Removal and Recycling of Precious Rare Earth Element from

Research Article pubs.acs.org/journal/ascecg

Removal and Recycling of Precious Rare Earth Element from Wastewater Samples Using Imprinted Magnetic Ordered Mesoporous Carbon Santanu Patra,† Ekta Roy,† Rashmi Madhuri,*,† and Prashant K. Sharma‡ †

Department of Applied Chemistry, Indian Institute of Technology (Indian School of Mines), Dhanbad, Jharkhand 826 004, India Functional Nanomaterials Research Laboratory, Department of Applied Physics, Indian Institute of Technology (Indian School of Mines), Dhanbad, Jharkhand 826 004, India

‡

S Supporting Information *

ABSTRACT: The present work is devoted toward the development of a highly efficient, low cost, selective and sensitive technique for the detection, removal and recovery of a popular rare earth element, i.e., gadolinium [Gd]. Herein, the magnetic ordered mesoporous carbon (OMMC) is prepared by a green synthesis approach and used as a core for the preparation of imprinted-OMMC using Gd(III) as a template. The prepared material has been used as a coating sorbent for solid phase microextraction (SPME) fiber as well as filled in a small sized micropipette tip to perform the microsolid phase extraction (μ-SPE) based study. The techniques have been explored for different purposes, i.e., preconcentration and trace level detection of Gd(III) has been done by SPME; however, μ-SPE is used for removal as well as recycling of Gd(III) from wastewater samples. The SPME fiber shows a higher preconcentration factor 1400 for Gd(III) with a limit of detection = 2.34 ng L−1, whereas the μ-SPE cartridge shows a higher adsorption capacity (30.2 μg g−1) and removal efficiency (90%) toward Gd(III). Both the techniques have been successfully applied to the preconcentration, detection and removal of Gd(III) from pathological laboratory wastewater, drinking water, sewage sludge, tap water, pond water, river water, human sera, fruits and vegetables and soil and water samples collected from the local coal mines. In addition, the μ-SPE cartridge was successfully applied for recycling of Gd(III) (in solid form) from both pathological laboratory wastewater and coal soil samples. KEYWORDS: Rare earth element, Imprinted-magnetic ordered mesoporous carbon, Solid phase microextraction, Microsolid phase extraction, Recovery

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were being recycled in 2011.4 Therefore, we are in a time where not only the recognition or removal but also the recycling of REEs is very crucial. Among these REEs, gadolinium (Gd) is widely employed and known for its major role as a magnetic resonance imaging (MRI) contrast agent.5 Gd-DTPA (gadopentetate) is the first commercially available contrast agent, introduced in 1988.6 A standard dosage for the enhanced MRI scan per infusion of commercially available contrast agents is 0.05 to 0.3 mmol per kg body weight.7 This signifies that about 1.0 g of Gd is applied for one MRI scan. As a result, a higher dose or large amount of anthropogenic Gd is released into our ecosystem every day. However, owing to their good water solubility and stable complex formation tendency, the commonly used wastewater treatment technologies have failed to remove Gd ions from

INTRODUCTION Rare earth elements (REEs) are known as future materials due to their importance and implications in some key technologies like medical diagnostics, petroleum refining, wind turbines, electrical car engines, etc. According to a recent report, use of rare earth elements has taken a steep increase and reached to 185 000 ton in the year 2015.1 In the last 30 years, we have observed a drastic increase in the demand for REEs, generally because of their increasing number of economically important applications in green energy technologies such as batteries (La), wind turbines (Sm, Dy, Pr, Nd), fluorescent and luminescent phosphor lamps (La, Gd, Tb, Eu, Yb), as car catalysts (Ce) and hybrid vehicles (Dy, La, Nd).2 European Union Commission and the US Department of Energy have enormously reported regarding our current and future needs of those REEs.3 They have considered these REEs as the most critical raw materials with the higher supply risk. As of their use, their reported commercial recycling or recovery processes are remaining low and insufficient. It is reported that only around 3% of the REEs © 2017 American Chemical Society

