Chalcophilic mobility

Cu may be reduced to Cu", especially if soft bases such as halides and S are present to stabilize the Cu" " ion. All are chalcophiles and tend to form insoluble sulfides in anaerobic conditions (pKs = 21.3-25.6, 19.4-26.6 and 36.1, respectively). They therefore tend to have low mobilities in submerged soils, especially Cu +, and accumulate. 

In contrast to refractory uranium and thorium, lead is a moderately volatile element. Uranium and thorium are lithophile, while lead can exhibit lithophile, siderophile, or chalcophile behavior. This means that in many cosmochemical situations, it is possible to strongly fractionate the daughter lead from parent uranium and thorium, a favorable situation for radiochronology. On the other hand, lead tends to be mobile at relatively low temperatures and can be either lost from a system or introduced at a later time. As already mentioned, uranium can also become mobile under oxidizing conditions. This means that the U-Th-Pb system is more susceptible to open-system behavior than several other commonly used dating techniques. However, as we discuss below, there are ways to recognize and account for the open-system behavior in many cases. 

The probability that sulfide is molten in some situations within the mantle raises the possibility that this melt may be an elfective metasomatic agent, and that sulfides observed in mantle-derived samples have been introduced in metasomatic events (e.g.. Fleet and Stone, 1990), rather than being trapped in peridotitic residue of partial melting events. Given the ability of sulfide melt to mobilize chalcophilic elements, a significant amount of experimental work has been conducted to evaluate the mobility of liquid sulfide at high pressures and temperatures, both to constrain... 

Soil redox potential is also critical in controlling elemental mobility. Some elements are much more soluble and mobile in one oxidation state than another (examples include Cr, Mn, Se, and others). The elements classified as chalcophiles (e.g., Hg, Cu, Pb, Cd, Zn, As, Se) form insoluble sulfide minerals in reducing environments where sulfide (S ) is generated from sulfate reduction (see Chapters 4 and 7). Mobility for chalcophiles is then extremely low unless oxidizing conditions are restored in the soil. Those elements that, in the sulfide form, have the very lowest solubility products (notably mercury, copper, lead, and cadmium) are the most Ukely to become highly immobile and unavailable in reduced soils. ... 

Copper occurs in soil solids and solutions almost exclusively as the divalent cation Cu ". However, reduction of Cu " (cupric) to Cu (cuprous) and Cu (metallic copper) is possible under reducing conditions, especially if halide or sulfide ions ( soft bases) are present to stabilize Cu" (a soft acid). Copper is classified as a chalcophile, owing to its tendency to associate with sulfide in the very insoluble minerals, CU2S and CuS. In reduced soils, then, copper has very low mobility. Most of the colloidal material of soils (oxides of Mn, Al, and Fe, silicate clays, and humus) adsorb strongly, and increasingly so as the pH is raised. For soils with high Cu accumula-... 

With regard to the potential release of metals from solid waste materials, changes in pH and redox conditions are of prime importance. It can be expected that changes from reducing to oxidizing conditions, which involve transformations of sulfides and a shift to more acid conditions, increase the mobility of typical B- or chalcophilic elements, such as Hg, Zn, Pb, Cu, and Cd. On the other hand, the mobility is character-... 

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