Divalent cadmium

The pH of a medium also impacts the formation of metal-phosphate precipitates. For example, divalent ionic cadmium (Cd2+) concentrations rapidly decline as both phosphate concentration and pH increase. Sandrin and Hoffman121 determined that when no phosphate is present in a commonly used mineral salts medium, the concentration of divalent ionic cadmium remains relatively constant until an abrupt decline above pH 8. When 15 mM inorganic phosphate is added to the medium, divalent cadmium ion concentrations rapidly decline at pH values above only 6. 

FIGURE 20.5 Distribution of molecular and ionic species of divalent cadmium at different pH values. (From U.S. EPA, Assessing the Geochemical Fate of Deep-Well-Injected Hazardous Waste A Reference Guide, EPA/625/6-89/025a, U.S. EPA, Cincinnati, OH, June 1990.)... 

The presence of free electrons means that metal-rich CdO should be an ra-typc semiconductor. It is possible to imagine that the divalent cadmium interstitials can take up an electron to form monovalent interstitials ... 

Figure 9—4 shows the polarization curves observed for the transfer reaction of cadmium ions (Cd Cd ) at a metallic cadmium electrode in a sulfuric acid solution. It has been proposed in the literature that the transfer of cadmium ions is a single elemental step involving divalent cadmium ions [Conway-Bockris, 1968]. The Tafel constant, a, obtained from the observed polarization curves in Fig. 9-4 agrees well with that derived for a single transfer step of divalent ions the Tafel constant is = (1- P) 1 in the anodic transfer and is a = z p = 1 in the cathodic transfer. 

Fig. 9-4. Anodic and cathodic polarization curves measured for transfer of divalent cadmium ions (dissolution-deposition) at a metallic cadmium electrode in a sulfate solution (0.005MCd + 0.4MS04 ) i (i )= anodic (cathodic) reaction current a = Tafel constant (transfer coefficient). [From Lorenz, 1954.]...

Christensen and Lun [57] developed a speciation procedure using a cation-exchange resin (Chelex 100) in a sequential batch/column/batch system for determining free divalent cadmium and cadmium complexes of various stabilities at the cadmium concentrations typically found in landfill leachates... 

The electrodeposition of cadmium, Cd, is possible in the basic and neutral EMICI-AICI3 ionic liquids [56, 58]. In the basic ionic liquid, cadmium dichloride, CdCl2, dissolves and forms a divalent cadmium chlorocomplex anion, CdCl [69], which is reducible to metallic Cd at around — 1.2 V. In the case of a neutral buffered ionic liquid, the electrodeposition of Cd is observed at around —1.0 V, which is slightly positive than in the basic ionic liquid probably caused by the formation of more reducible species, CdCl3. ... 

The electrodeposition of cadmium, Cd, has been studied in an acidic EMICl-ZnCl2 ionic liquid [92]. Cadmium dichloride, CdCl2, dissolves in the ionc liquid and forms a divalent cadmium species, which is reducible to metallic Cd prior to the bulk deposition of metallic Zn. The electrodeposition of Zn-Cd alloys was also reported. 

No compound with five-coordinate carbon as the central atom has yet been crystallized. Burgi, however, has studied a series of trigonal bipyramidal complexes of divalent cadmium with three equatorial sulfur ligands and with axial ligands iodide, hydroxyl, or sulfur groups ( Y and X). 

Cadmium is of particular note partly because there is more understanding regarding its mobilisation mechanism/ but also because of its toxicity and ability to accumulate up the food chain The mobilisation of cadmium is dependent upon the pH of the soil solution/ and unlike other heavy metals/ cadmium does not become fixed within the soil structure. Research has shown that the divalent cadmium ion will be displaced by hydrogen ions when the pH is lowered by one unit from pH 5 to 4. Concentrations of up to 1-2 g/1 have been obtained. 

Because the use of QDs as reporters in living cells may soon become conventional, the possibility of QD cytotoxicity is of interest and concern. Almost aU of the reports of QDs in living cells have revealed little or no obvious cytotoxicity or changes in ceUular differentiation [8,13]. Although QDs contain toxic elements, most importantly divalent cadmium, cytotoxicity issues may only become relevant for truly long-term (months to years) visualization of QDs in cells, a time period in which QD degradation could become significant. 