Received: April 12, 2017 Revised: May 3, 2017 Published: July 17, 2017 6910

DOI: 10.1021/acssuschemeng.7b01124 ACS Sustainable Chem. Eng. 2017, 5, 6910−6923

Research Article

ACS Sustainable Chemistry & Engineering

adsorbent of SPME fiber coating and found their superior performance as a preconcentration device.20 However, the other problem still present in this combination is “selectivity” and to sort out a very popular option is coupling of OMCSPME/SPE with artificial antibody, i.e., molecularly imprinted polymers (MIPs).21 MIPs are known to have predetermined selectivity for their template or target analyte molecules, and literature has reported great combination of SPME/SPE with MIPs.22,23 However, to the best of our knowledge, the unique and extraordinary combination of these three different techniques, i.e., SPME/SPE, MIPs, OMC, has not yet been tried. Therefore, in this work, we tried to design the imprinted ordered mesoporous magnetic carbon material for solid phase extraction and microextraction of Gd(III) from a complex matrix. For this, first, the ordered mesoporous magnetic carbon (OMMC) is prepared via a one-pot, economic, green softtemplate synthesis approach. For this, PluronicF-127 is used as a soft template, curcumin as a natural precursor and ferric nitrate as a precursor for a magnetic nanoparticle. The prepared OMMC is then modified with vinyl silane to generate double bond at their surface and later on used as a monomer for the molecular imprinting. The Gd(III)-imprinted OMMC is synthesized by Pickering emulsion method and the resulting nanomaterial possesses high magnetic property with good adsorption capacity. It is observed that via extraction technique, we can either separate a lot of materials via SPE (using large solvent) or could detect a trace-level of an analyte via SPME (using minimum solvent). Herein, our aim is to separate the large quantity of Gd(III) from the waste samples (recycling of REEs) as well as detect a trace-level of Gd(III) from a large volume of the complex matrix. For this, the imprinted-OMMC is packed in a micropipette tip as a replacement of large SPE columns. This mini-tip cartridge is advantageous in terms of simplicity, small bed volume, low consumption of adsorbent/ solvent, easy mode of operation and time efficiency. Similarly, for trace-level detection of Gd(III) from large sample volume, the same material (imprinted-OMMC) is coated on the silica fiber and used as SPME-device for preconcentration of Gd(III). In contrast to the other extraction devices, both SPME and SPE assemblies designed in this work are easy in carrying, can perform in-field analysis, and directly connected to the detection method, here an electrochemical cell. In addition, both the devices (μ-SPE and SPME) are successfully employed for preconcentration and separation of Gd(III) from the soil, fruits, vegetables and wastewater samples of different areas and human sera.

water. Therefore, they skip the wastewater treatment plants and are released into rivers and lakes.8 The first report for the contamination of environmental water by Gd ions was reported by Bau and Dulski in 1996.9 They have found an anomalously high concentration of Gd3+ in the rivers of Berlin, Germany. After that, a number of reports have been published for the contamination of rivers, lake waters, coastal seawater, groundwater and tap water in Europe,10 Australia,11 Asia,12 and North America.13 Verplanck et al. have revealed that a large gadolinium amount can be found in the treated wastewater than in the sewage sludge in the year 2010.14 It is worth mention that the supply of drinking water is associated with surface water and if surface water is being contaminated, it has an enormous chance to contaminate the drinking water and by this way enter our ecological system. Previously, some separation and detection methods like high-performance liquid chromatography, size exclusion chromatography and ion chromatography have been reported in the literature for analysis of Gd(III) in the water resources and used by some industries.15 In 2008, a hydrophilic interaction liquid chromatography (HILIC) hyphenated to electrospray ionization mass spectrometry (ESI-MS) technique was reported by Künnemeyer et al., which become very popular for the analysis of Gd(III) present in the contrast agents.16 However, owing to their high cost, complex set up and failure in direct analysis of Gd(III) in drinking water, samples have raised a question toward their applicability in real sample analysis. Nowadays, microsolid phase extraction (μ-SPE) and solid phase microextraction (SPME) techniques have become very popular in the field of separation science, due to their low analysis time, low required volume of organic solvents for the analytical protocol, easy setup and easy automation, i.e., able to connect any analytical instruments in a simple but effective way.17 These techniques have integrated two different processes in one, i.e., sampling and sample pretreatment. In addition, it is easy to operate and show high enrichment factors. The extraction ability or performance of SPME or μ-SPE depends upon nature of sorbent material either coated onto the fiber or packed in the microcolumns. Therefore, a search of sorbents having efficient and good adsorption capability is a key focus for current research on extraction based techniques. In this regard, carbonaceous nanomaterials have emerged with a better perspective as sorbent material in the field of SPME/ SPE. These nanomaterials possess excellent stability (chemical, mechanical and thermal), high surface area and outstanding pore geometry, which make them a suitable adsorption material.18 However, not only the high surface area but also ordered pore structure is required for rapid mass transfer resulting from the shorter equilibrium time, which lead toward shorter analysis time. Therefore, we need a suitable carbonaceous nanomaterial as a sorbent material, which has not only the largest adsorption capacity but also fast mass transfer. In recent years, ordered mesoporous carbon (OMC) has emerged as a material of unique combinations like uniformly distributed pores, ordered channels, chemical inertness, easily tunable pore sizes and high specific surface area,19 which are the basic need for an ideal candidate for adsorbents of SPME fiber or SPE columns. It could be assumed that OMC and SPME/ SPEs have mutual promoting properties, where OMC can improve the sensitivity and selectivity of the extraction in short analysis time and the extraction technique can explore the applications of OMC with a better perspective. Inspired by the phenomena, recently Zheng et al. applied OMC as an

