Highly efficient ultrasonic-assisted pre-concentration and simultaneous determination of trace amounts of Pb (II) and Cd (II) ions using modified magnetic natural clinoptilolite zeolite: Response surface methodology

Microchemical Journal 146 (2019) 498–508

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Highly efficient ultrasonic-assisted pre-concentration and simultaneous determination of trace amounts of Pb (II) and Cd (II) ions using modified magnetic natural clinoptilolite zeolite: Response surface methodology

T

T. Amiri-Yazani, R. Zare-Dorabei , M. Rabbani, A. Mollahosseini ⁎

Research Laboratory of Spectrometry & Micro and Nano Extraction, Department of Chemistry, Iran University of Science and Technology, Tehran 16846-13114, Iran

ARTICLE INFO

ABSTRACT

Keywords: Sonication Simultaneous pre-concentration Lead Cadmium Modified natural clinoptilolite zeolite

In this work, the synthesis of the magnetic natural clinoptilolite (CP) zeolite modified by cetyltrimethyl ammonium bromide hexadecyl trimethyl ammonium bromide (CTAB) and dithizone was utilized for simultaneous determination of trace amounts of lead (II) and cadmium (II) ions by flame atomic absorption spectrophotometry (FAAS). Characterization of the prepared adsorbent was carried out using various techniques such as XRD, FT-IR, SEM, EDS mapping and BET analyses. Central composite design (CCD) under response surface methodology (RSM) was used for obtaining the most important parameters and probable interactions between variables. In this favor, parameters including adsorbent dosage, pH, adsorption time and desorption time using ultrasound were optimized. Sonication had an important role in shortening the adsorption and desorption times of lead (II) and cadmium (II) ions by enhancing the dispersion of adsorbent in solution. Under optimum conditions, the proposed method represented linear ranges of 2.0–130.0 and 5.0–150.0 μg l−1 for lead and cadmium, respectively; while their corresponding values for limit of detection (LOD) using this method were found to be 0.93 and 1.08 μg l−1. Moreover, these ions exhibited relative standard deviation (RSD %) values of 2.13 and 2.78, respectively. Finally, the application of the synthesized material was evaluated for pre-concentration and determination of Pb (II) and Cd (II) ions from environmental water samples.

1. Introduction Heavy metals are known as the most important mineral pollutants [1–3]. Lead is the most wide-spread heavy metal and is considered a poison in the environment. Lead poisoning is usually chronic and brings about several changes in bone and bone marrow, causing disorders of red blood cells and consequently anemia [4–6]. Cadmium enters aqueous ecosystems through soil and bed rock erosion, atmospheric contaminant precipitants produced by industrial units, waste water from polluted regions and sludge and fertilizers used in agriculture [4–8]. When analyzing real samples, regarding the presence of other pollutants, negligible concentration of the analyte, complexity of the bed or sample matrix, and the necessity to achieve small values of LOD, certain techniques for pre-processing or sample preparation prior to identification and measurement must be inevitably employed [9–12]. Solidphase extraction [13–15] gets the highest priority among various separation and pre-concentration methods [16–18]. Due to economic and environmental benefits, magnetic solid-phase extraction (MSPE) [19–22] may be employed in order to eliminate the extra centrifuge

⁎

step and hence the increased operation rate [23]. Today, nanoparticles and nanostructures have attracted huge attention because of their specific and extensive applications, among them zeolite is a special and interest material in order to its unique spatial structure, high cation exchange capacity [24–26], structure maintenance at high temperatures, low cost [27] and its abundant distribution in the world. Zeolites consist of 3D porous crystals, comprising alumina silicates of alkaline metals (mostly sodium and potassium) and of alkaline earth metals (mainly calcium). The crystal body is actually a 3D tetrahedron frame of (SiAl)O4, creating a porous structure of 3–10 Å. The primary units contributing to a zeolite body are single and specific tetrahedrons [28]. They are connected to each other by oxygen atoms existing on their body corners in order to establish secondary structure units through the connection of which are created various multi-dimensional forms. Finally, connection of the latter ones helps to build infinite network of crystalline structures of different zeolites. The clinoptilolite (CP) is the most abundant natural zeolite [29,30]. Due to the occurrence of impurities within their structure, naturally

Corresponding author. E-mail address: [email protected] (R. Zare-Dorabei).

https://doi.org/10.1016/j.microc.2019.01.050 Received 9 December 2018; Received in revised form 19 January 2019; Accepted 21 January 2019 Available online 22 January 2019 0026-265X/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Complex formation by modified zeolite CTAB and dithizone.

in order to demonstrate the efficiency and performance of this analyzer [46].

