Phosphate oxyanion

Phosphorus oxyanions are entirely different from nitrogen oxyanions. First, the oxyanion species present is controlled by the pH also, phosphate oxyanions are generally not mobile in soil. However, sandy soils and soils high in phosphorus are exceptions to this rule. Any soil, though, can lose phosphate by erosion and this phosphate can cause environmental problems. Because of its unique chemistry, phosphorus will be discussed separately later. 

Compare the reactivity and movement of nitrogen and phosphate oxyanions in soil. 

Hanshaw RG, Hilkert SM, Hua J et al (2004) An indicator displacement system for fluorescent detection of phosphate oxyanions under physiological conditions. Tetrahedron Lett 45 8721-8724... 

By proton inventory, a technique that determines whether acid and base groups act simultaneously, we found that hydrolysis of 36 by artificial enzyme 44 involves two protons moving in the transition state [130]. Thus, ImH+ of 46 is hydrogen bonded to a phosphate oxyanion of bound substrate 36 water hydrogen bonded to the Im then attacks the phosphorus, and as the O-P bond forms the ImH+ proton transfers (along with the water proton) to produce the phosphorane monoanion 47. This then goes on to the cleaved product in later catalyzed steps before there is time for pseudo-rotation. These general conclusions have been described and summarized in several publications [131-137]. 

In both cases, the phosphate oxyanion is clearly chelated to the diprotonated sapphyrin core by anisotropic hydrogen-bonding interactions as well as by, presumably, charge effects. Taken together, these effects appear to result in strong binding, at least as inferred from bond distances the phosphate oxyanion is found 1.22 A and 1.38 A above the mean N5 macrocyclic plane in the case of these two complexes, respectively. ... 

J Structural Studies. As was true in the case of the halide anion recognition, important support for the proposed phosphate anion binding interaction has come from single crystal X-ray diffraction analyses. As shown in Figures 6 and 7, respectively, the diprotonated form of sapphyrin binds the monobasic forms of both phosphoric and phenylphosphoric acid. While, at least from the perspective of the coordinated atoms, these two structures bear a striking resemblance to those of the mono- and dihydrochloride salts alluded to above, in both cases it is of interest to note that in each relevant comparison pair, the phosphate oxyanion is far closer to sapphyrin plane. Specifically, in the case of the H2P04 structure, this chelated atom is found to reside 0.83 A (only ) above the macrocylic plane. Similarly, in the case of the 1 2 complex formed between diprotonated sapphyrin and... 

A. Abouimrane, M. Armandand and N. Ravet, Carbon nano-painting Application to non-phosphate oxyanions, eg. borates, in K. Zaghib, C.M. Julien and J. Prakash, (eds.). New Trends in Intercalation Compounds for Energy Storage Conversion, Vol. PV 2003-20, The Electrochemical Society, Pennington, NJ, 2003, p. 15. 

The most general method for the simultaneous analysis of oxyanions by gas chromatography is the formation of trimethylsilyl derivatives. Trimethylsilyl derivatives of silicate, carbonate, oxalate, borate, phosphite, phosphate, orthophosphate, arsenite, arsenate, sulfate and vanadate, usually as their ammonium salts, are readily prepared by reaction with BSTFA-TMCS (99 1). Fluoride can be derivatized in aqueous solution with triethylchlorosilane and the triethylfluorosilane formed extracted into an immiscible organic solvent for analysis by gas chromatography [685). 

Phosphate and other Oxyanion Ligands 6.8.6.5.1 Phosphates and phosphonate ligands... 

Chemical effects include stable compound formation and ionization, both of which decrease the population of free atoms in the sample vapour and thereby lower the measured absorbance. Examples of compound formation include reactions between alkaline earth metals and oxyanions such as aluminates, silicates and phosphates, as well as the formation of stable oxides of aluminium, vanadium, boron etc. 

Prehydrolysis method. Investigation of influence of added oxyanions such as phosphate, perchlorate, arsenate, chlorate, bromate, etc. on rate of crystallization. The overall crystallization time in the presence of additives reduced by about five times compared to the conventional prehydrolysis method (7,8)... 

Soils with AEC can be expected to exchange anions in the same way. However, in many soils, anions are present as oxyanions, which often react with soil components to form permanent covalent bonds and thus do not act as exchangeable anions. Phosphate anions are excellent examples of this type of interaction [11],... 

