Calcium Carbonate Dissolution Research Articles

Euendolithic Cyanobacteria and Proteobacteria Together Contribute to Trigger Bioerosion in Aquatic Environments.

Shellfish, mussels, snails, and other aquatic animals, which assimilate limestone (calcium carbonate, CaCO3) to build shells and skeletons, are effective carbon sinks that help mitigate the greenhouse effect. However, bioerosion, the dissolution of calcium carbonate and the release of carbon dioxide, hinders carbon sequestration process. The bioerosion of aquatic environments remains to be elucidated. In this study, the bioerosion of Bellamya spp. shells from the aquatic environment was taken as the research object. In situ microbial community structure analysis of the bioerosion shell from different geographical locations, laboratory-level infected culture, and validated experiments were conducted by coupling traditional observation and 16S rRNA sequencing analysis method. Results showed that bioeroders can implant into the CaCO3 layer of the snail shell, resulting in the formation of many small holes in the shell, which reduced the shell’s density and made the shell fragile. Results also showed that bioeroders were distributed in two major phyla, namely, Cyanobacteria and Proteobacteria. Cluster analysis showed that Cyanobacteria sp. and two unidentified genera (Burkholderiaceae and Raistonia) were the key bioeroders. Moreover, results suggested that the interaction of Cyanobacteria and other bacteria promoted the biological function of “shell bioerosion.” This study identified the causes of “shell bioerosion” in aquatic environments and provided some theoretical basis for preventing and controlling it in the aquatic industry. Results also provided new insights of cyanobacterial bioerosion of shells and microalgae carbon sequestration.

The Deep Ocean's Carbon Exhaust.

The deep ocean releases large amounts of old, pre‐industrial carbon dioxide (CO2) to the atmosphere through upwelling in the Southern Ocean, which counters the marine carbon uptake occurring elsewhere. This Southern Ocean CO2 release is relevant to the global climate because its changes could alter atmospheric CO2 levels on long time scales, and also affects the present‐day potential of the Southern Ocean to take up anthropogenic CO2. Here, year‐round profiling float measurements show that this CO2 release arises from a zonal band of upwelling waters between the Subantarctic Front and wintertime sea‐ice edge. This band of high CO2 subsurface water coincides with the outcropping of the 27.8 kg m−3 isoneutral density surface that characterizes Indo‐Pacific Deep Water (IPDW). It has a potential partial pressure of CO2 exceeding current atmospheric CO2 levels (∆PCO2) by 175 ± 32 μatm. Ship‐based measurements reveal that IPDW exhibits a distinct ∆PCO2 maximum in the ocean, which is set by remineralization of organic carbon and originates from the northern Pacific and Indian Ocean basins. Below this IPDW layer, the carbon content increases downwards, whereas ∆PCO2 decreases. Most of this vertical ∆PCO2 decline results from decreasing temperatures and increasing alkalinity due to an increased fraction of calcium carbonate dissolution. These two factors limit the CO2 outgassing from the high‐carbon content deep waters on more southerly surface outcrops. Our results imply that the response of Southern Ocean CO2 fluxes to possible future changes in upwelling are sensitive to the subsurface carbon chemistry set by the vertical remineralization and dissolution profiles.

Molecular Interaction Study on a New Application of Ionic Liquids as Dissolver toward Carbonate Scale

Latest advances of ionic liquids (ILs) have allowed to a new application on the dissolution of calcium carbonate (CaCO3) scales where the CaCO3 scale deposition have seriously severe threat in the petroleum field. In this study, the molecular interaction between CaCO3 and ILs n-pyridinium chloride [NPy][Cl] was studied experimentally in order to get a better understanding during the dissolution of scale. NMR and FTIR spectroscopy was used to study the molecular interaction between CaCO3 and [NPy][Cl] solution during the dissolution process. To further evaluate the result, the simulation study using Gaussian software was utilized to predict in detail the molecular interaction between [Npy][Cl] and CaCO3. The finding from this study showed that the metal complex was formed via ligand after dissolution scale process. Based on the findings, it can be clinched that [Npy][Cl] has potential to be used as a scale dissolver in the oilfield, especially in dissolving calcium carbonate scales.

