Calcite Surface Research Articles

Simulations of Dissolution of Initially Flat Calcite Surfaces: Retreat Velocity Control and Surface Roughness Scaling

The far-from-equilibrium alkaline dissolution of initially flat and defect-free calcite surfaces is modeled with kinetic Monte Carlo simulations of site detachment from a Kossel crystal and with scaling approaches. The surface retreat velocity is strongly dependent on the detachment rate of the highest coordination sites, which represent molecules in the middle of (101̅4) terraces. The comparison with a recent velocity estimate from digital holographic microscopy predicts the removal rate of those molecules between 2 × 10–7 and 4 × 10–6 s–1 at room temperature, which improves a previous estimate from a grain dissolution model. The activation energy for this molecular-scale process is estimated as 92 ± 6 kJ/mol (assuming the same prefactor ∼3 × 1010 s–1 of kink and step sites). The areal density of nonterrace sites (mostly steps and kinks) is also related to that removal rate, so we propose that the measurement of this density may provide independent estimates of the above quantities. Arrhenius plots of the retreat velocities obtained in high-temperature simulations predict the (macroscopic) activation energy of 69 ± 4 kJ/mol for dissolution of smooth surfaces. We also show that the scaling of the surface roughness in time and size is the same as in the Kardar–Parisi–Zhang (KPZ) equation of kinetic roughening. However, room-temperature roughening of initially smooth calcite surfaces is so slow that it is unlikely to be observed in typical experimental times and the alkaline conditions modeled here. Since the rates in acidic media are larger and arguing that the microscopic symmetries of the molecule detachment processes should be preserved, we suggest the investigation of KPZ scaling under those conditions.

Water-calcite (104) surface interactions using first-principles simulations

An in-depth understanding of the water and mineral interaction is indispensable for many fields including enhanced oil recovery, global carbon cycle, and environment sustainability. In this work, using first-principles calculations based on density functional theory supplemented with semiempirical DFT-D2 dispersion corrections, we study the structural, energetic, electronic, and optical properties of hydrated calcite (104) surfaces via an exhaustive sampling of all the adsorption sites with up to full (monolayer) water coverage. Our results suggest that on-top Ca surface sites are the most energetically favorable sites for water adsorption. Also, we found sizable differences in the binding energies among the configurations having the same number of adsorbed water molecules, highly likely due to the differences in the organization and tilting of Cas-Os octahedra along with the differences in the Cas-Ow bond distance and Cas-Ow-H bond angles. We also report intriguing variations in the optical absorption spectra of hydrated calcite (104) surfaces with respect to pristine (anhydrous) calcite (104) surface. Interestingly, we find significant variations in the optical absorption spectra among the hydrated calcite (104) surfaces having the same of adsorbed water molecules. The findings in this theoretical study would help in improving our fundamental understanding of hydrated calcite (104) surface that has impact on many processes in nature.

Selective adsorption of Na2ATP as an eco-friendly depressant on the calcite surface for effective flotation separation of cassiterite from calcite

Adenosine 5′-triphosphate disodium salt (Na 2 ATP) was used as the efficient depressant for the separation of cassiterite from calcite. The effect of Na 2 ATP on the flotation of cassiterite and calcite was evaluated by the micro-flotation experiments, and the results showed that an excellent separation performance was achieved at the reagents regime of 1.0 mM Na 2 ATP and 0.14 mM NaOL when pulp pH was 9.11. Moreover, the adsorption and depression mechanism of Na 2 ATP on calcite was investigated via a series of surface analyses. The contact angle analysis and zeta potential measurements showed that Na 2 ATP was strongly adsorbed onto the calcite surface rather than cassiterite surface, and that could still significantly enhance the hydrophilicity of the calcite after conditioned with NaOL. The FTIR analysis and XPS analysis results indicated that the interaction between Na 2 ATP and calcite was the chemical binding, and the calcium atom on the calcite surface and the phosphate group of Na 2 ATP was the major reaction unit in this processing. Furthermore, the pre-adsorption of Na 2 ATP on calcite surface would impede the interaction between NaOL and calcite, but the adsorption of NaOL on cassiterite surface could not be affected by the weak physical adsorption of Na 2 ATP. Therefore, Na 2 ATP could be utilized as a potential depressant for removing calcite from cassiterite. ● Na 2 ATP was chemisorbed onto the calcite surface rather than cassiterite surface. ● The chemisorption of Na 2 ATP on calcite surface could intensively impede the further adsorption of NaOL. ● Na 2 ATP could be selected as an effective depressant for removing the calcite from cassiterite.

