School of Engineering and Science - Jacobs University

waters, as evidenced by the dominant association of the REY with Fe-organic colloids
in river waters (e.g., Sholkovitz, 1995). The particle reactive nature of the rare earths
was well exploited by early workers conducting seawater analyses, in which the REY
were quantitatively removed and concentrated from seawater samples by coprecipitation
with Fe-hydroxides (e.g., Goldberg et al., 1963; Høgdahl et al., 1968;
Elderfield and Greaves, 1982). This scavenging by particulate matter is considered the
primary removal process for the REY in seawater (e.g., de Baar et al., 1985a; Elderfield,
1988), and it appears that particle coatings consisting of organic films and Fe-Mn
oxides are particularly important in this scavenging process (e.g., Byrne and Kim, 1990;
Sholkovitz et al., 1994, and references therein). Experimental work indicates these
scavenging reactions approach steady state within minutes (e.g., Byrne and Kim, 1990;
Koeppenkastrop and De Carlo, 1993; Bau, 1999), suggesting that equilibrium
conditions characterize REY partitioning between seawater and particle surfaces.
The general shape of the REY patterns shown in Figs. 4-6, with a strong
enrichment of the HREE over the LREE in seawater, can be attributed to the
competition between carbonate complexation in solution and reactive particle surfaces.
The exact process by which REY are adsorbed to particle surfaces is still a matter of
study, and Piasecki and Sverjensky (2008) provide a review of proposed sorption
mechanisms in addition to a triple-layer surface complexation model that is consistent
with X-ray studies. However, to a first approximation, particle surface adsorption
constants for individual REY can be described by their first hydrolysis constants (Erel
and Morgan, 1991; Bau et al., 1996). Using such an approach, calculated partition
coefficients between particle surfaces and seawater predict a preferential LREE
enrichment for the adsorbed component (Erel and Stolper; 1993), consistent with LREE
depletion in the solution phase and the observed REY patterns for world seawater. The
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