Speciation Of Ca Research Articles

Thermochemical Transformation of Calcium during Biomass Burning and the Effects on Postfire Aqueous Dissolution of Macronutrients.

Calcium is commonly the most abundant element in fire residues and its speciation largely determines the geochemical properties of fire residues and their effects on postfire soil chemistry. To explore the effects of biomass composition and fire conditions on ash Ca speciation, this study characterizes the speciation of Ca in charcoal and ash samples that were derived from different plant compartments and thermal conditions, using Ca K-edge X-ray absorption near edge spectroscopy. Results showed that biomass contains abundant organic Ca complexes, which were mineralized into fairchildite and calcite after heating at 450 to 600 °C and then CaO, as temperature increased to 750 °C. Apatite could be an abundant Ca species in fire residues if the Ca/P molar ratio of the biomass is small (<2). The mineralization of organic Ca to the identified Ca minerals during burning was negligibly affected by the oxygen level. Calcium speciation in prescribed fire residues resembled that of biomass ash burned at 550 °C with similar Ca/P molar ratios. Batch experiments showed that macronutrients (Ca, Mg, K, and P) were differentially released, as a result of different solubility of minerals in ashes and reprecipitation of minerals. The aqueous solubility of Ca, Mg, and P decreased as pH increased from 5 to 9, while K showed no pH dependency and was almost completely soluble. Results from this study improve our understanding of the chemistry of fire residues and their geochemical behaviors, which can help evaluate the impact of fire on postfire soil properties and macronutrient cycling.

(Invited) Analysis of Interfacial Species at Finite Bias - Calcium Borohydride in Tetrahydrofuran

To understand the operation of complex electrolytes we must determine which species are present and active at the interface with the electrode. We combine molecular dynamics sampling with continuum modeling to determine the speciation of Ca(BH4)2 in THF next to a model electrode (graphite). Mulitvalent ions in organic solvents with poor dielectric screening may exist in multiple coordinated states. We explore the free energy landscape associated with the interconversion of species in the bulk electrolyte using metadynamics simulations. We find that the chemical equilibrium between neutral and charged species is dominated by disproportionation following dimer formation - consistent with experimental findings. We also make use of clustering analysis to reveal distinct molecular conformations within constrained coordination states. To assess how the chemical equilibrium may be modified by the potential difference at the electrode, we develop a continuum model based on the modified Poisson-Boltzmann equation, with volume exclusion. This model also incorporates molecular-scale details through sampled free energy profiles of each relevant species within the solvent as a function of distance from the electrode. We find that a narrow (approximately 1 nm) region near the electrode, with highly variable speciation driven by potential difference, dominates the electrochemical activity of this electrolyte.

X-ray Absorption Spectroscopy as a Process Analytical Technology: Reaction Studies for the Manufacture of Sulfonate-Stabilized Calcium Carbonate Particles.

Process analytical technologies are widely used to inform process control by identifying relationships between reagents and products. Here, we present a novel process analytical technology system for operando XAS on multiphase multicomponent synthesis processes based on the combination of a conventional lab-scale agitated reactor with a liquid-jet cell. The preparation of sulfonate-stabilized CaCO3 particles from polyphasic Ca(OH)2 dispersions was monitored in real time by Ca K-edge XAS to identify changes in Ca speciation in the bulk solution/dispersion as a function of time and process conditions. Linear combination fitting of the spectra quantitatively resolved composition changes from the initial conversion of Ca(OH)2 to the Ca(R-SO3)2 surfactant to the ultimate formation of nCaCO3·mCa(R- SO3)2 particles. The system provides a novel tool with strong chemical specificity for probing multiphase synthesis processes at a molecular level, providing an avenue to establishing the relationships between critical quality attributes of a process and the quality and performance of the product.

