Monitoring of trace amounts of heavy metals in

Monitoring of trace amounts of heavy metals in different food and water samples by flame atomic absorption spectrophotometer after preconcentration by amine-functionalized graphene nanosheet Mohammad Behbahani, Nasim Akbari Ghareh Tapeh, Mojtaba Mahyari, Ali Reza Pourali, Bahareh Golrokh Amin & Ahmad Shaabani Environmental Monitoring and Assessment An International Journal Devoted to Progress in the Use of Monitoring Data in Assessing Environmental Risks to Man and the Environment ISSN 0167-6369 Environ Monit Assess DOI 10.1007/s10661-014-3924-1

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Author's personal copy Environ Monit Assess DOI 10.1007/s10661-014-3924-1

Monitoring of trace amounts of heavy metals in different food and water samples by flame atomic absorption spectrophotometer after preconcentration by amine-functionalized graphene nanosheet Mohammad Behbahani & Nasim Akbari Ghareh Tapeh & Mojtaba Mahyari & Ali Reza Pourali & Bahareh Golrokh Amin & Ahmad Shaabani

Received: 17 January 2014 / Accepted: 30 June 2014 # Springer International Publishing Switzerland 2014

Abstract We are introducing graphene oxide modified with amine groups as a new solid phase for extraction of heavy metal ions including cadmium(II), copper(II), nickel(II), zinc(II), and lead(II). Effects of pH value, flow rates, type, concentration, and volume of the eluent, breakthrough volume, and the effect of potentially interfering ions were studied. Under optimized conditions, the extraction efficiency is >97 %, the limit of detections are 0.03, 0.05, 0.2, 0.1, and 1 μg L−1 for the ions of cadmium, copper, nickel, zinc, and lead, respectively, and the adsorption capacities for these ions are 178, 142, 110, 125, and 210 mg g−1. The aminofunctionalized graphene oxide was characterized by thermogravimetric analysis, transmission electron microscopy, scanning electron microscopy, and Fourier transform infrared spectrometry. The proposed method was successfully applied in the analysis of environmental water and food samples. Good spiked recoveries over the range of 95.8–100.0 % were obtained. This work not o n l y p r o p os e s a us e f u l m e t h o d f o r s am pl e preconcentration but also reveals the great potential of modified graphene as an excellent sorbent material in analytical processes. M. Behbahani (*) : M. Mahyari : B. G. Amin : A. Shaabani Department of Chemistry, Shahid Beheshti University, Evin, Tehran, Iran e-mail: [email protected] N. A. G. Tapeh : A. R. Pourali Faculty of Chemistry, Damghan University, Damghan 36715/364, Iran

Keywords Amine-functionalized graphene oxide . Food and water samples . Solid phase extraction . Preconcentration . Heavy metals

Introduction The need for highly reliable methods and techniques for the determination of trace heavy transition metals has been recognized in analytical chemistry and environmental science. For that purpose, it is necessary to utilize either very sensitive instrumental technique or enrichment/separation methods for the quantification of low concentrations of metals. Because of the higher costs of the instrumental techniques like ICP-MS, generally, the preconcentration techniques are preferred (Miró et al. 2004). Various separation techniques including coprecipitation (Krishna et al. 2004; Liang et al. 2004), solvent extraction (Saito et al. 1998; Tuzen et al. 2002), membrane filtration (Soylak et al. 2004), cloud point extraction (Wen et al. 2009), and ion exchange (Bazzi et al. 2005) have been used for the traces of heavy metal ions in environmental samples. Solid phase extraction has also emerged as a powerful tool for separation/enrichment of heavy metal ions (Abdullin et al. 2000; Kobayashi 1994). Solid phase extraction based on adsorption is a recently used method that compensates solvent extraction disadvantages. The solid phase extraction technique reduces organic solvent usage and exposure, disposal costs, and extraction time for sample preparation. Solid phase extraction allows

