Cesium is shown in figure 1 below. 5

Cesium is a radioactive
element and it naturally occurs as 133Cs. It has a chemical symbol Cs and its
atomic number is 55.  Studies show that,
the concentration of cesium is 1.9 mg/kg in the earth crust and that of sea
water is about 0.5 micrograms per kilogram (?g/kg).1
Its melting point is 28.50
C and it is also an alkali metal.
2

There are 40 isotopes
of cesium, but the major isotopes are 11. Among the 11 major isotopes, only 133
Cs is stable. Radioactive isotopes of cesium
are produced mainly by nuclear fission.3 134Cs decays via beta emission and produces one beta particle per each
transformation.

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 Research shows that the mean
energy produced from the beta emission of 134 Cs decay is 0.157 MeV
and the mean energy produced from the emission of gamma rays is 0.698 MeV.135
Cs undergoes beta decay and produces one beta particle per transformation
with a mean energy of 0.188 MeV.4 Also, during the radioactive decay
of 137Cs, 92% of beta particle (?) are emitted via excited state producing 89% of gamma radiation. However, 8% of the beta particles are
emitted directly to the stable state of 137Ba. In addition, the stable
137 Ba produced emits a gamma radiation of 0.662 MeV energy. This is shown in figure 1 below. 5 

Figure 1: Diagram of 137 Cs radioactive decay5

It is very useful in brachytherapy for treating cancer. In addition, it is an excellent source for
medical applications such as instrument disinfection and radiotherapy.6   Also, it is
less expensive and has demonstrated good characteristics that makes it very
useful substitute for radium in treatment of malignant disease.7

1.2  Distribution
of Cesium into the Environment and its Impacts.

Research shows that, the spreading of radioactive cesium into the environment is
due to the release of 137Cs and 134 Cs.8  The release of 137Cs into the environment is via three routes. Nuclear weapons testing is one of the route that led to the pollution of the atmosphere. Developed countries such as United States, Russia, the United Kingdom,
France, and China were leading this radioactive
contamination with massive nuclear tests in the atmosphere. The
year 1963, marked a milestone with the passing of the Limited Test Ban Treaty
(LTBT), a treaty banning the testing of nuclear weapons in all global
environments, in exception of the underground.9 Though this treaty was passed putting a ban on all
nuclear tests in the atmosphere, France and China still continued nuclear
testing in the atmosphere.

The second category, the nuclear-armed states, represented by India,
South Africa, Pakistan, North Korea, and Israel came into force after this
period and  they continued testing nuclear weapons underground.10

With respect to all the nuclear tests carried-out in the atmosphere from
1945-1963, the USA and Russia were responsible for 82 % of all these tests.
Between 1951 and 1992, nuclear tests totaled an explosive yield of
approximately 530 Megatonnes, of which 83 % were due to the atmospheric nuclear
tests carried out.11 The development of nuclear devices such as nuclear bombs also led to the
radioactive pollution of the atmosphere.12

The discharge of waste effluents
generated from nuclear fuel-reprocessing
plants and nuclear
reactors led to the second route.13

The last route is through the accidental leaks of radioisotopes from nuclear power plants and this
route is an environmental concern since the radionuclides speedily distribute
into the environment. Recently, much attention was focused on the release of
large-scale radioactivity that occurred following the explosion at Chernobyl and Fukushima nuclear accident.14

The trace of radioactive substances in water bodies is
mainly due to radionuclides wash off from the water-catchment areas.
Radionuclides are rapidly redistributed and accumulated in aquatic plants,
fishes and soil sediments.15 Moreover, the contamination of
biological system has been due to the radiation exposure of aquatic organisms
and humans connected by food-chains within the hydrosphere.16 The
migration of 137Cs into water bodies led to contamination and damaged
many aquatic habitants.17

Cesium enters the human body and it spread throughout
the body at higher concentrations into the muscle tissues.18 Essentially, all cesium that is ingested is
absorbed into the bloodstream through the intestines.19    However, cesium is excreted from the body quickly.20 The accumulation of cesium
into the body poses health hazard from both gamma and beta radiation. This has
become a concern due to the increased likelihood for inducing cancer. Consequently, humans are at high risks since fish is a major source of
food to humans.21