Lattice Boltzmann study of dissolution in porous media: Comparison of VOP with VOF-curved boundary coupling

In dissolution processes, during geometry evolution, the two-phase interface is continuously changed. For tracking the position of the interface, the method of combining VOF with curved boundary schemes was presented in this research. Since the combination would be rather complex, in addition to the original approach, a simplified approximate approach was also introduced. Further, for simulating heterogeneous dissolution in lattice Boltzmann (LB) framework, the volume of pixel (VOP) method has been widely used by researchers. Since the area of the solid-fluid interface is not captured by this method, the study of its validity was also presented in this research. For this purpose, simulations of calcium carbonate dissolution by hydrochloric acid in simple and complex porous media were performed. In addition to the VOP method, the volume of fluid (VOF) method in which the solid-fluid interface is tracked, was also applied for all cases of simulations. The results revealed that from a dissolution perspective, the performance of the VOF-curved boundary combination is essentially similar to that of the VOF-bounce back combination. However, the algorithm of bounce back was found to be more than 10% computationally efficient. Also, comparison of the outcomes from VOP and VOF indicated that VOP intrinsically overestimates the surface area of reactions by about 25–35%, which fictitiously leads to 20–30% higher reaction rates and nearly 20–30% less dissolution times. Hence, it was deduced that the temporal outcomes of VOP may not be valid. However, from a non-temporal perspective, the dissolution patterns of both methods were found to be essentially similar.

Chemical reactions of carbonate-rich mudstones with aqueous CO2 and their impacts on rock's local microstructural and chemo-mechanical properties

Assessment of the structural integrity of carbonate-rich rocks subjected to acidic brine strongly relies on the understanding of the coupling between the chemical changes and the alteration of the mechanical properties of rocks. This study aims at an integrated microscale chemo-mechanical and microstructural characterization of carbonate mudstones exposed to CO2-rich brine under high pressure and high temperature conditions. Digital image analysis was performed on X-ray micro-computed tomography (micro-CT) images to realize microstructural evolution and spatial distribution of pores and different material phases. This approach showed three distinct regions of porosity and carbonate content (resulting from dissolution and precipitation of calcium carbonates), which are critical for understanding the mechanical alteration of the rocks being investigated. Detailed analysis of the micro-CT images also revealed the preferred precipitation of calcium carbonates in pores parallel to the flow/reaction direction. Microscale chemo-mechanical testing performed using coupled grid nanoindentation and scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) complemented the micro-CT results: dissolution zones close to the reacted surface were found to have lower hardness and indentation modulus while an increased values in adjacent precipitation zones was observed. Also, the thickness of the precipitation zones was found to be considerably higher in the direction of the bedding planes, which needs special attention in laminated rocks like shales and mudstones. The microstructural and mechanical properties of the microscale regions and the layered structure are expected to influence the overall mechanical properties and integrity of the rock. These results enhance our understanding of the microscale origin of the macroscopic mechanical alteration of carbonate-rich mudstones subjected to acidic brine.

Novel regimes of calcium carbonate dissolution in micron-scale confined spaces

We report the discovery of novel regimes of calcium carbonate dissolution in micron-scale confined spaces through microfluidic experiments. In the experiments, hydrochloric acid is injected into a microfluidic chamber with pre-deposited calcium carbonate solids. As the acid flow rate is decreased, the dissolution of calcium carbonate transits from a liquid/solid single-phase reactive transport regime to a gas/liquid/solid two-phase one. The two-phase regime further splits into two different regimes, one with fluctuating and the other with nonfluctuating gaseous CO2 phase. We name the three regimes as “Nongaseous”, “Gaseous Breathing”, and “Gaseous Nonbreathing”. The experimental observations are reproduced and interpreted by mathematical modeling. Results show that the regimes are controlled by the ratio of H+ concentration and saturated CO2 concentration, the Peclet number and the second Damkohler number. Dimensionless diagrams distinguishing different regimes are presented and an empirical relationship is proposed. Our results offer insights into various engineering and natural processes, e.g., geological carbon storage, petroleum reservoir acidizing, Karst dissolution, and others that are governed by small scale multiphase reactive flow physics.

