Cosorption Characteristics of SeO42â and Sr2+ Radioactive

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Co-sorption characteristics of SeO and Sr radioactive surrogates using 2D/2D graphene oxide-layered double hydroxide nanocomposites Paulmanickam Koilraj, Yuta Kamura, and Keiko Sasaki ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02056 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on October 5, 2018

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ACS Sustainable Chemistry & Engineering

Co-sorption characteristics of SeO42- and Sr2+ radioactive surrogates using 2D/2D graphene oxide-layered double hydroxide nanocomposites Paulmanickam Koilraj, Yuta Kamura, Keiko Sasaki* Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan Corresponding Author Tel./Fax. +81 92 802 3338 Email: [email protected] (K.S.); [email protected]; [email protected] (P.K.)

ABSTRACT

Co-sorption of anionic and cationic radioactive nuclides is highly desired towards the total cleaning of radioactive contaminated wastewater. A 2D/2D multifunctional nanocomposite of MgAl-LDH/graphene oxide (GO) was fabricated using coagulation and applied for the co-sorption of Sr2+ and SeO42- from aqueous solution. The co-sorption of Sr2+ and SeO42- was synergetically

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enhanced with the co-presence of each species and showed a maximum Sr2+ removal of 2.435 mmol/g of GO. The synergetic effect occurs only in the MgAl-LDH/GO nanocomposite because of the synchronized effect of MgAl-LDH, GO and alkaline cations, which was not present in pure GO. The SeO42- removal occurred by the interchange of NO3- anion from the LDH, while the removal of Sr2+ occurred through coordination with carboxyl/alkoxy (–COO-/-CO-) groups in GO by the ring opening of epoxides. The co-sorption efficiencies of Sr2+ and SeO42- were stable in the wide pH range of 4-10. The binary (Na2SeO4 + SrCl2) and ternary (Na2SeO4 + SrCl2 + M+/M2+ = other metal ions or An- = other negative ions) systems enhanced the co-sorption of Sr2+ and SeO42in the presence of other alkali and alkali earth metals and other anions compared with the single system. The Sr2+ and SeO42- sorption densities were superior to previously reported values. The combined multifunctional ability and environmentally benign nature of the MgAl-LDH/GO composite is promising as sustainable materials for the total remediation of Sr2+ and SeO42- a radioactive surrogates and can also be extended to wide combinations of divalent anions and cations.

KEYWORDS layered double hydroxide, graphene oxide, radioactive waste, strontium, selenate, co-sorption INTRODUCTION The depletion of fossil fuels and increased demand for electrical energy has forced our society to move to nuclear energy. The nuclear power plants produce a huge amount of radioactive contaminants as a byproduct. In Japan, the earthquake mediated Fukushima Daiichi Nuclear disaster injected a huge amount of radioactive contaminants in to the water stream.1-2 90Sr2+ is one of the major elements entered into the water, with a half-life period of 28.9 years, along with