Hasan, S Krishnaiah, A Ghosh, TK. Adsorption of Divalent Cadmium (Cd(II)) from Aqueous Solutions onto Chitosan-Coated Perlite Beads. Ind. Eng. Chem. Res., 2006, 45, 5066-5077. 

Group IIB-Transition- and Inner-Transition-Metal Bonds 8.3.3.2. Reactions of Carbonyls with Divalent Group-IIB Salts 8.3.3.2.I. Derivatives of Zinc and Cadmium. 

Metal toxicity is also affected by physiochemical factors, such as pH and the concentration of divalent cations. Adding divalent cations, such as zinc, has been reported to mitigate toxicity produced by other metals. For example, the addition of 60 pM zinc reduced toxicity in Pseudomonas putida caused by 3 mM cadmium.148 Zinc had no effect on cells grown in the absence of cadmium. Little is understood surrounding the mechanism of protection however, cadmium uptake was observed to be dependent on zinc concentration.149 Zinc was found to be a competitive inhibitor of cadmium uptake. 

Gasiorek K, Bauchinger M. 1981. Chromosome changes in human lymphocytes after separate and combined treatment with divalent salts of lead, cadmium, and zinc. Environ Mut 3 513-518. 

In mammals, as in yeast, several different metallothionein isoforms are known, each with a particular tissue distribution (Vasak and Hasler, 2000). Their synthesis is regulated at the level of transcription not only by copper (as well as the other divalent metal ions cadmium, mercury and zinc) but also by hormones, notably steroid hormones, that affect cellular differentiation. Intracellular copper accumulates in metallothionein in copper overload diseases, such as Wilson s disease, forming two distinct molecular forms one with 12 Cu(I) equivalents bound, in which all 20 thiolate ligands of the protein participate in metal binding the other with eight Cu(I)/ metallothionein a molecules, with between 12-14 cysteines involved in Cu(I) coordination (Pountney et ah, 1994). Although the role of specific metallothionein isoforms in zinc homeostasis and apoptosis is established, its primary function in copper metabolism remains enigmatic (Vasak and Hasler, 2000). 

Cadmium is a silver-white, blue-tinged, lustrous metal that melts at 321°C and boils at 767°C. This divalent element has an atomic weight of 112.4, an atomic number of 48, and a density of 8.642 g/cm3. It is insoluble in water, although its chloride and sulfate salts are freely soluble (Windholz et al. 1976 USPHS 1993). The availability of cadmium to living organisms from their immediate physical and chemical environs depends on numerous factors, including adsorption and desorption rates of cadmium from terrigenous materials, pH, Eh, chemical speciation, and many... 

Diffusion in the extrinsic region can readily be modified by doping, although knowledge of the mechanism by which the diffusion takes place is important if this is to be immediately successful. For example, sodium chloride structure materials that conduct by a vacancy mechanism can have the cation conductivity enhanced by doping with divalent cations, as these generate compensating cation vacancies. The inclusion of cadmium chloride into sodium chloride can be written ... 

Narang, K. K. et al., Synth. React, lnorg. Met.-Org. Chem., 1996, 26(4), 573 The explosive properties of a series of 5 amminecobalt(III) azides were examined in detail. Compounds were hexaamminecobalt triazide, pentaammineazidocobalt diazide, cis- and fram-tetraamminediazidocobalt azide, triamminecobalt triazide [1], A variety of hydrazine complexed azides and chloroazides of divalent metals have been prepared. Those of iron, manganese and copper could not be isolated cobalt, nickel, cadmium and zinc gave products stable at room temperature but more or less explosive on heating [2],... 

A similar technique was used by Soliman, who also used a silica gel matrix and a covalent linker to tie to a series of amines, mono-, di-, tri-, and tetra-amine [15]. Using a batch equilibrium technique, he measured the removal capacities (mmole/g) for divalent forms of cobalt, nickel, copper, zinc, cadmium, and lead. In general maximum removal values (at optimum pH values) were obtained for the tetra-amine species. 

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