- Email: [email protected]
O2 plasma-treated carbon nanotube membranes for Sr(II) and Cs(I) in water and wastewater: Fit-for-purpose water treatment
Journal Pre-proofs Selectivity of Ar/O2 plasma-treated carbon nanotube membranes for Sr(II) and Cs(I) in water and wastewater: Fit-for-purpose water treatment Sharafat Ali, Izaz Ali Shah, Haiou Huang PII: DOI: Reference:
S1383-5866(19)34765-3 https://doi.org/10.1016/j.seppur.2019.116352 SEPPUR 116352
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
17 October 2019 25 November 2019 25 November 2019
Please cite this article as: S. Ali, I. Ali Shah, H. Huang, Selectivity of Ar/O2 plasma-treated carbon nanotube membranes for Sr(II) and Cs(I) in water and wastewater: Fit-for-purpose water treatment, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116352
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Selectivity of Ar/O2 plasma-treated carbon nanotube membranes for Sr(II) and Cs(I) in water and wastewater: Fit-for-purpose water treatment
Sharafat Alia, Izaz Ali Shaha and Haiou Huanga,b,* a
State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of
Environment, Beijing Normal University, No. 19, Xinjiekouwai Street, Beijing, 100875, China b
Department of Environmental Health and Engineering, Bloomberg School of Public Health,
The John Hopkins University, 615 North Wolfe Street, MD 21205, USA *
Corresponding Author: E-mail address: [email protected] (H. Huang)
Abstract The need for high-performance and novel technologies for selective treatment of radionuclides in water has recently stimulated broad research. In this study, we examined the ability of carbon nanotube (CNT) membranes in selective removal of strontium (Sr2+) and cesium (Cs+) from water and wastewater. In single-metal systems that contained only Sr2+, Cs+ or their competitive metal ions, the distribution coefficients (Kd) of functionalized CNT membranes for divalent cations, i.e., Sr2+, Ca2+, and Mg2+, were measured as 4.41 mmol g-1, 2.14 mmol g-1, and 1.30 mmol g-1, respectively, and those for monovalent cations, i.e., Cs+, K+ and Na+, were 0.81 mmol g-1, 0.79 mmol g-1, and 0.81 mmol g-1, respectively. When Cs+ or Sr2+ co-existed with a competing cation, both pristine and functionalized CNT membranes showed excellent selectivity to Sr2+ over Mg2+ and Ca2+ while their selectivity to Cs+ over K+ and Na+ diminished. Moreover, the CNT membranes remained their selectivity for Sr2+ in a wastewater effluent despite the 1
complex water composition. Overall, the selectivity sequence of the CNT membranes for divalent cations support the occurrence of inner-sphere complexation on CNT surfaces, while that for the monovalent cations was probably determined by electrostatic interactions. This study demonstrates that Ar/O2 plasma treatment is a greener technology for the production of CNT functionalized membranes for selective removal of contaminants from water to achieve “fit-forpurpose” treatment. Keywords: Adsorptive membrane filtration; Distribution coefficient; Plasma functionalization; Radionuclides; Selective removal. 1. Introduction Radioactive contaminants are mainly released into the aquatic environment during the operation of nuclear power plants, reprocessing of spent fuel, testing of nuclear weapons and sometimes due to accidental waste discharge (e.g. Fukushima Daiichi Nuclear Power Plant Accident, Japan, 2011) [1-4]. Among different radionuclides, radiostrontium and radiocesium isotopes are found in high concentrations in seawater, groundwater and wastewater because of the increases in nuclear industrial activities and disasters such as Chernobyl and Fukushima Daiichi Nuclear Plant Accident [1, 5-8]. Once released into the environment, Sr and Cs isotopes have persistent impacts on human beings and aquatic organisms even at very low concentrations due to their long half-lives, e.g., 28.9 years for
90
Sr and 30.2 years for
137
Cs [9-11]. Since the chemical
properties of Sr are very similar to those of calcium (Ca) and magnesium (Mg), and the chemical properties of Cs are similar to those of potassium (K) and sodium (Na), when they enter the human body these radionuclides can easily replace the essential salts in the body and remain there for long time [12-14]. Therefore, direct or indirect exposure of humans and aquatic
2
organisms to radioactive Sr and Cs can lead to various conditions, for instance, anemia, leukemia and genetic disorders. Therefore, careful consideration regarding the effective treatment and separation of Sr and Cs isotopes from water is of immense importance. Sr and Cs isotopes exist in a different ionic state in water depending on the environmental conditions and water composition [15]. Sr is predominantly found in the form of exchangeable Sr2+ over a wide range of pH [16], while Cs exists in natural waters primarily as Cs+ ions and forms complexes with some organic molecules and weak complexes with nitrate, sulfate and halides. The bared Sr2+ has an ionic radius equal to 113 pm compared to 170 pm for bared Cs+ [17]. However, Sr2+ in water is surrounded by a hydration layer of 150 pm that is much thicker than the hydration layer of Cs+ which is only 49 pm. Comparing the structures of the hydration layers for both cations, each Sr2+ ion contains 6.4 water molecules, which is two times greater than that of Cs+ (2.1 water molecules per ion) [17]. Therefore, this is harder for the current water treatment technology to remove both Sr2+ and Cs+ from water because of their different chemical behaviors. Another major obstacle faced by the current water treatment technologies is that these radioactive contaminants exist in relatively low concentrations compared to co-existing cations with similar properties, such as sodium, potassium, calcium and magnesium. Thus, novel technologies are in immediate need to effectively separate these radioactive species from aqueous streams [18]. Recently, carbon nanotubes (CNTs) containing oxygenated functional groups have attracted the attention of many researchers, due to their excellent capability in selective adsorption of heavy metal ions from aqueous solution [19]. Although many previous studies have reported on the removal of Cd(II), Zn(II), Co(II), Mn(II), Fe(III), Cu(II), Pb(II), etc. by CNTs, there are few studies available on the removal of Sr2+ and Cs+ by CNTs. Lee et al. [20] evaluated the removal 3
of Cs+ from aqueous solutions by copper ferrocyanide functionalized multi-walled carbon nanotubes (CuFCs-MWCNTs) and found that, in the pH range of 7-11, CuFCs-MWCNTs possessed a high affinity to Cs+ in the presence of co-existing ions such as K+ and Na+. Similarly, Yavari et al. [21] reported effective adsorption of Cs+ from nuclear wastewater by oxidized MWCNTs treated with nitric acid. They also found that the adsorption capacity of oxidized MWCNTs is strongly dependent on the pH values and ionic strengths of the water. Like Cs+, Chen et al. [22] studied the adsorption of Sr2+ by oxidized MWCNTs and found that the adsorption percentage increased with increasing pH of the solution but decreased with increasing the ionic strengths of the solution. Despite the promising removal results obtained in the previous studies, the selectivity of functional CNT materials for removal of Sr2+ and Cs+ from complex water matrices has not yet been fully assessed. Therefore, it is unclear whether and to what extent the co-existence of similar metal ions would reduce the effectiveness of CNT for Sr or Cs removal. Therefore, this study was performed with three specific objectives: (1) to systematically assess the selectivity of pristine and plasma-functionalized MWCNT for Sr and Cs in the presence/absence of similar ions through adsorptive filtration technology (2) to explore the ability of MWCNTs for the filtration of Sr and Cs in realistic wastewater, and (3) to explore the relationship between the properties of Sr and Cs ions and their selectivity by CNT membranes. We fabricated pristine and plasma-treated CNT membranes by depositing the respective MWCNTs on the inside lumen of a hollow fiber membrane. The prepared membranes were subsequently assessed for selectively removing Sr2+ and Cs+ from aqueous solutions and secondary wastewater effluent. We also characterized the as-prepared membrane to understand the selectivity mechanisms, in conjunction with consideration of ionic properties of Sr and Cs. 4
The obtained results revealed a strong selectivity of the CNT membranes for Sr2+ under all water chemistry conditions studied. 2. Materials and methods 2.1. Chemicals Reagent-grade strontium nitrate, calcium nitrate, magnesium nitrate, cesium nitrate, sodium nitrate, and potassium nitrate were purchased from Aladdin Industrial Corporation, China. These salts were selected to evaluate the selectivity of CNT membranes for removal Sr2+ and Cs+ ions from aqueous solutions over other divalent or monovalent cations. Stock solutions of these salts were prepared by dissolving individual salts in ultrapure water obtained from a Milli-Q water system (E-POD, Millipore, France). A polyvinylchloride (PVC), hollow fiber membrane with a nominal pore size of 0.01μm was bought from Litree Purifying Technology Co., Ltd, China and was used as the substrate for preparing the CNT membranes. 2.2. Pristine and plasma-functionalized multi-walled carbon nanotubes A pristine multi-walled carbon nanotube (MWCNT) was purchased from Beijing Boyu Technology Corporation of High-tech New Materials, China. According to the manufacturer, this MWCNT was prepared by using a chemical vapor deposition method and possessed outer diameters less than 8 nm, lengths ranging between 10-20 µm, a specific surface area (SSA) of 500 m2 g-1, and purity of above 95%. The as-received MWCNT sample was further functionalized by treating the powder in a borosilicate glass tubular plasma reactor (3 cm in diameter, 118 cm long), following the previous methods reported in the literature [23-25]. During the plasma treatment, the gaseous mixture was injected into the reactor at flowrates of 70 Sccm3 min-1 (standard cubic centimeter per minute) for Ar and 40 Sccm3 min-1 for O2. The gas 5
pressure within the reactor was maintained at 2 Pa when a plasma power of 80 W was applied at a frequency of 13.56 MHz to produce the Ar/O2 plasma. After the treatment, the plasmafunctionalized MWCNT (P-MWCNT) powder was collected in an amber glass bottle and stored at the room temperature. 2.3. Fabrication of the CNT membrane Small U-shaped membrane modules were pre-made with the PVC membrane to obtain effective membrane areas of approximately 6.78 × 10-4 m2 for each module. Then, 20 mg of MWCNT or P-MWCNT was dispersed in 20 ml of ultrapure water and sonicated for 10 minutes at 150 W with a probe sonicator (Ultrasonic processor FS-250N, Shanghai, China). Subsequently, the MWCNT or P-MWCNT dispersion was filled into the internal lumen of a pre-made membrane modules by using a peristaltic pump (BT100-1L, Longer Pump, China) at a flow rate of 88 L m2 h-1 and in the inside-out filtration mode. The resulting membrane possessed a specific CNT loading of 29.4 g m-2. Finally, the prepared membrane modules were immersed in ultrapure water at room temperature for further testing. 2.4. Synthetic feed water and wastewater effluent Two types of synthetic feed solutions were used in this study to determine the selectivity of the CNT membrane for metal ions. The first type was single metal solutions containing Sr2+, Cs+ or one of the competing ions. In comparison, the second type was binary solutions comprising of either Sr2+ or Cs+ and one competing ion of identical valence. Both types of feed solutions were prepared by diluting the stock solutions of individual metal ions at a mass concentration of 100 mg L-1 into the ultrapure water to reach a final concentration of 3 mg L-1 for each metal ion. Moreover, the pH of the feed solutions was adjusted in a range of 3-11 by using 0.05 M HCl or 6
NaOH solution to determine the dependence of CNT’s metal ion selectivity upon the ionization of functional groups on the plasma-treated CNT. A wastewater effluent samples were collected from a local wastewater treatment plant for this study. After sampling, the effluent was immediately transferred to the laboratory and filtered through binder-free glass fiber filters (Whatman, GF/C, USA) to remove suspended and particulate matters larger than approximately 1.2 µm. Characteristics of the secondary wastewater effluent are presented in Table S2 (Supporting Information). In order to determine the selectivity of MWCNT and P-MWCNT for removal of Sr2+ and Cs+ from real wastewater effluent, we spiked Sr2+ and Cs+ separately into a secondary wastewater effluent to the desired concentration of 3 mg L-1 and placed the water overnight prior to membrane filtration experiments. 2.5. Bench-scale membrane filtration setup and protocol A bench scale filtration system was established for this study. The filtration system comprised of a feed water container, a peristaltic pump (BT 100-1L, Longer Pump, China) with a dual-channel pump head, two U-shaped CNT membrane modules, two digital pressure gauges (Asmik, Hangzhou Mike Technology Corporation of Sensing, China) and other accessories (Figure S6, Supporting Information). Prior to each filtration experiment, the CNT membranes were rinsed with ultrapure water for 30 minutes at a constant flowrate of 1 mL min-1. Later, the feed water was filtered through the membranes in the inside-out mode at a flowrate of 0.5 mL min-1 for 60 minutes, which yielded a cumulative throughput of 44 L m-2. The permeate samples were collected from each channel and used for further analyses. All filtration runs were conducted at a room temperature of 24 ± 2 °C in duplicate to ensure repeatability.
