosarcina pasteurii and an indigenous groundwater community under conditions required to precipitate large volumes of calcite

Ureolysis-driven calcite precipitation has potential to seal porosity and fracture networks in rocks thus preventing groundwater flow and contaminant transport. In this study urea hydrolysis and calcite precipitation rates for the model bacterium Sporosarcina pasteurii were compared with those of indigenous groundwater communities under conditions required to precipitate large volumes of calcite (up to 50 g L −1). We conducted microcosm experiments in oxic artificial and anoxic natural groundwaters (collected from the Permo-Triassic sandstone aquifer at Birmingham, UK) that were inoculated with aerobically grown S. pasteurii. The rate constants for urea hydrolysis, k urea, ranged between 0.06 and 3.29 d −1 and were only affected by inoculum density. Higher Ca 2+ concentration (50–500 mM Ca 2+) as well as differences in fO 2 did not inhibit the ureolytic activity of S. pasteurii and did not significantly impact k urea. These results demonstrate that S. pasteurii has potential to improve calcite precipitation in both oxic and anoxic groundwaters, especially if indigenous communities lack ureolytic activity. Urea hydrolysis by indigenous groundwater communities was investigated in anoxic, natural groundwaters amended with urea and CaCl 2. A notable increase in ureolysis rates was measured only when these communities were stimulated with dilute nutrients (with best results from blackstrap molasses). Furthermore, there was a considerable lag time (12–20 days) before ureolysis and calcite precipitation began. Calculated ureolysis rate constants, k urea, ranged between 0.03 and 0.05 d −1 and were similar to k urea values produced by S. pasteurii at low inoculum densities. Overall, this comparative study revealed that the growth of ureolytic microorganisms present within groundwaters can easily be stimulated to enhance rates of urea hydrolysis in the subsurface, and thus can be used to induce calcite precipitation in these environments. The time required for urea hydrolysis to begin is almost instantaneous if an inoculum of S. pasteurii is included, while it may take several weeks for ureolytic groundwater communities to grow and become ureolytically active.

The effect of dissolved sulphate on calcite precipitation kinetics and consequences for subsurface CO2 storage

Steady state calcite precipitation rates were measured in mixed-flow reactors at 25 °C and pH ∼9 in the presence and absence of aqueous sulphate. Two aqueous inlet solutions were used to provoke precipitation 1) containing NaHCO3 and Na2CO3 and 2) a second containing CaCl2. 0-20 mM of Na2SO4 was added to this second solution to assess the effects of the presence of aqueous sulphate on rates. The presence of aqueous sulphur is found to decrease calcite precipiation rates; the presence of 20 mM lowers calcite dissolution rates by a factor of 2 at a constant Ω of 2.6. The slowing of calcite precipitation may aid subsurface carbon storage efforts as it will slow pore clogging of injected rock formation. In addition as the rate limiting step of mineral carbonation is the dissolution of divalent-metal bearing silicate solids such as basaltic glass or olivine, it seems likely that a decrease of carbonate precipitation rates of a factor of 2 will negligibily effect mineral carbonation efforts. © 2010 Elsevier Ltd. © 2011 Published by Elsevier Ltd.

Calcite precipitation from aqueous solutions with different calcium and hydrogen carbonate concentrations

This paper describes the influence of different Ca 2 + and HCO 3 − concentrations on the precipitation of calcite in aqueous solutions. Mixtures of CaCl 2 and NaHCO 3 solutions with different concentrations were stirred, covering a wide range of supersaturation and precipitation of calcite. The resulting reduction of the Ca 2 + concentration was recorded as a function of time by measuring the electric conductivity and the pH value. The nucleation rate increased with increasing supersaturation and can be described with the classical theory of nucleation. For different solutions with similar values of supersaturation, the hydrogen carbonate/calcium ratio had no significant influence on the rate of nucleation. At a given calcium concentration the precipitation rate increased with increasing supersaturation. This effect was more pronounced at higher supersaturations. Measurements at similar values of supersaturation showed that the calcite precipitation rate increased with increasing hydrogen carbonate/calcium ratio. These results can be explained by applying a surface complexation model. The crystal surface concentrations of the two species > CaCO 3 − and > CO 3 − and the adsorption of CaCO 3 0 ion pairs are responsible for catalysing calcite precipitation.

