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Fig. 8.40 Experimental results for a Cu and b Ni adsorption isotherms on goethite, at pH 7.35, in
the presence of groundwater containing natural organic ligands, as compared to irradiated natural
groundwater and synthetic groundwater. Reprinted with permission from Buerge-Weirich et al.
( 2002 ). Copyright 2002 American Chemical Society
groundwater containing natural organic ligands, as compared to irradiated natural
and synthetic groundwater. Clearly, less Cu and Ni were adsorbed in the presence
of organic ligands than in the reference systems. In both cases, the isotherms do
not intercept the x-axis at the origin but at a value near 5 9 10 -8 M. This behavior
indicates the presence of strong complexing ligands in solution. When these
ligands were saturated, the amount of adsorbed metal increased linearly with total
metal concentration.
Dissolved inorganic trace metal (e.g., Hg, Ag, Cd, Ni, Zn) speciation and the
tendency to form stable soluble complexes in saline water was considered by
Turner ( 1996 ) and Turner et al. ( 2001 , 2002 ), who developed an empirical
equation to describe the direct relationship between the sediment-water distribu-
tion coefficient (K d ) and salinity. We focus on Hg sediment-water partitioning as a
result of a coupled speciation and a salting-out process, as reported by Turner et al.
( 2001 ). Hg has the tendency to form chloro-complexes. In a natural aqueous
system characterized by the presence of dissolved humic materials and salts, Hg is
complexed by the abundant cation or has a strong affinity for particulate OM.
Turner et al. ( 2001 ) designed their study recognizing that the nature and extent of
sediment-saline water partitioning of Hg(II) are governed by the hydrophobic
characteristics in the presence of OM, including a tendency to be salted out of
solution by seawater ions. They found that the increase in K d with increasing
salinity is at least partially the result of an increase in the proportion of the
relatively hydrophobic and lipophilic HgCl 2 complex, which is subject to salting
out.
Distribution coefficients (K d ) and (K oc ) defining 203 Hg(II) sorption to three
estuarine sediments are plotted against salinity in Fig. 8.41 . Despite different
Hg(II) concentrations and K oc values in each location, a common exponential
increase is evident. Salting out did not occur when Hg(II) is only in an ionic form,
so that an increase in Hg(II) dissolution in estuarine water may be explained by the
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