Determination of nickel and cobalt in reference plant materials by carbon slurry sampling GFAAS technique after their simultaneous preconcentration onto modified activated carbon

Journal of Food Composition and Analysis 26 (2012) 58–65

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Original Research Article

Determination of nickel and cobalt in reference plant materials by carbon slurry sampling GFAAS technique after their simultaneous preconcentration onto modified activated carbon Ryszard Dobrowolski *, Magdalena Otto Department of Analytical Chemistry and Instrumental Analysis, Faculty of Chemistry, Maria Curie-Sklodowska University, M. C. Sklodowska Sq. 3, 20-031 Lublin, Poland

A R T I C L E I N F O

A B S T R A C T

Article history: Received 4 November 2011 Received in revised form 5 March 2012 Accepted 10 March 2012

A novel method, in which dimethylglyoxime impregnated activated carbon (DMGC*) has been applied for preconcentration and further simultaneous determination of Ni(II) and Co(II) ions in the reference plant materials by the carbon slurry sampling graphite furnace atomic absorption spectrometry technique (GFAAS), is presented. Moreover, for comparison and evaluation, the activated carbons modified in different ways, e.g. oxidized with concentrated HNO3 solution (OC*) and outgassed at high temperature (OGC*), were prepared and carefully characterized, especially in terms of their selectivity. The method after preconcentration on DMGC* carbon has been successfully tested by the analysis of certified reference materials (spinach leaves: NIST 1570a and NCS ZC73013, mixed Polish herbs INCT-MPH-2, tea leaves INCT-TL-1), using carbon slurry sampling GFAAS and the standard calibration method. The obtained detection limits for nickel and cobalt were 0.02 and 0.001 mg/g, and quantification limits were 0.06 and 0.004, respectively. The precision (RSD% about 7%) and accuracy of nickel and cobalt determination by the described method were acceptable. ß 2012 Elsevier Inc. All rights reserved.

Keywords: Trace element determination Nickel Cobalt Modified activated carbons Preconcentration Food analysis Food safety Slurry sampling graphite furnace atomic absorption spectrometry Food composition

1. Introduction Nickel, as an activator of several enzymes, and cobalt, as a component of vitamin B12, are considered to be essential trace elements which play an important role in many body functions. However, depending on their concentration, they can be either crucial or toxic for living beings, including humans (Greenwood and Earnshows, 1988; Lison, 2007). Hence, from the analytical, environmental and public health points of view, it is important to develop sensitive and economical methods for determination of trace amounts of nickel and cobalt in natural water and food samples. Inductively coupled plasma optical emission spectrometry (ICP-OES) (Dolan and Capar, 2002; Guo et al., 2004) combined with flow injection analysis (FIA) (Yunes et al., 2003) or vapor generation (VG) technique (Cerutti et al., 2005), inductively coupled plasma-mass spectrometry (ICP-MS) (Millour et al., 2011), electrothermal atomic absorption spectrometry (ETAAS) (Cerutti et al., 2003; Shiowatana et al., 2000) and flame atomic absorption spectrometry (FAAS) (Haji Shabani et al., 2003; Sahin et al., 2010) are the most useful techniques for determination of

* Corresponding author. Tel.: +48 81 5375704; fax: +48 81 5333348. E-mail address: [email protected] (R. Dobrowolski). 0889-1575/$ – see front matter ß 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.jfca.2012.03.002

nickel and cobalt in food and environmental samples. However, low levels of these elements in natural samples are not compatible with the detection limits (LODs) of the above-mentioned techniques. Therefore, preconcentration and separation steps are required to improve detection capability. There are many methods of Ni(II) and Co(II) preconcentration from aqueous solution, including coprecipitation (Saracoglu and Soylak, 2010), microextraction (Dadfarnia et al., 2010; Shirani Bidabadi et al., 2009), electrodeposition, chelation (Ghaedi et al., 2009a; Narin and Soylak, 2003) membrane extraction (Aouarram et al., 2007) solid phase (Kiran et al., 2007; Tuzen et al., 2009) and cloud point extraction (Gil et al., 2008; Lemos et al., 2007). Different sorbents, such as Amberlite XAD resins (Ghaedi et al., 2009b), nanotubes (Duran et al., 2009), ion-imprinted polymers (Saraji and Yousefi, 2009), polyurethane foams (Jorgensen Cassella et al., 2001; Lemos et al., 2010), silica gels (Pourreza et al., 2010) and new low-cost adsorbents (Hannachi et al., 2010), have been used for the preconcentration of Ni(II) and Co(II) ions from various media. However, the most effective method for separation and enrichment of trace metals from aqueous solutions seems to be adsorption of heavy metals onto activated carbons because of their large surface area, high adsorption capacity, micro-pore character and high degree of surface reactivity (Dobrowolski, 1998). Generally, metal preconcentration on activated carbon is

