Metals natural occurrence

The zinc +2 ion, with its six-coordinate radius of 0.74 A, is almost identical in size to both the magnesium (0.72) and the copper (0.73) ions, but zinc is much more polarizing than the alkaline earth metal and consequently has a well-defined, albeit limited, coordination chemistry. In keeping with the much lower hardness of Zn2+ (77 = 10.88 eV) versus Mg2+ (32.55 eV),9 zinc has a much greater affinity for softer ligands than magnesium, a fact that is also reflected in the natural occurrence of zinc as sulfide ores. 

In 1866 Friedrich Wohler discovered a ruthenium mineral. When he analyzed the shining black grains of what seemed to be an unusual platinum mineral which Herr Waitz of Cassel had brought back from Borneo, he found it to be a sulfide of ruthenium and osmium. Wohler stated that this mineral, which he named laurite, presented the first example of the natural occurrence of sulfur compounds of the platinum metals (129). 

Table 3.1. Natural Occurrence of Selected Metals and Materials-Based Applications...

For the essential elements the amounts in the body are normally controlled by physiological mechanisms, but for the non-essential, non-beneficial elements there are no such controls and the amounts in the body generally reflect the natural occurrence of the elements in food and water. For many such elements we may consider that there is a base load in the human body which reflects the natural intake of the elements in the diet. For some elements, industrial, mining or other human activities, may release metals into the environment. Such activities may result in a civilization-related load being added to the natural base load in some circumstances this civilization-related load may be very much greater than the base load. For example, the natural concentrations of the highly toxic metal cadmium in soils are generally quite low, yet in the... 

Niobium metal is typically gray or dull silver in color. It is one of the refractory metals along with tantalum, tungsten, molybdenum, and rhenium, due to its very high melting point. It is estimated that niobium has a natural occurrence in Earth s crust of approximately 20 parts per milhon (ppm). The largest niobium-containing mineral reserves are located in Brazil and Canada. 

The long-range transport of Saharan and Asian dust has been identified as the dominant source of mineralic particles over the Atlantic, the Arctic, and the Pacific (SCOPE 1979 Rahn et al. 1979 Duce et al. 1980 Uematsu etal. 1983 Parrington and Zoller 1984). Other important sources of naturally emitted metal compounds are volcanoes (Zoller 1983), forest fires which may be of natural occurrence as well as originate from anthropogenic activities and exudations from vegetation (Pacyna 1986a, b). 

The tentative tolerable daily intakes proposed for certain metals provide a guideline for maximum tolerable exposure. In the case of essential elements, these levels exceed the daily requirements, but this should not be construed as an indication of any change in the recommended daily requirements. In the case of both essential and nonessential metals, the tentative tolerable intake reflects permissible human exposures to these substances as a result of natural occurrence in foods or various food processing practices, as well as exposure from drinking water. 

Natural occurrence. Rhodium is one of the rarest element in the Earth s crust with an abundance of 1 pg/kg (i.e., ppb wt.). Rhodium occurs in nature as a native metal along with other platinum-group metals in the native mineral iridosmine or in sulfide ores such as rhodite, sperrylite, and some copper-nickel ores. 

Natural occurrence. Platinum metal occurs free in nature as a native metal contaminated with small amounts of all the PGMs such as iridium, osmium, palladium, ruthenium, and rhodium. These native minerals are found in placer ore deposits. 

Quantitative analysis was soon employed to verify the law of constant composition. It had for many years been assumed by most chemists that different samples of the same pure compound would have identical compositions. In 1799, the Frenchman Joseph Proust (1754-1826) analysed basic copper carbonate of natural occurrence (malachite) and the same compound prepared in the laboratory. Both samples gave the same analytical results. He demonstrated a similar constancy of composition in many other compounds, and he showed that several metals form more than one oxide and sulphide, each of definite composition. 

People are exposed to lead tlirough breathing lead-polluted air and through the ingestion of lead in food and drink. The relative importance of each particular route of exposure is a matter of some uncertainty, and does of course vary considerably between individuals dependent upon their places of residence and work, and their dietary habits. Lead has always been present at low levels in food and drink due to the natural occurrence of the metal in rocks and soils. Deposition of lead aerosol may enhance the levels of lead in foodstuffs and water, but the magnitude of this is difficult to quantify. 

Walker, J.D., M. Enache, and J.C. Dearden. 2007. Quantitative cationic-activity relationships for predicting toxicity of metal ions from physicochemical properties and natural occurrence levels. QSAR Combin. Sci. 26 522-527. 

QICARs use the metal-Iigand bonding characteristics to predict metal ion toxicity (Newman et al., 1998). In general, the models developed for metals with the same valence were better than those combining mono-, di-, and trivalent metals. The metal ion characteristics included a softness parameter and the absolute value of the log of the first hydrolysis constant. The first stable reduced state also contributed to several two-variable models. Since most metals can interact in biological systems as cations and because toxicity of metals depends on cationic activity, the term (quantitative) cationic-activity relationships or (Q)CARs also describes the qualitative and quantitative relationships for predicting the bioconcentration, biosorption, or toxicity of metals, from their physicochemical properties and natural occurrence levels. 

Walker et al. (2007) developed 6 QSARs to predict the toxicity of 17 cations to sunflower seeds (F.l. Helianthus annuus Sunspot) from the metal s physical properties and natural occurrence levels (Table 5.4). The QSARs predicted EC50 values based on the cation concentration producing a 50% inhibition of radicle growth one day after emergence. The QSAR developed with density of the elements (p), enthalpy of formation of metal sulfides (AHJ, and the stability constants of metal ions with sulfate (log Ki [sulphate]) produced the highest adjusted (Table 5.20). For natural occurrence levels, the QSAR developed with metal concentrations in soil (log Msoi,), the median elemental composition of soils (mg X/kg soil), and the calculated mean of the elemental content in land plants (Land Plants) produced the highest adjusted... 

---