Comprehensive Groundwater Quality Analysis in Chronic Kidney Disease of Unknown Etiology (CKDu) Prevalence Areas of Sri Lanka to Investigate the Responsible Culprit

Chronic kidney disease of unknown etiology (CKDu) is a serious health problem in Sri Lanka especially among agricultural communities in dry zone since 19th century. In the present study, several water quality parameters were studied in well water samples and only four parameters, namely, hardness, cadmium level, fluoride level, and strontium level have shown a relationship with the CKDu prevalence. Higher percentage of well water samples collected from CKDu prevalent area in both pre and postmonsoon seasons have exceeded the WHO recommended levels of hardness. Further, water samples collected in postmonsoon seasons had significantly higher hardness than the well water samples collected in premonsoon season ( P < 0.05 ). This may be due to the dissolution of calcium carbonate by water recharge. Moreover, cadmium and fluoride contents have exceeded the recommended levels in high-risk area during the premonsoon period. Furthermore, according to principal component analysis (PCA), four clusters were identified depending on the different levels of fluoride, cadmium, hardness, and strontium contents. The control area (Am) fell in to separate cluster with low contents of fluoride, cadmium, hardness, and strontium than in CKDu prevalent area. Since it has been found that the above species are directly involved in renal damage, it can be concluded that a synergetic effect of cadmium, fluoride, hardness, and strontium in well water may be a main cause for CKDu in Sri Lanka.

Calcium Carbonate Dissolution Triggered by High Productivity During the Last Glacial–Interglacial Interval in the Deep Western South Atlantic

Studies reconstructing surface paleoproductivity and benthic environmental conditions allow us to measure the effectiveness of the biological pump, an important mechanism in the global climate system. In order to assess surface productivity changes and their effect on the seafloor, we studied the sediment core SAT-048A, spanning 43–5 ka, recovered from the continental slope (1,542 m water depth) of the southernmost Brazilian continental margin, deep western South Atlantic. We assessed the sea surface productivity, the organic matter flux to the seafloor, and calcite dissolution effects, based on micropaleontological (benthic and planktonic foraminifers, ostracods), geochemical (benthic δ13C isotopes), and sedimentological data (carbonate and bulk sand content). Superimposed on the induced changes related to the last glacial–interglacial transition, the reconstruction indicates a significant and positive correlation between the paleoproductivity proxies and the summer insolation. From the reconstructed data, it was possible to identify high (low) surface productivity, high (low) organic matter flux to the seafloor, and high (low) dissolution rates of planktonic Foraminifera tests during the glacial (postglacial). Furthermore, within the glacial, enhanced productivity was associated with higher insolation values, explained by increased northeasterly summer winds that promoted meandering and upwelling of the nutrient-rich South Atlantic Central Water. Statistical analyses support the idea that productivity is the main cause for seafloor calcium carbonate dissolution, as opposed to changes in the Atlantic Meridional Overturning Circulation (at least for the 25–4 ka period). Further efforts must be invested in the comprehension and quantification of the total organic matter and biogenic carbonate burial during time intervals with an enhanced biological pump, aiming to better understand their individual roles.