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EXPERIMENTAL SECTION

Chemicals and reagents, instruments used in the experiment and preparation of tyrosine monomer are given in the Supporting Information (Sections S1−S3). Preparation of Vinyl Silane Modified Magnetic Ordered Mesoporous Carbon (silane@OMMC). Herein, the silane modified OMMC was used as a functional monomer for synthesis of Gd(III)imprinted polymer. First, OMMC was fabricated using a green and economic synthesis approach in a single-step reaction. For the synthesis, 1.22 g of Pluronic F-127 was dissolved in 8.0 mL of water to form a micellar solution. In different vials, curcumin (2.25 g) was dissolved in ethanol/water mixture and ferric nitrate (0.25 g) was dissolved in distilled water. The curcumin, iron solution and Pluronic solutions were then mixed together at room temperature, leading to an immediate phase separation and left at room temperature for 2 days. After that, the upper liquid phase was removed and the solid was dried 6911


Research Article


Scheme 1. (A) Schematic Presentation Showing Solid Phase Microextraction Process; (B) Camera Picture of Solid Phase Microextraction Device Used in the Study; (C) Schematic Presentation Showing Microsolid Phase Extraction Process

in a vacuum oven at 80 °C. The dried samples were then placed in an oven for pyrolysis at a heating rate of 1 °C min−1 up to the final carbonization temperature of 800 °C, and kept for natural cooling. In addition, the nonmagnetic mesoporous carbons were also prepared by the same process, in the absence of ferric nitrate and termed as OMC. For the modification of prepared OMMC or OMC, 1.0 g of material was added to a reaction vessel containing 8.0 mL of triethoxy vinyl silane and 4.0 mL of glycerol. The mixture was heated at 90 °C for 2 h under magnetic stirring in the nitrogen atmosphere. The obtained precipitate (silane@OMMC or silane@OMC) was washed three times with deionized water and methanol, dried and stored at room temperature in the vacuum desiccator. Designing of Imprinted OMMC-Modified Silica Fiber Based SPME Assembly. Before the Gd(III)-imprinting on the silica fiber, the fibers were pretreated with DMSO to remove the impurities, if any, and rubbed gently with acetone-soaked tissue paper. After that, a 2.0 cm length of the cleaned silica fiber (diameter = 0.5 mm) was dipped in the vial containing silane@OMMC (4.0 mg dispersed in 4.0 mL DMSO) for 15 min and dried at room temperature. For synthesis of imprinted OMMC, Pickering emulsion polymerization was used with silane@OMMC-modified fiber as a solid support; a mixture of crosslinker (2.0 mmol, EGDMA) and toluene (100 μL) was taken as an oil

phase and mixture of vinyl derivative of tyrosine (preparation procedure reported in the Supporting Information, 1.0 mmol, another functional monomer), Gd(NO3)3 (1.0 mmol), CTAB (3.0 mL) and NaOH (0.25 mL) as aqueous phase. First, both oil phase and water phase were mixed in a 10.0 mL beaker and sonicated for 15.0 min, followed by addition of AIBN (0.1 mmol, as initiator). In the mixture, silane@OMMC modified fiber was immersed for 10 min and heated at 50 °C for 2.5 h. After polymerization, the resulting adduct polymer modified fiber was washed with distilled water and stored at room temperature in a vacuum desiccator. For the preparation of the homemade SPME device (Scheme 1A,B), the above-modified silica fiber was fixed to the tight-fit hole of the plunger of a plastic syringe (volume = 1.0 mL). The fiber attached to the plunger can move in and out of the syringe barrel by mechanical force. To extract template molecules from adduct polymer layer, the plunger containing modified silica fiber was immersed in the 0.1 M HCl (extraction solvent) and the fiber was left in contact with extraction solvent for 30 min. After extraction, the fiber was withdrawn from the extraction solvent and imprinted-OMMC-modified silica fiber was ready to use for further analysis. Similarly, the nonimprinted polymer (NIP)-OMMC-modified silica fiber was also prepared, in the absence of template molecule and used as control fiber. The 6912