Seasonal microclimate control of calcite fabrics, stable isotopes and trace elements in modern speleothem from St Michaels Cave, Gibraltar

Abstract Detailed monitoring of three drip sites in New St Michael's Cave, Gibraltar, reveals a strongly coherent seasonal pattern of dripwater chemistry despite each site having significantly different flow paths and discharge patterns. Calcite saturation is closely linked to regular seasonal variations in cave air pCO 2 caused by seasonally reversing ventilation driven by temperature difference between the cave interior and the air outside. A coupled model of CO 2 degassing and calcite precipitation links seasonal δ 13 C variations in coexisting dripwater, cave air CO 2 and speleothem calcite to large variations in pCO 2 that are driven by cave ventilation. The relationships between stable isotope ratios, Sr/Ca and speleothem fabrics across annually formed calcite laminae are consistent with a degassing–calcite precipitation process in which rapid degassing controls the δ 13 C of both drip water DIC and calcite whereas a much slower rate of calcite precipitation causes seasonal cycles of Sr in a more complex manner. By demonstrating the causes of laminated speleothem fabrics plus trace element and isotope cycles in modern speleothem from a closely monitored cave, this study provides clear links between the local microclimate and the proxy record provided by speleothem geochemistry. In Gibraltar, low cave air pCO 2 in summer is unusual compared to what has been revealed by cave monitoring carried out elsewhere and shows that caution is needed when linking paired speleothem fabrics to specific seasons without knowledge of local processes operating in the cave.

Cave air ventilation and CO2 outgassing by radon-222 modeling: How fast do caves breathe?

In general, the rate and timing of calcite precipitation is in part affected by variations in cave air CO2 concentrations. Knowledge of cave ventilation processes is required to quantify the effect variations in CO2 concentrations have on speleothem deposition rates and thus paleoclimate records. In this study we use radon-222 (222Rn) as a proxy of ventilation to estimate CO2 outgassing from the cave to the atmosphere, which can be used to infer relative speleothem deposition rates. Hollow Ridge Cave, a wild cave preserve in Marianna, Florida, is instrumented inside and out with multiple micro-meteorological sensor stations that record continuous physical and air chemistry time-series data. Our time series datasets indicate diurnal and seasonal variations in cave air 222Rn and CO2 concentrations, punctuated by events that provide clues to ventilation and drip water degassing mechanisms. Average cave air 222Rn and CO2 concentrations vary seasonally between winter (222Rn = 50 dpm L− 1, where 1 dpm L− 1 = 60 Bq m− 3; CO2 = 360 ppmv) and summer (222Rn = 1400 dpm L− 1; CO2 = 3900 ppmv). Large amplitude diurnal variations are observed during late summer and autumn (222Rn = 6 to 581 dpm L− 1; CO2 = 360 to 2500 ppmv). We employ a simple first-order 222Rn mass balance model to estimate cave air exchange rates with the outside atmosphere. Ventilation occurs via density driven flow and by winds across the entrances which create a ‘venturi’ effect. The most rapid ventilation occurs 25 m inside the cave near the entrance: 45 h− 1 (1.33 min turnover time). Farther inside (175 m) exchange is slower and maximum ventilation rates are 3 h− 1 (22 min turnover time). We estimate net CO2 flux from the epikarst to the cave atmosphere using a CO2 mass balance model tuned with the 222Rn model. Net CO2 flux from the epikarst is highest in summer (72 mmol m− 2 day− 1) and lowest in late autumn and winter (12 mmol m− 2 day− 1). Modeled ventilation and net CO2 fluxes are used to estimate net CO2 outgassing from the cave to the atmosphere. Average net CO2 outgassing is positive (net loss from the cave) and is highest in late summer and early autumn (about 4 mol h− 1) and lowest in winter (about 0.5 mol h− 1). Modeling of ventilation, net CO2 flux from the epikarst, and CO2 outgassing to the atmosphere from cave monitoring time-series can help better constrain paleoclimatic interpretations of speleothem geochemical records.

