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ISSN : 1226-0088(Print)
ISSN : 2288-7253(Online)

Membrane Journal Vol.32 No.1 pp.13-22
DOI : https://doi.org/10.14579/MEMBRANE_JOURNAL.2022.32.1.13

Recovery of Valuable Minerals from Sea Water by Membrane Separation and Adsorption Process: A Review

Sungsu Jeon^*, Rajkumar Patel^**†

^*Integrated Science and Engineering Division (ISED), Underwood International College, Yonsei University, Incheon 21983, South Korea
^**Energy and Environmental Science and Engineering (EESE), Integrated Science and Engineering Division (ISED), Underwood International College, Yonsei University, Incheon 21983, South Korea

^†Corresponding author(e-mail: rajkumar@yonsei.ac.kr; http://orcid.org/0000-0002-3820-141X)

Received February 15, 2022 ; Review ; Accepted February 22, 2022

Abstract

Ever increasing global energy demand gives rise to uncontrollable environmental pollution. Demand on fossil fuel and consequent carbon emission leads to global warming and climate change. Nuclear energy is an alternative source to generate clean energy but mining of nuclear fuel is associated with harmful chemicals. Mining of valuable minerals from sea water by membrane separation process is a cost effective along with environmental friendly process. Separation and adsorption based mining of valuable minerals from sea water are another efficient process. Recovery of actinides from rare earth elements are very challenging and expensive process. Pressure driven membrane separation process is economically more viable along with environmental process. In this review membrane separation process are based on polyether sulfone, polyamide, polyimide, polyamidoxine and hybrid membranes. In case of adsorption process, mainly amidoxime kind of adsorbent are discussed.

Key Words : sea water , membrane , polyamidoxine , adsorption

막 분리와 흡착 과정을 통한 해수로부터의 주요 광물 회수: 리뷰

전 성 수^*, 라즈쿠마 파텔^**†

^*연세대학교 언더우드학부 융합과학공학부
^**연세대학교 언더우드학부 융합과학공학부 에너지환경융합전공

초록

세계적인 에너지 수요의 증가는 통제할 수 없는 환경 오염을 야기하고 있다. 화석 연료에 대한 수요와 그로 인한 탄소 배출이 지구 온난화와 기후 변화로 이어진 것이다. 핵에너지는 청정 에너지를 생산하는 대체 자원이지만 핵연료 채굴은 유해한 화학물질과 관련이 있다. 반면에 막 분리 과정을 통해 바닷물에서 중요 광물을 채굴하는 것은 효율적이며 친환경적이 다. 분리와 흡착을 통해 해수로부터 주요 광물을 채굴하는 것은 또 다른 효율적인 과정이다. 희토류 원소에서 악티늄족을 회 수하는 것은 매우 어렵고 고비용의 과정이다. 압력 기반 막 분리 과정은 친환경적일 뿐만 아니라 경제적으로 실현가능한 과 정이기도 하다. 본 리뷰에서 다루는 막 공정에는 폴리에테르 설폰, 폴리아미드, 폴리이미드, 폴리아미독신 및 하이브리드 막 이 있다. 또한 흡착 공정의 경우, 주로 아미독심 종류의 흡착제가 논의될 것이다.

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1. Introduction

In recent year there is due to severe weather change there is drastic rise in temperature and scarcity of rainfall that cause uncontrollable forest fire. Currently there is urgent need to cut fossil fuel and replace it with alternative clean source of energy. Nuclear power is a substitute of fossil fuel in terms of renewable energy. Naturally is used as fuel in the nuclear plant which is present in the low concentration in sea water. Removal of nuclear effluents from the discharge of power plant mining of uranium from sea water is essential to control pollution of environment and economical power generation from nuclear power plant[1-6].

Extraction of radioactive mineral can be done by methods such as membrane separation, adsorption, precipitation and ion exchange methods. Membrane based separation method are efficient and economical for mining of the valuable minerals. Polymers based on amidomixe are widely used from the extraction of uranium present in sea water or wastewater due to its high selectivity and chelating ability. Surface area of the membrane are enhanced by preparing nanofiber membrane[7-10].

