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DECONTAMINATION OF WATER POLLUTED BY RADON USING MEMBRANES WITH NANOPORES
Pavel DANIHELKAa, Jana SUCHÁNKOVÁa, Marek ČÁSLAVSKÝb, Lubomír KŘÍŽb
VSB - Technical University of Ostrava, Ostrava, Czech Republic, EU, [email protected]
Vodní zdroje Chrudim, spol. s r. o., Chrudim, Czech Republic, EU
Abstract
The paper focuses on the possibility of utilization of membranes with nano-pores for decontamination of
radon-polluted groundwater. The idea of using nano-pores for the separation of radon resulted from the
hypothesis based on the assumption that the existence of nano-pores would support the rate and efficiency
of radon removal by increasing the phase interface area where the pollutant removal occurs and by
shortening or eliminating diffusion paths in the solid phase. The paper also presents the results of a pilot
experimental study on a basis of which a device was designed that has been patented. The device is
capable of removing radon from water in small water sources and uses hydrophobic membranes with nanopores that allow transition of radon atoms to the gaseous phase whereas the water surface in pores forms a
phase interface at which the separation occurs.
Keywords: nano-pores membrane, radon, water decontamination
1
1.1
INTRODUCTION
Radon as a health problem and its relation to nanotechnologies
In nature, radon (Rn-222) is continuously produced by the decay of radium, which is found virtually in all
rocks as a trace element. Acidic igneous rocks (such as granites, syenites or pegmatites) exhibit highest
concentrations of radium. Radon enters the groundwater by diffusion from the parent rock and by radioactive
decay of dissolved radium. Radon in water can migrate over long distances. Radon itself is not very
dangerous for human body [1] as after being inhaled it is in large part exhaled back.
The health risk is associated with the occurrence of short-period radon decay products (progeny), which are,
unlike gaseous radon, metallic in nature. In the air, the radon progeny attach to small dust particles and thus
create radioactive aerosols. After inhalation they settle in the bronchial tubes or lungs and continue to decay
irradiating the cells of the lower respiratory tract for a long time. Therefore, radon is a risk factor for lung
cancer. [2]. From the perspective of nano-science, it is also interesting that a large portion of solid decay
products of radon gas occur naturally in the form of nano-scale particles.
The estimation of the increment of airborne Rn-222 in a dwelling that arises from the use of water that
contains dissolved radon is a complex problem. It involves the solubility of radon in water, the amount of
water used in the dwelling, the volume of the dwelling, and the ventilation rate. The amount of waterborne
radon escaping into the air is different throughout a dwelling but is higher in areas of active water use such
as bathrooms and kitchens. It also depends on the radon concentration in the water and the activities that
are taking place. [3]
More than two-thirds of the bedrock of the Czech Republic consists of metamorphic or igneous rocks.
Therefore, increased attention should be paid to radon derived from bedrock and thence penetrating into
buildings [4]. The issue of removing radon from water is part of the radiation protection of the public in the
Czech Republic, but the issue of radon occurrence in individual water supply sources is not addressed
sufficiently.
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1.2
The principle of utilization of nanotechnology for removing radon from water
One of the options of radon removal from water is its adsorption by activated carbon. It is a process that can
be considered as nanotechnology as interactions occur at the molecular level. According to the European
Commission Recommendation of 18 October 2011, also material with the specific surface area by volume
greater than 60 m2 / cm3 is considered a nanomaterial. The principle of the method is therefore taking
advantage of the activated carbon enormous surface. The efficiency reaches up to 95% and it is the second
most frequent method of radon removal from water used in practice, aeration being the most frequent one. It
was originally assumed that adsorption on activated carbon could be an alternative to aeration, especially for
small water systems. However, it does not meet the requirements of Best Available Technology (BAT) and,
as a result, U.S. EPA did not find it suitable. It shifts the issue of polluted water to the issue of solid
radioactive waste that is newly formed when radon is captured and its solid decay products are subsequently
fixated on activated carbon. This relates to the requirements to place the activated carbon cartridges in an
isolated place in a building, as fully saturated carbon may be a source of ionizing radiation. Another negative
aspect is the fact that over time (in days or months), the efficiency of the method is reduced as the active
surface of pores clog not only with trapped dissolved gases, but also with sludge matters or with iron and
manganese precipitates. Activated carbon bed must then be replaced. [3]
Aeration techniques are also used for radon removal. They take advantage of the maximization of the gasliquid phase interface area in which the transition of radon from the water surface to the gaseous phase
occurs resulting in its removal. The direct contact of the two phases is realized in two ways. Either air is
bubbled through water, or water is sprayed in small droplets. Aeration is an effective and highly efficient
radon removal technique (efficiency up to 99%). However, it is not quite suitable for small water sources
where purchase, space or operating costs are not favorable for the user. The disadvantages of aeration may
include corrosion of technological parts or formation of mineral deposits. Another disadvantage that aeration
entails is the risk of microbial contamination of the water which can occur when the liquid and gaseous
phases are in direct contact.