The Multi-Analytical Characterization of Calcium Oxalate Phytolith Crystals from Grapevine after Treatment with Calcination

Calcium oxalate phytoliths are one of the most prominent types of Ca speciation in the plant kingdom, and they store extensive amounts of carbon in crystalline form. Ca phytoliths were investigated in the root, trunk, and bark of Vitis vinifera Chasselas from a vineyard in Alsace, France. A multi-analytical approach was used, which included SEM coupled with EDX spectroscopy, XRD, XRF, TGA, and 13C-NMR spectroscopy. These techniques revealed that phytoliths are composed of crystalline calcium oxalate monohydrate (whewellite). The whewellite crystals exhibited mostly equant or short-prismatic habits in all of the three studied grapevine parts, but bipyramidal crystals also occurred. Raphide crystals were only observed in the root, where they were abundant. Instead of using wet chemical procedures to extract the mineral components from the organic parts of the biomass, a thermal treatment via calcination was chosen. The suitable temperature of calcination was determined through TGA experiments. The calcination of the biomass samples at 250 °C enhanced the amounts of Ca phytoliths in the residual chars. The thermal treatment, however, affected the appearance of the Ca oxalate crystals by producing surfaces that displayed macroporosity and by creating fractures. For calcination at both 300 °C and 350 °C, Ca oxalate lost a molecule of carbon monoxide to form Ca carbonate, and the modifications of the original crystal surfaces were more pronounced than those observed after thermal treatment at 250 °C.

Electro-dewatering of steel industrial sludge: Performance and metal speciation

Electro-dewatering (EDW) of sludge has been regarded as an economical and efficient dewatering method in recent years. There have been some studies on municipal sludge and industrial sludge. However, the effects of industrial sludge with high metal and salty content on the EDW performance are still unclear. Here we investigate EDW in the characteristics of steel industrial sludge, which contains large amounts of salts and metals. First, the differences between the steel industrial sludge and the other sludge in EDW treatment were explored. Then, the mechanism of interaction between the transformation of metal speciation and dewatering performance was investigated. The results indicated that the optimal 40.09% moisture content (MC) could be obtained under the operating conditions of 60 V voltage and 2.6 bar pressure. Moreover, the pH variation and electro-redox behavior can lead to the transformation of metal speciation and release charged intermediates relating to metal, causing the rise of current in the EDW process. These phenomena were beneficial to continuous dewatering. By contrast, the speciation of Ca and Zn is mainly affected by electromigration and electroosmotic flow, but the speciation of Fe and Al is more affected by the oxidizing atmosphere. This work extended the application range of EDW and filled the gap of EDW for the industrial sludge field.

Накопление Cs-137 и Sr-90 культурой салата (Lactuca sativa L.) на почвах радиоактивно-загрязнённых участков бывшего Семипалатинского испытательного полигона

The paper presents the study of Cs-137 and Sr-90 transfer from soils of the former technical sites of the Semipalatinsk test site to the salad culture (Lactuca sativa L.) as an example of the leaf vege-table. The factors affecting the transfer of the radionuclides from soils with a different character of radioactive contamination to the salad culture were identified under the model vegetation ex-periment conditions. The variation ranges of Cs-137 and Sr-90 transfer factors values were 2 and 1 orders of magnitude respectively. With the use of the private correlation method, it was found that the content of biologically available forms of Cs-137 and Sr-90 in the soils of the former Semipala-tinsk test site, and, consequently, accumulation of these radionuclides by the salad, depended on their nonisotopic analogs K and Ca concentration in the soil, respectively (rxy–z=-0.81; n=4, p&lt;0.2 – for Cs-137 and gross K; rxy-z=-0.64; n=4, p&lt;0.2 – for Sr-90). There was a moderate positive relationship between the value of Cs-137 transfer factor and physical clay content (fraction &gt;0.01%) in the soil (ρ=0.47; n=12, p&lt;0.05), the relationship between value of Sr-90 transfer factor and the physical clay content was weak negative (ρ=-0.3; n=11, p&lt;0.05). The migration of Cs-137 and Sr-90 to the salad culture from the radioactively contaminated soils of the Semipalatinsk test site mainly depended on biologically available forms content of radionuclides and their non-isotopic analogs K and Ca speciation, respectively. The impact of physical clay on the accumulation of Cs-137 and Sr-90 by the salad culture was less pronounced, it could be due to the specific conditions for radioactive contamination of the soil surface in the territory of the former technical sites of the Semipalatinsk test site.

