Despite the broad range of interest and applications, controls on calcite surface charge in aqueous solution, especially at conditions relevant to natural systems, remain poorly understood. The ...primary data source to understand calcite surface charge comprises measurements of zeta potential. Here we collate and review previous measurements of zeta potential on natural and artificial calcite and carbonate as a resource for future studies, compare and contrast the results of these studies to determine key controls on zeta potential and where uncertainties remain, and report new measurements of zeta potential relevant to natural subsurface systems.
The results show that the potential determining ions (PDIs) for the carbonate mineral surface are the lattice ions Ca2+, Mg2+ and CO32−. The zeta potential is controlled by the concentration-dependent adsorption of these ions within the Stern layer, primarily at the Outer Helmholtz Plane (OHP). Given this, the Iso-Electric Point (IEP) at which the zeta potential is zero should be expressed as pCa (or pMg). It should not be reported as pH, similar to most metal oxides.
The pH does not directly control the zeta potential. Varying the pH whilst holding pCa constant yields constant zeta potential. The pH affects the zeta potential only by moderating the equilibrium pCa for a given CO2 partial pressure (pCO2). Experimental studies that appear to yield a systematic relationship between pH and zeta potential are most likely observing the relationship between pCa and zeta potential, with pCa responding to the change in pH. New data presented here show a consistent linear relationship between equilibrium pH and equilibrium pCa or pMg irrespective of sample used or solution ionic strength. The surface charge of calcite is weakly dependent on pH, through protonation and deprotonation reactions that occur within a hydrolysis layer immediately adjacent to the mineral surface. The Point of Zero Charge (PZC) at which the surface charge is zero could be expressed as pH, but surface complexation models suggest the surface is negatively charged over the pH range 5.5–11.
Several studies have suggested that SO42− is also a PDI for the calcite surface, but new data presented here indicate that the value of pSO4 may affect zeta potential only by moderating the equilibrium pCa. Natural carbonate typically yields a more negative zeta potential than synthetic calcite, most likely due to the presence of impurities including clays, organic matter, apatite, anhydrite or quartz, that yield a more negative zeta potential than pure calcite. New data presented here show that apparently identical natural carbonates display differing zeta potential behaviour, most likely due to the presence of small volumes of these impurities. It is important to ensure equilibrium, defined in terms of the concentration of PDIs, has been reached prior to taking measurements. Inconsistent values of zeta potential obtained in some studies may reflect a lack of equilibration.
The data collated and reported here have broad application in engineering processes including the manufacture of paper and cement, the geologic storage of nuclear waste and CO2, and the production of oil and gas.
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•The IEP should be expressed as pCa or pMg; not pH. The PZC can be expressed as pH•Ca2+ and Mg2+ behave identically and zeta potential varies linearly with pCa and pMg•pH varies linearly with pCa irrespective of sample type or solution ionic strength•pSO4 may affect zeta potential by moderating the equilibrium pCa•It is important to ensure equilibrium has been reached prior to taking measurements
One-part geopolymer similar to conventional Portland cement by just adding water is a new type of green cementitious material. Na2SiO3-anhydrous is an excellent powder activator for preparing ...one-part geopolymer, but its CO2-e emission and alkalinity are very high. In this study, the Na2CO3 was used to replace a portion of Na2SiO3-anhydrous, and the properties of one-part geopolymer with composite activators were investigated systematically. The possible influencing mechanism of the complex use of activators in one-part geopolymer was proposed. Test results indicate that the replacement level of Na2CO3 has slight impact on the fluidity and increases the final setting time significantly. This is directly related to the initial heat release rate and the type of network structures formed. The compressive strength decreases with increasing the Na2CO3 content. On the one hand, the quantity of gelatinous products reduces and calcite is produced as the new phase after replacing Na2SiO3-anhydrous by Na2CO3. On the other hand, the gathering of calcite and low-crystalline CaCO3 in some regions leads to the generation of many large micro-cracks, which explains the change in the porosity of the unfavorable pores. Judged from the cost per MPa and total CO2-e emission for a cubic meter of cementitious materials, one-part geopolymer activated by composite activators is cleaner than that activated by single Na2SiO3-anhydrous or Na2CO3. This study provides a guidance for preparing one-part geopolymer with lower alkalinity and potential environmental impact.
•One-part geopolymer was prepared by using the compound activator of Na2SiO3-anhydrous and Na2CO3.•The properties of one-part geopolymer and influencing mechanism of Na2CO3 were investigated.•Na2CO3 makes one-part geopolymer cleaner without sacrificing the performance and economic benefit.
