Food Anal. Methods DOI 10.1007/s12161-014-9964-x
Hybrid Amine-Functionalized Titania/Silica Nanoparticles for Solid-Phase Extraction of Lead, Copper, and Zinc from Food and Water Samples: Kinetics and Equilibrium Studies Maryam Rajabi & Behruz Barfi & Alireza Asghari & Farhood Najafi & Reza Aran
Received: 7 April 2014 / Accepted: 7 August 2014 # Springer Science+Business Media New York 2014
Abstract In the present study, hybrid amine-functionalized titania/silica nanoparticles were employed as a new and novel adsorbent for solid-phase extraction of Pb2+, Cu2+, and Zn2+ ions prior to their determination using flame atomic absorption spectrometry. Under the best conditions (including adsorbent, 0.4 g; eluent, 5.0 mL nitric acid (HNO3), 3.0 mol L−1, 1.0 mL min−1; and sample, pH 5.0, 3.0 mL min−1), detection limits, adsorption capacities, and preconcentration factors were 0.12–0.24 μg L−1, 7.1–20.7 mg g−1, and 200, respectively. To predict the adsorption isotherms, different isotherm models were studied and the obtained results showed that the Langmuir model is the most suitable one to explain the experimental data. The kinetics of the reaction followed pseudosecond-order kinetic model. Thermodynamic parameters like free energy (ΔG0) and enthalpy (ΔH0) confirmed the spontaneous and exothermic nature of the process. The method was successfully applied for determination of the analytes in different food and water samples. Keywords Hybrid amine-functionalized titania/silica nanoparticles . Food . Water . Solid-phase extraction . Kinetic . Thermodynamic
Introduction Heavy metals are known for their nonbiodegradability and accumulation in living systems, causing serious diseases and M. Rajabi (*) : B. Barfi : A. Asghari : R. Aran Department of Chemistry, Semnan University, Semnan 35195-363, Iran e-mail:
[email protected] F. Najafi Department of Resin and Additives, Institute for Color Science and Technology, Tehran, Iran
disorders. These metals are widely found in the environment and will eventually enter food chain, releasing from industrial activity (industrial wastes, metal manufacturing, incineration plants, electroplating, fuel combustion, inter alia) and farm work. Some heavy metals at trace concentrations are essential elements and play important roles in human metabolism. Meanwhile, at higher concentrations, most of them may be toxic. Zinc (Zn) and copper (Cu) species are commonly used in various industrial processes such as production of corrosionresistant alloys and brass for galvanizing steel, iron products, and dye pigments (Elinder et al. 1986; Knopfel et al. 2005). Also, these elements are involved in catalytic, enzymatic, and structural activities in the organism and have to be taken from outside the organism with food and water. However, their accumulation in tissues can cause progressively various toxic effects, including rheumatoid arthritis, abnormal pregnancies, malignancies, hypocalcemia, and bone resorption (Broos et al. 2007; Mertens et al. 2010). Lead (Pb) is also classified as a prevalent toxic metal, which constitutes a major environmental health problem. Typical symptoms of lead poisoning are abdominal pain, anemia, headaches and convulsions, kidney chronic nephritis, brain damage, and central nervous system disorders (Kazi et al. 2010; Korn et al. 2006). When the blood lead concentration is near 0.80 μg mL−1 or greater, basophilic stripping occurs in erythrocytes. Since these metal ions are widely used as chemical materials in modern society, they are widespread in the environmental water and food samples (Medeiros et al. 2012). Consequently, some international organizations, such as the Food and Agriculture Organization (FAO) and World Health Organization (WHO), have regulated the maximum allowable concentrations (MACs) or maximum permitted concentrations of heavy metals in foodstuffs for evaluating their safety (USEPA 1997, 2002a, b; WHO 1990, 1992, 1995). In this way, their accurate and sensitive determination using a simple
Food Anal. Methods