Calcium: A comprehensive review on quantification, interaction with milk proteins and implications for processing of dairy products.

Calcium (Ca) is a key micronutrient of high relevance for human nutrition that also influences the texture and taste of dairy products and their processability. In bovine milk, Ca is presented in several speciation forms, such as complexed with other milk components or free as ionic calcium while being distributed between colloidal and serum phases of milk. Partitioning of Ca between these phases is highly dynamic and influenced by factors, such as temperature, ionic strength, pH, and milk composition. Processing steps used during the manufacture of dairy products, such as preconditioning, concentration, acidification, salting, cooling, and heating, all contribute to modify Ca speciation and partition, thereby influencing product functionality, product yield, and fouling of equipment. This review aims to provide a comprehensive understanding of the influence of Ca partition on dairy products properties to support the development of kinetics models to reduce product losses and develop added-value products with improved functionality. To achieve this objective, approaches to separate milk phases, analytical approaches to determine Ca partition and speciation, the role of Ca on protein-protein interactions, and their influence on processing of dairy products are discussed.

Molecular mechanisms underlying the calcium-mediated uptake of hematite nanoparticles by the ciliate Tetrahymena thermophila

In aquatic ecosystems, the calcium (Ca) concentration varies greatly. It is well known that Ca affects the aggregation of nanoparticles (NPs) and thus their bioaccumulation. Nevertheless, Ca also plays critical roles in various biological processes, whose effects on NP accumulation in aquatic organisms remain unclear. In this study, the effects of Ca on the uptake of polyacrylate-coated hematite NPs (HemNPs) by the aquatic ciliate Tetrahymena thermophila were investigated. At all of the tested Ca concentrations, HemNPs were well dispersed in the experimental medium, excluding the possibility of Ca to influence HemNP bioaccumulation by aggregating the NPs. Instead, Ca was shown to induce the clathrin-mediated endocytosis and phagocytosis of HemNPs. Manipulation of the Ca speciation in the experimental medium as well as the influx and intracellular availability of Ca in T. thermophila indicated that HemNP uptake was regulated by the intracellular Ca level. The results of the proteomics analyses further showed that the binding of intracellular Ca to calmodulin altered the activity of proteins involved in clathrin-mediated endocytosis (calcineurin and dynamin) and phagocytosis (actin). Overall, the biologically inductive effects of Ca on NP accumulation in aquatic organisms should be considered when evaluating the environmental risks of NPs.

The fate of calcium in temperate forest soils: a Ca K-edge XANES study

Calcium (Ca) plays a crucial role for plant nutrition, soil aggregation, and soil organic matter (SOM) stabilization. Turnover and ecological functions of Ca in soils depend on soil Ca speciation. For the first time, we used synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy at the Ca K-edge (4038 eV) to investigate Ca speciation in soils. We present Ca K-edge XANES spectra of standard compounds with relevance in soils (e.g. calcite, dolomite, hydroxyapatite, anorthite, clay mineral-adsorbed Ca; Ca oxalate, formate, acetate, citrate, pectate, phytate). Calcium XANES spectra with good signal-to-noise ratios were acquired in fluorescence mode for Ca concentrations between 1 and 10 mg g−1. Most standard spectra differed markedly among each other, allowing the identification of different Ca species in soils and other environmental samples as well as Ca speciation by linear combination fitting. Calcium XANES spectra obtained for samples from different horizons of twelve temperate forest soils revealed a change from dominating lithogenic Ca to clay mineral-bound and/or organically bound Ca with advancing pedogenesis. O layer Ca was almost exclusively organically bound. With increasing SOM decomposition, shares of oxalate-bound Ca decreased. Oxalate-bound Ca was absent in calcareous, but not in silicate subsoil horizons, which can be explained by microbial decomposition in the former vs. stabilization by association to pedogenic minerals in the latter soils. Synchrotron-based Ca XANES spectroscopy is a promising novel tool to investigate the fate of Ca during pedogenesis and—when performed with high spatial resolution (µ-XANES), to study aggregation and SOM stabilization mechanisms produced by Ca.