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Figure 1. (A) XRD pattern of OMC, OMMC and imprinted-OMMC; (B) magnetic hysteresis loops of OMMC and imprinted-OMMC [Inset: camera picture of imprinted-OMMC in the absence (a) and presence (b) of magnet]; (C) FT-IR spectra of (i) adduct-OMMC and imprintedOMMC in the absence (ii) and presence (iii) of template; (D) adsorption capacity and (E) %removal efficiency of imprinted and nonimprinted OMMC; (F) XRD pattern of recovered GdCl3 salt. polymerization (temperature, time), extraction (solvent, temperature, volume and time) and rebinding (volume, time and pH) conditions reported in the Experimental Section are discussed in the Supporting Information (Sections S4 and S5). Preconcentration and Trace Level Estimation of Gd(III). For the estimation of Gd(III) from aqueous or complex matrices, a threestep process was used: (1) rebinding, (2) extraction and (3) detection. In the first step, rebinding of the template to the imprinted-OMMC fiber was carried out. For this, the imprinted-OMMC fiber was retracted from the syringe and immersed in the 70.0 mL of known concentrations of Gd(III) for 25 min, which leads to rebinding of template molecule to the MIP cavities present at the fiber surface. After this, the fiber was withdrawn from the template solution and placed in the syringe. For extraction of the template, 0.05 mL of extraction solvent was withdrawn to the syringe cap and Gd-bounded fiber was left in contact with extraction solvent for 30 min. After that, the extraction solvent containing template molecule was directly placed in the electrochemical cell containing 9.0 mL of KCl (pH 7.0) as a supporting electrolyte and concentration of Gd(III) was determined by the standard addition method at the optimized electrochemical parameters, i.e., accumulation potential −1.0 V, accumulation time 180 s, scan rate 100 mV s−1, pulse amplitude 25 mV and pulse width 50 ms. The analytical parameters used in the

experiment such as rebinding solvent pH, rebinding time, rebinding volume and electrochemical parameters are discussed in the Supporting Information (Sections S5 and S6). Designing of μ-SPE Device for Separation of Gd(III). For designing of the μ-SPE device, imprinted-OMMC monolithic polymers were used. The imprinted-OMMC was prepared via the same procedure mentioned previously, but herein, the fiber solid support was not used and the polymerization was carried out in a glass capillary. In brief, silane@OMMC (as a solid support), a mixture of cross-linker and toluene (as an oil phase) and a mixture of vinyl derivative of tyrosine, Gd(NO3)3, CTAB and NaOH (as aqueous phase) were mixed together and after addition of AIBN, the whole mixture was filled in the glass capillary, sealed and placed in an oven for 2.5 h at 50 °C. After polymerization, the resulting adduct polymer was washed, crushed to the fine powder and stored at room temperature in a vacuum desiccator. Similarly, the monolithic NIPs were also prepared, in the absence of template molecule. For the fabrication of the μ-SPE device, the prepared adduct polymer or monolithic NIPs were separately packed into a 1.0 mL micropipette tip (Scheme 1C). The micropipette tip was closed at both the ends by cotton plugs to fix the position of the adsorbents. The micropipette tip packed with polymers was first preconditioned and the template was extracted using 0.1 M HCl as the extraction 6913


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Figure 2. FE-SEM images of (A) OMC and imprinted-OMC: at lower (B) and higher (C) magnifications. FE-SEM images of (D) OMMC and imprinted-OMMC: at lower (E) and higher (F) magnifications. TEM image of OMMC and imprinted-OMMC: at lower (G and I) and higher (H and J) magnifications, respectively. solvent. For this, the polymer filled micropipette tip was fitted to the corresponding micropipette and 1.0 mL solvent was withdrawn through aspirating process of the micropipette and after 25 min of incubation, the extraction solvent was dispensed from the tip and them the μ-SPE device was ready to be used for sample analysis. The optimization of contact time, pH of the rebinding solution and the amount of imprinted-OMMC are discussed in the Supporting Information (Section S7). For separation of Gd(III) from aqueous solution, the prepared μSPE device was immersed in 4.0 mL of template solution (different concentrations, 1.0−50.0 μg L−1) and by 10 aspirating/dispensing cycles using micropipette tip syringe system, the template solution was retracted to the micropipette tip and incubated with MIP cavities for 25 min to rebind the template molecule. After that, the template solution was dispensed from the tip and extraction solvent was captured through a micropipette suction process. The extraction solvent with template was then evaporated in a rotary evaporator and the solid material was separated or recovered. Analytical Performance of SPME-Fiber and μ-SPE Cartridge. The analytical performance of the developed fiber was evaluated in terms of the calibration curve, linear range, limit of detection (LOD), limit of quantification (LOQ), accuracy, precision, enrichment factor (EF), sensitivity and selectivity. The LOD and LOQ were calculated at a signal-to-noise ratio of three (S/N = 3) and a signal-to-noise ratio of ten (S/N = 10), respectively. The precision of the method was investigated by analyzing four different concentration of Gd(III) in six independent series on the same day (intraday precision) and six consecutive days (interday precision). The analysis was done in triplicate containing known amounts of the Gd(III). Precision was determined in terms of relative standard deviation (RSD, %), and the accuracy was calculated according to the following equation:

Accuracy (%) = [(Cfound − Cadded)/Cadded] × 100

EF =

[Analyte]desorbed [Analyte]taken

(2)

Here [Analyte]desorbed represents the concentration of desorbed amount (i.e., Gd(III) amount eluted by fiber/optimized volume of the extracting solvent and [Analyte]taken is the known amount of analyte taken during the experiments. Real Sample Analysis and Recovery of Target Analyte. For the real time application of the proposed method, human blood and urine samples were collected from a local pathological laboratory and some soil samples were collected from various parts of local coal mines. The sample preparation techniques are reported in detail in the Supporting Information (Section S8). The samples were analyzed by both SPME and μ-SPE techniques. To study the purity of recovered material after μ-SPE, the following equation was used:

P = CVM /m0 × 100%

(3)

where P is the purity of the solid material (here Gd(III)) after evaporation; C and V represent the detected concentration of Gd(III) through SPME and volume of the solution; M and m0 are molar mass and obtained the mass of the corresponding Gd(III) solid product. For the purity analysis of the obtained precipitates, X-ray diffraction (XRD) analysis was used to determine the chemical compositions. Three parallel experiments were simultaneously conducted during the whole process to avoid random errors, and mean values of the analytical results would be treated as the final experimental results.

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RESULTS AND DISCUSSIONS Characterization of the Nanoparticles. X-ray diffraction (XRD) analysis of OMC, OMMC and imprinted-OMMC was performed, and the results are shown in Figure 1A. The XRD pattern of OMC exhibits two peaks at 22° and 43° for the presence of a graphitic layer [(002) and (100) planes] in the material, which supports the successful formation of OMC (JCPDS: 75-2078).24,25 In a similar manner, the OMMC shows the six characteristics peaks at around 31°, 36°, 43°, 53°, 57°

(1)

Here, Cfound = concentrations of the analyte after addition of a given amount of standard into the real sample, and Cadded = concentration of a known amount of the standard spiked into the real sample. The enrichment factor was calculated as 6914


Research Article


Table 1. Analytical Data for Determination of Gd(III) in aqueous, Blood Plasma, Serum and Urine Samples Using ImprintedOMMC Microsolid phase extraction (μ-SPE) Sample

Dilution

Aqueous Blood plasma Blood serum Urine

a

b

b

RSD (%)

LOD (ng L−1)

RSD (%)

1115.24

1.2

33.45

1.1

98.9−101.1

1123.59

1.3

33.70

1.0

98.9−101.0

1136.36

1.2

34.09

1.2

98.8−101.0

1136.36

1.1

34.09

1.3

Calibration equation

Recovery (%)

100.0−4000.0

Ip = (0.0269 ± 0.0008) C+(4.8810 ± 1.7972), R2 = 0.99, n = 7 Ip = (0.0267 ± 0.0008) C+(2.9744 ± 1.8311), R2 = 0.99, n = 7 Ip = (0.0264 ± 0.0009) C+(1.1275 ± 1.9260), R2 = 0.99, n = 7 Ip = (0.0264 ± 0.0009) C+(0.7979 ± 2.1320), R2= 0.99, n = 7

99.0−101.0

Ip = (0.641 ± 0.017) R2=0.99, n = 9 Ip = (0.636 ± 0.018) R2=0.99, n = 9 Ip = (0.637 ± 0.018) R2=0.99, n = 9 Ip = (0.635 ± 0.017) R2=0.99, n = 9

C+(87.048 ± 11.903),

99.1−100.9

7.800

1.1

2.340

1.3

C+(85.386 ± 12.128),

99.0−101.0

7.861

1.2

2.358

1.2

C+(83.765 ± 12.218),

99.1−101.1

7.849

1.2

2.354

1.0

C+(82.997 ± 11.534),

99.1−101.0

7.874

1.1

2.362

1.3

100

100.0−4000.0

100

100.0−4000.0

100

100.0−4000.0

Solid phase microextraction (SPME) Aqueous 6.96−891.54 Blood plasma Blood serum Urine

a LOQ (ng L−1)

Range (ng L−1)