Also, the radioactivity
emitted by cesium caused damage to the deoxyribonucleic acid (DNA) of humans.22  After the nuclear
accidents, there was large exposure of radioactive cesium (137Cs)  and this became a serious concern due to health
problems associated with it.23

 

1.3 Contamination of Land and Water from Nuclear Accidents

1.3.1 Chernobyl Accident

Studies shows that, Chernobyl and its surroundings (as
shown in figure 2 were contaminated with radionuclides. The nuclear accident that occurred On April 26, 1986, at the atomic power station in Ukraine resulted in the spread of significant amounts of radionuclides. A complete meltdown of the nuclear reactor took place,
and an estimated 150 million  radioactive
substances were distributed into the
atmosphere during the first week of the accident.24

The Chernobyl accident affected people about hundreds of kilometres away from the atomic power station
resulting into several cancerous conditions with higher incidence of tumour
type cancers. Thyroid tumours were the largest number of cancers in the
affected population.25 The flood plain of
Pripyat River was contaminated due to its closeness to the nuclear plant. 26
Several studies about Cs pollution of soil and water
surfaces have been investigated and it provided estimates of dissolved 137Cs
in Chernobyl. 27

About 450 different radioactive materials were distributed into the atmosphere affecting Ukraine, Belarus and
Russia. The radioactive cloud travelled to the Baltic states, Scandinavia,
Europe and radioactive materials were even detected over North American.
Millions of people were exposed to high quantities of radioisotopes with millions of children
suffering from thyroidal conditions.28

Moreover, after the deposition of radiocesium from the nuclear accident,
it led to the distribution of radionuclides in alpine which affected alpine
pastures that were used extensively in agricultural land for milk production.29
Also, aquatic system were reported to be contaminated  after the Chernobyl accident.30 Sequentially,
aquatic systems became the source of radionuclides due to the deposition of 137Cs
and 90Sr in the terrestrial areas which lead to the transportation
of higher amount of radionuclides transported to groundwater, lakes and river.31
Subsequently, the discharge of nuclear fuel from Chernobyl  nuclear accident led to the transportation of 137Cs
into the Siberian Arctic. Several studies show that the Barents Sea, Pechora
Sea, Kara Sea (Ob River) and the Yenisey River were largely polluted by 137Cs.32

 

 Figure 2: Chernobyl Accident
Radiation Contamination24

1.3.2 Fukushima Daichi
Nuclear Power Plant (FDNPP) Accident

The FDNPP is a nuclear accident that occurred
in Japan. This disaster was triggered by the tsunami following the T?hoku
earthquake on 11th March 2011 and affected several nuclear plants in Japan
concurrently, thereby causing drastic contamination in Fukushima .33 Consequently, radionuclides were distributed  into the atmosphere as a result of the nuclear
accident as shown in figure 3.34  The
nuclear accident  also led to the
speedily transportation and distribution of radionuclides such as  134Cs, 137Cs, 131I
and 90Sr into river sediments, potable water and food.35 After the
Fukushima disaster, most research conducted show that, drinking waters around
Fukushima were largely contaminated with large amount of radioactive cesium.36
However, 131I has a short half-life and can decay within six months but
the long half-life of 137Cs can cause severe damage to human life,
agriculture and stock farming for decades.37Also, the release of severe
gaseous radioactive materials into the atmosphere led to acute and chronic
health problems of the population of the Republic of Korea.38

 

Figure
3: Radiations in the environment around Fukushima34

 

1 .4 Adsorption of Cesium

Radioactive cesium in waste water has been an environmental concern in this era where nuclear power has been on the ascendancy. There have been several techniques to remove cesium from waste water to prevent contamination.39