Environmental versus biomineralization controls on the intratest variation in the trace element composition of the planktonic foraminifera G. inflata and G. scitula

The intratest variation in the chemical composition of Globorotalia scitula and G. inflata recovered from a sediment trap sample collected at 3000 m in the North Atlantic in early spring has been investigated using laser ablation inductively coupled plasma–mass spectrometry and electron microprobe. Mg/Ca, Li/Ca, B/Ca, Mn/Ca, and Ba/Ca vary by up to a factor of 10 through the test walls. Water column properties, including temperature and salinity, are well documented at the trap site, and the observed variations are too large to be explained by vertical migration of the foraminifera. However, changes in calcite precipitation rate, crystal structure, or the chemical composition of the internal calcification reservoir also cannot, by themselves, fully account for the pattern of intratest variability. Nevertheless, the average Mg/Ca for each chamber generally produces a Mg/Ca temperature that matches that measured in the water column. The exception is small, morphologically distinct G. inflata tests that have anomalously high Mg/Ca.

Abiotic versus biotic controls on the development of the Fairmont Hot Springs carbonate deposit, British Columbia, Canada

AbstractThe relict Fairmont Hot Springs deposit, formed largely of carbonates, covers an area of 0·5 km2, and is up to 16 m thick. The triangle‐shaped discharge apron, which broadens down‐valley, is divided into a proximal part with beds dipping at <10° and a distal part with beds dipping at 10° to 15°. The deposit is formed of the: (1) Basal Macrophyte; (2) Lower Carbonate; (3) Middle Clastic; (4) Upper Carbonate; and (5) Upper Clastic Sequences. Two charcoal samples embedded in the Lower Carbonate Sequence yielded dates of 8690 ± 90 and 8270 ± 70 cal yr bp, indicating that much of the deposit formed post‐glacially during the Early to Mid‐Holocene. Deposit aggradation ceased in the Mid to Late Holocene when the Fairmont Creek valley was incised. The Lower and Upper Carbonate Sequences, which are the thickest sequences, are composed of nearly equal parts of travertine (abiotic) and tufa (biotic), with feather dendrite travertine, radiating dendrite travertine and stromatolite tufa dominating. Competition between calcite precipitation rates and biotic growth rates controlled the distribution of tufa and travertine across the discharge apron. Calcite and biotic growth rates were controlled largely by flow velocity across the apron which, in turn, was controlled by topography and regular fluctuations in spring water discharge volume. During times of high spring discharge, slow sheet flow over the proximal part of the apron promoted stromatolite growth, whereas fast, turbulent flow on the distal part of the apron induced rapid feather dendrite formation. During times of low spring discharge, quiescent, shallow evaporative pools, conducive to radiating dendrite formation, formed on the proximal part of the apron, whereas slow flow on the distal part promoted stromatolite growth. Facies with high calcite supersaturation experienced rapid abiotic dendrite growth that precluded most biotic growth.

Oxygen isotopic fractionation during inorganic calcite precipitation ― Effects of temperature, precipitation rate and pH

Stable oxygen isotopic fractionation during inorganic calcite precipitation was experimentally studied by spontaneous precipitation at various pH (8.3 < pH < 10.5), precipitation rates (1.8 < log R < 4.4 μmol m − 2 h − 1 ) and temperatures (5, 25, and 40 °C) using the CO 2 diffusion technique. The results show that the apparent stable oxygen isotopic fractionation factor between calcite and water ( α calcite–water) is affected by temperature, the pH of the solution, and the precipitation rate of calcite. Isotopic equilibrium is not maintained during spontaneous precipitation from the solution. Under isotopic non-equilibrium conditions, at a constant temperature and precipitation rate, apparent 1000ln α calcite–water decreases with increasing pH of the solution. If the temperature and pH are held constant, apparent 1000ln α calcite–water values decrease with elevated precipitation rates of calcite. At pH = 8.3, oxygen isotopic fractionation between inorganically precipitated calcite and water as a function of the precipitation rate ( R) can be described by the expressions 1000 ln α calcite – water = - 1.102 log R + 34.56 1000 ln α calcite – water = - 1.094 log R + 30.87 1000 ln α calcite – water = - 0.534 log R + 26.80 at 5, 25, and 40 °C, respectively. The impact of precipitation rate on 1000ln α calcite–water value in our experiments clearly indicates a kinetic effect on oxygen isotopic fractionation during calcite precipitation from aqueous solution, even if calcite precipitated slowly from aqueous solution at the given temperature range. Our results support Coplen's work [Coplen T. B. (2007) Calibration of the calcite–water oxygen isotope geothermometer at Devils Hole, Nevada, a natural laboratory. Geochim. Cosmochim. Acta 71, 3948–3957], which indicates that the equilibrium oxygen isotopic fractionation factor might be greater than the commonly accepted value.

