Carbon-Dot-Decorated Layered Double Hydroxide Nanocomposites

Research Article pubs.acs.org/journal/ascecg

Carbon-Dot-Decorated Layered Double Hydroxide Nanocomposites as a Multifunctional Environmental Material for Co-immobilization of SeO42− and Sr2+ from Aqueous Solutions Paulmanickam Koilraj, Yuta Kamura, and Keiko Sasaki* Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: Co-immobilization of cationic and anionic radionuclides is highly desirable for total remediation of radioactive wastewater. Carbonaceous nanomaterials have received much attention in the field of water remediation and pollution control in recent years. However, the handling of these nanomaterials is challenging due to increased bioavailability and toxicity. In this work, MgAl-NO3 layered double hydroxide (LDH) was synthesized and modified using carbon nanodots (C-dot). The prepared materials were characterized using powder X-ray diffraction (PXRD), Fourier transform infrared (FT-IR), zeta potential, and transmission electron microscopy (TEM) observation. Adsorption of SeO42− and Sr2+ on MgAl-NO3-LDH/C-dot composites showed that the Sr2+ immobilization capacities increased with an increase in the amount of C-dot. The mechanism of Sr2+ adsorption on these composites occurs via coordination with the −COO− group of C-dot, whereas that of SeO42− occurs through ion exchange with NO3− in the interlayer galleries of LDH. The adsorption of Sr2+ and SeO42− was enhanced in both bicomponent (Sr2+ + SeO42−) and tricomponent systems (Sr2+ + SeO42− + M+/M2+ = coexisting cations or An− = coexisting anions) with the presence of other anion and cations. The MgAl-NO3-LDH/C-dot composites demonstrated that the high adsorption efficiency of Sr2+ and SeO42− than most of other materials reported. These results demonstrate that MgAl-NO3-LDH/C-dot composites are an effective adsorbent for total remediation of anionic and cationic radioactive nuclides from wastewater. KEYWORDS: Layered double hydroxide, Carbon dot, Nanocomposites, Radioactive waste, Adsorption, Ion exchange

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INTRODUCTION

Layered double hydroxides (LDHs) are a type of synthetic anionic clay that has a high anion exchange capacity (AEC ∼ 3 mequiv/g), labile interlayer anions, and large surface area. LDHs are widely used as catalysts, catalyst supports, and environmental materials.12,13 C-dot supported on LDH could enhance handling efficacy and could add an additional functionality (anion removal). The unitization of C-dot as a functional component for the remediation of aqueous pollutants is scarce except the C-dot fabricated on MgAl-LDH and MgFe-LDH for dye14 and Pb2+removal,15 respectively. However, to the best of our knowledge, co-immobilization of both anions and cations on LDH/C-dot composite materials has not yet been reported in the literature. The present study focuses the co-immobilization of anionic and cationic radioactive nuclide analogue using multifunctional materials for total remediation. Selenium nuclides (79Se) mainly exist in the environment in the highly soluble forms of selenite SeO32− (Se4+) and selenate SeO42− (Se6+) with a notably long half-life of 2.952 × 105 y.16 The International Research Institute

Nanomaterials have gained much attention in recent years for use in decontamination of aqueous toxic ions.1,2 Carbon nanodots (C-dot) have also shown promise in different fields of science. Carbon dots contain many functional groups (−COOH, −OH, −NH2) on their surfaces.3 The synthesis of C-dots involves simple protocols and also uses waste materials as a precursor instead of chemical precursors.4−6 Due to the presence of functional groups, easy preparation methods, and low toxicity, C-dots can be used in the removal of cations. However, handling of these nanomaterials for practical applications is challenging due to the difficulties in separation and reuse and/or the risk of secondary contamination from leaching of nanomaterials in the environment.7,8 Nanomaterials supported on a bulky solid support offer an effective and efficient protocol to prevent secondary effects during remediation of wastewater. Recently, C-dot immobilized on mesoporous organosilica9,10 and magnetic ferrite-MoS2-C-dot nanohybrids11 have been utilized for the removal of dichlorophenol and heavy metal ions (U(VI), Cu2+, Pb2+, and Hg2+)9−11 from aqueous solutions. However, a solid support that offers an additional functionality could be advantageous for remediation purposes. © XXXX American Chemical Society

Received: June 17, 2017 Revised: August 5, 2017 Published: August 21, 2017 A

DOI: 10.1021/acssuschemeng.7b01979 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

