Surface Kinetic Model Research Articles

A study on the application of a composite MIL88A(Fe)/TiO2 in a hexagonal photoreactor for phenol removal: Response surface methodology and kinetic modeling

AbstractThe application of a novel composite MIL88A(Fe)/TiO2 for phenol removal in a new hexagonal photoreactor design was investigated. The unique hexagonal shape of the reactor increases the surface area available for irradiation, leading to more efficient removal of contaminants. The composite was characterized using X ray diffraction (XRD), Fourier transform‐infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) images to determine its properties. Photocatalyst dosage, reaction time, phenol concentration, pH, and mL H2O2/L PW (phenol wastewater) were chosen as effective parameters on the process. To plan an experiment and maximize phenol removal, the response surface methodology (RSM) was applied. Ideal conditions for optimum efficiency (95.96%) include initial phenol concentration of 58 mg/L, pH of 7.51, reaction time of 68.61 min, mL H2O2/L PW of 0.18, and catalyst dosage of 0.4 g/L PW. Trapping experiments prove that ˙O2 and ˙OH produced in Fenton and photocatalytic processes are the predominant active radicals in this process. The kinetics was fitted with the first‐order, second‐order, n‐order, and Langmuir–Hinshelwood models using nonlinear least squares techniques. The n‐order model with n = 0.54 was found to be the most suitable model (R2 0.998), with a model constant of k = 0.11 (mol0.46/L0.46.s).

Quantification of classical and non-classical crystallization pathways in calcite precipitation

Crystal precipitation from aqueous solution occurs through multiple pathways. Besides the classical ion-by-ion addition, non-classical crystallization mechanisms, such as multi-ion polymer and nano-particle attachment, could be significant. These non-classical crystallization processes have been observed with advanced microscopy, yet detailed quantification of their contributions remains challenging. Building from paired Ca and Sr isotope observations, we develop a theoretical framework to quantify the contributions of classical and non-classical crystallization pathways on the precipitation of the calcium carbonate mineral calcite, a common precipitate in nature. We demonstrate that the classical crystallization pathway alone is insufficient to account for the observed isotope behaviors and, thus, the entire calcite precipitation process. We further present a surface kinetic model that incorporates non-classical crystallization pathways. This model enables the characterization of the roles of classical and non-classical crystallization mechanisms in calcite precipitation. The results suggest that the relative contribution of non-classical crystallization pathways increases with saturation state and can, under high supersaturation levels, be comparable to or greater than the classical pathway. The presented theoretical framework readily explains observed trace element partitioning and isotope fractionation behaviors during calcite precipitation and can be further expanded onto other mineral systems to gain insights into crystal growth mechanisms.

Response surface optimization, kinetic modeling, and thermodynamic study for ultrasound-assisted extraction of dietary fiber from mango peels and its structural characterization

Response surface optimization, kinetic modeling, and thermodynamic study for ultrasound-assisted extraction of dietary fiber from mango peels and its structural characterization

Surface and mass transfer kinetic and equilibrium modeling of Pb(II) ions adsorption on hydroxyapatite scaffold: Batch and fixed-bed column studies

