Calcium speciation

In the last section, a rough estimate for the solution speciation in the presence of a dissolved organic matter (DOM) was attempted. DOM is the MinteqAl equivalent of DOC. This involved the speciation of DOM as a function ofpH for two calcium concentrations. Subsequently the impact of DOM on calcium solubility and calcite precipitation was examined. Again, the calcium speciation was determined as a function of pH. 

Calcium consists of six stable isotopes, " Ca, Ca, Ca, Ca, Ca, and Ca, with average natural abundances of96.941,0.647,0.135, 2.086,0.004, and 0.187%, respectively. All stable calcium isotopes can be used as tracers apart from Ca with its high natural abundance. The least abundant isotopes Ca, Ca, and Ca are most suitable, in particular for intravenous administration. The lower the natural abundance, the smaller is the required dose and, thus, its effect on calcium homeostasis in plasma. For isotopic labeling, differences in calcium speciation between the isotopic label and calcium in the meal or compound to be tested must be taken into account. Calcium absorption from intrinsically and extrinsically labeled meals was found to be equivalent for wheat [75] and dairy products [76], but not for green leafy vegetables [77, 78]. Heaney et al. [79] found a difference in calcium absorption from labeled calcium chloride and tricaldum phosphate when added to soy milk, but no difference for calcium carbonate and calcium citrate [80]. In the latter studies, the calcium salts were directly synthesized from the label as the standard procedure for intrinsic labeling of chemical compounds. 

Baumgartner, L. P. (1991). Reinterpretation of Calcium Speciation in Supercritical CaCl2 Solutions. Trans., Am. Geophys. Union, Abs. 72(17), 308-309. 

Roselle, G. T. (1993). Anorthite Solubility and Calcium Speciation in Supercritical Calcium Chloride Solutions. Masters Degree, Univ. Wiscon.-Madison, Madison, Wl, 79 pp. 

Roselle, G.T. and Baumgartner, L.P (1995) Experimental determination of anorthite solubility and calcium speciation in supercritical chloride solutions at 2 kb from 400 to 600 °C. Geochim. Cosmochim. Acta, 59, 1539-1549. 

In arid and semi-arid soils with a pH range of 6-9, free calcium is the major Ca speciation form in soil solution. When pH > 9.2-9.5, CaP04 becomes a major calcium species in soil solution of neutral and calcareous soils, especially when the activity of H2P04 is > 10 5 M (Lindsay, 1979), such as after phosphate fertilizers are used. Lindsay (1979) further pointed out that CaS04° contributes significantly to the total calcium in solution when S042- is > 10 M. 

As a final example, we consider a fluid of known fluoride concentration whose calcium content is set by equilibrium with fluorite (CaF2). The speciation of fluorine provides for two solutions to this problem. In dilute solutions, in which the free ion I dominates, the reaction,... 

Geochemical speciation modelling indicated saturation with respect to gypsum and several carbonates, slight under-saturation with respect to calcium arsenate (Ca3[As04]2) and ferrihydrite. 

Equilibrium with precipitation. The previous example calculated carbonate speciation admitting unrestricted solubility of all species. Actually, it is easily verified that the calculated calcium and carbonate concentrations exceed calcium carbonate solubility as measured by the solubility product... 

Sterling, R.O. and Helble, J J. (2003) Reaction of arsenic vapor species with fly ash compounds kinetics and speciation of the reaction with calcium silicates. Chemosphere, 51(10), 1111-19. 

Morse J.W. and Berner R.A. (1979) The chemistry of calcium carbonate in the deep oceans. In Chemical Modelling—Speciation, Sorption, Solubility, and Kinetics in Aqueous Systems (ed. E. Jenne), pp. 499-535. Amer. Chem. Soc., Washington, D.C. 

In soil research, the term speciation is often applied to operationally defined fractionation of heavy metals into five or more components.25 Typically, water soluble, exchangeable, organically bound (which includes what is in biomass), amorphous oxide bound, crystalline oxide bound, and residual fractions are measured.26 Sometimes residual fractions are further subdivided according to particle size distributions to give amounts in sand, silt, and clay fractions. Similar fractionation procedures are often applied to aquatic sediments.27 In arid regions, often the calcium carbonate bound fractions of heavy metals are also measured.28 Because of the constraints of detection limits, generally only cadmium, copper, iron, manganese, and zinc are usually monitored by flame spectrometry in such heavy metal speciation studies.28... 

It has been shown that the major part of Ca and Mg are found in the soluble fraction (whey) of human milk, whereas only small amounts are present in insoluble caseins or in fat [11-13]. Calcium and Mg speciation studies in milk whey using size exclusion chromatography (SEC) combined with inductively coupled plasma atomic emission spectrometry (ICP-AES) [14] indicated that they were preferably associated with the milk nonprotein fraction. A further work [15] showed similar chromatographic profiles by SEC hyphenated with inductively coupled plasma mass spectrometry (ICP-MS), with Ca and Mg eluting in the lowmolecular-weight region (LMW) (<1.4 kDa). Thus, there seem to be weak associations, if any, of Ca2+ or Mg2+ with higher molecular mass biocompounds of milk. 

The effect of pH is illustrated by the example of the chemical speciation of hydrocarbonate, which is abundant and usually the most important anion of groundwaters. The hydrocarbonate content of groundwaters is usually high. Hydrocarbonate can be present as different species, depending on pH. It is shown in Figure 1.2, where the speciation of a typical Hungarian groundwater is shown at different pH values (467 mg/dm3 = 7.5 mmol/dm3 hydrocarbonate, 48 mg/dm3 = 1.2 mmol/dm3 calcium content). The chemical equilibria in a closed system are as follows. 

In soils, F can be found in four major fractions (1) dissolved in soil solution (2) sorbed to Al, Fe, and Mn oxides and hydroxides and carbonates (3) solid phases, such as fluorite and fluorophlogopite and (4) associated with organic compounds. The solubility of F in soil solution is variable and is affected by pH, speciation, adsorption and desorption reactions, and dissolution and precipitation reactions (Luther et al., 1996). Acidic conditions and low calcium carbonate content are favorable to F solubility and can therefore enhance both root uptake (Weinstein and Alscher-Herman, 1982) and migration to surface and ground water (Smith, 1983). These conditions can lead to human, plant, and animal health issues. Soils that do contain appreciable amounts of calcium carbonate and are neutral to slightly alkaline conditions can fix F as insoluble calcium fluoride (CaF2), and reduce its bioavailability and mobility (Kubota et al., 1982 Tracy et al., 1984 Reddy et al., 1993 Poulsen and Dudas, 1998). 

An important problem in the speciation analysis of arsenic is the above-mentioned co-precipitation of oxidized forms of arsenic with sparingly soluble compounds of iron or manganese, or the formation of iron and calcium arsenates... 

Calcium Phosphates—Speciation, Solubility, and Kinetic Considerations... 

Metals Lead, calcium, arsenic, methyl mercury, organotins (TBTO, i.e., bis(tris- -butyltin)oxide), nickel Appears to depend of speciation of metal [35]... 

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