7
2.6. Quantification of the membrane selectivity The selectivity of CNT membranes for Sr2+ and Cs+ in single-metal solutions was quantified with three types of parameters i.e., distribution coefficient Kd (mL.g–1), selectivity coefficient (K) and relative selectivity coefficient (K′) that are defined by the following equations [26]: ܭௗ ൌ
ሺ ି ሻ
ܭൌ భ
మ
ಾೈಿ
ܭԢ ൌ
ುషಾೈಿ
(1)
(2)
(3)
Where Co and Ce (mg L-1) represent the initial and equilibrium concentrations of metal ions in the feed solution and the composite permeate sample, respectively; m (g) is the mass of CNT loaded onto the substrate membrane and V (L) is the total volume of permeate passed through the CNT membrane; Kd1 represents the distribution coefficient of Sr2+ or Cs+ and Kd2 represents the distribution coefficient of a competing metal ion; KMWCNT and KP-MWCNT represent the selectivity coefficients of MWCNT and P-MWCNT, respectively. 2.7. Sample characterization and analysis The physiochemical properties of the MWCNT and the P-MWCNT were determined by using various techniques. An X-ray photoelectron spectroscopy (XPS) system (XPS, ESCALAB 250Xi, USA) was employed to investigate the elemental composition and surface chemical functional groups on MWCNT/P-MWCNT before and after plasma treatment, as well as after water filtration. An X-ray diffraction analyzer (D2 CRYSO-XRD, Bruker, Germany) was used to characterize the crystalline structure and atomic spacing of P-MWCNT. The electrokinetic
8
property of MWCNT dispersions was determined in a 10mM NaCl solution by using the laser Doppler velocimetry technique (Nano Brook 90 Plus PALS, Brookhaven, USA). The surface and cross section morphology of the virgin PVC membrane and the CNT composite membranes were evaluated by field emission scanning electron microscopy (SEM, Hitachi S-4800, Japan). The specimens of membranes for SEM imaging were obtained by freeze-fracturing the hollow fiber membranes in liquid hydrogen and then air dried, followed by sputter coating with gold to improve surface conductance. The hydrophobicity of virgin and CNT membranes was determined by the sessile drop method using a goniometer (JC200D2, Beijing Powereach Digital Technology Equipment Co. Ltd., China). Furthermore, the concentrations of dissolved metal ions (except for Cs+) in water samples were measured by using an inductively coupled plasma atomic emission spectrometry system (ICP-AES, SPECTRO ARCOS EOP, SPECTRO Analytical Instruments GmbH/Spectro, Germany), while the Cs concentration was measured by an inductively coupled plasma mass spectrometry system (ICP-MS, NexION 300X, PE, USA). 3. Results and discussions 3.1. Selectivity of CNT membranes in single-metal solution filtration The distribution coefficients Kd of Sr2+ and Cs+ and their competitive metal ions on the PMWCNT membrane were first investigated with separate filtration of the single-metal solutions. The results were also benchmarked with those obtained with the MWCNT membrane to investigate the effect of plasma functionalization on the membrane selectivity. Fig. 1 shows that the Kd values of the studied metal ions onto the CNT membranes generally increased with increasing solution pH. Comparing different CNT membranes, the Kd values of MWCNT membranes were 1.26 mmol g-1 to 3.64 mmol g-1 for Sr2, 0.49 mmol g-1 to 1.48 mmol g-1 for Ca2+ and 0.40 mmol g-1 to 1.21 mmol g-1 for Mg2+ in the pH range of 3.0-9.0 (Fig. 1a), while the Kd 9
values of P-MWCNT membranes were 1.90 mmol g-1 to 4.36 mmol g-1 for Sr2+, 0.96 mmol g-1 to 2.25 mmol g-1 for Ca2+ and 0.67 mmol g-1 to 1.48 mmol g-1 for Mg2+ (Fig. 1b). These results showed that the three divalent cations had greater adsorption on P-MWCNT than MWCNT in the studied pH range. The higher Kd values obtained with P-MWCNT membranes should be ascribed to the higher concentration of oxygenated functional groups available on the surface of CNTs for cation binding (Table S1). Also, regardless of the pH, both CNT membranes demonstrated distribution coefficients in the decreasing order of Sr2+ > Ca2+ > Mg2+. Likewise, both CNT membranes removed more monovalent cations, i.e., Cs+, Na+ and K+ from the single-metal solutions with increasing pH from 3.0 to 9.0 (Fig. 1c & 1d). However, the Kd values for the monovalent ions were consistently below 1.12 mmol g-1 for the P-MWCNT membrane and 0.76 mmol g-1 for the MWCNT membrane, which were close to those of Mg2+ and Ca2+, but much lower than those of Sr2+. These results suggest that the CNT membranes had higher adsorption capacity for divalent metal ions (Sr2+, in particular) than for monovalent metal ions. Comparing the studied monovalent ions, the Kd values for the MWCNT membrane followed the order of Cs+ > Na+ > K+ in the studied pH range. Meanwhile, the Kd values for the P-MWCNT membranes followed the order of Cs+ > Na+ > K+ at pH 3.0 and 9.0 and became similar at pH 5.0 and 7.0. Overall, the P-MWCNT adsorbed slightly more monovalent ions than the MWCNT membranes, but the difference was less pronounced than what was observed for divalent cations (Fig. 1).
10
Figure 1. Distribution coefficients of divalent cations (A & B) and monovalent cations (C & D) on MWCNT and P-MWCNT membranes as a function of feed solution pH. Spiked feed metal concentration = 3 mg L−1, permeate flux = 44 L m-2, filtration time = 60 min, and temperature = 25 ± 2 °C. The error bars represent the upper and lower values obtained in duplicate filtration experiments. Table 1 summarizes the Kd, K1, K2 and K′ values with respect to Sr2+ and Cs+ in the single-metal solutions at a neutral pH value (7.0). The selectivity coefficients (K) of Sr2+ over Mg2+ and Sr2+ over Ca2+ by MWCNT membrane in single-metal solution were 3.49 and 2.19 respectively at the neutral pH 7.0, while the K values of P-MWCNT membrane for Sr2+ over Mg2+ and Sr2+ over 11
Ca2+ were 3.17 and 1.93, respectively. On the other hand, the K values of Cs+ over K+ and Cs+ over Na+ by MWCNT membrane in single metal solution were 1.16 and 1.00, respectively, compared to P-MWCNT membranes with 1.01 and 1.00, respectively. These results confirmed that both membranes possessed strong selectivity to Sr2+ but not Cs+. Moreover, the P-MWCNT membrane had slightly stronger affinities to Sr2+ and Cs+ than the MWCNT membrane, as manifested by the K’ values. Table 1. Kd, K, and K′ values of K+, Na+, Mg2+ and Ca2+ with respect to Cs+ and Sr2+ at pH 7.0. MWCNT Metal ion
Kd (mL g-1)
P-MWCNT
KMWCNT
Kd (mL g-1)
KP-MWCNT
K′
Cs+
0.630
K+
0.540
1.16
0.795
1.01
1.14
Na+
0.630
1.00
0.810
1.00
1.00
Sr2+
2.919
Mg2+
0.836
3.49
1.305
3.17
1.00
Ca2+
1.329
2.19
2.145
1.93
1.13
0.810
4.140
Fig. 2 shows the effect of pH on the selectivity coefficient of P-MWCNT membranes. The K values of Sr2+ over Mg2+ in single-metal solutions fluctuate in a narrow range of 2.82 to 2.93 as the solution pH was adjusted from 3.0 to 9.0 (Fig. 2a). Likewise, the K values of Sr2+ over Ca2+ remained at around 2.0 in the same pH range (Fig. 2b). These results suggested that the variations in solution pH exerted similar impacts on the adsorption of the alkaline earth ions, thereby not affecting the high selectivity of P-MWCNT for Sr2+. In terms of alkaline ions, the K value of Cs+ over K+ by P-MWCNT membrane was greater (1.95) at pH 3.0 but decreased 12
dramatically to 1.05-1.19 after increasing the pH of the solution to 5.0 and above (Fig. 2c). Similarly, the K value of Cs+ over Na+ was relatively high at pH 3.0 (1.95) but decreased to 1.19 as the pH increased to 9.0 (Fig. 2d). The relative selectivity coefficient (K′), or the ratio of the corresponding K values provides a quantitative comparison of the relative selectivity of the P-MWCNT membrane over the MWCNT membrane for the same metal ion. Table 1 shows that the K′ values for Cs+/K+ and Cs+/Na+ are 1.14 and 1.00, respectively; both are greater than unity. This means that, for singlemetal solutions, the P-MWCNT membrane possessed slightly higher selectivity for Cs+ over K+ than the MWCNT membrane. Likewise, the K′ values for Sr2+/Mg2+ and Sr2+/Ca2+ are 1.00 and 1.13, respectively, indicating slightly greater selectivity possessed by the P-MWCNT membrane for Sr2+ over Ca2+.