This paper presents the results of a study on removal of Pb2+ ions by using hydroxyapatite (HAP) scaffold prepared by replication method. To this aim, the effects of suspension solid content and polymeric foam density on the properties of ceramic foam were investigated. The mean total porosity and apparent porosity of the optimized scaffolds were 92.67 % and 92.04 %, respectively. The Taguchi method was performed for designing batch experiments by considering three controllable variables, namely: (1) initial pH of the solution, (2) initial concentration of Pb2+ ions (C0), and (3) circulating rate of the solution (Ǭ), on the removal efficiency of Pb2+ ions as the response. Based on the obtained results from the ANOVA analysis, the removal efficiency of Pb2+ ions was mainly affected by the initial pH of the solution (90.88 %). The highest removal efficiency (99.1 %) of Pb2+ ions was obtained by optimizing the process under operating conditions of pH = 3.5, C0 = 50 mg.L−1, and Ǭ= 0.5 L.min−1. The Langmuir isotherm model with good accuracy was fitted to the equilibrium results, and the maximum adsorption capacity of 106.38 mg.g−1 was predicted for the HAP scaffold adsorbent. Kinetic studies showed that the mass transfer diffusion steps in the external film and inside the pores are fast, being the adsorption on the adsorbent sites the rate-limiting step of the kinetics, what is plausible given the high porosity of the material. Also, the primary mechanism for Pb2+ adsorption using HAP scaffold adsorbents was found to be dissolution-precipitation. The fixed-bed column adsorption experiments performed under the predicted optimum conditions of batch experiments. In comparison to the powder adsorbents tested in fixed-bed column, this adsorbent presents several advantages. These include the elimination of the requirement for mechanical separation of the shaped adsorbent from the solution after adsorption, prevention of bed clogging, and its high surface area derived from interconnected open pores.

Lithium partition and isotopic fractionation for cation exchange in clay: Insights from molecular simulations

Chemical weathering is one of the most important processes on the Earth’s surface, and lithium isotopes have been used as a promising tracer of silicate weathering, as the secondary clay minerals precipitated during silicate weathering are thought to preferentially incorporate the lighter 6Li isotope. However, to utilize lithium isotopes as a geochemical tracer, it is essential to quantitatively understand the partitioning of Li in clay minerals, including structurally bound Li and exchangeable interlayer ions, as well as the associated isotope fractionation. With classical molecular dynamic simulations and thermodynamic calculations, the cation exchange between Na-/K-montmorillonite and solvated Li in environmental solution are disclosed under different aqueous activity conditions. The Li+ ion, whose hydration ability is stronger than Na+ and K+ ions, is easier to swell the montmorillonite and shows less affinity for the clay. And the calculated cationic selectivity coefficients between Na+/K+ and Li+ ions are Kc (Li → Na) = 2.0, Kc (Li → K) = 10.0 for Arizona-type montmorillonite and Kc (Li → Na) = 1.0, Kc (Li → K) = 4.0 for Wyoming-type montmorillonite, which are comparable to reported experiments. The cationic partition of Li in the clay interlayer decreases when the hydration states of montmorillonite transform from triple-layer to bilayer and then to monolayer. Based on the density functional theory, the Li isotope fractionations for Li in clay interlayer sites and structural octahedral sites are further derived by thermodynamic calculations with path integral molecular dynamic simulation, which deals with the quantum anharmonicity. The behaviors of Li isotope in the bilayer hydration state closely resembles that in the aqueous solution, and only a slight Li isotope fractionation is found between the monolayer hydrated octahedral-substituted clay and solution phase (Δ7Limonolayer-aq = −2.0‰). While Li isotope fractionation between aqueous solution and mica structural sites is significant (Δ7Listructural-aq = −10.4‰) and shows an obvious quantum anharmonicity, which indicates the importance of the anharmonic treatment for Li isotope fractionation calculations. The newly calculated Li equilibrium isotope fractionation between clay and aqueous solution deviates from the experimentally determined fractionation factor, which can be attributed to the kinetic effects. Moreover, on the basis of the surface kinetic reaction model, the Li isotope fractionation becomes more pronounced with higher clay growth rate. By integrating the cationic selectivity coefficient and Li isotope fractionation, it is possible to constrain the fluid composition during Li-clay deposit and diagenesis, and to evaluate the intensity of chemical weathering processes as well as the related carbon cycle.

