The bioavailability of trace metals, their accumulation by organisms, and
their toxicity to terrestrial and aquatic organisms can be understood better
in terms of trace metal species. Although there are numerous chemical
techniques used for the determination of ionic species, most are tedious,
time-consuming, and not practical for routine analyses. Alternatives to these
chemical techniques are computer speciation models that often have an
incomplete database on metal–organic chemistry, yet scientists continue
to use these techniques for estimation of trace metal activities. In this
study we compare the cation exchange resin (Amberlite) and the MINTEQA2
geochemical model for the speciation of zinc (Zn2+)
and copper (Cu2+) in the aqueous phase of 11 soils
varying widely in chemical composition. The pHw(1:5) of
these soils ranged from 5·3 to 9·1, while the total soil aqueous
phase concentrations of Zn and Cu were in the ranges 1–71 and 5–74
µg/L, respectively. Preliminary equilibration studies revealed that
24-h contact between resin and sample solution was sucient to exchange all
free ions onto the resin bead. By using this equilibration time, the effects
of temperature, solution composition, and pH on the resin distribution
coeficients (Kd) for Zn and Cu
were investigated. These studies revealed that temperature did not
significantly influence the Zn and Cu resin distribution coecient. Solution
composition such as the presence of Ca, Mg, and Al, however, had a marked
effect on the Kd value. Although
the Kd value was not sensitive to
changes in solution pH (4·5–7·1), it increased at pH
>7·1. These results suggest that soil solution composition must be
considered during metal ion speciation using the resin beads. Changes in Zn
and Cu concentrations, however, did not influence the
Kd values of these metals. The
cation exchange resin method was modified for speciation of solutions
containing very low concentration (<50 µg/L) of Zn. Comparison of
the 2 speciation techniques demonstrated excellent agreement for free hydrated
Zn2+ and Cu2+ in the
soils studied.
Extraction of Aluminum and Iron from Bauxite: A Unique Closed-Loop Ore Refining Process Utilizing Oxalate Chemistry