100

6.96−891.54

100

6.96−891.54

100

6.96−891.54

LOQ = Limit of quantitation. bRSD = Relative standard deviation; LOD = Limit of detection.

interaction between the imprinted cavity and target analyte, i.e., Gd(III) and also enlighten a path toward the recycling and reusability of the prepared magnetic mesoporous carbons. The morphology of the nanoparticles was well studied by FE-SEM and TEM analysis (Figure 2). The recorded FE-SEM and TEM images clearly support the formation of uniformly spherical structures of mesoporous carbon. The diameter of OMC (dsphere) was found in the range of 50 nm; however, the magnetic OMC shows slightly larger particle size found to be around 60 nm. After polymerization, the diameter of both imprinted-OMC and imprinted OMMC has increased to 80 and 95 nm, respectively. The similar results were also obtained from TEM images, which clearly support and strengthen the uniform and spherical shaped synthesis of mesoporous carbons. From the FE-SEM and TEM images, the thickness of imprinted polymer layer could also be calculated and found to be ∼30−35 nm (gray color, outside the black sphere), which is sufficient enough for the effective and efficient binding of the target analyte to the imprinted layer. The selected area electron diffraction (SAED) pattern of imprinted-OMMC was also taken (Figure S9). The SAED pattern of imprinted-OMMC is indicative of high nanocrystallinity of the formed product and can be assigned to the cubic phase of Fe3O4. The concentric Debye−Scherrer rings can be indexed to the (220), (311) and (400) planes, and the d-spacing corresponding to the diffraction rings of the SAED pattern is in good agreement with the cubic phase of the Fe3O4 nanoparticle. The SAED results are in good accordance with XRD data. Hierarchical pore structures and high surface area are the prompt features of OMC. Herein, a nitrogen adsorption− desorption study was performed to understand the surface area and pore size of prepared OMC, OMMC, adduct-OMC/ OMMC and imprinted-OMC/OMMC. As shown in Table S1, the synthesized mesoporous materials exhibit high surface areas and narrow pore size distribution. Among the differentially designed mesoporous materials, the OMMC show the highest surface area of 741 m2 g−1. Interestingly, the coating of an imprinted polymer layer on the mesoporous carbons leads to a sufficient increase in their particle size; however, a significant decrease in surface area and pore volume was also found. The

and 63° corresponding to (220), (331), (400), (422), (511) and (440) planes of Fe3O4 (JCPDS No: 19-0629) and can be assigned to cubic crystal structure of Fe3O4 having space group Fd3m with lattice parameters a = b = c = 8.396 Å; α = β = γ = 90°. The additional peaks appearing at 22° and 43° correspond to the characteristic peaks of OMC. The appearance of both kinds of peaks supports the successful synthesis of magnetic mesoporous carbon material. After polymerization, the peak at 22° becomes somewhat broader but smaller in intensity. It may possible due to the overlapping of peaks at the similar position, which suggests the successful modification of polymer film over the surface of OMMC. To understand the magnetic property of prepared OMMC, before and after polymer modification, the vibrating sample magnetometer (VSM) was used. The magnetization curve is shown in Figure 1B. From the curve, the saturation magnetization value (at room temperature) of OMMC and imprinted-OMMC was found to be 139.3 and 105.5 emu g−1, respectively. Although there is a sufficient decrease in magnetization value after polymer modification, the present value is sufficient enough to respond well to the presence of an external magnet. The imprinted-OMMC suspended in the aqueous medium could be easily separated by the simple laboratory magnet within a few seconds and the medium become transparent (Figure 1, inset). Spectroscopic analysis (FT-IR) was carried out to understand the binding interaction between imprinted-OMMC and Gd(III) (Figure 1C). The FT-IR spectra of adduct-OMMC shows characteristics peak for O−H str./N−H str., C−H str., amide I, amide II, C−O str., C−O−C str. and N−H wagging at around 3500, 2910, 1650, 1510, 1250, 1100 and 660 cm−1 (Figure 1C, curve i. After extraction, the imprinted-OMMC shows similar peaks but slightly shifted toward higher wavenumber (curve ii, which confirms the template extraction from adduct-OMMC. To explore the rebinding capacity of imprinted-OMMC, the FT-IR spectrum of template rebound with imprinted-OMMC was also recorded. The higher wavenumber shifted peaks reassumed their positions in the curve iii (toward lower wavenumber), after rebinding of the template molecule. This supports the noncovalent binding 6915


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Table 2. Analytical Results of Gd(III) Preconcentration in Blood Serum Sample Using Imprinted-OMMC-Modified SPME Fiber

a

[Analyte]taken (ng L−1)

Sample volume (mL)