Aragonite dissolution protects calcite at the seafloor

In the open ocean, calcium carbonates are mainly found in two mineral forms. Calcite, the least soluble, is widespread at the seafloor, while aragonite, the more soluble, is rarely preserved in marine sediments. Despite its greater solubility, research has shown that aragonite, whose contribution to global pelagic calcification could be at par with that of calcite, is able to reach the deep-ocean. If large quantities of aragonite settle and dissolve at the seafloor, this represents a large source of alkalinity that buffers the deep ocean and favours the preservation of less soluble calcite, acting as a deep-sea, carbonate version of galvanization. Here, we investigate the role of aragonite dissolution on the early diagenesis of calcite-rich sediments using a novel 3D, micrometric-scale reactive-transport model combined with 3D, X-ray tomography structures of natural aragonite and calcite shells. Results highlight the important role of diffusive transport in benthic calcium carbonate dissolution, in agreement with recent work. We show that, locally, aragonite fluxes to the seafloor could be sufficient to suppress calcite dissolution in the top layer of the seabed, possibly causing calcite recrystallization. As aragonite producers are particularly vulnerable to ocean acidification, the proposed galvanizing effect of aragonite could be weakened in the future, and calcite dissolution at the sediment-water interface will have to cover a greater share of CO2 neutralization.

Light‐driven dynamics between calcification and production in functionally diverse coral reef calcifiers

AbstractCoral reef metabolism underpins ecosystem function and is defined by the processes of photosynthesis, respiration, calcification, and calcium carbonate dissolution. However, the relationships between these physiological processes at the organismal level and their interactions with light remain unclear. We examined metabolic rates across a range of photosynthesising calcifiers in the Caribbean: the scleractinian corals Acropora cervicornis, Orbicella faveolata, Porites astreoides, and Siderastrea siderea, and crustose coralline algae (CCA) under varying natural light conditions. Net photosynthesis and calcification showed a parabolic response to light across all species, with differences among massive corals, branching corals, and CCA that reflect their relative functional roles on the reef. At night, all organisms were net respiring, and most were net calcifying, although some incubations demonstrated instances of net calcium carbonate (CaCO3) dissolution. Peak metabolic rates at light‐saturation (maximum photosynthesis and calcification) and average dark rates (respiration and dark calcification) were positively correlated across species. Interspecies relationships among photosynthesis, respiration, and calcification indicate that calcification rates are linked to energy production at the organismal level in calcifying reef organisms. The species‐specific ratios of net calcification to photosynthesis varied with light over a diurnal cycle. The dynamic nature of calcification/photosynthesis ratios over a diurnal cycle questions the use of this metric as an indicator for reef function and health at the ecosystem scale unless temporal variability is accounted for, and a new metric is proposed. The complex light‐driven dynamics of metabolic processes in coral reef organisms indicate that a more comprehensive understanding of reef metabolism is needed for predicting the future impacts of global change.

Chemical Compositions in Modified Salinity Waterflooding of Calcium Carbonate Reservoirs: Experiment

Modified or low-salinity waterflooding of carbonate oil reservoirs is of considerable economic interest because of potentially inexpensive incremental oil production. The injected modified brine changes the surface chemistry of the carbonate rock and crude oil interfaces and detaches some of adhered crude oil. Composition design of brine modified to enhance oil recovery is determined by labor-intensive trial-and-error laboratory corefloods. Unfortunately, limestone, which predominantly consists of aqueous-reactive calcium carbonate, alters injected brine composition by mineral dissolution/precipitation. Accordingly, the rock reactivity hinders rational design of brines tailored to improve oil recovery. Previously, we presented a theoretical analysis of 1D, single-phase brine injection into calcium carbonate-rock that accounts for mineral dissolution, ion exchange, and dispersion (Yutkin et al. in SPE J 23(01):084–101, 2018. https://doi.org/10.2118/182829-PA). Here, we present the results of single-phase waterflood-brine experiments that verify the theoretical framework. We show that concentration histories eluted from Indiana limestone cores possess features characteristic of fast calcium carbonate dissolution, 2:1 ion exchange, and high dispersion. The injected brine reaches chemical equilibrium inside the porous rock even at injection rates higher than 3.5 times 10^{-3} m s^{-1} (1000 ft/day). Ion exchange results in salinity waves observed experimentally, while high dispersion is responsible for long concentration history tails. Using the verified theoretical framework, we briefly explore how these processes modify aqueous-phase composition during the injection of designer brines into a calcium-carbonate reservoir. Because of high salinity of the initial and injected brines, ion exchange affects injected concentrations only in high surface area carbonates/limestones, such as chalks. Calcium-carbonate dissolution only affects aqueous solution pH. The rock surface composition is affected by all processes.

