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Enhanced Sr adsorption performance of MnO2-alginate beads in seawater and evaluation of its mechanism
Accepted Manuscript Enhanced Sr adsorption performance of MnO2-alginate beads in seawater and evaluation of its mechanism Hye-Jin Hong, Byoung-Gyu Kim, Jeongsik Hong, Jungho Ryu, Taegong Ryu, Kang-Sup Chung, Hyunchul Kim, In-Su Park PII: DOI: Reference:
S1385-8947(17)30309-1 http://dx.doi.org/10.1016/j.cej.2017.02.132 CEJ 16571
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
21 December 2016 22 February 2017 24 February 2017
Please cite this article as: H-J. Hong, B-G. Kim, J. Hong, J. Ryu, T. Ryu, K-S. Chung, H. Kim, I-S. Park, Enhanced Sr adsorption performance of MnO2-alginate beads in seawater and evaluation of its mechanism, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.02.132
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Enhanced Sr adsorption performance of MnO2-alginate beads in seawater and evaluation of its mechanism
Hye-Jin Hong1, Byoung-Gyu Kim1, Jeongsik Hong1, Jungho Ryu 1, Taegong Ryu1, Kang-Sup Chung1, Hyunchul Kim2, In-Su Park1,*
1
Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 34132, Republic of Korea 2
Environmental Radioactivity Assessment Team, Korea Atomic Energy Research Institute (KAERI), Daejeon 34057, Republic of Korea
*Corresponding author Dr. In-Su Park Tel:+82-42-868-3071, Fax: +82-42-868-3411 E-mail: [email protected] 1
Abstract Alginate bead is a promising adsorbent for Sr ions in solution, but their adsorption performance is limited in seawater due to the competition with other concentrated ions. In this study, MnO2-alginate beads were prepared by immobilizing MnO2 in alginate, and their feasibility for Sr adsorption in seawater was evaluated. MnO2-alginate beads consisted of 16.1% MnO2 and 7.0% alginate, while plain alginate beads contained only 3.7% alginate. That is, immobilizing MnO2 in alginate induced a compact structure in the MnO2-alginate beads, which resulted in a lower amount of moisture. Sr adsorption on MnO2-alginate beads occurred predominantly on the alginate part and partially on the MnO2. Compared with plain alginate beads, MnO2-alginate beads exhibited significantly improved Sr adsorption efficiency in the presence of seawater with a concentration of Na (10,000 ppm), Mg (1,200 ppm), Ca (400 ppm) and K (400 ppm) due to its enhanced Sr selectivity against Na and increased number of Sr adsorption sites. Finally, MnO2-alginate beads exhibited four times higher Sr adsorption efficiency than plain alginate beads in real seawater. These results demonstrate that MnO2-alginate beads are a highly feasible adsorbent for Sr removal from seawater.
Keywords: strontium, alginate bead, hydrous manganese oxide, seawater, adsorption 2
1. Introduction Since the accident at Fukushima Daiichi nuclear plant at 2011, numerous radioactive compounds have been discharged into seawater. Radioactive strontium (Sr) is one of the fission pollutants from nuclear plant which emits beta rays and exhibits a long half-life of 29 to 30 years [1]. Additionally, radioactive Sr exhibits a similar chemistry with calcium (Ca) and accumulates in the bones of living organisms. Therefore, living organisms are exposed to beta rays for a lengthy period of lifetime [2]. Due to the long lifetime, high solubility, and bio-toxicity of Sr, separation and removal of this ion from seawater needs special attention. There are many technologies to separate Sr from a liquid source such as solvent extraction, chemical precipitation and adsorption [3-5]. All methods have their own advantages, but adsorption is appropriate for treatment of a dilute liquid source such as seawater. Previously, we investigated alginate microspheres as a Sr adsorbent. Ca ion cross-linked alginate microspheres exhibited excellent selectivity for Sr. Sr in the liquid phase quickly adsorbed by ion exchange with the Ca ions in the alginate microsphere [6]. However, Sr adsorption on alginate microspheres is limited in seawater due to the competition with other cations such as Na, Ca and Mg. In particular, the chemical properties of Ca ions are similar to those of Sr ions. Therefore, the concentrated Ca ions (400 mg/L) in seawater severely interferes with Sr adsorption on alginate microspheres. To improve the Sr adsorption performance of alginate microspheres in seawater, the Sr selectivity needs to be improved or the number of Sr binding site in the alginate beads should be increased. In this study, hydrous MnO2 is immobilized in alginate beads to improve the Sr adsorption capacity in seawater. Hydrous MnO2 is a cheap and effective material which is widely used for radio nucleotide removal [7]. The typical Kd value for Sr2+ adsorbed onto MnO2 is reported to be 100 mL/g, and the adsorption capacity of Co2+ onto synthetic nano-manganese 3
oxide is as high as 25 mg/g. This shows that MnO2 has good adsorption properties both for Sr2+ and Co 2+ ions. MnO2 possesses several crystallographic forms such as α-, β-, δ-, γ-, and ε-type, because the basic MnO6 octahedra are linked in different ways [8]. Among these forms, δ-MnO2 (also known as birnessite-type MnO2) has gained special attention as an adsorbent for Sr. Birnessite behaves as an ion-sieve material because of exchangeable alkali ions (Na or K) and crystallized water between sheets of lamellar structure. That is, Sr ions exchange with alkali ions in the interlayer or hydrogen in the hydroxyl group of the interlayer water molecules (or surface OH groups). Dyer et al. explained the Sr adsorption mechanism using an ion exchange reaction with Na or K ions at sites located on the crystal water sheet [9]. Additionally, Ghaly et al. proposed a Sr adsorption mechanism on birnessite via exchange between a Sr ion and hydrogen in the hydroxyl group of the interlayer water molecules [10]. In this paper, hydrous MnO2 was synthesized by alkali precipitation methods. The capacity and mechanism of Sr adsorption on MnO2 were investigated. Then, MnO2-alginate beads were prepared and its physico-chemical properties were characterized. Additionally, the Sr adsorption efficiency and Sr selectivity were evaluated and those of plain and MnO2-alginate beads were compared to identify the Sr adsorption mechanism. Finally, a Sr adsorption experiment is carried out in real seawater.
2. Materials and Methods 2.1. Materials For the synthesis of hydrous MnO2, manganese sulfate (MgSO4, Sigma-Aldrich, USA) and potassium permanganate (KMnO4, Sigma-Aldrich, USA) were used. Sodium alginic acid (Junsei, Japan, Product code: 13035-1201) was used for preparing the alginate beads. The molecular weight of used alginic acid is around 220,000. The ratio of the mannuronic and 4
glunonic acid is not available. Calcium chloride dihydrate (CaCl2·2H2O, Junsei, 99%>) was used for cross-linking of the alginate beads. To control the pH of the solution, 0.1 and 1 M hydrochloric acid (HCl, Junsei) and 5 M sodium hydroxide(NaOH, Junsei, 99%>) were applied. In the Sr adsorption experiment, strontium chloride hexahydrate (SrCl2 ·6H2O, Junsei, purity) was used as the Sr source. Sodium chloride (NaCl, OCI company), magnesium chloride hexahydrate (MgCl2·6H2O, Junsei), and potassium chloride (KCl, Junsei) were applied as competitive cations during adsorption. The
90
Sr was purchased from the National
Institute of Standards and Technology (NIST, Gaithersburg, MA, USA). Real seawater was prepared by filtering using a micro-filter with a 0.4 µm pore size before use. The composition of the seawater is shown in table 1.
2.2 Preparation of hydrous MnO2 Hydrous MnO2 was synthesized by the method described by Valsala et al. [11]. First, the mixed solution (0.1 M KMnO4 and 0.14 M MnSO4) was prepared and its color was dark violet. The initial solution pH was approximately 1~2 and it was controlled between 8~9.5 by using the 5 M NaOH solution. The solution pH decreased during the formation of the MnO2 powder. After 3 additions of the 5 M NaOH solution to control the pH , the solution pH remained constant and the final solution’s color changed to brown from purple. Brown hydrous MnO2 powder was settled, filtered with a 0.2 µm filter, washed thoroughly by DI water to remove unreacted chemicals and dried in a 60 °C oven.
2.3 Synthesis of plain and MnO2-alginate beads MnO2-alginate beads were synthesized by the following procedure. The prepared hydrous MnO2 was put into a 2 wt% sodium alginic acid solution. The ratio of MnO2 powder and 5
sodium alginic acid solution was 17:83 (w/w). The mixture was stirred to disperse the MnO2 powder in the solution. Next, the mixture was dropwise added to the 0.5 M CaCl2 solution using a syringe pump under vigorous stirring. The MnO2 and alginic acid mixture immediately changed to a spherical hydrogel. The synthesized MnO2-alginate beads was kept in 0.5 M CaCl2 solution for 12 hours and washed with DI water several times to remove unreacted Ca ions. Plain alginate beads were prepared with same procedure as the MnO2alginate beads except for the MnO2 powder addition.