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adsorbing chemical species directly onto a solid phase as an adsorbent provides an effective separation. The relatively high concentration factor and the ability of treating large volume samples compared to the other separation–preconcentration techniques are other advantages of the solid phase extraction. The sorbent material used, which is the core of solid phase extraction (SPE), determines the selectivity and sensitivity of the method. Although numerous substances have been applied as SPE sorbents, such as C18 silica (Jak et al. 2004), polymeric sorbents (Rao et al. 2004), and polyurethane foam (Lemos et al. 2007), their applicability is often limited only to a number of analytes. Reusability of the SPE columns is also a problem. Thus, developing new SPE adsorbents is of high value (Soylak et al. 1997, 2001; Soylak and Dogan 1996; Saracoglu et al. 2002). Carbon materials are well known for their high adsorption capacity. They have been proven to possess great potential as adsorbents for removing many kinds of environmental pollutants: Some examples are activated carbon (Chakrapani et al. 2001), fullerenes (Vallant et al. 2007), carbon nanotubes (CNTs) (Shamspur and Mostafavi 2009), carbon nanohorn (Zhu et al. 2009), and carbon nanocones/disks (Jimenez-Soto et al. 2009). Graphene, which is considered as the basic building block of all graphitic forms (including CNTs, graphite, and fullerene C60), is a single-atom-thick, two-dimensional carbon material (Rao et al. 2009). Compared with other graphitic forms, graphene possesses extraordinary electronic, thermal, and mechanical properties such as ultrahigh specific surface area, good thermal conductivity, fast mobility of charge carriers, and high values of Young’s modulus and fracture strength (Stoller et al. 2008; Lee et al. 2008; Balandin et al. 2008; Bolotin et al. 2008). To date, the unique planar structure of graphene provides tremendous potential applications in many fields. For example, graphene served as filler for the enhancement of mechanical and electrical properties in composite materials (Watcharotone et al. 2007). Graphene-based materials were also used as sensors for the sensitive and selective detection of biomolecules (Lu et al. 2009; Chang et al. 2010; Shan et al. 2009), and graphene/polymer composite is a candidate for super capacitors because of their high specific capacitance and good cycling stability (Zhang et al. 2010). The exceptional properties of graphene make it a superior candidate as an adsorbent for SPE. Firstly, graphene has a large specific surface area (theoretical value 2,630 m2 g−1 (Stoller et al.

2008)), suggesting a high sorption capacity. Specifically, both sides of the planar sheets of graphene are available for molecule adsorption, whereas for CNTs and fullerenes, steric hindrance may exist when molecules access their inner walls. Secondly, graphene can be easily modified with functional groups, especially via graphene oxide (GO) which has many reactive groups (Park and Ruoff 2009). Functionalization may further enhance the selectivity of SPE. Thirdly, CNTs usually contain trace amounts of metallic impurities that come from the metal catalysts used in their synthesis. These impurities may have negative effects on the applications of CNTs (Banks et al. 2006; Pumera and Miyahara 2009). Graphene, on the other hand, can be synthesized from graphite without the use of metal catalysts, thus obtaining pure material. Fourthly, the large delocalized π-electron system of graphene can form strong πstacking interaction with the benzene ring (Allen et al. 2010; Dreyer et al. 2010), which might make graphene a good choice as an adsorbent for the extraction of benzenoid-form compounds. In fact, the SPE technique using graphene as a novel adsorbent has been developed by Liu et al. (2011) for the determination of eight chlorophenols in aqueous samples. They demonstrated that the preconcentration technique is an efficient, simple, and nonexpensive extraction procedure for HPLC analysis. We extend its application to inorganic analysis and have obtained the consistent conclusion. In this work, graphene oxide has been modified with amine groups and used as a novel sorbent for heavy metal removal and these trace analysis. The effects of pH, flow rates, type, concentration, and volume of eluent for elution of cadmium, copper, nickel, zinc, and lead ions, break through volume and effect of potentially interfering ions on the separation, and determination of these heavy metals were investigated. The developed method was applied for determination of cadmium, copper, nickel, zinc, and lead ions in several real samples, and the accuracy of the method was confirmed by certified reference material.