Table 1 XRF information of natural clinoptilolite zeolite. Sample

CP zeolite

SiO2

Al2O3

Fe2O3

CaO

Na2O

K2 O

MgO

TiO2

2. Materials and methods

%

%

%

%

%

%

%

%

2.1. Chemical materials

69.28

10.43

0.49

3.56

0.73

1.27

0.50

0.166

FeCl2·4H2O, FeCl3·6H2O, ammonia, ethanol, the cationic CTAB surfactant and dithizone ligand were purchased from Merch Company (Germany). Clinoptilolite natural zeolite was provided from Kansaran Binaloud Company (Iran). XRF information of this zeolite is shown in the Table 1. Nitric acid as the eluent solvent, Pb(NO3)2 and Cd (NO3)2·4H2O were purchased from Merck Company (Germany). The required concentrations were prepared by diluting the initial solution with deionized water. Standard solutions (1000 mg L−1) of disrupting factors discussed in their corresponding section were made by solving appropriate amounts of their relevant salts. Also, pH of the solutions was adjusted by solving proportionate amounts of phosphoric acid, acetic acid and boric acid in water, followed by addition of sodium hydroxide solution (universal buffer). All crystal containers and required devices were placed in an acidic solution and were subsequently washed by washing material and deionized water, and were finally dried in the oven.

occurring zeolites have limited industrial applications. Moreover, their crystalline structure is not suitable for many such purposes due to their pore sizes. Hence, several attempts have been made to raise their capacity as adsorbents for heavy metal elimination, which is a major problem in different ecosystems, by modifying these nanostructured silicates. As a result of the replacement of the silicon by trivalent aluminum and consequently creation of a negative charge within their structure, zeolites are able to adsorb cationic surfactants [31,32] such as CTAB (cetyltrimethyl ammonium bromide hexadecyl trimethyl ammonium bromide) via ion exchange [33–35]. At concentrations below critical micelle concentration (CMC), positive sides of the surfactant monomers are adsorbed by negatively charged regions in zeolites, making a surfactant layer on their surface. A second surfactant layer is created after its concentration grows and reaches hydrophobia among hydrocarbon molecules of the surfactant [36]. Based on Fig. 1, in second layer, hydrocarbon chains penetrate into the first adsorbed layer and hence their positive side stands outwards. The ligand negative ions, especially as the opposite ions for heavy metal ions, are placed next to the second surfactant layer [37–41]. Diphenyl thiocarbazone (DTZ) is an appropriate ligand which makes good complexes with metals like lead, cadmium, cupper and mercury, because it bears nitrogen atoms as well as eNH and eSH groups. The reason why DTZ is placed on surface of the surfactant-modified zeolites is the adsorption capacity enhancement of the adsorbent for simultaneous extraction and pre-concentration of lead and cadmium, and in turn the increased sensitivity and selectivity are brought about by doing so [42–44]. Prior to decomposition process, experimental design was employed in this work so as to assume the influential parameters as dependent on one another and to evaluate how their interactions affect the solution. Thus, the number of experiments required to reach the optimum outcome is significantly reduced and the found optimum points agree well with the results of ‘one-variable-at-a-time’ method [45]. Finally, a flame atomic absorption spectrophotometry was used to analyze the pre-concentrated metals that are transferred from high volume and low concentration to low volumes and high concentration,

2.2. Equipment Analyses of lead and cadmium samples were done using flame atomic absorption spectrometry (Shimadzu, AA-6300, Japan) at wavelengths of 283.23 and 228.80 nm, respectively. In order to establish contact and mix metal bearing solutions with the adsorbent, ultrasonic bath (Elmasonic, S-60H, Germany) was used. pH measurements were carried out using pH meter (Metrohm, Switzerland). The balance used in this research was Sarturious with an accuracy of 0.00001 g. Memmert oven (Germany) was used to dry containers and other materials. Mechanical shaker (SHO-2D, Korea South) was employed to stir and make uniform solutions, set at a speed of 200 rpm. A strong electromagnet was utilized to gather the adsorbent in solutions. 2.3. Synthesis of adsorbent 2.3.1. Synthesis of magnetized adsorbent In order to magnetize the clinoptilolite zeolite, in situ co-precipitation method was employed. To this end, 2.32 g of FeCl3·6H2O and 0.8 g of FeCl2·4H2O together with 4 g of zeolite and 100 ml of deionized 499