and selective method is the most significant task in analysis (Yildiz et al. 2011). Flame atomic absorption spectrometry (FAAS) has been widely used for the determination of Pb, Zn, and Cu ions due to the relatively simple and inexpensive equipment required (Asghari and Mohammadi 2013; Ghaedi et al. 2013a, b, 2014; Ozturk et al. 2011; Rajabi et al. 2014; Wadhwa et al. 2014). However, direct determination of these ions at trace levels by FAAS is limited, not only due to insufficient sensitivity but also to matrix interferences (Wang et al. 2012; Welna et al. 2013). Due to some advantages such as high enrichment factors, emulsion absence, safety with respect to hazardous samples, flexibility, and automation ease (Das et al. 2012), solid-phase extraction (SPE) has found increasing application for the preconcentration of trace metals and the elimination of matrix interferences prior to FAAS determination. The extraction ability of SPE method is directly related to the properties of adsorbent materials, which determines the selectivity and sensitivity of the method. The main requirements of an adsorbent are as follows: (i) the possibility to extract a large number of trace elements over a wide pH range, (ii) the fast and quantitative adsorption and elution, (iii) high capacity, and (iv) regenerability (Camel 2003). In the SPE method, many different materials such as polymeric resins, modified resins (by complexing agent), silica, carbon nanotubes, metal oxides with nanometer sizes, and nanohybrid materials are used as solid phase (Ciftci 2010; Ghaedi et al. 2007, 2013a, b, 2010; Mahmoodi and Najafi 2012; Mahmoud et al. 2010; Yildiz et al. 2011). Recently, nanohybrid materials due to their excellent properties as new functional materials in photonic catalysts, highperformance electronic materials, and so on have attracted much attention. Consequently, many studies have been conducted to prepare hybrid colloids, especially those that are organic and inorganic (Choi and Bae 2007; Cong and Yu 2009; Umeda et al. 2009). Among a great variety of these materials, hybrid titania/silica nanoparticles are of particular interest for the adsorption of certain metal ions, because of their cation exchange properties. Although various functional groups with selective adsorption affinity to targeted adsorbates have been introduced to adsorbent by surface modifications (Zub et al. 2005), there is no report about the application of hybrid functionalized titania/silica nanoparticles (hybrid SiO2/TiO2-NH2 nanoparticles) as an adsorbent for heavy metal ions in the literature. In the present study, for the first time, an efficient SPE method (based on using hybrid SiO2/TiO2-NH2 nanoparticles as a novel adsorbent) combined with flame atomic absorption spectrometry was investigated for preconcentration and determination of Pb2+, Zn2+, and Cu2+ at trace levels in different food and water samples. Additionally, the adsorption kinetics and isotherms for these metal ions were also modeled to study the influence of various parameters on the adsorption process.
Experimental Instrumentation The evaluation of the metal ions content was performed using a Shimadzu atomic absorption spectrometer (AA-680, Japan) equipped with a hollow cathode lamp and a deuterium background corrector at respective wavelengths using an air–acetylene flame according to the instrument parameters recommended by the manufacturer. A PHS-3BW model pH meter (Bell, Italy) was used for pH adjustment. A LAMBDA CZs.ro multiflow peristaltic pump (LAMBDA, Switzerland) and a polytetrafluoroethylene (PTFE) column (25 mm×7.0 mm i.d.) were used with small natural cotton (Kave Company, Iran) at both ends. Chemicals and Reagents All chemicals were of analytical reagent-grade purity and purchased from Merck (Darmstadt, Germany). Stock solutions were prepared by dissolving appropriate amounts of ultrapure metal ions salt in double-distilled deionized water. The adsorbent (hybrid SiO2/TiO2-NH2 nanoparticles) was synthesized in our laboratory (Fig. 1) (Mahmoodi and Najafi 2012). Working standard solutions were prepared daily by suitable stepwise dilution of stock solutions with distilled water. The pH was adjusted by 0.10 mol L−1 nitric acid (HNO3) or 0.10 mol L−1 sodium hydroxide (NaOH). An acetic acid/acetate buffer (pH=5.0) was used. All laboratory glassware was cleaned with 5.0 % (v/v) HNO3 solution and then rinsed with double-distilled deionized water. Sample Preparation Lettuce and Coriander Samples The lettuce and coriander leaves were rinsed with doubledeionized distilled water and, after drying, were taken in small mesh. About 5 g of each oven-dried plant samples was transferred into a silica