Experimental Reagents and solutions All reagents were of analytical grade. Glasswares were cleaned by 10 % (v/v) HCl and then rinsed with deionized water. Graphite powder (325 mesh), 3-


aminopropyltriethoxysilane, potassium permanganate (KMnO4), concentrated sulfuric acid (H2SO4), hydrogen peroxide (H2O2), hydrochloric acid (HCl), nitric acid (HNO3), acetic acid (HOAC), sodium nitrate (NaNO3), and ethanol (EtOH) were purchased from Merck (Darmstadt, Germany, www.merck.de). Double distilled water from a Milli-Q purification system (Millipore, Bedford, MA, USA) was used for the preparation of solutions. A stock solution (100.0 mg L−1) of cadmium, nickel, copper, zinc, and lead ions was prepared by dissolving an appropriate amount of corresponding nitrate salts in double distilled water. Working standard solutions were prepared daily through serial dilutions of the stock solution with deionized water prior to analysis. Ore polymetallic gold Zidarovo-PMZrZ (206 BG 326) from Bulgaria was used as the reference material.

Apparatus Cadmium, copper, nickel, zinc, and lead concentrations were determined by an AA-680 Shimadzu (Kyoto, Japan) flame atomic absorption spectrometer (FAAS) in an air-acetylene flame, according to the user’s manual, provided by the manufacturer. Cadmium, copper, nickel, zinc, and lead hollow cathode lamps (HCL) were used as the radiation source with wavelengths of 228.8, 324.8, 232, 213.9, and 283 nm, respectively. The pH was measured at 25±1 °C with a digital WTW Metrohm 827 Ion analyzer (Herisau, Switzerland) equipped with a combined glass-calomel electrode. A peristaltic pump was obtained from Leybold (Cologne, Germany), and an adjustable vacuum gauge and controller were obtained from Analytichem International (Harber City, CA). The adjustable vacuum gauge allowed the control of the flow rate during extraction. The CHN analysis was performed on a Thermo Finnigan Flash EA112 elemental analyzer (Okehampton, UK). IR spectra were

Fig. 1 A scheme for synthesis of amine-functionalized graphene

recorded on a Bruker IFS-66 Fourier transform infrared (FTIR) spectrophotometer. The transmission electron microscopy (TEM) images were taken on a JEOL JEM-2100 F field emission transmission electron microscope. X-ray diffraction patterns were obtained on a Philips-PW 17C diffractometer with Cu Kα radiation. Thermogeravimetric analysis (TGA/DSC) was carried out on a Bahr STA-503 instrument under air atmosphere. Scanning electron microscopy (SEM) was performed by gently distributing the powder sample on the stainless steel stubs, using SEM (Philips, XL-30, Almelo, the Netherlands) instrument. Preparation of graphene oxide GO was prepared using Hummer’s method (Hummers and Offeman 1958). Graphite powder (1.00 g, 325 mesh) and H2SO4 (95 %) (23 mL) were added to a 250-mL conical flask, and the mixture was stirred with a magnetic bar. Sodium nitrate (0.50 g) was added, and the resulting mixture was cooled to 0 °C. Under vigorous agitation, KMnO4 (3.00 g) was added slowly, and the mixture was stirred for 1 h, while the temperature was kept below 35 °C. Then, H2O (45 mL) was added slowly to the reaction mixture, and the solution was stirred for 30 min at 90 °C. Next, H2O2 (10 mL of a 30 % solution) and deionized water (140 mL) were added to the mixture. Then, resulting precipitate was centrifuged and washed repeatedly with HCl (5 %, 3× 15 mL) and EtOH. The mixture was then vacuum dried at 60 °C. The GO was obtained as a brown powder. Preparation of amine-functionalized graphene oxide In a typical run, GO (0.05 g) and DMF (5.0 mL) in a round flask were subjected to ultrasonic pulse for 30 min, and then 3-aminopropyltriethoxysilane was added to the solution under nitrogen atmosphere. The


reaction mixture was heated to 80–110 °C and stirred for 40 h. For coupling reaction, 20 μL of triethylamine was typically added to the reaction mixture to promote the graft reaction. Then, the mixture was subjected to

centrifugation and thoroughly washed with toluene and THF for at least five times. The product was carefully collected and dried under vacuum at 60 °C until constant weight. Amine-functionalized graphene was