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several other radioactive elements.2-3 The 90Sr2+ can reach the food chain through plants, which can harm living organisms, especially humans. The fission products contain the 79SeO42- isotope, which has a half-life period of 2.952 x 105 years. The anionic radionuclides are highly mobile, entering into the aqueous stream due to rejection by the negatively charged materials in the earth’s crust along with trace concentrations of cations.4 Hence, the removal of both anionic and cationic radioactive contaminants is highly desired from the environmental point of view. Very few studies focused for the concurrent sorption of Sr2+ and SeO42-. Nie et al. studied the co-removal of Sr2+ and SeO42- on the surface of a goethite material.5 Very recently, carbon-nanodot modified layered double hydroxide composites demonstrated the co-sorption of Sr2+ and SeO42-.6,7 Graphene oxides (GO) are a type of (2D) carbonaceous nanomaterials that are extensively used for the decontamination of anions, cations and organics due to their high contents of hydroxyl, epoxy and carboxyl (-OH, C-O-C and –COO-) functional moieties. Removal of radioactive nuclides using pure GO has also been studied extensively.8-11 The nanosheets of GO tend to disperse in water, which makes them difficult to separate from the medium after remediation.12-13 The dispersed GO nanosheets released in the aqueous environment cause secondary water pollution, leading to bio-toxicity to living organisms.14-15 To reduce the bioavailability and to enhance the sorption efficiency, GO nanosheets are fabricated with different types of support and tremendously utilized for the removal of Sr2+. For instance, composite materials, such as magnetic polyaniline/GO,16 polyacrylamide-GO,17 ethylene diamine tetra-acetic acid-functionalized-GO,18 thiacalixarene-functionalized-GO,19 magnetite-GO,20 WO3-GO,21 and GO-bayberry tannin sponge22 have been prepared and utilized for the removal of Sr2+. Besides the prevention of GO nanosheet release, the solid supports that could provide some additional functionality to target another pollutant in the composite materials and thus are highly encouraged.


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Layered double hydroxides (LDHs) are anionic clays having the structure of [M(II)1xM(III)x(OH)2]

x+

(An-)x/n·mH2O, where M(II) and M(III) are a divalent and trivalent metal ions

respectively, and An- is the charge compensating anion. LDHs are well-known anion-exchangers and are broadly used for the remediation of diverse aqueous contaminants because of their superior anion exchange density and stability.23-26 The Fe2+ doped,27 MoS4 intercalated,28 no alkoxidecontaining sol-gel synthesized and grainy MgAl-LDHs,29-30 and aqueous dispersible magnetic GO/LDH composites31 have been reported for aqueous SeO42- removal. Based on this observation, it was assumed that combining anionic LDHs with GO could produce multifunctional materials that can effectively adsorb both anions and cations simultaneously. Recently, we have developed carbon-dot decorated MgAl-LDH based composite materials for the co-sorption of Sr2+ and SeO42.6 In the previous reports, LDH/GO-based composites have been synthesized and utilized for the treatment of diverse toxic ions. The GO mediated composites, such as three-dimensional MgAlGO,32 NiAl-LDH/GO composite,33 water-swellable MgAl-LDH/GO composite,34 and 3D MgAlLDH/GO aerogels,35 have been reported for the treatment of Pb2+, Cd2+, U(VI), AsO43- and methylene blue32-35 separately. Based on our understanding, the co-sorption of both anionic and cationic (SeO42- and Sr2+) pollutants has been not yet reported for GO-based sorbents. Very recently we have reported the synergetic removal of SeO42- and Sr2+ on the LDH based carbon-dot and graphene oxide composites.7 Herein, we report the synthesis of 2D/2D MgAl-LDH/GO composite materials by coagulation with varying GO contents, which were then studied for the co-sorption of Sr2+ and SeO42radioactive surrogates. The effects of time, concentration, pH of the medium, and the co-presence of anions and cations on the co-sorption have been examined. Based on diverse characterization techniques, a possible mechanism for the co-removal of Sr2+ and SeO42- is proposed.


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EXPERIMENTAL METHODS Preparation of GO and MgAl-LDH Graphene oxide (GO) was prepared from commercially available graphite through the modified Hummers method.36-37 The detailed synthesis and purification of GO is provided in the Supporting Information. As reported earlier, the co-precipitation at low supersaturation was utilized for the preparation MgAl-LDH support .6 In brief, 200 mL of aqueous metallic nitrate with a Mg2+/Al3+ atomic ratio of 2 was titrated with the base solution containing NaOH (2 M) and NaNO3 (0.2 M ) until to reach the pH 10 under vigorous stirring. The resulting mixture was kept at 80 °C (20 h), and the solid was centrifuged, washed and freeze-dried and are referred as MgAl-LDH.