Re-evaluating cloud chamber constraints on depositional ice growth in cirrus clouds – Part 1: Model description and sensitivity tests

Abstract. Ice growth from vapor deposition is an important process for the evolution of cirrus clouds, but the physics of depositional ice growth at the low temperatures (<235 K) characteristic of the upper troposphere and lower stratosphere is not well understood. Surface attachment kinetics, generally parameterized as a deposition coefficient αD, control ice crystal habit and also may limit growth rates in certain cases, but significant discrepancies between experimental measurements have not been satisfactorily explained. Experiments on single ice crystals have previously indicated the deposition coefficient is a function of temperature and supersaturation, consistent with growth mechanisms controlled by the crystal's surface characteristics. Here we use observations from cloud chamber experiments in the Aerosol Interactions and Dynamics in the Atmosphere (AIDA) aerosol and cloud chamber to evaluate surface kinetic models in realistic cirrus conditions. These experiments have rapidly changing temperature, pressure, and ice supersaturation such that depositional ice growth may evolve from diffusion limited to surface kinetics limited over the course of a single experiment. In Part 1, we describe the adaptation of a Lagrangian parcel model with the Diffusion Surface Kinetics Ice Crystal Evolution (DiSKICE) model (Zhang and Harrington, 2014) to the AIDA chamber experiments. We compare the observed ice water content and saturation ratios to that derived under varying assumptions for ice surface growth mechanisms for experiments simulating ice clouds between 180 and 235 K and pressures between 150 and 300 hPa. We found that both heterogeneous and homogeneous nucleation experiments at higher temperatures (>205 K) could generally be modeled consistently with either a constant deposition coefficient or the DiSKICE model assuming growth on isometric crystals via abundant surface dislocations. Lower-temperature experiments showed more significant deviations from any depositional growth model, with different ice growth rates for heterogeneous and homogeneous nucleation experiments.

Numerical Study of Aperture Shape Effects in Deep Cryogenic Etching of Silicon

Cryogenic etching of silicon in SF6/O2 plasma is the process of choice for fabrication of deep high aspect ratio structures with strict requirements for the sidewall angle, surface roughness and contamination, including MEMS, X-ray optics, and supercapacitors. The manufacturing of such structures, containing features of various size and shape, poses new challenges for the etching process optimization, since the key parameters of the resulting structures depend on the local aspect ratio and the shape of the mask aperture. As an aid in process optimization, we propose a three-dimensional Monte-Carlo simulator for the cryogenic etching of complex structures. It employs a surface kinetics model tuned to SF6/O2 process in 2.5D geometry and cubic voxel representations of the simulation domain. Systematic study on cryogenic etching of test structures with different mask shapes and aspect ratios is performed and principal mechanisms affecting etching results are identified.

YSZ/Ni double‐shell powder via surface electroless deposition tuned by an active kinetic model

AbstractYttria‐stabilized zirconia (YSZ) powder with surface Ni plating has promising application in many fields due to its large surface area, low density and excellent high temperature performance. In this study, YSZ/Ni double‐shell powder was fabricated via electroless deposition method, and an active surface kinetic model was developed to tune the surface area and nuclei loading quantitatively. A novel concept, active surface ratio (ASR, the ratio of active deposition surface area to projected area of ceramic powder), is proposed to intrinsically correlate deposition rate with nuclei topology on ceramic surface. The intrinsic relationship between microstructure of metallized ceramic surface and deposition duration was clarified. The active surface kinetic model reveals the correlation between the ASR and instantaneous deposition rate and provides necessary theoretical basis for effective control of ceramic surface modification process by electroless deposition.

Role of Phase Stabilization and Surface Orientation in 4,4′-Biphenyl-Dicarboxylic Acid Self-Assembly and Transformation on Silver Substrates