1.0 2.0 4.0 8.0 16.0 32.0 64.0 128.0 256.0

70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0

[Analyte]desorbed (ng L−1) ± SDa 1400 2800 5600 11200 22400 44800 89600 179200 358400

± ± ± ± ± ± ± ± ±

Volume eluted (mL)

1.9 2.6 2.8 3.3 3.7 4.7 6.7 8.1 9.2

0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05

b

Ef

Recovery (%)

1400 1400 1400 1400 1400 1400 1400 1400 1400

99.5 100.5 99.7 99.6 100.1 100.3 99.8 99.9 100.0

c

RSD (%) 0.13 0.09 0.05 0.03 0.02 0.01 0.01 0.005 0.003

SD = Standard deviation. bEf = Enrichment factor. cRSD = Relative standard deviation.

Analytical Performance of μ-SPE Cartridge. The adsorption of Gd(III) and removal was performed on the μSPE cartridge, as shown in Figure 1D, under optimized analytical parameters. As shown in the figure, initially the adsorption capacity increases with increasing the concentration of Gd(III) and became constant after a certain time. The constant point in the binding isotherm was termed as the “saturation point”, where the entire imprinting site was occupied by Gd(III) and the maximum binding capacity was obtained as 30.2 μg g−1. A similar batch binding study was performed with NIP-filled μ-SPE cartridge, where the maximum binding capacity was obtained as 3.2 μg g−1. The high value in the case of imprinted-OMMC filled μ-SPE cartridge is mainly due to the presence of cavities in the polymer matrix, which are absent in the case of nonimprinted OMMC. In addition to the μ-SPE, the larger cartridge (pipette tip of 5.0 mL) filled with imprinted-OMMC was also explored for bulk removal or recovery of Gd(III). In the bulk adsorption experiment, it was found that by using 1.0 g of imprintedOMMC, 499.0 mg g−1 adsorption capacity could be attained, which is high enough for the bulk removal or recovery of rare earth element. Furthermore, the removal efficiency of prepared materials was also studied, where a similar graphical trend was observed. The removal efficiency first increases linearly with increase in the concentration of Gd(III), and after a certain concentration of 10.0 μg L−1, starts decreasing in the case of imprintedOMMC. The maximum removal efficiency at Gd(III) concentration of 10.0 μg L−1 was obtained as 90.1%, which is higher enough for removal and recovery of rare earth element in enough amounts. In the case of nonimprinted-OMMC, the removal efficiency was much lower (95 99.1−100.6

-

-

5 cycles 5 cycles 5 cycles -

Recycling efficiency

98−103 95.0−103.5

-

-

-

Recovery (%)

91.8−95.2

-

-

-

-

68−69 -

Purity of recovered product

41 This work

22

37 38 39 40

35 36

29 30 31 32 33 26 34

ref.

a mIIP-CS/CNT = magnetic ion imprinted polymer-chitosan/carbon nanotube; Calix = calixarene; GO = graphene oxide; CBD = {[Ce(BTC)(H2O)].DMF}n; CMC= craboxymethyl cellulose; DTD OII = dual-template docking oriented ionic imprinting; IMS = imprinted mesoporous silica; 11-MVPsZ-SBA-15 = 11-molybdo-vanadophosphoric acid supported on Zr-modified mesoporous silica SBA-15; MSFPG = mesoporous silicas functionalized with phosphonic acid groups; DFCMN = diethylenetriamine-functionalized chitosan magnetic nanobased particles; IIP = ion imprinted polymer; HQ = 8hydroxyquinoline; As = Arthrobacter sp.; CP = chitosan powder; CB = chitosan beads; MRGO = magnetic reduced graphene oxide; HDEHP = bis(2-ethylhexyl)-phosphate; AES = atomic emission spectroscopy; ICP = inductively coupled plasma; OES = optical emission spectrometry; MS = mass spectroscopy; SWSV = square wave stripping voltammetry.

15. 16.

14.

Material

SN

Table 6. Comparison between Various Techniques Used for Detection or Removal of Rare Earth Elementsa

ACS Sustainable Chemistry & Engineering Research Article


Research Article


been performed by μ-SPE technique from pathological wastewater, soil, water and vegetable/fruits samples from coal mines. For the first time, we have compared the analytical performance of both techniques and draw a fruitful conclusion related to their future use. In the comparative study, it is very clear that the MIP based μ-SPE cartridge is suitable for the adsorption and recovery of Gd(III) on a large scale, whereas SPME technique can be used for preconcentration and trace level detection of the REEs. In addition, the SPME-fiber and μSPE cartridge have shown satisfactory regeneration performance, reusability and long-term storage ability, which can reduce the wastewater disposal expenses as well as the cost of these techniques.