13
Figure 2. Selectivity of P-MWCNT membranes for selected metal ions in water. A) Sr/Mg in single, binary and wastewater solution, B) Sr/Ca in single, binary and wastewater solution, C) Cs/K in single, binary and wastewater solution and D) Cs/Na in single, binary and wastewater solution. The error bars represent the upper and lower values obtained in duplicate filtration experiments. 3.3. Selectivity of the P-MWCNT in binary-metal solution filtration Considering the relatively high adsorption capacities exhibited by P-MWCNT membranes in single-metal solution filtration (Fig. 1), this study further determined the selectivity of the P-
14
MWCNT membranes during binary-metal solution filtration. Fig. 3 shows the Kd values of the PMWCNT membrane for Sr2+ and Cs+ when each metal ion coexisted with a competing ion of the same valence and an equal initial concentration of 3 mg L-1. In the pH range of 3.0-9.0, the PMWCNT membrane exhibited similar Kd towards Sr2+ in the coexistence of either Ca2+ or Mg2+ (Fig. 3a vs. Fig. 3b) and in both cases, Kd values increased with increasing pH. These results were slightly lower than those obtained with the single-metal solutions (Fig. 1b). For instance, at pH 7.0, Kd value was 4.14 mmol g-1 for Sr2+ in the absence of competing ions (Fig. 1b), but slightly decreased to 3.54 mmol g-1 and 3.63 mmol g-1 in the presence of Ca2+ and Mg2+, respectively (Fig. 3a-b). In the co-existence of Cs+ and K+, the respective Kd values for the two ions increased from 0.43 mmol g-1 to 0.67 mmol g-1 and 0.49 mmol g-1 to 0.79 mmol g-1 as the pH was elevated from 3.09.0 (Fig. 3c). These results were similar to those obtained with single-metal solutions (Fig. 1d), despite the presence of co-existing ions. Comparatively, the distribution coefficient of PMWCNT membrane toward Cs+ was suppressed when Na+ coexisted with Cs+ in the same feed solution, regardless of the solution pH (Fig. 3d). The competitive effect of Na+ on Cs+ adsorption appeared to disagree with the results obtained with the single-metal solutions, with which the distribution coefficients of Cs+ were either greater or equal to those of Na+ across the studied pH range. This discrepancy indicated the occurrence of different adsorption processes for PMWCNT in single- vs. binary-metal solutions. The selectivity coefficients of the P-MWCNT membranes were high for Sr2+ compared to Cs+ in the binary-metal solutions (Fig. 2). The K value of Sr2+ over Mg2+ at pH 3.0 was 3.34 and increased sharply to 5.75 at pH 5.0. Then, from pH 7.0 to 9.0, the K value of Sr2+ over Mg2+ remained at 3.13-3.38 (Fig. 2a). However, the K value of Sr2+ over Ca2+ was 1.00 at pH 3.0, and 15
then increased to 1.50-1.70 at higher pH (Fig. 2b). Moreover, the K values of Cs+ over K+ remained at 0.84-0.95 in the studied pH range (Fig. 2c), while those of Cs+ over Na+ remained at 0.46-0.58 (Fig. 2d). Overall, the pH of the binary metal solution did not exert significant effect on the selectivity of the P-MWCNT membranes for Cs+ and Sr2+ in the binary-metal solutions, except for the coexistence of Sr2+ and Mg2+ at a pH of 5.0. On the other hand, the filtration of binary-metal solutions consistently resulted in higher Sr2+ selectivity but lower Cs+ selectivity than the filtration of single-metal solutions. The opposite trends observed for the two ions suggested the existence of different adsorption mechanisms as depicted later.
16
Figure 3. Distribution coefficient of Sr2+ in the presence of Mg2+ (A) and Ca2+ (B) and the distribution coefficient of Cs+ in the presence of K+ (C) and Na+ (D) by the P-MWCNT membranes from binary-metal solutions at different pH values. Spiked feed metal concentration = 3 mg L−1 and permeate flux = 44 L m−2. Filtration time = 60 min, temperature = 25 ± 2 °C. The error bars represent the upper and lower values obtained in duplicate filtration experiments. 3.4. Selectivity of CNT membranes in wastewater filtration Given the diverse selectivity of CNT membranes for Sr2+ and Cs+ in synthetic solutions, a secondary wastewater effluent containing a suite of metal ions and organic matter (Table S2) was adopted in this study to further evaluate the selectivity of CNT membranes in complex aqueous solutions. Since the effluent did not contain Sr2+ or Cs+, 3 mg L−1 of each ion was spiked into the wastewater effluent to match the conditions used in the single-metal solution experiments. The separation performance of P-MWCNT membrane for Sr2+ in the wastewater effluent was similar to those observed in the single- and binary-metal solutions (Fig. 4b), although more competitive metal ions co-existed in the wastewater at higher concentrations than that of Sr2+. The Kd of P-MWCNT membrane for Sr2+ was 2.77 mmol g-1 to 3.58 mmol g-1 from wastewater effluent in the pH range of 3.0-9.0, greater than the competitive metal ions (Table S2) such as Mg2+ and Ca2+ (Fig. 4b). In contrast, the Kd values of P-MWCNT membrane was 0.49 mmol g-1 to 1.87 mmol g-1 for Mg2+ and 0.58 mmol g-1 to 1.54 mmol g-1 for Ca2+ in the same pH range. The Kd values of other metals such as Cu2+ and Zn2+ were above 4.00 mmol g-1 due to their high initial concentrations and/or strong affinity to the P-MWCNT membrane. Moreover, the Kd values of all metal ions onto the P-MWCNT membrane increased when the pH of the wastewater effluent was increased from 3.0-9.0. On the other side, the Kd values of 17
MWCNT membrane were only 2.32 mmol g-1 to 3.11 mmol g-1 for Sr2+, 0.40 to 1.21 mmol g-1 for Mg2+ and 0.49 mmol g-1 to 0.85 mmol g-1 for Ca2+ at the same pH range (Fig. 4a). These results justified that the P-MWCNT membrane had higher adsorption capacities for metal ions than the MWCNT membrane under the studied conditions. On the other hand, the Kd values of both MWCNT and P-MWCNT membranes to monovalent Cs+ in wastewater were negligible and greatly suppressed by the presence of other competitive metal ions (Fig. 4c-d). Specifically, the P-MWCNT membrane showed a Kd value of 0.22 mmol g-1 to 0.42 mmol g-1 to Cs+ in the pH range of 3.0-9.0, while the Kd values of competitive K+ and Na+ were 0.49 mmol g-1 to 1.35 mmol g-1 and 0.49 mmol g-1 to 1.41 mmol g-1, respectively (Fig. 4d). Herein, the lower Kd values of Cs+ than those of K+ and Na+ should be ascribed to various influencing factors such as high concentrations of competing ions. The chemical properties of the Cs+ may also play a role as elucidated later. The effect of pH on the selectivity of selected metal ions by P-MWCNT membranes in wastewater effluent is shown in Fig. 2. The K value of Sr2+ over Mg2+ in the wastewater was 5.60 at the lower pH of 3.0 and then decreased to 1.91 at the higher pH of 9.0 (Fig. 2a). Similarly, the K value of Sr2+ over Ca2+ was higher at lower pH (4.74 at pH 3.0) and decreased to 2.32 with increasing the pH to 9.0) (Fig. 2b). Overall, it was found that the K values of divalent metal ions in the wastewater effluent were greater at lower pH and decreased with increasing pH. However, the K values of monovalent cations in the wastewater effluent were less affected by pH and stayed at very low levels compared to those in single-metal solution and binary-metal solution (Fig. 2c-d). Among those ions, the K value of Cs+ over K+ in wastewater effluent was 0.45 at pH 3.0 and decreased to 0.31 at a higher pH of 9.0 (Fig. 2c). Similarly, the K value of Cs+ over Na+ in the wastewater effluent fluctuated between 0.45 and 0.29 in the pH range of 3.0-9.0 (Fig. 2d). 18
Collectively, the K values of divalent cations were greater than monovalent cations during CNT membrane filtration of the wastewater effluent.
Figure 4. Distribution coefficient of MWCNT membrane (A & C) and the P-MWCNT membranes (B & D) for Sr2+ and Cs+ spiked in a wastewater effluent as a function of pH. Initial concentrations of Sr2+ and Cs+ were 3 mg L−1, permeate flux of 44 L m−2, filtration time = 60 min, and temperature = 25 ± 2 °C. The error bars represent the upper and lower values obtained in duplicate filtration experiments.
19
3.5. Potential mechanisms for CNT’s selectivity for metal ions The XPS spectra (Fig. 5) shows the emerge of new peaks at binding energies of 134 eV, 399 eV and 1304 eV after the filtration of wastewater effluent are ascribed to Sr2+, Ca2+ and Mg2+ that were indeed adsorbed on the surface of P-MWCNT. The removal mechanisms of divalent cations by adsorbents usually involve inner-sphere complexation reactions between the metal ions and the electron-pair donor atoms available on the surface of the adsorbents [29]. Under the studied conditions, Sr2+, Ca2+ and Mg2+ were possibly removed from water by forming innersphere complexes on the surface of MWCNT and P-MWCNT. Because the binding sites on CNTs for divalent cations are also those for proton binding, the increase in pH would lower the concentration of proton in the feed solution, thus mitigating the competing effect of proton for surface complexation. This trend was indeed observed for the experimentally determined Kd values for Sr2+, Ca2+, and Mg2+ (Fig. 1a & 1b).
Figure 5. XPS spectra of the P-MWCNT membrane (A) before and (B) after filtration of the wastewater effluent.
20
In order to further explore the strong selectivity of MWCNT and P-MWCNT for Sr2+, major characteristics of the metal ions involved in this study were obtained from published data [17, 30-32] and are summarized in Table 2. As described above, the selectivity of both CNT membranes followed the sequence of Sr2+ > Ca2+ > Mg2+ for both single- and binary-metal solutions, regardless of solution pH (Fig. 5). Similar selectivity sequence has also been observed during the adsorption of earth alkali cations to titanium oxide surfaces [33-35] and follows the classical Hofmeister series for ion affinity to oxide surfaces [36]. This sequence is consistent with that of ionic radii of the studied ions and opposite to the sequence of absolute hardness and absolute electronegativity (Table 2). According to Nieboer and Richardson [37], alkali and alkaline earth ions belong to Class A metal ions that are considered to be oxygen-seeking or ‘hard’ in nature. This means that these ions prefer to bind to oxygenated functional groups on CNT surfaces. Therefore, plasma treatment provided more oxygenated sites on MWCNT, which allowed the P-MWCNT to remove more alkaline earth ions than the pristine MWCNT under similar conditions (Fig. 1). Moreover, for alkaline earth ions, the inner-sphere complexation process involves substitution of the water molecules in the hydration layer of the metal ions by the CNT functional groups. As the ionic radius increases from Mg2+ to Sr2+, the ionic bonding strength between the alkaline earth ions and the hydrating water decreases with the decreasing absolute hardness. This favors the substitution of hydrating water molecules by the CNT functional groups. As such, the CNT membranes preferentially removed Sr2+ over Mg2+ and Ca2+ from the studied water and wastewater effluent (Fig. 2).
21
Table 2. Major properties of metal ions used in this study. Parameters
Metal ions Sr2+
Ca2+
Mg2+
Cs+
K+
Na+
Ionic radius (Հ )a
1.25
1.00
0.72
1.70
1.38
1.02
Hydration energy (kJ mol-1)b
̶ 1380
̶ 1306
̶ 1828
̶ 376
̶ 271
̶ 365
Hydrated radius (nm)b
0.412
0.412
0.300
0.329
0.201
0.276
Hydration shell thickness (nm)
0.294
0.312
0.228
0.159
0.063
0.174
Absolute hardnessc
16.3
19.52
32.55
10.6
13.64
21.08
Absolute electronegativityd
27.3
31.39
47.59
14.5
17.99
26.21
Ionic indexe
3.70
4.02
5.70
0.58
0.72
0.85
a
[17]
b
[31]
c
[32]
d
[30]
e
[37]
On the other hand, the adsorption of CNT membranes towards monovalent cations was primarily induced by electrostatic or Coulombic attraction between negatively charged CNTs (Fig. S2, Supporting Information) and positively charged monovalent cations [29]. During this process, water molecules in the hydration layer of metal ions are maintained since no chemical bonding is formed between the metal ions and the functional groups on CNT surfaces. Thus, the stability of metal ion hydrates is no longer important, which is different from that of alkaline earth ions. Accordingly, the adsorption of alkali ions was stronger at higher solution pH (Fig. 1c & 1d) because the electrostatic attraction between the ions and the CNT surfaces increased as the CNT surfaces were more negatively charged (Fig. S2). Besides, pH decreasing also means that there were fewer protons in the aqueous solutions to compete with alkali ions for surface binding on CNTs. This also promoted ion adsorption by the membranes. 22
However, the inconsistent trends observed for the selectivity of CNTs for the alkali ions necessitate further scrutiny. For the binary-metal solutions, the selectivity sequence of the membranes followed a simple trend of Na+ > K+ > Cs+ (Fig. 2c & 2d). This sequence is in agreement to that of the strength of Coulombic attraction and indicates its dominant role during the adsorption process. For the single-metal solutions, the selectivity sequence at an acidic pH of 3.0 changed to Cs+ > Na+ > K+ (Fig. 1c & 1d). This suggests that non-Coulombic interaction may be important under this particular condition and can enhance the adsorption of Cs+. Nevertheless, a conclusive remark in this regard cannot be made here and further investigation is warranted. 4. Conclusion ·
Both CNT membranes prepared in this study effectively removed Sr2+ from the singlemetal solutions and showed higher distribution coefficients for Sr2+ than the competitive metal ions. P-MWCNT membrane displayed excellent selectivity for Sr2+ in the binary metal system and real wastewater effluent, while its selectivity for Cs+ was lower than for other competitive metal ions.