Fig. 2 A schematic illustration of extraction procedure of heavy metals by amine-functionalized graphene


obtained as a black powder. Figure 1 provides a scheme to illustrate the synthesis of amine-functionalized graphene oxide. Column method One hundred milligrams of amine-functionalized graphene was packed in a glass column with dimensions of 120 mm in length and 20 mm in diameter and blocked by two polypropylene filters at the ends to prevent the loss of the material during sample loading. Prior to extraction, the column was preconditioned successively with 5 mL of absolute ethanol, 5 mL of toluene, and 5 mL of absolute ethanol and then washed with double distilled water until it became free of organic solvents. Twenty-five milliliters of standard solution containing 2 mg L−1 cadmium, copper, nickel, zinc, and lead ions, after adjusting the pH of sample to 6.0 by adding HNO3 or NaOH solution, was passed through the column at a flow rate of 18 mL min−1 using a peristaltic pump. The sorbed cadmium, copper, nickel, zinc, and lead ions were eluted from the column with HCl (1 mol L−1) at a flow rate of 3 mL min−1. Afterward, the analytes in the eluent were determined by FAAS. Figure 2 provides a scheme for illustration of the mentioned extraction procedure. Sample pretreatment for metal analysis Food samples Fish The fish samples were bought from four local supermarkets in four different sites randomly in Tehran, Iran. After collection, four samples of fish from each supermarket were randomly selected, mixed, and

transferred into an ice bag and transported to the laboratory and stored at −20 °C prior to analysis. The muscle tissue of the fish sample was taken out quickly and was air dried in an oven at 70 °C for 48 h (Behbahani et al. 2013a, b). About 0.5 g of the sample was digested with 5 mL of concentrated HNO3 for 4 h at 100 °C. The resulted solution was filtered and transferred into a beaker and diluted to 250 mL before adjusting the pH to 6.0. Tomato, mushroom, and apple Food products, including tomato, mushroom, and apple, were collected from an agricultural land in south of Tehran, Iran. All of the samples were stored in plastic bags and brought to the laboratory for preparation and treatment. After washing the samples with distilled water, 5.0 g of each sample was grounded, homogenized, and dried at 80 °C and triturated in porcelain mortar. After fractionation of samples by sieving, sizes less than 20 μm were dissolved in 10 mL of 3 mol L−1 HNO3 solution and diluted with distilled water to a final volume of 250 mL, and then the pH was adjusted to 6.0 for further experiments (Behbahani et al. 2013a, b). Water samples Distilled, tap, and river water The sorbent was successfully used for determination of cadmium, copper, nickel, zinc, and lead in distilled, tap, and river water samples (from Shahrood River and Derka River (Ghaemshahr, Iran)). The polyethylene bottles filled with the samples were cleaned with detergent, water, diluted nitric acid, and water in sequence. The samples were immediately filtered through a cellulose filter membrane (pore size 0.45 μm) and were acidified to pH of 2.0 for storage.

Fig. 3 The FTIR of GO (blue line) and amine-functionalized graphene (red line)


Before analysis, the water samples (250 mL) were adjusted to pH of 6.0 according to optimized experimental conditions.

Results and discussion Sorbent characterization The synthesized amine-functionalized graphene oxide was characterized by FTIR spectroscopy, TEM, SEM, Fig. 4 The scanning electron microscopy (a) and transmission electron microscopy (b) graph of modified graphene

and TGA and elemental analysis (CHN). The FTIR spectra of amine-functionalized graphene and GO are shown in Fig. 3. FTIR spectroscopy is one of the important tools for characterization of GO (blue line). The weak bands at 3,410 cm−1, 1,610 cm−1, and 1,059 cm−1 show stretching vibrations of−COOH groups. After the modification of GO sheets with amine groups (red line), the appearance of a relatively strong band at 2,900 cm−1 (C–H band of 3-aminopropyltriethoxysilane) proves a successful modification of graphene oxide with amine groups. SEM image of the suspended modified

