1. Introduction
Nuclear energy is one of the most promising clean energy sources for
mankind in the future, and spent fuel contains a large amount of raw
material uranium, solid and gaseous radioactive elements produced by
fission as well. During spent fuel reprocessing, processes such as
shearing and dissolving, nitric acid recovery, process solution and
liquid waste evaporation, waste calcination and melting, etc. will all
produce process tail gas. The capture of the fission product129I in the nuclear fuel cycle is a growing priority
for nuclear wasteform research and development. Due to its long
half-life (t1/2-1.57×107 year) and
high mobility in most geological environments, its removal is a
difficult problem. 129I is of concern for spent
nuclear fuel reprocessing facilities and in this regard a review of129I immobilization has recently been
published1. Therefore,
the capture and storage of gaseous iodine in off-gas has become the most
important issue in the treatment of radioactive waste gas in the
world2.
In the current PUREX process, radioiodine can be eliminated from the
gaseous waste streams by counter-current scrubbing of the off-gas,
including alkaline washing method, Mercurex method and Iodox method,
etc3. But it has
shortcomings such as high corrosivity, high fluidity, difficult storage,
and secondary
pollution3. Therefore,
solid adsorption has been widely developed on iodine adsorption with the
advantages of economical, convenient and highly effective. Moreover,
using solid adsorbents to remove gaseous iodine can avoid the
complication in system designing and high maintenance costs. The solid
adsorbents mainly include activated carbon, inorganic porous materials,
metal-organic frameworks, and porous organic materials. Although organic
adsorption materials like metal-organic frameworks and porous organic
materials have large adsorption capacity, they are not stable enough
under water vapor conditions and are prone to adhesion, which are not
suitable under actual working conditions. In addition, their
manufacturing process is cumbersome. For these reasons, the currently
promising solid adsorbents for gaseous iodine adsorption are mainly
inorganic adsorbents, including activated
carbon3-8, silver-loaded
silica gel9-12,
silver-loaded
alumina13-15 and
silver-exchanged
zeolite16-22. Among
them, the silver-exchanged zeolites are most commonly used for iodine
capture, being considered as a benchmark sorbent.
Domestic and foreign scholars found that zeolites with silver nitrate
can capture molecular iodine and iodine alkyl compounds, with
decontamination factors higher than
10323-28. The saturated
adsorption capacity was 196.6 mg g-1 for elemental
iodine and the average utilization rate of silver atoms was greater than
86.5%29. But one
important disadvantage of Ag-based zeolites is that only a portion of
silver species participate in the I2 and
CH3I capturing processes, but all of the spent solid
sorbents need to be discarded after the adsorption
process30. Although AgI
and AgIO3 in the spent adsorbents can be converted back
to Ag0 nanoparticles by the treatment with molecular
hydrogen at high temperatures (ca. 773 K), the regenerated Ag
nanoparticles undergo gradual sintering during the regeneration process,
which leads to a significant decrease in their activity. Due to this
deleterious feature, the expensive adsorbents are typically discarded
after being recycled several
times30. In addition,
previous studies showed that the iodine adsorption performances of
silver-containing zeolites were readily influenced by nitrogen oxides,
which was due to the oxidation of metallic
silver22,31-33.
And increasing the maximum iodine adsorption capacity of the adsorbent
can reduce the amount of secondary solid waste after the adsorption of
radioactive iodine. There is an urgent need to find a zeolite adsorbent
that is cheap, recyclable, especially suitable for industrial
application.
All-silica zeolite has good hydrophobicity, acid resistance and thermal
stability, which is suitable under actual spent fuel reprocessing
conditions30,34,35.
More importantly, it is cheap than Ag-loaded zeolite which shows a
potential for iodine adsorption. Currently, the all-silica zeolites
tested for iodine adsorption are poorly documented. A.Hijazi et.al
studied the iodine adsorption on polyethyleneimine impregnated
nanosilica sorbents. Several nanoporous silica of SBA-15 and Aerosil
types were impregnated with branched polyethyleneimine with the aim to
evaluate their iodine adsorption performance. The adsorption capacities
of the adsorbents increased almost linearly with the N content provided
by polyethyleneimine below certain
threshold35. Tung Cao
Thanh Pham et. al discussed power silicalite-1 all-silica zeolite on
iodine adsorption performance. They observed silicalite-1 were stable in
5 M nitric acid and adsorbed iodine from highly acidic off-gas mixtures
to much greater extents than does activated carbon. The iodine
adsorption capacity for silicalite-1 was 480 mg g-1,
and after hydrophobicity intensification, it performed better up to 530
mg g-130. However, in
practical engineering applications, the adsorbent needs to be formed
into spherical or strip-shaped materials with a size of micrometers or
millimeters through a molding process, because powder materials can not
be directly used in engineering applications. In the molding process, it
is generally necessary to mix inorganic binders and adsorbent powders.
High-temperature calcination is performed to enhance the mechanical
strength of the spherical or strip-shaped materials and ensure that no
pulverization occurs under actual application conditions after
preliminary molding. Generally, inorganic materials used as binders do
not have gas molecule adsorption capacity, thus the maximum adsorption
capacity of the adsorbent after molding is generally lower than the
maximum adsorption capacity of the adsorbent powder.
In this work, a new kind of shaped silicalite-1 all-silica zeolite was
synthesized by a novel method with aluminum oxide as a binder which
could be directly used in industrial applications. It was strip-shaped
with a diameter of 3 mm and a length of about 1cm with a topological
structure of MFI molecular sieve. The adsorption properties for iodine
were investigated under the condition of reprocessing temperature about
348 K subsequently 36.