Molecular functionalization of nanoparticles and metallic substrates can be used to tune their properties for specific applications. However, polycrystalline substrates and nanoparticles exhibit surface planes with distinct crystallographic orientations. Therefore, the development of reliable strategies for molecular functionalization requires knowledge of the role of the surface plane orientation in the growth kinetics, structure, and properties of the molecular layer. Here, we apply a multiscale analysis to investigate the self-assembly of 4,4'-biphenyl-dicarboxylic acid (BDA) on Ag(111) and critically discuss the difference to Ag(100). Whereas the structural motifs for intact and fully deprotonated BDA are similar on both surfaces, the intermediate phases comprising partially deprotonated BDA differ in the structure and chemical composition. A real-time view of the phase transformations enables us to present a generalized picture of the phase transformations between the self-assembled molecular phases on the surfaces and underline important features such as the phase stabilization of the chemical composition and the mechanism of the related burst transformation. The influence of the substrate orientation on the structure of molecular layers and phase transformations provides the necessary background for developing functionalization strategies of the substrates displaying multiple surface planes and kinetic models for the growth near thermodynamic equilibrium.

Protein Folding Dynamics as Diffusion on a Free Energy Surface: Rate Equation Terms, Transition Paths, and Analysis of Single-Molecule Photon Trajectories.

The rates of protein (un)folding are often described as diffusion on the projection of a hyperdimensional energy landscape onto a few (ideally one) order parameters. Testing such an approximation by experiment requires resolving the reactive transition paths of individual molecules, which is now becoming feasible with advanced single-molecule spectroscopic techniques. This has also sparked the interest of theorists in better understanding reactive transition paths. Here we focus on these issues aiming to establish (i) practical guidelines for the mechanistic interpretation of transition path times (TPT) and (ii) methods to extract the free energy surface and protein dynamics from the maximum likelihood analysis of photon trajectories (MLA-PT). We represent the (un)folding rates as diffusion on a 1D free energy surface with the FRET efficiency as a reaction coordinate proxy. We then perform diffusive kinetic simulations on surfaces with two minima and a barrier, but with different shapes (curvatures, barrier height, and symmetry), coupled to stochastic simulations of photon emissions that reproduce current SM-FRET experiments. From the analysis of transition paths, we find that the TPT is inversely proportional to the barrier height (difference in free energy between minimum and barrier top) for any given surface shape, and that dividing the TPT into climb and descent segments provides key information about the barrier's symmetry. We also find that the original MLA-PT procedure used to determine the TPT from experiments underestimates its value, particularly for the cases with smaller barriers (e.g., fast folders), and we suggest a simple strategy to correct for this bias. Importantly, we also demonstrate that photon trajectories contain enough information to extract the 1D free energy surface's shape and dynamics (if TPT is >4-5-fold longer than the interphoton time) using the MLA-PT directly implemented with a diffusive free energy surface model. When dealing with real (unknown) experimental data, the comparison between the likelihoods of the free energy surface and discrete kinetic three-state models can be used to evaluate the statistical significance of the estimated free energy surface.

Modelling tritium adsorption and desorption from tungsten dust particles with a surface kinetic model

A kinetic surface model is presented and used to explain the loading and desorption kinetics of tritium retained in micrometre-sized tungsten (W) dust particles. The model describes the sticking of hydrogen isotopes from the gas phase to W surfaces and the desorption from W surfaces. The initial sticking coefficient is set to one and independent of the temperature. The activation energy for desorption depends on the hydrogen coverage of the surface and is parameterised with density functional theory (DFT) calculations for W(100), W(110) and W(111) surfaces. The DFT-parameterised model is successfully compared to experimental results showing that the amount of measured tritium as well as the desorption kinetic can be modelled with only tritium adsorbed on the surface of W dust particles. Then, the model is used to explore possible scenarios to remove the tritium from the W surfaces by exposing the tritiated surfaces to either deuterium and hydrogen. The simulations suggest that it can be possible to remove all the tritium trapped on the W surfaces even at room temperature as soon as the hydrogen or deuterium pressure is higher than the tritium pressure. This gives opportunity to build tritium removal scenarios for ITER.