crucible; mineralization was carried out by heating it slowly to 550 °C for 2 h and keeping at this temperature for further 2 h. The ash was moistened with 8 mL of water, followed by an addition of 5 mL of HNO3 and 3 mL of concentrated hydrochloric acid to ensure complete digestion. The mixture was heated on a hot plate to near dryness. The solid residue was dissolved in hot water and was filtered; keeping the pH at 5.0 made up to 100.0 mL by addition of diluted KOH, acetate buffer, and distilled water. Then, the proposed SPE method was applied. Grape Juice, Apple Juice, and Water Samples Water, grape, and apple juice samples were collected from different cities of Iran and analyzed as soon as possible after
Food Anal. Methods Fig. 1 Schematic diagram for the synthesis of hybrid aminefunctionalized silica/titania nanoparticles
sampling. The organic content was oxidized in the presence of 10.0 % (w/v) H2O2 and concentrated nitric acid, and after filtration with a filter paper (Whatman No. 42), the resultant was stored at 4 °C in the dark. Before use, pH of the filtrate was adjusted at 5.0 and the SPE method was then performed. Method Validation The proposed method was utilized to determine lead, zinc, and copper in three standard reference alloy materials: Al/Cu/Si (MBH-C55X), LTD (MBH-C51X), and NBS SRM 85b. The analysis of certified reference materials (alloy samples) was conducted as follows: 10–12 mL of HNO3/HCl (1:3) mixture with a few drops of concentrated hydrofluoric acid (HF) was added, and the mixture was heated to achieve a complete dissolution of the components. After digestion, the resulting solution was filtered using a filter paper Whatman No. 1 and HF solution was added to the filtrate. Finally, the solution was diluted with water to 100 mL in a 100-mL volumetric flask and the SPE method was then performed. Adsorption–Desorption Experiments pH-adjusted sample solutions (100 mL) were passed through the SPE column, containing 0.40 g of adsorbent at a flow rate of 3.0 mL min−1, to deposit the analytes on the adsorbent. The adsorbed metal ions were then eluted with 5.0 mL of HNO3 (3.0 mol L−1) and determined by FAAS. After adsorption process, 0.4 g of the metal ions of loaded adsorbent was treated with 5.0 mL of 3.0 mol L−1 nitric acid for 10 min in a thermostated rotary mechanical shaker and the amounts of desorbed metal ions were then determined by FAAS. Each adsorption–desorption experiment was repeated three times.
adsorption capacity was considered as the maximum metal quantity taken up by 0.40 g of solid phase and given in milligrams of metal ion per gram adsorbent. In order to determine this parameter, 0.40 g of the adsorbent was weighed and added to a mixture containing 2.0–30.0 mg of the target analytes into a 100.0-mL measuring flask. The final mixture was then shaken at room temperature for 30 min using an automatic shaker. After equilibration, the mixture was filtered and washed three times with 100.0 mL double-distilled water. The adsorption data were then subjected to different adsorption isotherms, namely Langmuir, Freundlich, and Temkin, to determine the adsorbent capacity for each metal. The kinetic of the metal ions adsorption from an aqueous phase by any adsorbent can be explained using kinetic models, examining the rate controlling mechanism based on the adsorption process such as chemical reaction, diffusion control, and mass transfer. In order to investigate the adsorption kinetics, the batch method was applied. The adsorption of metal ions was carried out by agitation of 0.40 mg of the adsorbent suspended in 100.0 mL solution with 0.1 mg L−1 concentrations of Pb2+, Zn2+, and Cu2+ with constant stirring rate at optimum conditions. The preconcentration process was performed in the time ranging from 2 to 90 min. Then, the mixture was filtered and the required parameters were calculated. Thermodynamic parameters related to the adsorption process, i.e., Gibbs free energy change (ΔG0, kJ mol−1), enthalpy change (ΔH 0 , kJ mol −1 ), and entropy change (ΔS 0 , J mol−1 K−1), are obtained from the experiments at various temperatures. At optimum conditions, by mixing 0.40 g of the adsorbent and 100 mL metal ion solutions with initial concentrations of 0.1 mg L−1, the SPE method was carried out using a batch method in a temperature-controllable water bath at 288.15, 298.15, and 308.15 K.
