Graphene oxide supported on amberlite resin for the analytical

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Graphene oxide supported on amberlite resin for the analytical method development for enhanced column preconcentration/sensitive FAAS determination of toxic metal ions in environmental samples Suneel Kumar, Aminul Islam, Hilal Ahmad, and Noushi Zaidi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00576 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

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Industrial & Engineering Chemistry Research

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Graphene oxide supported on amberlite resin for the analytical method development for enhanced column preconcentration/sensitive FAAS determination of toxic metal ions in environmental samples

Suneel Kumar, Aminul Islam, Hilal Ahmad, Noushi Zaidi Analytical Research Laboratory, Department of Chemistry, Aligarh Muslim University, Aligarh, India-202002

Abstract Graphene oxide (GO) layered nanosheets with two-dimensional carbon lattice is linked to styrenic carbon of porous XAD polymeric resin via azo spacer arm without utilizing any oxygen containing functionality for the first time. The adopted synthesis approach makes all the epoxy, hydroxyl, carbonyl and carboxyl groups on axial and basal plane of GO sheet available for coordination with metal ions and introduction of hydrophilic character as result highest preconcentration factor for Pb (500) was observed among the GO-composites used as solid phase extractant in column operation. The developed analytical methodology using XAD-GO for preconcentration of Pb, Cd and Zn ions permits the use of an economically viable less sensitive AAS for their trace determination in water samples due to the enhanced quantification limit. Application of polymer immobilized GO in a column sort-out the problem of escape of toxic GO into the environment and under optimized SPE conditions quantitative desorption of metal ion involves only 5.0 mL of 2 M HCl instead of any carcinogenic organic solvents. These two

Corresponding

author: Analytical Research Laboratory, Department of Chemistry, Aligarh Muslim University, Aligarh, India 202 002 Tel.+91 9358979659; E-mail address: [email protected]

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Industrial & Engineering Chemistry Research 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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components make this method a green one. The developed method was validated analyzing standard reference materials and recovery of the spiked analyte from real water samples.

Key Words: Amberlite XAD-16, Graphene oxide, Solid phase extraction, Preconcentration, Flame atomic absorption spectrometer, Environmental samples

Introduction The increasing levels of heavy toxic metals constituting PBT (persistent, bioaccumulative, toxic) chemicals in the natural ecosystem have been the prime threat owing to their environmental impact on biota. Their biochemical action includes interference with biological activity of different enzymes, altered ligand amino acid complexation and replacement of essential metal ions from their enzyme complexes.1-3 Sensitive and selective method which is also accurate and economical for the trace determination of heavy metals in environmental samples (from both industrial and biological) are essential for environmental monitoring and assessment of its level of exposure and effects onto the ecosystem. Despite the availability of highly sensitive, selective and rapid atomic spectroscopic and electro analytical techniques with simultaneous capability, direct trace determination of heavy metals in real samples with complex matrix can be complicated due to potential concomitant matrix effects.4,5 Moreover, high initial capital investment, maintenance and running cost of sophisticated instruments render the affordability of a routine analytical laboratory. Flame atomic absorption spectrometry (FAAS) being the cheaper technique amongst them and also has limitation of having low detection limit. Solid phase extraction (SPE) deals with the mentioned two issues due to its ability to initially concentrate the target trace element and secondly removing interferences thereby improving the detection limits,


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precision and accuracy of the developed analytical method and allowing the use of less sensitive and cost-effective technique. The target of presented research in SPE is the preparation of hydrophilic extractants of high surface area by enhancing the number of chelating sites on the extractant and their accessibility to target ions. Graphene oxide (GO), a carbon based nanomaterial9-11 with two-dimensional one or several atomic layered honeycomb carbon lattice, high specific surface area, excellent mechanical stability and abundant functional groups like epoxy (−COC), hydroxyl (−OH), carbonyl (−CO) and carboxyl (−COOH) that can be synthesized in a one-step oxidation of graphite without any metal catalyst has been considered a material of choice and substitute for CNT11,12 due to its hydrophilic character facilitating better surface contact with aqueous phase and active sites for metal ion complexation. Sorption studies of metal ions using GO13,14 and GO-nano-composite/hybrid15,16 by batch method suffers from their separation and recycling due and needs sonication for the redispertion before its use in other sorption cycle. In addition, there is a chance of loss due to the polydispersity during their separation leading to potential health threat to ecosystems. After its entry into the environment it adversely affects cell function via lowering cell viability and increasing reactive oxygen species (ROS) generation, which disturb the locomotive activity and promotion of Lewy bodies.17 Although, the difficulties of re-dispersion and loss of GO can be overcome by adopting dispersive magnetic SPE18,19 the limitation of the use of large sample volume is a challenge in batch method for metal ion preconcentration. Alternatively, column operation provides the benefits of higher number of theoretical plates for better extraction, high preconcentration factor, online coupling with determination techniques and relatively higher regenerability. However,