·
The adsorptive removal of Sr2+ and other alkaline earth ions by CNT membranes involved the formation of inner-sphere complexes, and therefore, the selectivity sequence increased with the ionic radii of the studied ions.
·
The increase in the initial pH of the feed solution promoted the adsorption of Sr2+ onto the CNT membranes, but had mixed effects on the selectivity of Sr2+ in both synthetic solution and wastewater effluent.
·
Adsorptive filtration of monovalent Cs+ and other alkali ions were primarily caused by electrostatic attraction between the cations and negatively charged CNT surfaces, and
23
therefore, was enhanced with elevating solution pH. Due to the unspecific nature of the interaction, the selectivity sequence of CNTs for alkali ions followed different trends under varying solution conditions. Acknowledgment The authors wish to thank Dr. Guling Zhang from Minzu University of China for providing the plasma treatment facility used in this work. Dr. Jianying Shang from China Agricultural University is acknowledged for providing access to ICP-AES used in this work. This research was financially supported by the special fund of State Key Joint Laboratory of Environmental Simulation and Pollution Control of China (18L01ESPC) and the Fundamental Research Funds for the Central Universities of China (310421111). References [1] S. Ding, Y. Yang, C. Li, H. Huang, L.A. Hou, The effects of organic fouling on the removal of radionuclides by reverse osmosis membranes, Water Res. 95 (2016) 174-184. [2] L. Wu, G. Zhang, Q. Wang, L.a. Hou, P. Gu, Removal of strontium from liquid waste using a hydraulic pellet co-precipitation microfiltration (HPC-MF) process, Desalination 349 (2014) 31-38. [3] D. Alby, C. Charnay, M. Heran, B. Prelot, J. Zajac, Recent developments in nanostructured inorganic materials for sorption of cesium and strontium: Synthesis and shaping, sorption capacity, mechanisms, and selectivity-A review, J Hazard Mater 344 (2018) 511-530. [4] F. Ma, Z. Li, H. Zhao, Y. Geng, W. Zhou, Q. Li, L. Zhang, Potential application of graphene oxide membranes for removal of Cs(I) and Sr(II) from high level-liquid waste, Separation and Purification Technology 188 (2017) 523-529. [5] 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. [6] T. Nur, P. Loganathan, J. Kandasamy, S. Vigneswaran, Removal of strontium from aqueous solutions and synthetic seawater using resorcinol formaldehyde polycondensate resin, Desalination 420 (2017) 283291. [7] J. Wang, S. Zhuang, Removal of cesium ions from aqueous solutions using various separation technologies, Reviews in Environmental Science and Bio/Technology 18 (2019) 231-269. 24
[8] J. Wang, S. Zhuang, Y. Liu, Metal hexacyanoferrates-based adsorbents for cesium removal, Coord. Chem. Rev. 374 (2018) 430-438. [9] A.J. O'Donnell, D.A. Lytle, S. Harmon, K. Vu, H. Chait, D.D. Dionysiou, Removal of strontium from drinking water by conventional treatment and lime softening in bench-scale studies, Water Res 103 (2016) 319-333. [10] Y. Zheng, J. Qiao, J. Yuan, J. Shen, A.-j. Wang, L. Niu, Electrochemical Removal of Radioactive Cesium from Nuclear Waste Using the Dendritic Copper Hexacyanoferrate/Carbon Nanotube Hybrids, Electrochimica Acta 257 (2017) 172-180. [11] B. Sarkar, S. Mandal, Y.F. Tsang, P. Kumar, K.H. Kim, Y.S. Ok, Designer carbon nanotubes for contaminant removal in water and wastewater: A critical review, Sci. Total Environ. 612 (2018) 561-581. [12] Y. Huang, W. Wang, Q. Feng, F. Dong, Preparation of magnetic clinoptilolite/CoFe2O4 composites for removal of Sr2+ from aqueous solutions: Kinetic, equilibrium, and thermodynamic studies, Journal of Saudi Chemical Society 21 (2017) 58-66. [13] M.R. Awual, T. Yaita, Y. Miyazaki, D. Matsumura, H. Shiwaku, T. Taguchi, A Reliable Hybrid Adsorbent for Efficient Radioactive Cesium Accumulation from Contaminated Wastewater, Sci Rep 6 (2016) 19937. [14] R. Kamaraj, S. Vasudevan, Evaluation of electrocoagulation process for the removal of strontium and cesium from aqueous solution, Chemical Engineering Research and Design 93 (2015) 522-530. [15] R.R. Sheha, S.H. El-Khouly, Adsorption and diffusion of cesium ions in zirconium(IV) iodomolybdate exchanger, Chemical Engineering Research and Design 91 (2013) 942-954. [16] I.A. B. Prelot, F. Marchandeau, J. Zajac,, On the real performance of cation exchange resins in wastewater treatment under conditions of cation competition: the case of heavy metal pollution, Environ. Sci. Pollut. Res. 21 (2014) 9334–9343. [17] Y. Marcus, A simple empirical model describing the thermodynamics of hydration of ions of widely varying charges, sizes, and shapes, Biophysieai Chemistr 51 (1994) 111-127. [18] A.K. Vipin, B. Fugetsu, I. Sakata, A. Isogai, M. Endo, M. Li, M.S. Dresselhaus, Cellulose nanofiber backboned Prussian blue nanoparticles as powerful adsorbents for the selective elimination of radioactive cesium, Sci Rep 6 (2016) 37009. [19] Ihsanullah, A. Abbas, A.M. Al-Amer, T. Laoui, M.J. Al-Marri, M.S. Nasser, M. Khraisheh, M.A. Atieh, Heavy metal removal from aqueous solution by advanced carbon nanotubes: Critical review of adsorption applications, Sep. Purif. Technol. 157 (2016) 141-161. [20] H.-K. Lee, J.W. Choi, W. Oh, S.-J. Choi, Sorption of cesium ions from aqueous solutions by multiwalled carbon nanotubes functionalized with copper ferrocyanide, Journal of Radioanalytical and Nuclear Chemistry (2016). [21] R. Yavari, Y.D. Huang, S.J. Ahmadi, Adsorption of cesium (I) from aqueous solution using oxidized multiwall carbon nanotubes, Journal of Radioanalytical and Nuclear Chemistry 287 (2010) 393-401.
25
[22] C. Chen, J. Hu, D. Xu, X. Tan, Y. Meng, X. Wang, Surface complexation modeling of Sr(II) and Eu(III) adsorption onto oxidized multiwall carbon nanotubes, J Colloid Interface Sci 323 (2008) 33-41. [23] D. Shao, J. Hu, X. Wang, Plasma Induced Grafting Multiwalled Carbon Nanotube with Chitosan and Its Application for Removal of UO 22+, Cu2+, and Pb2+ from Aqueous Solutions, PLASMA PROCESS POLYM 7 (2010) 977-985. [24] E.-s. Lee, Y.-K. Lim, Y.-s. Chun, B.-Y. Wang, D.-S. Lim, Characteristics of hydrogen plasma treated carbon nanotubes and their influence on the mechanical properties of polyetherimide-based nanocomposites, Carbon 118 (2017) 650-658. [25] M. Usman Farid, H.-Y. Luan, Y. Wang, H. Huang, A.K. An, R. Jalil Khan, Increased adsorption of aqueous zinc species by Ar/O 2 plasma-treated carbon nanotubes immobilized in hollow-fiber ultrafiltration membrane, Chem. Eng. J. 325 (2017) 239-248. [26] K. Li, Q. Gao, G. Yadavalli, X. Shen, H. Lei, B. Han, K. Xia, C. Zhou, Selective Adsorption of Gd(3+) on a Magnetically Retrievable Imprinted Chitosan/Carbon Nanotube Composite with High Capacity, ACS Appl Mater Interfaces 7 (2015) 21047-21055. [27] S. Gómez, N.M. Rendtorff, E.F. Aglietti, Y. Sakka, G. Suárez, Surface modification of multiwall carbon nanotubes by sulfonitric treatment, Appl. Surf. Sci. 379 (2016) 264-269. [28] Y.X. Bin Zhang, Yifang Zheng, Lizong Dai, Mingqiu Zhang, Jin Yang, Yujie Chen, Xudong Chen and, J. Zhou, A Facile Synthesis of Polypyrrole/Carbon Nanotube Composites with Ultrathin, Uniform and Thickness-Tunable Polypyrrole Shells, Nanoscale Research Letters 6 (2011) 431. [29] S. Warner, The Inner-Sphere Complex: A key to Understanding surface reactivity, Advance in Chemistry 244 (1995) 1-32. [30] R.G. Pearson, Phenomenological theory of ion salvation-effective radii of hydrated ions, Inorg. Chem. 27 (1988) 734-740. [31] B. Tansel, Significance of thermodynamic and physical characteristics on permeation of ions during membrane separation: Hydrated radius, hydration free energy and viscous effects, Sep. Purif. Technol. 86 (2012) 119-126. [32] N. Li, L. Zhang, Y. Chen, M. Fang, J. Zhang, H. Wang, Highly Efficient, Irreversible and Selective Ion Exchange Property of Layered Titanate Nanostructures, Advanced Functional Materials 22 (2012) 835-841. [33] X. Tan, M. Fang, L. Tan, H. Liu, X. Ye, T. Hayat, X. Wang, Core–shell hierarchical C@Na2Ti3O7·9H2O nanostructures for the efficient removal of radionuclides, Environmental Science: Nano 5 (2018) 1140-1149. [34] B. Filipowicz, M. Pruszynski, S. Krajewski, A. Bilewicz, Adsorption of (137)Cs on titanate nanostructures, J Radioanal Nucl Chem 301 (2014) 889-895. [35] S.P. S. Kasap, H. Tel, Titanate nanotubes: preparation, characterization and application in adsorption of strontium ion from aqueous solution, Radiochim. Acta 100 (2012) 925–929. [36] Y. Marcus, Ionic Radii in Auqeous Solutions, Chem. Rev. 88 (1988) 1475-1498. 26
[37] E. Nieboer, S.H.D. Richardson, THE REPLACEMENT OF THE NONDESCRIPT TERM 'HEAVY METALS' BY A BIOLOGICALLY AND CHEMICALLY SIGNIFICANT CLASSIFICATION OF METAL IONS, Environmental Pollution (Series B) 1 (1980) 3-26.
27
Highlights: ·
The selective removal of Sr2+ and Cs+ by P-MWCNT membranes were studied.
·
The P-MWCNT membrane showed superb selectivity to Sr2+ over other metal ions.
·
The selectivity of Cs+ was depressed by the presence of coexisted metal ions.