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achieved simply by adjusting pH to the appropriate value or by application of a chelating agent (Camel, 2003). In the case of nickel and cobalt enrichment on activated carbon, this is done after chelating with: pyrocatechol violet (Narin et al., 2000), 8-hydroxyquinoline (Shiowatana et al., 2000), 4-(2 pyridylazo) resorcinol (Chakrapani et al., 1998), cupferron (Yaman and Gucer, 1998), 1,10phenanthroline (Mikuła and Puzio, 2007), diethyldithiocarbamate (Kilicel and Gokalp, 2005), ammonium pyrrolidinedithiocarbamate (Soylak et al., 1996), dithioxamide (Ghaedi et al., 2007a), or N,N0 ethylenebis (ethane sulfonamide) (Karacan and Aslantas, 2008). The disadvantage of these procedures is that they require series of complexation steps (Daorattanachai et al., 2005). On the other hand, the surface of activated carbon can be easily modified in various ways, which can give them adsorption capacity and selectivity for specific metal ions. These modifications can be accomplished, for example, by treatment with solutions of acids and alkalis, outgassing at high temperature (above 1100 8C) and impregnation with metal salts or organic compounds (Dobrowolski, 1998). The use of metal chelating activated carbons could give higher selectivity and the large enrichment factors of such separation and preconcentration techniques. For this reason, some activated carbons modified with chelating agents for Ni(II) and Co(II) ions adsorption have been proposed, e.g. ammonium pyrrolidinedithiocarbamate (APDC) (Daorattanachai et al., 2005), bis salicyl aldehyde, 1,3 propandiimine (BSPDI) (Ghaedi et al., 2008), 4,6-dihydroxy-2-mercaptopyrimidine (DHMP) (Ghaedi et al., 2007b), and 2-{[1-(2-hydroxynaphthyl) methylidene] amino} benzoic acid (HNMABA) (Kiran et al., 2007). The slurry sampling graphite furnace atomic absorption spectrometry technique (GFAAS) seems to be very attractive for trace element determination (Ferreira et al., 2010), especially in the case of direct introduction of carbon slurry with the adsorbed trace elements after preconcentration into the electrothermal graphite atomizer. Additionally, this technique is irreplaceable when the desorption of an analyte from activated carbons is incomplete, despite the application of a very aggressive desorption medium. While almost all analyte is adsorbed onto activated carbon, the batch system of preconcentration is suitable for further determination of trace metals by the carbon slurry sampling GFAAS technique. For this reason, new specific adsorbents are needed (Dobrowolski, 1998). The aim of this study was to compare and evaluate a modified activated carbon application for the enrichment of Ni(II) and Co(II) ions from aqueous solutions. For this purpose, modification of powdered activated carbon by treatment with the concentrated HNO3 solution, outgassing treatment at high temperature and impregnation with dimethylglyoxime (DMG) solution of outgassed carbon was proposed. The basic parameters affecting the adsorption capacity of Ni(II) and Co(II) ions on the modified activated carbons were studied in detail. The effects of activated carbons modification have been determined by setting the initial runs of adsorption isotherms. Taking further analytical application of the prepared carbons into account, removing nickel and cobalt from the modified activated carbons was studied using inorganic acids with respect to their concentrations. The influence of chlorides and nitrates on the adsorption ability of Ni(II) and Co(II) ions onto the modified activated carbons for diluted aqueous solutions was also studied with respect to the determination of nickel and cobalt in solid materials after digestion steps in the analytical procedure, which usually involves the application of aqua regia. 2. Experimental 2.1. Reagents and materials The powdered activated carbon, Medical Carbon (Carbo Medicinalis), produced from charcoal by Gryfskand in Hajno´wka,

59

Poland, after pretreatment with hydrochloric and hydrofluoric acids (POCH, Gliwice, Poland), which reduced the ash contents significantly, was applied as the initial adsorbent. Hydrochloric acid Suprapur (36%) and nitric acid Suprapur (65%) were purchased from Merck (Darmstadt, Germany). Nickel stock solution of 495 mg/L concentration was prepared by proper dissolution of powdered Ni(NO3)26H2O (POCH, Gliwice, Poland) in 0.05 mol/L HCl. Cobalt stock solutions were prepared by proper dilution of 1000 mg/L standard solution (Merck, Darmstadt, Germany). The calibration curve was established using the standard solutions of Ni(II) and Co(II) prepared in 1 mol/L HNO3 by dilution from 1000 mg/L stock solution (Merck, Darmstadt, Germany). The calibration standard was not submitted to the preconcentration procedure. Sodium chloride and potassium nitrate, as interfering agents, were purchased from POCH (Gliwice, Poland). NaOH (Merck, Darmstadt, Germany) and HCl solutions were used for pH adjustment. DMG solution (1%, w/w) was prepared by dissolving the powdered DMG (POCH, Gliwice, Poland) in ethanol (POCH, Gliwice, Poland). Iridium (SCP Science, Quebec, Canada) and niobium (CPI International, Santa Rosa, USA) standard solutions diluted in appropriate way in 5% (v/v) HNO3 and tungsten standard solution (Fluka, Buchs SG, Switzerland) diluted in Milli-Q water were used for modification of graphite tubes. Throughout all analytical work, Milli-Q water of resistivity not less than 18.2 MV cm (Millipore, Billerica, MA, USA) was used. All solvents and reagents were of analytical-reagent grade, and the presence of nickel and cobalt was not detected in the working range. Certified reference materials: spinach leaves NIST 1570a (National Institute of Standards and Technology, Gaithersburg, USA) and NCS ZC73013 (China National Analysis Center for Iron and Steel, Beijing, China); mixed Polish herbs INCT-MPH-2 and tea leaves INCT-TL-1 (Institute of Nuclear Chemistry and Technology, Warsaw, Poland) were analyzed. 2.2. Apparatus A UV–vis spectrophotometer (Cary 50 Bio, Varian, Australia) has been used for the spectrophotometric data measuring. Measurements of nickel and cobalt concentrations in the studied adsorption system and their determination in certified reference materials after its preconcentration on DMGC* carbon were carried out using an AAS 3 (Carl Zeiss, Jena, Germany) atomic absorption spectrometer equipped with deuterium-lamp background correction, an EA 3 electrothermal atomizer and an MPE autosampler. The conditions of hollow cathode lamps work for Ni and Co (Varian, Australia) and time temperature programs are shown in Table 1. The pyrolytic graphite coated tubes (Perkin Elmer, Waltham, MA, USA) modified by Ir/Nb and Ir/W as permanent modifiers were used for nickel and cobalt determination, respectively. Pure argon (99.999% pure, Air Products, Warsaw, Poland) was employed as the inert gas with a flow rate of 280 mL/min except the atomization stage. The pH of initial nickel and cobalt solutions as well as of carbon suspensions was measured using a glass electrode combined with a calomel electrode SenTix 81 (WTW GmbH, Weilheim, Germany). 2.3. Activated carbon modification and measurements Two portions of the commercially available powdered activated carbon (C*) were chemically modified by oxidation with concentrated nitric acid and outgassing treatment in the argon atmosphere at high temperature (1100 8C). Then the portion of activated carbon obtained by outgassing treatment was impregnated by dimethylglyoxime solution. The procedure of oxidation was as follows: 400 mL of concentrated nitric acid was added to 40 g of