The effect of vacancy defects on the adsorption of methane on calcite 104 surface

Density functional theory calculations are used to study the effect of vacancy defects (ionic Ca and charge neutral CO 3 ) on the adsorption properties of CH 4 molecule on different calcite surfaces. Both types of vacancy defect results in strong adsorption of the gas molecules as compared to defect-free sample. For example, for the 104 surface of the material the methane molecules gets adsorbed stronger by 63.6% and 24.4% for Ca and CO 3 vacancy defects, respectively. Such enhanced adsorption of the CH 4 molecule due to the presence of the vacancy defects is also obtained for the other surface symmetries. Electronic structure calculations show that the latter is due to the orbital-overlap/hybridization between the organic molecules and the substrate. Adsorption capacity of the calcite also increases due to the presence of the defect. Vacancy defects also create energy barriers for the migration of CH 4 on the surface of calcite as revealed in our nudged elastic band calculations. The effect on the CH 4 adsorption and migration becomes more pronounced in the case of Ca vacancy defect. These findings could be of practical importance for, e.g., estimating ultimate gas recovery.

Haldor Topsoe and Yanchang form joint venture and build methanol catalyst facility in China

The flotation separation of fluorite and calcite poses a long-standing challenge in the mineral processing industry because of their similar physicochemical characteristics. This study uses sodium oleate (NaOL) as the collector and Pb-water glass as the depressant to improve the fluorite-calcite separation efficiency. The effect of Pb-water glass was analyzed using micro-flotation experiments, calculations of solution chemistry, X-ray photoelectron spectroscopy (XPS), zeta potential and time of flight secondary ion mass spectrometry (ToF-SIMS). The results of micro-flotation experiments show that Pb-water glass exhibits a stronger selective depression effect on calcite than water glass alone, but is considerably less sensitive to fluorite flotation. The calculations of solution chemistry, XPS and zeta potential results show that the improved depression effect can be attributed to lead ions, which not only increase the adsorption of silicate species on the calcite surface but also improve the formation of Pb-silicate polymer species. The ToF-SIMS results further show that the adsorption of Pb-water glass on the calcite surface interferes with the adsorption of NaOL on the calcite surface, thereby reducing the floatability of calcite.

Molecular Origin of Wettability Alteration of Subsurface Porous Media upon Gas Pressure Variations

Upon extraction/injection of a large quantity of gas from/into a subsurface system in shale gas production or carbon sequestration, the gas pressure varies remarkably, which may significantly change the wettability of porous media involved. Mechanistic understanding of such changes is critical for designing and optimizing a related subsurface engineering process. Using molecular dynamics simulations, we have calculated the contact angle of a water droplet on various solid surfaces (kerogen, pyrophyllite, calcite, gibbsite, and montmorillonite) as a function of CO2 or CH4 gas pressure up to 200 atm at a temperature of 300 K. The calculation reveals a complex behavior of surface wettability alteration by gas pressure variation depending on surface chemistry and structure, and molecular interactions of fluid molecules with surfaces. As the CO2 gas pressure increases, a partially hydrophilic kerogen surface becomes highly hydrophobic, while a calcite surface becomes more hydrophilic. Considering kerogen and calcite being the major components of a shale formation, we postulate that the wettability alteration of a solid surface induced by a gas pressure change may play an important role in fluid flows in shale gas production and geological carbon sequestration.