Mining of precious minerals form sea water is done by adsorption method. Silica based materials like MCM-41, SBA-15 are widely used for uranium (VI) sorbent mainly due to chemical stability, large pore size and higher surface area. Silica microsphere with amidoxime functional group are another class of sorbent materials for precious minerals[11-18]. This review is broadly divided into two membrane based mining and removal or extraction by adsorbents.

2. Membrane Based Separation

Generation of clean energy is quite essential in order to control environmental pollution. Mining of earth is an age old process but with continuous depletion of important minerals, now a day there is lot of focus on mining of mineral resources from sea water. Nuclear energy is an alternative source of clean energy for which liquid fluoride thorium reactors (LFTR) process is next generation technology. It is about 250 time more efficient then uranium based process and thorium is he fuel used in this reactor. Thorium can be extracted by solid extraction process with higher efficiency and selectivity using covalent organic framework (COF) as the solid adsorbent[19]. [NH4]⁺ [COF SO₃^- is an excellent adsorbent of thorium and selectivity over uranium is more than nine times. It captures 395 mg/g of Th(IV).

2.1. Polyether sulfone

Suresh et al. worked for creating robust uranium-selective membrane absorbers to sequester uranium from seawater[20]. They represented the synthesis and characterization of polyacid-functionalized membranes which can extract uranium. Fig. 2 represents polyether sulfonegraft- membranes prepared by the “grafting-from” technique using photopolymerization under UV light exposure.

Generally, membrane permeability becomes smaller when polymerization time and degree of grafting increase. Plotting DI water is used to measure the pure water permeability coefficient of the membrane. The permeability coefficient of the unmodified PES membranes for pure water was 223 ± 20 L m^-2 h^-1 bar^-1, and a values of 232 ± 12, 145 ± 20, and 287 ± 87 L m^-2 h^-1 bar^-1 are obtained when grafting of side chain, times are 10, 20, and 30 minutes, respectively. Fig. 3 represents the permeability changes between 0 and 25 min, and Fig 4. represents Langmuir adsorption model.

The equilibrium adsorption experiments were conducted with uranium-spiked seawater to check the membrane affinity and capacity towards uranyl in presence of competing ions. The equilibrium pH was adjusted at 8.2 for matching the real conditions of seawater. The ionic strength of the solution, pH, and swelling of the polymer network had effects on the membrane capacity. In synthetic seawater (pH 4.08), 15.3 mg of U was the maximum capacity, and in DI water, 13.3 mg of U was the capacity for the prepared membranes. Also, a lower degree of grafting lets membranes recover much more uranium for a given feed condition during filtration. This study showed the way to use polymer-grafted membrane adsorbers for getting uranium from seawater.

Yang et al. conducted a study on a way to extract uranium from seawater by positively charged CMPs (conjugated microporous polymers)[21]. It can be improved by solving problems on poor selectivity, antifouling, and slow sorption rate. In the experiment, synthesized CMPs (modified with oxime and carboxyl) showed fast sorption for uranium (0.46 mg g⁻¹ day⁻¹), a high selectivity against vanadium and other ions, and with a high sorption capacity ratio of U/V (8.4) in real seawater. Also, the GMPs skeleton has a high affinity for uranium, which is absorbed by the interaction of oxime/carboxyl ligands with hydantoin even with low concentration in seawater. In addition, antifouling against E. coli and S. aureus could be found remarkably. This work showed a promising material for uranium extraction by designing absorbents.