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direct use of GO20-22 or support coated GO23 in SPE column/cartridge develops back pressure and requires a high pressure peristaltic pump, this may cause its escape while possibility of leaching of GO from the support will be evident in the later case. A radical way to circumvent these obstacles is to chemically immobilize GO onto a porous polymeric beads24 to render a rugged systems. Amberlite XAD resins have been much established as a coploymer backbone for the immobilization of chelating ligand owing to its physical superiority in terms of porosity, uniform pore size distribution, high surface area and chemical stability towards acids, bases, and oxidizing agents over other sorbents. In the presented paper, our ultimate aim is to synthesize XAD-GO solid phase extractant using azo linking in order achieve more hydrophilic functionality which helps for better metal complexation. The advantage of azo coupling is that it is linked to styrenic carbon of porous XAD polymeric resin through azo spacer arm without utilizing any oxygen containing functional group for the first time. Hence, by this approach all the epoxy, hydroxyl, carbonyl and carboxyl groups on axial and basal plane of GO sheet available for coordination with metal ions as well it introduce more hydrophilic character. The prepared XAD-GO resin used to explore for the development of column SPE method for preconcentration and determination of Pb, Cd and Zn ions in large amount of different environmental water samples. The developed method was also validated by using certified reference material.

Materials and methods Reagents and Solutions


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All reagents used in the proposed research were of analytical reagent grade. Graphite powder was purchased from Otto chemie Pvt. Ltd. (Mumbai, India). Metal Stock solutions of 1000 mg L-1 were obtained from Merck (Darmstadt, Germany). Amberlite XAD-16 (particle size 20-60, pore size 100 Å, pore volume 1.82 mL g-1, and surface area 900 m2 g-1) was purchased from Sigma-Aldrich (Steinem, Germany). A standard reference material (SRM) vehicle exhaust particulates (NIES 8) was obtained from the National Institute of Environmental Studies (Ibaraki, Japan). Buffer solutions with appropriate volumes of containing suitable amounts of hydrochloric acid (HCl)-glycine for pH 2.0, acetic acid-sodium acetate for pH 3.75-5.75, disodium hydrogen phosphate-HCl for pH 7.4 and ammonia-ammonium chloride for pH 8.0-10.0 were used for pH adjustment. Nitric acid (HNO3), HCl, perchloric acid (HClO4) and hydrogen peroxide (H2O2) used were procured from Merck (Mumbai, India). For the dilution of the samples, triply distilled water (TDW) was used. Instrumentation For the determination of Pb, Cd and Zn, an atomic absorption spectrometer, GBC 932+ (Dandenong, Australia) with deuterium background correction was used at a wavelength of 217.0, 228.8 and 213.9 nm, respectively. An Orion 2 star model pH meter from Thermo Scientific (Waltham, MA, USA) was used for pH measurement. A glass column (1×10 cm) obtained from J-SIL Scientific Industries (Agra, UP, India) was used for dynamic studies. FT-IR and FIR spectral studies were done on a PerkinElmer Spectrum Two spectrometer (Waltham, MA, USA), using a KBr disk method in the range between 500 and 4000 cm-1 with the resolution of 2.0 cm-1 and Perkin Elmer spectrometer, Frontier (MA, USA) in a polyethylene pellet under nitrogen atmosphere at room temperature (27ºC), respectively. Quantachrome Instrument (Boynton Beach, FL, USA) was used for Brunauer-Emmett-Teller (BET) surface area analysis



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and pore size measurements. Thermal gravimetric analysis (TGA) was carried out on the Shimadzu TGA/DTA (Kyoto, Honshu, Japan). For the surface morphology the micrographs were recorded using Scanning electron microscopy (SEM) (Jeol JSM- 6510LV, Tokyo, Japan). Transmission electron microscope (TEM) images were recorded (Jeol JEM-2100 microscope, MA, USA) at a maximum accelerating voltage of 200 KV.