Biosorption of Methylene Blue Dye from Textile-Industry Wastewater onto Sugarcane Bagasse: Response Surface Modeling, Isotherms, Kinetic and Thermodynamic Modeling

This research focuses on the viability of removing basic dye methylene blue (MB) from water solutions by the application of sugarcane bagasse (SCB), an economic natural biosorbent. Batch experiments were conducted based on solution pH, contact period, initial adsorbate dye concentration, biosorbent amount, and temperature as independent variables by employing a central composite design approach of response surface methodology (RSM). With the use of RSM, the optimization of the MB biosorption process was carried out for maximum MB removal. The biosorption equilibrium of the dye solution was achieved after a contact time of 90 min and explained by the Langmuir and Freundlich isotherms. The biosorption data is employed for the kinetic modeling, from pseudo-first- and pseudo-second-order kinetic equation. Thermodynamic parameters (Enthalpy change ΔH, entropy change ΔS, and Gibbs free energy ΔG) were studied, which showed that biosorption is natural and endothermic in nature by reducing color randomness at the liquid and solid interface. Biosorbent characterization explained in this analysis is performed through Fourier-transform infrared spectroscopy before and after MB biosorption. Characteristic tests and separation factor proved that SCB was able to be used as an option to commercial biosorbents for the aqueous solution and removal of MB from wastewater.

Investigation of culture conditions by Response Surface Methodology and kinetic modeling for exopolysaccharide production by Klebsiella oxytoca ICCF 419 strain, using lactose as substrate

The objective of this work was to optimize the bioprocess parameters, using Klebsiella oxytoca ICCF 419 to obtain an exopolysaccharide based on lactose as substrate. A kinetic study was employed and Logistic and Gompertz models were applied to describe the polysaccharide production, in relation with biomass growth and substrate consumption. The RSM methodology based on Central Composite Rotatable Design was used to evaluate and optimize the effect of lactose, corn extract, KH2PO4 and citric acid concentrations as independent variables on the polysaccharide production, biomass growth and substrate consumption as the response functions. The interaction effects and optimal parameters were obtained using Design Expert Software (version 9.0.6.2). The significance of the variables and their interactions was tested by means of ANOVA analysis with a 99% confidence level. The optimum culture conditions were determined and the model prediction was compared with experimental results. At an initial value of 23.45 for the C/N in the fermentation medium, the strain produces 17.41 g/L of crude polysaccharide and 2.53 g/L dry biomass. The EPS production was significantly influenced by lactose, corn extract and KH2PO4, while the citric acid had no influence. The biomass growth was influenced by the corn extract, KH2PO4 and citric acid.

CO2 Capture Exploration on Potassium Hydroxide Employing Response Surface Methodology, Isotherm and Kinetic Models

In this research, KOH has been evaluated as a solid adsorbent for carbon dioxide (CO2) capture. The effect of pressure, temperature, and KOH loading on CO2 adsorption in a fixed-bed reactor were investigated. Response Surface Methodology (RSM) based on the central composite design (CCD) was used to evaluate the effects of operating parameters on adsorption capacity in order to achieve the optimum conditions. The experimental values of the responses were in decent agreement with the predicted result of regression models. Techniques such as Fourier Transform InfraRed (FT-IR) spectroscopy and X-ray diffraction (XRD) were used to study the consider KOH sorbent. The results show that CO2 adsorption is improved with the loading of 0.5 g of KOH. The maximum CO2 adsorption capacity was acquired for KOH at temperature 45°C and pressure 6 bars. The Freundlich model was found to be the best for fitting the adsorption of CO2 owing to the closeness of the R2 to unity. Furthermore, the kinetic study specified that the first-order model is well-fitted with the experimental data. Overall, the very high surface area of KOH adsorbent makes this adsorbent new promising material for CO2 capture.