2.4 Characterization of synthesized adsorbents The morphology of MnO2 powder was characterized by scanning electron microscopy (SEM, EPMA 1610, Shimadzu). The crystalline structure of the composite was analyzed by X-ray diffraction (XRD, smartlab, Rigaku). Thermogravimetric analysis (TGA, Labsys EVO, Setaram) was carried out in a nitrogen gas flow from room temperature to 1400 °C. Fourier transmission infrared spectra (FTIR, FTIR 4100, Jasco) was performed to analyze the chemical structure.
2.5 Sr adsorption experiments In the Sr adsorption experiment, the same volume of plain and MnO2-alginate beads was put into 0.2 L of Sr solution in a bottle and it was shaken at 120 rpm, 25 °C for 7 days. After adsorption, the solution was filtered with a 0.45 µm syringe filter and then used for Sr concentration analysis. For safety reasons, non-radioactive Sr was used in all Sr adsorption experiments. One experiment was carried out to prove similar adsorption behavior of stable Sr and
90
Sr. The
adsorption of the radioactive isotope of Sr (90Sr) was also determined to prove the reliability 6
of MnO2-alginate bead. Approximately 10 Bq/L of solution. The activity of descendant
90
90
90
Sr was spiked in a 50 ppm stable Sr
Sr (half-life: 28.8 yr) through Cerenkov radiation emitted by its
Y (half-life: 64 h) was determined using a liquid scintillation counter
(Quantulus 1220, PerkinElmer, USA). The
90
Y makes secular equilibrium with
90
Sr after a
two-week in-growth period. The counting efficiency was determined using a mixed source of 90
Sr and 90Y in secular equilibrium.
Stable (non-radioactive) Sr and the other cations concentrations were measured via inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Perkin-Elmer, USA). Sr and cation uptake, and Sr adsorption efficiency were calculated using the following equations;
qe (mg / g ) =
(C0 − Ce ) ×V m
C Sr adsorption(%) = 1 − e C0
× 100
where qe is the adsorption capacity (mg/g), C0 (mg/L) and Ce (mg/L) denote the initial and equilibrium concentration, respectively, V is the solution volume (L), and m is the adsorbent mass (g).
3. Results and Discussion
3.1 Hydrous MnO2 Highly efficient hydrous MnO2 powder was prepared and its characteristics, Sr adsorption capacity and mechanisms were investigated before immobilization of MnO2 into the alginate 7
beads. The phase of hydrous MnO2 was characterized by XRD, as shown in Figure 1 (a). Significant XRD peaks at 2θ=12.15°, 36.98° and 66.27° can be well assigned to the (001), (111) and (020) planes of MnO2 with a K-birnessite structure (JCPDS 80-1098, a=5.149 Å, b=2.843 Å, c=7.176 Å), respectively [12]. The prepared MnO2 has a layered structure and the basal plane spacing calculated from the (001) plane is ~0.73 nm. The broadness of other diffraction peaks indicates the nanoscaled crystal size of the prepared MnO2. Elemental analysis discovered that K and Na intercalated into the MnO2 layers. The amount of K and Na in MnO2 is 1.24 mmol/g and 0.54 mmol/g, respectively. Figure 1 (b) shows an FE-SEM image of the synthesized MnO2. The SEM image demonstrates that the MnO2 products have uniform particles with diameters of 300-600 nm, and their surface is covered with protruding ultrathin nanosheets [13]. Figure 1 (c) shows the Sr sorption isotherm of the MnO2 product. The uptake of Sr increased up to approximately 400 mg/L of Sr2+ equilibrium concentration and this MnO2 showed approximately 110 mg/g of maximum Sr adsorption capacity. Interestingly, the amount of the released K and Na during Sr adsorption increased linearly with the Sr uptake (mmol/g) until 100 mg/L of initial Sr concentration. Figure 1(d) shows the correlation between Sr uptake and K, Na release from MnO2 as molar concentration. The slope is almost 0.5, which means that the adsorption of 1 mol of Sr ions caused the release of 2 mol of Na and/or K from MnO2. This result indicates that Sr intercalates via an ion-exchange reaction with Na or K. When the Sr concentration increased to greater than 100 mg/L, the Sr uptake increased rapidly regardless of the amount of released K or Na. That is, Sr exchanged with hydrogen in the hydroxyl group of the interlayer water molecules (or surface OH groups) after consumption of all of the Na or K in MnO2.
8
3.2 MnO2-alginate beads Figure 2 (a) and (b) shows the prepared plain alginate and MnO2-alginate. Transparent plain alginate became black after immobilization of MnO2 into the alginate. The size of both beads was approximately 2.5~2.7 mm. Figure 2 (c) shows the TG analysis of the plain and MnO2-alginate beads. Both alginate beads exhibited a sharp weight loss at approximately 100~150 °C due to the evaporation of moisture. Weight losses in the plain alginate beads and MnO2-alginate beads were almost 95% and 80%, respectively. The moisture content in both the plain and MnO2-alginate beads is very high because they are hydrogel-type adsorbents. Alginate polymer is degraded at 150~420 °C so the alginate content can be derived by considering the weight loss between 150~500 °C [14, 15]. The alginate contents of the plain and MnO2-alginate beads were 3.7% and 7.0%, respectively. This indicates that the MnO2alginate beads contained more alginic acid than the plain beads even though MnO2 was immobilized in the MnO2-alginate beads. That is, immobilizing MnO2 in alginate induced a compact structure in the MnO2-alginate beads, which resulted in a lower amount of moisture. Therefore, the MnO2-alginate beads would have more Sr adsorption sites than the plain alginate beads. Due to the higher alginate content, as well as the immobilized MnO2, the MnO2-alginate beads exhibited a higher density of Sr adsorption sites compared with plain alginate beads. Only the moisture content was decreased after immobilization of the MnO2. Hydrous MnO2 powder also showed an approximately 15% weight loss at 100~200 °C due to water evaporation and a slight weight loss between 200~700 °C due to loss of the crystalline H2O containing in the MnO2 layer. At approximately 750~800 °C, there is a slight decrease in weight associated with the transformation of MnO2 to Mn2O3 [16]. Hydrous MnO2 maintained its phase until 750 °C. MnO2-alginate beads exhibited 13.3% remaining weight at 750 °C. Considering that moisture of hydrous MnO2 (22%) at room temperature, 9
the MnO2 content in MnO2-alginate beads is 16.1%. Figure 2(d) shows the FTIR spectrum of the MnO2 powder, the plain beads and the MnO2alginate beads. Hydrous MnO2 powder shows two absorption bands at 475 and 600 cm-1 which correspond to the characteristic stretching collision of O-Mn-O. The broad band at 3000~4000 cm-1 indicates the presence of plenty of hydroxyl groups (-OH) on hydrous MnO2 [5]. At 1625 cm-1, the H2O deformation peak is observed. It is evident that the prepared MnO2 is in hydrous form. Plain alginate beads also exhibits many absorption bands attributed to various functional groups. A dominant peak at 3000 ~ 3500 cm-1 indicates the existence of the –OH group. The absorption peaks at 1613 and 1417 cm−1 represent the asymme tric and symmetric stretching vibrations of the carboxylate (-COOH) group, respectivel y, [17]. Additionally, the 1027 cm-1 and 1097 cm-1 peaks indicate the presence of OC -OH [18]. The 3610 and 1670 cm-1 peaks indicate the deformation of the hydroxyl (OH -) and carboxylate (-COO) due to the electrostatic interaction attraction with the Ca ion after cross-linking [6]. MnO2-alginate beads show a similar FTIR spectrum as MnO2 due to its high MnO2 content. Peaks belong to plain alginate beads are almost not detectable in the FTIR spectrum. Only the hydroxyl peak (3000~3500 cm-1) is slightly remaining after immobilized in the alginate beads. It revealed that hydrous MnO2 was immobilized in the alginate beads, without unexpected chemical reaction in the organic structure. Figure 3(a) shows the Sr adsorption isotherm of the MnO2-alginate beads. Two isotherm models were applied for analysis of Sr adsorption on the MnO2-alginate beads. They are the Langmuir and Freundlich isotherm models(See supporting information, SI). Sr adsorption on MnO2-alginate beads was well-fit with both isotherm models, but the Langmuir model showed a higher linear regression coefficient (R2=0.99) than the Freundlich mode1 (table 2). The Langmuir adsorption model assumes homogeneous adsorption of adsorbate on substrate 10
with monolayer formation rather Freundlich model is empirical heterogeneous adsorption [19]. So it seems that Sr adsorption on MnO2-alginate bead is homogeneous adsorption. The maximum Sr adsorption capacity (qmax) derived from the Langmuir isotherm is 102.0 mg/g. Sr adsorption capacities of other Sr adsorbents including ion exchange resin are described (table S1 in SI). Among adsorbents, MnO2-alginate bead exhibited excellent maximum Sr adsorption capacity. Several adsorbents such as zeolite exhibited even higher Sr adsorption performance than MnO2-alginate beads. However powder form of adsorbents could not be applied seawater directly due to recovery and handling problem, and then MnO2-alginate bead is considered as effective and highly promising Sr adsorbents. During Sr adsorption on the MnO2-alginate beads, Ca, K and Na were released into solution (Figure 3(a)). It is already known that Sr adsorbed on alginate by ion exchange with Ca in a 1(Sr):1(Ca) molar ratio [6], whereas Sr adsorption on MnO2 powder caused the release of K or Na by the ratio of 1(Sr):2(K or Na) in the low Sr concentration range (<100 mg/L) (section 3.1). From the data analysis of Sr uptake and the released amount of Ca, K and Na, the Sr adsorption mechanism on MnO2-alginate could be determined. The released amount of Ca increased as Sr uptake increased in all concentration ranges. On the other hand, the K concentration increased until the Sr equilibrium concentration reached 25 mg/L and then remained constant, while Na was almost not released into solution. This result indicates that Sr adsorption on MnO2-alginate beads is mainly due to the alginate component. That is, only a limited amount of Sr was adsorbed on the immobilized MnO2 compared with its powdered form because of the blocking of the Sr solution into the trapped MnO2 in the hydrogel-type alginate beads. Figure 3(b) exhibits the Sr uptake and the released Ca, K and Na as molar concentration. Because Sr adsorption on MnO2 powder caused release of K or Na by the ratio of 1(Sr):2(K 11
or Na), the K and Na concentration was divided by 2. Sr uptake of MnO2-alginate beads was slightly higher than the sum of the released Ca and (K+Na)/2, but the trend was closely followed. From this result, the Sr adsorption mechanism of MnO2-alginate beads was determined. Among the 102 mg/g of Sr uptake, 8.8 mg/g (0.1 mmol/g) of the Sr uptake is contributed by the MnO2 and approximately 94 mg/g is achieved by the alginate part of the MnO2-alginate beads.