13C

NMR was recorded on Bruker

Avance III HD (Germany).

Preparation of GO immobilized XAD-16 resin Modified Hummers method was used for the preparation of GO from natural graphite powders.25 The immobilization of GO onto XAD-16 resin matrix was done through azo coupling. The diazotization of XAD-16 resin was carried out as mentioned in the earlier work of our lab.26 The 3.0 mL of GO solution (2 mg mL-1) along with 3g of diazotized resin was added into mixture of NaOH and methanol (1:2) and kept at a temperature of 0-2 °C for 48 h and, then washed with TDW to remove the alkali and oven dried for 24 h. The prepared resin was abbreviated as XADGO. The synthetic scheme was depicted in Figure 1.

Collection and pretreatment of samples and SRM River water (The Ganga, Narora), drinking water (Municipal water supply of city; Aligarh, India) and tap water (Laboratory, Department of Chemistry and city, Aligarh) were collected and filtered through a cellulose membrane (Millipore) of 0.45μm pore size, acidified to pH 2.0 with HNO3 and stored in precleaned polyethylene bottles. SRM solution was prepared as already mentioned in our previous research.26, 27


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Optimized column procedure for preconcentration/determination of metal ions A column (10×2 cm) was packed with 0.2 g of XAD-GO resin after swelling in TDW for 1 h and preconditioned by passing 5.0 mL of buffer solution pH 8.0±0.1 before subjected to further experimental studies. Sample solution contains 5µg of Pb, Cd and Zn ions adjusted to pH 8.0±0.1 was passed through the column at a flow rate of 5.0 mL min-1. Flow rate was manually adjusted by connecting a suction apparatus between running water taps and column end via a small ‘T’ shaped glass tube. Sorbed analyte ions were eluted with 5.0 mL of 2 M HCl at a flow rate of 2.0 mL min-1 from column and subsequently determined by FAAS.

Result and discussion Characterization FT-IR spectrum of XAD-GO (Figure S1) shows a broad band at 3436 cm-1 due to structural O-H (-COOH and -OH) stretching vibrations of GO. Peaks at 2925.89 and 2856 cm-1 indicate the presence of sp3 C−H and sp2 C−H stretching vibrations, respectively. The peak appearing at 1698.25 cm-1 corresponds to carbonyl (C=O) stretching vibrations.28 The azo (-N=N-) spacer band appeared at 1606.96 cm-1 29 indicates the immobilization of GO onto XAD resin. The band at 1447.65 and 1488.39 cm-1may be assigned as C=C bond stretching. Peaks at 1276.7 and 1117.93 cm-1, and 1350 cm-1 may be attributed to C-O stretching vibrations and O-H deformation. The peaks at 39.60, 96.87, 174.96, 223.97 and 145.06 in the solid state 13C NMR spectrum (Figure S2) corresponding to styrenic -CH and -CH2, aromatic carbon, carboxylic carbon, carbonylic carbon and carbon of azo group (C-N=N-), respectively confirm the successful immobilization of GO onto the XAD matrix.30 The XRD pattern of graphite and GO was shown in Figure 2. In the XRD pattern of graphite a sharp and highly symmetrical



