|Year : 2020 | Volume
| Issue : 2 | Page : 100-107
Comparative evaluation of the radioactivity removal efficiency of different commercially available reverse osmosis membranes
Vinod Kumar1, Swayang Siddha Nayak2, Deeksha Katyal3
1 University School of Environment Management (USEM), Guru Gobind Singh Indraprastha University, Dwarka; Division of CBRN Defence, Institute of Nuclear Medicine and Allied Sciences, Timarpur, Delhi, India
2 Division of CBRN Defence, Institute of Nuclear Medicine and Allied Sciences, Timarpur, Delhi, India
3 University School of Environment Management (USEM), Guru Gobind Singh Indraprastha University, Dwarka, India
|Date of Submission||27-Apr-2020|
|Date of Decision||25-May-2020|
|Date of Acceptance||26-Jun-2020|
|Date of Web Publication||27-Aug-2020|
Dr. Deeksha Katyal
School of Environment Management, Guru Gobind Singh Indraprastha University, Dwarka, Delhi - 110 078
Source of Support: None, Conflict of Interest: None
Reverse osmosis (RO) is a water purification process that uses a semi-permeable membrane to remove ions from potable water. It has high rejection throughput, low energy consumption, and negligible pollution load when compared to conventional treatment methods. Comparative percentage salt rejection (SR) efficiency for surrogates of cesium and molybdenum was performed using commercially available membranes. Polyamide, polysulfone, polyamide–polysulfone composite, and cellulose acetate were subjected to various operating conditions in a domestically developed high-pressure membrane test cell. Five different concentrations of surrogate salts ranging from 100 to 500 ppm and varied pressures of 15–17 kg/cm2 combined with varying temperatures of the feed solution from 25°C to 45°C were used in this experimental work. It was found that the %SR efficiency of these membranes increased with the increase in salt concentration and feed temperature. However, a significant decrease in SR was observed with increasing pressure. A comparative study of these commercially available RO membranes was also performed against short-lived radioisotope Technetium-99m, and was determined by activity counts of feed and filtered samples using a well counter. The results suggest that the rejection efficiency was found to be highest in the case of polyamide–polysulfone composite, followed by polysulfone, polyamide, and cellulose acetate.
Keywords: Cellulose acetate membrane, high-pressure membrane test cell, radio-contamination, technetium-99m pertechnetate, thin film composite, well counter
|How to cite this article:|
Kumar V, Nayak SS, Katyal D. Comparative evaluation of the radioactivity removal efficiency of different commercially available reverse osmosis membranes. Radiat Prot Environ 2020;43:100-7
|How to cite this URL:|
Kumar V, Nayak SS, Katyal D. Comparative evaluation of the radioactivity removal efficiency of different commercially available reverse osmosis membranes. Radiat Prot Environ [serial online] 2020 [cited 2020 Oct 30];43:100-7. Available from: https://www.rpe.org.in/text.asp?2020/43/2/100/293622
| Introduction|| |
One of the major problems the world is facing in the 21st century is concerned with issues related to water contamination. Recently, there has been a global concern over radioactive pollution in water and the focus has now shifted toward the development of advanced decontamination systems to combat the issue.,
In the past two decades, problem of elevated levels of naturally occurring radionuclides such as radium isotopes (226 Ra and228 Ra) and uranium (238 U and235 U) in ground and surface water has been observed in several areas and states in the USA, Canada, Greece, India, and Morocco.,, Studies have shown that when exposed to high-level radiations, humans become more susceptible to different health conditions.,,
Other sources of radiations can be identified as medical (nuclear medicine, interventional fluoroscopy, computed tomography, etc.), industrial, and occupational. Nuclear accidents such as Three Mile Island in the USA (1979), Chernobyl in Ukraine (1986), and Fukushima in Japan (2011) have been majorly responsible for exposing the environment to harmful radiations from radioisotopes such as iodine (131 I) and cesium (137 Cs).,,, Therefore, it is crucial to understand the techniques involved in selective detection, quantification, and extraction of radioactive metal ions for the efficient management of nuclear and radiological emergencies.,,
Many conventional water treatment methods have been incorporated in the past to treat water contaminated with heavy metals and radioisotopes such as a combination of chemical oxidation, coagulation–flocculation, sand filtration, and disinfection., There are some studies about the influence of conventional treatments applied at water treatment plants to reduce radioactivity.,,,,, However, these methods have several limiting factors including thermal instability, formation of toxic intermediates, and high energy consumption.