·
The selectivity of divalent cations resulted from inner-sphere complexation.
·
While the selectivity of monovalent cations resulted from electrostatic interactions.
28
Graphical Abstract
29
Conflict of interest The authors of this research paper declare no conflict of interest.
30
S1383-5866(19)34765-3 https://doi.org/10.1016/j.seppur.2019.116352 SEPPUR 116352
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
17 October 2019 25 November 2019 25 November 2019
Please cite this article as: S. Ali, I. Ali Shah, H. Huang, Selectivity of Ar/O2 plasma-treated carbon nanotube membranes for Sr(II) and Cs(I) in water and wastewater: Fit-for-purpose water treatment, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116352
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Selectivity of Ar/O2 plasma-treated carbon nanotube membranes for Sr(II) and Cs(I) in water and wastewater: Fit-for-purpose water treatment
Sharafat Alia, Izaz Ali Shaha and Haiou Huanga,b,* a
State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of
Environment, Beijing Normal University, No. 19, Xinjiekouwai Street, Beijing, 100875, China b
Department of Environmental Health and Engineering, Bloomberg School of Public Health,
The John Hopkins University, 615 North Wolfe Street, MD 21205, USA *
Corresponding Author: E-mail address: [email protected] (H. Huang)
Abstract The need for high-performance and novel technologies for selective treatment of radionuclides in water has recently stimulated broad research. In this study, we examined the ability of carbon nanotube (CNT) membranes in selective removal of strontium (Sr2+) and cesium (Cs+) from water and wastewater. In single-metal systems that contained only Sr2+, Cs+ or their competitive metal ions, the distribution coefficients (Kd) of functionalized CNT membranes for divalent cations, i.e., Sr2+, Ca2+, and Mg2+, were measured as 4.41 mmol g-1, 2.14 mmol g-1, and 1.30 mmol g-1, respectively, and those for monovalent cations, i.e., Cs+, K+ and Na+, were 0.81 mmol g-1, 0.79 mmol g-1, and 0.81 mmol g-1, respectively. When Cs+ or Sr2+ co-existed with a competing cation, both pristine and functionalized CNT membranes showed excellent selectivity to Sr2+ over Mg2+ and Ca2+ while their selectivity to Cs+ over K+ and Na+ diminished. Moreover, the CNT membranes remained their selectivity for Sr2+ in a wastewater effluent despite the 1
complex water composition. Overall, the selectivity sequence of the CNT membranes for divalent cations support the occurrence of inner-sphere complexation on CNT surfaces, while that for the monovalent cations was probably determined by electrostatic interactions. This study demonstrates that Ar/O2 plasma treatment is a greener technology for the production of CNT functionalized membranes for selective removal of contaminants from water to achieve “fit-forpurpose” treatment. Keywords: Adsorptive membrane filtration; Distribution coefficient; Plasma functionalization; Radionuclides; Selective removal. 1. Introduction Radioactive contaminants are mainly released into the aquatic environment during the operation of nuclear power plants, reprocessing of spent fuel, testing of nuclear weapons and sometimes due to accidental waste discharge (e.g. Fukushima Daiichi Nuclear Power Plant Accident, Japan, 2011) [1-4]. Among different radionuclides, radiostrontium and radiocesium isotopes are found in high concentrations in seawater, groundwater and wastewater because of the increases in nuclear industrial activities and disasters such as Chernobyl and Fukushima Daiichi Nuclear Plant Accident [1, 5-8]. Once released into the environment, Sr and Cs isotopes have persistent impacts on human beings and aquatic organisms even at very low concentrations due to their long half-lives, e.g., 28.9 years for
90
Sr and 30.2 years for
137
Cs [9-11]. Since the chemical
properties of Sr are very similar to those of calcium (Ca) and magnesium (Mg), and the chemical properties of Cs are similar to those of potassium (K) and sodium (Na), when they enter the human body these radionuclides can easily replace the essential salts in the body and remain there for long time [12-14]. Therefore, direct or indirect exposure of humans and aquatic
2
organisms to radioactive Sr and Cs can lead to various conditions, for instance, anemia, leukemia and genetic disorders. Therefore, careful consideration regarding the effective treatment and separation of Sr and Cs isotopes from water is of immense importance. Sr and Cs isotopes exist in a different ionic state in water depending on the environmental conditions and water composition [15]. Sr is predominantly found in the form of exchangeable Sr2+ over a wide range of pH [16], while Cs exists in natural waters primarily as Cs+ ions and forms complexes with some organic molecules and weak complexes with nitrate, sulfate and halides. The bared Sr2+ has an ionic radius equal to 113 pm compared to 170 pm for bared Cs+ [17]. However, Sr2+ in water is surrounded by a hydration layer of 150 pm that is much thicker than the hydration layer of Cs+ which is only 49 pm. Comparing the structures of the hydration layers for both cations, each Sr2+ ion contains 6.4 water molecules, which is two times greater than that of Cs+ (2.1 water molecules per ion) [17]. Therefore, this is harder for the current water treatment technology to remove both Sr2+ and Cs+ from water because of their different chemical behaviors. Another major obstacle faced by the current water treatment technologies is that these radioactive contaminants exist in relatively low concentrations compared to co-existing cations with similar properties, such as sodium, potassium, calcium and magnesium. Thus, novel technologies are in immediate need to effectively separate these radioactive species from aqueous streams [18]. Recently, carbon nanotubes (CNTs) containing oxygenated functional groups have attracted the attention of many researchers, due to their excellent capability in selective adsorption of heavy metal ions from aqueous solution [19]. Although many previous studies have reported on the removal of Cd(II), Zn(II), Co(II), Mn(II), Fe(III), Cu(II), Pb(II), etc. by CNTs, there are few studies available on the removal of Sr2+ and Cs+ by CNTs. Lee et al. [20] evaluated the removal 3
of Cs+ from aqueous solutions by copper ferrocyanide functionalized multi-walled carbon nanotubes (CuFCs-MWCNTs) and found that, in the pH range of 7-11, CuFCs-MWCNTs possessed a high affinity to Cs+ in the presence of co-existing ions such as K+ and Na+. Similarly, Yavari et al. [21] reported effective adsorption of Cs+ from nuclear wastewater by oxidized MWCNTs treated with nitric acid. They also found that the adsorption capacity of oxidized MWCNTs is strongly dependent on the pH values and ionic strengths of the water. Like Cs+, Chen et al. [22] studied the adsorption of Sr2+ by oxidized MWCNTs and found that the adsorption percentage increased with increasing pH of the solution but decreased with increasing the ionic strengths of the solution. Despite the promising removal results obtained in the previous studies, the selectivity of functional CNT materials for removal of Sr2+ and Cs+ from complex water matrices has not yet been fully assessed. Therefore, it is unclear whether and to what extent the co-existence of similar metal ions would reduce the effectiveness of CNT for Sr or Cs removal. Therefore, this study was performed with three specific objectives: (1) to systematically assess the selectivity of pristine and plasma-functionalized MWCNT for Sr and Cs in the presence/absence of similar ions through adsorptive filtration technology (2) to explore the ability of MWCNTs for the filtration of Sr and Cs in realistic wastewater, and (3) to explore the relationship between the properties of Sr and Cs ions and their selectivity by CNT membranes. We fabricated pristine and plasma-treated CNT membranes by depositing the respective MWCNTs on the inside lumen of a hollow fiber membrane. The prepared membranes were subsequently assessed for selectively removing Sr2+ and Cs+ from aqueous solutions and secondary wastewater effluent. We also characterized the as-prepared membrane to understand the selectivity mechanisms, in conjunction with consideration of ionic properties of Sr and Cs. 4
The obtained results revealed a strong selectivity of the CNT membranes for Sr2+ under all water chemistry conditions studied. 2. Materials and methods 2.1. Chemicals Reagent-grade strontium nitrate, calcium nitrate, magnesium nitrate, cesium nitrate, sodium nitrate, and potassium nitrate were purchased from Aladdin Industrial Corporation, China. These salts were selected to evaluate the selectivity of CNT membranes for removal Sr2+ and Cs+ ions from aqueous solutions over other divalent or monovalent cations. Stock solutions of these salts were prepared by dissolving individual salts in ultrapure water obtained from a Milli-Q water system (E-POD, Millipore, France). A polyvinylchloride (PVC), hollow fiber membrane with a nominal pore size of 0.01μm was bought from Litree Purifying Technology Co., Ltd, China and was used as the substrate for preparing the CNT membranes. 2.2. Pristine and plasma-functionalized multi-walled carbon nanotubes A pristine multi-walled carbon nanotube (MWCNT) was purchased from Beijing Boyu Technology Corporation of High-tech New Materials, China. According to the manufacturer, this MWCNT was prepared by using a chemical vapor deposition method and possessed outer diameters less than 8 nm, lengths ranging between 10-20 µm, a specific surface area (SSA) of 500 m2 g-1, and purity of above 95%. The as-received MWCNT sample was further functionalized by treating the powder in a borosilicate glass tubular plasma reactor (3 cm in diameter, 118 cm long), following the previous methods reported in the literature [23-25]. During the plasma treatment, the gaseous mixture was injected into the reactor at flowrates of 70 Sccm3 min-1 (standard cubic centimeter per minute) for Ar and 40 Sccm3 min-1 for O2. The gas 5
pressure within the reactor was maintained at 2 Pa when a plasma power of 80 W was applied at a frequency of 13.56 MHz to produce the Ar/O2 plasma. After the treatment, the plasmafunctionalized MWCNT (P-MWCNT) powder was collected in an amber glass bottle and stored at the room temperature. 2.3. Fabrication of the CNT membrane Small U-shaped membrane modules were pre-made with the PVC membrane to obtain effective membrane areas of approximately 6.78 × 10-4 m2 for each module. Then, 20 mg of MWCNT or P-MWCNT was dispersed in 20 ml of ultrapure water and sonicated for 10 minutes at 150 W with a probe sonicator (Ultrasonic processor FS-250N, Shanghai, China). Subsequently, the MWCNT or P-MWCNT dispersion was filled into the internal lumen of a pre-made membrane modules by using a peristaltic pump (BT100-1L, Longer Pump, China) at a flow rate of 88 L m2 h-1 and in the inside-out filtration mode. The resulting membrane possessed a specific CNT loading of 29.4 g m-2. Finally, the prepared membrane modules were immersed in ultrapure water at room temperature for further testing. 2.4. Synthetic feed water and wastewater effluent Two types of synthetic feed solutions were used in this study to determine the selectivity of the CNT membrane for metal ions. The first type was single metal solutions containing Sr2+, Cs+ or one of the competing ions. In comparison, the second type was binary solutions comprising of either Sr2+ or Cs+ and one competing ion of identical valence. Both types of feed solutions were prepared by diluting the stock solutions of individual metal ions at a mass concentration of 100 mg L-1 into the ultrapure water to reach a final concentration of 3 mg L-1 for each metal ion. Moreover, the pH of the feed solutions was adjusted in a range of 3-11 by using 0.05 M HCl or 6
NaOH solution to determine the dependence of CNT’s metal ion selectivity upon the ionization of functional groups on the plasma-treated CNT. A wastewater effluent samples were collected from a local wastewater treatment plant for this study. After sampling, the effluent was immediately transferred to the laboratory and filtered through binder-free glass fiber filters (Whatman, GF/C, USA) to remove suspended and particulate matters larger than approximately 1.2 µm. Characteristics of the secondary wastewater effluent are presented in Table S2 (Supporting Information). In order to determine the selectivity of MWCNT and P-MWCNT for removal of Sr2+ and Cs+ from real wastewater effluent, we spiked Sr2+ and Cs+ separately into a secondary wastewater effluent to the desired concentration of 3 mg L-1 and placed the water overnight prior to membrane filtration experiments. 2.5. Bench-scale membrane filtration setup and protocol A bench scale filtration system was established for this study. The filtration system comprised of a feed water container, a peristaltic pump (BT 100-1L, Longer Pump, China) with a dual-channel pump head, two U-shaped CNT membrane modules, two digital pressure gauges (Asmik, Hangzhou Mike Technology Corporation of Sensing, China) and other accessories (Figure S6, Supporting Information). Prior to each filtration experiment, the CNT membranes were rinsed with ultrapure water for 30 minutes at a constant flowrate of 1 mL min-1. Later, the feed water was filtered through the membranes in the inside-out mode at a flowrate of 0.5 mL min-1 for 60 minutes, which yielded a cumulative throughput of 44 L m-2. The permeate samples were collected from each channel and used for further analyses. All filtration runs were conducted at a room temperature of 24 ± 2 °C in duplicate to ensure repeatability.