(a)

(b)

Author's personal copy Environ Monit Assess Fig. 5 The TG plot of graphene oxide (A) and aminefunctionalized graphene oxide (B)

Heavy metal adsorption–desorption studies

graphene sheets is shown in Fig. 4a. The exfoliated graphene sheets with lateral dimensions of several micrometers were observed by SEM image. TEM (Fig. 4b) is one of the most powerful techniques used to investigate single layers and the level of the dispersion of graphene sheets in aqueous solution. The thermal stability of GO and amine-functionalized graphene oxide has been investigated by thermal analysis. According to these results in Fig. 5, the difference between weight loss of amine-functionalized graphene oxide and GO proves the modification of GO with amine groups.

100

Retention (%)

80

Optimization of the adsorption steps Among the tested variables, pH was found to be the most critical parameter for the adsorption of metals on the modified graphene. To evaluate the effect of pH on the retention efficiency, the pH of 25 mL of sample solution containing 2 mg L−1 of cadmium, copper, nickel, zinc, and lead ions was adjusted to fit in the range of 2–9. As can be seen in Fig. 6, the quantitative adsorption of cadmium, copper, nickel, zinc, and lead ions on the

Cadmium Copper Nickel Zinc Lead

60 40 20 0 2

3

4

5

6

7

8

9

pH Fig. 6 The effect of pH of sample solutions on retention of lead ions by amine-functionalized graphene (conditions: sample concentration, 2 mg L−1; sample volume, 25 mL)


amine-functionalized graphene was obtained in the pH of 6.0. Due to hydrolysis of the heavy metals, pH above 9.0 was not tested. To optimize the sample flow rate, 25 mL solutions of 2 mg L−1 cadmium, copper, nickel, zinc, and lead ions were adjusted to pH of 6.0 and then passed through the column at flow rates in the range of 1–22 mL min−1 with a peristaltic pump. The obtained results demonstrated that the sample flow rate variation in the ranges of 1– 18 mL min−1 had no effect on the simultaneous retention of the heavy metals on amine-functionalized graphene oxide. Optimization of the elution conditions For desorption of cadmium, copper, nickel, zinc, and lead ions from amine-functionalized graphene, series of selected eluents, such as HNO3, HCl, and CH3COOH at different concentrations, were used. As shown in Table 1, it was eventually found that HCl (1 mol L−1) provided an effective elution of cadmium, copper, nickel, zinc, and lead ions from amine-functionalized graphene oxide. The effect of eluent volume on the recovery of the heavy metals was also studied. As Table 1 shows, quantitative recovery could be obtained with 2.5 mL of HCl (1 mol L−1). Therefore, volumes of 2.5 mL of eluent for desorption of cadmium, copper, nickel, zinc, and lead ions were used in the remaining experiments. The influence of the eluent flow rate on metal recovery was also studied. The results demonstrated that the quantitative recoveries for cadmium, copper,

nickel, zinc, and lead ions were obtained at a flow rate range of 0.5–3.0 mL min−1 with HCl (1 mol L−1). Effect of the volume of sample solutions Due to low concentrations of heavy metals, preconcentration was performed on large volumes of real samples. Therefore, the maximum volume of sample solution was investigated by increasing the volume of metal ion solution with a constant amount of ions (0.1 mg of cadmium, copper, nickel, zinc, and lead ions). Sample solution volumes of 50, 100, 300, 500, 600, 700, 800, 900, 1,000, 1,100, and 1,200 mL containing cadmium, copper, nickel, zinc, and lead ions were passed through the column. The results (Fig. 7) demonstrated that the simultaneous quantitative recovery of cadmium, copper, nickel, zinc, and lead ions on amine-functionalized graphene oxide can be obtained for sample volume up to 800 mL. Effect of the potentially interfering ions To investigate the effect of various cations found in natural samples, elements that are known as alkaline, alkaline earth, and transition metals were added to 100 mL of solution containing 0.01 mg of cadmium, 0.01 mg of copper, 0.01 mg of nickel, 0.01 mg of zinc, and 0.01 mg of lead ions. The degrees of tolerance for some alkaline, alkaline earth, and transition metal ions are presented in Table 2. From the tolerance data, it can be seen that the potentially interfering ions have no