One such novel methods is the application
of macrocyclic ligand, o-benzo-p-xylyl-22-crown-6-ether (OBPX22C6) as shown in figure 5 and it is very useful  in the
adsorption of cesium  from
wastewater. There is a high
propensity of the  ? electron of this crown ether to interact with d-f hybrid orbital electron of cesium. Furthermore,  research shows
that the d-f hybrid orbital is absent in sodium(Na) and potassium (K) ions and thus, the selectivity of the
macrocyclic ligand for cesium is highly optimal.40 Easy scaling up
and low power consumption  in nuclear
waste water processing are the main advantages of the d-f
hybrid adsorbent and less waste is produced 41 Research shows
that, the adsorption of cesium with OBPX22C6 is better at low pH ranges .42 Notwithstanding,
the high efficiency of the OBPX22C6 in the adsorption of cesium, is affected by
co-existing metal ions. 43

Figure 5: Synthetic route for the producing o-benzo-p-xylyl-22-crown-6-ether. 40

Similarly, 1,3-(2,4-diethylheptylethoxy)oxy-2,4-crown-6-Calix4arene(Calix4arene-R14) is
found to be one of the most promising extractant of cesium among crown
ethers.This is because the cavity of its ligand match well with the ionic
radius of  cesium. Calix4areneR14 has a
higher affinity to remove cesium from wastewater.44

Also, ion-exchangers are another kind of adsorbants that has been very useful in removing cesium due
to their chemical and radiation stabilities, granulometric properties suitable
for column operation and high ion-exchange capacity and adsorption efficiency 45 Furthermore, inorganic ion exchangers demonstrated good selectivity in the processing of waste water containing cesium and the recovery of valuable metals. Inorganic ion exchangers of
the class of tetravalent metal acid (TMA) salts  such as zirconium phosphate (Zr(HPO4)2.H2O),
titanium phosphate (Ti3(PO4)4 )and tin
phosphate(Sn3(PO4)4 have emerged as promising advanced materials owing to their important
applications as ion exchangers in separation science. There is an interest in zirconium-based
ion-exchangers because of their good selectivity, reproducibility and excellent ion-exchange behavior.

Nevertheless, ion-exchange studies with crystalline materials are often
complicated because of the formation of new crystalline phases. Properties of amorphous ion
exchangers such as easy preparation and their granular nature make them suitable for
column operation and more preferred than crystalline forms. Besides, they are quite stable in acidic medium.46 The adsorption of cesium unceasingly increased to
attain a maximum percent at pH of 3. There was a change in the
mechanism involved in initial and final stages of adsorption process at higher pH values and adsorption was almost constant throughout
this pH range. This behavior may be due to the change in the mechanism involved in initial and final stages of
adsorption process. Since its cation has a large ionic radius and a small
hydration number, it has the ability to compress the electric double layer
around adsorbent particles and reduces the electrokinetic potential that favour
its adsorption.47

Also, insoluble hexacyanoferrate is another method
that have been employed for removing cesium from liquid waste water
containing radionuclides such as cesium. Research shows that insoluble hexacyanoferrate
shows high affinity and selectivity for cesium ions. 

But this method has certain disadvantages such as
pressure drop and filtration when used in column application. However, this can
be reduced by attaching supporting material such as polyacrylonitrile (PAN).
PAN is an organic binder with good solubility and immobilization properties and
strong adhesive forces with inorganic materials. Other studies conducted on
PAN-based hexacyanoferrate and PAN-based potassium nickel hexacyanoferrate(II)
(PAN-KNiFC) shows high selectivity for removal of cesium from liquid waste water.48

Furthermore, copper ferrocyanide(CuFC) is an inorganic
metal that has been used extensively to remove cesium from liquid waste due to
its good adsorption property for cesium. It also has high affinity for cesium
over a wide pH range. However, CuFC adsorption capacity may be reduced by mass
or volume occupied by the support materials and this causes the kinetics of
adsorption to be slower due to the mass transfer within the material.49