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diffraction peak was appeared at 2Θ=26.5° which was assigned to the (002) reflection plane with interlayer spacing of 0.34 nm. This interprets the crystalline nature of graphite. In GO XRD pattern, a strong peak was observed at 2Θ=10.6° corresponds to interlayer spacing of 0.834 nm. The shifting in 2Θ from 26.5°to10.6° and increase in the layer to layer distance from 0.34 nm to 0.834 nm depicts the conversion of graphite to GO.31 It is further supported by Raman spectra (Figure 3), shows the characteristic D and G bands of GO at 1344.6 and 1588.6 cm-1, respectively. The multipoint BET surface area of XAD-GO was found to be 772.83 m² g-1 and comparable to the previous reported literature as in GO immobilized polystyrene (PS-GO) it was 12.06 m² g-1 30, glycidylmethacrylate-diethylenetriamine-GO (p-DETAGO) gives 51.063 m² g-1 32 and in case of chitosan-GO it was found to be 4.232 m² g-1.33 TGA curve (Figure S3) shows that the material can be used up to 260 °C without any degradation. Decoration of XAD resin surface with GO sheets can be observed from the SEM images (Figure 4a) while disordered stacking of GO layers evidenced by different degree of transparency was observed in TEM images (Figure 4b). Metal ion complexation with the donor atom of GO were indicated by the M-O vibrational frequency band in the region of 120-290 cm-1 in FIR spectrum (Figure S4).34 The presence of Pb, Cd and Zn in energy-dispersive X-ray analysis (EDS) spectra (Figure 5) obtained from SEM for micro-compositional analysis also supports of their chelation.

Optimization of experimental parameters The experimental variables such as pH, column flow rate, eluent type and volume, and loading concentration of metal ions were optimized by adopting a univariate approach (one parameter kept constant and other one is varied) for preconcentration of metal ions. Sorption of metal ions onto chelating resin was affected by the acidity/basicity of the sample solution and it is important


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to optimize the solution pH in order to achieve effective sorption. For sorption experiment, 0.1 g of XAD-GO resin equilibrated for 120 minute with 50 mL of solution containing 0.5 mM of Pb, Cd and Zn ions individually at the pH range of 2.0-9.0 (Figure 6). Extent of sorption of all the studied metal ions increases from pH 2.0 onward. For Pb and Cd, sorption attains its maximum at pH 8.0 and decreases thereafter while for Zn it increases continuously up to pH 9.0.This is because the oxygen (-COO-) of the GO was negatively charged after deprotonation while the metal ions were positively charged and hence, strong electrostatic interaction takes place.14 It was observed that Pb forms precipitate at pH 9.0 and hence, pH > 9.0 for Pb was not studied. At pH 9.0 and 10 formation of hydroxide complexes (MOH+) and M(OH)2 decrease the positive charge of hydroxide complexes and thereby lessens the electrostatic attraction which results lowering of sorption capacity, where M denotes metals (Pb, Cd and Zn ions). At pH ˃10 more negative species Pb(OH)3− is formed and functional group also get deprotonated. This forms negative charge on the GO surface, decreasing the metal sorption. Although the sorption capacity of Zn ions slightly increases from pH 8.0 to 9.0 but the magnitude of increase was not too much that’s is why pH 9.0 was avoided for further studies for Zn ions. Hence, all the studies for Pb, Cd and Zn ions were carried out at pH 8.0. The point of zero charge (pHpzc) has been calculated for better understanding the sorption behaviour35,36 and the value of pHpzc was found to be 6.2 (Figure S5) which is less than that of pH (8.0). Thus, it gives the idea of negatively charged surface groups at above pHpzc, favored the strong electrostatic interaction with metal ion.35,36 Column flow rate was optimized (using recommended column procedure) by passing 100 mL of solution contain 5 μg of metal ion at a flow rate range of 2.0-6.0 mL min-1. It was observed that quantitative sorption of metal ions was unaffected up to a flow rate of 5.0 mL min-1. On increasing flow rate from 5 to 6 mL min-1 the decrease in flow rate was observed from 99 to 84



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% for Pb, 100 to 78 for Cd and 98.6 to 71 for Zn, respectively. Hence, a flow rate of 5.0 mL min1

was set for further subsequent column experiments. For the quantitative recovery of sorbed Pb,

Cd and Zn ions, varying concentration and volume of HCl and HNO3 were tested out as eluent. Experimentally, it was found that 5.0 mL of 2 M HCl gives the recovery >98% for all the three metal ions. Hence, 5.0 mL of 2 M HCl was selected as a final eluent. The XAD-GO can be regenerated up to 53 sorption/elution cycles without any decrease in uptake capacity and after that a slight loss (5%) in sorption capacity was observed up to 60th cycle. Hence, the resin can be utilized for multiple use in the preconcentration/determination of studied metal ion from different environmental samples.