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Fig. 2. XRD patterns of the natural CP zeolite: (a) original, (b) magnetic CP, (c) modified with CTAB and DTZ.

water were poured in a 3-mouthed balloon held by a grip, of which one mouth was blocked by a cork, the middle one was connected to a balloon so as to control the amount of the entering gas, and the last one was used as nitrogen entrance path, all in order to make the reaction take place in the absence of oxygen. The balloon was then placed in ultrasonic bath for 2 h at room temperature. In order to prevent any temperature rising in the bath, deionized water was gradually introduced and after an hour, 20 ml of 25% ammonia was injected through the plugged mouth by a syringe which caused the color of the solution turn to black (pH = 10.87). The solution was remained in the bath for another 1 h and was then poured inside a beaker to be placed on a shaker at 175 rpm for 12 h. Using the electromagnet, the precipitating portion was washed with deionized water several times so that unreacted materials were removed and hence pH was no longer in the alkaline region. Eventually, the resulting precipitate was put in the oven for 4 h to complete dry.

First, certain concentration and volume of metal solutions containing lead and cadmium were prepared and added to certain amounts of adsorbent at a known pH in an Erlenmeyer flask which was later exposed to ultrasonic bath so as to be stirred well. Next, the supernatant was discarded by an electromagnet, and the precipitate was mixed with 0.5 ml of 1 M nitric acid, followed by exposure to ultrasonic bath for a certain time. Thereafter, supernatant was gathered by an electromagnet and insulin syringes. Finally, concentrations of lead and cadmium ions in the resulting solution were measured using an atomic absorption spectroscopy device and the microsampler arranged on it. 2.3.4. Identification and determination of synthesized adsorbent Investigation of the pores and specific surface area of the synthesized adsorbent was conducted using BET method from nitrogen gas adsorption-desorption isotherms using a Micromeritics absorbance detector (model ASAP 2020, USA). Also, Fourier transform infrared spectroscopy (FT-IR) was carried out by Shimadzu 8400 s instrument. XRD device (Pert Pro 'X, Panalytical Company, FE-SEM equipment, model SIGMA VP 500, ZEISS Company, Germany), and finally EDS detector (Oxford Instrument Company) were utilized.

2.3.2. Modifying of magnetized adsorbent Initially, 364 mg of CTAB was weighed and placed inside a balloon, followed by addition of 2 ml of ethanol and 8 ml of deionized water to obtain 10 ml of 0.1 molar solution. Then 3 g of magnetized zeolite and 10 ml of deionized were added and the mixture was kept on a shaker at 150 rpm for 24 h and finally dried. In the next step, 6.4 mg of dithizone was weighed and put inside a 25-ml balloon, followed by addition of 2 ml of ammonia (25%) and 23 ml of deionized water to gain 25 ml of its 1 mmol L−1 solution so as to blend it with modified magnetic zeolite with CTAB. Then, the mixture was kept at room temperature on a shaker at 150 rpm for 24 h and finally was dried in the oven [47].

3. Results and discussion 3.1. Adsorbent characterization Prepared adsorbents were identified by X-ray diffraction pattern (XRD), FT-IR, nitrogen adsorption-desorption, scanning electron microscopy (SEM) and EDS techniques.

2.3.3. Solution preparation and magnetic solid-phase extraction using the synthetized adsorbent The extraction method was conducted with the help of solid phase for simultaneous extraction and non-continuous pre-concentration of lead and cadmium using the magnetized clinoptilolite natural zeolite modified by CTAB and dithizone as adsorbent. Pb(NO3)2 and Cd (NO3)2·4H2O were used to prepare lead and cadmium solutions, respectively. Initially, certain amount of each salt was weighed, poured into a 250-ml balloon, followed by addition of 1 M nitric acid to the required volume so as to obtain base solutions (mother solutions) of 1000 mg L−1 of Pb2+ and Cd2+. In order to prepare other solutions needed for further experiments, deionized water was employed for diluting the mother solution.

