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Section 4 <br />Discussion <br />1.4 <br />1.2 <br />1.0 <br />0.8 <br />0.8 <br />0.4 <br />0.2 <br />110.0 <br />412 <br />-0.4 <br />-0.8 <br />-0.8 <br />-1.0 <br />X V4VVR4 <br />_•'`•_•• H3VO40 (aq) OWSWR3 <br />•_ r SJWR3 <br />VOH2+ ••••_? * 07FVVR3 <br />oSVWR2 <br />H2VO4" (aq) W • ? <br />VO+2 • <br />(aq) HVO,i z(?q) <br />V+3 <br />VOH+2 _ YJQH)30(aq) <br />0 2 4 8 pH 8 10 12 14 <br />Figure 4-3 - Eh-pH Diagram for the System V-O-H at 25° C and 1 atm.. <br />4.4 Uranium <br />The uranium Kd values reported in Table 3-7 (24-87 L/kg) are generally consistent <br />with the values reported in the literature (60-100 L/kg). Under oxidizing conditions <br />and within the pH range of 7.2 to 8 su, uranium exists as either a cation (UO20H+), a <br />neutral species (U02(OH)20), or an anion (UO2(OH)3-) (see Figure 4-4). At pH 7.2 to <br />7.4 su (sample WSWR3) the most predominant species is the cation, while at pH 8 the <br />anion predominates (neglecting other possible species such as uranylcarbonates and <br />uranylphosphates). The cationic form would tend to adsorb to most mineral surfaces, <br />with the exception of iron hydroxide, which is positively charged at pH 7.2-7.4. The <br />anion would adsorb mostly onto iron hydroxide, but the degree of adsorption (and <br />Kd) would tend to decrease as the pHZp, is approached (8.5 su). One sample had a <br />vertical isotherm, which suggests a solubility control. As U+6 minerals are very <br />soluble and U+4 is unstable at an Eh of >400 mv, control by a pure phase uranium <br />mineral is unlikely. However, control by a solid-solution phase is possible. Davis et al. <br />(2005) found that at the Hanford Site, uranium is controlled partly by the formation of <br />calcite. Uranium substitutes for calcium within calcite. Therefore, uranium can be <br />removed from solution when calcite precipitates, which can occur in response to <br />carbon dioxide degassing, mixing of groundwaters and other processes. <br />• <br />• <br /> <br /> <br />4-3 <br />TA64986-Denison Mines\Task Order 3- DMO Sampling and Analysis Plan%Task 3.12- Soil, Ore, Rock Data Assessment Report%Sunday Rods ReportWtadhments%Attenuation Report_mrking draft 040309.doc