Speciation in the Atmosphere

6 CHEMICAL REACTIVITY TOWARDS DIFFERENT LIGAND CLASSES. 71 

1 Macrocycle/Chelate Effects and Enthalpic, Entropic Contributions. 76 

Abstract This chapter reports an analysis of literature dedicated to the speciation of cadmium in various environmental compartments, i.e., atmosphere, natural waters, soils and sediments. The difficulty of the cadmium speciation studies, due to the variability of composition of different natural systems and to the low cadmium concentration in the environment, is highlighted. As an alternative approach, cadmium behavior is assessed by modelling its reactivity towards the main classes of ligands usually present in natural systems. The stability of cadmium complexes with 

Department of Inorganic, Analytical and Physical Chemistry, University of Messina, Viale F. Stagno d Alcontres 31,1-98166 Messina, Italy 

Sigel (eds.). Cadmium From Toxicity to Essentiality, Metal Ions in Life Sciences 11, DOI 10.1007/978-94-007-5179-8 3, 

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Environmental Fate. Zinc partitions to the air, water, and soil (Callahan et al. 1979 Guy and Chakrabarti 1976 Houba et al. 1983 Pita and Hyne 1975). Zinc occurs in the environment mainly in the +2 oxidation state (Lindsey 1979). Adsorption is the dominant fate of zinc, resulting in enrichment of zinc in suspended and bed sediments (Callahan et al. 1979). The mobility of zinc in soil has been characterized (Base and Sharp 1983 Bergkvist et al. 1989 EPA 1980d Hermann and Neumann-Mahikau 1985 Kalbasi et al. 1978 Saeed and Fox 1977 Tyler and McBride 1982). No estimate for the atmospheric lifetime of zinc is available. Development of pertinent data on the atmospheric processes important for zinc speciation in the atmosphere would be helpful. Development of this information would permit construction of a comprehensive model for the transport and interaction of zinc not only in air but in other media as well. Transformation in air and water can occur as a result of changes in chemical speciation (Anderson et al. 1988 Callahan et al. 1979 EPA 1980d Stokinger 1981). Data that describe the transformation processes for zinc in soil or the fate of zinc in soil are needed. A model of zinc flux from all environmental compartments would be useful for providing information on the overall environmental fate of zinc. 

A little information is available on the chemical forms of radionuclides in routine emissions from nuclear facilities and on chemical speciation in the atmosphere. The chemical identity of radionuclides affecting their mobilities and biological effects in environmental media must be made clear. 

The high concentration of oxygen in the atmosphere plays a central role in the photochemistry and chemical reactivity of the atmosphere. Atmospheric oxygen also defines the oxidation reduction potential of surface waters saturated with oxygen. The presence of oxygen defines the speciation of many other aquatic species in surface waters. 

Sprovieri F, Pirrone N, Gardfeldt K, Sommar J. 2003. Atmospheric mercury speciation in the marine boundary layer along 6000 km cruise path over the Mediterranean Sea. Atmos Environ 37(S1) 63-72. 

Not only do microorganisms have a profound influence on the concentration of elements in the atmospheric, aquatic and terrestrial environments (Lovelock, 1979), but the concentration and chemical speciation of these elements predicates which organisms can compete and survive within a given environment. Therefore, the cycling and mobility of trace elements will be linked to the cycling of bulk elements and correlations to the various bulk element cycles can be postulated. 

In addition to aerosol age, phosphorus aerosol speciation is also affected by the humidity of the ambient environment (Van Voris et al. 1987). Washout and rainout processes transport both the reaction products of vapor phase phosphorus and unreacted particles of phosphorus to water and land (Berkowitz et al. 1981). Because of its lower water solubility, physical state (gas), and slower reactivity, phosphine formed during the combustion of white phosphorus or released to the atmosphere from other media persists in the atmosphere longer than other reaction products. 

The availability of iron as a nutrient for phytoplankton growth is dependent on its chemical speciation (Wells et al., 1995). Thus processes which alter iron solubility in the atmospheric aerosols have the potential to influence bioavailability... 

Harrison, R.M. (1986) Chemical speciation and reaction pathways of metals in the atmosphere. In Toxic Metals in the Atmosphere (eds Nriagu, J.O. and Davidson, C.I.). Wiley, Chichester. 

Most barium released to the environment from industrial sources is in forms that do not become widely dispersed (Ng and Patterson 1982). In the atmosphere, barium is likely to be present in particulate form (EPA 1984). Although chemical reactions may cause changes in speciation of barium in air, the main mechanisms for the removal of barium compounds from the atmosphere are likely to be wet and dry deposition (EPA 1984). 

The particular chemical form in which an element exists in water is its speciation. For example, an element can be present as a simple hydrated ion, as a molecule, as a complex with another ion or molecule, and so forth. From what was said previously, bare ions or bare polar molecules do not exist in water. At the least, they would be solvated species. Species of an element are distinguishable from one another stoichibmetrically, structurally, and energetically. In addition to aqueous species, one can distinguish elements in different phases, for example, as gaseous species, as solid phases, or in adsorbed states, and on the basis of particle sizes. In the atmosphere, for example, speciation extends over liquid, gas, and aerosol phases (Seinfeld, 1986). The notion of chemical speciation is central to equilibrium and kinetic aspects of aquatic chemistry, as will be evident throughout this book. 

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