Fabrication of MgAl-LDH/GO nanocomposites The fabrication of MgAl-LDH on the surface of GO was performed by coagulation/flocculation, as reported earlier with slight modification.38 In detail, around 0.5 g of MgAl-LDH was dispersed in 50 mL of the GO suspension, which was prepared by sonication at 28 kHz for 3 h. The amount of GO in the aqueous suspension was adjusted to obtain 5, 10 and 20% in the resulting products. The mixed solution was stirred for 2 h under a N2 atmosphere, and then filtered, washed and freeze dried for 24 h and the solids are termed as MgAl-LDH/GO (x %), where x = 5, 10 and 20% GOcontaining composites, respectively.

Co-sorption of Sr2+ and SeO42- on to different MgAl-LDH/GO composite materials The co-sorption of Sr2+ and SeO42- was investigated in batch tests onto MgAl-LDH, MgAlLDH/GO (x%) nanocomposites and GO by dispersing in 0.984 mM Na2SeO4 containing 0.485


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mM SrCl2. The synergism between the removal of Sr2+ and SeO42- was studied under the same conditions with and without each other ion. The sorption isotherm studies were conducted on MgAl-LDH/GO composites (5 and 20% of GO) in a single electrolytic solution containing 0.1 to 5.0 mM Sr2+ and 0.1 to 5.0 SeO42- solution, separately. The influence of pH on the co-sorption of Sr2+ and SeO42- was examined using in 0.984 mM Na2SeO4 containing 0.485 mM SrCl2 on MgAlLDH/GO (5%) composites. The pH of the medium was changed using dilute HCl and NaOH. The effect of the co-cation on Sr2+ and SeO42- co-sorption over MgAl-LDH/GO (5%) nanocomposites was studied in 0.485 mM Sr2+ solution containing 0.5 mM nitrate salts of alkali (Li+, Na+, K+ and Cs+) and alkaline earth metals (Mg2+ and Ca2+) separately with and without the addition of Na2SeO4. Likewise, the influence co- anion were studied in the mixed solutions containing 0.984 mM Na2SeO4 with 1.0 mM sodium salts of NO3-, Cl-, F-, SO42-, CO32-, and H2PO4- separately by the presence and absence of Sr2+. In all the cases, the solid–to-liquid ratio was maintained at 1 g/L; the mixture was shaken at 100 rpm for 3 h (pH=5.95; except pH variation) at 25 °C and separated by membrane filtration. The desorption of Sr2+ and SeO42- from MgAl-LDH/GO (5%) nanocomposites were conducted in the mixed solution containing NaNO3 (0.5 M) and HNO3 (3.0 mM) for 12 h.6 The successive sorption-desorption performance were evaluated up to fourth cycle. The residual Mg, Al, Se and Sr concentrations in the supernatant were calculated by ICP-OES.

Analysis of nanocomposite materials The PXRD patterns of solid products were measured in Rigaku, Ultima IV diffractometer (Akishima, Japan). The Cu Kα radiation was used as X-ray source with the acceleration voltage of 40 kV and the applied current of 40 mA and the scanning speed was 2°/min with a step size of 0.02°. The nature of interlayer anions in the as-synthesized MgAl-LDH or MgAl-LDH/GO (x %)


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composite and after the co-sorption of Sr2+ and SeO42- were accumulated (JASCO FT/IR-670 Plus, Tokyo, Japan) by grinding 2 wt% solid materials in KBr. TEM images of the composite materials were obtained on a JEM-2100HCKM, JEOL microscopy (Akishima, Japan). Zeta potential analyses were conducted on a Malvern ZETASIZER NANO-ZS instrument (Kobe, Japan). The pH of the MgAl-LDH, MgAl-LDH/GO (x %; x = 5, 10 and 20) composites and GO suspensions used for zeta potential analysis were 8.65, 8.01, 7.83, 7.76 and 3.36 respectively.