Due to the fact that both of these technologies have limitations for use in small water sources, other
possibilities of nanotechnology were searched for. Based on the hypothesis that we can significantly
increase the active surface and, to a certain extent, we are able to work with pressure systems when we
apply to the surface of contaminated water a hydrophobic membrane with holes of such dimensions that the
water surface tension does not allow, even if it is slightly over-pressured (0.1 – 0.2 MPa), to penetrate the
liquid phase through the membrane, while enough area of the water surface is kept in direct contact with the
gas on the other side of the membrane to allow radon diffuse rapidly to the gaseous phase. It turned out that
membranes in the shape of fibers (large active surface) with a pore size of tens to hundreds of nanometers
are suitable for such purposes.
2
2.1
EXPERIMENTAL PART
Preparation of experiment
To verify the described hypothesis, a field experiment was prepared preceded by several considerations and
preliminary experiments whose results are briefly described in the following text.
A membrane can be defined as a selective barrier between two environments that allows transport of
selected particles. If a membrane is inserted between the liquid phase and the gaseous phase, a mechanical
separation of the two phases occurs. Places inside the membrane wall - transverse pores - are the only
points of direct contact of the two phases. Depending on the pressure on one or the other side of the
membrane and depending on the surface tension of water on the surface of the membrane material, the
pores contain either liquid or gaseous phase.
A dynamic equilibrium establishes at the liquid-gas phase interface due to diffusion. The decontamination to
be successful, there must be a one-way flow of the material to be separated, in this case from the liquid to
23. - 25. 10. 2012, Brno, Czech Republic, EU
the gas phase. Concentration gradient prevents equilibration. Therefore, the concentration of the separated
substance (radon) at the input of the gaseous phase to the system must be zero or significantly lower
compared to the concentration of that substance in the liquid. If this condition is met and if the flow of the
gaseous phase occurs, then there will be no gradual equalization of the concentrations and equilibration at
the phase interface. The decontamination process will run efficiently and continuously.
As the efficiency of the static process of contaminant removal controlled by diffusion itself is not sufficient for
real use in practice, several steps to promote the process based on previous experiments were
recommended. They include the introduction of dynamic conditions in both phases, ie., the implementation of
a forced flow of the gas and liquid phases through the system, and the flow of the two phases in opposite
directions to each other. To maximize the active surface, a membrane in the form of hollow fibers with its
walls containing the nano-pores was chosen (Figure 1).
Fig. 1: Electrone microscope image of a membrane with nano-pores
Due to the fact that the diffusion rate of radon in water is several orders of magnitude lower than in the air
(diffusion coefficient of radon is 1.15.10-9 m2s-1 in water and 1.02.10-5 m2s-1 in the air, at 18 °C [5]), and
to prevent the penetration of water into hollow fibers, a hydrophobic membrane material was given
preference. The hydrophobic material supports that the gas phase be contained in the pores. This is
advantageous to support the process of radon removal as radon does not have to diffuse through the liquid
phase in the membrane wall.
2.2
Description of experiments
The experiments were conducted using a special module with a membrane. The module consisted of a
cylindrical tube, a membrane and of components that allow supply and discharge of the liquid and the
gaseous phase. The functional module element consisted of a hydrophobic membrane – a bundle of
polyethylene hollow fibers. The bundle with approximately 1 500 fibers was inserted in the module
longitudinal axis and crossed the center of the module along its whole length. The module of 75 cm had a
total of four holes. Two holes were in the longitudinal axis of the module allowing access to the fibers. Two
holes were in the module housing, each at a distance of 3.5 cm from the tube edges. Holes in the housing
module were located on a the same line parallel to the longitudinal axis of the module and allowed the flow of
media between fibers. All openings together served for the input and output of the media, in this case, water
and air. All inlets and outlets were equipped with regulatory elements. The module housing with the outer
diameter of 6 cm served also as a membrane protection.
The aim of the experiment was to verify the possibility of utilizing membrane with nano-pores for radon
separation from water. The effectiveness of the membrane was tested in such a way that the activity
concentration of radon was measured in the water at the inlet and outlet to / from the module at specific
water flow rates. To verify the effect of water flow rate on the efficiency of the separation process, the
experiments were performed at different flow rates.