There is a promising scope of membrane technology in the effective removal of heavy metals and radiocontaminants from aqueous solutions.,,, Studies have suggested the effective use of reverse osmosis (RO) in removing heavy metals such as chromium (Cr), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), arsenic (As), lead (Pb), and mercury (Hg) and radioisotopes such as226 Ra,228 Ra238 U, and235 U., When compared to conventional treatment methods, RO has lower ecotoxicity and energy consumption than conventional methods.,,,,, It is fundamentally a pressure-driven water treatment technology where under certain hydraulic operating pressure of about 10–20 bar,, ionic particles separate out from water through a semipermeable membrane. The mechanisms of separation of species are based on processes relating to their size and shape, their ionic charge, and their interactions with the membrane itself.,,
There are a number of commercially available RO membranes used for removal of heavy metal ions and radioisotopes from aqueous solutions. Among them, the most commonly available are cellulose acetate, polyamide, polysulfone, and polyamide–polysulfone composites.
The present work aims to study the comparative efficiency of commercially available RO membranes in removing radioactive metal ions from contaminated water using an in-house developed bespoke test cell, as elaborated in membrane filtration setup The commercially available RO membranes (polysulfone, polyamide–polysulfone composite, polyamide, and cellulose acetate) were obtained from a local Indian market in Kashmiri Gate, New Delhi, for the same. Sodium chloride (NaCl) was used as a standard to study the salt rejection (SR) efficiency of the commercially available RO membranes in the test cell due to its high solubility in water, low reactivity, high equivalent weight, and easy availability. Surrogates such as cesium chloride (CsCl) and sodium molybdate dihydrate (Na2 MoO4.2H2O) were used that have identical chemical properties and size, as like radioactive metal ions of cesium and molybdenum, respectively. This was done to avoid hazardous accidents during handling and disposal of long-lived radioactive waste in the above experimental settings. However, to study the radioactivity removal efficiency of commercially available RO membranes using counting technique, a comparatively safer short-lived radioisotope was used. Technetium-99m (99m Tc) pertechnetate with a half-life of 6 h, was obtained from Regional Center for Radiopharmaceuticals, Board of Radiation and Isotope Technology (BRIT), and Institute of Nuclear Medicine and Allied Sciences (INMAS). It was found that the above-mentioned commercially available RO membranes removed the salts/radioisotopes efficiently in varying degrees.
| Materials and Methods|| |
Analytical grade salts of NaCl (molecular weight: 58.44 g/mol), CsCl (molecular weight: 168.36 g/mol), and Na2 MoO4.2H2O (molecular weight: 241.95 g/mol) were obtained from Sigma-Aldrich (Honeywell Int. Inc., Seelze, Germany). 19.5 mCi of99m Tc was obtained from Regional Center for Radiopharmaceuticals, BRIT, and INMAS, Brig. S.K. Mazumdar Marg, Delhi, India. All aqueous solutions were prepared in deionized water that had been passed through Milli-Q® Synergy 185 water purification system (Millipore, Saint-Quentin-en-Yvelines, France).