separated by centrifugation at 11 000 rpm for 20 min, washed three times with decarbonated water, and freeze-dried for 24 h to obtain powdery MgAl-NO3-LDH. MgAl-NO3-LDH/C-Dot Composite Preparation. The MgAlNO3-LDH/C-dot composite was prepared by the colloidal deposition method. Approximately 0.50 g of MgAl-NO3-LDH powder was mixed with 12.5 mL of ultrapure water containing certain amount of purified C-dot to obtain 5, 10, and 20% (wt/wt) C-dot on a MgAl-NO3-LDH support in the final composite. The mixture was stirred for 2 h, separated by filtration, washed with water, and freezedried for 24 h. The obtained product is referred to as MgAl-NO3LDH/C-dot (x%), where x = 5, 10, and 20 for composite materials denotes the addition of 5, 10, and 20% C-dot, respectively. Adsorption of Sr2+ and SeO42− onto MgAl-NO3-LDH and MgAl-NO3-LDH/C-Dot Composites. Approximately 20 mg of either MgAl-NO3-LDH or MgAl-NO3-LDH/C-dot (x%) was added to 20 mL of mixed solution containing 1 mM SeO42− (Na2SeO4) and 0.1 mM Sr2+ (SrCl2). All the experiments were performed using a solid-toliquid ratio of 1 g/L under a N2 atmosphere, agitated at 100 rpm for 2 h at a stock solution pH of 5.95 at 25 °C unless otherwise stated. The adsorption studies on C-dot were performed by dialysis method using a semipermeable membrane bag (Wako, pore size 15−50 Å) similar to the previously reported procedure.14 Kinetics studies were performed by collecting samples at different time intervals at the conditions mentioned above. Sr2+ adsorption isotherm studies were performed on MgAl-NO3-LDH/C-dot (20%) composite using 0.01−1 mM Sr2+ by retaining 1 mM SeO42−. The effect of coexisting cations on Sr2+ adsorption was studied using a mixture of 0.1 mM Sr2+ with 0.1 mM LiCl, NaNO3, KNO3, Mg(NO3)2, and Ca(NO3)2 separately under the same condition with and without 1 mM SeO42−. The influence of coexisting anion on the adsorption of SeO42− was studied using 1.0 mM SeO42− containing 1.0 mM NaCl, NaNO3, NaF, Na2SO4, Na2CO3, and NaH2PO4 separately with and without 0.1 mM Sr2+. Desorption studies were performed in four different medium such as deionized water, 3.0 mM HNO3, 0.5 mM NaNO3, and the mixture of 0.5 mM NaNO3 + 3.0 mM HNO3, respectively, for 24 h. After adsorption of Sr2+ and SeO42− on MgAl-NO3-LDH/C-dot (20%) composites, the cyclic adsorption−desorption studies were performed in the mixture of 0.5 mM NaNO3 + 3.0 mM HNO3. In all the cases, the solid products after adsorption and desorption of Sr2+ and SeO42− were separated using membrane filtration. The residual Sr, Se, Mg, and Al concentrations were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES; PerkinElmer 8500, Yokohama, Japan) after acidification. Characterization of Solid Residues. Powder X-ray diffraction (PXRD) patterns of the composite materials were collected (Rigaku, Ultima IV, Akishima, Japan) using Cu Kα radiation with 40 kV acceleration voltage and 40 mA applied current at a 2°/min scanning speed and 0.02° step size. Fourier transform infrared (FT-IR) spectra of the MgAl-NO3-LDH or MgAl-NO3-LDH/C-dot (x%) and the solid residues after sorption of SeO42− and Sr2+ were collected in transmission mode (JASCO FT/IR-670 Plus, Tokyo, Japan). The pellets were prepared by mixing 2 wt % of the sample in spectral grade KBr. Transmission electron microscopy (TEM) images of the solid products were observed on a transmission electron microscope (JEM-2100HCKM, JEOL, Akishima, Japan) by dispersing them in ethanol. Particle size and zeta potential measurements were performed on a Malvern ZETASIZER NANO-ZS (Kobe, Japan) using the dynamic light scattering method. X-ray photoelectron spectra (XPS) were collected on an ESCA 5800 (ULVAC-PHI, Inc., Kanagawa, Japan) using a monochromated Al Kα X-ray source at 200 W, and the data were analyzed with Casa XPS software (version 2.3.12.8). Binding energies (EB) were calibrated using EB [C 1s] = 284.6 eV for adventitious carbon.

for Nuclear Decommission (IRID) in Fukushima, Japan, has selected 79Se as one of the most important radioactive nuclides in evaluation for purification and disposal of radioactive wastes. As such, 90Sr is a beta radiation emitter with t1/2 = 28.8 y and is primarily present in the aqueous environment as Sr2+.17 90Sr is mobile and reaches the food chain through plant sources.18 As an example, the Fukushima Daichi nuclear disaster caused severe 90Sr radioactive contamination in water sources, and the advanced liquid processing system (ALPS) has been used in treatment of radioactive nuclides in the contaminated area. Upstream of the ALPS system, 90Sr is assumed to be removed by adsorption on chabazite (herscherite-H). However, for the treated water, the concentration of 90Sr was not completely removed in the upstream of the process. Therefore, in the downstream areas, the primarily anionic adsorbents must be cleaned up, but the small adsorption capacity of cationic targets is needed to add onto the adsorbents. The synthesis of LDH modifiers is meaningful for the customization of effective bifunctional materials other than hydroxyapatite and other existing adsorbents. The sorption process has merits due to its efficiency in low concentrations, production of low amounts of waste for disposal, and economic viability. Sorption of Sr2+ has been achieved using different cation exchange materials, such as clay/oxide-based zeolite,19,20 titanium dioxide,21 titano-silicate,22 titanium niobate,19 thiostannate,23 sericite,24 gibbsite,25 modified layered double hydroxides,26,27 and hydroxyapatite.28 Aside from carbonaceous composites such as magnetite-reduced graphene oxide,29 thiacalixarene-functionalized graphene oxide (GO), and hydroxyapatite−graphene oxide,30 the use of activated carbon31 and carbon nanotubes32 have also been reported in the literature. Similarly, SeO42− removal has been performed using metal− organic frameworks,33 LDHs,34 and carbon nanospheres.35 The co-immobilization of both Sr2+ and SeO42− is highly desirable and is challenging in terms of environmental concerns. Few studies are available in the literature on combined Sr2+ and SeO42− removal, except for the report by Nie et al.36 In this work, we report the synthesis of C-dot-decorated MgAl-NO3-LDH composites using a colloidal deposition method and an application for co-immobilization of Sr2+ and SeO42− from wastewater. Parameters such as contact time, adsorption isotherm, and effect of coexisting cations were studied in detail. The mechanism of Sr2+ and SeO42− removal in the composite materials was evaluated using different physicochemical analyses.