23. - 25. 10. 2012, Brno, Czech Republic, EU
The arrangement of experimental apparatus
The module was installed in an upright position with the counter-current flow of the liquid and gaseous
phases. Water pumped from the well passed through the module and then was discharged into the drainage
system outside the workplace. The radon activity concentration measurement was analyzed according to the
methodology for Radim 3W radon monitor.
Due to the fact that there are two ways of how to arrange the module (according to whether one of the media
flows either outside or inside the fiber), it was necessary to compare the effectiveness of both arrangements.
In the first arrangement, the water entered the inside of the fibers and the air flowed at the outside of the
fibers. In the second arrangement, the air entered into the fibers and the water flowed between the fibers in
the space between the bundle and the module housing. The flow rates are shown in the next chapter.
3
RESULTS AND DISCUSSION
The first part of the experiment was performed in the arrangement with water flowing inside the fibers and
the air flowing outside of them. Measurement was carried out at flow rates in the range of 0.002 to 0.1 l/s.
The measurement results are shown in Figure 2 below.
Fig. 2: Separation efficiency at configuration with water flowing inside the fibers
The results of the second part of the measurement when the water flowed outside the fibers and the air
flowed inside of them are shown in Figure 3. The experiment was performed at flow rates of 0.05 to 0.2 l/s.
Fig. 3: Separation efficiency at configuration with water flowing outside the fibers
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The presented results are of a pilot nature and demonstrate the difference between the two configurations of
the systems. As the data show, a higher water flow rate through the module and a higher efficiency were
achieved with the second arrangement. In the first case, high pressure losses seem to have occurred when
the fluid flowed through the fibers that can be represented by very narrow capillaries with high hydraulic
resistance. The radon separation process was hindered and a higher efficiency was achieved only at very
low flow rates. Low efficiency could also be caused the condensation of water vapor from the air at the outer
wall of the membrane as a result of the temperature gradient between the temperature of the well water
inside the fibers and the ambient temperature. The resulting droplets may have significantly slow down the
diffusion from the membrane to the ambient space. The process could not be considered as promising for
practical use. In the second case, twice as high efficiency was achieved at a flow rate that was two times as
high than the maximum flow rate in the first variant. At lower flow rates, the efficiency of up to 65% was
achieved. Based on these pilot results, it was decided that the follow-up experiments would be carried out in
the arrangement where the water would flow outside the fibers and the air inside of them.
The use of membranes with nano-pores in the form of fibers proved promising for the radon removal from
water by the transition of radon into the gaseous phase. In follow-up research a number of other modules
were created that were gradually technically adapted to the point where the efficiency of the final module
(Figure 4) was high enough to reduce the contamination of real pollution below the limits required applicable
legislature. The limits are set out in the Decree No. 307/2002 Coll. [6] and are based on values required for
drinking water. The limit in question is the radon activity concentration, which is 300, or 50 Bq/l, respectively.
If the activity concentration is higher than 300 Bq/l, the water must not be distributed. If the radon content is
higher than 50 Bq/l, it is required to assess whether it would be appropriate to remove radon from the water,
i.e., costs and benefits of such measures are compared.
Fig. 4: Patented radon removal device
4
CONCLUSION
Out of the two arrangements of the system tested, the arrangement with the flow of the gaseous phase
inside the fibers and the liquid phase outside the fibers proved better for decontamination. Based on the
results of the experiments, a number of other separation modules were prepared. Subsequent testing and
technical improvements of the gradually developed modules resulted in the construction of the final device
that has been successfully patented. It is a device designed for the decontamination of small water sources
from radon, particularly suitable for installation in areas with granitic bedrock where groundwater is used in
households, and thus it can help reduce risks to human health.
The results of the experiment confirmed the hypothesis formulated by the authors. It was experimentally
confirmed that the use of nano-pores in the membrane helps to maximize the active surface and, together
23. - 25. 10. 2012, Brno, Czech Republic, EU
with a particular arrangement of the resultant device, it supports radon removal from water. The applied
membrane with nano-pores therefore allows a relatively rapid and effective transfer of the contaminant in a
compact device.
ACKNOWLEDGMENT
Experiments described in this paper were realized thanks to financial support of Ministry of Industry
and Trade of the Czech Republic, project number 2A-1TP1/044.
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Česká geologická služba. Radonové riziko [online]. c2012, [cit. 2012-08-30].
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