Determination of calcium and sulfate species in aerosols associated with the conversion of its species through reaction processes in the atmosphere and its influence on cloud condensation nuclei activation

Calcium (Ca) species in aerosols collected in Higashi–Hiroshima in southwestern Japan were determined by X-ray absorption near-edge structure (XANES) analysis, and their size distributions and seasonal variation were investigated. Calcium K-edge XANES results showed that gypsum (CaSO4⋅2H2O) and calcium oxalate (CaC2O4⋅2H2O) were the major Ca species that were formed by reactions of mineral particles in the atmosphere. These species exhibit similar hygroscopicity to that of calcite (CaCO3), which is an original Ca species in mineral particles. In addition, high hygroscopic Ca species, such as calcium nitrate [Ca(NO3)2⋅4H2O] and calcium chloride (CaCl2⋅2H2O) were considered minor Ca species. Moreover, the formation of CaSO4⋅2H2O by the reaction of mineral particles suppresses the formation of hygroscopic sulfate aerosol species, such as ammonium sulfate [(NH4)2SO4], reducing the number of cloud condensation nuclei (CCN). Sulfur K-edge XANES also confirmed that CaSO4⋅2H2O was a major species of sulfate in coarse particles. By contrast, high hygroscopic sulfate, such as (NH4)2SO4, was major species in fine particle. In addition, CaSO4⋅2H2O fraction to total sulfate increased during the sampling period, which was affected by a dust event. Consequently, the amount of hygroscopic sulfate in fine particles was decreased by the reaction, forming CaSO4⋅2H2O in coarse particles in the presence of mineral dust. Although these facts have been indicated mostly by modeling and laboratory studies, the present research suggested that the reaction of CaCO3 to form CaSO4⋅2H2O will reduce the amount of hygroscopic sulfate aerosols based on natural sample measurement in terms of the speciation of Ca and sulfate in aerosols using XANES spectroscopy.

Contribution of carbonates and oxalates to the calcium cycle in three beech temperate forest ecosystems with contrasting soil calcium availability

Calcium (Ca) is an essential macronutrient which plays a fundamental role in ecosystem structure and function. In temperate forest ecosystems, Ca oxalate is ubiquitous, existing in live and decomposing biomass pools. Calcium in minerals occurs principally as carbonate and silicate. Because of structural differences between its different forms, Ca speciation can influence the Ca biogeochemical cycle. In this study, we compared three beech temperate forests of contrasting soil Ca availability, i.e., dystric cambisol (DC), eutric cambisol, and rendzic leptosol (RL), but with similar climates, atmospheric depositions, species compositions and management. The pools and fluxes of total Ca (Cat) and Ca oxalate (Caox) were assessed in plant samples (aboveground and belowground tree tissues, forest floor). Pools and fluxes of Cat were also quantified in the atmospheric inputs (dry and wet deposits), in the soil (total and exchangeable fractions) and in the soil solutions at different depths. The soil solutions were measured monthly for 4 years to study the seasonal dynamics of Ca fluxes. A budget of dissolved Ca was also determined for the forest floor and soil layers. Regarding the global Ca cycle at the stand scale, our study highlighted: (i) the predominant role of biological recycling in the Ca cycle in forest ecosystems for all soil types; the turnover rates of fine roots and leaves were approximately 43 and 39% of the total Ca taken up by trees each year, revealing that only 18% accumulated in the perennial tissues; (ii) the vegetation was Ca-enriched in the high Ca soil (RL) compared to the low-Ca soil (DC); (iii) the existence of an unexpected available Ca source in DC due to upwelling by the diffusion of Ca issued from the dissolution of carbonates in-depth; (iv) a higher production of dissolved Ca (from carbonate dissolution) in RL compared to DC, resulting in an important loss of Ca by drainage (280 kg ha−1 year vs. 14 kg ha−1 year); (v) monthly measurements demonstrated that the seasonal dynamics was mainly linked to biological activity. In addition, our study showed that the distribution of Caox differs greatly between vegetation compartments, ranging from null (stem and branch wood) to up to 40% (stem and branch bark and fine roots) of the Cat in the biomass. Vegetation is a factory of Ca oxalate, and the return of this Caox enriched vegetation to the soil influences the Ca mobility in the forest floor, as demonstrated by a Caox residence time shorter than that of Cat in the DC.