The two important oxyanions in soil are nitrate and phosphate. Nitrate (N03 ) is the predominant oxyanion of nitrogen however, nitrite (N02 ) can also occur in the soil solution. Phosphate can exist as one of three species,... 

It is reasonable to expect that because anions and most colloidal particles in temperate region soils have a negative charge, they will repel each other. The consequence is that anions will pass through soil and will not be adsorbed or even retarded. For the simple anions and some of the oxyanions, this is exactly what happens. All the halides, nitrite, nitrate, bicarbonate, and carbonate act in this fashion. However, there are some oxyanions that do not act as expected, and chief among them is phosphate. 

Many important soil components are not present as simple cations or anions but as oxyanions that include both important metals and nonmetals. The most common and important metal oxyanion is molybdate (Mo042 ). The most common and important nonmetal oxyanions are those of carbon (e.g., bicarbonate [HC03 ] and carbonate [C032-]), nitrogen (e.g., nitrate [N03 ] and nitrite [NQ2 ]), and phosphorus (e.g., monobasic phosphate [H2P04 ], dibasic... 

Boron and arsenic are natural components of soil and are both present as oxyanions. Boron is present as boric acid or borate polymers, and arsenic is present as arsenate. While boron is weakly held by soil, arsenic is similar to phosphate in its interactions with soil constituents. Boron is an essential nutrient for plants however, it is also toxic to plants at relatively low levels. Arsenic is toxic. The laboratory chemistry of both of these elements is well understood, but their environmental chemistry, speciation and movement, is less well understood [23-27],... 

Inorganic components in soil are extracted with water, acidic solutions containing highly soluble ligands and chelates, and basic solutions. Acidic solutions are typically used for extraction of metals and metal ions in both exchangeable and nonexchangeable forms. Basic solutions are used much less commonly, although they are important for oxyanions, particularly phosphate. 

The large specific surface areas of the Fe solid phases (Fe(II,III)(hydr)oxides, FeS2, FeS, Fe-silicates) and their surface chemical reactivities facilitate specific adsorption of various solutes. This is one of the causes for the interdependence of the iron cycle with that of many other elements, above all with heavy metals, some metalloids, and oxyanions such as phosphate. 

Fe(III)(hydr)oxides introduced into the lake and formed within the lake - Strong affinity (surface complex formation) for heavy metals, phosphates, silicates and oxyanions of As, Se Fe(III) oxides even if present in small proportions can exert significant removal of trace elements. - At the oxic-anoxic boundary of a lake (see Chapter 9.6) Fe(III) oxides may represent a large part of settling particles. Internal cycling of Fe by reductive dissolution and by oxidation-precipitation is coupled to the cycling of metal ions as discussed in Chapter 9. 

Phosphate must be applied as fertilizer to the soil. Ideally it is added in quantities sufficient to guarantee optimal yields, but not in excess in order to avoid P transportation into other compartments of the ecosystem. The amount added should be based on an accurate estimation of the plant-available fraction of P already present in a soil.This is an old and difficult task and a large number of extraction methods have been used since intensive land use was practised. Recently methods have been worked out in which a strip of filter paper impregnated with an Fe oxide (2-line ferri-hydrite) is dipped into a soil suspension and the amount of P adsorbed by the paper is taken as being plant-available (Sissingh,1988 Van der Zee et ah, 1987 Sharpley, 1993 Sharpley et ah,1994 Kuo and Jellum, 1994 Myers et ah 1997). Anion and cation resins extracted more P from four heavily fertilized soils than from goethite (Delgado Torrent, 2000). Other oxyanions adsorbed by soil Fe oxides are silicate, arsenate, chromate, selenite ( ) and sulphate. Adsorption of sulphate led to a release of OH ions and was substantially lowered once the Fe oxides were selectively removed (Fig.16.17). 

The oxyanion hole geometry of three complexes is visuahzed in Figures 4.2. 4. Figure 4.2 displays the active site of trypsin complexed with a peptide inhibitor [41]. In Figure 4.3, the active site of chymotrypsin complexed with a neutral aldehyde adduct is displayed [43], and in Figure 4.4, cutinase (a lipase) with a covalently bound phosphate, a transition state analog is depicted [63]. 

Figure 4.4 Structure of the oxyanion hole of cutinase, with a covalently bound transition state analog, diethyl phosphate (PDB 2CUT).

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