Membrane filtration setup
A high pressure membrane test cell (HPMTC) which was able to accommodate a very high transmembrane pressure, up to 1500–2000 psi, with a stainless steel (SS316 grade material) cell, was developed domestically for the membrane filtration tests as shown in [Figure 1]. It consisted of four parallel arrangements of SS cells having an active area of 53 mm2. A pressure support system having an electric motor (1 hp) with a back pressure regulator connected to a feed inlet and a concentrate outlet system was also present. The pressure was adjusted using a gauge and needle valve to the desired pressure range. Prior to all filtration experiments, commercially available RO membranes (polysulfone, polyamide–polysulfone composite, polyamide, and cellulose acetate) were stabilized for at least 4 h by filtrating deionized water to obtain a stable water flux. The properties of different RO membranes have been summarized in [Table 1]. The initial temperature of the feed solution and conductivity measurements were done using sensors to avoid cross-contamination, as elaborated in instruments used.
|Table 1: Properties of different commercially available reverse osmosis membranes used in the experiment|
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Multiline 3620 Set G multi-parameter kit from Merck & Co., United States, was used to measure the conductivity and temperature of individual water samples (feed and filtered).
Well counter and dose calibrator
PNS2 well counter (NaI-Tl) by Para Electronics Enterprises (I) Pvt. Ltd., Mumbai, and a dose calibrator VIK-202 from Comecer Medical System Pvt. Ltd., Mumbai, were used to measure the activity of the water samples (feed and filtered).
Percentage salt rejection efficiency of RO membranes using cold surrogates
Disc-shaped circular cutouts of the four commercially available membranes (polysulfone, polyamide–polysulfone composite, polyamide-hollow fiber, and cellulose acetate) were placed in the four SS cells 1, 2, 3, and 4, respectively. For the experiment, three sets of five different concentrations, namely 100 ppm, 200 ppm, 300 ppm, 400 ppm, and 500 ppm were prepared in fixed volumes to cover a wide range of concentrations reasonably. Suitable quantities of solutes (Na+, Cs+, and Mo6+) from salts of NaCl, CsCl, and Na2 MoO4.2H2O were weighed and mixed in 5 L of deionized water for the same. The behavior of membranes against Na+ was observed as the standard.
The initial conductivity (μS/cm) of the feed was measured using a conductivity meter under constant pressure of 17 kg/cm2, and the temperatures of both the feed solution and the final solution were maintained at approximately 25°C. After the module was run for at least an hour, filtered samples were collected from the four outlet points and conductivity of each was measured. The module was flushed each time with clean deionized water to ensure complete reflux of concentrates. Further, the effect of variable pressure (15, 16, and 17 kg/cm2) and temperature (25°C, 30°C, and 35°C) was tested. The conductivity of feed (Cf) and filtered samples (Cp) was measured for all three surrogates and the % SR was calculated using Eq. (1).
Percentage radioactivity removal efficiency for99m Tc
The initial activity of99m Tc was measured at 19.5 mCi at 11:25 a.m. The radiation field was measured using a dosimeter at 21 mR/h. The inlet pressure was adjusted to 17 kg/cm2, and the temperature of about 35°C was maintained in HPMC to test the removal efficiency of99m Tc. The activity was introduced into 10 L of deionized water via a 10 ml syringe. The flow of filtered water was observed through the four outlets and 10 ml of each filtered water sample was collected at around 12:25 p.m. Simultaneously, 10 ml of sample was withdrawn from the feed. The activity counts of filtered water samples with respect to the feed water was measured at a given time using a dose calibrator. It was found that the activity of 10 ml of feed water contains 0.8 μCi of99m Tc. The filtered water samples did not show any activity in the dose calibrator. Activity counts of 1ml, 2ml, and 3ml samples were taken for the feed and four filtered water samples at the well counter (without Pb shield – background level of 42), as shown in [Table 2]. Using the data obtained, activity concentrations of filtered samples, as shown in [Table 3], were calculated using Eq. (2) and percentage removal of each membrane against the radioisotope was determined using Eq. (3).