3.1.1. XRD results In order to investigate the structure of the synthesized compounds, XRD analysis was employed. X-ray diffraction patterns of CP, magnetic CP and modified magnetic CP are illustrated in Fig. 2. In X-ray diffraction pattern of CP, major diffraction peaks occur at 2θ = 7.46, 11.19, 14.94, 15.84, 17.36, 17.53, 22.36, 22.48, 22.49, 22.84 and 23.21° [48], whereas diffraction peaks of Fe3O4 nanoparticles occur at 2θ = 30, 34, 43, 53, 57 and 63° [49]. Comparing the diffraction patterns of magnetic CP and pure CP determined that the peaks related to pure CP in magnetic CP have not been changed at all; indeed, the zeolite crystal structure of CP in loading and produced Fe3O4 magnetic CP has not been changed. This is in accordance with the results of other magnetic zeolites. The reason for the observed reduction in peak 500

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zeolite modified with a surfactant, and represents that the peak intensity of the magnetic zeolite has been reduced. 3.1.3. Study on SEM images Scanning electron microscopy imaging is employed so as to delineate the morphology of the synthesized materials. SEM images of pure CP, magnetic CP and modified magnetic CP with CTAB and DTZ are depicted in Fig. 4. As it is apparent from Fig. 4a, pure zeolite was observed as separated particles or agglomerates having large dimensions. Fig. 4b and c indicates that the morphology of zeolite has retained after modifying with Fe3O4 nanoparticles, CTAB and DTZ. EDS spectrum of magnetic zeolite modified with CTAB and DTZ is shown in Fig. 4d, indicating that Al, Si and O are the elements related to the original structure of zeolite, whereas N, C and S are the elements of CTAB and DTZ. Also, Fe and O occur as constituent elements of Fe3O4 nanoparticles. Results of these investigations confirmed that the adsorbent was made of Fe3O4 nanoparticles, CTAB and DTZ on CP zeolite. Moreover, EDS mapping of the natural CP with CTAB and DTZ is illustrated in Fig. 5. As it is obvious, each of the constituent elements of the magnetic CP modified with CTAB and DTZ are marked by a corresponding color.

Fig. 3. FT-IR spectra of natural CP zeolite: (a) original, (b) Fe3O4, (c) magnetic zeolite, (d) CTAB, (e) magnetic zeolite + CTAB, (f) DTZ, (g) modified with CTAB and DTZ.

3.1.4. BET studies In order to investigate the porosity of the media and to obtain the pore size distribution and surface area of pure zeolite, magnetic zeolite and modified magnetic zeolite, samples were examined carefully by nitrogen gas adsorption-desorption analysis. Categories of IUPAC: type-II was absorbed, indicating that this compound has the characteristics of nanoporous materials. As it is apparent from Fig. 6a, the isotherm diagram of CP matches the IUPAC category coming from type-II adsorption and given this, the compound has the characteristics of nanoporous materials. Depending upon Fig. 6b and c, it could realize that after loading Fe3O4 particles along with the organic materials, type-II structure still remains unchanged. Results of area, volume and size of the average porosity of pure CP, magnetic and modified CP are given in the Table 2. Specific surface area and the volume of clinoptilolite were enhanced after using Fe3O4 composites. In fact, deposition of iron oxide nanoparticles on zeolite led to the formation of secondary pores. Considering the increased surface area of the adsorbent and enhanced volume of the pores as well as the reduced average diameter of the porosity of pure CP, it could be concluded that magnetic nanoparticles of Fe3O4 are situated on the surface of the zeolite. Comparing the results of magnetic CP with those of modified magnetic CP revealed that the surface area and pore volume of the modified magnetic adsorbent structure declined in comparison with magnetic CP, which represents loading of CTAB and DTZ on the magnetic clinoptilolite structure.