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MATERIALS AND METHODS

Preparation and Purification of Carbon Dots (C-dot). The C-dot material was prepared using the previously reported procedure.37 In brief, 1.0 g of citric acid and 1.0 g of urea were dissolved in 10 mL of ultrapure water. The mixture was microwaved at 500 W and treated for 10 min. The obtained black solid was ground well, and the powdery products were dispersed in water. The mixed solution was dialyzed using a dialysis membrane bag (Wako, pore size 15−50 Å) to remove soluble intermediates and unreacted precursors. The leaching solution was changed at regular time intervals with fresh ultrapure water until the color disappeared. Photographs of C-dot before and after purification are supplied in the Supporting Information (Figure S1). The dialysis-purified solution was freeze-dried for 24 h to obtain a powdery C-dot material. Preparation of MgAl-NO3-LDH. The coprecipitation method was used to synthesize MgAl-NO3-LDH. In detail, approximately 200 mL of decarbonated solution containing a mixture of 0.04 M Mg(NO3)2· 6H2O and 0.02 M Al(NO3)3·9H2O was pH-adjusted to 10 using a mixed solution of 2 M NaOH and 0.2 M NaNO3 under vigorous stirring that was continued for 30 min in a N2 atmosphere. The obtained slurry was aged at 80 °C for 20 h. The solid residues were

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RESULTS AND DISCUSSION Characterization of MgAl-NO3-LDHs and MgAl-NO3LDH/C-Dot Composites. The synthesized MgAl-NO3-LDHs and MgAl-NO3-LDH/C-dot (x%) composites containing different mass ratios were characterized via different physicochemical analyses. The color appearance of the composites was B


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ACS Sustainable Chemistry & Engineering changed from white (MgAl-NO3-LDH) to brown and finally to black with increases in the C-dot mass ratio (Figure S2). The PXRD of purified C-dot (Figure 1A) showed a broad

corresponding to CO and N−H stretching vibrations, which increased with increasing C-dot contents. These results confirm the integration of C-dot on the LDH in the composite materials. Zeta potential measurements showed that the MgAl-NO3LDH had a positive surface charge of 31.4 mV, and the C-dot had a negative surface charge of −33.7 mV due to the presence of surface −COO− groups (Figure 1C). Conversely, the MgAlNO3-LDH/C-dot of 5, 10, and 20 wt % composites showed surface charges of 31.1, 19.5, and −16.4 mV, respectively. The shift in the surface charge from positive to negative with the increase in the C-dot content was due to the interaction of surface −COO− with positively charged LDHs. The masking of the positive charge of LDH even in the presence of an 80% mass ratio of LDH offers a clue that the C-dot predominantly occupies the surface and edges of the LDH crystallites due to its larger size compared with the interlayer spacing of LDH.39 TEM images showed that the diameter of C-dot was approximately 3−5 nm (Figure 2Aa) and that MgAl-NO3-LDH

Figure 1. (A) PXRD patterns, (B) FT-IR spectra, and (C) zeta potential of (a) MgAl-NO3-LDH, (b−d) MgAl-NO3-LDH/C-dot (x%; x = 5, 10, 20) composites, and (e) C-dot.

reflection at 2θ, where 27.2° corresponds to amorphous/nanocarbon.38 The PXRD of MgAl-NO3-LDH confirmed that the LDH was crystallized in nitrate form (d003 spacing of 8.94 Å). The MgAl-NO3-LDH/C-dot (x%) composite showed d003 spacing of 8.88, 8.91, and 8.96 Å for composites containing 5, 10, and 20% C-dot, respectively (Figure 1A). A slight change in the d003 spacing of these composite materials could be due to the release of an amount of NO3− from the interlayer of LDH or the adsorption of a small amount of carbonate during the C-dot deposition method. It worth noting that at a lower content of C-dot (5%), the d003 spacing changed drastically from 8.94 to 8.88 Å due to the incorporation of a large amount of CO32− in the absence of a charge-balancing C-dot in the medium. However, the CO32− interference in the release of NO3− decreased with an increase in the C-dot content, as supported by PXRD measurements (Figure 1A). Alternatively, the C-dot acts as a barrier to minimize the contact of CO32− with LDHs at higher C-dot contents. The C-dots are larger than the interlayer spacing and occupy the surface and edges of LDH crystallites by acting as a barrier to the contact of CO32− with LDH for ion exchange. The PXRD peak fitting of the obtained solid residues supports the claim of reduction in the CO32− contamination with an increase in the C-dot content (Figure S3). The FT-IR spectrum of C-dot showed peaks at 3192 and 3449 cm−1 corresponding to −OH stretching vibration for C-dot and MgAl-NO3-LDH/C-dot (20%), respectively (Figure 1B). The peak at 2926 cm−1 corresponds to C−H stretching vibration. The peaks that appeared at 1701, 1667, and 1557 cm−1 were attributed to CO, N−H, and C−O stretching vibrations, respectively.39 The peak at 1361 cm−1 was ascribed to C−O symmetric stretching vibration. The peak at 1272 cm−1 is attributed to C−O−C stretching vibrations.39 The bands depicted at 1187 and 1067 cm−1 correspond to −C−O stretching vibrations. The MgAl-NO3-LDH and MgAl-NO3-LDH/C-dot (x%) showed a band centered at 1384 cm−1 assigned to asymmetric stretching vibration of NO3−.40 MgAl-NO3-LDH/ C-dot (x%) composites showed peaks at 1701 and 1667 cm−1

Figure 2. (A) TEM images of (a) C-dot, (b) MgAl-NO3-LDH, and (c) MgAl-NO3-LDH/C-dot (20%). (B) TEM EDX elemental mapping on MgAl-NO3-LDH/C-dot (20%).