3.3 Sr adsorption behavior of plain alginate beads vs. MnO2-alginate beads In this section, the Sr adsorption behavior of plain and MnO2-alginate beads are investigated by applying an equal volume (equal number) of beads. Because alginate beads are a hydrogel-type adsorbent, almost 90% of the beads consist of moisture. In this case, dry or wet weight could be an inappropriate unit in evaluating the adsorbent. Therefore, we applied 3 cm3 of beads as a unit system for Sr adsorption. Sr adsorption efficiency and Sr selectivity are discussed to evaluate two hydrogel type adsorbents. The Sr adsorption kinetic and isotherm analysis based on unit volume(L) of plain and MnO2-alginate bead were described (figure S12, table S2-3 in SI).
3.3.1 Sr adsorption efficiency Figure 4 shows the Sr adsorption efficiencies of plain and MnO2-alginate beads according to the initial Sr concentration. When the initial Sr concentration was lower than 10 mg/L, both alginate beads adsorbed the Sr completely. After that, Sr adsorption efficiency decreased because of an insufficient number of Sr binding sites. At a Sr concentration of 200 mg/L, the MnO2-alginate beads showed a Sr adsorption capacity of 51.8%. Considering that the Sr adsorption efficiency of plain alginate beads was only 23.4%, the MnO2-alginate beads 12
showed a significantly improved Sr adsorption capacity. Since MnO2-alginate beads have a high alginate content and MnO2, it exhibits enhanced Sr adsorption efficiency.
3.3.2 Sr selectivity Figure 5(a) shows the Sr adsorption efficiency of plain alginate beads, MnO2-alginate beads and MnO2 powder in the presence of co-existing cations. To simulate seawater, a seawater concentration of Na (10,000 ppm), Mg (1,200 ppm), Ca (400 ppm) and K (400 ppm) was spiked separately in 7 mg/L of Sr solution,. In pure Sr solution, plain alginate beads showed an excellent Sr adsorption efficiency of 93.8%. However, it was severely influenced and decreased with the presence of other cations. In particular, Na and Ca caused a significant decrease of Sr adsorption on the plain alginate beads. Na is a monovalent cation which is not competitive compared with divalent cations such as Ca or Mg, but its concentration is approximately 10,000 ppm. The extremely high concentration caused the Sr adsorption to decrease. Additionally, Ca exhibits similar chemical characteristics as Sr, such as valence, absolute hardness, and ionic radius, and Ca shows a similar ion exchange behavior to Sr [20]. Therefore, Ca in seawater caused the most severe interference to Sr adsorption on plain alginate beads. MnO2-alginate beads showed much better Sr adsorption efficiency than plain alginate beads in conditions containing Na, Mg and Ca. The coexistence of K ions did not severely affect Sr adsorption like the other ions. MnO2-alginate beads exhibited 58.0% and 72.2% Sr adsorption efficiency in the presence of Ca and Mg, respectively. These values were 2.5 and 1.5 times improved compared with those of plain alginate beads. However, the improved Sr adsorption performance of the MnO2-alginate beads was not caused by MnO2 powder because the MnO2 did not have Sr selectivity for Mg or Ca adsorption like the plain alginate beads. 13
Figure 5(b) shows the adsorption efficiency of Na, Mg, Ca and K on the MnO2 powder, the plain alginate beads and the MnO2-alginate beads in this experiment. The plain alginate beads and the MnO2 powder adsorbed 1~4% of the Na, Ca, Mg and K during Sr adsorption. MnO2alginate beads showed a slightly higher Mg, Ca and K adsorption efficiency. Not only was the Sr adsorption efficiency higher, but the Mg and Ca adsorption efficiencies were also improved after immobilization of MnO2. This means that MnO2-alginate beads did not show improved Sr selectivity against Ca or Mg. However, the increased number of Sr adsorption sites in MnO2-alginate resulted in the enhanced Sr adsorption performance in the presence of Ca or Mg. With 10,000 ppm Na, MnO2-alginate beads showed a Sr adsorption efficiency of 72.5% while that of the plain alginate was only 22.4%. MnO2 powder showed a 60% Sr adsorption efficiency despite the extremely high Na concentration. This result revealed that MnO2 exhibited a higher selectivity to Sr against Na. The increase number of Sr adsorption sites due to the high alginate content, as well as the immobilization of MnO2 in the alginate beads, led to the increase of Sr selectivity in conditions with co-existing Na.
3.4 Sr adsorption in real seawater Finally, the Sr adsorption efficiency of plain and MnO2-alginate beads was evaluated in real seawater. Figure 6 shows the Sr adsorption efficiency of plain and MnO2-alginate beads according to reaction time. In DI water, 7 mg/L of Sr was completely adsorbed by both alginate beads. However, the Sr adsorption efficiency of plain and MnO2-alginate beads was significantly decreased by competition with the cations in seawater. Plain alginate beads exhibited only a Sr adsorption efficiency of 13.4%. MnO2-alginate beads also showed a decreased Sr adsorption efficiency (44.3%) in seawater, but it was much higher than that of 14
the plain beads. Because MnO2-alginate beads exhibited many Sr adsorption sites as well as a high selectivity for Sr against Na, it shows an improved Sr adsorption performance in real seawater. This result proved that MnO2-alginate beads are a feasible material to Sr removal in a complex medium such as seawater.
3.5 Sr adsorption behavior of stable Sr and radio-active Sr For the comparison of Sr adsorption behavior of stable Sr and radioactive Sr, 90Sr is spiked in water and the Sr adsorption capacity of stable Sr and 90Sr is evaluated. Table 3 shows Sr the adsorption capacity of plain and MnO2-alginate beads in the presence of 10 Bq/L of Sr. MnO2-alginate beads show a Sr removal efficiency of 97%, whereas the plain alginate beads show only 84% Sr removal. Stable Sr and 90Sr show similar Sr adsorption efficiencies, which proved that the adsorption behavior of radioactive
90
Sr on plain and MnO2-alginate beads is
similar to the behavior of stable Sr.
4. Conclusions
Alginate beads are a promising Sr adsorbent due to their high Sr adsorption performance and abundance of raw material. However, the Sr adsorption performance of alginate beads is limited in a complex medium, such as seawater and radioactive wastewaters, due to the competition of other concentrated ions. In this study, we highly improved the Sr adsorption performance of alginate beads by immobilizing hydrous MnO2 in the beads and the Sr adsorption behavior of the improved beads was evaluated in a seawater medium. MnO2alginate beads consisted of 13.2% MnO2 and 7.0% alginate. Compared with plain alginate beads (alginate content = 3.7%), MnO2-alginate beads had a higher alginate content. Because the MnO2-alginate beads have a compact structure, it showed a higher Sr adsorption 15
efficiency compared with plain alginate beads. Sr adsorption on MnO2-alginate beads predominantly occurred on the alginate part and only 8% of the Sr adsorption was contributed by the MnO2. Compared with plain alginate beads, MnO2-alginate beads exhibited a much higher Sr adsorption efficiency in the presence of seawater with a concentration of Na(10,000 ppm), Mg(1,200 ppm), Ca(400 ppm) and K(400ppm). Because MnO2 exhibits excellent Sr selectivity against Na, the MnO2-alginate beads show a better Sr adsorption performance in a solution containing Na. Additionally, the high alginate content provided sufficient binding sites and the Sr adsorption performance is therefore improved in the presence of Ca or Mg. Finally, 44% Sr removal was obtained in seawater with the MnO2-alginate beads, whereas plain alginate beads show only 10% Sr removal. These results prove that MnO2-alginate beads are a reliable material for Sr adsorption in seawater.