High-Speed Atomic Force Microscopy of the Structure and Dynamics of Calcite Nanoscale Etch Pits.

Calcite dissolution is initiated by the formation of a nanoscale etch pit followed by step edge propagation and hence strongly influenced by the interactions between surface diffusing ions and step edges. However, such atomic-scale dynamics are mostly inaccessible with current imaging tools. Here, we overcome this limitation by using our recent development of high-speed frequency modulation atomic force microscopy. By visualizing atomic-scale structural changes of the etch pits at the calcite surface in water, we found the existence of mobile and less-mobile surface adsorption layers (SALs) in the etch pits. We also found that some etch pits maintain their size for a long time without expansion, and their step edges are often associated with less-mobile SALs, suggesting their step stabilization effect.

Antiscaling Evaluation and Quantum Chemical Studies of Nitrogen-Free Organophosphorus Compounds for Oilfield Scale Management

Nonpolymeric aminomethylenephosphonates are widely used as powerful scale inhibitors in the petroleum industry. However, most of these inhibitors have certain drawbacks, such as low biodegradability and incompatibilities with high calcium brines. Therefore, there is a great need to explore more biodegradable phosphonated oilfield scale inhibitors affording high calcium-ion tolerance. In this project, known and new nitrogen-free phosphonates have been tested as scale inhibitors against carbonate and sulfate scales according to the Heidrun oilfield, Norway. The considered nitrogen-free scale inhibitors are 1,2,4-phosphonobutanetricarboxylic acid (PBTCA), hydroxyphosphonoacetic acid (HPAA), phosphonoacetic acid (PAA), and 3-phosphonopropanoic acid (PPA). A high-pressure dynamic tube-blocking test, calcium tolerance, thermal aging, and seawater biodegradation were used to assess the antiscaling performance of these inhibitors. A very good to excellent performance of all nitrogen-free phosphonate scale inhibitors has been observed against the calcite scaling. A biodegradable naturally occurring PAA displayed a very good calcite inhibition efficiency and afforded excellent thermal stability at 130 °C for 7 days under anaerobic conditions. PAA also gave outstanding tolerance activity with all concentrations up to 10 000 ppm calcium ions. Density functional theory (DFT) simulations predicted higher affinities of the commercial SIs compared to the nitrogen-free molecules, which is in line with their calcium compatibilities. The high calcium tolerance of nitrogen-free molecules makes them more efficient than commercial inhibitors. Further, DFT solid-state simulations reveal that the affinities of the nitrogen-free molecules for the calcite surface are higher than the barite surface, which agrees well with the experimental fail inhibitor concentration (FIC) data. The sluggish and complicated kinetics of the barite scale formation compared to the calcite scale explain well the high concentrations of the nitrogen-free molecules required for barite inhibition. In summary, our results showed that the nitrogen-free molecules show good potential as scale inhibitors for both calcite and barite. However, for the latter scale, further optimization is needed for optimal performance.

Enhanced Flotation Separation of Cassiterite from Calcite Using Metal-Inorganic Complex Depressant

At present, the research on flotation separation of cassiterite and gangue minerals is mainly focused on the development of new collectors or depressants, while the research on combined depressants is rare. In this study, the flotation separation of cassiterite and the typical gangue mineral, calcite, was investigated using the metal-inorganic complex depressant AlSS, composed of aluminum sulfate (Al2(SO4)3) and water glass (Na2SiO3·4H2O). The flotation results indicated that, with the assistance of Al3+, the inhibition effect of SS (water glass) on calcite is significantly enhanced, while the inhibition effect on cassiterite is almost unchanged. The zeta potential and adsorption measurements proved that the formation of the negatively charged colloidal particles composed of Alm(OH)n3m−n and SiOm(OH)n4−2m−n tended to adsorb on the positively charged calcite surface, inhibiting the adsorption of sodium oleate (NaOL), while the adsorption of the colloidal particles on the negatively charged cassiterite surface is relatively less. The contact angle measurements indicate that the contact angle of cassiterite in the NaOL + AlSS solution is obviously larger than that of calcite, which indicates that cassiterite has better floatability in this system. The X-ray photoelectron spectroscopy (XPS) analysis confirms the Al and Si species are adsorbed on the surface of calcite, but not on the cassiterite. In addition, the adsorption of AlSS on cassiterite and calcite changes the chemical environment on the calcite surface, indicating that the adsorption of AlSS on the mineral surface is more inclined to be through a chemical reaction.

