Journal of Radioanalytical and Nuclear Chemistry (2022) 331:5817–5824
https://doi.org/10.1007/s10967-022-08650-1
Study on the radon absorption performance of microwave
regenerated activated carbon
Xiyu Yang1 · Jian Shan1
· Songbo Tan1 · Fengdi Qin1
Received: 26 August 2022 / Accepted: 27 October 2022 / Published online: 15 November 2022
© Akadémiai Kiadó, Budapest, Hungary 2022
Abstract
Activated carbon is widely used in 222Rn removal. Meanwhile, the regeneration of activated carbon is key to improve its
utilization in 222Rn removal. The goal of this study is to find the most appropriate conditions of the regeneration of activated
carbon for the removal of 222Rn and to provide a reference for the application of microwave-assisted regeneration of saturated
activated carbon. In this paper, the self-developed microwave device is used to perform multiple regeneration experiments at
different gas flow rates, and the 222Rn absorption coefficient of activated carbon is determined by dynamic adsorption. The
relative standard deviation of 3.4% suggests that the measured dynamic adsorption coefficient is stable, reliable, and well
reproducible. When the flow rate is 4 L ­min−1, the dynamic adsorption coefficient of 222Rn, generated by the activated carbon
after three cycles through open-loop regeneration, reaches the maximum value, around 20.8% higher than the fresh activated
carbon. In the closed-loop regeneration mode (6 L ­min−1 flow rate), the dynamic adsorption coefficient is 19.1% higher than
that of fresh activated carbon. The aperture characterization shows that the micropores are most abundant ranging from 5 to
6 Å. Moreover, at 8 and 10 L ­min−1, the pore diameter of the activated carbon is in the range of 5 to 6 Å, about 20.0% and
30.0% higher than nine cycles. Therefore, three to four cycles times, using closed-loop regeneration at a low gas flow rate
(4 L ­min−1) can improve the 222Rn adsorption and the utilization rate of activated carbon most effectively.
Keywords Radon adsorption · Microwave regeneration · Closed-loop regeneration method · Open-loop regeneration
method · Dynamic adsorption coefficient
Introduction
222
Rn is a colorless and tasteless, natural radioactive gas
produced by various decay chains of uranium and thorium.
It mainly comes from the soil, building materials, and water,
and is also the main source of natural radiation [1]. Aerosols
carrying 222Rn and its products inhaled in the respiratory
system can damage organs and cause lung cancer. According
to the International Agency for Research on Cancer, 222Rn
is the second-largest substance that can cause lung cancer
[2]. Therefore, its removal from the natural environment is
particularly important.
Adsorption is widely used in industrial and research practice for the separation of many substances including metals.
* Jian Shan
shanjian0666@163.com
1
School of Nuclear Science and Technology,
University of South China, Hengyang 421001, Hunan,
The People’s Republic of China
It is also widely used in the removal of harmful air pollutants and toxic volatile organic compounds [3]. In addition,
activated carbon also plays an important role in the removal
of radionuclides from the environment, such as the removal
of U(VI) and Th(IV) ions in water media [4]. At the same
time, activated carbon is one of the most common adsorbents used for the removal and reduction of 222Rn removal
due to its good adsorption capacity and low production cost
[5, 6]. However, when the adsorption of 222Rn reaches the
equilibrium state, the saturated activated carbon is unable to
adsorb more of it. To reuse activated carbon for continuous
222
Rn prevention, saturated activated carbon will be heated
and regenerated. The traditional heating method achieves
efficient regeneration through the indirect heating of conduction and convection, mainly depending on the thermal
conductivity of the material. This method has the disadvantages of long heating time, uneven heating, and high energy
consumption [7]. To widen the application range of the activated carbon adsorption process, more efficient regeneration
methods are needed.
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Journal of Radioanalytical and Nuclear Chemistry (2022) 331:5817–5824
The main difference between microwave heating and traditional heating is the heat generation method. In microwave
devices, radiations are converted into heat by dipole rotation and ion conduction [8]. When a high-frequency voltage
is applied, the response of the materials with a permanent
dipole moment or an induced dipole changes their direction, opposite to the direction of the applied field. Synchronous stirring of the molecule then generates heat. Therefore,
microwave heating is fast and has low energy consumption
[9–11]. At present, microwave heating is widely applied in
the preparation and modification of activated carbon [12],
however, only a few studies are reported on the desorption and regeneration of saturated activated carbon through
this technology. This study compared the 222Rn adsorption
capacity of activated carbon after the open-loop and closedloop regeneration method under different conditions, thus,
providing reference for the engineering application and
promoting the use of microwave regeneration devices for
activated carbon regeneration.
material is cylindrical polyTeflon and can load up to 1200 g
of activated carbon.