Removing of Cadmium Ions and Reactive red Dye from Simulated Wastewater Using Eggshell as an Eco-Friendly Material

The research aims to use eggshells (ES) as civilian residues in the process of removing cadmium ions and reactive red dye according to international standards limits. Synthetic solutions were prepared for cadmium ions and reactive red dye using 0.2 g non-calcined and calcined ES at various temperatures (25, 250, 500, 750, and 1000 ℃) as an adsorbent. The result showed the removal of cadmium ion was (60, 100%) for non-calcined and calcined ES, respectively, with the initial concentration of cd2+ (10 ppm). The removal of reactive red dye was (18.5, 98%) using non-calcined and calcined ES, respectively, at a concentration of red dye (50 ppm). The best removal time was 90 min. XRD and FTIR spectroscopy were performed and the results were identical to the main components of ES and changed with temperature increasing due to dissolution of calcium carbonate (CaCO3).

Meteoric water contribution to sea ice formation and its control of the surface water carbonate cycle on the Wandel Sea shelf, northeastern Greenland

An influx of glacial meltwater has the ability to alter the properties of marine surface waters and their ability to exchange CO2 through changes to water column stratification and the inorganic carbon system. Here, we report how inputs of meteoric water affect the physical and biogeochemical properties of both the water column and the sea ice cover on the Wandel Sea shelf, northeastern Greenland, during spring 2015. The observed depleted δ18O–H2O in the water column, with surface water values as low as –16.3 ‰, suggests a strong input of meteoric water (i.e., water derived from atmospheric precipitation). Depleted δ18O–H2O observed within sea ice (from –21.5 to –8.0 ‰) reflects its formation from surface water that was already depleted isotopically. In addition, a thick snow cover, as present during the study, promotes the formation of snow ice as well as insulates the ice cover. Within sea ice, the resulting relatively warm temperature and low salinity impedes ikaite formation. However, measurements of total dissolved inorganic carbon and total alkalinity indicate that, in both sea ice and the water column, the dissolution of calcium carbonate was the main process affecting the carbonate system. This finding suggests that inputs of glacial meltwater deliver glacier-derived carbonate minerals to the ocean which become incorporated within the ice structure, increasing calcium carbonate dissolution in the water column in the absence of ikaite precipitation within the sea ice. If widespread in glacial-fed waters, bedrock carbonate minerals could increase CO2 sequestration in glacial catchments despite the weakening of the sea ice carbon pump.

A three dimensional kinetic Monte Carlo defect-free crystal dissolution model for biological systems, with application to uncertainty analysis and robust optimization

The objective of this study was to construct a novel kinetic Monte Carlo (kMC) model to predict the complete dissolution of nanoscale crystals. The developed framework was designed to predict the dissolution of a wide variety of crystalline minerals, regardless of their composition and crystal structure. The proposed framework was used to explore ways of enhancing crystal dissolution processes by assessing the variability from environmental uncertainties and by performing robust optimization to improve the dissolution performance. The approach was used to simulate calcium carbonate dissolution within the human gastrointestinal system. Polynomial chaos expansions (PCEs) were used to propagate the parametric uncertainty through the kMC model. Robust optimization was subsequently performed to determine the crystal design parameters that achieve target dissolution specifications using low-order PCE coefficient models. The results showcased the applicability of the kMC crystal dissolution model and the need to account for dissolution uncertainty within key biological applications.