This study explored the effect of indigenous bacteria present in the soil to stabilized swelling behavior and improving the mechanical property of expansive soil. The objective of the research is to ...investigate the effectiveness of the biostimulation microbial induced calcite precipitation (MICP) for controlling the swelling-shrinkage behavior and improving shear strength of expansive soil. An attempt was made to develop an effective procedure to culture the indigenous bacteria for treating clays with varying plasticity and improve their engineering behavior. The detailed procedure has been investigated to effectively apply the MICP technique in clay soil, considering its low permeable nature. The applicability of biostimulation to clayey soils in minimizing their swelling potential and improving the strength is assessed. Both macroscale and microscale studies were conducted on untreated and biostimulated soils to observe changes in plasticity, strength, swelling, mineralogical, chemical characteristics. The present method has shown an effective alternative to improve the road pavement subgrade without affecting the eco-system of natural soil. The method investigated the effective way of providing the enrichment and cementation solution in clayey soil, which is the major concern in current literature. The study confirms that the calcite content has been increased with biostimulated MICP treatment up to 205% in the treated specimens and which future increased the unconfined compressive strength and split tensile strength. A reduction in the swelling pressure and swell strain is also observed. The results show that a cost-effect and eco-friendly method can be deployed for stabilizing the road pavement subgrades. The statistical assessment using multivariate analysis and hierarchical clustering dendrogram has been carried out to investigate the effect of the MICP treatment protocol on different soil and engineering parameters.
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•Mesoporous activated carbon was produced from industrial laundry sewage sludge (AC).•Surface area of AC decreased with increase of activation temperature.•Kinetic, isotherm and ...thermodynamic studies for Remazol Brilliant Blue R removal were evaluated.•AC showed maximum monolayer adsorption capacity of 33.5mgg−1 at 25°C.•Spent activated carbon (ACsp) was submitted to thermal regeneration cycles.
Mesoporous activated carbon (AC) was prepared from sewage sludge of industrial laundry by slow pyrolysis followed of physical activation with CO2 and applied to remove reactive dye Remazol Brillant Blue R (RBBR) from aqueous solutions. The effect of activation temperature was investigated. The materials were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and N2 adsorption-desorption isotherms. The ACs prepared at activation temperatures of 750°C (AC750), 800°C (AC800) and 850°C (AC850) exhibited BET surface areas (SBET) of 159m2g−1, 156m2g−1 and 65m2g−1, respectively, and mesoporous features with average pore diameters ranging from 4.56 to 5.88nm. The SEM images indicated the formation of cavities on the surfaces of ACs with increasing temperature. The XRD patterns showed the presence of minerals in ACs and sewage sludge such as quartz, muscovite, dolomite and calcite. Inorganic and organic contents were evidenced by TGA, and surface groups by FT-IR. The values of pHPZC indicated basic characteristics for AC750, and acidic for AC800 and AC850. The AC750 was applied in the RBBR adsorption from aqueous solution. Adsorption kinetics and adsorption equilibrium showed that the pseudo-second order and Freundlich models were best fitted to experimental data. The Langmuir monolayer maximum adsorption capacity was 33.47mgg−1. The thermodynamic parameters indicated the adsorption is a spontaneous process and favored at higher temperatures (endothermic reaction). The thermal regeneration of spent AC750 showed it can be reused on adsorption processes.
The gasification of biomass is one of the most prominent technologies for the conversion of the raw material feedstock to polymers, useful chemical substances, and energy. The main engineering ...challenge during the processing of wastes is the presence of tars in gaseous reaction products, which could make this operation methodology unsuccessfully due to the blockage of separating particle filters, fuel line flow, and substantial transfer losses. Catalytic hydrocarbon cracking appears to be a promising developing approach for their optimal removal. However, it is still highly desirable to enhance the catalysts’ activity kinetics, selectivity, stability, resistance to (ir)reversible coke deposition, and regeneration solutions. The purpose of this Review is to provide a comparative systematic evaluation of the various natural, synthetic, and hybrid ways to convert the model molecular compounds into benzene, toluene, xylene, (poly)aromatics, syngas, and others. The recent scientific progress, including calcite, dolomite, lime, magnesite, olivine, char, nonmetallic activated carbons, supported alkali, noble, and transition metals, and (metal-promoted) zeolites, is presented. A special concentrated attention is paid to effectiveness, related to hydrogenation, peculiar pore structure, and formulations’ suitable acidity. The role of catalysis is described, recommendations for prospective catalyzed mechanisms are provided, and future technical feasibility is discussed as well.
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•S-(2-hydroxyamino)-2-oxoethyl-N,N-dimethyl-dithiocarbamate (HAOMDC) is designed.•The hydroxamate-functionalized tip is firstly exploited by self-assemble of HAOMDC on Au ...tip.•Adhesion forces of the protonated and deprotonated hydroxamate groups to minerals are compared.•Relationship between adhesion and flotation separation is established.•Octyl hydroxamic acid binds to surface iron atoms of wolframite via Fe-O bonding.