7
2.6. Quantification of the membrane selectivity The selectivity of CNT membranes for Sr2+ and Cs+ in single-metal solutions was quantified with three types of parameters i.e., distribution coefficient Kd (mL.g–1), selectivity coefficient (K) and relative selectivity coefficient (K′) that are defined by the following equations [26]: ܭௗ ൌ
ሺ ି ሻ
ܭൌ భ
మ
ಾೈಿ
ܭԢ ൌ
ುషಾೈಿ
(1)
(2)
(3)
Where Co and Ce (mg L-1) represent the initial and equilibrium concentrations of metal ions in the feed solution and the composite permeate sample, respectively; m (g) is the mass of CNT loaded onto the substrate membrane and V (L) is the total volume of permeate passed through the CNT membrane; Kd1 represents the distribution coefficient of Sr2+ or Cs+ and Kd2 represents the distribution coefficient of a competing metal ion; KMWCNT and KP-MWCNT represent the selectivity coefficients of MWCNT and P-MWCNT, respectively. 2.7. Sample characterization and analysis The physiochemical properties of the MWCNT and the P-MWCNT were determined by using various techniques. An X-ray photoelectron spectroscopy (XPS) system (XPS, ESCALAB 250Xi, USA) was employed to investigate the elemental composition and surface chemical functional groups on MWCNT/P-MWCNT before and after plasma treatment, as well as after water filtration. An X-ray diffraction analyzer (D2 CRYSO-XRD, Bruker, Germany) was used to characterize the crystalline structure and atomic spacing of P-MWCNT. The electrokinetic
8
property of MWCNT dispersions was determined in a 10mM NaCl solution by using the laser Doppler velocimetry technique (Nano Brook 90 Plus PALS, Brookhaven, USA). The surface and cross section morphology of the virgin PVC membrane and the CNT composite membranes were evaluated by field emission scanning electron microscopy (SEM, Hitachi S-4800, Japan). The specimens of membranes for SEM imaging were obtained by freeze-fracturing the hollow fiber membranes in liquid hydrogen and then air dried, followed by sputter coating with gold to improve surface conductance. The hydrophobicity of virgin and CNT membranes was determined by the sessile drop method using a goniometer (JC200D2, Beijing Powereach Digital Technology Equipment Co. Ltd., China). Furthermore, the concentrations of dissolved metal ions (except for Cs+) in water samples were measured by using an inductively coupled plasma atomic emission spectrometry system (ICP-AES, SPECTRO ARCOS EOP, SPECTRO Analytical Instruments GmbH/Spectro, Germany), while the Cs concentration was measured by an inductively coupled plasma mass spectrometry system (ICP-MS, NexION 300X, PE, USA). 3. Results and discussions 3.1. Selectivity of CNT membranes in single-metal solution filtration The distribution coefficients Kd of Sr2+ and Cs+ and their competitive metal ions on the PMWCNT membrane were first investigated with separate filtration of the single-metal solutions. The results were also benchmarked with those obtained with the MWCNT membrane to investigate the effect of plasma functionalization on the membrane selectivity. Fig. 1 shows that the Kd values of the studied metal ions onto the CNT membranes generally increased with increasing solution pH. Comparing different CNT membranes, the Kd values of MWCNT membranes were 1.26 mmol g-1 to 3.64 mmol g-1 for Sr2, 0.49 mmol g-1 to 1.48 mmol g-1 for Ca2+ and 0.40 mmol g-1 to 1.21 mmol g-1 for Mg2+ in the pH range of 3.0-9.0 (Fig. 1a), while the Kd 9
values of P-MWCNT membranes were 1.90 mmol g-1 to 4.36 mmol g-1 for Sr2+, 0.96 mmol g-1 to 2.25 mmol g-1 for Ca2+ and 0.67 mmol g-1 to 1.48 mmol g-1 for Mg2+ (Fig. 1b). These results showed that the three divalent cations had greater adsorption on P-MWCNT than MWCNT in the studied pH range. The higher Kd values obtained with P-MWCNT membranes should be ascribed to the higher concentration of oxygenated functional groups available on the surface of CNTs for cation binding (Table S1). Also, regardless of the pH, both CNT membranes demonstrated distribution coefficients in the decreasing order of Sr2+ > Ca2+ > Mg2+. Likewise, both CNT membranes removed more monovalent cations, i.e., Cs+, Na+ and K+ from the single-metal solutions with increasing pH from 3.0 to 9.0 (Fig. 1c & 1d). However, the Kd values for the monovalent ions were consistently below 1.12 mmol g-1 for the P-MWCNT membrane and 0.76 mmol g-1 for the MWCNT membrane, which were close to those of Mg2+ and Ca2+, but much lower than those of Sr2+. These results suggest that the CNT membranes had higher adsorption capacity for divalent metal ions (Sr2+, in particular) than for monovalent metal ions. Comparing the studied monovalent ions, the Kd values for the MWCNT membrane followed the order of Cs+ > Na+ > K+ in the studied pH range. Meanwhile, the Kd values for the P-MWCNT membranes followed the order of Cs+ > Na+ > K+ at pH 3.0 and 9.0 and became similar at pH 5.0 and 7.0. Overall, the P-MWCNT adsorbed slightly more monovalent ions than the MWCNT membranes, but the difference was less pronounced than what was observed for divalent cations (Fig. 1).
10
Figure 1. Distribution coefficients of divalent cations (A & B) and monovalent cations (C & D) on MWCNT and P-MWCNT membranes as a function of feed solution pH. Spiked feed metal concentration = 3 mg L−1, permeate flux = 44 L m-2, filtration time = 60 min, and temperature = 25 ± 2 °C. The error bars represent the upper and lower values obtained in duplicate filtration experiments. Table 1 summarizes the Kd, K1, K2 and K′ values with respect to Sr2+ and Cs+ in the single-metal solutions at a neutral pH value (7.0). The selectivity coefficients (K) of Sr2+ over Mg2+ and Sr2+ over Ca2+ by MWCNT membrane in single-metal solution were 3.49 and 2.19 respectively at the neutral pH 7.0, while the K values of P-MWCNT membrane for Sr2+ over Mg2+ and Sr2+ over 11
Ca2+ were 3.17 and 1.93, respectively. On the other hand, the K values of Cs+ over K+ and Cs+ over Na+ by MWCNT membrane in single metal solution were 1.16 and 1.00, respectively, compared to P-MWCNT membranes with 1.01 and 1.00, respectively. These results confirmed that both membranes possessed strong selectivity to Sr2+ but not Cs+. Moreover, the P-MWCNT membrane had slightly stronger affinities to Sr2+ and Cs+ than the MWCNT membrane, as manifested by the K’ values. Table 1. Kd, K, and K′ values of K+, Na+, Mg2+ and Ca2+ with respect to Cs+ and Sr2+ at pH 7.0. MWCNT Metal ion
Kd (mL g-1)
P-MWCNT
KMWCNT
Kd (mL g-1)
KP-MWCNT
K′
Cs+
0.630
K+
0.540
1.16
0.795
1.01
1.14
Na+
0.630
1.00
0.810
1.00
1.00
Sr2+
2.919
Mg2+
0.836
3.49
1.305
3.17
1.00
Ca2+
1.329
2.19
2.145
1.93
1.13
0.810
4.140
Fig. 2 shows the effect of pH on the selectivity coefficient of P-MWCNT membranes. The K values of Sr2+ over Mg2+ in single-metal solutions fluctuate in a narrow range of 2.82 to 2.93 as the solution pH was adjusted from 3.0 to 9.0 (Fig. 2a). Likewise, the K values of Sr2+ over Ca2+ remained at around 2.0 in the same pH range (Fig. 2b). These results suggested that the variations in solution pH exerted similar impacts on the adsorption of the alkaline earth ions, thereby not affecting the high selectivity of P-MWCNT for Sr2+. In terms of alkaline ions, the K value of Cs+ over K+ by P-MWCNT membrane was greater (1.95) at pH 3.0 but decreased 12
dramatically to 1.05-1.19 after increasing the pH of the solution to 5.0 and above (Fig. 2c). Similarly, the K value of Cs+ over Na+ was relatively high at pH 3.0 (1.95) but decreased to 1.19 as the pH increased to 9.0 (Fig. 2d). The relative selectivity coefficient (K′), or the ratio of the corresponding K values provides a quantitative comparison of the relative selectivity of the P-MWCNT membrane over the MWCNT membrane for the same metal ion. Table 1 shows that the K′ values for Cs+/K+ and Cs+/Na+ are 1.14 and 1.00, respectively; both are greater than unity. This means that, for singlemetal solutions, the P-MWCNT membrane possessed slightly higher selectivity for Cs+ over K+ than the MWCNT membrane. Likewise, the K′ values for Sr2+/Mg2+ and Sr2+/Ca2+ are 1.00 and 1.13, respectively, indicating slightly greater selectivity possessed by the P-MWCNT membrane for Sr2+ over Ca2+.