Equilibrium, Kinetic, and Thermodynamic Studies
Results and Discussion
Equilibrium data, commonly known as sorption isotherms, are basic requirements for the design of adsorption systems. These data provide suitable information on the capacity of the adsorbent. The capacity of the adsorbent is an important factor to determine how much sorbent is required to quantitatively adsorb a specific amount of metal ion from solution. The
Optimization of Effective Parameters on the SPE Efficiency Effect of pH pH plays an important role on the quantitative recovery of the metal ions during the SPE procedure. Hence, influence of the
Food Anal. Methods
pH on the ions recovery in the range of 2.0–10.0 was investigated and the respective results are shown in Fig. 2. As can be seen, with raising pH, the recoveries enhance and reach the maximum value at pH 5.0. The low adsorption, which took place in the acidic region, can be attributed to the high tendency of proton in comparison to metal ions for binding to the active sites of adsorbent. For high pH values, the retention decreases again because of the competition between the formation of hydroxylated complexes of the metal ions and the active sites of the adsorbent. Hence, further experiments were performed at pH 5.0. Effect of the Amount of Adsorbent To investigate the optimum amount of the adsorbent on quantitative extraction of the understudy metal ions, extraction was conducted by varying the amounts of adsorbent from 0.20 to 0.60 g. Under the predetermined elution conditions (5 mL HNO3, 1 mol L−1), the best recoveries were obtained at 0.40 g of the adsorbent. It seems that at higher amounts of this quantity, the predetermined volume of elution solvent is not sufficient for an efficient elution of the target ions. It is clear that, at lower amounts of the adsorbent, recoveries decrease due to insufficient active sites for analytes retention. Therefore, 0.40 g of the adsorbent was used in the subsequent experiments. Effect of the Type, Concentration, and Volume of the Eluent The other important factors that affect the SPE efficiency are the type, concentration, volume, and flow rate of the eluent used for releasing the metal ions from the adsorbent surface. Optimization of the elution conditions was performed based on the obtained maximum recoveries with the minimal concentration and volume of the eluent.
In order to select a suitable eluent, different acids such as HCl, H2SO4, H3PO4, and HNO3 were tested and the obtained results showed that HNO3 was better than other eluents to desorb the analytes. The effect of eluent concentration for simultaneous elution of the analytes from the adsorbent was studied. The eluent concentration was studied between 1.0 and 6.0 mol L−1. In theory, higher acid concentration leads to higher recovery. However, when the acid concentration was larger than 3.0 mol L−1, the results showed no significant improvement. Moreover, in order to avoid the possible damage of the functionalized adsorbent, lower concentrations of HNO3 should be used. Therefore, a solution of 3.0 mol L−1 HNO3 was used as the eluent to achieve the best recoveries. Influence of the eluent volume was investigated, ranging from 2.0 to 10.0 mL, because the eluent volume plays a key role in SPE studies to achieve higher preconcentration factors. At eluent volumes lower than 5.0 mL, recoveries of the metal ions were not quantitative due to insufficient eluent volume. In this way, 5.0 mL of 3.0 mol L−1 HNO3 solution was found to be satisfactory for subsequent works. Effect of the Flow Rates of Eluent and Sample The flow rate of the sample affects the analytes adsorption, whereas the flow rate of the eluent affects the recoveries of the analytes. The flow rate of sample not only affects adsorption of the analytes but also controls the analysis time. The flow rate was examined in the range of 1.0–6.0 mL min−1. It was proved that the retention of the ions was not changed up to 3.0 mL min−1, and the recovery for the analytes was lower than 90 % over a 3.0 mL min−1 flow rate. Thus, the sample flow rate was set at 3.0 mL min−1. For quantitative desorption of the analytes with a small eluent volume, a low elution flow rate should be used. This provides sufficient time for equilibrium between the adsorbent and the eluent. In this way, effect of the elution flow rate on recovery of the analytes was investigated in the range of 0.5– 3.0 mL min−1. The results indicated that the analytes can be quantitatively recovered with a flow rate of 1.0 mL min−1. Hence, this flow rate was selected for further experiments. Effect of Sample Volume
Fig. 2 Effect of pH on the recoveries of Pb2+, Cu2+, and Zn2+ ions. Experimental conditions are as follows: adsorbent, 0.4 g; eluent, 5.0 mL HNO3, 3.0 mol L−1, 1.0 mL min−1; and sample, 100.0 mL, 3.0 mL min−1
One of the important parameters in the SPE method, which determines the maximum introduced volume of aqueous sample to the adsorbent, is the breakthrough volume. The higher breakthrough volumes would correspond to more suitable adsorbent efficiency. To explore the possibility of high enrichment of the analytes from a large sample volume, the breakthrough volume was investigated in the range of 50.0– 1,250.0 mL. The results showed that the breakthrough volume
Food Anal. Methods Fig. 3 Effect of sample volume on the recoveries of Pb2+, Cu2+, and Zn2+ ions. Experimental conditions are as follows: adsorbent, 0.4 g; eluent, 5.0 mL HNO3, 3.0 mol L−1, 1.0 mL min−1; and sample, 50.0 mL, pH 5.0, 3.0 mL min−1
could be up to 1,000.0 mL for Pb2+ and 750.0 mL for Zn2+ and Cu2+ ions (Fig. 3). However, considering the analysis time, a sample volume of 100 mL was used for the analysis of the real samples.
