Oxyanions common

The ionic potentials of the nonmetal elements in Groups IIIA to VIA exceed 100 nm for their common oxidation states, and therefore these elements form oxyanions instead of hydrolytic species in soil solutions. The same tendency is observed for the Group VIB metals chromium and molybdenum. Examples of inorganic oxyanions commonly found in the aqueous phases of soil are B(OH)J, CO, NOJ, H3Si04, POj", SOj", AsOl", SeOf", MoOl , and, when the oxyanion is multivalent, some of the protonated forms. The qualitative and mechanistic features of the adsorption of these oxyanions—a topic on which there is an abundant literature —are the principal concerns of the present section. Quantitative models of inorganic oxyanion adsorption are described in Chap. 5. 

Common anions may be grouped as follows monatomic anions, oxyanions, and special anions. There are special endings for the first two groups the third group is small enough to be memorized. 

Other elements have similar sets of oxyanions, but not all have four different oxyanions. You should learn the names of the seven ions ending in -ate for the most common elements. These are the most important oxyanions. (Use the rules given above for remembering the others.) These ions are presented in Table 6-5. Note that the ones with central atoms in odd periodic groups have odd charges and those in even periodic groups have even charges. 

It is possible for an element and a compound of that element or for two compounds containing a common element to react by combination. The most common type in general chemistry is the reaction of a metal oxide with a nonmetal oxide to produce a salt with an oxyanion. For example,... 

Fig. 1.7. Ionic radius r, and charge z, of common forms of elements in water. The solid lines divide (a) elements with Z/r <0.03 pm-1, which form soluble hydrated cations such as Ca2+ (b) ones with 7Jr >0.12 pm-1, soluble as oxyanions such as S042 and (c) those of intermediate Z/r, which form oxides or hydroxides insoluble around neutral pH. (Reproduced with permission from P.A. Cox (1989), see Further Reading.)...

Another type of hypervalency is encountered in textbook descriptions of the oxyanions of common laboratory acids. Generations of chemistry students have been taught that the correct representations of these species are in terms of resonance-delocalized hypervalent Lewis-structure diagrams, such as sulfate (S042-),... 

Structure (3.226c), for example, depicts a central heptavalent Cl atom (Fa = 7), exceeding the normal valence octet by six electrons (These excess electrons are assumed to be accommodated in chlorine 3d orbitals, whereas d-orbital participation is prevented in first-row compounds.) Hypervalent structures such as (3.226a)-(3.226c) are claimed to be justified by the electroneutrality principle, which stipulates that second-row central atoms have zero formal charge (whereas first-row oxyanion Lewis structures commonly violate this principle).148... 

Table 3.36. Geometries and NBO/NRT descriptors of common oxyanions XOmn (see Fig. 3.91), showing symmetry, bond length Rxo, NRT bond order bxo and central-atom valency Vx, atomic charges Qx and Qo, and d-orbital occupancy dx for representative first- and second-row species...

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... 

TABLE 6.2. Common Oxyanions in Soil, Their Chemical Characteristics... 

Give some examples of anions including oxyanions that are common in soil. 

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. 

As for all elements, the distribution of Mo in the environment depends critically on chemical speciation, including oxidation state (Bertine and Turekian 1973 Morford and Emerson 1999). However, Mo is somewhat unusual in both respects. In terms of ligand coordination. Mo is one of a small number of transition metals that commonly form oxy anions and coordinate only weakly with other environmentally common ligands such as Cl" or OH". Other such metals include Cr and W, which sit above and below Mo, respectively, in Group VI of the Periodic Table, as well as Tc, Re, Os and U. Hence, Mo chemistry has some analogies with these metals, as well as with nonmetals such as S, Se, P and As which also form oxyanions. 

Lipases belong to the subclass of serine hydrolases, and their structure and reaction mechanism are well understood. Their common a/p-hydrolase enzyme fold is characterized by an a-helix that is connected with a sharp turn, referred to as the nucleophilic elbow, to the middle of a P-sheet array. All lipases possess an identical catalytic triad consisting of an Asp or Gin residue, a His and a nucleophilic Ser [14]. The latter residue is located at the nucleophilic elbow and is found in the middle of the highly conserved Gly—AAl—Ser—AA2—Gly sequence in which amino acids AAl and AA2 can vary. The His residue is spatially located at one side of the Ser residue, whereas at the opposite side of the Ser a negative charge can be stabilized in the so-called oxyanion hole by a series of hydrogen bond interactions. The catalytic mechanism of the class of a/P-hydrolases is briefly discussed below using CALB as a typical example, since this is the most commonly applied lipase in polymerization reactions [15]. 

Oxyanions of sufficient basicity will catalyze the hydrolysis of all but the least reactive esters but since the latter include the esters of the common aliphatic alcohols, early attempts to detect the reaction were negative or inconclusive. Dawson and Lowson274, claimed to have detected catalysis by acetate ion of the hydrolysis of ethyl acetate as early as 1927, but the extent of catalysis observed was too small to rule out the possibility that salt or solvent effects were, in fact, responsible. 

The reaction of diketene with some 4-substituted 3-oxobutanoate esters also provides a route to pyran-4-ones, though some attention to the reaction conditions is necessary to avoid the competitive formation of ethyl 2,4-dihydroxybenzoates (79JCS(Pi)529). The two products are considered to arise from a common intermediate (419), cyclization of which can be envisaged through nucleophilic attack by an oxyanion or a carbanion (Scheme 139). 

This coordination number (Figure 4) is an order of magnitude less common than for one- and two-coordinate oxyanions (Figures 2 and 3). The coordination number does not occur for X02-type... 

Precipitation refers to the process of adding one or more chemical reagents to water so that dissolved contaminants are transformed (precipitated) into insoluble solids (precipitates) (US EPA, 2002b, 17). The precipitates can then be collected and removed from the water by filtration, flotation, centrifugation, or other methods. For arsenic in water, precipitation typically involves reactions between arsenic oxyanions and dissolved cations. A common example is the precipitation of calcium arsenates from the addition of lime to a wastewater containing dissolved As(V). 

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