Stable calcium isotope speciation and calcium oxalate production within beech tree (Fagus sylvatica L.) organs

In this study, we linked Ca speciation with isotope composition in plants. To do this, we performed leachate experiments to access the soluble Ca, structurally bound Ca and insoluble Ca (i.e., water and weak acid resistant) within beech tree organs (Fagus sylvatica L.). Ca isotopic measurements were combined with infrared spectroscopy and calcium oxalate biomineralization identification. The results from our study indicate that bark and leaves are the most enriched in monohydrated calcium oxalate crystals (whewellite), which are observable in parenchyma and sclerenchyma tissues, whereas roots and wood are enriched in structurally bound Ca. Our leaching experiments also show decreasing δ44/40Ca isotopic signatures in the order of soluble Ca > structurally bound Ca > insoluble Ca. This finding implies that because leaves degrade faster than wooden organs and because Ca linked to pectate decomposes faster than Ca linked to oxalate crystals, differential Ca isotopic signatures are expected to be observed during litter degradation.

Nanomapping and speciation of C and Ca in thermally treated lignocellulosic cell walls using scanning transmission X-ray microscopy and K-edge XANES

The carbon matrix in cell walls of lignocellulosic plants has high recalcitrance to chemical and biological decomposition. Thermal treatments, such as torrefaction and pyrolysis are therefore of interest to pre-process biomass. Reed canary grass (RCG) as biomass model was treated at 90, 300, 400 and 500°C in N2 atmosphere. The induced cell wall changes, especially for C and Ca and their speciation and nanomapping were studied using synchrotron based (1) scanning transmission X-ray microscopy (STXM) C 1s near edge X-ray absorption fine structure (NEXAFS) spectroscopy, (2) STXM Ca K-edge NEXAFS, and (3) Ca K-edge X-ray absorption of near edge structure (XANES) spectroscopy.Clusters of nano-based pixels having different spectral features were identified using principal component analysis of mosaics of C 1s NEXAFS images. At 300°C there were only minor changes and peaks related to lignin were only slight reduced. At 400°C, the chemical change was substantial and most resonances related to CO and CH functional groups declined considerably. Finally, at 500°C there were two clusters indicating that the structural integrity of the cell wall was lost and aromatic CC resonances related to quinones appeared. These results were confirmed by FT-IR. The nanomapping also indicated that Ca oxalate has a sub-micron particle size distribution, most sizes <200nm. These may act as seed particles during combustion. Ca speciation according to Ca K-edge XANES showed that oxalate (CaC2O4) was the primary phase of Ca and thermal treatment induced minor Ca phase transition into butschliite (K2Ca(CO3)2) and hydroxyapatite (Ca5(PO4)3OH).