13
Figure 2. Selectivity of P-MWCNT membranes for selected metal ions in water. A) Sr/Mg in single, binary and wastewater solution, B) Sr/Ca in single, binary and wastewater solution, C) Cs/K in single, binary and wastewater solution and D) Cs/Na in single, binary and wastewater solution. The error bars represent the upper and lower values obtained in duplicate filtration experiments. 3.3. Selectivity of the P-MWCNT in binary-metal solution filtration Considering the relatively high adsorption capacities exhibited by P-MWCNT membranes in single-metal solution filtration (Fig. 1), this study further determined the selectivity of the P-
14
MWCNT membranes during binary-metal solution filtration. Fig. 3 shows the Kd values of the PMWCNT membrane for Sr2+ and Cs+ when each metal ion coexisted with a competing ion of the same valence and an equal initial concentration of 3 mg L-1. In the pH range of 3.0-9.0, the PMWCNT membrane exhibited similar Kd towards Sr2+ in the coexistence of either Ca2+ or Mg2+ (Fig. 3a vs. Fig. 3b) and in both cases, Kd values increased with increasing pH. These results were slightly lower than those obtained with the single-metal solutions (Fig. 1b). For instance, at pH 7.0, Kd value was 4.14 mmol g-1 for Sr2+ in the absence of competing ions (Fig. 1b), but slightly decreased to 3.54 mmol g-1 and 3.63 mmol g-1 in the presence of Ca2+ and Mg2+, respectively (Fig. 3a-b). In the co-existence of Cs+ and K+, the respective Kd values for the two ions increased from 0.43 mmol g-1 to 0.67 mmol g-1 and 0.49 mmol g-1 to 0.79 mmol g-1 as the pH was elevated from 3.09.0 (Fig. 3c). These results were similar to those obtained with single-metal solutions (Fig. 1d), despite the presence of co-existing ions. Comparatively, the distribution coefficient of PMWCNT membrane toward Cs+ was suppressed when Na+ coexisted with Cs+ in the same feed solution, regardless of the solution pH (Fig. 3d). The competitive effect of Na+ on Cs+ adsorption appeared to disagree with the results obtained with the single-metal solutions, with which the distribution coefficients of Cs+ were either greater or equal to those of Na+ across the studied pH range. This discrepancy indicated the occurrence of different adsorption processes for PMWCNT in single- vs. binary-metal solutions. The selectivity coefficients of the P-MWCNT membranes were high for Sr2+ compared to Cs+ in the binary-metal solutions (Fig. 2). The K value of Sr2+ over Mg2+ at pH 3.0 was 3.34 and increased sharply to 5.75 at pH 5.0. Then, from pH 7.0 to 9.0, the K value of Sr2+ over Mg2+ remained at 3.13-3.38 (Fig. 2a). However, the K value of Sr2+ over Ca2+ was 1.00 at pH 3.0, and 15
then increased to 1.50-1.70 at higher pH (Fig. 2b). Moreover, the K values of Cs+ over K+ remained at 0.84-0.95 in the studied pH range (Fig. 2c), while those of Cs+ over Na+ remained at 0.46-0.58 (Fig. 2d). Overall, the pH of the binary metal solution did not exert significant effect on the selectivity of the P-MWCNT membranes for Cs+ and Sr2+ in the binary-metal solutions, except for the coexistence of Sr2+ and Mg2+ at a pH of 5.0. On the other hand, the filtration of binary-metal solutions consistently resulted in higher Sr2+ selectivity but lower Cs+ selectivity than the filtration of single-metal solutions. The opposite trends observed for the two ions suggested the existence of different adsorption mechanisms as depicted later.
16
Figure 3. Distribution coefficient of Sr2+ in the presence of Mg2+ (A) and Ca2+ (B) and the distribution coefficient of Cs+ in the presence of K+ (C) and Na+ (D) by the P-MWCNT membranes from binary-metal solutions at different pH values. Spiked feed metal concentration = 3 mg L−1 and permeate flux = 44 L m−2. Filtration time = 60 min, temperature = 25 ± 2 °C. The error bars represent the upper and lower values obtained in duplicate filtration experiments. 3.4. Selectivity of CNT membranes in wastewater filtration Given the diverse selectivity of CNT membranes for Sr2+ and Cs+ in synthetic solutions, a secondary wastewater effluent containing a suite of metal ions and organic matter (Table S2) was adopted in this study to further evaluate the selectivity of CNT membranes in complex aqueous solutions. Since the effluent did not contain Sr2+ or Cs+, 3 mg L−1 of each ion was spiked into the wastewater effluent to match the conditions used in the single-metal solution experiments. The separation performance of P-MWCNT membrane for Sr2+ in the wastewater effluent was similar to those observed in the single- and binary-metal solutions (Fig. 4b), although more competitive metal ions co-existed in the wastewater at higher concentrations than that of Sr2+. The Kd of P-MWCNT membrane for Sr2+ was 2.77 mmol g-1 to 3.58 mmol g-1 from wastewater effluent in the pH range of 3.0-9.0, greater than the competitive metal ions (Table S2) such as Mg2+ and Ca2+ (Fig. 4b). In contrast, the Kd values of P-MWCNT membrane was 0.49 mmol g-1 to 1.87 mmol g-1 for Mg2+ and 0.58 mmol g-1 to 1.54 mmol g-1 for Ca2+ in the same pH range. The Kd values of other metals such as Cu2+ and Zn2+ were above 4.00 mmol g-1 due to their high initial concentrations and/or strong affinity to the P-MWCNT membrane. Moreover, the Kd values of all metal ions onto the P-MWCNT membrane increased when the pH of the wastewater effluent was increased from 3.0-9.0. On the other side, the Kd values of 17
MWCNT membrane were only 2.32 mmol g-1 to 3.11 mmol g-1 for Sr2+, 0.40 to 1.21 mmol g-1 for Mg2+ and 0.49 mmol g-1 to 0.85 mmol g-1 for Ca2+ at the same pH range (Fig. 4a). These results justified that the P-MWCNT membrane had higher adsorption capacities for metal ions than the MWCNT membrane under the studied conditions. On the other hand, the Kd values of both MWCNT and P-MWCNT membranes to monovalent Cs+ in wastewater were negligible and greatly suppressed by the presence of other competitive metal ions (Fig. 4c-d). Specifically, the P-MWCNT membrane showed a Kd value of 0.22 mmol g-1 to 0.42 mmol g-1 to Cs+ in the pH range of 3.0-9.0, while the Kd values of competitive K+ and Na+ were 0.49 mmol g-1 to 1.35 mmol g-1 and 0.49 mmol g-1 to 1.41 mmol g-1, respectively (Fig. 4d). Herein, the lower Kd values of Cs+ than those of K+ and Na+ should be ascribed to various influencing factors such as high concentrations of competing ions. The chemical properties of the Cs+ may also play a role as elucidated later. The effect of pH on the selectivity of selected metal ions by P-MWCNT membranes in wastewater effluent is shown in Fig. 2. The K value of Sr2+ over Mg2+ in the wastewater was 5.60 at the lower pH of 3.0 and then decreased to 1.91 at the higher pH of 9.0 (Fig. 2a). Similarly, the K value of Sr2+ over Ca2+ was higher at lower pH (4.74 at pH 3.0) and decreased to 2.32 with increasing the pH to 9.0) (Fig. 2b). Overall, it was found that the K values of divalent metal ions in the wastewater effluent were greater at lower pH and decreased with increasing pH. However, the K values of monovalent cations in the wastewater effluent were less affected by pH and stayed at very low levels compared to those in single-metal solution and binary-metal solution (Fig. 2c-d). Among those ions, the K value of Cs+ over K+ in wastewater effluent was 0.45 at pH 3.0 and decreased to 0.31 at a higher pH of 9.0 (Fig. 2c). Similarly, the K value of Cs+ over Na+ in the wastewater effluent fluctuated between 0.45 and 0.29 in the pH range of 3.0-9.0 (Fig. 2d). 18
Collectively, the K values of divalent cations were greater than monovalent cations during CNT membrane filtration of the wastewater effluent.
Figure 4. Distribution coefficient of MWCNT membrane (A & C) and the P-MWCNT membranes (B & D) for Sr2+ and Cs+ spiked in a wastewater effluent as a function of pH. Initial concentrations of Sr2+ and Cs+ were 3 mg L−1, permeate flux of 44 L m−2, filtration time = 60 min, and temperature = 25 ± 2 °C. The error bars represent the upper and lower values obtained in duplicate filtration experiments.
19
3.5. Potential mechanisms for CNT’s selectivity for metal ions The XPS spectra (Fig. 5) shows the emerge of new peaks at binding energies of 134 eV, 399 eV and 1304 eV after the filtration of wastewater effluent are ascribed to Sr2+, Ca2+ and Mg2+ that were indeed adsorbed on the surface of P-MWCNT. The removal mechanisms of divalent cations by adsorbents usually involve inner-sphere complexation reactions between the metal ions and the electron-pair donor atoms available on the surface of the adsorbents [29]. Under the studied conditions, Sr2+, Ca2+ and Mg2+ were possibly removed from water by forming innersphere complexes on the surface of MWCNT and P-MWCNT. Because the binding sites on CNTs for divalent cations are also those for proton binding, the increase in pH would lower the concentration of proton in the feed solution, thus mitigating the competing effect of proton for surface complexation. This trend was indeed observed for the experimentally determined Kd values for Sr2+, Ca2+, and Mg2+ (Fig. 1a & 1b).
Figure 5. XPS spectra of the P-MWCNT membrane (A) before and (B) after filtration of the wastewater effluent.
20
In order to further explore the strong selectivity of MWCNT and P-MWCNT for Sr2+, major characteristics of the metal ions involved in this study were obtained from published data [17, 30-32] and are summarized in Table 2. As described above, the selectivity of both CNT membranes followed the sequence of Sr2+ > Ca2+ > Mg2+ for both single- and binary-metal solutions, regardless of solution pH (Fig. 5). Similar selectivity sequence has also been observed during the adsorption of earth alkali cations to titanium oxide surfaces [33-35] and follows the classical Hofmeister series for ion affinity to oxide surfaces [36]. This sequence is consistent with that of ionic radii of the studied ions and opposite to the sequence of absolute hardness and absolute electronegativity (Table 2). According to Nieboer and Richardson [37], alkali and alkaline earth ions belong to Class A metal ions that are considered to be oxygen-seeking or ‘hard’ in nature. This means that these ions prefer to bind to oxygenated functional groups on CNT surfaces. Therefore, plasma treatment provided more oxygenated sites on MWCNT, which allowed the P-MWCNT to remove more alkaline earth ions than the pristine MWCNT under similar conditions (Fig. 1). Moreover, for alkaline earth ions, the inner-sphere complexation process involves substitution of the water molecules in the hydration layer of the metal ions by the CNT functional groups. As the ionic radius increases from Mg2+ to Sr2+, the ionic bonding strength between the alkaline earth ions and the hydrating water decreases with the decreasing absolute hardness. This favors the substitution of hydrating water molecules by the CNT functional groups. As such, the CNT membranes preferentially removed Sr2+ over Mg2+ and Ca2+ from the studied water and wastewater effluent (Fig. 2).
21
Table 2. Major properties of metal ions used in this study. Parameters
Metal ions Sr2+
Ca2+
Mg2+
Cs+
K+
Na+
Ionic radius (Հ )a
1.25
1.00
0.72
1.70
1.38
1.02
Hydration energy (kJ mol-1)b
̶ 1380
̶ 1306
̶ 1828
̶ 376
̶ 271
̶ 365
Hydrated radius (nm)b
0.412
0.412
0.300
0.329
0.201
0.276
Hydration shell thickness (nm)
0.294
0.312
0.228
0.159
0.063
0.174
Absolute hardnessc
16.3
19.52
32.55
10.6
13.64
21.08
Absolute electronegativityd
27.3
31.39
47.59
14.5
17.99
26.21
Ionic indexe
3.70
4.02
5.70
0.58
0.72
0.85
a
[17]
b
[31]
c
[32]
d
[30]
e
[37]
On the other hand, the adsorption of CNT membranes towards monovalent cations was primarily induced by electrostatic or Coulombic attraction between negatively charged CNTs (Fig. S2, Supporting Information) and positively charged monovalent cations [29]. During this process, water molecules in the hydration layer of metal ions are maintained since no chemical bonding is formed between the metal ions and the functional groups on CNT surfaces. Thus, the stability of metal ion hydrates is no longer important, which is different from that of alkaline earth ions. Accordingly, the adsorption of alkali ions was stronger at higher solution pH (Fig. 1c & 1d) because the electrostatic attraction between the ions and the CNT surfaces increased as the CNT surfaces were more negatively charged (Fig. S2). Besides, pH decreasing also means that there were fewer protons in the aqueous solutions to compete with alkali ions for surface binding on CNTs. This also promoted ion adsorption by the membranes. 22
However, the inconsistent trends observed for the selectivity of CNTs for the alkali ions necessitate further scrutiny. For the binary-metal solutions, the selectivity sequence of the membranes followed a simple trend of Na+ > K+ > Cs+ (Fig. 2c & 2d). This sequence is in agreement to that of the strength of Coulombic attraction and indicates its dominant role during the adsorption process. For the single-metal solutions, the selectivity sequence at an acidic pH of 3.0 changed to Cs+ > Na+ > K+ (Fig. 1c & 1d). This suggests that non-Coulombic interaction may be important under this particular condition and can enhance the adsorption of Cs+. Nevertheless, a conclusive remark in this regard cannot be made here and further investigation is warranted. 4. Conclusion ·
Both CNT membranes prepared in this study effectively removed Sr2+ from the singlemetal solutions and showed higher distribution coefficients for Sr2+ than the competitive metal ions. P-MWCNT membrane displayed excellent selectivity for Sr2+ in the binary metal system and real wastewater effluent, while its selectivity for Cs+ was lower than for other competitive metal ions.