Matrix Effect One of the main problems in atomic absorption spectrometric determination and preconcentration separation studies of heavy metal ions is the matrix interferences. Hence, interferences that may be concomitant with interested metal ions were investigated. Solutions containing 50.0 μg L−1 of these analytes and different concentrations of other ions were prepared and subjected to the proposed SPE method. A foreign ion was considered as interference when its presence caused a variation of more than 5.0 % in the absorbance of the sample. Among the tested ions, Ni2+, Cr2+, Ba2+, Cd2+, Ca2+, Co2+, K+, Ag+, Na+, Li+, NO3−, Br−, I−, and SO42− at the concentration of 1,000.0 times; Fe2+ and Fe3+ at the concentration of 350.0 times; Mn2+ at the concentration of 275.0 times higher than those of three tested ions; as well as F− and Cl− at the concentration of 750.0 times higher than those of three tested ions did not show any interferences. These results demonstrated that the adsorbent was efficient at retaining the analytes and their effective separation from the potentially interfering matrix constituents.
Analytical Performances of the Method Under the optimum conditions, precision (in terms of relative standard deviations (RSDs)), linear ranges (LRs), limits of detection (LODs), limits of quantification (LOQs), and preconcentration factors (PFs) of the proposed SPE method were investigated (Table 1). Three replicate extractions were performed for each concentration level. A series of working solutions containing a mixture of the analytes at eight concentration levels in the range of 5.0– 800.0 μg L−1 were used to prepare calibration curves. The regression equations of calibration curves showed good linearity in the extensive range of concentrations. LOD and LOQ were statistically calculated as three and ten times of the standard deviations (seven replicate extractions of the minimum detectable concentration of the analytes) divided on the calibration slope. The PF was defined as the ratio between the volume of the initial sample (100.0 mL) and the final volume obtained after the extraction step (500.0 μL), before FAAS analysis. Also, the adsorption capacity (as the largest amounts of analytes can be adsorbed on the fixed amounts of the adsorbent) was adopted from the method recommended by contacting 5.0 mL of the metal ion solutions with the concentrations between 0.1 and 1.5 mg L−1 (Table 1). To evaluate the usefulness of the adsorbent, the method was compared to different solid adsorbents which have been reported recently in the literature. As can be seen in Table 2,
Table 1 The analytical characteristic of the method at the optimum conditions Ions
LODa (μ g L−1)
LOQa (μ g L−1)
LR (μ g L−1)
Correlation coefficient
RSDa (%)
PF
Adsorption capacity (mg g−1)
Pb2+
0.12
0.39
8–700
0.9946
3.1
200
19.1
2+
0.13 0.24
0.41 0.72
5–120 5–250
0.9923 0.9928
3.6 3.1
200 200
20.7 7.1
Zn Cu2+
Experimental conditions are as follows: adsorbent, 0.4 g; eluent, 5.0 mL HNO3, 3.0 mol L−1 , 1.0 mL min−1 ; and sample, pH 5.0, 3.0 mL min−1 a
n=7
Food Anal. Methods Table 2 Comparison of analytical features of the proposed method with other preconcentration methods for Pb2+, Zn2+, and Cu2+ ions PF
Sample Detection Reference volume (mL) system
Cu2+, Ni2+, Cd2+, Zn2+ 0.1–0.3
100
500
FAAS
(Ozcelik et al. 2012)
Octadecyl silica/DBzDA18C6
Ag1+, Pb2+, Pd2+
1.8–8.0
110
1,000
FAAS
Amberlite XAD-1180 resin/1-(2-thiazolylazo)-2-naphthol 2,4-Dinitrophenylhydrazine-SDS-Al2O3
Co2+, Cu2+, Cd2+, Pb2+, Mn2+, Ni2+ Pb2+ Zn2+ Pb2+
0.1–3.6
50–300 250–1,500
FAAS
2.8 1.6 0.61
63
200
FAAS
(Khayatian and Hassanpoor 2012) (Yilmaz and Kartal 2012) (Ghaedi et al. 2009)
125
250
FAAS
(Wang et al. 2012)
FAAS
(Kumar et al. 2000)
–
FAAS
(Soylak et al. 2011)
100
FAAS