appeared as platelet-like morphologies with an average diameter of ∼100 nm (Figure 2Ab). The C-dot on MgAl-NO3-LDH was attached by electrostatic interaction with the π-electrons of the aromatic ring and/or by ionic interaction of the negatively charged −COO− functional group with positively charged LDH nanosheets. The C-dot in the nanocomposite was well-dispersed on the layered LDH nanosheets (Figure 2Ac). TEM-EDX analysis showed that the elemental mapping of C is significantly overlapped with Mg, Al, O, and N, which are the main constituents in LDH. This result indeed confirms that the C-dot was homogeneously integrated on the surface of LDHs (Figure 2B). The ICP-OES results showed that the composite materials had nearly similar Mg/Al atomic ratios, indicating the chemical stability of LDHs (Table 1). The anion exchange capacity (AEC) of MgAl LDHs in the composite materials decreases with increasing amount of inactive C-dot content. The increase in the carbon and nitrogen content confirms the increase in the amount of C-dot in the final composites (Table 1). Co-immobilization of Sr2+ and SeO42− on MgAl-NO3LDH/C-Dot Composites. Effect of C-Dot Content in MgAlNO3-LDH/C-Dot Composite on Adsorption of Sr2+ and SeO42−. Adsorption studies indicated that the sorption of Sr2+ C


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ACS Sustainable Chemistry & Engineering Table 1. Elemental Compositions of MgAl-NO3-LDH/C-dot Compositesa

a

materials

Mg/Al atomic ratiob

AECb (mmol/g)

exchangeable NO3− (mmol/g)c

C (mmol/g)d

N (mmol/g)d

MgAl-NO3-LDH MgAl-NO3-LDH/C-dot (5%) MgAl-NO3-LDH/C-dot (10%) MgAl-NO3-LDH/C-dot (20%)

2.23 2.35 2.34 2.34

3.27 3.39 3.10 2.84

3.01 2.13 1.89 1.58

0.34 3.00 4.97 7.86

3.10 2.67 3.04 3.83

Calculated based on composite. bICP-OES. cIon chromatography (exchanged with 0.1 M Na2CO3). dCHN analysis.

Figure 3. Co-immobilization of (A) Sr2+ and (B) SeO42− on MgAl-NO3-LDH/C-dot composites (x%; x = 5, 10, and 20). Initial conc 0.89 mM SeO42−+ 0.096 mM Sr2+; solid-to-liquid ratio 1.0 g/L; time 2 h.

on MgAl-NO3-LDH was notably low and near 0.004 mmol/g but increased with an increase in the amount of C-dot (Figure 3A). Among the materials studied, MgAl-NO3-LDH/C-dot (20%) showed the highest Sr2+ adsorption capacity at 0.031 mmol/g of composite or 0.157 mmol/g of C-dot. The reason for such an increase in the adsorption capacity of Sr2+ was due to the increase in the amount of C-dot content. The mechanism of Sr2+ adsorption on the MgAl-NO3-LDH/C-dot composite is due to the coordination of the −COO− and −NH2 functional groups of C-dot with Sr2+. However, the SeO42− adsorption on MgAl-NO3-LDH was 0.879 mmol/g. Conversely, the SeO42− adsorption capacities of MgAl-NO3-LDH/C-dot (x%) normalized to the mass of MgAl-NO3-LDH were decreased with the increase in the amount of C-dot and showed an SeO42− adsorption of 0.728 mmol/g of LDH for MgAl-NO3-LDH/C-dot (20%). It should be noted that the PXRD peak fitting analysis of MgAl-NO3-LDH and MgAl-NO3-LDH/C-dot (20%) showed that NO3− is the predominant intercalated species (Figure S3). The reason for the decrease in the SeO42− adsorption capacity on the MgAl-NO3-LDH/C-dot might be due to partial charge neutralization of MgAl-NO3-LDH with the negatively charged C-dots (Figure 3B). The zeta potential results also supported the claim of the decrease in the surface charge with increasing C-dot content (Figure 1C). It is worth mentioning that the C-dot alone showed a SeO42− adsorption capacity of 0.183 mmol/g. The sorption of SeO42− on C-dot was more satisfactorily explained by adsorption on the −NH3+ and −OH+ functional groups present on the surface of C-dot.35 The effect of C-dot on the adsorption of Sr2+ and SeO42− in the MgAl-NO3-LDH/C-dot composites was statistically evaluated in different physicochemical analyses. A graphical representation of the charge balance of LDH by different anionic species on MgAl-NO3-LDH/C-dot composites is given in Figure 4, and this reveals that the decrease in the SeO42− adsorption capacity was due to a decrease in the total AEC of MgAl-NO3-LDH/C-dot composites (Table 1 and Figure 4) as well as an increase in the charge compensation of LDHs with

Figure 4. Graphical representation of anionic charge utilization in different MgAl-NO3-LDH/C-dot composites.

the −COO− groups of C-dot. Moreover, the decrease in the CO32− contamination during both synthesis and ion exchange was suppressed with an increase in the C-dot. The Sr2+ adsorption increased linearly with increasing C-dot content. However, the statistical charge distribution of C-dot could not be derived due to difficulty in the calculation of total negative charge on the surface of the C-dot present in MgAl-NO3-LDH/C-dot composites. Effect of Microwave Irradiation Time during C-Dot Preparation on Adsorption of Sr2+. To enhance the amount of surface functional groups of C-dot, the microwave irradiation time was increased from 10 to 30 min under the assumption that higher irradiation times could produce larger numbers of surface functional groups. The PXRD of C-dot prepared at different microwave time intervals are given in the Supporting Information (Figure S4). The results showed that an increase in the crystallinity of graphitic carbon was observed on the C-dot synthesized using 30 min of microwave irradiation. The Sr2+ and SeO42− adsorption results showed that the microwave D


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phenomenon, the MgAl-NO3-LDH was contacted with water at different time intervals, which indeed showed higher dissolution of Al3+ (Figure 6D). The structurally unincorporated Al3+ could be the reason for such release in the solution regardless of reaction conditions. This observation was further supported by an increase in the Mg/Al atomic ratio after fabrication of C-dot on MgAl-NO3-LDH (Table 1). Moreover, the dissolved Al3+ and Mg2+ concentrations were high in the adsorption solution containing only SeO42− electrolyte (data not shown), whereas leaching was suppressed in the copresence of Sr2+. Adsorption Isotherm of Sr2+ on MgAl-NO3-LDH/C-Dot Composites. The Sr2+ adsorption isotherm on MgAl-NO3LDH/C-dot (20%) composite materials indicated that the maximum sorption capacity was 0.444 mmol/g of C-dot (Figure 7). The removal of Sr2+ occurred via a multilayer adsorption mechanism, which follows the Freundlich adsorption isotherm model. During the adsorption isotherm of Sr2+, the SeO42− concentration was fixed at 0.90 mM. The results of SeO42− adsorption showed that the adsorption capacity was not altered with increases in the concentration of SrCl2. The experimental Sr2+ adsorption data were fitted using the nonlinear Langmuir and Freundlich adsorption models, where the Langmuir equation is represented as follows,

irradiation time had minimal influence on the adsorption capacity (Figure 5). Hence, C-dot prepared by microwave treatment for 10 min was used in further studies.