Acknowledgment
This research was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources(KIGAM) funded by the Ministry of Science, ICT and Future Planning of Korea.
16
References
[1] A. Sachse, A. Merceille, Y. Barré, A. Grandjean, F. Fajula, A. Galarneau, Macroporous LTA-monoliths for in-flow removal of radioactive strontium from aqueous effluents: Application to the case of Fukushima, Micropor Mesopor Mat 164 (2012) 251-258. [2] M.E. Kitto, J.C. Marrantino, E.M. Fielman, D.K. Haines, T.M. Semkow, B. Abdul, Longterm monitoring of radioactivity in fish from New York waters, J Environ Radioactiv 146 (2016) 44-50. [3] C. Xu, J. Wang, J. Chen, Solvent Extraction of Strontium and Cesium: A Review of Recent Progress, Solvent Extr Ion Exc 30 (2012) 623-650. [4] J.-G. Cao, P. Gu, J. Zhao, D. Zhang, Y. Deng, Removal of strontium from an aqueous solution using co-precipitation followed by microfiltration (CPMF), J Radioanal Nucl Chem 285 (2010) 539-546. [5] D. Jaganyi, M. Altaf, I. Wekesa, Synthesis and characterization of whisker-shaped MnO2 nanostructure at room temperature, Applied Nanoscience 3 (2013) 329-333. [6] H.-J. Hong, J. Ryu, I.-S. Park, T. Ryu, K.-S. Chung, B.-G. Kim, Investigation of the strontium (Sr(II)) adsorption of an alginate microsphere as a low-cost adsorbent for removal and recovery from seawater, J Environ Manage 165 (2016) 263-270. [7] Y. Park, W.S. Shin, S.-J. Choi, Sorptive removal of cobalt, strontium and cesium onto manganese and iron oxide-coated montmorillonite from groundwater, J Radioanal Nucl Chem 292 (2012) 837-852. [8] S. Devaraj, N. Munichandraiah, Effect of Crystallographic Structure of MnO2 on Its Electrochemical Capacitance Properties, J Phys Chem C 112 (2008) 4406-4417. [9] A. Dyer, M. Pillinger, R. Harjula, S. Amin, Sorption characteristics of radionuclides on synthetic birnessite-type layered manganese oxides, J Mater Chem 10 (2000) 1867-1874. [10] M. Ghaly, F.M.S.E. El-Dars, M.M. Hegazy, R.O. Abdel Rahman, Evaluation of synthetic Birnessite utilization as a sorbent for cobalt and strontium removal from aqueous solution, Chem Eng J 284 (2016) 1373-1385. [11] T.P. Valsala, A. Joseph, N.L. Sonar, M.S. Sonavane, J.G. Shah, K. Raj, V. Venugopal, Separation of strontium from low level radioactive waste solutions using hydrous manganese dioxide composite materials, J Nucl Mater 404 (2010) 138-143. [12] X. Zhang, P. Yu, H. Zhang, D. Zhang, X. Sun, Y. Ma, Rapid hydrothermal synthesis of hierarchical nanostructures assembled from ultrathin birnessite-type MnO2 nanosheets for supercapacitor applications, Electrochim Acta 89 (2013) 523-529. [13] X. Zhang, W. Miao, C. Li, X. Sun, K. Wang, Y. Ma, Microwave-assisted rapid synthesis of birnessite-type MnO2 nanoparticles for high performance supercapacitor applications, Mater Res Bull 71 (2015) 111-115. [14] X.Q. Xu, H. Shen, J.R. Xu, M.Q. Xie, X.J. Li, The colloidal stability and core-shell structure of magnetite nanoparticles coated with alginate, Appl Surf Sci 253 (2006) 21582164. [15] D. Wu, J. Zhao, L. Zhang, Q. Wu, Y. Yang, Lanthanum adsorption using iron oxide loaded calcium alginate beads, Hydrometallurgy 101 (2010) 76-83. [16] P. Ragupathy, D.H. Park, G. Campet, H.N. Vasan, S.-J. Hwang, J.-H. Choy, N. Munichandraiah, Remarkable Capacity Retention of Nanostructured Manganese Oxide upon Cycling as an Electrode Material for Supercapacitor, J Phys Chem C 113 (2009) 6303-6309. [17] S.K. Bajpai, S. Sharma, Investigation of swelling/degradation behaviour of alginate beads crosslinked with Ca2+ and Ba2+ ions, React Funct Polym 59 (2004) 129-140. [18] D. Song, S.-J. Park, H.W. Kang, S.B. Park, J.-I. Han, Recovery of Lithium(I), 17
Strontium(II), and Lanthanum(III) Using Ca–Alginate Beads, J Chem Eng Data 58 (2013) 2455-2464. [19] H.-j. Hong, H. Kim, K. Baek, J.-W. Yang, Removal of arsenate, chromate and ferricyanide by cationic surfactant modified powdered activated carbon, Desalination 223 (2008) 221-228. [20] N. Li, L. Zhang, Y. Chen, M. Fang, J. Zhang, H. Wang, Highly Efficient, Irreversible and Selective Ion Exchange Property of Layered Titanate Nanostructures, Adv Funct Mater 22 (2012) 835-841. [21] J. Ryu, S. Kim, H.-J. Hong, J. Hong, M. Kim, T. Ryu, I.-S. Park, K.-S. Chung, J.S. Jang, B.-G. Kim, Strontium ion (Sr2+) separation from seawater by hydrothermally structured titanate nanotubes: Removal vs. recovery, Chem Eng J 304 (2016) 503-510. [22] L. Zhang, J. Wei, X. Zhao, F. Li, F. Jiang, Adsorption characteristics of strontium on synthesized antimony silicate, Chem Eng J 277 (2015) 378-387. [23] L. Zhang, J. Wei, X. Zhao, F. Li, F. Jiang, M. Zhang, Strontium(II) adsorption on Sb(III)/Sb2O5 Chem Eng J 267 (2015) 245-252. [24] L. Zhang, J. Wei, X. Zhao, F. Li, F. Jiang, M. Zhang, X. Cheng, Removal of strontium(II) and cobalt(II) from acidic solution by manganese antimonate, Chem Eng J 302 (2016) 733743. [25] M. Hafizi, H. Abolghasemi, M. Moradi, S.A. Milani, Strontium Adsorption from Sulfuric Acid Solution by Dowex 50W-X Resins, Chinese Journal of Chemical Engineering 19 (2011) 267-272.
18
List of figures
Figure 1. XRD pattern (a), SEM image (b), Sr adsorption isotherm (c) and relation between Sr uptake and Na release (d) in hydrous MnO2. Figure 2. Picture of plain alginate bead (a) and MnO2-alginate bead (b), TG-analysis and composition (c), FTIR spectrum (d) of plain and MnO2 immobilized alginate bead. Figure 3. Sr adsorption isotherm of MnO2-alginate bead (dry bead weight base) (a) and corelation of Sr uptake and released amount of Ca, K, Na (b). Figure 4. Sr adsorption on plain and MnO2-alginate bead according to initial Sr concentration. Figure 5. Sr adsorption efficiency on plain and MnO2 alginate bead with co-existing cations (a), Cation adsorption capacity on plain and MnO2 immobilized alginate bead (b). Figure 6. Sr adsorption efficiency of plain and MnO2-alginate bead in real seawater.
19
Figure 1
20
Figure 2
21
Figure 3
22
Figure 4.
23
Figure 5
24
Figure 6
25
List of tables
Table 1. Composition of seawater. Element Concentration
Na
Mg
K
Ca
Sr
Mn
B
9829.5
1187.0
332.2
319.9
6.1
0.01
3.7
(mg/L)
Table 2. Sr adsorption isotherm model constants of MnO2-alginate bead
Langmuir
Freundlich
KL
qmax
R2
KF
n
R2
0.048
102.0
0.99
8.26
2.23
0.976
Table 3. Stable Sr and 90Sr removal efficiency by plain and MnO2-alginate bead.
Adsorbent
Initial stable Sr (mg/L)
Radioactivity (Bq/L)
Stable Sr removal (%)
Sr removal (%)
Plain alginate bead
7
10
84
87
MnO2-alginate bead
7
10
97
96
26
90
Graphical abstract
27
Highlights 1. Adsorption capacity of alginate for strontium(Sr) ion is limited in seawater. 2. Compact structured MnO2-alginate bead contains the increased adsorption sites. 3. MnO2 in MnO2-alginate bead induced enhancement in Sr selectivity against Na. 4. MnO2-alginate bead adsorbs more amount of Sr than plain alginate bead in seawater.