2.2. Polyamide

Gao et al. had research on efficient membranes for extracting actinides from rare earth elements (REEs) and purifying wastewater with toxic heavy metal ions (HMIs)[22]. They used peptide-carbon hybrid membrane to recover uranium and thorium from REES by using high pressure-driven filtration. It was composed of peptides, paper pulp, and activated carbon. This hybrid membrane had high saturation absorption capacities (Qm) and high distribution coefficients (4 to 5 orders of magnitude higher than others). It is attributed to the combination of SAC and peptides. Moreover, it could reduce the concentrations of HMIs to purify wastewater, which can lead to further developments of water purification applications. This study on the constituents of the membranes and removing actinides from REEs showed significance for resolving a scant supply of nuclear fuels as well as radioactive contamination in REE.

2.3. Polyimide

Luo et al. synthesized a microporous membrane derived from biomass, based on the interfacial synthesis of excellent metal–phenolic networks (MPNs) to improve the limitation on a low concentration of uranium[23]. They formed robust supramolecular networks with uranium on commercial porous membranes by using natural biomass polyphenols. These membranes showed the high quality of kinetics, capacity, selectivity, and renewability in the testing. Polyphenol- based membrane functionalization leads to effective absorption of uranium from real water which indicates the economy viability of the materials. In the marine test the total long term uranium adsorption from 10 L sea water is 27. 81μg which is almost nine times higher than conventional methods. Desorption of the adsorbed uranium on the membrane surface is almost 97 % without affecting the membrane efficiency. This work anticipated the possibility to make the best use of these materials to extract uranium from seawater economically.

2.4. Polyamidoxine

Although adsorbents based on amidoxime are well established for excellent uranium adsorption but bacterial fouling is a key issues need to be addressed[24]. In this situation, Sun et al. explored an antibiofouling ultrathin poly(amidoxime) membrane (AUPM) with high antibacterial property and a great recovery performance. Fig. 4 shows its fabrication process of the membrane.

The experiment on higher adsorption capacity of the AUPM membrane indicated that adding S-CNC could improve the uranium adsorption performance with improved hydrophilicity. Also, the enhanced mechanical property of the AUPM having tensile strength of 17.21 MPa for membrane thickness of 10 μm helps to with stand of harsh seawater environment. This ultrathin AUPM had an improved specific surface area and increased uranium adsorption speed. On the other hand, the antibiofouling property was measured through an experiment using E. coli, S. aureus, and V. alginolyticus. This antibacterial ability can kill bacteria and prevent biofouling, ultimately increasing U sorption capacity, due to the broadspectrum antibacterial quaternary ammonium groups of the quaternized chitosan (Q-CS). Fig. 5 represents the result of the experiment for measuring antibacterial performances of AUPM.

Because of both the great mechanical strength and a good antibiofouling property of AUPM, fabricating adsorbents efficient for the removal of U was possible in both alkaline and acidic wastewater. AUPM membrane treated water with 100 ppb U concentration in pure water having pH about 5 and in seawater with pH 8 for 48 h recover 93.7% U and 91.4% U, respectively.

Furthermore, U-absorption of AUPM increased 37.4% reaching from 6.39 to 8.78 mg/g, even maintaining constant tensile strength during 10 cycles of absorption- desorption. This AUPM has a highly efficient U-recovery capacity appliable to both natural seawater and U-containing waste seawater or fresh water, and it would provide a way to fabricate low-cost U-adsorbents with antibacterial property and a strong mechanical performance.

2.5. Hybrid

Radioactive contamination is needed to be discussed in the nuclear industry. Mainly based on quick pollution element detection, its not easy to push the limit as low as possible to make it highly sensitive Trace of uranyl ions are monitored by functionalized solid nanochannels[25]. Fig. 5 and 6 represent the operational principle of this platform.

Concentration signal can be converted into transmembrane which is visible by alternation of surface conditions in nanochannels. A detection limit of the platform was 1 fM, and uranyl removal property was shown. The mesenchymal stem cells with uranyl could have high viability with the membranes. The visible recemt change reversibly reflected a little difference in UO₂²⁺ concentration. In addition, applying the membrane in culture media with UO₂²⁺ improving MSCs’ spreading and proliferation indicated that the system could be applied in the removal of UO₂²⁺. This study made a path to controlling global radioactive contamination of water.