Table 1 The effect of type, concentration, and volume of eluent on extraction efficiency of the study heavy metals Eluent

Concentration (mol L−1)

Volume (mL)

R%±S Copper

Lead

Cadmium

Nickel

Zinc

HNO3

3

10

87.0±1.0

76.0±1.0

91.0±1.0

79.0±1.2

92.0±1.0

HCl

3

10

99.0±1.0

99.0±1.0

99.0±1.0

99.0±1.0

99.0±1.0

CH3COOH

3

10

37.0±1.4

42.0±1.2

38.0±1.3

49.0±1.2

52.0±1.3

HCl

2

10

99.0±1.0

99.0±1.0

99.0±1.0

99.0±1.0

99.0±1.0

HCl

1

10

99.0±1.0

99.0±1.0

99.0±1.0

99.0±1.0

99.0±1.0

HCl

0.5

10

72.0±1.2

67.0±1.4

82.0±1.3

71.0±1.5

62.0±1.3

HCl

1

5

99.0±0.8

99.0±1.0

99.0±1.0

99.0±1.0

99.0±1.0

HCl

1

3

99.0±1.0

99.0±1.0

99.0±1.0

99.0±1.0

99.0±1.0

HCl

1

2.5

99.0±0.8

99.0±1.0

99.0±1.0

99.0±1.0

99.0±1.0

HCl

1

2

80.0±1.3

78.0±1.5

71.0±1.3

69.0±1.3

54.0±1.0

R recovery, S standard deviation


110 100

Recovery (%)

Fig. 7 The effect of sample volume on the recovery of the heavy metals on aminefunctionalized graphene (0.1 mg of cadmium, copper, nickel, zinc, and lead ions)

Cadmium Copper Nickel Zinc Lead

90 80 70 60 50 40 0

200

400

600

800

1000

1200

1400

Sample volume (mL) significant effects on preconcentration of cadmium, copper, nickel, zinc, and lead ions at pH of 6.0.

by using a FAAS. The amount of adsorbed heavy metal ions was calculated using the following equation: Q ¼ ½ðC o −C A Þ V =m

Adsorption capacity The concentration of the metal ions in the aqueous phases after desired treatment periods was measured

where Q is the amount of metal ions adsorbed onto unit amount of the composites (mg g−1), Co and CA are the concentrations of metal ions in the initial solution and in

Table 2 The tolerance limit of various potentially interfering ions on determination of the study heavy metals Foreign ion