28
S1385-8947(17)30309-1 http://dx.doi.org/10.1016/j.cej.2017.02.132 CEJ 16571
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
21 December 2016 22 February 2017 24 February 2017
Please cite this article as: H-J. Hong, B-G. Kim, J. Hong, J. Ryu, T. Ryu, K-S. Chung, H. Kim, I-S. Park, Enhanced Sr adsorption performance of MnO2-alginate beads in seawater and evaluation of its mechanism, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.02.132
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Enhanced Sr adsorption performance of MnO2-alginate beads in seawater and evaluation of its mechanism
Hye-Jin Hong1, Byoung-Gyu Kim1, Jeongsik Hong1, Jungho Ryu 1, Taegong Ryu1, Kang-Sup Chung1, Hyunchul Kim2, In-Su Park1,*
1
Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 34132, Republic of Korea 2
Environmental Radioactivity Assessment Team, Korea Atomic Energy Research Institute (KAERI), Daejeon 34057, Republic of Korea
*Corresponding author Dr. In-Su Park Tel:+82-42-868-3071, Fax: +82-42-868-3411 E-mail: [email protected] 1
Abstract Alginate bead is a promising adsorbent for Sr ions in solution, but their adsorption performance is limited in seawater due to the competition with other concentrated ions. In this study, MnO2-alginate beads were prepared by immobilizing MnO2 in alginate, and their feasibility for Sr adsorption in seawater was evaluated. MnO2-alginate beads consisted of 16.1% MnO2 and 7.0% alginate, while plain alginate beads contained only 3.7% alginate. That is, immobilizing MnO2 in alginate induced a compact structure in the MnO2-alginate beads, which resulted in a lower amount of moisture. Sr adsorption on MnO2-alginate beads occurred predominantly on the alginate part and partially on the MnO2. Compared with plain alginate beads, MnO2-alginate beads exhibited significantly improved Sr adsorption efficiency in the presence of seawater with a concentration of Na (10,000 ppm), Mg (1,200 ppm), Ca (400 ppm) and K (400 ppm) due to its enhanced Sr selectivity against Na and increased number of Sr adsorption sites. Finally, MnO2-alginate beads exhibited four times higher Sr adsorption efficiency than plain alginate beads in real seawater. These results demonstrate that MnO2-alginate beads are a highly feasible adsorbent for Sr removal from seawater.
Keywords: strontium, alginate bead, hydrous manganese oxide, seawater, adsorption 2
1. Introduction Since the accident at Fukushima Daiichi nuclear plant at 2011, numerous radioactive compounds have been discharged into seawater. Radioactive strontium (Sr) is one of the fission pollutants from nuclear plant which emits beta rays and exhibits a long half-life of 29 to 30 years [1]. Additionally, radioactive Sr exhibits a similar chemistry with calcium (Ca) and accumulates in the bones of living organisms. Therefore, living organisms are exposed to beta rays for a lengthy period of lifetime [2]. Due to the long lifetime, high solubility, and bio-toxicity of Sr, separation and removal of this ion from seawater needs special attention. There are many technologies to separate Sr from a liquid source such as solvent extraction, chemical precipitation and adsorption [3-5]. All methods have their own advantages, but adsorption is appropriate for treatment of a dilute liquid source such as seawater. Previously, we investigated alginate microspheres as a Sr adsorbent. Ca ion cross-linked alginate microspheres exhibited excellent selectivity for Sr. Sr in the liquid phase quickly adsorbed by ion exchange with the Ca ions in the alginate microsphere [6]. However, Sr adsorption on alginate microspheres is limited in seawater due to the competition with other cations such as Na, Ca and Mg. In particular, the chemical properties of Ca ions are similar to those of Sr ions. Therefore, the concentrated Ca ions (400 mg/L) in seawater severely interferes with Sr adsorption on alginate microspheres. To improve the Sr adsorption performance of alginate microspheres in seawater, the Sr selectivity needs to be improved or the number of Sr binding site in the alginate beads should be increased. In this study, hydrous MnO2 is immobilized in alginate beads to improve the Sr adsorption capacity in seawater. Hydrous MnO2 is a cheap and effective material which is widely used for radio nucleotide removal [7]. The typical Kd value for Sr2+ adsorbed onto MnO2 is reported to be 100 mL/g, and the adsorption capacity of Co2+ onto synthetic nano-manganese 3
oxide is as high as 25 mg/g. This shows that MnO2 has good adsorption properties both for Sr2+ and Co 2+ ions. MnO2 possesses several crystallographic forms such as α-, β-, δ-, γ-, and ε-type, because the basic MnO6 octahedra are linked in different ways [8]. Among these forms, δ-MnO2 (also known as birnessite-type MnO2) has gained special attention as an adsorbent for Sr. Birnessite behaves as an ion-sieve material because of exchangeable alkali ions (Na or K) and crystallized water between sheets of lamellar structure. That is, Sr ions exchange with alkali ions in the interlayer or hydrogen in the hydroxyl group of the interlayer water molecules (or surface OH groups). Dyer et al. explained the Sr adsorption mechanism using an ion exchange reaction with Na or K ions at sites located on the crystal water sheet [9]. Additionally, Ghaly et al. proposed a Sr adsorption mechanism on birnessite via exchange between a Sr ion and hydrogen in the hydroxyl group of the interlayer water molecules [10]. In this paper, hydrous MnO2 was synthesized by alkali precipitation methods. The capacity and mechanism of Sr adsorption on MnO2 were investigated. Then, MnO2-alginate beads were prepared and its physico-chemical properties were characterized. Additionally, the Sr adsorption efficiency and Sr selectivity were evaluated and those of plain and MnO2-alginate beads were compared to identify the Sr adsorption mechanism. Finally, a Sr adsorption experiment is carried out in real seawater.
2. Materials and Methods 2.1. Materials For the synthesis of hydrous MnO2, manganese sulfate (MgSO4, Sigma-Aldrich, USA) and potassium permanganate (KMnO4, Sigma-Aldrich, USA) were used. Sodium alginic acid (Junsei, Japan, Product code: 13035-1201) was used for preparing the alginate beads. The molecular weight of used alginic acid is around 220,000. The ratio of the mannuronic and 4
glunonic acid is not available. Calcium chloride dihydrate (CaCl2·2H2O, Junsei, 99%>) was used for cross-linking of the alginate beads. To control the pH of the solution, 0.1 and 1 M hydrochloric acid (HCl, Junsei) and 5 M sodium hydroxide(NaOH, Junsei, 99%>) were applied. In the Sr adsorption experiment, strontium chloride hexahydrate (SrCl2 ·6H2O, Junsei, purity) was used as the Sr source. Sodium chloride (NaCl, OCI company), magnesium chloride hexahydrate (MgCl2·6H2O, Junsei), and potassium chloride (KCl, Junsei) were applied as competitive cations during adsorption. The
90
Sr was purchased from the National
Institute of Standards and Technology (NIST, Gaithersburg, MA, USA). Real seawater was prepared by filtering using a micro-filter with a 0.4 µm pore size before use. The composition of the seawater is shown in table 1.
2.2 Preparation of hydrous MnO2 Hydrous MnO2 was synthesized by the method described by Valsala et al. [11]. First, the mixed solution (0.1 M KMnO4 and 0.14 M MnSO4) was prepared and its color was dark violet. The initial solution pH was approximately 1~2 and it was controlled between 8~9.5 by using the 5 M NaOH solution. The solution pH decreased during the formation of the MnO2 powder. After 3 additions of the 5 M NaOH solution to control the pH , the solution pH remained constant and the final solution’s color changed to brown from purple. Brown hydrous MnO2 powder was settled, filtered with a 0.2 µm filter, washed thoroughly by DI water to remove unreacted chemicals and dried in a 60 °C oven.
2.3 Synthesis of plain and MnO2-alginate beads MnO2-alginate beads were synthesized by the following procedure. The prepared hydrous MnO2 was put into a 2 wt% sodium alginic acid solution. The ratio of MnO2 powder and 5
sodium alginic acid solution was 17:83 (w/w). The mixture was stirred to disperse the MnO2 powder in the solution. Next, the mixture was dropwise added to the 0.5 M CaCl2 solution using a syringe pump under vigorous stirring. The MnO2 and alginic acid mixture immediately changed to a spherical hydrogel. The synthesized MnO2-alginate beads was kept in 0.5 M CaCl2 solution for 12 hours and washed with DI water several times to remove unreacted Ca ions. Plain alginate beads were prepared with same procedure as the MnO2alginate beads except for the MnO2 powder addition.