intensity of diffraction pattern for magnetic CP is pore closure by magnetic nanoparticles and organic material after peak modification and broadening processes. This is true whenever locations of the peaks have not been changed. According to X-Pert software, zeolite code number is (00-039-1383) with monoclinic structure and space group of C2/m. For Fe3O4, the code number is (01-088-0315) with a cubic structure and Fd-3m space group. 3.1.2. FT-IR study In order to ensure the synthesis of modified magnetic zeolite and to determine the surface functional groups of the adsorbent, every step of the synthesis was monitored by FT-IR spectrophotometer, as exhibited in Fig. 3. The spectrum of pure CP represented in Fig. 3a displays bending vibrations of SieOeSi and SieOeAl at 606 cm−1, SieO stretching vibration at 796 cm−1, asymmetric vibrations of SieOeSi at 1078 cm−1, bending vibration of surface hydroxyl groups (eOH) at 1638 cm−1 and stretching vibration of the surface hydroxyl groups (eOH) at 3446 cm−1 [50]. As it is obvious from Fig. 3b showing FT-IR spectrum of Fe3O4, FeeO stretching vibration of Fe3O4 appears at 580–422 cm−1 region of the spectrum [51]. Regarding Fig. 3c, the presence of FeeO stretching vibration of functional groups can also be confirmed by comparing the spectrum of Fe3O4 with that of magnetic zeolite in the range of 580–422 cm−1. Eventually, it substantiates the formation of magnetic zeolite of clinoptilolite. Fig. 3d illustrates the spectrum of CTAB surfactant. This spectrum represents the vibrations of CeH, CeC and NeC at 2921, 2851 and 1491 cm−1, respectively [52,53]. FT-IR spectrum of magnetic zeolite + CTAB is displayed in Fig. 3e. According to this figure, main peaks of CTAB overlap with those of magnetic zeolite, but the observed increase in peak intensities of 2921 and 2851 cm−1 as well as the reduction in peak intensity of the magnetic zeolite reveals the fact that CTAB load is on magnetic zeolite. Fig. 3f demonstrates the spectrum of dithizone at 750, 1379, 1491, 1494 and 1500 cm−1. It also indicates the existence of benzene ring stretching vibration at 888 cm−1, stretching state of N]N and NH stretching vibration in the range of 2900–3100 cm−1 [54–57]. Fig. 3g exhibits FT-IR spectrums of magnetic zeolites modified with CTAB and dithizone. Comparing the spectra of magnetic zeolites modified with CTAB and dithizone with that of pure dithizone, it can be observed that NH peak has shifted towards higher energies and hence overlaps with broad peak of eOH. This factor confirms that dithizone has sat on the surface of the

3.2. Optimization method In order to optimize the procedure, the following parameters were optimized using “one-variable-a-time” method. 3.2.1. Type, concentration and volume of eluent solvent The eluent solvent must be capable of desorbing analytes from the surface without any destructive influences on structures of the species. In order to select the most appropriate solvent, four solutions of 20 μg l−1 of Pb2+ and Cd2+ having the volume of 200 ml were prepared and subsequently 8 mg of the adsorbent was introduced to each solution, followed by placement in ultrasonic bath for 3 min. Finally, the adsorbent was collected by means of an electromagnet. 0.1 M eluent solvents of HCl, NaOH, EDTA and HNO3 were provided and 1 ml of each solvent was separately introduced to the adsorbent, followed by the exposure of the resulting solution to ultrasonic bath for 4 min as the desorption step. The electromagnet was applied again and flame atomic 501

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Fig. 4. SEM images of natural clinoptilolite zeolite: (a) original, (b) magnetic zeolite, (c) modified with CTAB and DTZ, (d) EDS spectrum of magnetic zeolite modified with CTAB and DTZ.

adsorption device was employed for measurements and finding the recovery efficiency. Results are represented in Fig. 7 at which a close look reveals that the highest efficiency was achieved when nitric acid was used as the eluent solvent. The observed decrease in recovery efficiency of the metal ions when using other eluent solvents might be attributed to precipitation formation resulting from interaction of metal ions with anionic bases in eluent solvents. The extraction percentage was highest when nitric acid was used, this shows that this solvent does not have a destructive effect on the analyte structure, while other solvents reduced the concentration factor and ultimately the efficiency. This decrease was due to the formation of metal hydroxide sediment by sodium hydroxide or the formation of complex, and finally the extraction was not performed well. In order to determine the minimum concentration of the required nitric acid for washing, a series of experiments similar to those of the previous step were conducted whose corresponding outcomes are exhibited in Table 3. Accordingly, the highest percentage of extraction was related to 2 M nitric acid. However, considering the negligible difference between the result of 2 M with that of 1 M nitric acid and taking into account the economic aspects as well as the lower environmental damage, 1 M nitric acid was utilized in this study for further experiments. At low concentrations, due to applying low concentration of the acid needed for complete washing of metal cations from the supposed adsorbent, the extraction efficiency was dropped. The volume of eluent solvent must be at its minimum possible amount so that pre-concentration of the measured species could be