Efficient cesium encapsulation from contaminated water by cellulosic biomass based activated wood charcoal

In this study, cost-effective cellulosic biomass based activated wood charcoal was developed from Japanese Sugi tree (Cryptomeria japonica) by concentrated nitric acid modification for adsorption of Cs from contaminated water. The physicochemical properties of specimens were investigated using N2 adsorption-desorption isotherms (BET method), FESEM, FTIR, and XPS spectra analysis. The experimental results revealed that the surface area of the raw wood charcoal was significantly decreased after boiling nitric acid modification. However, several oxygen-containing acidic function groups (-COOH, –CO) were introduced on the surface. The adsorption study confirmed that the equilibrium contact time was 1 h, the optimum adsorption pH was neutral to alkaline and the suitable adsorbent dose was 1:100 (solid: liquid). The maximum Cs was removed when the concentration of Na and K were lower (5.0 mM) with Cs in solution. The Cs adsorption processes well approved by the Langmuir isotherm and pseudo-second-order kinetic models and the maximum adsorption capacity was 35.46 mgg−1. The Cs adsorption mechanism was clearly described and it was assumed that the adsorption was strongly followed by chemisorptions mechanism based on the adsorbent surface properties, kinetic model and Langmuir isotherm model. Most importantly, about 98% of volume reduction was obtained by burning (500 °C) the Cs adsorbed charcoal, which ensured safe storage and disposal of radioactive waste. Therefore, this study can offer a guideline to produce a functional adsorbent for effective Cs removal and safe radioactive waste disposal.

Comparison of hydroxide‐based adsorbents of Mg(OH)2 and Ca(OH)2 for CO2 capture: utilization of response surface methodology, kinetic, and isotherm modeling

AbstractCO2 capture using hydroxide‐based adsorbents is studied experimentally for two adsorbents of Mg(OH)2 and Ca(OH)2. The adsorbents are characterized before and after the adsorption process by X‐ray diffraction and Fourier transform infrared spectroscopy analyses. The experiments were carried out in the ranges of 25–65°C, 2–10 bar, and 0.5–2.5 g for temperature, initial pressure, and weight of adsorbent, respectively. Based on the response surface methodology with a central composite design approach, predicted CO2 adsorption capacities are fitted well with R2 values of 0.981 and 0.929 for Mg(OH)2 and Ca(OH)2, respectively. The Freundlich isotherm model with R2 values of 0.985 and 0.994 is the best model for adsorption capacity prediction of Mg(OH)2 and Ca(OH)2, respectively. Also, the pseudo‐second‐order kinetic model with R2 values more than 0.98 is selected as the best model for both adsorbents. From the thermodynamic parameters, the reactive adsorption process is found to be exothermic and spontaneous. Due to higher adsorption percentage and capacity at the same conditions, Ca(OH)2 is more efficient than Mg(OH)2 for CO2 capture at studied ranges. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd.

Chemical Reduction of Nitrate by Zero-Valent Iron: Shrinking-Core versus Surface Kinetics Models.

Zero valent iron (ZVI) is being used in permeable reactive barriers (PRB) for the removal of oxidant contaminants, from nitrate to chlorinated organics. A sound design of these barriers requires a good understanding of kinetics. Here we present a study of the kinetics of nitrate reduction under relatively low values of pH, from 2 to 4.5. We use a particle size of 0.42 mm, which is within the recommended size for PRBs (0.2 mm to 2.0 mm). In order to avoid possible mass-transfer limitations, a well-stirred reactor coupled with a fluidized bed reactor was used. The experiments were performed at constant pH values using a pH controller that allows to accurately track the amount of acid added. Since the reduction of to by the oxidation of ZVI will always be present for these pH values, blank experiments (without nitrate) were performed and the rate of this reduction obtained. This rate of reduction was studied using three kinetic models: a regular empirical one, the Shrinking-Core Model (SCM), and the Surface Kinetics Model (SKM). The best performance was obtained from the SKM model. Therefore, this model was also used to study the results for the nitrate reduction, also with satisfactory results. In both cases, some assumptions are introduced to maintain a moderate number of fitting parameters.

Cesium and Strontium Contamination of Nuclear Plant Stainless Steel: Implications for Decommissioning and Waste Minimization.