·
The adsorptive removal of Sr2+ and other alkaline earth ions by CNT membranes involved the formation of inner-sphere complexes, and therefore, the selectivity sequence increased with the ionic radii of the studied ions.
·
The increase in the initial pH of the feed solution promoted the adsorption of Sr2+ onto the CNT membranes, but had mixed effects on the selectivity of Sr2+ in both synthetic solution and wastewater effluent.
·
Adsorptive filtration of monovalent Cs+ and other alkali ions were primarily caused by electrostatic attraction between the cations and negatively charged CNT surfaces, and
23
therefore, was enhanced with elevating solution pH. Due to the unspecific nature of the interaction, the selectivity sequence of CNTs for alkali ions followed different trends under varying solution conditions. Acknowledgment The authors wish to thank Dr. Guling Zhang from Minzu University of China for providing the plasma treatment facility used in this work. Dr. Jianying Shang from China Agricultural University is acknowledged for providing access to ICP-AES used in this work. This research was financially supported by the special fund of State Key Joint Laboratory of Environmental Simulation and Pollution Control of China (18L01ESPC) and the Fundamental Research Funds for the Central Universities of China (310421111). References [1] S. Ding, Y. Yang, C. Li, H. Huang, L.A. Hou, The effects of organic fouling on the removal of radionuclides by reverse osmosis membranes, Water Res. 95 (2016) 174-184. [2] L. Wu, G. Zhang, Q. Wang, L.a. Hou, P. Gu, Removal of strontium from liquid waste using a hydraulic pellet co-precipitation microfiltration (HPC-MF) process, Desalination 349 (2014) 31-38. [3] D. Alby, C. Charnay, M. Heran, B. Prelot, J. Zajac, Recent developments in nanostructured inorganic materials for sorption of cesium and strontium: Synthesis and shaping, sorption capacity, mechanisms, and selectivity-A review, J Hazard Mater 344 (2018) 511-530. [4] F. Ma, Z. Li, H. Zhao, Y. Geng, W. Zhou, Q. Li, L. Zhang, Potential application of graphene oxide membranes for removal of Cs(I) and Sr(II) from high level-liquid waste, Separation and Purification Technology 188 (2017) 523-529. [5] 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. [6] T. Nur, P. Loganathan, J. Kandasamy, S. Vigneswaran, Removal of strontium from aqueous solutions and synthetic seawater using resorcinol formaldehyde polycondensate resin, Desalination 420 (2017) 283291. [7] J. Wang, S. Zhuang, Removal of cesium ions from aqueous solutions using various separation technologies, Reviews in Environmental Science and Bio/Technology 18 (2019) 231-269. 24
[8] J. Wang, S. Zhuang, Y. Liu, Metal hexacyanoferrates-based adsorbents for cesium removal, Coord. Chem. Rev. 374 (2018) 430-438. [9] A.J. O'Donnell, D.A. Lytle, S. Harmon, K. Vu, H. Chait, D.D. Dionysiou, Removal of strontium from drinking water by conventional treatment and lime softening in bench-scale studies, Water Res 103 (2016) 319-333. [10] Y. Zheng, J. Qiao, J. Yuan, J. Shen, A.-j. Wang, L. Niu, Electrochemical Removal of Radioactive Cesium from Nuclear Waste Using the Dendritic Copper Hexacyanoferrate/Carbon Nanotube Hybrids, Electrochimica Acta 257 (2017) 172-180. [11] B. Sarkar, S. Mandal, Y.F. Tsang, P. Kumar, K.H. Kim, Y.S. Ok, Designer carbon nanotubes for contaminant removal in water and wastewater: A critical review, Sci. Total Environ. 612 (2018) 561-581. [12] Y. Huang, W. Wang, Q. Feng, F. Dong, Preparation of magnetic clinoptilolite/CoFe2O4 composites for removal of Sr2+ from aqueous solutions: Kinetic, equilibrium, and thermodynamic studies, Journal of Saudi Chemical Society 21 (2017) 58-66. [13] M.R. Awual, T. Yaita, Y. Miyazaki, D. Matsumura, H. Shiwaku, T. Taguchi, A Reliable Hybrid Adsorbent for Efficient Radioactive Cesium Accumulation from Contaminated Wastewater, Sci Rep 6 (2016) 19937. [14] R. Kamaraj, S. Vasudevan, Evaluation of electrocoagulation process for the removal of strontium and cesium from aqueous solution, Chemical Engineering Research and Design 93 (2015) 522-530. [15] R.R. Sheha, S.H. El-Khouly, Adsorption and diffusion of cesium ions in zirconium(IV) iodomolybdate exchanger, Chemical Engineering Research and Design 91 (2013) 942-954. [16] I.A. B. Prelot, F. Marchandeau, J. Zajac,, On the real performance of cation exchange resins in wastewater treatment under conditions of cation competition: the case of heavy metal pollution, Environ. Sci. Pollut. Res. 21 (2014) 9334–9343. [17] Y. Marcus, A simple empirical model describing the thermodynamics of hydration of ions of widely varying charges, sizes, and shapes, Biophysieai Chemistr 51 (1994) 111-127. [18] A.K. Vipin, B. Fugetsu, I. Sakata, A. Isogai, M. Endo, M. Li, M.S. Dresselhaus, Cellulose nanofiber backboned Prussian blue nanoparticles as powerful adsorbents for the selective elimination of radioactive cesium, Sci Rep 6 (2016) 37009. [19] Ihsanullah, A. Abbas, A.M. Al-Amer, T. Laoui, M.J. Al-Marri, M.S. Nasser, M. Khraisheh, M.A. Atieh, Heavy metal removal from aqueous solution by advanced carbon nanotubes: Critical review of adsorption applications, Sep. Purif. Technol. 157 (2016) 141-161. [20] H.-K. Lee, J.W. Choi, W. Oh, S.-J. Choi, Sorption of cesium ions from aqueous solutions by multiwalled carbon nanotubes functionalized with copper ferrocyanide, Journal of Radioanalytical and Nuclear Chemistry (2016). [21] R. Yavari, Y.D. Huang, S.J. Ahmadi, Adsorption of cesium (I) from aqueous solution using oxidized multiwall carbon nanotubes, Journal of Radioanalytical and Nuclear Chemistry 287 (2010) 393-401.
25
[22] C. Chen, J. Hu, D. Xu, X. Tan, Y. Meng, X. Wang, Surface complexation modeling of Sr(II) and Eu(III) adsorption onto oxidized multiwall carbon nanotubes, J Colloid Interface Sci 323 (2008) 33-41. [23] D. Shao, J. Hu, X. Wang, Plasma Induced Grafting Multiwalled Carbon Nanotube with Chitosan and Its Application for Removal of UO 22+, Cu2+, and Pb2+ from Aqueous Solutions, PLASMA PROCESS POLYM 7 (2010) 977-985. [24] E.-s. Lee, Y.-K. Lim, Y.-s. Chun, B.-Y. Wang, D.-S. Lim, Characteristics of hydrogen plasma treated carbon nanotubes and their influence on the mechanical properties of polyetherimide-based nanocomposites, Carbon 118 (2017) 650-658. [25] M. Usman Farid, H.-Y. Luan, Y. Wang, H. Huang, A.K. An, R. Jalil Khan, Increased adsorption of aqueous zinc species by Ar/O 2 plasma-treated carbon nanotubes immobilized in hollow-fiber ultrafiltration membrane, Chem. Eng. J. 325 (2017) 239-248. [26] K. Li, Q. Gao, G. Yadavalli, X. Shen, H. Lei, B. Han, K. Xia, C. Zhou, Selective Adsorption of Gd(3+) on a Magnetically Retrievable Imprinted Chitosan/Carbon Nanotube Composite with High Capacity, ACS Appl Mater Interfaces 7 (2015) 21047-21055. [27] S. Gómez, N.M. Rendtorff, E.F. Aglietti, Y. Sakka, G. Suárez, Surface modification of multiwall carbon nanotubes by sulfonitric treatment, Appl. Surf. Sci. 379 (2016) 264-269. [28] Y.X. Bin Zhang, Yifang Zheng, Lizong Dai, Mingqiu Zhang, Jin Yang, Yujie Chen, Xudong Chen and, J. Zhou, A Facile Synthesis of Polypyrrole/Carbon Nanotube Composites with Ultrathin, Uniform and Thickness-Tunable Polypyrrole Shells, Nanoscale Research Letters 6 (2011) 431. [29] S. Warner, The Inner-Sphere Complex: A key to Understanding surface reactivity, Advance in Chemistry 244 (1995) 1-32. [30] R.G. Pearson, Phenomenological theory of ion salvation-effective radii of hydrated ions, Inorg. Chem. 27 (1988) 734-740. [31] B. Tansel, Significance of thermodynamic and physical characteristics on permeation of ions during membrane separation: Hydrated radius, hydration free energy and viscous effects, Sep. Purif. Technol. 86 (2012) 119-126. [32] N. Li, L. Zhang, Y. Chen, M. Fang, J. Zhang, H. Wang, Highly Efficient, Irreversible and Selective Ion Exchange Property of Layered Titanate Nanostructures, Advanced Functional Materials 22 (2012) 835-841. [33] X. Tan, M. Fang, L. Tan, H. Liu, X. Ye, T. Hayat, X. Wang, Core–shell hierarchical C@Na2Ti3O7·9H2O nanostructures for the efficient removal of radionuclides, Environmental Science: Nano 5 (2018) 1140-1149. [34] B. Filipowicz, M. Pruszynski, S. Krajewski, A. Bilewicz, Adsorption of (137)Cs on titanate nanostructures, J Radioanal Nucl Chem 301 (2014) 889-895. [35] S.P. S. Kasap, H. Tel, Titanate nanotubes: preparation, characterization and application in adsorption of strontium ion from aqueous solution, Radiochim. Acta 100 (2012) 925–929. [36] Y. Marcus, Ionic Radii in Auqeous Solutions, Chem. Rev. 88 (1988) 1475-1498. 26
[37] E. Nieboer, S.H.D. Richardson, THE REPLACEMENT OF THE NONDESCRIPT TERM 'HEAVY METALS' BY A BIOLOGICALLY AND CHEMICALLY SIGNIFICANT CLASSIFICATION OF METAL IONS, Environmental Pollution (Series B) 1 (1980) 3-26.
27
Highlights: ·
The selective removal of Sr2+ and Cs+ by P-MWCNT membranes were studied.
·
The P-MWCNT membrane showed superb selectivity to Sr2+ over other metal ions.
·
The selectivity of Cs+ was depressed by the presence of coexisted metal ions.
·
The selectivity of divalent cations resulted from inner-sphere complexation.
·
While the selectivity of monovalent cations resulted from electrostatic interactions.
28
Graphical Abstract
29
Conflict of interest The authors of this research paper declare no conflict of interest.
30