Dissolved lanthanide concentrations

Dissolved lanthanide concentrations (pmol/kg) in the ocean water masses ... 

Large scale fractionation accompanies the seasonal cycle of the dissolved lanthanide concentrations in Chesapeake Bay. This is illustrated in fig. 37 which presents the time series of the Nd/Yb ratio for bottom waters (and upper pore waters). This light to heavy lanthanide ratio increases in the spring and is followed by an equally large decrease in the fall. At its maximum in the summer, the Nd/Yb ratio is three times greater than its winter baseline ratio. Hence, the lighter lanthanides are preferentially released in the spring and summer and then preferentially removed in the fall. 

By comparison with natural samples, lanthanide-bearing species from manufactured sources are typically much simpler analytical targets. The samples are often more readily dissolved and, because many of them are rare-earth-based materials, preconcentration steps can sometimes be eliminated. Recent reports have apphed analytical separation methods to determine lanthanide concentrations in metals (Kobayashi et al. 1992), alloys (Al-Shawi and Dahl 1996), and magnets (Saraswati 1993), in high-purity rare-earth oxides (Stijfhoom et al. 1993, Yin et al. 1998, W. Li et al. 1997, 1998, Wu et al. 1997, Peng et al. 1997), and in optical materials (Bruzzoniti et al. 1996). 

Fig. 1. (a) The concentration of dissolved lanthanides in the surface waters of the Sargasso Sea. A composite of data measured by TIMS (Sholkovitz and Schneider 1991) and INAA (De Baar et al. 1983). Note classic sawtooth abundance pattern. Pm does not exist in nature, (b) Shale-normalized pattern of the composite seawater shown in (a) using shale concentrations of table 1. Tb, being inconsistent, probably reflects an incorrect concentration of the seawater. 

Fig. 10. The calculated lanthanide concentrations of river colloids normalized against the dissolved (0.22 filtrate) lanthanide concentrations for the Connecticut River water collected on 20 July 1992 and on 17 December 1992. From Sholkovitz (1995). Dissolved refers to <0.22 nm filtrate and calculated colloid refers to the difference between the dissolved composition and the composition of two ultrafiltrates (5K and 50K nominal wt. cut-offs).

The large-scale removal of dissolved lanthanides in the low salinity region is an ubiquitous feature of estuaries (Martin et al. 1976, Goldstein and Jacobsen 1988c, Sholkovitz and Elderfield 1988, Elderfield et al. 1990, Sholkovitz et al. 1992, Sholkovitz 1993, 1995). As illustrated for the Amazon and Fly (fig. 12), the Nd concentrations of surface waters decrease sharply between the river (S 0) and a salinity of 5-7. In the case of the Amazon, Nd removal represents approximately 95% of the river-borne dissolved flux. Of all the trace element distributions reported for estuaries, the lanthanides stand out as being the most similar to dissolved Fe and humic acids, both of which undergo extensive removal in estuaries. Since there is also a strong correlation between... 

In summary, profiles of dissolved and particulate lanthanide concentrations in the Sargasso Sea (Sholkovitz et al. 1994) provide the following conclusions ... 

The relationship between dissolved lanthanides and nutrients has received considerable discussion in the literature. Unlike for many other trace metals, there appears to be no metabolic requirement for lanthanides by plankton. At pmol/kg levels such a requirement is unlikely. Hence, surface water depletion must reflect a general type of scavenging onto newly formed surfaces (plankton). The extremely low concentrations of lanthanides in the calcium carbonate or siliceous skeletal matter of plankton suggest that such... 

The oxalates obtained above, alternatively, are digested with sodium hydroxide converting the rare earth metals to hydroxides. Cerium forms a tetravalent hydroxide, Ce(OH)4, which is insoluble in dilute nitric acid. When dilute nitric acid is added to this rare earth hydroxide mixture, cerium(lV) hydroxide forms an insoluble basic nitrate, which is filtered out from the solution. Cerium also may be removed by several other procedures. One such method involves calcining rare earth hydroxides at 500°C in air. Cerium converts to tetravalent oxide, Ce02, while other lanthanides are oxidized to triva-lent oxides. The oxides are dissolved in moderately concentrated nitric acid. Ceric nitrate so formed and any remaining thorium nitrate present is now removed from the nitrate solution hy contact with tributyl pbospbate in a countercurrent. 

Di-iso-decylphosphoric Acid The DIDPA Process An(III) and Ln(III) can be partitioned using the DIDPA solvent (DIDPA and TBP, respectively dissolved at 0.5 and 0.1 M in n-dodecane) in a two-step process approach. First coextracted and costripped in a 4 M nitric acid solution in a first DIDPA cycle (see Section 3.3.1.1.4), the An(III) + Ln(III) fraction is partitioned in a second cycle after denitration of the An(III) + Ln(III) product by formic acid to reduce the nitric acid concentration to at least 0.5 M. In this second DIDPA cycle, An(III) and Ln(III) are first coextracted by the DIDPA solvent, and the An(III) are selectively stripped by DTPA (0.05-0.1 M) in a solution buffered at pH 3 with lactic acid (1 M). The triva-lent lanthanides are further stripped with a 4 M nitric acid solution (134). 

The SETFICS process (Solvent Extraction for Trivalent /-elements Intragroup Separation in CMPO-Complexant System) was initially proposed by research teams of the former Japan Nuclear Cycle Development Institute (JNC, today JAEA) to separate An(III) from PUREX raffinates. It uses a TRUEX solvent (composed of CMPO and TBP, respectively dissolved at 0.2 and 1.2 M in -dodecane) to coextract trivalent actinides and lanthanides, and a sodium nitrate concentrated solution (4 M NaN03) containing DTPA (0.05 M) to selectively strip the TPEs at pH 2 and keep the Ln(III) extracted by the TRUEX solvent (239). However, the DFs for heavy Ln(III) are rather poor. An optimized version of the SETFICS process has recently been proposed as an alternative process to extraction chromatography for the recovery of Am(III) and Cm(III) in the New Extraction System for TRU Recovery (NEXT) process. NEXT basically consists of a front-end crystallization of uranium, a simplified PUREX process using TBP for the recovery of U, Np, and Pu, and a back-end Am(III) + Cm(III) recovery step (240, 241). 

After initial concentration by crushing, grinding and froth flotation, bastnasite is treated with 10% HCl to remove calcite, by which time the mixture contains around 70% lanthanide oxides. This is roasted to oxidize the cerium content to Ce on further extraction with HCl, the Ce remains as Ce02, whilst the lanthanides in the (-T3) state dissolve as a solution of the chlorides. 

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