Recently, the application of electrocoagulation
process for removing cesium from aqueous solution have demonstrated high
removal efficiency. In this method, different anode materials such as
aluminium, iron, magnesium and zinc with galvanized iron as cathode. This
method shows 96% removal of cesium with magnesium as cathode and the adsorption
process follows second order kinetics model with good correlation of Langmuir
and Freundlich adsorption isotherm model were studied using experimental data.
Langmuir adsorption isotherm favours monolayer coverage of adsorbed molecules for
adsorption of cesium.50 Also, silicate-based multifunctional nanostructured
materials with magnetite and Prussian blue shows effective removal of cesium
ions from aqueous media by avoiding the use of organic solvents. This method is
also effective even at high concentration of sodium chloride which allows their
application in the removal of radioactive 137Cs. It is easy and has
quick recovery of the pollutant -loaded adsorbents by means of a magnet.51

Clay colloids are also one of the effective means of adsorption of cesium and significantly
enhancing its transport in ground water. Clay colloids exhibit small size, hence
they have the ability to remain suspended in groundwater
and be transported. Also, their large surface area enhances adsorption capacity for low solubility radionuclides such as cesium.
Owing to the very small hydration energy of cesium, it is able to
preferentially interact and adsorb to the surface of clay colloids.52 The coefficient of
distribution of cesium and the percentage adsorption of it increases with pH. This observation may be due to the presence of amphoteric OH groups on the surface of clay colloids that can either
take up a proton at low pH or leave one proton at a high pH. Thus, at higher
pH, the negative charge on the clay increases and hence the adsorption of
cesium ion increases. Though the clay colloids are effective in adsorption of
cesium, their efficiency is drastically reduced in the presence of high sodium
and calcium ions. The bivalency of calcium confers on it a higher reduction
power on the coefficient of distribution of cesium than sodium ions.53

However, these techniques have some disadvantages
associated with their application. Several techniques such as Prussian blue,
zeolites, ammonium molybdo phosphate -polyacrylonitrile (AMP-PAN), potassium
nickel hexacyanoferrate-polyacrylonitrile (KNiFC-PAN) shows that, in the process
of removing cesium ions from aqueous waste water, other metal ions can be
exchanged and causes inability to remove Cs under high ionic strength
conditions such as sea water.54,55

1.5 Heteropoly acids (HPAs)

Heteropoly acids are complex acids containing
oxygen, hydrogen, non-metals and metals such as molybdenum, vanadium or
tungsten. They form a conjugate anion known as polyoxometalate and they have a
well-known structures such as Keggin (HnXM12O40)
as shown in Figure 4 and Dawson, HnX2M18O62,
structures.56 The X in their Keggin
structure represents the heteroatom while M represent the central atom. The
heteroatom is either tungsten (W) or molybdenum (Mo) while the central atom is
either phosphorus (P) or silicon (Si).57

When the heteroatom of HPAs is tungsten or molybdenum, it makes them highly active in many catalytic reactions such as condensation, hydration and polymerization.58 Heteropolyacids demonstrate unique
properties such as high thermal stability and high proton mobility which is attributed to the ability to catalyze
several acids both in heterogenous and homogenous catalysis systems.  Phosphotungstic acid (H3PW12O40) is known to be the strongest HPA with Keggin structure
and it is widely used in hydrolysis and dehydration.59 Hence, HPAs have been widely used as
acid, oxidation and electrophilic catalysis for several reactions and
application in industries. Also, they are soluble in non-polar solvents, hence
they are environmentally friendly and harmless. Therefore, they are solid acids
that can replace harmful solutions such as H2SO4 and HF.60

HPAs’ unique catalytic activity is the result of super acidity. The
strong acidity of HPAs is caused by delocalization of negative charge on large
anions with Keggin structure. HPA’s also have the ability to form insoluble salts with
alkaline metal cations.61 

Pure HPAs have certain limitations in catalytic
applications due to their low surface area and nonporosity. However, to improve
their catalytic activity, it is important to immobilize them onto a large
surface area and a better pore support such as silica, zeolites, mesoporous
carbon, alumina, etc. Increasing their acid sites will also enhance better
catalytic activity. Also, their negatively charged heteropolyanions enables them
to be immobilized onto an ion-exchanged resins.62

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