Oxyanion species

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. 

Another oxyanion of interest to soil chemists is that of tungsten, which is in itself important but is also important because it forms polymers and reacts with both molybdenum and phosphorus oxyanions to form mixed polymers. Because tungsten species are toxic, its oxyanionic species, including polymers, are of interest [28],... 

A 0.5-gram mass of either the organo-treated or inorganic cation exchanged zeolite and 50 mL of 10 mM/L arsenate or chromate aqueous solutions were placed into Erlenmeyer flasks and mechanically shaken in reciprocating mode to attain equilibrium. Different equilibrium periods for individual zeolite modifications and both aqueous oxyanions species have been established. The adsorption isotherm experiments were conducted using above mass/ volume ratio of samples with an initial metal concentrations ranged from 0.5 to 100 mM/L at laboratory temperature. The... 

From the above description, it becomes apparent that with such disorder, the sorption of both heavy metal cations and oxyanionic species is favoured. Owing to the lack of knowledge of the structure of the C-S-H gel itself, however, it is unlikely that an exact binding mechanism could be discerned at present... 

Heavy metal cations precipitate readily as hydroxides or carbonates in alkaline media. Dissolved carbonate content will be limited by calcite precipitation or by conversion of hydroxyl AFm to carbonate AFm. Hydroxide ions, on the other hand, are abundant. Here only the solids that may be present under oxic conditions will be discussed. Figure 5a shows the total dissolved heavy metal cation concentrations that would prevail if hydroxide precipitation were to be the dominant solubility-controlling process. Figure 5b shows the solubility of Ca metallate species, as these are likely to act as solubility-controlling phases for oxyanionic species. 

Table 1 Polyatomic Oxyanion Species, their Shape, Point Group Symmetry and Representative Examples that are Known...

The pattern of one-coordinate oxyanions for the pyramidal X03 and X04 type oxyanion species is somewhat different. X-atom coordination only occurs significantly via the sulfur atom of the... 

Phosphorus and vanadium, which are typically present in seawater as dissolved oxyanion species, have been shown to exhibit systematic plume-particle P Fe and V Fe variations which differ from one ocean basin to another (e.g., Trefry and Metz, 1989 Feely et al., 1990). This has led to the hypothesis (Feely et al., 1998) that (i) plume P Fe and V Fe ratios may be directly linked to local deep-ocean dissolved phosphate concentrations and (ii) ridge-flank metalliferous sediments, preserved under oxic diagenesis, might faithfully record temporal variations in plume-particle P Fe... 

Once they have reached higher pH, reducing conditions of the intestinal tract (Davis et al, 1992), sulhdes should be more stable, and may actually precipitate if reduced sulfur is present. Other solids, such as hydroxides or hydroxy-sulfates of aluminum, and possibly iron, may also precipitate. The increased pH should also lead to the increased sorption onto particulates of various metals and metalloids such as lead and copper (Smith, 1999). However, in vitro tests (Ruby et al, 1993) indicate that the increased complexing with unprotonated organic acids and enzymes helps offset the pH-driven precipitation and sorption of the base metals that were dominantly chloride-complexed in the stomach fluids. Arsenic and other oxyanionic species are likely to be sorbed as the stomach acids are neutralized, but may be partially desorbed once higher pH values are reached in the intestine (Ruby et al, 1996). 

Chemical leach tests of the <50 p.m size fraction of dust samples collected around Owens Lake, using water (Reheis etal, 2001, and our unpublished data) and SLFs (our unpubhshed data), show that the dusts are sufficiently aUcahne and reactive to shift the pH of water and SLF to values near 10.5 and 9.5, respectively. Arsenic, chromium, vanadium, molybdenum, hthium, zinc, and other trace metals or metalloids are readily solubilized from the dusts. The trace metals or metalloids leached in the greatest quantities are those that form oxyanion species or abundant carbonate complexes in solution, and that are therefore mobilized most effectively under the alkaline conditions generated by the alkaline dusts. 

Chemical leach tests on the bulk settled dust samples showed that the dusts are quite chemically reactive. Leach solutions have high alkali-nities, due to the rapid partial dissolution of calcium hydroxide from concrete particles. Indoor dust samples produced higher pH levels (11.8-12.4) and alkalinities (—600 mg CaCOa) than outdoor dusts (pH 8.2-10.4 alkalinity —30mgL CaCOa), indicating that outdoor dust samples had reacted with rainfall or other water prior to collection. Thurston et al (2002) found that the leachate pH of the dusts decreased with decreasing particle size. Some metals or metalloids in the dusts (aluminum, chromium, antimony, molybdenum, barium, copper, zinc, cobalt, nickel) are readily leached by deionized water many of these form oxyanion species or carbonate complexes that are most mobile at the alkaline pH s generated by the leachates. 

As Ip increases above values as low as 8.5, and we enter the upper-left portion of Fig. 3.4, the bonding between core cation and associated oxygen or hydroxyl is even stronger and largely covalent. The result is the formation of oxyanionic species such as silicate, selenate, borate, carbonate, arsenate, and sulfate, which, because of their relatively low charge densities as oxyanions, form rather weak bonds with cations and are soluble. 

Other oxyanionic species are present in sea water, such as SO, CrO -, and, especially, H2PO4 and HPO2-. All these anions are taken up by tunicates1261. They are not assimilated, however, but are rapidly turned over. Laboratory attempts to measure vanadate uptake in the presence of phosphate generally show inhibition of uptake of the essential element. This observation can be explained by the formation of vanadate-phosphate complexes which are not bound at the uptake sites49). At the high levels of phosphate used, the equilibria would be shifted away from monomeric vanadate, whereas in sea water these moderately stable complexes would remain virtually fully dissociated. 

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