Preliminary evaluation of a constructed wetland for treating extremely alkaline (pH 12) steel slag drainage

High pH (> 12) leachates are an environmental problem associated with drainage from lime (CaO)-rich industrial residues such as steel slags, lime spoil and coal combustion residues. Recent research has highlighted the potential for natural ('volunteer') wetlands to buffer extremely alkaline influent waters. This appears ascribable to high CO(2) partial pressures in the wetland waters from microbial respiration, which accelerates precipitation of calcium carbonate (CaCO(3)), and the high specific surface area for mineral precipitation offered by macrophytes. The research presented here builds on this and provides preliminary evaluation of a constructed wetland built in March 2008 to buffer drainage from steel slag heaps in north-east England. The drainage water from the slag mounds is characterised by a mean pH of 11.9, high concentrations of Ca (up to 700 mg/L), total alkalinity (up to 800 mg/L as CaCO(3)) and are slightly brackish (Na = 300 mg/L; Cl = 400 mg/L) reflecting native groundwaters at this coastal setting. Documented calcite precipitation rates (mean of 5 g CaCO(3)/m(2)/day) from nearby volunteer sites receiving steel slag drainage were used to scale the constructed wetland planted with Phragmites australis; a species found to spontaneously grow in the vicinity of the discharge. Improved performance of the wetland during summer months may at least in part be due to biological activity which enhances rates of calcite precipitation and thus lowering of pH. Secondary Ca-rich precipitates also serve as a sink for some trace elements present at low concentrations in the slag leachate such as Ni and V. The implications for scaling and applying constructed wetlands for highly alkaline drainage are discussed.

Factors controlling present-day tufa dynamics in the Monasterio de Piedra Natural Park (Iberian Range, Spain): depositional environmental settings, sedimentation rates and hydrochemistry

The tufa record and hydrochemical characteristics of the River Piedra in the Monasterio de Piedra Natural Park (NE Spain) were studied for 6 years. The mean discharge of this river was 1.22 m3/s. The water was supersaturated with calcium carbonate. The HCO3 −, Ca2+ and TDIC concentrations decreased along the 0.5-km-long studied stretch, whereas the calcite SI showed no systematic downstream or seasonal variation over the same stretch. Several sedimentary subenvironments exist in which four broad types of tufa facies form: (1) Dense laminated tufa (stromatolites), (2) Dense to porous, massive tufa, (3) Porous, coarsely laminated tufa with bryophytes and algae, and (4) Dense, hard, laminated deposits in caves. The half-yearly period thickness and weight of sediment accumulated on 14 tablets installed in several subenvironments showed that the deposition rate was greater in fast flowing river areas and in stepped waterfalls, and lower in slow flowing or standing river areas and in spray and splash areas. Mechanical CO2 outgassing is the main factor controlling calcite precipitation on the river bed and in waterfalls, but this process does not explain the seasonal changes in depositional rates. The deposition rates showed a half-yearly period pattern recorded in all fluvial subenvironments persistent over time (5.26 mm, 0.86 g/cm2 in warm periods; 2.26 mm, 0.13 g/cm2 in cool periods). Mass balance calculations showed higher calcite mass values in warm (21.58 mg/L) than in cool (13.68 mg/L) periods. This biannual variation is mainly attributed to the seasonal differences in temperature that caused changes in inorganic calcite precipitation rate and in biomass and the correlative photosynthetic activity. Tufa sedimentation was therefore controlled by both physicochemical and biological processes. The results of this study may help test depositional rates and their environmental controls and thus assess the climatic and hydrological significance of ancient tufas in semi-arid conditions, in particular in the Quaternary.