done by higher coefficients. In order to find the minimum volume of the nitric acid required for complete washing, a series of experiments akin to previous steps were designed and performed, with Table 4 unfolding the outcomes. As can be seen from this table, at values lower than 0.5 ml, the bond between the ligand and metal ions were not completely broken and hence the analytes were not appropriately recovered. At higher volumes, the recovery percentage fell, just as the concentration factor did, as a consequence of concentration deduction. 3.2.2. Volume of the initial solution To select the appropriate initial volume of the solution, 4 solutions of Pb2+ and Cd2+ at a concentration of 20 μg l−1 with different volumes were provided and 8.0 mg of the adsorbent was added and subsequently exposed to the ultrasonic bath for 3 min. The electromagnet separated the adsorbent and to address this problem, 0.5 ml of 1 M nitric acid was poured on the absorber to be placed in ultrasonic bath afterwards for desorption purposes, followed by the use of electromagnet again. The atomic absorption device helped measuring the recovery percentage in each case, and the results are exhibited in Table 5. It proved that the volume of 100 ml of lead and cadmium metal cations possesses the highest extraction efficiency. Then, central composite rotatable design was employed in order to investigate other influential parameters affecting the simultaneous extraction process of the solid phase of lead and cadmium [58,59]. Four parameters including pH, adsorption time, adsorbent amount (in mg) and desorption time were probed as the main factors Table 6. 502

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Fig. 5. EDS mapping of natural CP zeolite with CTAB and DTZ.

P-value is less than 0.0001 which means the model is logical and there is a probability of only 0.01% that P-value is a result of noise. In this experiment, values of the Lack of Fit (LOF) corresponding to the extraction of lead and cadmium were equal to 0.9623 and 0.9816, respectively; which means LOF is not significant compared to the net error. Also, the greatest impact arises from desorption time (highest F value) and the most significant interaction occurs between the adsorbent amount and adsorption time. The most appropriate statistical model was selected with regard to the high values of F and response (R2) and low value of standard error,

which was further considered as the appropriate response model. Normal plots of the extraction of lead and cadmium ions are depicted in Fig. S1, suggesting that the conducted experiments have been carried out with the minimum error, and agree well with the proposed model. 3.2.3. Impact of pH In studies of solid phase extraction, pH plays an integral role in formation of metal cation complexes, their stability and extraction. In addition, the ability to create a complex between the ligand and 503

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Fig. 6. Nitrogen adsorption-desorption patterns of the natural clinoptilolite zeolite: (a) original, (b) magnetic clinoptilolite, (c) modified with CTAB and DTZ.

Table 2 Surface area, volume and pore size of the prepared adsorbents. Sample Clinoptilolite zeolite Magnetic zeolite Magnetic zeolite modified

Pore diameter BJH (nm) 12.4583 10.7963 13.5663

Total pore volume BJH (cm3/g) 0.065605 0.192352 0.162872

Surface area BJH (m2/g) 21.0638 71.2660 49.3910

aforementioned ions highly depends on pH of the medium, due to the presence of eNH and eSH groups. Fig. S2 represents the influence of the pH as the only varying factor. Accordingly, an increase in pH value results in a rise in extraction efficiency, till the pH values reach 5. Inappropriateness of the pH values below 5 for extraction of metal ions

Fig. 7. Effect of the type of eluent solvents on determination of Pb (II) and Cd (II).

504

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Table 3 Concentration of eluent solvent. Concentration of the nitric acid (molar)

Extraction% (Cd)

Extraction% (Pb)

88 85.5 67 61

90 87.5 70 65

2 1 0.5 0.1

Table 7 Analytical parameters in simultaneous measurement of lead and cadmium (pH = 5, adsorbent dosage = 7.12 mg, adsorption time = 3.30 min, desorption time = 4 min, and concentrations of the ions: lead and cadmium = 20 μg l−1, 100 ml, eluent solvent = 0.5 ml of 1 M nitric acid). Analytical parameters Correlation coefficient (R ) Relative standard deviation (% RSD)a Limit of detection (μg l−1)b Linear range (μg l−1) Enrichment factorc Calibration equation (μg l−1)

Table 4 Volume of the eluent solvent. Nitric acid volume (ml)

Extraction% (Cd)

Extraction% (Pb)

7.5 24.5 30.62 18.2 14.5

8.46 27.81 42.17 21.17 16

0.3 0.4 0.5 0.7 1

pH Adsorbent dosage Adsorption time Desorption time

Units

mg min min

0.9997 2.78

0.9998 2.13

1.08 5–150 146.709 y = 0.1515x + 0.0124

0.93 2–130 160.74 y = 0.1771x + 0.0588

S=SD=

b

LOD = 3S / m. C EF = Ce .

