796
K. M. G. Mostofa et al.
Fig. 5 Conditional
binding constants (log K)
for rare earth elements
(REE)–Leonardite coal
humic acid standard (LHA)
and REE–SRFA complex
formation at pH 7 with
various ionic strength (a) and
the effect pH on log K for
complexation of LHA (b) and
SRFA (c) with REE. Error
bars (0.16 log units) are only
shown for pH 7 (a), but apply
to all log Kc values. Nominal
total concentrations are 100
nM REE; 20 mg L −1 SRFA,
130 nM EDTA for SRFA
experiments (b and c). Data
source Sonke and Salters
(2006)
Log K
Rare earth elements
(b)
(a)
(c)
Log K
Ionic radius (angstrom)
Fig. 6 Log K for lanthanides and organic
ligands of various sources at pH 6, 7, and
8 as a function of ionic radius (sixfold
hydrated), including Sc and Y transition metals. Log K for Y and Leonardite
coal humic acid standard (LHA) values
are represented by singular data points,
floating just below the main rare earth elements (REE) data trends at an ionic radius
of 0.900 Å. Log K for lanthanides and
Elliot soil humic acid standard (EHA) values observes at pH 9, and 0.01 mol L −1 I
and extrapolated log K values for SRFA
and LHA under those conditions. EDTA
conditional binding constants for 0.01 mol
L −1 I are included for comparison. Data
source Sonke and Salters (2006)