Experimental methods
Introduction to the experimental equipment
The microwave regeneration device was developed by our
team. It consists of a cuboid structure of 76 cm long, 63.5 cm
wide, and 83.5 cm high. The schematic diagram of the operational principle of the device is shown in Fig. 1. Two microwave generators are operating independently, to achieve the
adjustable microwave power of 0 ~ 2100 W. The microwave
power of 300 W is used in this study. The device has a side
length of 23 cm and a 51 cm high positive eight-sided column
furnace with a rotating activated carbon bed bracket, which
can hold up to 10 activated carbon beds. The carbon bed
Principle of microwave heating and regeneration
AN electromagnetic wave has frequencies in the range of 3106
to 3109 Hz. The frequencies of civilian microwaves used for
heating are 915 MHz and 2450 MHz. When microwaves
encounter different materials, they are reflected, adsorbed, and
penetrate the material, depending on the dielectric constant,
dielectric loss coefficient, specific heat, shape, and moisture
content of the material.
The microwave power of activated carbon adsorption is
represented by Pc(W)[13, 14]:
Pc = P0 𝜂 = 𝜀0 𝜀1 E2 𝜋f
(1)
where P0 is the total microwave power, 𝜂 is microwave
absorption efficiency of activated carbon, 𝜀0 is the dielectric constant of free space, 𝜀1 is the dielectric loss constant,
E is the electric field strength, and f is the electromagnetic
field frequency.
The number of molecules adsorbed on the surface of the
activated carbon depends on the number of colliding molecules with the activated carbon and their residence time on its
surface. When the number of colliding molecules that collide
with the activated carbon per unit of time is represented by
n, their average residence time is represented by 𝜏 , and the
adsorption capacity per unit of the surface area is represented
by 𝜎:
𝜎 = n𝜏
(2)
The basic equations can be derived from the notion of
"ideal" disorder in the molecular theory of gas dynamics:
n=
1
n𝜇
4 0 0
(3)
where 𝜇0 is the average velocity of all molecules, which varies with temperature and molecular weight, and n0 represents
the number of molecules per unit volume.
According to the scientific laws of the ideal gas:
n0 =
Np
N
=
V
RT
(4)
where N is the Avogadro constant, V is the volume per mole,
p is the pressure of the ideal gas, R is the gas constant, and
T is the absolute temperature. According to the Maxwell
velocity distribution law, 𝜇0 can be acquired by the following formula (5):
√
8RT
(5)
𝜇0 =
𝜋M
Fig.1 Schematic diagram of the microwave regeneration device
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Journal of Radioanalytical and Nuclear Chemistry (2022) 331:5817–5824
differential equations. When the time is 0, the unit pulse
value of the adsorbent mass is shown by the following
formula:
where M is the molecular weight of the gas.
Therefore, according to formulas (4), (5), and (3):
n= √
Np
2𝜋MRT
(6)
The relation between T and τ was given by Frenkel in
1924:
Q
𝜏 = 𝜏0 e RT
(7)
In this equation , τ0 is the vibration period of the molecule
in the adsorbed state, Q is adsorption heat, the energy needed
to desorb from activated carbon. According to formulas (6),
(7), and (2), an expression for the adsorption capacity per
unit surface area σ can be obtained:
Q
Np
𝜏0 e RT
𝜎=√
2𝜋MRT
5819
(8)
In the process of 222Rn adsorption by activated carbon,
M and τ0 are constant. When the temperature increases, the
222
Rn gas adsorbed on the activated carbon decrease. Therefore, when heating the 222Rn activated carbon, more 222Rn
disorients from the activated carbon as the temperature rises.
Yn =
nn F n−1 tn−1 (− nFt
e kM )
(n − 1)!(kM)
(10)
When the derivative of (10) is equal to zero, the time
required for adsorption mass concentration to reach its maximum value can be obtained as follows:
tmax =
(n − 1)kM
nF
(11)
Since the concentration curve given by (10) is almost
symmetrical, it can be assumed that t­max occurs at the midpoint of the curve. Thus, this value is equivalent to the halfpenetration time (t). At the same time, when n → ∞, n−1
→1,
n
the dynamic adsorption coefficient k can be obtained using
the following equation:
k=
Ft
M
(12)
Theoretical principle of dynamic adsorption
The dynamic adsorption experiment is shown in Fig. 3
The activated carbon was dried in an oven before the measurement. The pumps of Rad7 were open during the process,
the “sniff” mode of measurement was set and the measurement period was 5 min.