Figure 5. Effect of microwave irradiation for preparation of C-dot on co-immobilization of (A) Sr2+ and (B) SeO42− over MgAl-NO3-LDH/ C-dot (20%) composites. Initial conc 0.89 mM SeO42− + 0.096 mM Sr2+; solid-to-liquid ratio 1.0 g/L; time 2 h.

Effect of Contact Time on Adsorption of Sr2+ and SeO42−. Contact time studies revealed that the Sr2+ adsorption on MgAl-NO3-LDH/C-dot (20%) reached equilibrium within 30 min (Figure 6A), whereas SeO42− removal required 1 h (Figure 6B). The rapid adsorption of Sr2+ was due to the coordination, which mainly occurs on the C-dot present on the surface of MgAl-NO3-LDH. However, the SeO42− removal occurred through ion exchange that needed to overcome the diffusional problem associated with layered materials for ion exchange of nitrate present in the interlayer galleries. At equilibrium, the pH of the medium was increased from 6.0 to 7.4 (Figure 6C), which means that the treated solution does not require any pH adjustment before discharge. The increase in the pH of the medium can be better explained by the dissolution of Mg2+ ions for alkalization. The Mg2+ dissolution was high in MgAl-NO3LDH/C-dot (20%) (Figure 6D). Conversely, the dissolution of Al3+ unusually followed the reverse order and produced higher leaching for MgAl-NO3-LDH (Figure 6D). To understand this

Q e = Q max

LCeq 1 + LCeq

(1)

where Qe (mmol/g) is the amount of Sr2+ adsorbed per unit mass of C-dot, Ceq (mmol/L) is the equilibrium concentration, L is the Langmuir constant, and Qmax (mmol/g) is the maximum amount of Sr2+ adsorption coverage. Adsorption of solute species on heterogeneous surfaces or multilayer adsorption processes can be better explained by the Freundlich isotherm. The general form of the Freundlich adsorption isotherm is stated as follows,

Q e = KFCe1/ n

(2)

where KF is the Freundlich constant related to the Sr2+ adsorption capacity at an equilibrium concentration equal to 1 and n is the adsorption intensity. The degree of data fitting with the nonlinear model can be best described by the regression

Figure 6. Effect of contact time on MgAl-NO3-LDH and MgAl-NO3-LDH/C-dot (20%) composites: (A) Sr2+ concentration, (B) SeO42− concentration, (C) pH, (D) Mg2+ concentration, and (E) Al3+ concentration. Initial conc 0.89 mM SeO42− + 0.096 mM Sr2+; solid-to-liquid ratio 1.0 g/L. E


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on the observations it can be concluded that the Sr2+ was partially coprecipitated with Mg2+ as Mg{Sr}(OH)2, which may be the reason for such enhanced Sr2+ adsorption from the solution.42 At lower pH of the medium higher dissolution Mg2+ occurs from solid for the alkalization of solution and it was precipitated with Sr2+ at higher pH (Figure 8B). Mechanism of Sr2+ Adsorption on MgAl-NO3-LDH/C-Dot Composites. The mechanism of Sr2+ and SeO42− adsorption on MgAl-NO3-LDH/C-dot was evaluated via different physiochemical analyses. The PXRD patterns of MgAl-NO3-LDH/ C-dot (20%) composite after adsorption of the SeO42− and Sr2+ mixture showed a change in d003 spacing from 8.96 to 9.04 Å (Figure 9A). The increase in the d003 spacing values was due to the ion exchange of higher ionic radius SeO42− (2.43 Å) with smaller NO3− (2.00 Å) present in the interlayer galleries of LDH.43 FT-IR analysis showed that the peak at 1384 cm−1 corresponding to the υ3 asymmetric stretching vibration mode of NO3− disappeared and a new peak centered at 867 cm−1 corresponding to Se−O appeared after adsorption (Figure 9B). These observations suggest that SeO42− removal on these MgAl-NO3-LDH/C-dot composites occurs via anion-exchange.34 Furthermore, the blueshift in the −CO stretching vibration from 1702 to 1697 cm−1 after adsorption indicates that the removal of Sr2+ occurred by coordination of the metal ion with the carboxyl group of C-dot as follows

Figure 7. Sr2+ adsorption isotherm onto MgAl-NO3-LDH and MgAlNO3-LDH/C-dot (20%) composite. Initial conc 0.90 mM SeO42− + 0.0096−0.970 mM SeO42−; solid to liquid ratio 1.0 g/L; time 2 h.

coefficient R2. The coefficient values were deduced using the following equation:41 R2 =

∑ (Q cal − Q̅ e)2 ∑ (Q cal − Q̅ e)2 + ∑ (Q cal − Q e)2

(3)