Laboratory and field tests of CO2–water injection into the Ogachi hot dry rock site, Japan

Abstract This paper reports the results of laboratory and field experiments of CO 2 sequestration into the Ogachi hot dry rock(HDR; the temperature is 200 degree C) site, where a part of CO 2 will be expected to be fixed as carbonates by interaction with rocks (Georeactor; Ca extraction from rocks and carbonate fixation). In 2007, CO 2 dissolved water (river water with dry ice) was directly injected into OGC-2 (from September 2nd to 9th) and Run #2(from September 11th to 16th). Several tracers were also injected at the same time. Water samples are collected at the depth of ca. 800 m by a sampler (500 ml in volume) and monitored for their chemical and isotopic compositions. During the Run #2 experiment, river water was injected into OGC-1 at 2 days after injection of CO 2 water into OGC-2. During the field experiments, dissolution or precipitation rates of calcite were determined by using a technique of ”in site analyses”. Calcite crystals covered with Ti rod or Au film is hold in a crystal cell and set in a crystal sonde. The crystal sonde is then put into OGC-2 and water samples at the certain depth is introduced into the sonde. After 1 hour, the sonde is recovered and the calcite crystal is observed by a newly developed phase shift interferometer to analyze the dissolution or precipitation rates of calcite from the reservoir fluids. The “in situ analyses” show that calcite precipitation was observed within 2 day after the injection. This supports the view that most of CO 2 injected might be fixed as carbonate.

ジオリアクターによる CO2 固定化試験:雄勝高温岩体での原位置試験

Field experiments of CO2 sequestration into the Ogachi hot dry rock (HDR; the temperature is 200 °C) site were performed to investigate mineralization of a part of CO2 as carbonates through interaction with rocks (Georeactor; Ca extraction from rocks and carbonate fixation). In 2007, carbonated water (1 wt% CO2; river water with dry ice) with several tracers was directly injected into well OGC-2, Test #1 (from September 2nd to 9th) and Test #2 (from September 11th to 16th). During the Test #2 experiment, additional river water was injected into well OGC-1, 100 m apart from well OGC-2, at 2 days after the injection of CO2 water into well OGC-2 so as to push back carbonated water around this well. Water samples were collected at the depth of ~850 m by a sampler (500 ml in volume) and monitored for their chemical and isotopic compositions. The CO2 concentrations in fluids collected decreased with duration time and were almost 2/3 of the expected concentration from tracer concentrations. This means that CO2 injected into well OGC-2 can be removed from fluid by carbonate fixation. During the field experiments, dissolution or precipitation rates of calcite were determined by using a technique of “in situ observation of crystallization”. Calcite crystals covered with Au film were hold in a crystal cell and set in a crystal sonde. The crystal sonde was filled with He gas under a certain pressure and then put into well OGC-2. The fluid at depth of -850 m is introduced into the sonde and calcite can start to react with them. After 1 hour, the sonde was recovered to the surface, where the fluid in the sonde is pushed to outside by He gas. The recovered calcite crystal is analyzed for the dissolution or precipitation rates by a newly developed phase shift interferometer. The “in situ analyses” show that calcite precipitation was observed within 2 days after the injection. This probably indicates that the most of CO2 injected has been fixed as carbonate, supporting our idea that HDR system can be useful for CO2 sequestration from inland emission sources.