222
Microwave regeneration experiment
dYi
−Fn
=
(Y − Yi+1 )
dt
kM i
In this study, the activated carbon was adsorbed and regenerated nine times in two modes; open loop and closed loop, as
shown in Fig. 4a and b, respectively. Gas-flow rate regulation in the open-loop and closed-loop regeneration methods
was 4, 6, and 8 L ­min−1, and 6, 8, and 10 L ­min−1, respectively. A saturated activated carbon sample was placed in
the proposed device. After regeneration, 25 g of the material
was randomly sampled to measure the adsorption coefficient.
Rn adsorption coefficient is a key parameter to characterize the 222Rn absorption capacity of activated carbon.
Dynamic adsorption of 222Rn is generally associated with
activated carbon. In dynamic adsorption, activated carbon
is divided into n-block carbon layers, as shown in Fig. 2.
The mass balance of each stage adsorbent is shown by the
following formula:
(9)
In the equation, k is the dynamic adsorption coefficient
(L ­g−1), F is the flow rate of the carbon column (L ­min−1),
Yi is the volume fraction of adsorbent leaving i th layer in
the gas phase, M is the total mass of the adsorbent (g), and t
is the time (min). The concentration of the adsorbent along
stage n of the carbon column is obtained by solving the n
Fig. 2 Schematic diagram of the carbon layer
Fig.3 Schematic diagram of the dynamic adsorption experiment of
activated carbon for 222Rn
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Journal of Radioanalytical and Nuclear Chemistry (2022) 331:5817–5824
Fig.4 Schematic diagram
of the microwave open-loop
regeneration experiment (a) and
closed-loop regeneration experiment (b)
Results
Tendency of dynamic adsorption coefficient
with regeneration times
Dynamic 222Rn absorption coefficient of fresh
activated carbon
Dynamic 222Rn absorption coefficient of fresh coconut
shell activated carbon is measured five times, which is
shown in Table 1. The calculated value is 3.03 ± 0.1 L ­g−1.
The relative standard deviation of 3.40% indicates that
the 222Rn dynamic adsorption coefficient is stable, reliable, and highly reproducible. It is noted that there is a
competition between water and 222Rn molecules during
dynamic adsorption. However, 222Rn molecules reach
adsorption equilibrium faster than water molecules, therefore, the competitive influence of water molecules can
be ignored. Since the 222Rn-containing air entering the
activated carbon bed is not removed, the measurement
results are slightly lower than the actual value.
The trend for the dynamic adsorption coefficient of the openloop and closed-loop regeneration activated carbon absorption of 222Rn with the regeneration times is shown in Fig. 5.
The dynamic adsorption coefficient of activated carbon
222
Rn increases and then decreases, and reaches the maximum value after the third or fourth regeneration, however, its
maximum value decreases when the flow velocity increases.
In the open-loop regeneration method, when the flow rate is
4 L ­min−1, the dynamic adsorption coefficient of activated
carbon on 222Rn after three cycles increases by 20.8% compared with fresh activated carbon. When the flow rate is
8 L ­min−1, the dynamic adsorption coefficient of activated
carbon decreases to the lowest value by 20.5% compared
with fresh activated carbon. In the closed-loop regeneration
method, the 222Rn dynamic adsorption coefficient reaches the
maximum value, 19.1% higher than fresh activated carbon,
after the third regeneration and flow rate of 6 L ­min−1. When
the flow rate increases to 10 L ­min−1, the dynamic adsorption
Table 1 Measurement results of dynamic adsorption factor of activated carbon 222Rn absorption
Activated
carbon
Quantity (g)
Half penetration Air flow rate
time (min)
(L ­min−1)
Dynamic 222Rn absorption coefficient (L ­g−1)
Average value
Standard bias
Relative
standard
deviation
N1
N2
N3
N4
N5
80.50
15.10
25.00
25.00
25.00
285.53
49.12
95.79
85.72
83.73
3.19
2.92
3.06
3.01
2.94
3.03
0.10
3.40%
13
0.90
0.90
0.80
0.88
0.88
Journal of Radioanalytical and Nuclear Chemistry (2022) 331:5817–5824
5821
Fig.5 Variation of dynamic adsorption coefficient with regeneration times in open-loop regeneration method (a) and closed-loop regeneration
method (b)
coefficient of regenerated activated carbon drops to the lowest value, 4.0% lower than fresh activated carbon. The main
reason for the above trend is that the pore structure of activated carbon changes after microwave irradiation. The high
heat generated by microwaves collapses the part of the existing pore wall of activated carbon, thereby, producing many
new microporous structures, conducive to the adsorption of
222
Rn. Therefore, the dynamic 222Rn absorption coefficient
of activated carbon reaches its peak value after three or four
times of microwave recycling. When the number of recycling
and regeneration increases further, the activated carbon pore
wall collapse phenomenon is too intense due to the continuous heat accumulation, and the micropore volume is gradually
reduced to the lowest value. In addition, the oxygen content is
the main influencing factor in the open-loop and closed-loop
regeneration of activated carbon. In the open-loop regeneration
method, a large amount of air enters the carbon bed, oxidizing
the activated carbon at a high temperature, and part of the activated carbon burns. The phenomenon of charcoal burning is
relatively strong, resulting in the serious collapse of activated
carbon pores and a reduction in the volume of micropores.
