The use of the composite adsorbents composed of benzo-15-crown-5
(abbreviated as BC15) and benzo-18-crown-6 (BC18) for the simultaneous
recovery of vanadium (V), uranium (U), and lithium (Li) from seawater
has been proposed for industrial applications. The adsorption and
desorption behavior of these elements on BC15 and BC18 has been examined
in various types of aqueous solutions over a wide temperature range.
As a result, it was shown that BC15 and BC18 have sufficient adsorption
ability for the simultaneous recovery of V, U, and Li from seawater.
Moreover, it was seen that the distribution coefficients (
K
d
) of V decrease with an increase in [HCl]
T
(subscript T: total concentration), indicating that the anionic
V species such as H
2
V
4
O
13
4–
are exponentially changed into the cationic V species such as V
3+
, VO
2+
, and VO
2
+
under the
condition [HCl]
T
= 1.0 M, and the complexation reactions
between BC15 (or BC18) and the initial V structures are inhibited.
Besides, it was reasonably shown that the adsorption mechanism is
the path through the electrostatic interaction between the anionic
V species such as H
2
V
4
O
13
4–
, and the −C–O–C– single bond that the
electron density is eccentrically located in ether functional groups
in crown ether rings in BC15 and BC18 (or the −C–OH
single bond that the electron density is eccentrically located in
bisphenol A in BC15 and BC18). Then, the chromatography experiment
of V, U, and Li on BC15 (or BC18) at 298 K was carried out by flowing
seawater, 1.0 × 10
–2
M HCl, and 1.0 M HCl in
sequence. The first peak of V can be separated from the plateau of
Li and the first and second peaks of U in the case of the BC15 system.
The recovery ratios of V and U were more than 80%. On the other hand,
entirely overlapping chromatograms were obtained in the case of the
BC18 system, and accordingly, the recovery ratios of V and U were
much lower. In short, the separation efficiency of V with BC15 is
more pre-eminent than that with BC18. Judging from these results,
the durability of BC15 was finally assessed for industrial applications,
that is, the aforementioned chromatography experiment was repeatedly
carried out to check whether V, U, and Li were stably and mutually
separated from seawater or not. The evidence that the recovery performances
of V, U, and Li from seawater do not decrease at all after at least
five cycle tests was provided. This indicates that this information
will be valuable for the development of a practical chromatographic
technology to simultaneously recover V, U, and Li from seawater.