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60 Table 1 Instrumental basic working parameters.

3. Results and discussion 3.1. Effect of pH

Parameter

Ni

Co

Wavelength [nm] Lamp current [mA] Spectral bandpass [nm] Injection volume [ml] Pyrolysis temperature [8C] Atomization temperature [8C] Cleaning temperature [8C]

232 5 0.2 20 900 2400 2600

240.7 5 0.2 20 900 2400 2600

the activated carbon and the suspension was heated at 90 8C until dry. The residue was washed with distilled water until conductivity of the water eluent was close to that of the distilled water. The outgassing treatment was carried out in the fluidized furnace at 1100 8C in argon atmosphere. The impregnation treatment was as follows: 100 mL of 1% DMG in ethanol solution (w/w) were added to 20 g of the outgassed activated carbon and the suspension was put into the ultrasonic bath for 45 min and filtered through the filter disk. Then the sorbent was dried in a laboratory dryer at 120 8C until the constant weight was reached. In this way, three different activated carbons were obtained: OC*-oxidized by concentrated nitric acid carbon, OGC*-outgassed and DMGC*impregnated by dimethylglyoxime. The presence of DMG on the activated carbon surface was confirmed after its desorption by ethanol and its content was further determined using the UV–vis spectrophotometer. Optimization of Ni(II) and Co(II) ions adsorption onto the modified activated carbons was carried out at 25 8C. Individual measuring points were obtained for the adsorption system: 50 mL of nickel and cobalt solution and 0.2 g of activated carbon. The equilibrium adsorption uptake in the solid phase a, (mg/g), was calculated as follows: a¼

ðci  cÞ  V m

(1)

where ci is the initial Ni(II) or Co(II) concentration (mg/L), c is the equilibrium Ni(II) or Co(II) concentration (mg/L), V is the volume of the solution (L) and m is the mass of the adsorbent (g). After the equilibrium was reached (20 min), the liquid phase was separated by a centrifuge and nickel and cobalt were determined. 2.4. Determination of Ni(II) and Co(II) in certified reference materials Certified reference materials (spinach leaves: NIST 1570a and NCS ZC73013, mixed Polish herbs INCT-MPH-2, tea leaves INCT-TL1) were used for validation. The samples were digested as follows: for the microwave digestion 0.5 g of samples was weighted accurately into the Teflon vessels and 7 mL of HNO3 were added. The above mixture was digested in the microwave system (Mars 5, Matthews, USA). After digestion completion, the volume of the digested samples was made up to 25 mL with Milli-Q water. The blanks were prepared in the same way as the sample. Then the digested samples were evaporated almost to dryness, made up to a volume of 50 mL with Milli-Q water and adjusted to a pH value of 7 with sodium hydroxide. The preconcentration-separation procedure given above was applied for the samples using 0.2 g of the DMGC* carbon. After the equilibrium was reached, the suspensions were filtered through the filter disk and dried in a laboratory dryer at 120 8C until the constant weight was obtained. Nickel and cobalt loaded carbons were then analyzed using the carbon slurry sampling GFAAS technique by preparing the carbon slurry of 0.2– 1% for nickel and 1–3% for cobalt with 5% HNO3 as a liquid medium. The carbon slurries were homogenized using the Vortex agitator just before each measurement, which reduced drastically the error caused by the lack of slurries stability.