Separation process facilitated by selective adsorption of chemical reagents can be modulated through an in-depth understanding of reagent-surface interaction, which is of both fundamental and practical importance in a wide range of industrial applications. Hydroxamate is a class of widely used reagents in chemical and mineral engineering that can selectively adsorb on certain minerals (e.g., malachite, wolframite and rare-earth minerals) to facilitate their separation from polymetallic ores by froth flotation. In this study, the hydroxamate-mineral interaction has been probed at nanoscale using atomic force microscopy (AFM), where the hydroxamate-functionalized tips were fabricated by self-assembly of a novel reagent S-(2-hydroxyamino)-2-oxoethyl-N,N-dimethyl-dithiocarbamate (HAOMDC) on Au tips. The adhesion energy between hydroxamate-functionalized tip and wolframite was found much higher than that with calcite and quartz, attributed to the stronger bonding between hydroxamate groups and surface Fe atoms of wolframite. Under alkaline condition, the deprotonated hydroxamate groups, due to stronger electron donating power, exhibited enhanced adhesion on wolframite and calcite as compared to that under acidic condition. X-ray photoelectron spectroscopy (XPS) results showed the adsorption of octyl hydroxamic acid (OHA) on wolframite possibly via the formation of five-membered-ring structure. The AFM force results support the froth flotation data that OHA exhibits superior selectivity for wolframite against calcite and quartz. This work provides a useful approach to evaluate the reagent-solid interaction in selective separation processes, with implications for the development of novel chemical additives in many engineering and environmental applications.
•Selective collectors for scheelite and effective depressants of calcium-containing gangue minerals have been theoretically studied or tested at laboratory stage.•Novel concepts and specific ...processes have been studied to increase flotation efficiency of fine and very scheelite and wolframite.•Combined beneficiation techniques have been investigated and applied for complex wolframite and scheelite ores.•Associated valuable metals in the form of sulphides are normally floated prior to recovery of scheelite or wolframite.
Scheelite (CaWO4) and wolframite ((Fe, Mn)WO4) are the only tungsten minerals mined commercially and are mainly found in five types of deposits: skarn, vein/stockwork, porphyry, disseminated and stratabound. Gravity concentration and flotation are the beneficiation techniques most commonly applied to scheelite ore, and gravity and/or magnetic separation for wolframite ore. With increasing exploitation, the primary tungsten resources become poorer in grade, finer in grain size and more complex in mineralogy, which makes the processing of the tungsten ores more difficult. Research and investigations into various approaches to improving the beneficiation efficiency of tungsten ores are reviewed in this paper, including selective flotation of scheelite from other calcium containing minerals such as calcite and fluorite using more selective reagents, very fine scheelite and wolframite flotation, optimization of concentration of complex tungsten ores by a combined gravity-magnetic-flotation process and recovery of associated Mo, Cu and Bi sulphides from tungsten ores.
•Significant calcite precipitation obtained without external calcium source.•Optimum compressive strength enhancement at intermediate cell concentration.•Crack healing and water absorption most ...efficient at highest cell concentration.•Reasons for the existence of the optimum concentrations provided.
The efficient crack healing capability of microbial concrete leads to an improvement in its various mechanical properties such as compressive strength, water absorption and water permeability. Studies on microbial concrete have reported that the enhancement of the compressive strength is maximum at a particular bacterial concentration, which is not necessarily the highest amongst the considered levels of bacterial concentrations. So far, the reason for the existence of such an optimal bacterial concentration for the increase in the compressive strength of concrete remains unexplored. In this paper, an attempt has been made to establish the cause of the presence of this optimal bacterial concentration. Three different bacterial concentrations of Bacillus subtilis have been used in this study, namely 103 cells/ml, 105 cells/ml and 107 cells/ml of water. Results indicate that though the higher bacterial concentration of 107 cells/ml is more efficient for crack healing, the best performance in compressive strength enhancement is achieved with the bacterial concentration of 105 cells/ml. It is seen that for a given bacterial type and mortar mix, the different calcite precipitation patterns inside the mortar matrix at varying levels of bacterial concentrations constitute the reason for the existence of the optimal bacterial concentration for compressive strength enhancement.
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•Scheelite and calcite floated well using sodium oleate as collector.•Sodium phytate exhibited strong depressant effect on calcite flotation.•Sodium phytate had little negative effect ...on scheelite flotation.•Sodium phytate could selectively adsorb on calcite through chemisorption.
In this paper, sodium phytate, a phosphate ester salt rich in phosphate groups, was evaluated as a flotation depressant for selective separation of scheelite from calcite. Its adsorption mechanism on the mineral surface was also investigated through zeta potential and FTIR spectra analysis. The micro flotation results of both single and mixed binary minerals showed that sodium phytate exhibited strong depressant effect on calcite flotation while had little influence on scheelite flotation, when it was added before the collector sodium oleate. Using sodium phytate as depressant improved the flotation separation of scheelite from calcite in the pH range of 7–12. The results of zeta potential measurements and FTIR spectra analysis demonstrated that sodium phytate adsorbed on the calcite surface and hindered the subsequent chemisorption of sodium oleate. The chelating adsorption mainly consisted of the interactions between Ca2+ on calcite surface and the PO tetrahedron in sodium phytate. Sodium phytate did not adsorb on the surface of scheelite and did not hinder the subsequent coverage by the collector sodium oleate. Based on these findings, sodium phytate has the potential to act as an effective depressant in flotation separation of scheelite from calcite.
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