This work
Adsorbent
Analyte
Silica gel/N-(2-aminoethyl)salicylaldimine
Graphene/dithizone
2+
Amberlite XAD-2/o-aminophenol
LOD (μ g L−1)
2+
2+
Cu , Co , Cd , 2.5–25.0 Ni2+, Zn2+, Pb2+ Cu2+, Zn2+, Pb2+, Cd2+ 0.25–8.9
Multiwalled carbon nanotube/(E)-N1(4-nitro-benzylidene)-N2-(2-((E)-4nitrobenzylideneamino)ethyl)ethane-1,2-diamine Amine-functionalized titania/silica nanohybrid Cu2+, Zn2+, Pb2+
40–100 1,000 20–80
0.12–0.24 200
FAAS flame atomic absorption spectrometry
the presented method exhibits a high preconcentration factor, low LOD, and good RSD in the most cases than the previously reported methods, which offer the better sensitivity and extraction efficiency than some of the other SPE methods. The proposed method was also utilized to determine Pb2+, 2+ Zn , and Cu2+ in three standard reference alloy materials: Al/ Cu/Si (MBH-C55X), LTD (MBH-C51X), and NBS SRM 85b (Table 3). As can be seen, the results are in good agreement with the certified values. It can be concluded that the proposed SPE method is free from interferences of the various constituents. Analysis of Food and Real Water Samples The suitability of the method for analysis of food and real water samples was studied by spiked lettuce and coriander leaves, grape and apple juices, and water samples with different concentrations of Pb2+, Zn2+, and Cu2+ ions. For the analysis of these samples, the standard addition method was used and the analytical results were given in Table 4. The
method was found to be an effective approach to improve the sensitivity of FAAS technique for determination of the understudy metal ions in different real samples. A good agreement was obtained between the added and measured amounts of the metal ions (recovery >92.3 %). These results confirmed the validity of the proposed SPE method (Table 4). Adsorption Study Adsorption Isotherms Adsorption isotherms are mathematical models that describe the distribution of the adsorbed solutes among liquid and sorbent, based on a set of assumptions that are mainly related to the heterogeneity/homogeneity of adsorbents, the type of coverage, and the possibility of interaction between the adsorbed solutes. The Langmuir isotherm model is one of the most wellknown and applied adsorption isotherms. This model assumes that there is no interaction between the adsorbed solutes, and
Table 3 Determination of lead, zinc, and copper in three industrial alloys Alloy
Al/Cu/Si (MBH-C55X) LTD (MBH-C51X) NBS SRM 85b
Certified value (%)
Found using the proposed method
Recovery (%)
Pb2+
Zn2+
Cu2+
Pb2+
Zn2+
Cu2+
Pb2+
Zn2+
Cu2+
0.19 0.08 0.021
2.46 0.15 0.04
3.16 0.13 3.99
0.181±(0.006) 0.079±(0.002) 0.0212±(0.001)
2.39±(0.07) 0.149±(0.005) 0.039±(0.001)
3.03±(0.09) 0.127±(0.004) 3.79±(0.1)
95 99 101
97 100 98
96 98 95
Alloy contents are as follows: (1) Al/Cu/Si (3.16 % Cu, 0.1 % Mg, 8.78 % Si, 0.64 % Fe, 0.18 % Mn, 0.67 % Ni, 2.46 % Zn, 0.19 % Pb, 0.26 % Sn, 0.09 % Ti, 0.11 % Cr, and 83.36 % Al), (2) LTD (0.13 % Cu, 0.04 % Mg, 0.48 % Si, 0.41 % Fe, 0.2 % Mn, 0.12 % Ni, 0.15 % Zn, 0.08 % Pb, 0.008 % Sn, 0.09 % Ti, 0.06 % Cr, and 98.23 % Al), (3) NBS SRM 85b (93.097 % Al, 0.61 % Mn, 0.18 % Si, 3.99 % Cu, 0.084 % Ni, 0.211 % Cr, 0.006 % V, 0.022 % Ti, 0.019 % Ga, 0.24 % Fe, 0.021 % Pb, 1.49 % Mg, and 0.040 % Zn). (Numbers in parenthesis are the standard deviation for four replicate measurements)
Food Anal. Methods Table 4 Application of the proposed method for determination of the metal ions in different samples (N=4) Ion
Added (μg L−1)
Tap watera Foundb (μg L−1)
Pb2+
Zn2+
Cu2+
0
Recovery (%)
6.3±3.2
40 80 0 40 80 0 40 80
Grape juice
43.2±3.1 84.2±2.6 42.5±2.8 81.5±2.6 120.6±2.4 8.2±3.4 48.1±2.8 87.0±2.5
− 92.3 97.4 − 97.5 97.6 − 99.8 98.5
Found (μg L−1)
Apple juice Recovery (%)
ND
−
40.4±2.8 78.8±2.1 16.3±3.1 56.1±2.1 96.2±2.8 4.2±3.1 43.3±2.2 81.1±3.1