RESULTS AND DISCUSSION Characterizations of MgAl-LDH/GO nanocomposites The MgAl-LDH/GO composites were prepared by co-precipitation and analyzed by different physicochemical analyses. PXRD showed that the MgAl-LDH was crystallized in a hexagonal hydrotalcite-like structure (Figure 1A). The d003-spacing of MgAl-LDH was 8.68 Å, which is related to the NO3- anion. The presence of large number of monovalent NO3- anions in their interlayer galleries for charge compensation causes strong electrostatic repulsive forces among them that exerts the change in the orientation of NO3- anions resulting larger interlayer spacing.39 Conversely, the MgAl-LDH/GO composites indicated the interlayer distance of 8.71, 8.68 and 8.60 Å for the 5, 10 and 20% GO-containing materials, respectively. A small increase in the interlayer distance was observed after compositing with LDH. However, the decrease in the d003 spacing observed with an increase in the GO loading may be due to the increase in the release of partial NO3- due to the partial charge neutralization with carboxylate/alkoxide anions of GO. FT-IR of the GO showed a vibration peak at 3600-3000 cm-1 corresponding to the O-H stretching vibrations of free and alkoxide O-H (Figure 1B). The peaks present at 1731, and 1620 cm-1 are ascribed to carboxylic C=O and aromatic C=C stretching vibrations of the GO backbone,


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respectively. The peaks centered at 1363, 1226 and 1053 cm-1 can be ascribed to C-O stretching vibrations of carboxyl, epoxy/ether and alkoxide functional, respectively.40 The as-synthesized MgAl-LDH and their corresponding MgAl-LDH/GO composites showed bands at 3467 and 1625 cm-1 related to the vibration of O-H water stretching and bending, respectively. The interlayer NO3ions in the solid residues showed three bands centered at 1384, 826 and 663 cm-1 corresponding to asymmetric and symmetric stretching vibrations. The presence of GO in the MgAl-LDH/GO composites could not be determined clearly due to the overlap of water bending vibrations with the aromatic C=C of GO. Similarly, the C-O stretching vibrations are mostly merged with the CO stretching vibrations of contaminated CO32-. However, a new peak appeared at 1126 cm-1 after fabrication with GO, related to the C-O stretching of the alkoxide/alkoxy functional group (COH), and their intensity increased with increase in the GO contents in the composite. The zeta potential of GO and MgAl-LDH were -21.0 mV and +36.7, respectively (Figure 1C). The negative charge of GO is due to the availability of –COO-/-C-O- groups on their surface. The MgAl-LDH, after fabrication with different amounts of GO (5, 10 and 20%), indicates the positive surface charges of 33.8, 31.1 and 31.7 mV, respectively, which are very close to the parent MgAlLDH (Figure 1C). This illustrates that the MgAl-LDH surface has been exposed to decoration by smaller LDH crystallites or the nanosheets of LDH on larger GO sheets, as shown in Figure 1C (inset).


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-

-

-

2 NO3

4 NO3

(B)

3 NO3

C=O -H2O bent

(A)

(e)

(e) (d) (c)

Tansmittance (%)

(d)

Intensity (a.u)

(c)

(b)

(b)

(a)

(a)

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70 4000

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-1

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-O-H

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-40

(a)

-30

-20

-10

0

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40

50

Zeta potential (mV)

Figure 1. (A) Powder X-ray diffraction patterns, (B) FTIR, and (C) surface potential measurement of as-synthesized (a) MgAl-LDH, (b-d) nanocomposites of MgAl-LDH/GO containing 5,10 and 20% of GO, respectively and (e) GO. TEM micrographic images of MgAl-LDH showed aggregated hexagonal nanosheets with the lateral dimension of ~100 nm, while GO showed the larger nanoheets (Figure 2A). The MgAlLDH/GO (5%) nanocomposites clearly indicate the decoration by LDH on the surface of the larger GO (Figure 2A), which is reliable with the results of surface potential measurements (Figure 1C). TEM-EDS elemental analysis of the nanocomposites indicates that the element distribution of C is nearly same to the LDH constituents of Mg, Al, O and N that confirms the homogeneous nature of the composite (Figure 2B & C).