Atomistic Insight into the Behavior of Ions at an Oil-Bearing Hydrated Calcite Surface: Implication to Ion-Engineered Waterflooding

This research provides an atomistic picture of the role of ions in modulating the microstructural features of an oil-contaminated calcite surface. This is of crucial importance for the rational design of ion-engineered waterflooding, a promising technique for enhancing oil recovery from carbonate reservoirs. Inspired by a conventional lab-scale procedure, an integrated series of molecular dynamics (MD) simulations were carried out to resolve the relative contribution of the major ionic constituent of natural brines (i.e., Na+, Cl–, Mg2+, Ca2+, and SO42–) when soaking an oil-bearing calcite surface in different electrolyte solutions of same salinity, namely, CaCl2, MgCl2, Na2SO4, MgSO4, and deionized water (DW). In all cases, we observed the gradual detachment of polar oil molecules (mimicked by decanoate) already paired to Na+ cations, covering the calcite substrate in such a way that carboxylate groups were in contact with the treating solution. The appearance of such a negatively charged interface in conjunction with the bare calcite/brine surface is a likely nanometric source for the disparity of surface characteristics of oil-bearing rocks reported in the literature. The MD results showed the affinity of divalent cations (Ca2+ or Mg2+) for pairing with negatively charged carboxylate functional groups, thereby facilitating desorption of decanoates. Having a compact solvation shell, Mg2+ was not as effective as Ca2+ cations; however, its performance was enhanced in the presence of sulfate anions. We further figured out the tendency of SO42– anions to shielding Na+ sites over the calcite surface, thus limiting the chance of readsorption of carboxylate compounds. Consistent with lab experiences, sulfate was found to assist the access of magnesium cations to the calcite surface as well. Altogether, the present results provide us with a molecular-level validation for the well-known multicomponent ion exchange mechanism proposed for the wettability alteration of carbonate rocks through enriching the divalent ionic content of the injection brine solution.

Nanoscale Wear and Mechanical Properties of Calcite: Effects of Stearic Acid Modification and Water Vapor.

Understanding the wear of mineral fillers is crucial for controlling industrial processes, and in the present work, we examine the wear resistance and nanomechanical properties of bare calcite and stearic acid-modified calcite surfaces under dry and humid conditions at the nanoscale. Measurements under different loads allow us to probe the situation in the absence and presence of abrasive wear. The sliding motion is in general characterized by irregular stick-slip events that at higher loads lead to abrasion of the brittle calcite surface. Bare calcite is hydrophilic, and under humid conditions, a thin water layer is present on the surface. This water layer does not affect the friction force. However, it slightly decreases the wear depth and strongly influences the distribution of wear particles. In contrast, stearic acid-modified surfaces are hydrophobic. Nevertheless, humidity affects the wear characteristics by decreasing the binding strength of stearic acid at higher humidity. A complete monolayer coverage of calcite by stearic acid results in a significant reduction in wear but only a moderate reduction in friction forces at low humidity and no reduction at 75% relative humidity (RH). Thus, our data suggest that the wear reduction does not result from a lowering of the friction force but rather from an increased ductility of the surface region as offered by the stearic acid layer. An incomplete monolayer of stearic acid on the calcite surface provides no reduction in wear regardless of the RH investigated. Clearly, the wear properties of modified calcite surfaces depend crucially on the packing density of the surface modifier and also on the air humidity.