where the % desorption value was almost constant up to 90 extraction/rebinding cycles (Figure 3D). Only a 3% change in % desorption value can be observed after the 90th run, which proves the reusability of the proposed MIP based μ-SPE cartridge. The results confirm the excellent reusability of the proposed devices. The storage capacity or leaching test of MIP-SPME fiber and the μ-SPE cartridge was also studied by storing them at room temperature for 90 days (Figure 3E,F). After 90 days, they can store ∼97% recovery for Gd(III) extraction. In addition, to explore the temperature based stability of MIP-SPME fiber and μ-SPE cartridge, they were stored at 4 and 50 °C for 90 days. After 90 days of storage in extreme conditions, the fiber and cartridge have shown 95% recovery for Gd(III) extraction. The study suggests the nonleaching tendency with a good storage capacity of the proposed fiber and μ-SPE cartridge. Comparison with Earlier Reported Methods for REEs Detection and/or Extraction. Selective recognition of REEs is a real challenge for a large range of applications in the analytical field (from extraction to detection and quantification). For that purpose, REEs extraction and recycling have been increasingly developed over the last 15 years on the principle of imprinted polymers.28 After going through the previously reported literature for the detection, removal and recovery of REEs, we found very few studies related to the detection or recovery of REEs (Table 6). As shown in the comparative table, the majority of work have been dedicated toward the designing of REEs adsorbent, which depends on the inductively coupled plasma atomic emission spectroscopy (ICP-AES) or inductively coupled plasma mass spectrometry (ICP-MS) for detection or analysis of Gd(III). ICP-MS or ICPAES are very sensitive techniques but requires skilled persons for the analysis, proper laboratory based sample preparation and high instrument cost (Table 6). In our laboratory, earlier, we developed a very simple and easy square wave voltammetry based technique using dendrite structure for detection and preconcentration of Eu(III) from the coal mines; however, recovery of Eu(III) was not explored at that time.40,22 Raju et al. have reported an SPE method using bis(2-ethylhexyl)phosphate (HDEHP) coated reverse phase C18 support for the preconcentration of Gd and Gd contrast agents.41 They have successfully detected Gd(III) in various water samples; however, have not discussed anything related to recovery adsorbed Gd(III) from water samples. Related to the recovery of REEs, to the best of our knowledge, only two literature sources have been found from the same group of researchers dedicated toward Dy26 and Nd33 extraction. From the comparative table, it can be easily concluded that our method is first of its kind, simple in use, highly selective, cost-effective, and has high regeneration ability (90 cycles). Here, the single material was applied for detection, removal and recovery purpose. The regenerated material has a very high purity ∼95%, which suggests the reusability of the recovered precious material in different fields. Along with this, it is the first study where recovery of RRE is being done from wastewater samples.

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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.7b01124. Reagents, instrumentation, preparation of tyrosine monomer; optimization of: polymerization temperature, polymerization time, SPME and μ-SPE analytical parameters (extraction solvent, pH, extraction temperature, extraction time, extraction volume, rebinding volume, rebinding time, pH of rebinding solvent, accumulation time, pH of the supporting electrolyte, accumulation potential, accumulation time); SAED pattern of imprinted-OMMC; nitrogen adsorption and desorption result; optimization of calcination temperature; process for sample preparation; intraday and interday precision and accuracy results; dilution factor; adsorption and kinetics experiments; Langmuir and Freundlich adsorption isotherm model; isotherm and kinetic parameters; competitive experiment and selectivity results; calculation and results of distribution coefficient, selectivity coefficient and relative selectivity coefficient; recycling or regeneration of μ-SPE cartridge and fiber (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +91 9471191640. Email: [email protected] (R. Madhuri). ORCID

Rashmi Madhuri: 0000-0003-3600-2924 Prashant K. Sharma: 0000-0001-5283-0901 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Authors are thankful to DST, BRNS and ISM for sponsoring the research projects to Dr. Rashmi Madhuri (No. SERB/F/ 2798/2016-17; SB/FT/CS-155/2012; FRS/43/2013-2014/ AC; 34/14/21/2014-BRNS) and Dr. Prashant K. Sharma (No. SR/FTP/PS-157/2011; FRS/34/2012-2013/APH; 34/ 14/21/2014-BRNS).

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CONCLUSION In summary, we have successfully synthesized the imprintedOMMC for recognition, removal as well as recovery of Gd(III) through μ-SPE and SPME techniques. Herein, the high extraction efficiency, enrichment factor and good LOD value are obtained for Gd(III) using SPME technique. However, the high yield recovery of pure Gd(III) in the form of GdCl3 has

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REFERENCES

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