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Figure 2. (A) TEM micrographs of (a) MgAl-LDH, (b) MgAl-LDH/GO (5%) composite and (c) GO (solid red arrow = LDH; dotted blue arrow = GO); (B, C) TEM-EDS elemental mapping of the nanocomposites of MgAl-LDH/GO containing 5 and 20% GO.

Table 1. Elemental compositions and specific surface areas of GO, MgAl-LDH and MgAlLDH/GO nanocompositesa


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Mg/Al metal ratiob

(mmol/g)b

Exchangeable NO3- (mmol/g)c

MgAl-LDH

2.38

3.493

MgAl-LDH/GO (5%)

2.35

MgAl-LDH/GO (10%) MgAl-LDH/GO (20%)

Materials

GO

C (mmol/g)d

N (mmol/g)d

Specific surface area (m2/g)

2.835

0.42

2.56

5.0

3.344

2.164

2.47

2.28

62.5

2.35

3.140

2.105

4.14

2.17

50.2

2.33

2.789

1.831

7.70

1.91

18.7

-

-

-

48.55

0.36

11.6

AEC

a

calculated based on composite; bICP analysis; cIon chromatographic analysis were performed by exchanging the interlayer anion with 0.1 mM Na2CO3; dCHN analysis.

Elemental analysis indicates that the anion exchange capacity (AEC) of the MgAl-LDH/GO composites was reduced with an increase in the amount of GO that directly relates to the content of NO3- present in the LDHs (Table 1). Additionally, increased carbon amount was detected in the composite containing larger amount of GO. The specific surface area of the MgAl-LDH/GO composites were higher than their MgAl-LDH and GO counterparts, which increased with decrease in the GO content (Table 1). The outcome of the result is in good agreement with the speculation which 5% GO delaminated GO sheets the best. The homogeneous formation of the nanocomposite might be the reason for such enhancement in the specific surface area of the corresponding MgAl-LDH/GO nanocomposites.

Effect of GO content on the co-sorption of Sr2+ and SeO42- on MgAl-LDH/GO nanocomposites


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The Sr2+ and SeO42- co-sorption onto parent MgAl-LDH, GO and MgAl-LDH/GO (x%) composites are shown in Figure 3A. The sorption density of Sr2+ enhanced with the increase in the content of GO and reached 0.206 mmol/g for the MgAl-LDH/GO (20%) composites, which is slightly larger than GO (0.187 mmol/g). Conversely, the MgAl-LDH showed the poor sorption value of 0.006 mmol/g. Interestingly, the sorption capacity values of Sr2+ normalized to GO were 1.413, 1.150 and 1.031 mmol/g for MgAl-LDH/GO composites containing 5, 10 and 20% GO, respectively, which are far higher than that of the parent GO. This suggests the presence of cooperative effect between the GO and MgAl-LDH in the composite. Moreover, such an effect was higher in the MgAl-LDH/GO (5%) composite than MgAl-LDH/GO (20%) because of the high dispersion of GO in the resulting composite, which evidenced by the specific surface area of the corresponding samples (Table 1). The observed Sr2+ sorption capacity is slightly lesser than the similar type of MgAl-LDH/GO composite reported, which is due to the difference in the sorbent fabrication process.7 Conversely, the sorption of SeO42- decreased with increasing GO contents, which is due to the decrease in the LDH contents as well as increased charge compensation of

1.5

mmol/g of composite mmol/g of graphene oxide

(A)

1.0

2+

2-

0.5

SeO4 adsorption capacity (mmol/g)

positive charge in LDH by GO. Sr adsorption capacity (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.0

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10 %

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100 %

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(B)

mmol/g of composite mmol/g of LDH

0.8

0.6

0.4

0.2

0.0

0%

Graphene oxide content (w/w, %)

5%

10 %

20 %

100 %


Figure 3 Co-sorption of (A) Sr2+ cation and (B) SeO42- anion onto MgAl-LDH, GO and MgAlLDH/GO nanocomposites.