Concentration = (0.8/count of feed sample × count of filtered sample) (2)
| Results and Discussion|| |
Salt solutions of NaCl, CsCl, and Na2 MoO4.2H2O were treated using bespoke HPMTC module. Performance of the commercially available RO membranes (polyamide–polysulfone, polysulfone, polyamide, and cellulose acetate) was examined by performing experiments under different operating conditions. The resulting %SR is represented in [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], under various operating conditions. The SR can be found by the following Eq. (1). Furthermore, the percentage radioactivity removal efficiency for the membranes was found using Eq. (3) and is represented in [Table 4].
|Figure 2: Effect of solute concentration (Na+) on % salt rejection efficiency (pressure: 17 kg/cm2, temperature: 25°C)|
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|Figure 3: (a) Effect of solute concentration of Cs+ on % salt rejection efficiency (pressure: 17 kg/cm2, temperature: 25°C). (b) Effect of solute concentration of Mo6+ on % salt rejection efficiency (pressure: 17 kg/cm2, temperature: 25°C)|
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|Figure 4: Effect of temperature on % salt rejection efficiency (Na+) (solute concentration: 100 ppm, pressure: kg/cm2)|
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|Figure 5: (a) Effect of temperature on % salt rejection efficiency for Cs+ (solute concentration: 100 ppm, pressure: kg/cm2). (b) Effect of temperature on % salt rejection efficiency for Mo6+ (solute concentration: 100 ppm, pressure: kg/cm2)|
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|Figure 6: Effect of pressure on % salt rejection efficieency for Na+ (solute concentration: 100 ppm, temperature: 25°C)|
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|Figure 7: (a) Effect of pressure on % salt rejection efficiency for Cs+ (solute concentration: 100 ppm, temperature: 25°C). (b) Effect of pressure on % salt rejection efficiency for Mo6+ (solute concentration: 100 ppm, temperature: 25°C)|
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Effect of salt concentration
The effect of salt concentration (taking Na+ as standard) in feed solution on %SR was studied for different commercially available RO membranes (polyamide–polysulfone, polysulfone, polyamide, and cellulose acetate). The experiment was carried out keeping a constant temperature of 25°C and pressure at 17 kg/cm2. As shown in [Figure 2], as the concentration of the salt increases from 100 ppm to 500 ppm, a decrease in Na+ ions from 99.84% ± 0.01% to 82.98% ± 1% was observed in case of polyamide–polysulfone composite. Similarly, decreasing trends were observed for other RO membranes such as in the case of polysulfone (99.06% ± 0.02%–77.44% ± 1%) and polyamide (99.03% ± 0.02%–77.35% ± 1%) and for cellulose acetate (97.04% ± 0.02%–73.77% ± 1%), SR of Cs+ and Mo6+ was tested and observations were evaluated [Figure 3]a and b]. For Cs+, the highest SR was achieved by polysulfone–polyamide composite at 99.02% ± 0.15% (100 ppm) and reduced to 83.97% ± 1% (500 ppm). The membrane exhibited similar properties for Mo6+. The efficiency reached its highest at 99.14% ± 0.01% (100 ppm) and lowest at 83.88% ± 1% (500 ppm). Cellulose acetate membrane showed the least SR efficiency for all the five concentration ranges in case of both Cs+ and Mo6+. A small decrease in SR was observed from 300 to 400 ppm of salt concentration for all the four membranes. This is possibly because when the salt concentration increases in feed water, the salt passage through the membrane increases. These results are in agreement with studies by Mohammad et al., where the effect of operating conditions on RO membranes was analyzed using different concentrations of Na2 CO3, temperature, and pressure. Alternative sentence could be “Similar conclusions were derived by Shamel and Chung while studying the SR efficiency of RO membranes in sea water”.