However, the oxygen content in the closed-loop regeneration
method is limited and the charcoal burning process is less. At
the same flow rate, the dynamic adsorption coefficient after
multiple regenerations (more than four times) is higher than
that of activated carbon after open-loop regeneration.
Micromorphological characterization of microwave
regenerated activated carbon
The micromorphology of fresh activated carbon and microwave regenerated activated carbon are analyzed by using
scanning electron microscopy (SEM), as shown in Fig. 6.
The activated carbon sample is regenerated three times at
an airflow rate of 8 L ­min−1.
From the comparison, the pore structure of the activated
carbon surface after microwave regeneration is much richer
than that of fresh activated carbon, and a large number of
attachments in and around pores are also removed after
microwave regeneration. In addition, during the microwave
regeneration process, the surface of the activated carbon
produces many cracks, and most of the occluded pores are
also opened. The reason for these changes may be: (1) the
activated carbon absorbs microwave energy in the internal
"microdomain explosion" which cracks the surface of the
activated carbon. (2) Under microwave irradiation, carrying
gas flowing through the activated carbon bed may be taken
away in the activated carbon pore with subtle impurities,
thereby, increasing the microporous volume of activated
carbon.
Surface characteristic parameters of microwave
regenerated activated carbon
NXVY represents the activated carbon analysis after the Xth
microwave closed-loop regeneration at a gas flow rate of
YL·min−1. The surface characteristic parameters of activated
carbon after microwave closed-loop regeneration are shown
in Table 2.
At a gas flow rate of 8 L·min−1, the BET surface area,
microporous area, and pore volume of the ninth-cycle regenerated activated carbon are reduced by 5.9%, 5.8%, and
11.1%, respectively, compared with the third regenerated
activated carbon. Similarly, at a gas flow rate of 10 L ­min−1,
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Journal of Radioanalytical and Nuclear Chemistry (2022) 331:5817–5824
Fig.6 SEM images of fresh activated carbon of 20 K times (a), regenerated activated carbon of 20 K times (b), fresh activated carbon of 50 K
times (c) and regenerated activated carbon of 50 K times (d)
Table 2 Surface characteristic parameters of activated carbon after
microwave closed-loop regeneration
Samples
BET surface area
­(m2/g)
Microporous area
­(m2/g)
Pore
volume
­(cm3/g)
N3V8
N9V8
N3V10
N9V10
909.92
855.86
892.68
834.03
749.06
705.12
732.59
687.98
0.38
0.34
0.37
0.32
the BET surface area, microporous area, and pore volume
of the ninth-cycle regenerated activated carbon are 6.5%,
6.1%, and 14.4% lower than the third regenerated activated
carbon, respectively. At the same flow rate, these parameters
of the third-cycle regenerated activated carbon are all higher
than that of ninth-cycle regenerated activated carbon. For
all the above changes, the most likely reason is that with
the increasing number of microwave regeneration cycles,
13
activated carbon internal new micropore wall collapses
under continuous high temperature, therefore, the area and
volume of micropores show a decreasing trend. This also
corresponds well to the trend of the 222Rn dynamic adsorption coefficient of activated carbon, that is, the dynamic
adsorption coefficient of activated carbon 222Rn after three
to four cycles of regeneration reaches the maximum value.