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-

2-

-

-COO /-C-O (GO) 2CO3 included or charge balanced by GO during ion exchange

SeO4 adsorbed 2CO3 included during synthesis DH l-L gA M

Materials

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/ DH -L %) l gA (5 M O G / DH ) -L % l 0 gA (1 M O G / DH ) l-L % gA (20 M O G

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Anion exchange capacity (meq/g of composite)

Figure 4. Anionic charge compensation by different species in the MgAl-LDH and MgAlLDH/GO nanocomposites. The positive charge of MgAl-LDH in the nanocomposites was balanced by different anionic species that were evaluated by multiple analytical techniques, as shown in Figure 4. The anion exchange capacity (AEC) of the composites decreased with an increase in the GO contents due to the decreased contents of LDHs. The decrease in the charge utilization of the –COO-/-C-Ofunctional groups at higher GO contents hints at the formation of heterogeneous composites by lesser ring opening of epoxides that reduced the normalized sorption capacity of Sr2+ for higher GO-containing composites. In addition, a decrease in the charge compensation of LDHs by atmospheric CO2 or the –C-O- of GO was observed for higher GO-containing samples. This indicates the increase in heterogeneity after the co-sorption of Sr2+ and SeO42- in higher GOcontaining composite materials.

Co-sorption of Sr2+ and SeO42- from single and binary solution.


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The co-sorption of Sr2+ and SeO42- was studied with and without the co-presence of the other ion, and the results are given in Figure 5. The MgAl-LDH/GO (x%) composites, irrespective of GO content, showed increased sorption capacity of Sr2+ in the binary solution, which were higher than those of the single electrolytic solutions. The increase in the Sr2+ sorption capacity with the presence of SeO42- is due to a synergetic effect. The sorption of negatively charged SeO42on the LDH provided a net negative charge on their surface, which enhanced the sorption of Sr2+. Correspondingly, the sorption capacity of SeO42- was also enhanced in the binary solutions due to synergetic effect of Sr2+, similar to the sorption occurring in LDH/C-dot composites.6 The synergetic effect decreases with increasing GO content, which may be due to the formation of a highly homogeneous composite for the low GO-containing sample and a heterogeneous composite

1.5 (A)

2+

0.5 mM Sr 2+ 20.5 mM Sr + 1.0 mM SeO4

1.0

1.00 (B)

2-

1.0 mM SeO4 2+ 20.5 mM Sr + 1.0 mM SeO4

0.75

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0.25

2-

2+

0.5

SeO4 adsorption capacity (mmol/g of LDH)

for higher GO contents. Sr adsorption capacity (mmol/g of GO)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.0 0%

5%

10 %

20 %

100 %

0.00 0%


5%

10 %

20 %

100 %


Figure 5. Sorption efficiency of (A) Sr2+ cation and (B) SeO42- anion on to as-synthesized MgAlLDH, GO and different MgAl-LDH/GO composites with and without SeO42- and Sr2+, respectively. Influence of time on the co-sorption of Sr2+ and SeO42-.