K+

Tolerable concentration Ratio X/Cd, Cu, Ni, Zn, Pb

R%±S Cadmium

Copper

Nickel

Lead

Zinc

10,000

99.0±1.0

99.0±1.0

98.0±1.2

99.0±1.0

99.0±1.0

+

Na

10,000

99.0±1.0

98.0±1.5

98.0±1.4

99.0±1.0

99.0±1.0

Cs+

10,000

98.0±1.5

98.0±1.2

98.0±1.4

98.0±1.5

99.0±1.0

Ca2+

1,000

97.0±1.4

96.0±1.8

98.0±1.0

97.0±1.3

97.0±1.2

Mg2+

1,000

96.0±1.2

97.0±1.3

97.0±1.1

97.0±1.5

98.0±1.0

Al3+

700

97.0±1.0

96.0±1.0

96.0±1.5

96.0±1.5

97.0±1.0

3+

Cr

800

97.0±1.2

96.0±1.0

96.0±1.4

97.0±1.2

96.0±1.5

Fe2+

800

97.0±1.0

96.0±1.3

96.0±1.2

96.0±1.5

97.0±1.2

As3+

500

97.0±1.2

96.0±1.5

97.0±1.3

96.0±1.5

97.0±1.2

Sb3+

700

96.0±1.5

96.0±1.3

97.0±1.2

97.0±1.3

98.0±1.0

Hg2+

600

96.0±1.3

97.0±1.5

97.0±1.5

96.0±1.2

96.0±1.5

2+

Mn

700

97.0±1.2

96.0±1.4

97.0±1.6

96.0±1.2

96.0±1.3

CrO42−

1,000

98.0±1.3

97.0±1.1

96.0±1.3

97.0±1.2

97.0±1.0

PO43−

1,000

97.0±1.3

97.0±1.5

97.0±1.5

96.0±1.4

96.0±1.4

Conditions: sample pH, 6.0; sample volume, 100 mL, 0.01 mg of each metal; sample flow rate, 18 mL min−1 ; eluent, 2.5 mL of 1 mol L−1 HCl; eluent flow rate, 3 mL min−1 . X indicates concentration of diverse ions R recovery, S standard deviation

Author's personal copy Environ Monit Assess Fig. 8 Effect of the study heavy metal equilibrium concentrations on the adsorption capacity of amine-functionalized graphene (adsorption isotherm)

250 Cadmium Copper Nickel Zinc Lead

Q (mg g-1)

200

150

100

50

0 0

50 100 150 200 250 300 350 400 Initial conecntration of heavy metals (mg L-1)

450

the aqueous phase after adsorption, respectively (mg L−1), V is the volume of the aqueous phase (L), and m is the weight of the composites (g). The heavy metal ion adsorption capacities of the amine-functionalized graphene oxide are given as a function of initial concentration of heavy metal ions within the aqueous phase in Fig. 8. These adsorption curves were obtained from experiments where adsorptions from the single heavy metal aqueous solutions were studied. It was observed that the amount of adsorption was significantly increased with initial heavy metal concentration in the studied concentration range. The maximum adsorption capacities of the aminefunctionalized graphene oxide in the studied range at optimum adsorption conditions are 178, 142, 110, 125, and 210 mg g−1 for cadmium, copper, nickel, zinc, and lead, respectively.

defined as CLOD =3Sb /m, where Sb is the standard deviation of ten replicate blank signals and m is the slope of the calibration curve after preconcentration, for a sample volume of 250 mL, was found to be 0.03 μg L−1 for cadmium, 0.05 μg L−1 for copper, 0.2 μg L−1 for nickel, 0.1 μg L−1 for zinc, and 1 μg L−1 for lead ions, respectively. The relative standard deviations for eight separate column experiments for determination of 7.5 μg of cadmium, copper, nickel, zinc, and lead ions in 250 mL of water were 2.4, 2.9, 2.7, 3.1, and 2.4 %, respectively.

Analytical performance

Table 3 The analysis of cadmium, copper, nickel, zinc, and lead in certified reference material

Under the optimized conditions, calibration curves were sketched for the determination of cadmium, copper, nickel, zinc, and lead ions according to the general procedure. Linearity was maintained at 0.1–100 μg L−1 for cadmium, 0.1–100 μg L −1 for copper, 0.9– 170 μg L−1 for nickel, 0.5–150 μg L−1 for zinc, and 1.5–350 μg L−1 for lead ions in initial solution. The correlation of determination (r2) was 0.992 for cadmium, 0.996 for copper, 0.997 for nickel, 0.997 for zinc, and 0.995 for lead ions. The limit of detection which is

Sample

Validation of the method The concentration of cadmium, copper, nickel, zinc, and lead ions obtained by amine-functionalized graphene oxide was compared to the standard material. For this

Element Concentration (mg kg−1)

Relative error (%)

Certified Found Ore Polymetallic gold Zidarovo PMZrZ(206 BG 326)

210.20 −0.8

Cd

212.0

Pb

5.47

5.35

−2.2

Ni

12.0

11.70

−2.5

Cu

0.51

0.49

−3.9

Zn

4.06

3.90

−3.9

Author's personal copy Environ Monit Assess Table 4 Analysis of cadmium, copper, nickel, zinc, and lead in different food (a) and water (b) samples Sample