2.4 Characterization of synthesized adsorbents The morphology of MnO2 powder was characterized by scanning electron microscopy (SEM, EPMA 1610, Shimadzu). The crystalline structure of the composite was analyzed by X-ray diffraction (XRD, smartlab, Rigaku). Thermogravimetric analysis (TGA, Labsys EVO, Setaram) was carried out in a nitrogen gas flow from room temperature to 1400 °C. Fourier transmission infrared spectra (FTIR, FTIR 4100, Jasco) was performed to analyze the chemical structure.
2.5 Sr adsorption experiments In the Sr adsorption experiment, the same volume of plain and MnO2-alginate beads was put into 0.2 L of Sr solution in a bottle and it was shaken at 120 rpm, 25 °C for 7 days. After adsorption, the solution was filtered with a 0.45 µm syringe filter and then used for Sr concentration analysis. For safety reasons, non-radioactive Sr was used in all Sr adsorption experiments. One experiment was carried out to prove similar adsorption behavior of stable Sr and
90
Sr. The
adsorption of the radioactive isotope of Sr (90Sr) was also determined to prove the reliability 6
of MnO2-alginate bead. Approximately 10 Bq/L of solution. The activity of descendant
90
90
90
Sr was spiked in a 50 ppm stable Sr
Sr (half-life: 28.8 yr) through Cerenkov radiation emitted by its
Y (half-life: 64 h) was determined using a liquid scintillation counter
(Quantulus 1220, PerkinElmer, USA). The
90
Y makes secular equilibrium with
90
Sr after a
two-week in-growth period. The counting efficiency was determined using a mixed source of 90
Sr and 90Y in secular equilibrium.
Stable (non-radioactive) Sr and the other cations concentrations were measured via inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Perkin-Elmer, USA). Sr and cation uptake, and Sr adsorption efficiency were calculated using the following equations;
qe (mg / g ) =
(C0 − Ce ) ×V m
C Sr adsorption(%) = 1 − e C0
× 100
where qe is the adsorption capacity (mg/g), C0 (mg/L) and Ce (mg/L) denote the initial and equilibrium concentration, respectively, V is the solution volume (L), and m is the adsorbent mass (g).
3. Results and Discussion
3.1 Hydrous MnO2 Highly efficient hydrous MnO2 powder was prepared and its characteristics, Sr adsorption capacity and mechanisms were investigated before immobilization of MnO2 into the alginate 7
beads. The phase of hydrous MnO2 was characterized by XRD, as shown in Figure 1 (a). Significant XRD peaks at 2θ=12.15°, 36.98° and 66.27° can be well assigned to the (001), (111) and (020) planes of MnO2 with a K-birnessite structure (JCPDS 80-1098, a=5.149 Å, b=2.843 Å, c=7.176 Å), respectively [12]. The prepared MnO2 has a layered structure and the basal plane spacing calculated from the (001) plane is ~0.73 nm. The broadness of other diffraction peaks indicates the nanoscaled crystal size of the prepared MnO2. Elemental analysis discovered that K and Na intercalated into the MnO2 layers. The amount of K and Na in MnO2 is 1.24 mmol/g and 0.54 mmol/g, respectively. Figure 1 (b) shows an FE-SEM image of the synthesized MnO2. The SEM image demonstrates that the MnO2 products have uniform particles with diameters of 300-600 nm, and their surface is covered with protruding ultrathin nanosheets [13]. Figure 1 (c) shows the Sr sorption isotherm of the MnO2 product. The uptake of Sr increased up to approximately 400 mg/L of Sr2+ equilibrium concentration and this MnO2 showed approximately 110 mg/g of maximum Sr adsorption capacity. Interestingly, the amount of the released K and Na during Sr adsorption increased linearly with the Sr uptake (mmol/g) until 100 mg/L of initial Sr concentration. Figure 1(d) shows the correlation between Sr uptake and K, Na release from MnO2 as molar concentration. The slope is almost 0.5, which means that the adsorption of 1 mol of Sr ions caused the release of 2 mol of Na and/or K from MnO2. This result indicates that Sr intercalates via an ion-exchange reaction with Na or K. When the Sr concentration increased to greater than 100 mg/L, the Sr uptake increased rapidly regardless of the amount of released K or Na. That is, Sr exchanged with hydrogen in the hydroxyl group of the interlayer water molecules (or surface OH groups) after consumption of all of the Na or K in MnO2.
8
3.2 MnO2-alginate beads Figure 2 (a) and (b) shows the prepared plain alginate and MnO2-alginate. Transparent plain alginate became black after immobilization of MnO2 into the alginate. The size of both beads was approximately 2.5~2.7 mm. Figure 2 (c) shows the TG analysis of the plain and MnO2-alginate beads. Both alginate beads exhibited a sharp weight loss at approximately 100~150 °C due to the evaporation of moisture. Weight losses in the plain alginate beads and MnO2-alginate beads were almost 95% and 80%, respectively. The moisture content in both the plain and MnO2-alginate beads is very high because they are hydrogel-type adsorbents. Alginate polymer is degraded at 150~420 °C so the alginate content can be derived by considering the weight loss between 150~500 °C [14, 15]. The alginate contents of the plain and MnO2-alginate beads were 3.7% and 7.0%, respectively. This indicates that the MnO2alginate beads contained more alginic acid than the plain beads even though MnO2 was immobilized in the MnO2-alginate beads. That is, immobilizing MnO2 in alginate induced a compact structure in the MnO2-alginate beads, which resulted in a lower amount of moisture. Therefore, the MnO2-alginate beads would have more Sr adsorption sites than the plain alginate beads. Due to the higher alginate content, as well as the immobilized MnO2, the MnO2-alginate beads exhibited a higher density of Sr adsorption sites compared with plain alginate beads. Only the moisture content was decreased after immobilization of the MnO2. Hydrous MnO2 powder also showed an approximately 15% weight loss at 100~200 °C due to water evaporation and a slight weight loss between 200~700 °C due to loss of the crystalline H2O containing in the MnO2 layer. At approximately 750~800 °C, there is a slight decrease in weight associated with the transformation of MnO2 to Mn2O3 [16]. Hydrous MnO2 maintained its phase until 750 °C. MnO2-alginate beads exhibited 13.3% remaining weight at 750 °C. Considering that moisture of hydrous MnO2 (22%) at room temperature, 9
the MnO2 content in MnO2-alginate beads is 16.1%. Figure 2(d) shows the FTIR spectrum of the MnO2 powder, the plain beads and the MnO2alginate beads. Hydrous MnO2 powder shows two absorption bands at 475 and 600 cm-1 which correspond to the characteristic stretching collision of O-Mn-O. The broad band at 3000~4000 cm-1 indicates the presence of plenty of hydroxyl groups (-OH) on hydrous MnO2 [5]. At 1625 cm-1, the H2O deformation peak is observed. It is evident that the prepared MnO2 is in hydrous form. Plain alginate beads also exhibits many absorption bands attributed to various functional groups. A dominant peak at 3000 ~ 3500 cm-1 indicates the existence of the –OH group. The absorption peaks at 1613 and 1417 cm−1 represent the asymme tric and symmetric stretching vibrations of the carboxylate (-COOH) group, respectivel y, [17]. Additionally, the 1027 cm-1 and 1097 cm-1 peaks indicate the presence of OC -OH [18]. The 3610 and 1670 cm-1 peaks indicate the deformation of the hydroxyl (OH -) and carboxylate (-COO) due to the electrostatic interaction attraction with the Ca ion after cross-linking [6]. MnO2-alginate beads show a similar FTIR spectrum as MnO2 due to its high MnO2 content. Peaks belong to plain alginate beads are almost not detectable in the FTIR spectrum. Only the hydroxyl peak (3000~3500 cm-1) is slightly remaining after immobilized in the alginate beads. It revealed that hydrous MnO2 was immobilized in the alginate beads, without unexpected chemical reaction in the organic structure. Figure 3(a) shows the Sr adsorption isotherm of the MnO2-alginate beads. Two isotherm models were applied for analysis of Sr adsorption on the MnO2-alginate beads. They are the Langmuir and Freundlich isotherm models(See supporting information, SI). Sr adsorption on MnO2-alginate beads was well-fit with both isotherm models, but the Langmuir model showed a higher linear regression coefficient (R2=0.99) than the Freundlich mode1 (table 2). The Langmuir adsorption model assumes homogeneous adsorption of adsorbate on substrate 10
with monolayer formation rather Freundlich model is empirical heterogeneous adsorption [19]. So it seems that Sr adsorption on MnO2-alginate bead is homogeneous adsorption. The maximum Sr adsorption capacity (qmax) derived from the Langmuir isotherm is 102.0 mg/g. Sr adsorption capacities of other Sr adsorbents including ion exchange resin are described (table S1 in SI). Among adsorbents, MnO2-alginate bead exhibited excellent maximum Sr adsorption capacity. Several adsorbents such as zeolite exhibited even higher Sr adsorption performance than MnO2-alginate beads. However powder form of adsorbents could not be applied seawater directly due to recovery and handling problem, and then MnO2-alginate bead is considered as effective and highly promising Sr adsorbents. During Sr adsorption on the MnO2-alginate beads, Ca, K and Na were released into solution (Figure 3(a)). It is already known that Sr adsorbed on alginate by ion exchange with Ca in a 1(Sr):1(Ca) molar ratio [6], whereas Sr adsorption on MnO2 powder caused the release of K or Na by the ratio of 1(Sr):2(K or Na) in the low Sr concentration range (<100 mg/L) (section 3.1). From the data analysis of Sr uptake and the released amount of Ca, K and Na, the Sr adsorption mechanism on MnO2-alginate could be determined. The released amount of Ca increased as Sr uptake increased in all concentration ranges. On the other hand, the K concentration increased until the Sr equilibrium concentration reached 25 mg/L and then remained constant, while Na was almost not released into solution. This result indicates that Sr adsorption on MnO2-alginate beads is mainly due to the alginate component. That is, only a limited amount of Sr was adsorbed on the immobilized MnO2 compared with its powdered form because of the blocking of the Sr solution into the trapped MnO2 in the hydrogel-type alginate beads. Figure 3(b) exhibits the Sr uptake and the released Ca, K and Na as molar concentration. Because Sr adsorption on MnO2 powder caused release of K or Na by the ratio of 1(Sr):2(K 11
or Na), the K and Na concentration was divided by 2. Sr uptake of MnO2-alginate beads was slightly higher than the sum of the released Ca and (K+Na)/2, but the trend was closely followed. From this result, the Sr adsorption mechanism of MnO2-alginate beads was determined. Among the 102 mg/g of Sr uptake, 8.8 mg/g (0.1 mmol/g) of the Sr uptake is contributed by the MnO2 and approximately 94 mg/g is achieved by the alginate part of the MnO2-alginate beads.