(x n

x )2 1

V=

S X

× 100 (n = 7).

figure, extraction was enhanced as longer desorption times were applied, particularly when approaching 4 min. This is because of direct relationship between desorption time and the response. The observed increase occurs since by increasing desorption time, the possibility of interaction between the metal ion and washing solvent is fortified and hence, desorption process takes place more appropriately. Sonication could be considered as a beneficial tool in accelerate mixing, dispersion of adsorbent into the solution, and increasing the diffusion coefficients which confirms the strong association of ultrasound with mass transfer between the absorbent and metal ions [60].

Extraction% (Pb)

40 80 38.75 37

45 85 41.25 40

Table 6 Variables and their levels selected in central composite design. Variables

The amount of Pb(II)

S

Extraction% (Cd)

50 100 200 250

a

c

Table 5 Volume of the initial solution. Initial solution volume (ml)

The amount of Cd(II) 2

3.3. Analytical figures of merit

Factor levels −α

−1

0

+1

+α

2 2 0.30 1

3 4 1.30 2

4 6 2.30 3

5 8 3.30 4

6 10 4.30 5

Under optimum conditions, analytical figures of merit for the proposed methods, including linear range (LDR), coefficient of determination (R2), limit of detection (LOD), relative standard deviation (RSD) and enrichment factor were fulfilled for the assessment procedure Table 7. 3.4. Effect of interfering ions In order to assess the selectivity of the method for measuring trace amounts of lead and cadmium, interference effects of other ions were studied on efficiency of the target ions. To this end, 100 ml of a solution containing 20 μg l−1 of lead and cadmium ions and 7.12 mg of adsorbent with an adjusted pH of 5, involving certain ratios of interfering ions (5000, 4000, 3000, 1500, 750, 100, 75 and 5) was prepared under optimum conditions and subsequently based on the mentioned procedure, atomic adsorption was evaluated and recovery was calculated in presence of the interfering ion. The obtained results are indicated in Table 8, indicate that silver(I) is less disturbing compared to zinc(II) because its charge is different with analyte as well as its higher solubility. On the other hand, based on Pearson law (HSAB, hard-soft Lewis

can be attributed to the competition between the metal and hydronium ions. Besides, at pH values above 5 the extraction efficiency undergoes a decrease due to the formation of precipitated metal hydroxides. 3.2.4. Impact of adsorbent amount Diagrams represented in Fig. S3 display the effect of adsorbent amount as the only varying parameter. As it is obvious from this figure, raising the amount of the adsorbent will partially enhance the extraction efficiency. It was predicted that by increasing the adsorbent amount to an average value of 7.11 mg, larger numbers of the empty sites would be available to the metal ions and hence, the extraction procedure is improved. At values higher than 7.11 mg, the free sites are almost fully occupied and will no longer affect the extraction efficiency.

Table 8 Effect of interfering ions on recovery of the desired ion (concentrations of the ions: lead and cadmium = 20 μg l−1).

3.2.5. Impact of adsorption time using ultrasound Diagrams depicted in Fig. S4 illustrate the impact of adsorption time as the only varying effective parameter. Results proved that as the adsorption time goes up, the extraction efficiency is partially increased. It was predicted that increasing the stirring time until 3.30 min would strengthen the probability of interaction between the adsorbent and the metal ion in solution; hence, greater amounts of adsorbed material could be removed from the medium.

Interfering ions K+, Na+, Ag+, Cl− SO42−, Ca2+ Mg2+, F− NO3−, CH3COO− Mn2+, Ni2+ PO43−, CO32− Zn2+, Cu2+ Hg2+

3.2.6. Impact of desorption time using ultrasound Diagrams of Fig. S5 illustrate the effect of desorption time, indicating that it is an important parameter. As it is apparent from this 505

Interfering% (Cd) 1.01 1.26 0.96 2.34 1.52 3.09 1.30 2.69

Interfering% (Pb) 1.40 2.35 2.04 1.56 1.02 2.05 3.02 2.75

Tolerable concentration ratio (X/Cd, Pb) 5000 4000 3000 1500 750 100 75 5

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Table 9 Extraction and simultaneous determination of lead and cadmium in water samples. Sample Spring water Well Water Mineral water Petrochemical water

a

Added (μg l−1)

Found (μg l−1) FAAS Cd(II)

Found (μg l−1) FAAS Pb(II)

Recovery (%) Cd(II)

Recovery (%) Pb(II)

0 10 20 0 10 20 0 10 20 0 10 20

1.42 ± 2.47a 10.98 ± 3.14 20.95 ± 2.37 3.12 ± 3.10 13.32 ± 1.69 23.21 ± 0.71 > LOD 10.15 ± 2.09 20.12 ± 0.86 17.47 ± 1.73 27.87 ± 2.96 37.52 ± 3.34