The pH value of the initial solutions has significant influence on the quantitative adsorption of trace metal ions on activated carbons (Chen and Lin, 2001). It affects the retention of the metal ions onto the activated carbon. The effect of pH on nickel and cobalt ions adsorption onto the modified activated carbons was studied in the initial pH range of 1.5–11.5, keeping the other parameters constant. In Fig. 1 the adsorption abilities of nickel and cobalt as a function of the initial and equilibrium pH are shown. The optimum equilibrium pHeq for adsorption of Ni(II) and Co(II) onto the prepared activated carbons is included in a wide range of pHeq values. The OC* carbon reaches the maximum adsorption for the pHeq value larger than 2.2, which corresponds to the initial pHin larger than 2.1. For the OGC* and DMGC* carbons, the shift of nickel maximum adsorption ability towards alkaline pHeq is observed. It is reached after pHeq of 6.8 and 8.0, respectively. These pHeq values correspond to the pHin larger than 4.2 for both carbons. In turn, in the case of cobalt its maximum adsorption ability is obtained in the pHeq larger than 6.8 for the OGC* carbon and 7.5 for the DMGC* carbon, which corresponds to the pHin larger than 6.0. Considering these results and the possibility of nickel and cobalt hydroxides precipitation after pH of 8 (Dobrowolski, 1998), the initial pHin for Ni(II) and Co(II) ions adsorption of 3.5 for the OC* carbon, 6.0 and 7.0 for the OGC* carbon and 6.5 and 7.5 for the DMGC* carbon, respectively have been recommended for subsequent experiments. These different optimum pH values of Ni(II) and Co(II) ions adsorption may be caused by the presence of various functional groups on the surface of activated carbons, mainly acidic in the case of oxidized carbon and alkaline in the case of outgassed carbon (Dobrowolski, 1998). Moreover, the surface charge density of oxidized carbon is negative up to pH around 3.5 units, while the outgassed carbon indicates a positive surface charge in a very wide range of pH values (Dobrowolski et al., 1986). 3.2. Kinetic study The time effect on adsorption of Ni(II) and Co(II) onto the modified activated carbons is shown in Fig. 2. The studies suggest that the adsorption process is rather quick and close to the equilibrium after 10 min in all cases. After that time, slight changes of adsorption values are observed within the precision limits of measuring technique. Finally, the shaking time of 20 min was chosen to ensure adsorption equilibrium in all cases. 3.3. Effect of NO3 and Cl In order to assess the possibility of analytical applications of the presented preconcentration procedure, the effect of oxidants and reducing agents, e.g. NO3 and Cl, which can interfere in the determination of Ni(II) and Co(II) ions in solid materials after digestion with e.g. aqua regia, was studied under the optimized conditions. In Fig. 3 the influence of nitrates and chlorides on the adsorption of Ni(II) and Co(II) ions is shown. In the case of OGC* and DMGC* carbons, these ions do not interfere in the sorption of Ni(II) and Co(II) ions and at higher concentrations of these interferents the adsorption ability increases slightly, except the DMGC* carbon, where the 0.001 mol/L concentration of chlorides causes slight decrease of Ni(II) and Co(II) adsorption. In turn, in the case of OC* carbon, the drastic decrease of Ni(II) and Co(II) adsorption, even over 90%, is observed. This may be caused by the competitiveness of Cl, NO3, Na(I), K(I), Ni(II) and Co(II) ions towards the adsorption centers of the activated carbon.

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Fig. 1. The influence of initial and equilibrium pH on (a) Ni(II) and (b) Co(II) adsorption onto OC* (^), OGC* (&), DMGC* (~); m = 0.2 g, V = 50 mL, t = 20 min, T = 25 8C, CNi(OC*) = 4.95 mg/L, CNi(OGC*) = 0.23 mg/L, CNi(DMGC*) = 0.46 mg/L, CCo(OC*) = 5.00 mg/L, CCo(OGC*) = 0.30 mg/L, CCo(DMGC*) = 0.50 mg/L.

3.4. Equilibrium isotherm study The adsorption ability of modified activated carbons has been determined by studying the initial runs of adsorption isotherms of Ni(II) and Co(II) ions from the aqueous solutions, which is shown in Fig. 4. The best sorption ability for both types of ions is exhibited by the OC* carbon and the worst by the OGC* carbon. The maximum sorption capacity evaluated from these adsorption isotherms equals 7.8 and 11.1 mg/g (0.13 and 0.19 mmol/g) for the OC* carbon, 4.0 and 3.1 mg/g (0.07 and 0.05 mmol/g) for the OGC* carbon and 5.1 and 4.6 mg/g (0.09 and 0.08 mmol/g) for the DMGC* carbon for Ni(II) and Co(II) ions, respectively. As follows the way of activated carbon modification affects their adsorption capacities. It is worth to mention that sorption capacity for the DMGC* carbon is much higher when compared with the BSPDI impregnated activated carbon (2.1 mg/g for both ions) (Ghaedi et al., 2008). The great adsorption properties of OC* carbon towards Ni(II) and Co(II) ions are probably caused by the presence of humic acids on its surface, which in the form of Ni(II) and Co(II) complexes can adsorb on the carbon active centers (Dobrowolski and Otto, 2010). Despite excellent sorption capacity, that carbon has not been selected for enrichment of nickel and cobalt ions from certified reference materials because of insufficient selectivity in NO3 and Cl matrix. 3.5. Desorption study The desorption kinetic studies of Ni(II) and Co(II) ions by 6 M HCl from the modified activated carbons and the desorption studies of Ni(II) and Co(II) in relation to the HCl and HNO3 concentration (Fig. 5) show that in the case of the OC* carbon desorption of the studied metal ions is the fastest and the largest. The desorption of Ni(II) ions after 10 min exceeds 85%, and after 50 min reaches almost 100%. In the case of Co(II) ions the desorption reaches about 90%. For the OGC* carbon only 60% of nickel and 35% of cobalt desorb from the activated carbon to 6 mol/