Geochemical and hydrological controls on biannual lamination of tufa deposits

Fluvial tufa deposits in southwest Japan commonly develop biannual lamination consisting of dense summer layers and porous winter layers, and the clearness of the laminae varies among the sites. The laminae have been largely attributed to a seasonally variable inorganic precipitation rate of calcite. This rate-controlled hypothesis was examined by using quantitative data for calcite packing-density (CPD) and the precipitation rate of calcite (PWP rate) calculated from water chemistry. The results for four tufa-depositing sites in SW Japan show that a positive correlation between CPD and PWP rate becomes less certain with increasing PWP rate. In the temperature realm of SW Japan, tufas develop regular distinct seasonal change in CPD when deposited in water containing Ca values less than 65 mg/l, which results in a relatively low precipitation rate. The CPD of tufa deposits rarely exceeds 65%, owing to pore space between fine-grained calcite crystals and to porosity derived from decomposed cyanobacteria and other microorganisms. By increasing the Ca content to more than 65 mg/l, the CPD often attains an upper limit and becomes insensitive to seasonal changes in the PWP rate. Therefore, seasonal variations in CPD at sites with a higher Ca content are unclear, as seen in two examples from tropical islands in southern Japan and in one locality in a temperate climate. The flow rate and microbial density on the tufa surface are subordinate factors with respect to the CPD. Seasonal changes in these two factors often enhance the porous/dense contrast of biannual lamination in SW Japan.

The influence of temperature and seawater composition on calcite crystal growth mechanisms and kinetics: Implications for Mg incorporation in calcite lattice

The composition of carbonate minerals formed in past and present oceans is assumed to be significantly controlled by temperature and seawater composition. To determine if and how temperature is kinetically responsible for the amount of Mg incorporated in calcite, we quantified the influence of temperature and specific dissolved components on the complex mechanism of calcite precipitation in seawater. A kinetic study was carried out in artificial seawater and NaCl–CaCl 2 solutions, each having a total ionic strength of 0.7 M. The constant addition technique was used to maintain [Ca 2+] at 10.5 mmol kg −1 while [ CO 3 2 - ] was varied to isolate the role of this variable on the precipitation rate of calcite. Our results show that the overall reaction of calcite precipitation in both seawater and NaCl–CaCl 2 solutions is dominated by the following reaction: Ca 2 + + CO 3 2 - ↔ k b , k f Ca CO 3 where k f and k b are the forward and backward reaction rate constants, respectively, while the net precipitation rate R, can be described at any temperature by R = k f a Ca 2 + n 1 a CO 3 2 - n 2 - k b or in its logarithmic form Log (R + k b ) = Log K f + n 2 Log [ CO 3 2 - ] where n i are the partial reaction orders with respect to the participating ions, a the ion activity, γ the activity coefficients, and K f = k f ( a Ca 2 + ) n 1 ( γ CO 3 2 - ) n 2 is a constant at a given temperature. We find that, irrespective of the presence of Mg, SO 4, and other specific seawater components known calcite reaction rate inhibitors, the partial reaction order with respect to carbonate ion concentration changes from 2 to 5 while the rate constant K f, increases by 3–4 orders of magnitude when temperature varies from 5 to 70 °C. The observed variations of the kinetic mechanism resulting from the temperature changes are correlated with the variable amount of Mg incorporated in the formed calcites. Moreover, at a given temperature, the increase in the saturation state enhances the rate of calcite precipitation without influencing the reaction mechanism and without changing the amount of Mg incorporated in the growing lattice. Thus, the results of this experimental study are consistent with present-day abiotic marine carbonates where low-Mg calcite cements are mainly associated with cool water while high-Mg carbonates are dominantly found in warm-water environments. This suggests that the apparent inverse relationship between the global average paleo-temperature and the Mg/Ca ratio in past formed marine carbonate may correspond to major changes in seawater saturation state or (Mg/Ca) ratios that in turn should reflect significant changes in the relative seawater geochemical cycles of these cations.

Calcite precipitation instability under laminar, open-channel flow

We present a 2D numerical model for the growth of calcite from supersaturated aqueous solutions under laminar, open-channel flow conditions. The model couples solution chemistry, precipitation at solution/calcite interfaces, hydrodynamics, diffusion and degassing. The model output is compared with experimental results obtained using an oversaturated calcite solution produced by mixing CaCl 2 and Na 2CO 3. The precipitation rate is observed to increase when the supersaturated solution flows over an obstruction, leading to a growth instability that causes the formation of terraces. At relatively high flow rates, the most important mechanism for this behaviour seems to be hydrodynamic advection of dissolved species either towards or away from the calcite surface, depending on location relative to the obstruction, which deforms the concentration gradients. At lower flow rates, steepening of diffusion gradients around protrusions becomes important. Enhanced degassing over the obstruction due to shallowing and pressure drop is not important on small scales. Diffusion controlled transport close to the calcite surface can lead to a fingering-type growth instability, which generates porous textures. Our results are consistent with existing diffusive boundary layer theory, but for flow over non-smooth surfaces, simple calcite precipitation models that include empirical correlations between fluid flow rate and calcite precipitation rate are inaccurate.