2+

where Qcal (mmol/g) is the amount of Sr adsorption calculated from the isotherm model, and Q̅ e is the average of Qe (mmol/g). The results revealed that the adsorption of Sr2+ followed the Freundlich isotherm rather than Langmuir model, as supported by the good correlation coefficient values (Table 2). The applicability of the Freundlich model suggests that the adsorption of Sr2+ occurs on heterogeneous surfaces, probably due to surface complexation at higher concentrations. The adsorption of Sr2+ was favorable on C-dots supported on MgAl-NO3-LDH nanosheets, as indicated by n > 1 (Table 2). Effect of pH on the Adsorption of Sr2+ and SeO42−. The evaluation of Sr2+ and SeO42− adsorption on MgAl-NO3-LDH/ C-dot (20%) composite as a function of solution pH is an important parameter in terms of environmental concern. As the wastewater, systems have a wide range of pH depending on the origin. The SeO42− adsorption results showed that the sorption capacity was almost constant in a wide pH range of 4−8 (Figure 8A). A decrease in the SeO42− adsorption capacity was observed above pH 8 due to competitive adsorption of OH− and/or electrostatic repulsion rendered by the deprotonated negatively charged surfaces. Conversely, an increase in the SeO42− adsorption capacity was observed below pH 4 owing to the protonation of SeO42− to HSeO4−. On the other hand, the decrease in the Sr2+ adsorption capacity was observed at lower pH (pH < 4). The protonation of functional −COO− groups to −COOH is the main cause for such hampering the adsorption capacity of Sr2+ at lower pH. Interestingly, at very higher pH values (>10), the enhanced Sr2+ adsorption capacity was observed. The critical analysis of the solution data revealed that the increase in the pH of the medium as well as decrease in the dissolved Mg2+concentration was observed at this region. Based

TEM EDX elemental mapping results of MgAl-NO3-LDH/ C-dot (20%) after adsorption of Sr2+ SeO42− are given in Figure 9C. The elemental distribution indicates that the composition of SeO42− is greatly overlapped with Mg and Al, suggesting ion exchange with NO3− in LDH. Conversely, the elemental distribution of Sr2+ was directly related to carbon. It is noted the distribution of carbon and Sr2+ suggests that C-dot was aggregated during adsorption of Sr2+ and formed separate nanometer-sized islands on the surface of MgAl-NO3-LDH. The XPS survey spectra of MgAl-NO3-LDH/C-dot (20%) showed the presence of Sr and Se orbitals after adsorption of Sr2+ and SeO42− (Figure 10A). The N 1s region of MgAl-NO3LDH/C-dot (20%) before adsorption showed three peaks centered at 397.4 (surface −NH2), 399.3 (aromatic N; CC−N) and a high-energy peak at 406.8 eV (NO3−).44,45 Conversely, after adsorption of Sr2+ and SeO42−, the spectra showed only two peaks with BE values of 397.4 (surface −NH2) and 399.3 eV (aromatic N; CC−N), respectively (Figure 10B). The disappearance of the high-energy NO3− peak after adsorption suggests the ion exchange of NO3− with SeO42−. Effect of Coexisting Anions and Cations on Adsorption of SeO42− and Sr2+. To understand the effect of SeO42− on adsorption of Sr2+ in the bicomponent system, experiments were conducted using the MgAl-NO3-LDH/C-dot (20%) composite with and without 0.89 mM Na2SeO4. Independently, the effect of different coexisting cations on Sr2+ adsorption was also studied (Figure 11A) in the same manner. The results indicated

Table 2. Langmuir and Freundlich Parameters and Constants for Sorption of Sr2+ over MgAl-NO3-LDH and MgAl-NO3-LDH/ C-Dot (20%) Composite Langmuir

Freundlich 2

adsorbents

Qe(exp) (mmol/g)

Qe(cal) (mmol/g)

L (L/mg)

RL

MgAl-NO3-LDH MgAl-NO3-LDH-C-dot (20%)

0.010 0.444

0.013 0.450

3.726 9.171

0.8966 0.9634

F

KF (mmol/g (L/mmol)1/n)

n

RF2

0.011 0.464

1.915 2.294

0.8925 0.9788


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Figure 8. Effect of pH on the adsorption of Sr2+ and SeO42−: (A) Sr2+ and SeO42− adsorption capacity and (B) dissolved metal concentration and equilibrium pH. Initial conc 0.89 Mm SeO42− + 0.096 mM Sr2+; material MgAl-NO3-LDH/C-dot (20%) composite; solid-to-liquid ratio 1.0 g/L; time 2 h.

Figure 9. (A) PXRD patterns, (B) FT-IR spectra, and (C) TEM EDX elemental mapping of MgAl-NO3-LDH/C-dot (20%) composite after adsorption of 0.096 mM Sr2+ and 0.89 mM SeO42−.

that the removal of Sr2+ from the one-component system without SeO42− was lower in the copresence of other alkali and alkaline earth metals. Divalent cations such as Mg2+ and Ca2+ had a predominant effect on the adsorption of Sr2+ compared with that of monovalent cations. In the case of Mg2+ and Ca2+, the adsorption capacity was halved due to the similar affinity of divalent Mg2+, Ca2+, and Sr2+ with C-dot. The influential order of other cations on the adsorption of Sr2+ is given as follows:

groups in C-dot for adsorption of other divalent cations in the one-component system.

Conversely, the adsorption capacities of Sr2+ were enhanced in the two-component system (with SeO42−) in all cases except for in the presence of Ca2+ (Figure 11A). The enhancement in the adsorption capacity of Sr2+ in the presence of SeO42− might be due to the formation of an ion-pair complex of Sr2+ with the negative surface of LDH rendered by ion-exchanged SeO42− (LDH-SeO42−:Sr2+). Recently, Luo et al. reported an enhancement in the adsorption capacity of Sr2+ in the presence of ReO4− over surfactant-modified montmorillonite due to a synergic

Li+ < K+ < Na + < Mg 2 + < Ca 2 +

In other words, the decrease in the adsorption capacity in the presence of M2+ can be better explained by the use of carboxylic G


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Figure 10. XPS spectra of MgAl-NO3-LDH/C-dot (20%) before and after adsorption of 0.096 mM Sr2+ and 0.89 mM SeO42−: (A) survey scan and (B) N 1s regions.

