Uranium natural mobility

Implications of the Natural Mobility of Uranium on the Storage of Nuclear Waste... 

Langmuir D (1978) Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits. Geochim Cosmochim Acta 42 547-569 Langmuir D, Herman JS (1980) The mobility of thorium in natural waters at low temperatures. Geochim Cosmochim Acta 44 1753-1766... 

Silver is usually found in extremely low concentrations in natural waters because of its low crustal abundance and low mobility in water (USEPA 1980). One of the highest silver concentrations recorded in freshwater (38 pg/L) occurred in the Colorado River at Loma, Colorado, downstream of an abandoned gold-copper-silver mine, an oil shale extraction plant, a gasoline and coke refinery, and a uranium processing facility (USEPA 1980). The maximum recorded value of silver in tapwater in the United States was 26 pg/L — significantly higher than finished water from the treatment plant (maximum of 5.0 pg/L) — because of the use of tin-silver solders for joining copper pipes in the home, office, or factory (USEPA 1980). 

As the data in Fig. 5.12 show, and as was pointed out by Bruno et at. (1991), the half time for the dissolution reaction of U02 in the pH-range of most natural waters and under reducing conditions is in the order of days. If we compare this with typical residence times of undisturbed ground waters (years) we can conclude that the dissolution of U02(s) and the mobility of uranium under these conditions is thermodynamically and not kinetically controlled. 

If the environmental condition of the ore zone changes as a result of either natural or manmade causes, the uranium can become very mobile. This is evidenced by the in situ mining process in which relatively minor modifications are made to the ground-water chemistry to produce a solution that can rapidly dissolve uranium and maintain uranium solution concentrations of hundreds of parts per million. We have shown in our laboratory experiments that when this uranium-rich solution contacts aquifer sediments containing minerals capable of reducing uranium from (VI) to (IV), large portions of the uranium are rapidly removed from solution and immobilized. 

Other examples of redox-sensitive elements include heavy elements such as uranium, plutonium, and neptunium, all of which can exist in multiple oxidation states in natural waters. Redox conditions in natural waters are also indirectly important for solute species associated with redox-sensitive elements. For example, dissolution of iron (hydr)oxides under reducing conditions may lead to the solubilization and hence mobilization of associated solid phase species, e.g. arsenate, phosphate (see Sections 3.3.2.1, 3.3.3.2, and 3.3.4.1). 

Sorption in most soils attains a maximum when the neutral hydroxy complex of uranium is at a maximum. However, at pH 6 and above, and in the presence of high carbonate or hydroxide concentrations, uranium may form anionic complexes such as [U02(0H)4]. The mobility of anionic uranium complexes in soil is dependent upon the nature of the soil. For example, the decrease in sorption in soil with little anion-exchange capacity may result in increased mobility however, increased sorption in soil with high anion-exchange may result in decreased mobility (Allard et al. 1982 Ames et al. 1982 Brookins et al. 1993 Ho and Doern 1985 Hsi and Langmuir 1985 Tichnor 1994). 

Natural attenuation encompasses processes that lead to reduction of the mass, toxicity, mobility, or volume of contaminants without human intervention. The US EPA has recently published guidelines for the use of MNA for a variety of contaminated sites (US EPA, 1997). For inorganic constiments, the most potentially important processes include dispersion and immobilization (reversible and irreversible sorption, co-precipi-tation, and precipitation) (Brady et al, 1998). Studies of remediation options at UMTRA sites (Jove-Colon et al, 2001) and the Hanford Site (Kelley et al, 2002) have addressed the viability of adopting an MNA approach for uranium and strontium, respectively. As discussed below, different approaches are required to establish the viabihty of MNA for these radioelements. 

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