3.3 Sr adsorption behavior of plain alginate beads vs. MnO2-alginate beads In this section, the Sr adsorption behavior of plain and MnO2-alginate beads are investigated by applying an equal volume (equal number) of beads. Because alginate beads are a hydrogel-type adsorbent, almost 90% of the beads consist of moisture. In this case, dry or wet weight could be an inappropriate unit in evaluating the adsorbent. Therefore, we applied 3 cm3 of beads as a unit system for Sr adsorption. Sr adsorption efficiency and Sr selectivity are discussed to evaluate two hydrogel type adsorbents. The Sr adsorption kinetic and isotherm analysis based on unit volume(L) of plain and MnO2-alginate bead were described (figure S12, table S2-3 in SI).
3.3.1 Sr adsorption efficiency Figure 4 shows the Sr adsorption efficiencies of plain and MnO2-alginate beads according to the initial Sr concentration. When the initial Sr concentration was lower than 10 mg/L, both alginate beads adsorbed the Sr completely. After that, Sr adsorption efficiency decreased because of an insufficient number of Sr binding sites. At a Sr concentration of 200 mg/L, the MnO2-alginate beads showed a Sr adsorption capacity of 51.8%. Considering that the Sr adsorption efficiency of plain alginate beads was only 23.4%, the MnO2-alginate beads 12
showed a significantly improved Sr adsorption capacity. Since MnO2-alginate beads have a high alginate content and MnO2, it exhibits enhanced Sr adsorption efficiency.
3.3.2 Sr selectivity Figure 5(a) shows the Sr adsorption efficiency of plain alginate beads, MnO2-alginate beads and MnO2 powder in the presence of co-existing cations. To simulate seawater, a seawater concentration of Na (10,000 ppm), Mg (1,200 ppm), Ca (400 ppm) and K (400 ppm) was spiked separately in 7 mg/L of Sr solution,. In pure Sr solution, plain alginate beads showed an excellent Sr adsorption efficiency of 93.8%. However, it was severely influenced and decreased with the presence of other cations. In particular, Na and Ca caused a significant decrease of Sr adsorption on the plain alginate beads. Na is a monovalent cation which is not competitive compared with divalent cations such as Ca or Mg, but its concentration is approximately 10,000 ppm. The extremely high concentration caused the Sr adsorption to decrease. Additionally, Ca exhibits similar chemical characteristics as Sr, such as valence, absolute hardness, and ionic radius, and Ca shows a similar ion exchange behavior to Sr [20]. Therefore, Ca in seawater caused the most severe interference to Sr adsorption on plain alginate beads. MnO2-alginate beads showed much better Sr adsorption efficiency than plain alginate beads in conditions containing Na, Mg and Ca. The coexistence of K ions did not severely affect Sr adsorption like the other ions. MnO2-alginate beads exhibited 58.0% and 72.2% Sr adsorption efficiency in the presence of Ca and Mg, respectively. These values were 2.5 and 1.5 times improved compared with those of plain alginate beads. However, the improved Sr adsorption performance of the MnO2-alginate beads was not caused by MnO2 powder because the MnO2 did not have Sr selectivity for Mg or Ca adsorption like the plain alginate beads. 13
Figure 5(b) shows the adsorption efficiency of Na, Mg, Ca and K on the MnO2 powder, the plain alginate beads and the MnO2-alginate beads in this experiment. The plain alginate beads and the MnO2 powder adsorbed 1~4% of the Na, Ca, Mg and K during Sr adsorption. MnO2alginate beads showed a slightly higher Mg, Ca and K adsorption efficiency. Not only was the Sr adsorption efficiency higher, but the Mg and Ca adsorption efficiencies were also improved after immobilization of MnO2. This means that MnO2-alginate beads did not show improved Sr selectivity against Ca or Mg. However, the increased number of Sr adsorption sites in MnO2-alginate resulted in the enhanced Sr adsorption performance in the presence of Ca or Mg. With 10,000 ppm Na, MnO2-alginate beads showed a Sr adsorption efficiency of 72.5% while that of the plain alginate was only 22.4%. MnO2 powder showed a 60% Sr adsorption efficiency despite the extremely high Na concentration. This result revealed that MnO2 exhibited a higher selectivity to Sr against Na. The increase number of Sr adsorption sites due to the high alginate content, as well as the immobilization of MnO2 in the alginate beads, led to the increase of Sr selectivity in conditions with co-existing Na.
3.4 Sr adsorption in real seawater Finally, the Sr adsorption efficiency of plain and MnO2-alginate beads was evaluated in real seawater. Figure 6 shows the Sr adsorption efficiency of plain and MnO2-alginate beads according to reaction time. In DI water, 7 mg/L of Sr was completely adsorbed by both alginate beads. However, the Sr adsorption efficiency of plain and MnO2-alginate beads was significantly decreased by competition with the cations in seawater. Plain alginate beads exhibited only a Sr adsorption efficiency of 13.4%. MnO2-alginate beads also showed a decreased Sr adsorption efficiency (44.3%) in seawater, but it was much higher than that of 14
the plain beads. Because MnO2-alginate beads exhibited many Sr adsorption sites as well as a high selectivity for Sr against Na, it shows an improved Sr adsorption performance in real seawater. This result proved that MnO2-alginate beads are a feasible material to Sr removal in a complex medium such as seawater.
3.5 Sr adsorption behavior of stable Sr and radio-active Sr For the comparison of Sr adsorption behavior of stable Sr and radioactive Sr, 90Sr is spiked in water and the Sr adsorption capacity of stable Sr and 90Sr is evaluated. Table 3 shows Sr the adsorption capacity of plain and MnO2-alginate beads in the presence of 10 Bq/L of Sr. MnO2-alginate beads show a Sr removal efficiency of 97%, whereas the plain alginate beads show only 84% Sr removal. Stable Sr and 90Sr show similar Sr adsorption efficiencies, which proved that the adsorption behavior of radioactive
90
Sr on plain and MnO2-alginate beads is
similar to the behavior of stable Sr.
4. Conclusions
Alginate beads are a promising Sr adsorbent due to their high Sr adsorption performance and abundance of raw material. However, the Sr adsorption performance of alginate beads is limited in a complex medium, such as seawater and radioactive wastewaters, due to the competition of other concentrated ions. In this study, we highly improved the Sr adsorption performance of alginate beads by immobilizing hydrous MnO2 in the beads and the Sr adsorption behavior of the improved beads was evaluated in a seawater medium. MnO2alginate beads consisted of 13.2% MnO2 and 7.0% alginate. Compared with plain alginate beads (alginate content = 3.7%), MnO2-alginate beads had a higher alginate content. Because the MnO2-alginate beads have a compact structure, it showed a higher Sr adsorption 15
efficiency compared with plain alginate beads. Sr adsorption on MnO2-alginate beads predominantly occurred on the alginate part and only 8% of the Sr adsorption was contributed by the MnO2. Compared with plain alginate beads, MnO2-alginate beads exhibited a much higher Sr adsorption efficiency in the presence of seawater with a concentration of Na(10,000 ppm), Mg(1,200 ppm), Ca(400 ppm) and K(400ppm). Because MnO2 exhibits excellent Sr selectivity against Na, the MnO2-alginate beads show a better Sr adsorption performance in a solution containing Na. Additionally, the high alginate content provided sufficient binding sites and the Sr adsorption performance is therefore improved in the presence of Ca or Mg. Finally, 44% Sr removal was obtained in seawater with the MnO2-alginate beads, whereas plain alginate beads show only 10% Sr removal. These results prove that MnO2-alginate beads are a reliable material for Sr adsorption in seawater.