Pore structure of microwave regenerated activated
carbon
The pore structure of the microwave-assisted closed-loop
recycled activated carbon is analyzed by Mack Instruments, USA (3-Flex). N
­ xVy represents the Xth microwave
cycle regeneration of activated carbon at a gas flow rate of
YL·min−1. The adsorption and desorption isotherm and the
aperture distribution curves are shown in Fig. 7.
The adsorption isotherm of all activated carbon samples
belongs to type I isotherm, and the adsorption process is
Journal of Radioanalytical and Nuclear Chemistry (2022) 331:5817–5824
5823
Conclusion
reversible, suggesting physical adsorption. And with the
increase in relative pressure, the final saturation adsorption amount of the regenerated activated carbon samples
is significantly higher than that of fresh activated carbon.
This further validates that the adsorption performance of
microwave recycling regeneration on activated carbon can
improve to a great extent.
According to the aperture distribution curve, all samples show the first peak at the aperture width range of
5 Å < d < 6 Å, and a second peak close to 8 Å, however, the
value is much smaller than the first peak. When the flow rate
is the same, the pore structure after three cycles in this pore
size range is significantly more than that after nine times
recycling, that is, when V = 8 L ­min−1, 10 L ­min−1, activated
carbon after three cycles, the pore increment in aperture
range of 5 Å to 6 Å is about 20% and 30% higher than that
after nine cycles. At the same gas flow rate, there is an obvious positive correlation between the 222Rn dynamic adsorption coefficient of activated carbon and the pore increment in
the aperture range of 5 Å to 6 Å. Moreover, with the increase
in the microwave cycle frequency of 222Rn absorption activated carbon, its aperture increased. When the number of
cycles is more than three times, with the increasing cycle
times, a new microporous hole wall produced in the previous
microwave regeneration collapses under the action of high
energy, thus, gradually linking adjacent pores together and
increasing the pore aperture, which is not conducive to the
adsorption of 222Rn. This eventually leads to the phenomenon that after multiple cycles (more than four), the dynamic
adsorption coefficient of 222Rn decreases.
(a) 350
This study explores the dynamic 222Rn absorption coefficient of microwave open-loop and closed-loop regenerative
activated carbon at different flow rates and characterized
their pore structure. Combined characterization results of
the dynamic adsorption coefficient and the pore structure,
the following conclusions are obtained:
(1) With the increase in regeneration times, both dynamic
222
Rn absorption coefficient of microwave open-loop
and closed-loop regenerative activated carbon show
an increasing trend and then decreasing, reaching the
maximum value after the third or fourth cycle, and the
maximum value decreases with the increase in flow
rate.
(2) When the number of microwave regeneration cycles
and gas flow rate are the same, the dynamic 222Rn
absorption coefficient of activated carbon samples
obtained by the closed-loop regeneration is higher than
that of the open-loop regeneration method.
(3) The aperture of activated carbon used is mainly in
the order of 5 Å < d < 6 Å and 6.5 Å < d < 10 Å. At an
aperture range of 5 Å < d < 6 Å, the content of pore
structure is positively related to the dynamic absorption
coefficient.
(4) The activated carbon samples after three cycles have
the highest pore content in the range of 5 Å < d < 6 Å.
However, when the cycle is more than three times, the
pore aperture range at 5 Å < d < 6 Å becomes larger.
(b)
500
250
200
150
N9V8
100
N3V8
N3V10
50
N9V10
N0V0
0
0.0
0.2
0.4
0.6
Relative Pressure (P/Po )
0.8
1.0
Incremental Surface Area (m²/g)
Quantity Adsorbed (cm³/g )
300
N9V8
N3V10
N3V8
N9V10
N0V0
400
300
200
100
0
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5 10.0
Pore Width (Å)
Fig.7 Adsorption and desorption isotherm (a) and aperture distribution curves (b) of different activated carbon samples
13
5824
Therefore, the three to four cycles and closed-loop
regeneration method at a low gas flow rate are conducive to
improving the micropore content of activated carbon which
plays a main role in the 222Rn adsorption process, to obtain
the regenerated activated carbon with the best 222Rn adsorption performance.
Acknowledgements This study was supported by the National Natural Science Foundation of China (12075112). The authors would like
to thank all the reviewers who participated in the review, as well as
MJEditor (www.​mjedi​tor.​com) for providing English editing services
during the preparation of this manuscript.
Declarations
Journal of Radioanalytical and Nuclear Chemistry (2022) 331:5817–5824
6.
7.
8.
9.
10.
11.
Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
12.
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