3. Adsorption Based Removal

Semiconductor industry is highly dependent on gallium arsenide (GaAs) and gallium nitride (GaN). Vanadium is essential material to prepare ferrous and non-ferrous alloy and vanadium redox battery. Zhao et al. studied the extraction of Ga (III) and V (V) by adsorption on to commercial amidoxime resin (LSC700) and found that 29.24 and 22.60 mg/g are adsorbed respectively[26]. Amidoxime group present in LSC700 form complex with [Ga(OH)4]^-. This mode of interaction is presented in Fig. 8 and 9.

3.1. Amidoxime

Using amidoxime-based resins for uranium recovery has been in the spotlight due to their high affinity[27]. Hamza et al. applied magnetic chitosan micro-particles modified by amidoxime-grafting. The amidoximation of chitosan had an effect on optimum adsorption at pH 4 for U(VI) and pH 5 for Eu(III). The sorbent treated as magnetic micro-particles showed fast metal sorption. Uptake kinetics were fast: 60–90 min was enough to reach the equilibrium, as the resistance was limited by the thin layer of polymer and small size adsorbent particles. Maximum sorption capacity got to 1.5 mmol U g^-1 and 2.47 mmol Eu g^-1 under optimal conditions. Sorption tests were conducted in bi-component solutions in equimolar quantities, showing that U(VI) were enriched at pH 4.9 with retention of capacities of sorption and Eu(III) metal binding is selective at pH 2.3 with an extreme decrease in sorption capacities. By using 0.5 M HCl solutions, about 95% of Eu(III) and all U(VI) were desorbed within 30–90 min. Moreover, the sorbent is efficient for recycling, maintained more than 5 cycles of sorption and desorption. It is mentioned that magnetic chitosan microparticles functionalized with amidoxime would be useful for metal recovery and separation. More studies are needed on mining of leachates in chromatographic columns packed with functionalized magnetic chitosan beads.

For the specific adsorption of uranyl ions onto a poly(acrylamidoxime), identifying the way they interact with uranyl (UO₂²⁺) is needed to study a better extractant[28]. A problem of the amidoxime sorbents is competition from vanadium ions (VO₂⁺) that damages the sorbent by stripping them. Therefore, the adjacent binding sites in di(amidoxime) functionalized with imidazole group interactions with UO₂²⁺ and VO₂⁺ ions are studied. Analyzing the structure ultimately indicated that cation exchange with acidic protons let diamidoxime ligands possible to coordinate UO₂²⁺ and VO₂⁺ ions in water (UO₂²⁺ with two adjacent amidoximate functional groups / VO²⁺ with only one group). This difference between two ions means that even if both UO₂²⁺ and VO₂⁺ coordinates to the ligand, the interactions and results in geometries will be very different. The structures also showed that the R-group of amidoxime would be a factor to affect the interactions of amidoximes with vanadium and uranium ions.

For the advancement of amidoxime-based absorbents for uranium extraction, an investigation of the influence of temperature on U(VI) and V(V) absorption was needed[29]. Kuo et al. had some experiments to figure out temperature effects on U and V adsorption from seawater using two efficients adsorbents (AF1 and AI8). Analyzing the values of enthalpy and entropy of the adsorption of U(VI) and V(V) with amidoxime, it can be concluded that the sorption of U(VI) was very endothermic, but the sorption of V(V) showed lower temperature sensitivity. It was tested at 8, 20, and 31°C, and the sorption of U produced entropies of 314 ± 21 and 320 ± 36 J K^-1 mol^-1 and enthalpies of 57 ± 6.0 and 59 ± 11 kJ mol^-1. On the other hand, the sorption of V showed entropies of 164 ± 20 and 103 ± 19 J K^-1 mol^-1 and enthalpies of 6.1 ± 5.9 and –11 ± 5.7 kJ mol^-1. These results revealed that amidoxime-based adsorbents can lead the improvements of uranium sequestration performance by increasing selectivity in warmer waters and U adsorption capacities. To conclude, the high temperature dependence showed that warmer seawater with amidoxime-based adsorbents can make seawater uranium extraction more efficient.