Fig. 2. Adsorption kinetics of (a) Ni(II) and (b) Co(II) onto OC* (^), OGC* (&), DMGC* (~); m = 0.2 g, V = 50 mL, pH optimal, T = 25 8C, CNi(OC*) = 4.95 mg/L, CNi(OGC*) = 0.23 mg/L, CNi(DMGC*) = 2.07 mg/L, CCo(OC*) = 5.00 mg/L, CCo(OGC*) = 0.30 mg/L, CCo(DMGC*) = 1.50 mg/L.

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Fig. 3. The influence of NO3 and Cl on (a) Ni(II) and (b) Co(II) adsorption onto OC* (^), OGC* (&), DMGC* (~); m = 0.2 g, V = 50 mL, pH optimal, t = 20 min, T = 25 8C, CNi(OC*) = 9.9 mg/L, CNi(OGC*) = 6.4 mg/L, CNi(DMGC*) = 9.2 mg/L, CCo(OC*) = 10.0 mg/L, CCo(OGC*) = 12.0 mg/L, CCo(DMGC*) = 15.0 mg/L.

L HCl solution. The adsorption of Ni(II) and Co(II) ions on the modified activated carbons is probably carried out according to the ion exchange and the chelates formation in the case of carbon OC* and surface precipitation with the basic surface functional groups

in the case of OGC* and DMGC* carbons, which for the DMGC* carbon is reinforced by a complexation reaction of nickel and cobalt ions with DMG. It is confirmed by the increase of nickel and cobalt desorption from this carbon by HCl and HNO3 due to good solubility of the Ni(II)-DMG and Co(II)-DMG complex. The desorption studies of Ni(II) and Co(II) ions with respect to the concentration of HCl and HNO3 confirmed that even the application of concentrated nitric or hydrochloric acid did not cause the total desorption of the studied ions from the modified activated carbons. From the analytical point of view, low desorption of Ni(II) and Co(II) from the activated carbons indicates that the most effective technique for determination of nickel and cobalt in the environmental samples after their enrichment on the activated carbon, is the slurry sampling atomic absorption spectrometry with electrothermal atomization (GFAAS). 3.6. Influence of activated carbon on the analytical signal of nickel and cobalt

Fig. 4. Initial runs of adsorption isotherms of (a) Ni(II) and (b) Co(II) onto OC* (^), OGC* (&), DMGC* (~); m = 0.2 g, V = 50 mL, pH optimal, t = 20 min, T = 25 8C.

The activated carbon matrix influence on the shape of the nickel and cobalt analytical signal after their adsorption from aqueous solutions and determination using the carbon slurry sampling GFAAS technique was carried out by comparing the shapes of the analytical signals obtained for Ni(II) of the 30.0 mg/ L and Co(II) of 12.5 mg/L standard solutions and those obtained for the suspensions of activated carbon containing about 29.9 mg/g of Ni(II) and 12.7 mg/g of Co(II). In Fig. 6 despite the same integrated absorbance, the decrease and slight delay of nickel and cobalt analytical signals formation, in the case of slurry sampling of activated carbon containing these ions towards analytical signals obtained for their standard solution, are observed. It is probably caused by the changes in the kinetics of analyte evaporation from the graphite tube surface in the case of standard solutions and the evaporation of nickel and cobalt from the activated carbon particles in the case of slurry sampling.

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Fig. 5. Desorption of (a) Ni(II) and (b) Co(II) from OC* (^), OGC* (&), DMGC* (~) with respect to HNO3 and HCl concentration; m = 0.005 g, V = 1.25 mL, t = 2 h, T = 25 8C, ANi(OC*) = 123.8 mg/g, ANi(OGC*) = 56.0 mg/g, ANi(DMGC*) = 163.9 mg/g, ACo(OC*) = 123.7 mg/g, ACo(OGC*) = 123.8 mg/g, ACoDMGC*) = 123.2 mg/g.

Table 2 Analytical characteristics of the proposed procedure.

Curve slope (aqueous solutions) Curve slope (carbon slurries) LOD [mg/g] LOQ [mg/g] RSD [%]

Ni

Co

0.0072 0.0072 0.02 0.06 6

0.0121 0.0121 0.001 0.004 7

3.7. Validation studies The calibration curves for nickel and cobalt determination have been established with a blank and five calibration solutions in the concentration range of 5.0–40.0 and 5.0–30.0 mg/L, respectively. The comparison of calibration curve slopes obtained for aqueous solutions and various percentage slurries prepared from the

Fig. 6. The influence of DMGC* loaded activated carbon on analytical signal of (a) Ni(II) and (b) Co(II); (- - -) standard solution, (—) activated carbon, CNi = 30.0 mg/L, CNi(DMGC*) = 29.9 mg/L, CCo = 12.5 mg/L, CCo(DMGC*) = 12.7 mg/L, mNi(DMGC*) = 0.01036 g, mCo(DMGC*) = 0.02230 g, V = 1.8 mL.