Effect of Foreign Ions Many cations and anions mostly present in water sometimes interfere samples were studied in order to check the interference in the selective determination of Pb, Cd and Zn ions. For this, a 50 mL solution containing 5 µg of analyte ion and different concentration of interfering ions, at pH 8.0 were preconcentrate using XAD-GO column at a flow rate of 5.0 mL min-1. The sorbed analyte was subsequently eluted and determined by FAAS. The results are shown in Table 1. It is evident that these interfering ions do not affect the uptake of Pb, Cd and Zn ions in the preconcentration studies up to a certain tolerance level. The tolerance limit is set as the concentration of foreign ion leading to relative error smaller than ±5% in the determination of analytes. Hence, the prepared resin shows a great application in the field of preconcentration/determination of metal ions using SPE coupled with FAAS. The preconcentration of Pb, Cd and Zn ions was not significantly affected even in the presence of alkali and alkaline earth metals and common matrix anions which are often observed to interfere


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in the subsequent metal ion FAAS determination and it also helps in sample preparation for ICPAES.

Preconcentration Studies The object of this study is to preconcentrate target analyte ions from large sample volumes into the limits of calibration range of FAAS prior to its determination. Since, direct determination of metal ions in the dilute samples is too challenging because of their low concentration. Hence, it requires a preconcentration step before subjected to FAAS determination. Following the recommended column procedure an increasing sample volume in the range of 0.75-2.6 L containing a constant amount of analyte ion (5 µg) were preconcentrate. The results obtained are shown in Table 2, gives the preconcentration limits of 2.0, 5.88 and 5.88 µg L-1 for the maximum volume of 2.5 L, 0.85 L and 0.85 L for Pb, Cd and Zn ions, respectively, where quantitative recovery of analytes were achieved. The preconcentration limit is defined as the lowest concentration of metal ion which can be quantitatively sorbed by sorbent from a large sample volume. Eluting the same sorbed metal ion amount into minimum volume (eluent) gives the high preconcentration factor, which is defined as the ratio of sample to eluent volumes. The binding affinity order of metal ions with GO are as follows: Pb(II) > Cd(II) > Zn(II). This is also relevant to the metal electronegativity and first stability constant of the associated metal hydroxide and metal acetate.13. The value of first stability constant of metal hydroxide and metal acetate is log K1=7.82, 4.17 and 4.4, log K1= 2.52, 1.5 and 1.5 for Pb, Cd and Zn ions respectively.13 Hence, in preconcentration studies, the effects of sample volume were different for Pb, Cd and Zn ions.

Analytical Method Validation



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To validate the proposed research methodology, several parameters limit of detection (LOD), limit of quantification (LOQ), correlation coefficient (R2) were studied. Under the set optimized conditions, calibration curves were plotted using least square method which results regression equation and correlation coefficient of Y = 0.042XPb - 0.008 (R² = 0.986) for Pb, Y = 0.274XCd 0.057 (R² = 0.989) for Cd and Y = 0.311XZn - 0.077 (R² = 0.990) for Zn. Concentrations of samples used to obtain calibration curve were 0.41-2000 µg L-1, 0.41-1800 µg L-1 0.13-1500 µg L-1 for Pb, Cd and Zn ions respectively. The LOD (3s/m) and LOQ (10s/m) 37 for (N=15,) where N is the no of blank runs were found to be 0.41, 0.41, 0.13 µg L-1 and 1.38, 1.36, 0.42 µg L-1for Pb, Cd and Zn ions, respectively. The accuracy of the method was checked by analyzing SRM, following recommended column procedure. Calculated Student’s t (t-test) values for Pb, Cd and Zn ions were observed to be less than the critical Student’s t value of 4.303 at 95 % confidence level for N=3 (Table 3). Hence, the mean concentration values were not statistically significant from the certified values indicating that absence of systematic errors and demonstrate the accuracy of the proposed method. The reliability of the developed method was also studied by the analyzing of spiked river water, tap water and city drinking water, with the known amount (5μg) of Pb, Cd and Zn ions. The percentage recoveries of spiked analyte were found to be 95103 with a relative standard deviation (RSD)

Graphene oxide supported on amberlite resin for the analytical

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