Figure 11. Influence of competitive (a) cations on adsorption of Sr2+ with and without 0.89 mM SeO42− (solution conc 0.096 mM Sr2+ + 0.1 mM other cations) and (b) anions on adsorption of SeO42− with and without Sr2+ 0.096 mM (solution conc 0.89 mM Sr2+ + 1.0 mM other anions). Materials MgAl-NO3-LDH/C-dot (20%) composite; solid-to-liquid ratio 1.0 g/L; time 2 h.

effect.46 The unaltered adsorption capacity of Sr2+ in the presence of other cations might be due to the synergetic selective ion-pair formation of Sr2+ with SeO42−. The reason for such enhancement in the Sr2+ adsorption in the two-component system (with SeO42−) was evaluated in surface charging studies. The zeta potential of the MgAl-NO3LDH/C-dot (20%) composite in 0.096 mM SrCl2 solution showed that the surface charges were shifted in a slightly positive direction, from −26.4 to −25.0 mV, which might be caused by the surface-adsorbed Sr2+ (Figure S5). However, in the Na2SeO4 solution, the surface charges of the composite were more negative (−32.6 mV) than in the original MgAlNO3-LDH/C-dot (20%) due to the adsorption of SeO42−. Interestingly, in the two-component system (Sr2+ and SeO42−), the surface charge increased further in the negative direction and reached −35.5 mV due to the synergic effect, which enhanced the adsorption capacity of Sr2+ in the two-component system. The effect of coexisting anions on the adsorption of SeO42− were studied with and without the presence of 0.096 mM SrCl2, and the results are given in Figure 11B. This indicates that the monovalent anions such as NO3− and Cl− had positive effect and showed slightly higher SeO42−adsorption capacity in the one-component system (without Sr2+). Similar observations were reported earlier on phosphate adsorption over La-porous carbon composites.47 Conversely, the multivalent anions such

as sulfate, carbonate, and phosphate had a detrimental effect on the adsorption of SeO42−. In the case of a two-component system (with Sr2+), the SeO42− adsorption capacity was increased with the presence of Sr2+ irrespective valence of anions. The enhancement in the SeO42− adsorption capacity in two-component system (with Sr2+) may be due to the formation of ion-pair of SeO42− with adsorbed Sr2+. Effect of Desorbing Medium on the Recovery of Sr2+ and SeO42− and Recyclability. Regeneration of eco-materials is highly desired in terms of reusability and economic viability. The MgAl-NO3-LDH/C-dot (20%) after adsorption of SeO42− and Sr2+ was regenerated in four different matrixes (Figure S6). In water, desorption efficiency of Sr2+ (43.2%) and SeO42− (16.7%) was very poor due to the absence of anion and cation for ion exchange. On the other hand, the 3.0 mM HNO3 exhibited greater Sr2+ desorption (89.1%) and showed poor SeO42− desorption (1.5%) efficiencies due to the presence of lesser concentration of anion in the matrix. Further, it is known that the selectivity of divalent cations are always higher than the monovalent anions toward LDHs. The desorption medium such as 0.5 M NaNO3 and the mixture of 0.5 M NaNO3 + 3.0 mM HNO3 showed >87% Sr2+ desorption and around 55.7% and 85.9% SeO42− desorption efficiencies, respectively. Because of higher Sr2+ and SeO42−desorption efficiency the mixture of 0.5 M NaNO3 + 3.0 mM HNO3 have been selected as desorbing matrix for recyclability studies. H


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listed adsorbents (Table 4).27,28,60−67 These results promised that the MgAl-NO3-LDH/C-dot (20%) composite is a highly efficient multifunctional environmental material for the total remediation of aqueous SeO42− and Sr2+. To summarize, the MgAl-NO3-LDH/C-dot (20%) composite materials act as an effective adsorbent for co-immobilization of anionic and cationic radioactive nuclide analogs Sr2+ and SeO42−. The results suggest that MgAl-NO3-LDH/C-dot (20%) is an efficient, environmentally friendly, and economically viable adsorbent for total remediation of aqueous radionuclides and that the scope of the MgAl-NO3-LDH/C-dot (20%) composite is not limited and could be extended to removal of a variety of aqueous cationic and anionic species by altering the functional groups of the C-dot.

Recycling performance of both Sr2+ and SeO42− targets on MgAl-NO3-LDH/C-dot composites is challenging due to their opposite charges. The cyclic adsorption−desorption cycles were conducted up to four cycles in the mixture of 0.5 M NaNO3 + 3.0 mM HNO3 solution, and the results are given in Figure 12A and B. Results depicts that the Sr2+ adsorption efficiency was decreased with increase in the cycle number, which may be due to the release of C-dot by protonation of functional −COO− to −COOH. Interestingly, SeO42− adsorption gained 57% of its original capacity in the second cycle. This is due to the exchange of contaminated CO32− from the interlayer galleries of LDH with NO3− under the acidic conditions during desorption.48 With further increase in the number of cycles, a slight decrease in the SeO42− adsorption capacity was observed because of the weathering of LDH under such acidic condition. Comparison of SeO42− and Sr2+ Adsorption Capacities with Other Materials. The SeO42− and Sr2+ adsorption capacities of MgAl-NO3-LDH/C-dot (20%) were compared with different adsorbent materials reported in the literature (Tables 3 and 4). The adsorption capacity of SeO42− onto MgAl-NO3-LDH/ C-dot (20%) was achieved to 0.581 mmol/g (45.8 mg/g), which is greatly superior to most of the adsorbents listed in Table 3.34,35,49−59 Likewise, the adsorption of Sr2+ was also related with various adsorbents. It also showed higher adsorption capacity of 0.444 mmol/g of C-dot (38.9 mg/g) than other

■

CONCLUSIONS The MgAl-NO3-LDH/C-dot composite with different contents of C-dot was prepared by the colloidal deposition method and characterized using PXRD, FT-IR, zeta potential measurements, and TEM observation. The deposition of C-dot on LDH occurred through electrostatic interaction with the π-electrons of aromatic rings and ionic interaction with −COO− groups. TEM observation suggested that the C-dot was distributed homogeneously on the surface of LDH nanosheets.