Supply‐controlled calcium carbonate dissolution decouples the seasonal dissolved oxygen andpHminima in Chesapeake Bay

AbstractAcidification can present a stress on organisms and habitats in estuaries in addition to hypoxia. Although oxygen and pH decreases are generally coupled due to aerobic respiration, pH dynamics may be more complex given the multiple modes of buffering in the carbonate system. We studied the seasonal cycle of dissolved oxygen (DO), pH, dissolved inorganic carbon, total alkalinity, and calcium ion (Ca2+) along the main channel of Chesapeake Bay from May to October in 2016. Contrary to the expected co‐occurrence of seasonal DO and pH declines in subsurface water, we found that the pH decline ended in June while the DO decline continued until August in mid‐Chesapeake Bay. We discovered that aerobic respiration was strong from May to August, but carbonate dissolution was minor in May and June and became substantial in August, which buffered further pH declines and caused the seasonal DO and pH minima mismatch. The rate of calcium carbonate (CaCO3) dissolution was not primarily controlled by the saturation state in bottom water, but was instead likely controlled by the supply of CaCO3particles. The seasonal variability of Ca2+addition in the mid‐bay was connected to Ca2+removal in the upper bay, and the timing of high carbonate dissolution coincided with peak seasonal biomass of upper Bay submerged aquatic vegetation. This study suggests a mechanism for a novel decoupling of DO and pH in estuarine waters associated with CaCO3, but future studies are needed to fully investigate the seasonality of physical transport and cycling of CaCO3.

Hydrological and hydrogeological characteristics and environmental assessment of Hashilan Wetland, a national heritage in NW Iran

In the present study, 6 water and 16 sediment samples were collected from Hashilan Wetland (Kermanshah Province, Iran) to specify physio-chemical properties of water and sediment and to evaluate the water pollution load with reference to the water physicochemical parameters. According to the Piper diagram, Ca2+–Mg2+–HCO3− hydrochemical facies is dominant due to calcium carbonate dissolution in the geological formations of the study area. The wetland water had a natural to slightly alkaline pH. Although TDS and EC of the water samples fall under freshwater category, their values were higher downstream of the wetland as a result of stagnant water, low water depth, more evaporation, and impact of agricultural runoff. Based on the water quality index (WQI), all water samples were ‘suitable’ for irrigation purposes, while 50% of the samples were ‘unsuitable’ for drinking. Comprehensive Pollution Index (CPI), and Sodium Adsorption Ratio (SAR) also indicated suitability of the wetland water for irrigation. According to the Wilcox diagram, salinity is higher downstream of the wetland due to introduction of agricultural runoff. It can be concluded that the wetland water could be used for irrigational purposes, but regular monitoring is recommended to promote sustainable use of natural resources and conserve the vulnerable aquatic ecosystem.

Self-Hybrid Transition Metal Oxide Nanosheets Synthesized by a Facile Programmable and Scalable Carbonate-Template Method.

2D transition metal oxides (TMO) nanosheets have attracted considerable attention in both fundamental research and practical applications. Herein, a convenient programmable and scalable carbonate crystals templating synthesis is developed to produce high-quality self-hybrid TMO nanosheets (Si-WO3- x , Tax Oy , Mnx Oy ) and their respective polymetallic oxide hybrid nanosheets with tunable composition, low-cost and high-yield. Taking tungsten oxide nanosheets as example, silicotungstic acid precursor is in situ converted into tungsten oxide nanosheets like scales on the surface of calcium carbonate crystals through the simple soaking-drying-calcination process, and after selectively dissolving calcium carbonate by etching, the dispersive tungsten oxide nanosheets with unique self-hybrid Si-doped h-WO3 /ε-WO3 /WO2 compositions are obtained, which show excellent acetone gas-sensing performances at low temperatures. This carbonate-template method opens up the possibility to economically produce various functional TMO nanosheets with specific compositions for diverse applications.

