38
Peter Stille and Graham Shields
and Elderfield 1988; Elderfield et al. 1990). River waters that are enriched in
colloidal phases display higher REE concentrations than rivers that are poor in
such phases. The influence of the suspended load (including colloidal phases) on
REE concentration is shown in Fig. 3.6. This diagram shows the behaviour of the
REE in river water in successive experiments. River water, which was filtered
through <21a m and <0.451.tin filters, shows concentrations that are almost identical
to those of the original, untreated sample. Use ofa <0.2~tm filter reduces the REE
concentration of the river water by about 60 to 80%. This means, therefore, that a
significant portion of the REEs exist in suspended form in the <0.451.tm fraction.
REE concentration is seen to decrease with pore size. It is scarcely possible to
filter off all such colloidal particles. Therefore, REE concentrations for river water
always represent a product of mixing between dissolved ions and REE integrated
into particles. Iron, in the form of iron hydroxide (Fe(OH)3) and its other oxides
and hydroxides, is the single most important component of colloidal particles. The
concentration of Fe in river water is also controlled by pH. The amount of
colloidal particles and hence the Fe content is directly correlatable with the
concentration of the REE (Fig. 3.7).
River water, which has been filtered through <2~m filters, shows an almost
identical REE pattern to that of the unfiltered sample, while increasing
fractionation of the REE can be observed with further decreases in pore size.
Depleiion of light and heavy REE can be seen in the fine fraction. These HREE
3000
y 2 0 0 0
o
E
"o l(x~
z
0
I ~ Q . - 9 i
2
0
I
I
I
0
&
6
8
10
Fe (}.Jmol / kg )
Fig. 3.7. Relationship between Fe and Nd concentrations in filtered river water. (Eiderfield
et al. 1990)