Mechanism for enhanced oil recovery from carbonate reservoirs by adding copper ions to seawater

In existing or operating oil fields, enhanced oil recovery (EOR) methods are employed to increase production, which can contribute to meet fossil fuel demand. This research proposes a new EOR method by adding Cu(II) ions into the injected seawater, which shows a significant incremental oil production in carbonate reservoirs. An important remark in this study is the Cu(II) incorporation into the cores, with mineralization of atacamite on the carbonate surface, and copper complexation with the acid fraction of the oil. In addition, we observed oil removal up to the last monolayer on calcite surfaces. Based on these observations and first-principles calculations, we propose an EOR mechanism where the acidic oil desorbs from the rock surface by forming acid-copper complexes that are dragged by the injected fluids. In summary, we propose a simple and cost-effective EOR method with transition metals, which may significantly increase the yield of low-recovery carbonate reservoirs and reduce the environmental impact of exploring new reservoirs.

Water Orientation at the Calcite-Water Interface.

Mineral-water interfaces play an important role in many natural as well as technological fields. Fundamental properties of these interfaces are governed by the presence of the interfacial water and its specific structure at the surface. Calcite is particularly interesting as a dominant rock-forming mineral in the earth’s crust. Here, we combine atomic force microscopy, sum-frequency generation spectroscopy, and molecular dynamics simulations to determine the position and orientation of the water molecules in the hydration layers of the calcite surface with high resolution. While atomic force microscopy provides detailed information about the position of the water molecules at the interface, sum-frequency generation spectroscopy can deduce the orientation of the water molecules. Comparison of the calcite-water interface to the interfaces of magnesite-water, magnesite-ethanol, and calcite-ethanol reveals a comprehensive picture with opposite water orientations in the first and second layer of the interface, which is corroborated by the molecular dynamics simulations.

A novel green depressant for flotation separation of scheelite from calcite

Polyaspartic acid (PASP) was used as a novel environmental-friendly depressant, and its inhibition effect on flotation performance of scheelite and calcite using sodium oleate (NaOL) as a collector was investigated by ways of flotation experiments, zeta potential measurement, contact angle measurement and infrared spectroscopic analysis (IR). The results show that PASP exhibits stronger inhibition capability and selectivity than acidified water glass, and the flotation separation of scheelite from calcite can be realized in the presence of 6 mg/L PASP and 20 mg/L NaOL at pH>6. In the presence of PASP, the zeta potential of calcite surface almost keeps unchanged after adding NaOL, and the contact angle decreases sharply. Therefore, it is indicated that PASP significantly decreases the adsorption of NaOL on calcite surface. By contrast, it has no distinct effect on the adsorption of NaOL on the scheelite surface, which is further confirmed by IR results.

Sulfate and Molybdate Incorporation at the Calcite-Water Interface: Insights from Ab Initio Molecular Dynamics.

Sulfur and molybdenum trace impurities in speleothems (stalagmites and stalactites) can provide long and continuous records of volcanic activity, which are important for past climatic and environmental reconstructions. However, the chemistry governing the incorporation of the trace element-bearing species into the calcium carbonate phases forming speleothems is not well understood. Our previous work has shown that substitution of tetrahedral oxyanions [XO4]2– (X = S and Mo) replacing [CO3]2– in CaCO3 bulk phases (except perhaps for vaterite) is thermodynamically unfavorable with respect to the formation of competing phases, due to the larger size and different shape of the [XO4]2– tetrahedral anions in comparison with the flat [CO3]2– anions, which implied that most of the incorporation would happen at the surface rather than at the bulk of the mineral. Here, we present an ab initio molecular dynamics study, exploring the incorporation of these impurities at the mineral–water interface. We show that the oxyanion substitution at the aqueous calcite (10.4) surface is clearly favored over bulk incorporation, due to the lower structural strain on the calcium carbonate solid. Incorporation at surface step sites is even more favorable for both oxyanions, thanks to the additional interface space afforded by the surface line defect to accommodate the tetrahedral anion. Differences between sulfate and molybdate substitutions can be mostly explained by the size of the anions. The molybdate oxyanion is more difficult to incorporate in the calcite bulk than the smaller sulfate oxyanion. However, when molybdate is substituted at the surface, the elastic cost is avoided because the oxyanion protrudes out of the surface and gains stability via the interaction with water at the interface, which in balance results in more favorable surface substitution for molybdate than for sulfate. The detailed molecular-level insights provided by our calculations will be useful to understand the chemical basis of S- and Mo-based speleothem records.