Effect of feed solution temperature
The effect of varying feed solution temperatures (i.e., 25°C, 35°C, and 45°C) on the SR efficiency of the different commercially available RO membranes was studied. Taking salt concentration at 100 ppm, the operating pressure was kept at 17 kg/cm2. In the case of Na+ ions, it was observed that the %SR decreased with increasing temperature of the feed solution, i.e., from 25°C to 45°C, as shown in [Figure 4]. The SR efficiency of the RO membranes was found to be as follows polyamide–polysulfone (99.84% ± 0.01% to 98.66% ± 0.02%), polysulfone (99.06% ± 0.02%–98.21% ± 0.01%), polyamide from 99.03% ± 0.02% to 98.04% ± 0.03%, and cellulose acetate from 97.04% ± 0.02% to 95.81 ± 0.06%. Similar trends can be observed for Cs+ and Mo6+, as seen in [Figure 5]a and b. Polyamide–polysulfone composites were highly efficient in rejecting salts from the module at 25°C. In case of both Cs+ and Mo6+, %SR was found to be 98.66% ± 0.02% and 99.14% ± 0.01%, respectively. Polyamide and cellulose acetate membranes temperatures are thermally unstable above 35°C and undergo degradation.,,
Effect of pressure
The effect of pressure on SR for the four RO membranes was studied under constant temperature of 25°C and salt concentration of 100 ppm. It was observed that the increase of pressure from 15 to 17 kg/cm2 results in an increase of SR from 97.66 ± 0.02% to 99.48% ± 0.01% in case of polyamide–polysulfone composite, followed by polysulfone from 97.91% ± 0.02% to 99.06% ± 0.02%, for polyamide from 97.3% ± 0.48% to 99.03 ± 0.02%, and from 95.81% ± 0.06% to 97.04% ± 0.02% for cellulose acetate. The effect of pressure on the SR is shown in [Figure 6]. As RO membranes do not completely retain dissolved salts in feed water, there is always some salt passage through the membrane. Therefore, there is a subsequent decrease in the salt passage with increasing feed water pressure. The permeate concentration is diluted by the higher rate of water flow through the membrane, resulting in an increase in SR. A similar trend was observed for Cs+ and Mo 6+, as seen in [Figure 7]a and b, where polyamide–polysulfone had the highest %SR for both Cs+ and Mo6+ at 98.66% ± 0.02% and 99.14% ± 0.01%, respectively under 17 kg/cm2. The rejection output was the least in the case of cellulose acetate membranes as it undergoes compaction under high operating pressures.
Alternatively Unlike thin-film composites, Cellulose acetate and polyamide membranes, work within a narrow pH range and are comparatively less thermally stable. Yet, cellulose acetate membranes are preferred over other types of RO membranes for the commercial system due to their resistance to fouling under high-pressure conditions, and cost-effectiveness.
Radioactivity removal efficiency
While very few studies related to the removal of99m Tc from the contaminated water have been reported, that using RO membrane technology is almost rare, the results based on the experiment on radioisotope 99mTc suggest that the removal efficiency of individual membranes is in the order polyamide–polysulfone composite (99.59%) > polysulfone (99.58%) > polyamide (99.39%) > cellulose acetate (98.89%), as seen in [Table 4].
| Conclusions|| |
Commercially available RO membranes such as cellulose acetate, polyamide, or thin-film composites (polysulfone–polyamide) can be effectively employed for the treatment of wastewater laden with heavy metals and radioisotopes. It was found that polysulfone–polyamide composites have superior SR efficiency when compared to other membranes under different operating conditions such as salt concentration, pressure, and temperature. This kind of evaluation and comparison of the different available commercial RO membranes will be helpful in developing water purification devices that target radioactive decontamination. Further studies are warranted with other radioisotopes and heavy metals to prove the ultimate removal efficiency of these commercially available RO membranes.
The authors are grateful to Dean, USEM, Guru Gobind Singh Indraprastha University and Director, INMAS, DRDO (Defence Research and Development Organization), for providing the financial support. The authors would also like to acknowledge DST-TMD under the project no. DST/TMD/EWO/WTI/2K18/05 (C), for the financial support of the work.
Financial support and sponsorship
This study was financially supported by Dean, USEM, Guru Gobind Singh Indraprastha University and Director, INMAS, DRDO (Defence Research and Development Organization) and also by DST-TMD under the project no. DST/TMD/EWO/WTI/2K18/05 (C).
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2], [Table 3], [Table 4]