Fig. 7. Relationship between integrated absorbance of Ni (^) and Co (&) and percentage of activated carbon slurry concentration.

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Table 3 Results of nickel and cobalt determination in certified reference material samples by slurry sampling GFAAS technique after preconcentration on DMGC* carbon. Certified reference materials

Ni Certified value [mg/g]

Slurry sampling [mg/g]

Certified value [mg/g]

Slurry sampling [mg/g]

Spinach leaves NIST 1570a Spinach NCS ZC73013 Mixed Polish herbs INCT-MPH-2 Tea leaves INCT-TL-1

2.14  0.10 0.92  0.12 1.57  0.16 6.12  0.52

2.13  0.07* 0.90  0.06* 1.49  0.07* 6.06  0.37*

0.39  0.05 0.22  0.03 0.21  0.03 0.39  0.04

0.42  0.02* 0.25  0.02* 0.23  0.01* 0.41  0.02*

*

Co

Standard deviation (n = 5).

DMGC* adsorbed with Ni and Co carbon confirmed that in the case of the absorbance measurements it is possible to use the standard calibration method for quantitative determination of nickel and cobalt after enrichment on activated carbons. The suitable data, such as: curve slopes, limit of detection (LOD, calculated as the average of 5 blank samples signals plus three times as large standard deviation of the signals obtained from 5 blank samples, expressed on the basis of sample treatment for 5% carbon slurry, 0.2 g adsorbent for adsorption system and 0.5 g samples for digestion), limit of quantifications (LOQ, calculated as the average of 5 blank samples signals plus ten times as large standard deviation of the signals obtained from 5 blank samples also expressed on the basis of sample treatment) are presented in Table 2. The application of high slurry concentration can improve the detection limit of the analytical procedure. The obtained data for LOQs are similar for those obtained using ICP-MS (Millour et al., 2011). The studies confirmed that the slurry concentrations up to 10% could be used in both cases (Fig. 7) (Shiowatana et al., 2000). Due to great sorption capacity for Ni(II) and Co(II) ions and good selectivity, the DMGC* carbon has been selected for enrichment and determination of these ions from certified reference materials. In order to assess the accuracy and validity of the developed procedure, the method was applied for separation and determination of nickel and cobalt in spinach leaves (NIST 1570a and NCS ZC73013), mixed Polish herbs (INCTMPH-2) and tea leaves (INCT-TL-1). The analytical data are summarized in Table 3. The obtained data are in good agreement with certified values, that was confirmed by using Student’s ttest (Konieczka and Namies´nik, 2009). The precision of nickel and cobalt determination by the presented method can be regarded as acceptable for this application. The relative standard deviations (RSD%) for five replicate measurements were about 7%. 4. Conclusions The application of activated carbons for enrichment of Ni(II) and Co(II) requires careful modification in order to obtain a selective adsorbent. Generally, chemical modification of the activated carbons improves their adsorption capacity. The studied activated carbons are characterized by different nickel and cobalt sorption capacities. The OC* carbon possesses the largest maximum sorption capacity, but it is the worst for analytical application because of its nonselectivity towards other ions. Although the adsorption capacities of the OGC* and DMGC* carbons are lower than the OC* carbon, the Ni(II) and Co(II) adsorption onto them was almost independent of nitrates and chlorides. The application of DMG loaded activated carbon is a sensitive, useful and accurate method for the Ni(II) and Co(II) ions preconcentration from aqueous solutions and their determination in food samples using the carbon slurry sampling GFAAS technique. This technique seems to be excellent for this purpose. Additionally, the application of concentrated carbon slurry can improve its limit detection significantly.