Stainless steels can become contaminated with radionuclides at nuclear sites. Their disposal as radioactive waste would be costly. If the nature of steel contamination could be understood, effective decontamination strategies could be designed and implemented during nuclear site decommissioning in an effort to release the steels from regulatory control. Here, batch uptake experiments have been used to understand Sr and Cs (fission product radionuclides) uptake onto AISI Type 304 stainless steel under conditions representative of spent nuclear fuel storage (alkaline ponds) and PUREX nuclear fuel reprocessing (HNO3). Solution (ICP-MS) and surface measurements (GD-OES depth profiling, TOF-SIMS, and XPS) and kinetic modeling of Sr and Cs removal from solution were used to characterize their uptake onto the steel and define the chemical composition and structure of the passive layer formed on the steel surfaces. Under passivating conditions (when the steel was exposed to solutions representative of alkaline ponds and 3 and 6 M HNO3), Sr and Cs were maintained at the steel surface by sorption/selective incorporation into the Cr-rich passive film. In 12 M HNO3, corrosion and severe intergranular attack led to Sr diffusion into the passive layer and steel bulk. In HNO3, Sr and Cs accumulation was also commensurate with corrosion product (Fe and Cr) readsorption, and in the 12 M HNO3 system, XPS documented the presence of Sr and Cs chromates.

Understanding the enhanced rates of hydrogen evolution on dissolving magnesium

Despite the growing interest in Mg and its alloys, their use has been largely limited due to their high reactivity in aqueous environments. Improving the understanding of the basic principles of Mg corrosion represents the first step to explain and, eventually, improve the corrosion behaviour of Mg alloys. Herein an original mechanistic surface kinetic DFT model that clarifies the mechanism of anomalous HE on anodically polarised Mg is presented. In accordance with several experimental observations, this model describes anomalous HE proceeding at the regions dominated by anodic dissolution via the reaction of an Mg*H intermediate with water. The Mg*H intermediates undergo oxidation upon anodic polarisation, resulting in hydrogen evolution and Mg dissolution. Furthermore, it is revealed that increasing rates of an electrochemical cathodic reaction are possible within a dissolving anode.

Boric acid and borate incorporation in inorganic calcite inferred from B/Ca, boron isotopes and surface kinetic modeling

The boron concentration (B/Ca ratio) and isotopic composition (δ11B) of biogenic calcite are widely applied to reconstruct past changes in seawater carbonate chemistry. Knowledge of B incorporation pathways into calcite is critical for these applications and for improving the theoretical basis of B proxies. While the canonical interpretation of δ11B holds that B in calcite predominantly derives from dissolved borate anion in seawater, recent studies of the B content, coordination, and isotopic composition in calcite suggest more complex B incorporation pathways. To provide new insights into these pathways, here we present δ11B of inorganic calcite precipitated from saline solutions of varying pH, calcium and dissolved inorganic carbon (DIC) concentration for which B/Ca data were previously reported by Uchikawa et al. (2015). Results show that calcite δ11B significantly increases with increasing pH and decreases with increasing [Ca2+] and [DIC]. In combination, these experiments show that the difference in δ11B between solid calcite and aqueous borate linearly decreases with increasing calcite precipitation rate. To interpret these data, we present the first application of surface kinetic modeling (SKM) to boron incorporation. The SKM can simultaneously explain rate-dependent B/Ca and δ11B patterns observed in our and previously published inorganic calcite precipitation experiments when both aqueous borate and boric acid contribute to boron in inorganic calcite. If the B incorporation mechanism shown here for inorganic calcite is applicable to biogenic calcite, precipitation rate variations could modify δ11B patterns by changing the contributions of aqueous boric acid and borate to boron in calcite. However, better knowledge of biogenic calcite precipitation mechanisms and rates is needed to assess the importance of this effect for applications of B proxies in biogenic carbonates.