Figure 12. Recycle ability of (A) Sr2+ and (B) SeO42− on MgAl-NO3-LDH/C-dot (20%). Adsorption condition: initial conc 0.89 mM SeO42− + 0.096 mM Sr2+; solid-to-liquid ratio 1.0 g/L; time 2 h. Desorption conditions: matrix mixture of 0.5 M NaNO3 + 3.0 mM HNO3; solid-to-liquid ratio 1.0 g/L; time 24 h.

Table 3. Comparison of SeO42− Adsorption Capacity with Other Materials adsorbent

mass of adsorbent (g/L)

magnetic nanoparticle−graphene oxide composite Al2O3−chitosan nanocomposite chitosan−montmorillonite composites Fe−Mn hydrous oxides Nano MnFe2O4 CuFe2O4 nanoparticle Mn3O4 nanomaterials maghemite manganese oxide MgAl-CO3-LDH (alkoxide free sol−gel synthesis, pH = 7) MgAl-LDH MgFeAl LDH carbon nanoshpere MgAl-NO3-LDH MgAl-NO3-LDH/C-dot (20%)

1 1.75 0.5 2 2.5 0.4 2.5 1 20 2 20 10 0.44 1 1

concentration (mg/L) 0.05−500 0.11−10 5−500 0.25−10 1−25 0.25−10 0.79 0.5−100 5−500 4−395 39−1974 1 70 70

I

SeO42− sorption capacity (mg/g)

ref

15.118 20.11 18.4 19.84 0.769 5.97 0.934 ∼0.47 1.396 45 0.026 111 2.20 69.4 (0.879 mmol/g) 45.9 (0.581 mmol/g)

49 50 51 52 53 54 55 56 57 34 58 59 35 present study present study


Research Article

ACS Sustainable Chemistry & Engineering Table 4. Comparison of Sr2+ Adsorption Capacity with Other Materials adsorbent

mass of adsorbent (g/L)

graphene oxide hydroxyapatite dolomite Saccharomyces cerevisiae immobilized magnetic chitosan zeolite 4A magnetic Fe3O4 particle-modified sawdust montmorillonite ammonium molybdophosphate−polyacrylonitrile CuFe2O4 nanoparticles Mg-aminoclay−humic acid hybrid MgAl-NO3-LDH MgAl-NO3-LDH/C-dot (20%) (based on C-dot)

0.038 10 10 2 40 2 20

ref

0−1000 10−50 5−300 1 5−50 88−1752 88−2629 150 55.6 8.4 0.8−85

23.8 2.4 1.172 36.97 0.025 12.59 13.3 15.8 23.04 0.12 0.35 (0.004 mmol/g) 38.9 (0.444 mmol/g)

60 28 61 62 63 64 65 66 67 27 present study present study

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support was provided to K.S. by Japan Society for the Promotion of Science (JSPS) Research Funding (16H02435, 15F15380) and to P.K. by JSPS Postdoctoral Fellowship for Foreign Researchers (P15380). The authors thank Advanced Analytical Center, Kyushu University, for X-ray photoelectron spectroscopy measurements and Ultramicroscopy Research Center, Kyushu University, for TEM observation.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng. 7b01979. Digital micrographs of carbon-dot suspension, powdered C-dot, MgAl-NO3-LDH, and MgAl-NO3-LDH/C-dot composite. PXRD peak fitting of MgAl-NO3-LDH/C-dot composite and PXRD of C-dot. Surface charge of MgAlNO3-LDH/C-dot (20%) composite and their desorption efficiencies in different medium (PDF)

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Sr2+ adsorption capacity (mg/g)

5 5 1 0.2

The MgAl-NO3-LDH/C-dot composite showed enhanced performance of Sr2+ adsorption with an increase in the amount of C-dot. The adsorption of Sr2+ on the composite mainly occurs though coordination of Sr2+ with the −COO− present on the surface of negatively charged C-dot, as supported by FT-IR, TEM elemental mapping, and XPS analyses. Conversely, the removal of SeO42− occurs via ion exchange with NO3−. The decrease in the SeO42− adsorption capacity of the MgAl-NO3LDH/C-dot composite was due to partial charge compensation of LDH with C-dot. The Sr2+ and SeO42− adsorption capacities are almost constant in the wide pH range (4−8). The adsorption capacities of Sr2+ and SeO42− were enhanced in the of bicomponent system (Sr2+ + SeO42−) and tricomponent system (Sr2+ + SeO42− + M+/M2+ = coexisting cations or An− = coexisting anions) with the presence other coexisting anions and cations. These materials can be recyclable up to four cycle without significant loss in their adsorption capacities. These results suggest that the MgAl-NO3-LDH/C-dot composite is an efficient adsorbent for co-immobilization of radioactive nuclides Sr2+ and SeO42−. Modification of anionic LDHs into a multifunctional adsorbent for removal of both anions and cations is promising for use in water remediation. In the future, C-dot could be obtained by valorizing the waste materials, which offer a sustainable double green benefit.

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concentration (mg/L)

AUTHOR INFORMATION

Corresponding Author

*Mailing address: Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, 744, Motooka, Fukuoka 819-0395, Japan. Tel./Fax.: +81 92 802 3338. E-mail: [email protected] (K.S.). Email: koilraj@mine. kyushu-u.ac.jp; [email protected] (P.K.). ORCID

Paulmanickam Koilraj: 0000-0002-4643-7772 J


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