Controls on water drop volume at speleothem drip sites: An experimental study

Summary The growth of speleothem under cave drip sites is closely related to local climate events and provides an increasingly important means of deciphering past climate change. Since calcite precipitation rates depend on the water discharge at the drip site, the drip rate and mass of water drops detaching from stalactites are fundamental controls on speleothem growth and we have investigated factors that control the volume of water drops in this environment. The classical investigations on the volume of falling water drops are reviewed but there have been no measurements of the volume of drops detaching from curved surfaces equivalent to tips of stalactites. In this study we have used an acoustic drop counting method to measure the variation of the mass of water drops detaching from tubes (representing ‘soda straw’ stalactites) and artificial stalactites with spherical terminations (representing massive stalactites) as a function of tube radius or surface curvature and the drip rate. The experimental method corroborates classical measurements of drop mass detaching from tubes and, for massive stalactites, we derive a simple empirical relationship between drop mass and radius of curvature of a spherical surface, based on 100,000 drop counts from artificial stalactites with 19 different radii ranging from 3.6 mm to 500 mm. The results of this study allow discharge to be calculated from drip interval measurements and provide a quantitative basis for theoretical modelling of speleothem growth from drip sites.

Reactivity of Alkaline Lignite Fly Ashes Towards CO2 in Water

The reaction kinetics between alkaline lignite fly ashes and CO2 (pCO2 = 0.01--0.03 MPa)were studied in a laboratory CO2 flow-through reactor at 25--75 degrees C. The reaction is characterized by three phases that can be separated according to the predominating buffering systems and the rates of CO2 uptake. Phase I (pH > 12, < 30 min) is characterized by the dissolution of lime, the onset of calcite precipitation and a maximum uptake, the rate of which seems to be limited by dissolution of CO2. Phase II (pH < 10.5, 10--60 min) is dominated by the carbonation reaction. CO2 uptake in phase III (pH < 8.3) is controlled by the dissolution of periclase (MgO) leading to the formation of dissolved magnesium-bicarbonate. Phase I could be significantly extended by increasing the solid-liquid ratios and temperature, respectively. At 75 degrees C the rate of calcite precipitation was doubled leading to the neutralization of approximately 0.23 kg CO2 per kg fly ash within 4.5 h, which corresponds to nearly 90% of the total acid neutralizing capacity.

δ44Ca evolution in a carbonate aquifer and its bearing on the equilibrium isotope fractionation factor for calcite

To improve understanding of Ca isotope transport during water-rock interaction on the continents, we measured dissolved δ 44Ca values along a 236 km flow path in the Madison aquifer, South Dakota, where fluids have chemically evolved according to dolomite and anhydrite dissolution, calcite precipitation, and Ca-for-Na ion-exchange over a timescale spanning ~ 15 kyr. We used a reactive transport model employing rate data constrained from major ion mass-balances to evaluate the extent to which calcite precipitation and ion-exchange fractionate Ca isotopes. Elevated δ 44Ca values during the initial and final stages of water transport possibly result from calcite precipitation under supersaturated conditions and Ca-for-Na ion-exchange, respectively. However, for the bulk of the flow path, δ 44Ca values evolve by mixing between anhydrite and dolomite dissolution, with no fractionation during calcite precipitation under saturated conditions. We attribute the absence of Ca isotope fractionation to the long timescale of water-rock interaction and slow rate of calcite precipitation, which have enabled fluids to chemically and isotopically equilibrate with calcite. We therefore conclude that the equilibrium Ca isotope fractionation factor between calcite and water ( Δ cal–w) is very close to zero. To the extent that the Madison aquifer typifies other groundwater systems where calcite slowly precipitates from solutions at or near chemical equilibrium, this study suggests that groundwater contributions to δ 44Ca variability on the continents can be modeled according to simple mixing theory without invoking isotope discrimination.