Ions, Complexes, and Chemical Speciation

Essentially, this Table is based upon the distribution of electrons amongst four sets of orbitals labelled s, p, d, and f and is comprised of the main group elements, with the completion of s and p orbitals, the transition elements, with the completion of electron shells for the d orbitals, and then the inner-transition elements, known as the lantha-nons and actinons, with the completion of f orbitals. All of these transition and inner-transition elements are metals in their native state, whereas the elements to the top right-hand side of the main group of the Periodic Table tend to be non-metals. 

There are many intricate details of the bonds formed between combinations of elements of the Periodic Table in order to make molecules but, at its simplest level, there are three types of bonding  

Ionic bonding where an element which has lost electrons to form positively charged species is ionically bonded to another element, which has gained electrons to form a negatively charged species. This is classical electrostatic attraction, 

Covalent bonding wherein two atoms of the same, or of differing elements, share one or more electron(s) from each atom in order to unite to form an electron shared bond between the two atoms. 

Dative covalent bonding wherein a similar electron shared bond occurs but all of the electrons are provided by the atom at one end of the bond. 

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Prediction of the chemistry of plutonium in near-neutral aqueous media is highly dependent on understanding reactions that may be occurring in such media. One of the most important parameters is the stability and nature of complexes formed by plutonium in its four common oxidation states. Because Pu(III), Pu(IV), and Pu(VI) are readily hydrolysed, complexation reactions generally are studied in mildly to strongly acidic media. Data determined in acid media (and frequently at high concentrations of plutonium) then are used to predict the chemical speciation of plutonium at near-neutral pH and low concentrations of the metal ion. 

The chemical speciation of cyanides varies according to their source. Specific terms used to describe cyanide include free cyanide, cyanide ion, simple cyanides, complex cyanides, nitriles, cyanogens, and total cyanide. The most common forms of cyanide in the environment are free cyanide, metallocyanide complexes, and synthetic nitriles. A brief description of each cyanide species follows (Smith et al. 1978, 1979 Towill et al. 1978 Egekeze and Oehme 1980 USEPA 1980, 1989 Davis 1981 Leduc 1981, 1984 Leduc etal. 1982 Simovic and Snodgrass 1985 Ballantyne 1987a Homan 1987 Marrs and Ballantyne 1987). 

As noted above, biouptake involves a series of elementary processes that take place in the external medium, in the interphasial region, and within the cell itself. One of the most important characteristics of the medium is the chemical speciation of the bioactive element or compound under consideration. Speci-ation not only includes complexation of metal ions by various types of ligands, but also the distribution over different oxidation states, e.g. Fe(II) and Fe(III), and protonation/deprotonation of organic and inorganic acids of intermediate strength. The relationship between speciation and the direct or indirect bioavailability1 of certain species has received a lot of recent attention. 

Rates of reductive dissolution of transition metal oxide/hydroxide minerals are controlled by rates of surface chemical reactions under most conditions of environmental and geochemical interest. This paper examines the mechanisms of reductive dissolution through a discussion of relevant elementary reaction processes. Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is an important determinant of rates of individual surface chemical reactions and overall rates of reductive dissolution. 

The fete of radionuclides in the marine environment is similar to that of the stable isotopes, being dependent on chemical speciation, including redox state, solubility, and tendency to form complex ions. For example, Pu and Am are particle reac-... 

The actual form in which a contaminant molecule or ion is present in natural water, as result of a change in the coordinative relationship, emphasizes a specific chemical speciation. A chemical species is defined by lUPAC as the isotopic composition, electronic or oxidation state, and/or complex or molecular stracture, and the speciation of an element as the distribution of an element amongst defined chemical species in a system (Templeton et al. 2000). 

These three types of surface species—inner-sphere complex, outer-sphere complex, and diffuse-layer—represent three modes of adsorption of small aqueous ions that contribute to the formation of the electrochemical double layer on clay mineral surfaces. No inference of special planes containing adsorbed ions is required by these surface chemical speciation concepts, nor is detailed molecular structure implied, other than the general notions of surface complexes and vicinal dissociated ions. It is sometimes convenient, although not necessary, to group the two types of surface complex into a Stern layer to distinguish them from diffuse-layer ions [18]. This geometric partitioning of surface species, however, should not be taken to mean that diffuse-layer ions necessarily approach a particle surface less closely than do Stern-layer ions. 

Physico-chemical speciation refers to the various physical and chemical forms in which an element may exist in the system. In oceanic waters, it is difficult to determine chemical species directly. Whereas some individual species can be analysed, others can only be inferred from thermodynamic equilibrium models as exemplified by the speciation of carbonic acid in Figure 9. Often an element is fractionated into various forms that behave similarly under a given physical (e.g., filtration) or chemical (e.g., ion exchange) operation. The resulting partition of the element is highly dependent upon the procedure utilised, and so known as operationally defined. In the following discussion, speciation will be exemplified with respect to size distribution, complexation characteristics, redox behaviour and methylation reactions. 

Another approach to the assessment of ion speciation in the resin and solution phases with equal ility for the determination of formation constants of complexes in these two environments is provided by NMR spectroscopy. The method can provide sharp, well-separated signals for each successive complex species formed, the peak area being proportional to the atomic concentration of the element being measured, irrespective of its complex form. For a completely labile system, a well-defined chemical shift change is often observed with successive complex formation. These features make this analytical procedure well suited for study of metal ion complexation by the ion-exchanger phase. 

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