Cadmium transport properties

Stolzberg [143] has reviewed the potential inaccuracies of anodic stripping voltammetry and differential pulse polarography in determining trace metal speciation, and thereby bio-availability and transport properties of trace metals in natural waters. In particular it is stressed that nonuniform distribution of metal-ligand species within the polarographic cell represents another limitation inherent in electrochemical measurement of speciation. Examples relate to the differential pulse polarographic behaviour of cadmium complexes of NTA and EDTA in seawater. 

In general, the cadmium halides show in their crystal structure the relation between polarizing effect and si/e of anion. The tluoride has tile smallest and least polarizable anion of Ihe lour and forms a cubic structure, while the mure polarizable heavy halides have hexagonal layer structures, increasingly covalent and al increasing distances apart in inxler down tire periodic table, in solution the halides exhibit anomalous thermal and transport properties, due primarily to the presence of complex ions, such as CDlr and CdBr r. especially in concentrated solutions or those containing excess halide ions. 

Cd " and Mn may also be substrates for ion efflux mechanisms such as the Na-Ca exchanger (Frame and Milanick 1991). It has long been appreciated that a number of metals, and in particular and Mn ", inhibit this exchanger (Philipson 1985). Recent studies by Frame and Milanick (1991) demonstrate that this inhibition is competitive, and that these metals are in fact transported as substitutes for Ca. The for Ca, Cd ", and Mn uptake by ferret red blood cells is roughly similar, —10//M, as is the (Frame and Milanick 1991), indicating that the transport properties of cadmium and manganese are similar to those of calcium in this system. 

As a rule, simulations consider emissions of heavy metals from anthropogenic and natural sources, transport in the atmosphere and deposition to the underlying surface (Figure 6). It is assumed that lead and cadmium are transported in the atmosphere only as a part of aerosol particles. Besides, chemical transformations of these metals do not change removal properties of their particles-carriers. On the contrary, mercury enters the atmosphere in different physical and chemical forms and undergoes numerous transformations during its pathway in the atmosphere (Ilyn et al., 2002 2004 Ilyin and Travnikov, 2003). 

Cadmium shares chemical properties with zinc and mercury, but in contrast to mercury, it is incapable of environmental methylation, due to the instability of the monoalkyl derivate. Similarities and differences also exist in the metabolism of Zn, Cd, and Hg. Metallothioneins and other Cd-binding proteins hold or transport Cd, Zn, and Hg within the body. Metallothioneins are metal-binding proteins of relatively low molecular mass with a high content of cysteine residues that have a particular affinity for cadmium, as well as for zinc and copper, and can affect its toxicity. 

This section will consider in greater detail specific examples of particular types of nanomaterials interacting with different media in the environment. The fate and transport of carbon-based nanomaterials, including carbon nanotubes and fuUerenes, in aqueous environments and the properties of commercial oxide nanoparticles that affect their removal in water will be discussed. Nanomaterial exposure to soils and porous media, focusing on transport and retention, as well as environmental interactions of cadmium selenide (CdSe) quantum dots with biofilms will be presented. These specific examples provide an idea of the types of environmental interactions that must be considered, and illustrate that environmental impacts of nanomaterials cannot be generalized, but rather, are dependent on properties of the material in question and the environment to which it is exposed or transported. 

Inorganic pollutants include (a) cationic heavy metals such as lead, cadmium, and nickel, (b) anionic metals and inorganics such as arsenic, chromium, selenium, nitrate and fluoride, and (c) radionuclides such as strontium and uranium. The geochemistry of these pollutants can widely vary and it depends on the specific pollutant type and soil/sediment properties. The speciation and transport of these pollutants also depend on the dynamic changes in the pH and redox potential of the soil that occurs under applied electric potential. The dominant transport process... 

Metallothioneins (MTs) are small (6000-7000 Da), intracellular, cysteine-rich ( 33% of the amino acids are cysteines) metal binding proteins that were first discovered in 1957 in equine kidney cortex. The subsequent purification of this protein identified it as the only known native cadmium-containing protein. Further studies showed this protein to bind both essential (e.g., Cu and Zn) and nonessential (e.g., Cd and Hg) metal ions and to be truly ubiquitously distributed in nature [215-218]. Additionally, MT biosynthesis is induced at the transcriptional level by a wide range of factors, which includes heavy metal ions that were subsequently found bound to the protein. All of the above factors suggest a role for this protein in heavy metal homeostasis, transport and detoxification [215-218]. Despite 55 years of extensive studies on MTs, which has included the high resolution characterization of the 3D stmcture and metal binding properties [134,136,138,151,154,218-222], the essential physiological functional role(s) of MT remains elusive. Of particular note in these studies was the determination of the NMR solution structure of a mammalian MT in a tour deforce effort by the Wiithrich lab which resulted in the reinterpretation and correction of the mily mammalian MT crystal structure currently available [223,224]. 

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