Soil Calcium Speciation at Different Geomorphological Positions in the Yaji Karst Experimental Site in Guilin, China

Abstract: Limestone soil in karst areas is the product of weathering and carbonate rock dissolution in tropical and subtropical regions. Limestone soil in karst regions has a Ca-rich alkaline geochemical environment, but the characteristics of calcium speciation in limestone soil remain unclear. Here, to study changes in calcium speciation in soil across seasons in a karst area, different geomorphological positions and soil samples were collected from the Yaji karst experimental site. Using European Community Bureau of Reference (BCR) methods, we analyzed and discuss soil calcium speciation and seasonal migration characteristics. We found that total soil Ca content was 2.80–11.75 g kg-1, with an average of 5.25±0.68 g kg-1 (mean±SE). The order of content of each Ca speciation is extractable-Ca>reducible-Ca>residual-Ca>oxidizable-Ca. In addition to oxidizable-Ca, other Ca species had a positive correlation with total calcium. The calcium content and various forms of calcium content decreased gradually with...

Sequential extraction of calcium in lake sediments for investigating the cycle of phosphorus in water environment

The role of calcium (Ca) in the cycle of phosphorus in water environment depends on the speciation of Ca in sediments, and reliable Ca sequential extraction method is useful for obtaining information about Ca transport processes from sediment to water. Experiments have been carried out to develop a simple but robust Ca sequential extraction method for identification of Ca portioning in sediments. The sequential extraction protocol was modified for extractants, extracting time, concentration and pH of the reagents in the first three steps of Tessier sequential procedure. With optimum conditions, there were 1:25 sediment to solution ratio and 3 h of extracting time shaken with NH4AC (1 mol L−1, pH 7.0) (exchangeable Ca fraction); 1:25 sediment to solution ratio and extracted with NH4AC (1 mol L−1, pH 5.0) for 5 h (carbonate-bound Ca fraction); and 1:30 sediment to solution ratio and extracted with NH2OH·HCl (0.5 mol L−1, pH 1.5) for 6 h (Fe and Mn oxides bound Ca fraction). The recovery for Tessier procedure ranged form 35.6 to 71.1 %, while the recovery for the modified Tessier procedure ranged form 46.1 to 86.9 %. These results revealed that Tessier procedure underestimated the proportion of Ca in exchangeable fraction. In sediments, Ca was mainly exchangeable and carbonate bound, which was considered as the most liable fractions.

Determination of the calcium species in coal chars by Ca K-edge XANES analysis

Ca-based additives have been widely used as a sulfur adsorbent during coal pyrolysis and gasification. The Ca speciation and evolution during the pyrolysis of coal with Ca additives have attracted great attention. In this paper, Ca species in the coal chars prepared from the pyrolysis of Ca(OH)2 or CaCO3-added coals are studied by using Ca K-edge X-ray absorption near-edge structural spectroscopy. The results demonstrate that Ca(OH)2, CaSO4, CaS and CaO coexist in the Ca(OH)2-added chars, while Ca(OH)2 and CaSO4 are the main species in the Ca(OH)2-added chars. Besides, a carboxyl-bound Ca is also formed during both the pyrolysis for the Ca(OH)2-added and the CaCO3-added coals. A detailed discussion about the Ca speciation is given.

The effects of road salt application on the accumulation and speciation of cations and anions in an urban environment

AbstractThis study aimed to assess the quality of road runoff in heavy traffic and residential areas of Hamedan province, in the western region of Iran. To investigate the effects of salt pollution, 14 runoff samples were collected from streets during the winter and spring of 2009. Streets received different amounts of deicing salt depending on the traffic density. Results showed that deicing salt has major affects on sodium (Na+), calcium (Ca2+), magnesium (Mg2+), chloride (Cl−), bicarbonate (), sulfate () and phosphate () pollution. This is important because this type of watercourse contamination can lead to serious problems, such as eutrophication. We also found an increase in lead (Pb2+) concentration, which was not attributable to the application of deicing agents, but that was due to the combustion of leaded gasoline. Speciation of Ca, Mg, Pb and P chemical species in the samples was calculated using Visual Minteq 2.6 program. The results indicated that Ca and Mg were mainly present as free ions, Pb was mainly present as PbCO3 (aq), while the dominant P species were and .