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The influence of contact times on the co-sorption of both cationic Sr2+ and anionic SeO42- onto parent MgAl-LDH, GO and their composites containing different GO content are shown in Figure 6. The co-sorption of Sr2+ increased with increased contact time and reached equilibrium in less than 1 h. The sorption capacities of Sr2+ on MgAl-LDH and GO were 0.011 and 0.186 mmol/g, respectively, which was increased to 1.443, and 1.015 mmol/g of GO for the composite containing 5 and 20% GO. The observance of lesser Sr2+ sorption capacity on MgAl-LDH/GO (20%) composite (Figure 6A) is due to the formation of non-homogeneous composites, as stated earlier. Conversely, the removal capacity of SeO42- decreased with the increase in GO loading (Figure 6B). It could be reasonably interpreted that the partial charge compensation of LDH occurs due to the carboxyl or alkoxide (-COO-/-C-O-) functional groups of GO, which is enhanced with the increase in the GO loading. The Sr2+ and SeO42- sorption kinetics on GO, MgAl-LDH and MgAlLDH/GO nanocomposites were analyzed and the details are given in the Supporting Information (Figure S1 and Table S1). It shows that the equilibrium rate constant for the sorption of SeO42- is enhanced with increase in the GO contents owing to increased dispersion of LDH particles on the surface of GO. Conversely, the Sr2+ removal was very rapid on MgAl-LDH than their GO nanocomposites (Table S1). The dissolution of Mg2+ increased with increasing GO content (Figure 6C), which may be due to the consumption of OH- by GO for the ring opening of epoxides. Conversely, a little change in the dissolution of Al3+ occurs, irrespective of the GO contents (Figure 6D).


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1.0

MgAl-LDH MgAl-LDH/GO (5 %) MgAl-LDH/GO (20 %) GO

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(B)

1.0

0.8

0.6 MgAl-LDH MgAl-LDH/GO (5 %) MgAl-LDH/GO (20 %) GO

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2+

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Sr adsorption capacity (mmol/g of GO)


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50

100

150

200

250

0

50

Time (min)

100

150

200

250

Time (min)

Figure 6 Effect of sorption time on the co-removal of (A) Sr2+ cation and (B) SeO42- anion on MgAl-LDH/GO nanocomposites and their precursor; dissolved (C) Mg2+ and (D) Al3+ concentrations.

Sorption isotherm of Sr2+ and SeO42- in single electrolytic solution. The Sr2+ and SeO42- sorption isotherms onto MgAl-LDH/GO composites containing 5 and 20 % GO were explored in single electrolytic solutions, as shown in Figure 7. The MgAl-LDH/GO


16

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(5%) composite showed larger sorption capacity of Sr2+ than MgAl-LDH/GO (20%) composite, which is due to highly dispersed nature of GO in the former than the latter. The sorption density was enhanced with the increase in the Sr2+ initial concentration. In contrast, the MgAl-LDH/GO (5%) showed higher normalized SeO42- sorption capacity than that of the MgAl-LDH/GO (20%) composite. It is reasonable to interpret this based on the ratio of charge compensation of the functional groups of GO nanosheets with LDHs. At larger GO-containing composites, the number of –COO-/C-O- groups charge increased for charge compensation with LDH positive charge,


thereby reducing their SeO42- sorption capacity of LDH. 2.5

(A) 2.0

1.5

1.0

0.0 0

1

2

3

4

1.00

(B)

0.75

0.50

0.25

MgAl-LDH/GO (5%) (exp) MgAl-LDH/GO (20%) (exp) Langmuir fit Freundlich fit

2-

MgAl-LDH/GO (5%) (exp) MgAl-LDH/GO (20%) (exp) Langmuir fit Freundlich fit

0.5

2+

Sr adsorption capacity (mmol/g of GO)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5

0.00 0

Equilibrium concenration (mM)

1

2

3

4

5

Equilibrium concenration (mM)

Figure 7. Sorption isotherms of (A) Sr2+ cation and (B) SeO42- anion on MgAl-LDH/GO (5%) and MgAl-LDH/GO (20%) composites in the single electrolytic solution. The experimental sorption results Sr2+ and SeO42- were fit well with both Langmuir and Freundlich non-linear sorption isotherm models. The Langmuir equation is denoted as: 𝑄e = 𝑄max

𝐿𝐶eq 1+𝐿𝐶eq

,

(1)

where, Qe (mmol/g) - quantity of Sr2+ and SeO42- adsorbed per gram of GO and LDH, respectively; Ceq (mmol/L) - equilibrium solution concentration; L - Langmuir constant; and Qmax (mmol/g) - theoretical maximum removal capacity of Sr2+ and SeO42-.