Element

Real sample (μg kg−1)

Added (μg kg−1)

Found (μg kg−1)

Cadmium

5.3

10.0

15.1

98.7

Copper

78.3

10.0

86.5

98.0

Recovery (%)

A Fish

Tomato

Mushroom

Apple

Nickel

52.2

10.0

61.3

98.5

Lead

38.5

10.0

46.7

96.3

Zinc

81.7

10.0

89.5

97.6

Cadmium

–

10.0

9.8

98.0

Copper

8.1

10.0

17.8

98.3

Nickel

5.1

10.0

14.7

97.4

Lead

7.2

10.0

16.9

98.2 98.5

Zinc

9.8

10.0

19.5

Cadmium

3.5

10.0

12.9

95.5

Copper

19.1

10.0

28.5

97.9

Nickel

5.4

10.0

15.1

98.0

Lead

6.1

10.0

15.8

98.1

Zinc

18.7

10.0

27.5

95.8

Cadmium

–

10.0

9.7

97.0

Copper

17.6

10.0

17.2

97.7

Nickel

6.1

10.0

15.8

98.1

Lead

4.2

10.0

13.9

97.9

Zinc

16.5

10.0

26.1

98.5

B Distilled water

Tap water

Shahrood river water

Derka river water

Cadmium

–

10.0

9.9

99.0

Copper

–

10.0

10.0

100.0

Nickel

–

10.0

9.9

99.0

Lead

–

10.0

9.9

99.0

Zinc

–

10.0

10.0

100.0

Cadmium

–

10.0

9.8

98.0

Copper

10.1

10.0

10.0

99.0 96.7

Nickel

2.3

10.0

11.9

Lead

–

10.0

9.9

99.0

Zinc

6.8

10.0

16.3

97.0

Cadmium

2.9

10.0

12.6

97.7

Copper

21.1

10.0

30.5

98.1

Nickel

45.1

10.0

54.2

98.4

Lead

10.2

10.0

19.8

98.0

Zinc

19.2

10.0

28.5

97.6

Cadmium

3.3

10.0

12.9

97.0

Copper

18.9

10.0

28.5

98.6

Nickel

34.2

10.0

42.5

96.2

Lead

10.3

10.0

19.9

98.0

Zinc

20.5

10.0

29.8

97.7

Conditions: sample pH, 6.0; sample volume, 250 mL; sample flow rate, 18 mL min−1 ; eluent, 2.5 mL of 1 mol L−1 HCl; eluent flow rate, 3 mL min−1


reason, the concentration of the heavy metal ions was determined at optimum conditions in standard reference material (Ore Polymetallic gold Zidarovo-PMZrZ (206 BG 326)). As it can be seen in Table 3, good correlation was achieved between estimated content by the present method and reference materials. Therefore, aminefunctionalized graphene oxide can be used as a reliable solid phase for extraction and determination of cadmium, copper, nickel, zinc, and lead ions in real samples. Determination of cadmium, copper, nickel, zinc, and lead in real samples Since natural samples have complex matrices, nonspecific background absorption was caused by interfering species of the sample matrix. To reduce this undesirable effect, modified graphene oxide was applied for selective extraction of cadmium, copper, nickel, zinc, and lead ions in pH of 6.0. Table 4 shows the cadmium, copper, nickel, zinc, and lead ion recoveries in various food and water samples.

Conclusions The proposed SPE method is easy, safe, and economic for the preconcentration and determination of trace amounts of toxic heavy metals in different samples. Due to a relatively high preconcentration factor, toxic heavy metals at trace level can be determined accurately. The amine-functionalized graphene oxide showed high tolerance to interferences from the matrix ions, and in a nutshell, the important features of the proposed method were its high adsorption capacity, good preconcentration factor, and low detection limit which are distinctive among other SPE methods. This work not only proposes a useful method for environmental water and food samples pretreatment but also reveals the great potential of graphene as an excellent sorbent material in analytical processes.

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