Computer Simulation of the Interactions of Glyphosate with Metal Ions in Phloem

Essential nutrients such as trace metal ions, amino acids, and sugars are transported in the phloem from leaves to other parts of the plant. The major chelating agents in phloem include nicotianamine, histidine, cysteine, glutamic acid, and citrate. A computer model for the speciation of metal ions in phloem has been used to assess the degree to which the widely used herbicide glyphosate binds to Fe(3+), Fe(2+), Cu(2+), Zn(2+), Mn(2+), Ca(2+), and Mg(2+) in this fluid over the pH range of 8 to 6.5. The calculations show that glyphosate is largely unable to compete effectively with the biological chelating agents in phloem. At a typical phloem pH of 8, 1.5 mM glyphosate binds 8.4% of the total Fe(3+), 3.4% of the total Mn(2+), and 2.3% of the total Mg(2+) but has almost no effect on the speciation of Ca(2+), Cu(2+), Zn(2+), and Fe(2+). As the pH decreases to 6.5, there are some major shifts of the metal ions among the biological chelators, but only modest increases in glyphosate binding to 6% for Fe(2+) and 2% for Zn(2+). The calculations also indicate that over 90% of the glyphosate in phloem is not bound to any metal ion and that none of the metal-glyphosate complexes exceed their solubility limits.

Effects of oxalate and phosphate on electrokinetic removal of arsenic from mine tailings

Electrokinetic remediation process can be used to separate arsenic from mine tailings and prevent it from leaching into the environment. In this process, oxalate and phosphate solutions might serve as mobility-enhancing agents. The influence of oxalate and phosphate ions was investigated using three approaches. First, desorption tests showed that oxalate is more effective extractant than phosphate. Secondly, equilibrium modeling was used to predict the electric charge distributions of As and co-existing elements under the prevailing pH and redox gradient. Modeling results suggested that oxalate affects the fate of As indirectly by mobilizing As-bearing minerals such as Fe oxyhydroxides, while phosphate influences mainly the speciation of Ca, Cu, Mg and Mn. Thirdly, electrokinetic experiments showed that in the tailings section near the anode, As removal was 30% with oxalate and 48% with phosphate, after 20d. Due to the accumulation of As in the middle section, the respective overall removals in the entire tailings material were only 6% and 12%. The results encouraged to use equilibrium modeling for selecting enhancement agents and their concentration and for predicting accumulation of As between electrodes. However, the co-influence of all major elements and desorption/adsorption kinetics have to be taken into account.

Neutralization of Calcite in Mineral Aerosols by Acidic Sulfur Species Collected in China and Japan Studied by Ca K-edge X-ray Absorption Near-Edge Structure

Calcium species in mineral aerosols collected simultaneously in Aksu (near the Taklimakan Desert), Qingdao (eastern China), and Tsukuba (Japan) during dust and nondust periods were determined using Ca K-edge X-ray absorption near-edge structure (XANES). From the fitting of XANES spectra, it was found that (i) calcite and gypsum were the main Ca species in the aerosol samples, and (ii) the gypsum fraction versus total Ca minerals [Gyp]/[Ca2+]t increased progressively in the order Aksu < Qingdao < Tsukuba. Surface-sensitive XANES in the conversion electron yield mode (CEY) showed that the gypsum is formed selectively at the surface of mineral aerosols for all the samples except for that taken in Aksu during the dust period. The decrease of the [Gyp]/[Ca2+]t ratio with an increase in particle size showed that the neutralization effect proceeds from the particle surface. For the Aksu sample in the dust period, however, (i) the [Gyp]/[Ca2+]t ratios obtained by XANES measured in the fluorescence (FL; regarded as bulk analysis) and CEY modes were similar and (ii) size dependence was not found, showing that neutralization is not important for the sample because of the large supply of mineral aerosol with little neutralization effect in Aksu. It was also found that the pH of the aerosol and the ratio of (NH4)2SO4 to gypsum were positively and negatively correlated with the Ca (or calcite) content, respectively. The speciation of Ca by XANES revealed the neutralization processes of acidic sulfur species by calcite during the long-range transport of mineral aerosols.