17


Page 18 of 42

Multilayer sorption of sorbate species or sorption on energetically different heterogeneous surfaces could be accurately described by the Freundlich sorption model. The non-linear Freundlich sorption isotherm equation is mathematically represented as, 1/𝑛

(2)

𝑄𝑒 = 𝐾𝐹 𝐶𝑒

Where, KF - Freundlich isotherm constant, which is correlated to the sorption density at equilibrium concentration of 1 and n - sorption intensity.

Table 2. Sorption of Sr2+ and SeO42- over the multifunctional MgAl-LDH/GO composites in the single electrolytic solution and their Langmuir and Freundlich parameters. Sorbents Ions

Langmuir Qmax(exp)

Qmax(cal)

Freundlich L

RL2

KF (mmol/g

n

RF2

0.9014

(L/mmol)1/n)

(mmol/g) (mmol/g) (L/mg) SeO42- MgAlLDH/GO (5%)

0.842

0.842

772.9

0.977 0.806

8.119

MgAlLDH/GO (20%)

0.806

0.807

905.9

0.942 0.777

13.044 0.925

MgAlLDH/GO (5%)

2.281

2.435

1.582

0.954 2.048

2.686

0.991

MgAlLDH/GO (20%)

1.680

1.568

4.656

0.951 1.548

3.645

0.994

Sr2+

The Sr2+ sorption isotherm indicated that the Freundlich isotherm model was better suited than the Langmuir adsorption isotherm, evidenced by better R2 (Table 2). The pertinence of the Freundlich isotherm indicates that the cationic Sr2+ sorption happens on the energetically different


18

Page 19 of 42

surfaces, possibly as a result of increase in the content of surface coordination compound by the increase in the ring opening of epoxides at the higher ionic strength of the medium. Moreover, the removal of Sr2+ was favorable on MgAl-LDH/GO composites, evidenced by n > 1 (Table 2). Conversely, the sorption of SeO42- follows Langmuir monolayer adsorption. This indicates that the sorption of SeO42- occurred by the ion-exchange of NO3- from the LDH without any secondary chemical interactions.

2 0.4 1

2+

0

0.2

2-

2+

Sr 2SeO4

0.0 2

4

6 8 Initial pH

10

12

8 1.5 6 2+

Mg 3+ Al Equilibrium pH

1.0

0.5

4

Equilibrium pH

0.6

(B)

3+

3

2+

0.8

2.0

Mg /Al concenration (mmol/L)

1.0

(A)


Influence of pH on the co-sorption of Sr2+ and SeO42- on to MgAl-LDH/GO (5%) composite Sr adsorption capacity (mmol/g of GO)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

0.0

0 2

4

6 8 Initial pH

10

12

Figure 8. Influence of initial pH on Sr2+ and SeO42- co-sorption over MgAl-LDH/GO (5%) composite: (A) co-sorption density of Sr2+ and SeO42- and (B) leached Mg2+ and Al3+ and equilibrium pH. The assessment of the pH effect on the co-sorption of Sr2+ and SeO42- is very important to identify the suitability of the MgAl-LDH/GO composites. The real effluent stream contains variable pH depending upon the origin; hence, sorbents that perform in a broad range of pH are highly desired. The pH influence on the co-sorption of cationic Sr2+ and anionic SeO42- onto the MgAl-LDH/GO (5%) nanocomposite is shown in Figure 8A. The Sr2+ sorption density was almost constant in a broad range of pH (4-10). The protonation of carboxyl or alkoxy groups (–COO-/-C-


19


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O-) or the increase in the competition of dissolved Mg2+ ions (Figure 8B) may be decreased the removal capacity of Sr2+ at lower pH (

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