Interactions between stearic acid and calcite surfaces: Experimental and computer simulation studies

Calcite surface was modified with stearic acids, and the interaction between them was investigated both experimentally and theoretically. Stearic-acid-modified calcite powders were investigated with Fourier transform infrared spectroscopy, X-ray diffraction, zeta potential analyser and contact angle measurement. Then, the density functional theory calculations were performed to explore the interaction at the atomic scale. The experimental results and simulation indicated that stearic acids interact with calcite surface via chelation between the double-bond O atom of –COOH and the Ca atom on calcite surface. The terminal methyl group of the stearic acids decreases the hydrophilicity of the calcite surface, and the interaction between different crystal faces of calcite is different.

Phosphorus recycling potential by synthetic and waste materials enriched with dairy wastewater: A comparative physicochemical study

Recovery of the essential macro-nutrient phosphorus (P) from P-rich waste streams is of global interest, with dwindling sources of P fertilizers threatening food security. We present a comparative physicochemical study of potential P adsorbents: two types of synthetic layered double hydroxide (LDH) and two recycled water treatment residuals (WTRs). Adsorbents were mixed with dairy WW, pre-clarified with either nanocomposites or centrifugation. LDHs practically removed all P (and higher N and organic C). In contrast, WTRs removed 50–80% P. WW-treated WTRs, particularly Fe-WTR, displayed substantially higher P lability than WW-treated LDHs, and inorganic P-loaded forms (e.g., 90 vs. 0.5 mg P kg−1). Adsorbent analyses with SEM, PXRD, FTIR, and 31P solid-state NMR contributed structural and chemical insights. Adsorbents mixture with dairy WW led to chemical and structural alterations, including Ca absorption and co-precipitation with P. Yet, high porosity and predominating adsorbed P species indicated rapid surface adsorption reactions onto the LDHs. The WTRs presented multiple sorption pools, including amorphous oxides, calcite and silicate surfaces, and disequilibrium with Ca-P minerals, accounting for weak P sorption. In addition, it appears Fe-based WTR was most sensitive to organic compounds’ impact. Thus, WW-treated WTRs can be employed as labile P sources, while LDH adsorbents can be employed as excellent purifiers for polluted streams.

Experimental Investigation of the Effect of Acid and Base on Wettability Alteration of a Calcite Surface

Wettability alteration toward a favorable wetting condition is one major mechanism of enhancing oil recovery in carbonates. Obtaining the desired wettability stipulates that a clear understanding of the original wetting properties of the rock surface be provided. In fact, the aim of this study is to elucidate the effect of acid and base upon the wettability alteration of a calcite surface by investigating the individual effects of an organic acid (stearic acid) and a base (N,N-dimethyldodecylamine, N,N-DMDA) at different concentrations as well as their combinations on calcite wetting properties. Contact angle, ζ-potential, Fourier transform infrared spectroscopy, and thermogravimetric analysis (TGA) were conducted to determine changes in wetting properties of the calcite surface. Moreover, interfacial tension (IFT) measurements were used to evaluate the interfacial activity of the model oil systems. Overall, the results revealed that by increasing the acid concentration, wettability of a calcite surface shifted toward more oil-wetness and adsorption of stearic acid on a calcite surface increased. Treating calcite surfaces with the base revealed that a low concentration of N,N-DMDA was able to shift the calcite wettability from a water-wet state to a slightly oil-wet state. In addition, TGA results showed low adsorption of the base with a high surface area occupied by the base molecule, indicating a horizontal orientation of the base on the calcite surface. For acid–base mixtures, at a constant acid concentration, the wettability alteration toward oil-wetness increased in the presence of the base, as the base-to-acid ratio increased up to 1, as a result of which oil-wetness decreased. Moreover, IFT measurements indicated lower IFT of acid–base mixtures compared to those of individual acidic and basic model oil solutions, which could be attributed to the synergistic effect between the acid and base by forming acid–base complexes. The presence of the base hindered acid adsorption on the calcite surface. However, the basic component could contribute to wettability alteration by forming acid–base complexes. These complexes could adsorb on the calcite surface and affect its wetting preferences. In addition, the base molecules may adsorb on the calcite surface at high base-to-acid ratios. The results suggested that the ratio of base to acid would determine the contribution of acid molecules, base molecules, and acid–base complexes in wettability alteration of calcite surfaces. Hence, possible mechanisms of wettability alteration by acid–base combinations were proposed at different base-to-acid ratios.