100.5 97.0 − 99.0 99.8 − 95.5 92.3
Found (μg L−1)
Coriander Recovery (%) −
2.3±2.1 42.1±3.0 83.2±2.9 18.5±2.3 59.1±2.2 97.9±3.2 7.2±2.7 46.9±3.1 85.7±2.8
99.5 101.1 − 101.5 99.2 − 99.2 98.1
Found (μg L−1) 3.8±2.9 43.1±3.2 84.1±3.0 4.5±2.4 43.2±2.8 79.8±2.9 11.6±2.8 51.1±3.1 89.5±3.0
Lettuce Recovery (%) −
Found (μg L−1)
Recovery (%) −
4.8±2.8
98.2 100.3 − 96.8 94.1 − 98.8 97.4
43.1±2.4 82.7±2.3 68.4±2.5 106.8±2.8 146.8±2.6 18.4±2.7 56.3±2.9 95.6±3.0
95.7 97.4 − 96.0 98.0 − 94.8 96.5
For extraction conditions, see Table 1 ND not detected a
Semnan, Iran
b
Mean±relative standard deviation based on the three replicates
the adsorption is localized in a monolayer. The Freundlich isotherm model describes that the ratio of adsorbed solute on the solid surface to the solute concentration is a function of the solution concentration. This model is an empirical model to describe the multilayer adsorption on the adsorbent. The Temkin isotherm model suggests an equal distribution of binding energies over a number of exchange sites on the surface. The calculated Langmuir, Freundlich, and Temkin parameters and regression coefficients are presented in Table 5. The validity of each isotherm model can be checked by the fitness of the straight line (R2). Accordingly and as shown in Table 5, the R2 of the Langmuir isotherm was greater than that of the Freundlich and Temkin isotherms for the adsorption of all investigated metal ions. This indicates that the adsorptions of metal ions on the adsorbent are better described by the Langmuir model than the Freundlich and Temkin models.
Adsorption Kinetics To examine the controlling mechanism of the adsorption process, different kinetic models such as pseudo-first order and pseudo-second order, intraparticle diffusion, and Elovich equation are used. However, generally, two kinetic models, namely pseudo-first order and pseudo-second order, are used in literatures. As can be seen in Table 6, the R2 of the pseudo-secondorder model was greater than that of the other models for the adsorption of all examined metal ions. Therefore, pseudosecond-order model is the more valid for the adsorption kinetic process description. Thermodynamic Studies The values of thermodynamic parameters for analyte adsorption on the adsorbent can be determined from the temperature
Table 5 Isotherm parameters of adsorption at optimum conditions Ion
Langmuir, Cq e ¼ qC e þ K L q1 e
Pb2+ Zn2+ Cu2+
max
Freundlich, logqe =logKf +nlogCe
Temkin, qe =BlnA+BlnCe
max
R2
qmax (mg g−1)
KL (L mg−1)
R2
n
Kf
R2
A
B
0.9976 0.9936 0.9927
19.1 20.7 7.1
0.120 0.962 0.092
0.9801 0.9963 0.9608
1.36 0.341 0.521
1.98 0.342 0.551
0.9159 0.8818 0.9441
2.95 0.262 0.152
2.99 0.472 0.231
qe amount of adsorbed metal per unit weight of the adsorbent (mg g−1 ) at equilibrium conditions, Ce equilibrium concentration in the solution (mg L−1 ), qmax maximum adsorption capacity (mg g−1 ), KL adsorption equilibrium constant (L mg−1 ), Kf Freundlich constant (mg1−n Ln g−1 ), n heterogeneity factor (is also known as the Freundlich coefficient), B is equal to RT/b with R being the universal gas constant (8.314 J mol−1 K−1 ), T absolute temperature in Kelvin, A equilibrium binding constant, R2 correlation coefficient
Food Anal. Methods Table 6 Kinetic parameters of adsorption at optimum conditions Ion
k1 t Pseudo-first order, log ðqe −qt Þ ¼ logqe −2:303 Pseudo-second order, qt ¼ k 21q2 þ t
e
Intraparticle diffusion, Elovich, qt ¼ 1a lnð1 þ abt Þ 1 qe t qt =Kdifft1/2 +C
qe (mg g−1)
R2
k2
qe (mg g−1)
R2
kdiff
C
R2
b
Pb2+ 0.18
0.86
0.9778
0.24
0.65
0.9958
0.06
0.09
0.8062
5.74
0.26
0.9506
Zn2+ 0.11 Cu2+ 0.17
0.07 0.45
0.9763 0.9703
4.03 0.66
0.69 0.69
0.9953 0.9965
0.01 0.04
0.64 0.39
0.8861 0.8407
58.82 9.90
4.35 1.75
0.9338 0.7861