Evidence of supra-lysoclinal dissolution of pelagic calcium carbonate in the late Quaternary in the western South Atlantic

Even at intermediate depths, where the environment is adequate for carbonate accumulations, a loss of calcareous microfossils may occur by the dissolution of CaCO3. Changes in the microfossil preservation indexes are a proxy for identifying oceanographic variations in the past. The goal of this study is to identify minor changes in the degree of preservation of foraminiferal assemblages in a supra-lysoclinal zone and their probable causes. Late Quaternary samples from three cores collected at intermediate water depths (1514, 1894 and 2091 m) on the Pelotas Basin continental slope were examined using the standard methodology for calcareous (foraminifera and coccolith) paleontological analyses. The age models were obtained based on AMS 14C dates and on the oxygen stable isotope (δ18O) record in foraminifera. Dissolution variations were identified using the coccolith-based CEX dissolution index, the carbonate content, foraminifera fragmentation index, the relationship between resistant and fragile planktonic foraminifera, and the abundance of fragile foraminifera Orbulina universa and Hoeglundina spp. Finally, the planktonic/benthic foraminifera ratio (P/B) was evaluated as a dissolution proxy, demonstrating non-applicability. Overall, the Marine Isotope Stages (MIS) 5 and MIS 1 (interglacials) show the best preservations, while MIS 4, 3 and 2 (glacials) show relatively poorer preservation. Although the cores were obtained well above the modern and the inferred glacial lysocline, periods of relative greater dissolution were identified which can be attributed to the advance of Upper Circumpolar Deep Water (UCDW). The best preservation indexes are attributed to the influence of North Atlantic Deep Water (NADW).

Leaching of trace metals (Pb) from contaminated tailings amended with iron oxides and manure: New insight from a modelling approach

Reclamation measurements are commonly applied to mitigate the leaching of metal pollutants in order to reduce the risk for humans and the environment. The stabilization of mine tailings can be performed by amending with organic or inorganic materials. In a recent laboratory microcosm experiment (Thouin et al., 2019), the addition of a mining slurry called ochre and manure, either alone or in combination, drastically reduced the leaching of several metal pollutants, notably Pb. Nevertheless, the biogeochemical processes involved in the immobilization of metal pollutants remain unknown, preventing the management of this remediation technique from being optimized and its extension to other sites. To fill this gap, a multicomponent mixing model was developed to simulate and forecast the impact of amendments on the leaching of metal pollutants. This model accounts for the following biogeochemical processes: kinetically-controlled dissolution/precipitation reactions, sorption reactions (i.e. surface complexation reactions), water-gas interactions and microbial respiration with an explicit microbial growth. For all treatments, simulations revealed that Pb reactivity followed dynamic patterns driven by watering steps. The decrease in Pb concentration in the leachates of amended tailings compared to untreated tailings was also accurately reproduced. In untreated tailings, Pb reactivity is mainly controlled by the dissolution of Pb-bearing mineral phases. These reactions were maintained in thermodynamic disequilibrium due to the renewal of pore solution at each watering step. In amended tailings, this pattern was strengthened as the iron oxides contributed by ochre maintained a low Pb concentration in pore solution by sorbing released Pb. Sorption reactions were enhanced by the increase in pH induced by the dissolution of calcium carbonate initially present in ochre. The latter reaction was partially counterbalanced in tailings amended with manure as organic matter provided sufficient energy to fuel microbial aerobic respiration, leading to the release of protons. Pb desorption was promoted by this pH drop. The magnitude of these reactions was not strictly proportional to the amount of manure added. For 0.15% by weight, aerobic respiration did not occur whereas its yield was similar for 1% and 2%. By providing a better understanding of the effect of amendment, this multicomponent mixing model is a powerful tool to optimize the reclamation of tailings, in order to limit contaminant transfer to the environment.

Non-lithifying microbial ecosystem dissolves peritidal lime sand

Microbialites accrete where environmental conditions and microbial metabolisms promote lithification, commonly through carbonate cementation. On Little Ambergris Cay, Turks and Caicos Islands, microbial mats occur widely in peritidal environments above ooid sand but do not become lithified or preserved. Sediment cores and porewater geochemistry indicated that aerobic respiration and sulfide oxidation inhibit lithification and dissolve calcium carbonate sand despite widespread aragonite precipitation from platform surface waters. Here, we report that in tidally pumped environments, microbial metabolisms can negate the effects of taphonomically-favorable seawater chemistry on carbonate mineral saturation and microbialite development.