3.2. Polyethylene fiber

Oyola et al. described the problem of lower uranium concentration than other metal salts, so they suggested grafting different acid monomers on polyethylene (PE) fibers for the uranium absorption[30]. These absorbents were prepared by using PE fiber with high surface area having hollow gear morphology. Acrylonitrile (AN) along with several acidic monomers were co-grafted in order to enhance the hydrophilicity of the adsorbent. It was found that it affected the uranium absorption in this order: acrylic acid (AA) < vinyl sulfonic acid (VSA) < methacrylic acid (MAA) < itaconic acid (ITA) < vinyl phosphonic acid (VPA). On the other hand, under a more realistic testing situation, the uranium absorption capacity increased in the order: MAA < AA (Mohr’s salt) < VSA < ITA (Mohr’s salt) < ITA < VPA, because of lower co-polymerization of the monomers. The increased capacity of acrylic acid was attributed to adding Mohr’s salt, and this trend is connected to the higher the grafting of AN then the number of amidoxime (AO) groups on each of the adsorbents will be higher. These recently suggested AN-acid monomer grafted adsorbents can lower the cost for extracting uranium and increase efficiency with their higher absorption capacity.

4. Conclusions

There is growing global concern of energy crisis due to ever increasing dependency on fossil energy. Nuclear energy is alternative clean energy source which is dependent on nuclear fuel. Unfortunately, there is no abundant mineral source on earth available to supply to uranium based light water reactors. Liquid fluoride thorium reactors are the alternative reactor of the future which is much more efficient then uranium based reactor. Valuable mineral can be mined from seawater which is abundantly available but economic viability is the key factor. In order to extract these mineral membrane separation and adsorption are very economical. Aldoxime functionl group attached to membrane or adsorbent are excellent material for extraction of actinides based minerals. This review is mainly divided into two category of membrane based separation and adsorption.

Figures

Fig. 1.

Schematic representation of mineral extraction

Fig. 2.

Fig. 3.

Degree of grafting increases linearly with the length of the reaction. The permeability coefficient, A, of the membranes decreases with an increase in the length of reaction (left). Scanning electron microscopy images of the membranes modified for 0, 10, 20, and 30 min (right). Data at 0 min reaction time represents pristine PES membranes. The membranes exhibit pore plugging due to polymer growth through 20 min of UV irradiation. The permeability coefficient of the membrane irradiated for 30 min increases in both the mean value and its relative error, which is supported by the widened pores shown in the 30 min SEM image (Reproduced with permission from Suresh et al., 20, Copyright 2020, American Chemical Society).

Fig 4.

Fig. 5.

(a, b) Different antibacterial performances of AUPM against a few common types of bacteria. (c) The enhanced U-adsorption capacity of the AUPM by the improved antibiofouling in seawater that was not filtered with 0.45 μm filter membrane to remove bacteria/microorganisms (Reproduced with permission from Sun et al., 24, Copyright 2021, American Chemical Society).

Fig. 6.

Fig. 7.

(a) Illustration of the modification and detection processes. (b) SEM images of a single conical nanochannel from the base side (upper) and a cross section of the typical conical PI nanochannel (below). (c) Survey scan XPS spectra of the PI membrane before and after the modification. (d) I–V curves of the single conical nanochannel before and after the modification and that of a detection process of uranyl ion (1 mM) in aqueous solution (pH 5). (e) Zeta potential of PI membranes. (f) Contact angles of the PI membranes at each experimental stage of 71.2 ± 1.7, 58.2 ± 1.9, 37.9 ± 1.3, and 53.5 ± 2.1° and photographs (insets) illustrating the shape of water droplet on the PI membranes (Reproduced with permission from Yang et al., 25, Copyright 2021, American Chemical Society).

Fig. 8.

Fig. 9.

Tables

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