Acknowledgment
This research was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources(KIGAM) funded by the Ministry of Science, ICT and Future Planning of Korea.
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References
[1] A. Sachse, A. Merceille, Y. Barré, A. Grandjean, F. Fajula, A. Galarneau, Macroporous LTA-monoliths for in-flow removal of radioactive strontium from aqueous effluents: Application to the case of Fukushima, Micropor Mesopor Mat 164 (2012) 251-258. [2] M.E. Kitto, J.C. Marrantino, E.M. Fielman, D.K. Haines, T.M. Semkow, B. Abdul, Longterm monitoring of radioactivity in fish from New York waters, J Environ Radioactiv 146 (2016) 44-50. [3] C. Xu, J. Wang, J. Chen, Solvent Extraction of Strontium and Cesium: A Review of Recent Progress, Solvent Extr Ion Exc 30 (2012) 623-650. [4] J.-G. Cao, P. Gu, J. Zhao, D. Zhang, Y. Deng, Removal of strontium from an aqueous solution using co-precipitation followed by microfiltration (CPMF), J Radioanal Nucl Chem 285 (2010) 539-546. [5] D. Jaganyi, M. Altaf, I. Wekesa, Synthesis and characterization of whisker-shaped MnO2 nanostructure at room temperature, Applied Nanoscience 3 (2013) 329-333. [6] H.-J. Hong, J. Ryu, I.-S. Park, T. Ryu, K.-S. Chung, B.-G. Kim, Investigation of the strontium (Sr(II)) adsorption of an alginate microsphere as a low-cost adsorbent for removal and recovery from seawater, J Environ Manage 165 (2016) 263-270. [7] Y. Park, W.S. Shin, S.-J. Choi, Sorptive removal of cobalt, strontium and cesium onto manganese and iron oxide-coated montmorillonite from groundwater, J Radioanal Nucl Chem 292 (2012) 837-852. [8] S. Devaraj, N. Munichandraiah, Effect of Crystallographic Structure of MnO2 on Its Electrochemical Capacitance Properties, J Phys Chem C 112 (2008) 4406-4417. [9] A. Dyer, M. Pillinger, R. Harjula, S. Amin, Sorption characteristics of radionuclides on synthetic birnessite-type layered manganese oxides, J Mater Chem 10 (2000) 1867-1874. [10] M. Ghaly, F.M.S.E. El-Dars, M.M. Hegazy, R.O. Abdel Rahman, Evaluation of synthetic Birnessite utilization as a sorbent for cobalt and strontium removal from aqueous solution, Chem Eng J 284 (2016) 1373-1385. [11] T.P. Valsala, A. Joseph, N.L. Sonar, M.S. Sonavane, J.G. Shah, K. Raj, V. Venugopal, Separation of strontium from low level radioactive waste solutions using hydrous manganese dioxide composite materials, J Nucl Mater 404 (2010) 138-143. [12] X. Zhang, P. Yu, H. Zhang, D. Zhang, X. Sun, Y. Ma, Rapid hydrothermal synthesis of hierarchical nanostructures assembled from ultrathin birnessite-type MnO2 nanosheets for supercapacitor applications, Electrochim Acta 89 (2013) 523-529. [13] X. Zhang, W. Miao, C. Li, X. Sun, K. Wang, Y. Ma, Microwave-assisted rapid synthesis of birnessite-type MnO2 nanoparticles for high performance supercapacitor applications, Mater Res Bull 71 (2015) 111-115. [14] X.Q. Xu, H. Shen, J.R. Xu, M.Q. Xie, X.J. Li, The colloidal stability and core-shell structure of magnetite nanoparticles coated with alginate, Appl Surf Sci 253 (2006) 21582164. [15] D. Wu, J. Zhao, L. Zhang, Q. Wu, Y. Yang, Lanthanum adsorption using iron oxide loaded calcium alginate beads, Hydrometallurgy 101 (2010) 76-83. [16] P. Ragupathy, D.H. Park, G. Campet, H.N. Vasan, S.-J. Hwang, J.-H. Choy, N. Munichandraiah, Remarkable Capacity Retention of Nanostructured Manganese Oxide upon Cycling as an Electrode Material for Supercapacitor, J Phys Chem C 113 (2009) 6303-6309. [17] S.K. Bajpai, S. Sharma, Investigation of swelling/degradation behaviour of alginate beads crosslinked with Ca2+ and Ba2+ ions, React Funct Polym 59 (2004) 129-140. [18] D. Song, S.-J. Park, H.W. Kang, S.B. Park, J.-I. Han, Recovery of Lithium(I), 17
Strontium(II), and Lanthanum(III) Using Ca–Alginate Beads, J Chem Eng Data 58 (2013) 2455-2464. [19] H.-j. Hong, H. Kim, K. Baek, J.-W. Yang, Removal of arsenate, chromate and ferricyanide by cationic surfactant modified powdered activated carbon, Desalination 223 (2008) 221-228. [20] N. Li, L. Zhang, Y. Chen, M. Fang, J. Zhang, H. Wang, Highly Efficient, Irreversible and Selective Ion Exchange Property of Layered Titanate Nanostructures, Adv Funct Mater 22 (2012) 835-841. [21] J. Ryu, S. Kim, H.-J. Hong, J. Hong, M. Kim, T. Ryu, I.-S. Park, K.-S. Chung, J.S. Jang, B.-G. Kim, Strontium ion (Sr2+) separation from seawater by hydrothermally structured titanate nanotubes: Removal vs. recovery, Chem Eng J 304 (2016) 503-510. [22] L. Zhang, J. Wei, X. Zhao, F. Li, F. Jiang, Adsorption characteristics of strontium on synthesized antimony silicate, Chem Eng J 277 (2015) 378-387. [23] L. Zhang, J. Wei, X. Zhao, F. Li, F. Jiang, M. Zhang, Strontium(II) adsorption on Sb(III)/Sb2O5 Chem Eng J 267 (2015) 245-252. [24] L. Zhang, J. Wei, X. Zhao, F. Li, F. Jiang, M. Zhang, X. Cheng, Removal of strontium(II) and cobalt(II) from acidic solution by manganese antimonate, Chem Eng J 302 (2016) 733743. [25] M. Hafizi, H. Abolghasemi, M. Moradi, S.A. Milani, Strontium Adsorption from Sulfuric Acid Solution by Dowex 50W-X Resins, Chinese Journal of Chemical Engineering 19 (2011) 267-272.
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List of figures
Figure 1. XRD pattern (a), SEM image (b), Sr adsorption isotherm (c) and relation between Sr uptake and Na release (d) in hydrous MnO2. Figure 2. Picture of plain alginate bead (a) and MnO2-alginate bead (b), TG-analysis and composition (c), FTIR spectrum (d) of plain and MnO2 immobilized alginate bead. Figure 3. Sr adsorption isotherm of MnO2-alginate bead (dry bead weight base) (a) and corelation of Sr uptake and released amount of Ca, K, Na (b). Figure 4. Sr adsorption on plain and MnO2-alginate bead according to initial Sr concentration. Figure 5. Sr adsorption efficiency on plain and MnO2 alginate bead with co-existing cations (a), Cation adsorption capacity on plain and MnO2 immobilized alginate bead (b). Figure 6. Sr adsorption efficiency of plain and MnO2-alginate bead in real seawater.
19
Figure 1
20
Figure 2
21
Figure 3
22
Figure 4.
23
Figure 5
24
Figure 6
25
List of tables
Table 1. Composition of seawater. Element Concentration
Na
Mg
K
Ca
Sr
Mn
B
9829.5
1187.0
332.2
319.9
6.1
0.01
3.7
(mg/L)
Table 2. Sr adsorption isotherm model constants of MnO2-alginate bead
Langmuir
Freundlich
KL
qmax
R2
KF
n
R2
0.048
102.0
0.99
8.26
2.23
0.976
Table 3. Stable Sr and 90Sr removal efficiency by plain and MnO2-alginate bead.
Adsorbent
Initial stable Sr (mg/L)
Radioactivity (Bq/L)
Stable Sr removal (%)
Sr removal (%)
Plain alginate bead
7
10
84
87
MnO2-alginate bead
7
10
97
96
26
90
Graphical abstract
27
Highlights 1. Adsorption capacity of alginate for strontium(Sr) ion is limited in seawater. 2. Compact structured MnO2-alginate bead contains the increased adsorption sites. 3. MnO2 in MnO2-alginate bead induced enhancement in Sr selectivity against Na. 4. MnO2-alginate bead adsorbs more amount of Sr than plain alginate bead in seawater.
28