References Aouarram, A., Galindo-Riano, M.D., Garcia-Vargas, M., Stitou, M., El Yousfi, F., 2007. A permeation liquid membrane system for determination of nickel in seawater. Talanta 71, 165–170. Camel, V., 2003. Solid phase extraction of trace elements. Spectrochimica Acta Part B 58, 1177–1233. Cerutti, S., Moyano, S., Gasquez, J.A., Stripeikis, J., Olsina, R.A., Martinez, L.D., 2003. On-line preconcentration of cobalt in drinking water using a minicolumn packed with activated carbon coupled to electrothermal atomic absorption spectrometric determination. Spectrochimica Acta Part B 58, 2015–2021. Cerutti, S., Moyano, S., Marrero, J., Smichowski, P., Martinez, L.D., 2005. On-line preconcentration of nickel on activated carbon prior to its determination by vapor generation associated to inductively coupled plasma optical emission spectrometry. Journal of Analytical Atomic Spectrometry 20, 559–561. Chakrapani, G., Murty, D.S.R., Mohanta, P.L., Rangaswamy, R., 1998. Sorption of PAR–metal complexes on activated carbon as a rapid preconcentration method for the determination of Cu, Co, Cd, Cr, Ni Pb and V in ground water. Journal of Geochemical Exploration 63, 145–152. Chen, J.P., Lin, M., 2001. Surface charge and metal ion adsorption on an H-type activated carbon: experimental observation and modeling simulation by the surface complex formation approach. Carbon 39, 1491–1504. Dadfarnia, S., Shabani, A.M.H., Bidabadi, M.S., Jafari, A.A., 2010. A novel ionic liquid/ micro-volume back extraction procedure combined with flame atomic absorption spectrometry for determination of trace nickel in samples of nutritional interest. Journal of Hazardous Materials 173, 534–538. Daorattanachai, P., Unob, F., Imyim, A., 2005. Multi-element preconcentration of heavy metal ions from aqueous solution by APDC impregnated activated carbon. Talanta 67, 59–64. Dobrowolski, R., 1998. Application of activated carbons for the enrichment of toxic metals and their determination by atomic spectroscopy. In: Da˛browski, A. (Ed.), Adsorption and Its Application in Industry and Environmental Protection. Elsevier Inc., Amsterdam, Netherlands, pp. 777–806. Dobrowolski, R., Jaroniec, M., Kosmulski, M., 1986. Study of Cd(II) adsorption from aqueous solution on activated carbons. Carbon 24, 15–20. Dobrowolski, R., Otto, M., 2010. Study of chromium(VI) adsorption onto modified activated carbons with respect to analytical application. Adsorption 16, 279– 286. Dolan, S.P., Capar, S.G., 2002. Multi-element analysis of food by microwave digestion and inductively coupled plasma-atomic emission spectrometry. Journal of Food Composition and Analysis 15, 593–615. Duran, A., Tuzen, M., Soylak, M., 2009. Preconcentration of some trace elements via using multiwalled carbon nanotubes as solid phase extraction adsorbent. Journal of Hazardous Materials 169, 466–471. Ferreira, S.L.C., Miro, M., da Silva, E.G.P., Matos, G.D., dos Reis, P.S., Brandao, G.C., dos Santos, W.N.L., Duarte, A.T., Vale, M.G.R., Araujo, R.G.O., 2010. Slurry samplingan analytical strategy for the determination of metals and metalloids by spectroanalytical techniques. Applied Spectroscopy Reviews 45, 44–62. Ghaedi, M., Ahmadi, F., Soylak, M., 2007a. Preconcentration and separation of nickel copper and cobalt using solid phase extraction and their determination in some real samples. Journal of Hazardous Materials 147, 226–231. Ghaedi, M., Ahmadi, F., Shokrollahi, A., 2007b. Simultaneous preconcentration and determination of copper, nickel cobalt and lead ions content by flame atomic absorption spectrometry. Journal of Hazardous Materials 142, 272– 278. Ghaedi, M., Karamia, B., Ehsani Sh Marahel, F., Soylak, M., 2009b. Preconcentration– separation of Co2+, Ni2+ Cu2+ and Cd2+ in real samples by solid phase extraction of a calix[4] resorcinarene modified Amberlite XAD-16 resin. Journal of Hazardous Materials 172, 802–808. Ghaedi, M., Shokrollahi, A., Kianfar, A.H., Mirsadeghi, A.S., Pourfarokhi, A., Soylak, M., 2008. The determination of some heavy metals in food samples by flame atomic absorption spectrometry after their separation-preconcentration on bis salicyl aldehyde, 1,3 propan diimine (BSPDI) loaded on activated carbon. Journal of Hazardous Materials 154, 128–134. Ghaedi, M., Tavallali, H., Shokrollahi, A., Zahedi, M., Montazerozohori, M., Soylak, M., 2009a. Flame atomic absorption spectrometric determination of zinc, nickel, iron and lead in different matrixes after solid phase extraction on sodium dodecyl sulfate (SDS)-coated alumina as their bis (2-hydroxyacetophenone)1,3-propanediimine chelates. Journal of Hazardous Materials 166, 1441–1448. Gil, R.A., Gasquez, J.A., Olsina, R., Martinez, L.D., Cerutti, S., 2008. Cloud point extraction for cobalt preconcentration with on-line phase separation in a