k1
a
R2
qt amounts of metal ions adsorbed (mg g−1 ) at any instance of time, k1 rate constant of the pseudo-first-order adsorption operation (min−1 ), k2 pseudosecond-order rate constant of adsorption (g mg−1 min−1 ), Kdiff intraparticle diffusion rate constant, C intercept, a initial adsorption rate, b a constant related to the activation energy and the heat of adsorption
dependence. The values of ΔH0 were calculated from the slopes and intercepts of linear regression of ln Ce versus 1/T using the Clausius–Clapeyron equation (Eq. (1)) ΔH 0 ln C eq ¼ þD ð1Þ RT
where ΔH0 is the standard enthalpy, T is the absolute temperature (K), R is the universal gas constant (J mol−1 K−1), and D is the intercept of the plot of ln Ce versus 1/T. The other parameters (Kc, ΔS0, ΔG0) were calculated using the following equations: Kc ¼
C ad; eq C eq
ln K c ¼
ð2Þ
ΔS 0 ΔH 0 − R RT
where Kc is the equilibrium constant of the adsorption, ΔG0 is the standard Gibbs free energy, ΔS0 is the standard entropy, Cad, eq is the concentration of the metal ions on the adsorbent at equilibrium, and Ceq is the equilibrium concentration of the metal ions in solution (mg L−1). The values of ΔH0 and ΔS0 were determined from the slopes and intercepts of the plots of ln Kc versus 1/T (figure not shown) and were listed in Table 7. As can be seen in Table 7, the negative values of ΔS0 correspond to a decrease in the degree of freedom of the adsorbed species, while the negative enthalpy changes (ΔH0) indicate that adsorption followed exothermic processes. The negative values of ΔG0 suggest that the sorption process is spontaneous. The observed increase in the negative values of ΔG0 with increasing temperature (Table 7) indicates that the sorption becomes more favorable at higher temperature and it can be attributed to the dehydration effect of both metal ions and active sites, which facilitates the interaction between the targeted metal ions and adsorbent.
ð3Þ Conclusion
ΔG0 ¼ −RT ln K c Table 7 Thermodynamic parameters of adsorption at optimum conditions
ð4Þ
Ion
The results of the present study provided the important information about the suitability of hybrid SiO 2/TiO 2-NH 2
Parameters ΔH0 (kJ mol−1)
ΔS0 (J mol−1 K−1)
T (K)
ΔG0 (kJ mol−1)
Kc
Pb2+
−35.4
−0.13
Zn2+
−64.3
−0.23
Cu2+
−53.4
−0.16
288.15 298.15 308.15 288.15 298.15 308.15 288.15 298.15 308.15
−1.76 −4.05 −5.17 −0.98 −2.74 −5.40 −5.88 −6.35 −11.14
6.08 7.13 7.34 1.53 3.78 8.18 17.17 52.50 92.15
Food Anal. Methods
nanoparticles as a novel and efficient adsorbent for solidphase extraction of lead, copper, and zinc from different food and water samples. The adsorbent efficiency in adsorption process of the metal ions was tested using the batch sorption technique. The adsorption isotherms were well fitted into the Langmuir isotherm model, which confirm adsorption with monolayer coverage. Satisfactory results were obtained from attempts for the quantitative recovery of the analytes from different real samples, and the obtained results confirmed the adsorbent capability for food and water samples analysis. Application of the proposed adsorbent also provides comparable preconcentration factors and lower detection limits for determination of Pb2+, Zn2+, and Cu2+ ions in the mentioned samples, in comparison with other SPE methods. Compliance with Ethics Requirements Conflict of Interest Maryam Rajabi declares that she has no conflict of interest. Behruz Barfi declares that he has no conflict of interest. Alireza Asghari declares that he has no conflict of interest. Reza Aran declares that he has no conflict of interest. Farhood Najafi declares that he has no conflict of interest. This article does not contain any studies with human or animal subjects.
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