Influence of lead ion pretreatment surface modification on reverse flotation separation of fluorite and calcite

The flotation separation of fluorite and calcite poses a long-standing challenge in the mineral processing industry. Conventional flotation exhibits a poor separation for high-calcite-type fluorite ores. In this study, solution chemical calculations and lead ion adsorption experiments were performed to test the effects and mechanisms of lead ion pretreatment surface modification on the reverse flotation separability of fluorite and calcite. The effect of lead ion pretreatment was studied by X-ray photoelectron spectroscopy (XPS). Reverse flotation experiments to separate fluorite and calcite were designed and the results were analyzed by fourier transform infrared spectroscopy (FT-IR) and time of flight secondary ion mass spectrometry (ToF-SIMS). The flotation results showed that the grade of the fluorite concentrate significantly improved after surface modification via lead ion pretreatment compared with that prior to pretreatment. Results of the solution chemical calculations, lead ion adsorption experiments, XPS, FT-IR and ToF-SIMS analyses showed that a large number of lead ions adsorbed on the calcite surface during the modification process and precipitates formed, including those of Pb3(CO3)2(OH)2 and Pb(OH)2, which enhanced the adsorption of calcite and the collector. In contrast, the adsorption capacity of lead ions on the surface of fluorite was poor and had little effect on the fluorite surface composition. These results provide a theoretical basis and promising new strategy for the reverse flotation separation of fluorite and calcite.

Why is calcite a strong phosphorus sink in freshwater? Investigating the adsorption mechanism using batch experiments and surface complexation modeling

One of the primary drivers of Phosphorus (P) limitation in aquatic systems is P adsorption to sediments. Sediments adsorb more P in freshwater compared to other natural solutions, but the mechanism driving this difference is poorly understood. To provide insights into the mechanism, we conducted batch experiments of P adsorption to calcite in freshwater and seawater, and used computer software to develop complexation models. Our simulations revealed three main reasons that, combining together, may explain the greater P adsorption to calcite in freshwater vs. seawater. First, aqueous speciation of P makes a difference. The ion pair CaPO4− is much more abundant in freshwater; although seawater has more Ca2+ ions, MgHPO40 and NaHPO40 are more thermodynamically favored. Second, the adsorbing species of P make a difference. The ion pair CaPO4− (the preferred adsorbate in freshwater) is able to access adsorption sites that are not available to HPO42− (the preferred adsorbate in seawater), thereby raising the maximum concentration of P that can adsorb to the calcite surface in freshwater. Third, water chemistry affects the competition among ions for surface sites. Other ions (including P) compete more effectively against CO32− when immersed in freshwater vs. seawater, even when the concentration of HCO3−/CO32− is higher in freshwater vs. seawater. In addition, we found that under oligotrophic conditions, P adsorption is driven by the higher energy adsorption sites, and by the lower energy sites in eutrophic conditions. This study is the first to model P adsorption mechanisms to calcite in freshwater and seawater.