R. Dobrowolski, M. Otto / Journal of Food Composition and Analysis 26 (2012) 58–65 knotted reactor followed by ETAAS determination in drinking waters. Talanta 76, 669–673. Greenwood, M.M., Earnshows, A., 1988. Chemistry of the Elements. Elsevier Inc., Amsterdam, Netherlands. Guo, Y., Din, B., Liu, Y., Chang, X., Meng, S., Liu, J., 2004. Preconcentration and determination of trace elements with 2-aminoacetylthiophenol functionalized Amberlite XAD-2 by inductively coupled plasma-atomic emission spectrometry. Talanta 62, 209–215. Haji Shabani, A.M., Dadfarnia, S., Dehghan, K., 2003. On-line preconcentration and determination of cobalt by chelating microcolumns and flow injection atomic spectrometry. Talanta 59, 719–725. Hannachi, Y., Affanacivich Shapovalov, N., Hannachi, A., 2010. Adsorption of nickel from aqueous solution by the use of low-cost adsorbents. Korean Journal of Chemical Engineering 27, 152–158. Jorgensen Cassella, R., Salim, V.A., Jesuino, L.S., Santelli, R.E., Ferreira, S.L.C., de Carvalho, M.S., 2001. Flow injection determination of cobalt after its sorption onto polyurethane foam loaded with 2-(2-thiazolylazo)-p-cresol (TAC). Talanta 54, 61–67. Karacan, M.S., Aslantas, N., 2008. Simultaneous preconcentration and removal of iron, chromium, nickel with NN0 -etylenebis-(ethane sulfonamide) ligand on activated carbon in aqueous solution and determination by ICP-OES. Journal of Hazardous Materials 155, 551–557. Kilicel, F., Gokalp, F., 2005. AAS determination of some trace heavy metals in Lake Van water after preconcentration by complexation with surface functional groups on activated carbon. Fresenius Environmental Bulletin 14, 124–129. Kiran, K., Suresh Kumar, K., Suvardhan, K., Janardhanam, K., Chiranjeevi, P., 2007. Preconcentration and solid phase extraction method for the determination of Co, Cu, Ni Zn and Cd in environment al and biological samples using activated carbon by FAAS. Journal of Hazardous Materials 147, 15–20. Konieczka, P., Namies´nik, J., 2009. Quality Assurance and Quality Control in the Analytical Chemical Laboratory: A Practical Approach. CRC Press, Boston, MA, USA. Lemos, V.A., Franca, R.S., Moreira, B.O., 2007. Cloud point extraction for Co and Ni determination in water samples by flame atomic absorption spectrometry. Separation and Purification Technology 54, 349–354. Lemos, V.A., Santos, L.N., Bezerra, M.A., 2010. Determination of cobalt and manganese in food seasonings by flame atomic absorption spectrometry after preconcentration with 2-hydroxyacetophenone-functionalized polyurethane foam. Journal of Food Composition and Analysis 23, 277–281. Lison, D., 2007. In: Nordberg, G.F., Fowler, B.A., Nordberg, M., Friberg, L.T. (Eds.), Handbook on the Toxicology of Metals. 3rd ed. Academic Press Inc., Waltham, MA, USA, pp. 511–528. Mikuła, B., Puzio, B., 2007. Determination of trace metals by ICP-OES in plant materials after preconcentration of 1,10-phenanthroline complexes on activated carbon. Talanta 71, 136–140. Millour, S., Noel, L., Kadar, A., Chekri, R., Vastel, C., Guerin, T., 2011. Simultaneous analysis of 21 elements in foodstuffs by ICP-MS after closed-vessel microwave

65

digestion: method validation. Journal of Food Composition and Analysis 24, 111–120. Narin, I., Soylak, M., 2003. Enrichment and determinations of nickel(II), cadmium(II), copper(II), cobalt(II) and lead(II) ions in natural waters, table salts tea and urine samples as pyrrolydine dithiocarbamate chelates by membrane filtration–flame atomic absorption spectrometry combination. Analytica Chimica Acta 493, 205–212. Narin, I., Soylak, M., Elci, L., Dogan, M., 2000. Determination of trace metal ions by AAS in natural water samples after preconcentration of pyrocatechol violet complexes on an activated carbon column. Talanta 52, 1041–1046. Pourreza, N., Zolgharnein, J., Kiasat, A.R., Dastyar, T., 2010. Silica gel–polyethylene glycol as a new adsorbent for solid phase extraction of cobalt and nickel and determination by flame atomic absorption spectrometry. Talanta 81, 773–777. Sahin, C.A., Efecinar, M., Satıroglu, N., 2010. Combination of cloud point extraction and flame atomic absorption spectrometry for preconcentration and determination of nickel and manganese ions in water and food samples. Journal of Hazardous Materials 176, 672–677. Saracoglu, S., Soylak, M., 2010. Carrier element-free coprecipitation (CEFC) method for separation and pre-concentration of some metal ions in natural water and soil samples. Food and Chemical Toxicology 48, 1328–1333. Saraji, M., Yousefi, H., 2009. Selective solid-phase extraction of Ni(II) by an ionimprinted polymer from water samples. Journal of Hazardous Materials 167, 1152–1157. Shiowatana, J., Benyatianb, K., Siripinyanond, A., 2000. Determination of Cd, Co, Hg and Ni in seawater after enrichment on activated carbon by slurry sampling electrothermal AAS. Atomic Spectroscopy 21, 179–186. Shirani Bidabadi, M., Dadfarnia, S., Hajishabani, A.M., 2009. Solidified floating organic drop microextraction (SFODME) for simultaneous separation/preconcentration and determination of cobalt and nickel by graphite furnace atomic absorption spectrometry (GFAAS). Journal of Hazardous Materials 166, 291– 296. Soylak, M., Elci, L., Dogan, M., 1996. Determination of some trace metal impurities in refined and unrefined salts after preconcentration onto activated carbon. Fresenius Environmental Bulletin 5, 148–155. Tuzen, M., Soylak, M., Citak, D., Ferreira, H.S., Korn, M.G.A., Bezerra, M.A., 2009. A preconcentration system for determination of copper and nickel in water and food samples employing flame atomic absorption spectrometry. Journal of Hazardous Materials 162, 1041–1045. Yaman, M., Gucer, S., 1998. Determination of nickel in vegetable matrices by atomic absorption spectrometry after preconcentration on activated carbon. Annali di Chimica 88, 555–565. Yunes, N., Moyano, S., Cerutti, S., Gasquez, J.A., Martinez, L.D., 2003. On-line preconcentration and determination of nickel in natural water samples by flow injection-inductively coupled plasma optical emission spectrometry (FI-ICPOES). Talanta 59, 943–949.