Soluble phosphate species

Phosphate reacts and forms insoluble compounds with many metals, particularly iron, aluminum, and calcium. Under acid soil conditions, both iron and aluminum become more soluble, and thus as soil pH decreases, its phosphate fixing power increases. This means that iron and aluminum react with phosphate to form insoluble species that are not available to plants. Under basic conditions, high concentrations of calcium exist and insoluble calcium phosphates form. Insoluble phosphate species are also formed with other metals that happen to be present however, the three mentioned are generally present in the highest concentration, and so they represent the major reactants with phosphate. Iron, aluminum, and calcium phosphates can also occur as coatings on soil particles. 

The solubility of metal-phosphates in soils is highly pH dependent because of the protonation potential of the phosphate species, and the various Kip values of a number... 

Fig. 10.8. Simple biogeochemical model for metal mineral transformations in the mycorhizosphere (the roles of the plant and other microorganisms contributing to the overall process are not shown). (1) Proton-promoted (proton pump, cation-anion antiport, organic anion efflux, dissociation of organic acids) and ligand-promoted (e.g. organic adds) dissolution of metal minerals. (2) Release of anionic (e.g. phosphate) nutrients and metal cations. (3) Nutrient uptake. (4) Intra- and extracellular sequestration of toxic metals biosorption, transport, compartmentation, predpitation etc. (5) Immobilization of metals as oxalates. (6) Binding of soluble metal species to soil constituents, e.g. clay minerals, metal oxides, humic substances.

Arsenic uptake from soil (i.e., arsenic bioavailability) depends on soluble arsenic species present in soil, on soil properties (i.e., the type and amount of the sorbent components of the soil), redox (Ej,) and pH conditions and microbiological activity (see Section 6.4.2.1). Uptake into plants further depends on phosphate (often originating from fertilizers) and vanadate levels, as the behavior of arsenate is similar to that of phosphates and vanadates (see also Section 6.6.1). 

PHREEQC2 is distributed with different databases. The database Minteq.dat was chosen for the modelling of the column experiments. Constants of the protonation reactions for all oxoanions are included in this database. It also contains constants of various soluble phosphate and chromate species (complexes with all major cations and anions). The formation of soluble arsenate complexes is not considered because this seems not to be necessary (compare Cullen and Reimer, 1989). 

The secretions of the seminal vesicles vary greatly between species in their normal volumes and chemical compositions, and in the extent to which they contribute to the total seminal plasma. In comparison with prostatic secretions, vesicular fluids tend to be more alkaline, higher in dry weight, and richer in bicarbonate, potassium, acid-soluble phosphate compounds, and especially proteins and polypeptides. In several species seminal vesicle secretions contain well over 200 mg/ml of total protein which in good measure accounts for the high viscosity and stickiness of these fluids. 

To summarize uranium complexing in hydrothermal solutions, the predominant species will depend on the concentration of complexing anions, which is, in turn, dependent on temperature and pH. The activity of fluoride in many uranium mineralizing systems appears to be significant, as is indicated by the abundance of fluorite and other fluoride-containing gangue minerals. In these systems uranyl fluoride complexes would predominate in acid to neutral solutions. At low temperatures carbonate complexes predominate in alkaline solutions, but, as temperature increases, carbonate complexes become less important. Phosphate complexes may be important in nearneutral solutions in which as little as 0.1 ppm phosphate is present. As temperature increases, hydroxide complexes become more important. At temperatures of 300°C and above hydroxide complexes may be the only soluble uranium species. 

Phosphorus species. Soluble phosphates precipitate lithium phosphate, more soluble in NH4CI than in H2O alone (distinction from Mg ). In dilute solutions the phosphate is not precipitated until the solution is boiled. The sensitivity of the test is increased by adding NaOH, forming a double phosphate of Na and Li. The phosphate dissolved in HCl is not at once reprecipitated on neutrahzation with NH3 (distinction from at least Ca " through Ra ty. Ethanol promotes precipitation. 

The electroreduction of phosphates in this melt by voltammetry and chronopotentiometry was studied by Franks and Inman.They reported that the reduction of POt" is a reversible two-electron process. The final product of the reaction is phosphorus that is produced through subsequent chemical reactions involving soluble reduced species (POa"). Two steps were observed in the reduction of P207. The results indicated that both P04 and P20 are present in solution. PO " and P20 were also present in the melt upon addition of PaO " or PaOfo ... 

Cosolvents ana Surfactants Many nonvolatile polar substances cannot be dissolved at moderate temperatures in nonpolar fluids such as CO9. Cosolvents (also called entrainers, modifiers, moderators) such as alcohols and acetone have been added to fluids to raise the solvent strength. The addition of only 2 mol % of the complexing agent tri-/i-butyl phosphate (TBP) to CO9 increases the solubility ofnydro-quinone by a factor of 250 due to Lewis acid-base interactions. Veiy recently, surfac tants have been used to form reverse micelles, microemulsions, and polymeric latexes in SCFs including CO9. These organized molecular assemblies can dissolve hydrophilic solutes and ionic species such as amino acids and even proteins. Examples of surfactant tails which interact favorably with CO9 include fluoroethers, fluoroacrylates, fluoroalkanes, propylene oxides, and siloxanes. 

Spectrophotometric methods may often be applied directly to the solvent extract utilising the absorption of the extracted species in the ultraviolet or visible region. A typical example is the extraction and determination of nickel as dimethylglyoximate in chloroform by measuring the absorption of the complex at 366 nm. Direct measurement of absorbance may also be made with appropriate ion association complexes, e.g. the ferroin anionic detergent system, but improved results can sometimes be obtained by developing a chelate complex after extraction. An example is the extraction of uranyl nitrate from nitric acid into tributyl phosphate and the subsequent addition of dibenzoylmethane to the solvent to form a soluble coloured chelate. 

This is illustrated in Scheme VI. The protected glyceryl derivatives are insoluble in aqueous media and appear to be hydrolytically stable. The deprotected species (structure 27) is water-soluble and hydrolyzes in aqueous media at neutral pH at 37°C to give glycerol, phosphate, and ammonia. The free hydroxyl units of the deprotected polymer provide sites for the covalent attachment of drug molecules. Water insolubility can be imparted by the use of appropriate hydro-phobic cosubstituent groups to generate solid, erodible materials. 

The progress of precipitation is revealed by the concentration/time curves for zinc and phosphate, since both these species are present initially in solution. There should be maxima for the soluble aluminium, calcium and fluoride which are extracted from the glass, but because of the early onset of precipitation these are not observed. Precipitation is accompanied by an increase in pH when it reaches 1-8, at which juncture 50% of both zinc and phosphate have been precipitated, the cement paste gels (5 minutes after preparation). 

The composition of the leachates does not correspond to the composition of the cement at all (Wilson Batchelor, 1967a,b). The predominant species eluted are the soluble sodium salts of phosphate and fluorides, although sodium is only a minor constituent of the cement. For one example of cement examined, the leachate contained 0-28 % sodium and 0-20% phosphate (expressed as a percentage of the amount of the species contained in the cement). For the major constituents of the glass the figures were 0 07% fluoride, 0 02% Al Oj, 0 01 % SiOj and 0 003% CaO. 

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