> LOD 10.25 ± 2.76 20.18 ± 1.53 > LOD 10.11 ± 1.38 20.08 ± 0.83 > LOD 10.10 ± 1.36 20.03 ± 0.43 20.6 ± 3.62 30.96 ± 3.18 40.82 ± 3.76

– 96.15 97.81 – 101.52 100.39 – 101.5 100.6 – 101.46 100.13

– 102.5 100.9 – 101.1 100.4 – 101 100.15 – 101.18 100.54

Relative standard deviation % (n = 3).

Table 10 Comparison of analytical features of the pre-concentration methods. Method

Metals

Limit of detection (μg l

SPE modified-ZnO nanopowders

−1

Pb Cd Pb Cd Cd Pb

0.15 0.21 0.13 0.29 0.04 0.01

Ni Pb Cd

MSPE Perygnii immobilized γ-Fe2O3 nanoparticles MSPE modified magnetic natural clinoptilolite zeolite

SPE ofloxacin-modified-silica gel SPE L-cystine modified zeolite USA-CP-MSPE amine functionalized silica aerogel nanoadsorbent SPE magnetic ion–imprinted nanocomposite SPE aspartic acid-modified magnetic nanoparticles MSPE Cmicaceus immobilized γ-Fe2O3 nanoparticles

a b

Initial solution −1

)

(μg l

)

Eluent solvent

Analysis method

Ref.

(ml)

1

40

0.3 ml HNO3 3 M

FAAS

[61]

1

200

2 ml HCl 0.5 M

ICP-OES

[62]

5 1

100 10

2 ml HNO3 2 M 0.1 ml HNO3 1 M

FAAS ETAASa

[61] [63]

0.06 0.6 0.04

10 300 1000

200 100 100

FAAS ICP-OESb ICP-OES

[65] [66] [67]

Cd

0.054

1000

100

ICP-OES

[67]

Pb Cd

0.93 1.08

20

100

2 ml EDTA 2 ml HNO3 2 M 5 ml of HCl 1 M and 5 ml HNO31 M 5 ml of HCl 1 M and 5 ml HNO31 M 0.5 ml HNO3 1 M

FAAS

This study

Electrothermal atomic absorption spectrometry(ETAAS). Inductively coupled plasma optical emission spectrometry (ICP-OES).

acid and base), mercury is a soft metal and can react with sulfur. Dithizone has a soft structure due to the presence of sulfur groups, which can easily absorb mercury metal that can be a serious nuisance for lead and cadmium ions. But due to high absorption capacity of the adsorbent and the desired pH adjustment and the wavelength of atomic absorption instrument on lead and cadmium ions, so this concentration does not interact and does not have a significant effect on the simultaneous extraction and pre-concentration of lead ions and cadmium. As it is obvious from this table, results divulge that until this concentration, the aforementioned ions impose no considerable impact on simultaneous pre-concentration and extraction of lead and cadmium ions.

capacity. Overall, simplicity and economic benefits are among the privileges of the proposed adsorbent recovery procedure. 3.6. Determination of lead (II) and cadmium (II) in real samples In order to evaluate the performance of the proposed method for simultaneous determination of lead and cadmium in real samples having different matrices, it was applied to spring, well, mineral and petrochemical water samples under optimum conditions. In order to verify the accuracy and appropriateness of the method, additional standard criteria were considered while extracting and recovering the aforementioned cations and to this end, certain amounts of lead and cadmium ions were added to each of the samples and subsequently extraction and recovery of these ions from the solutions were performed. Results of these experiments are shown in Table 9. According to the outcomes, the proposed method looks appropriate for the analysis of the real samples. Table 10 shows a general comparison of analytical features of the pre-concentration methods. These results show that the use of the MSPE method (magnetic solid-phase extraction) make it more simple.

3.5. Recovery studies Metal desorption and adsorbent recovery are considered as major problems regarding the possibility of reusing the absorbent. Adsorbent recovery under optimum conditions was achieved using the proposed working method. Then, desorption of lead and cadmium ions was performed using 1 M nitric acid. Next, the adsorbent was rinsed several times by deionized water for reuse. Eventually, pre-concentration procedure was conducted on the washed adsorbent several times using the listed methods. Fig. S6 represents the results of the tests. Reportedly, adsorbent reuse displayed no significant reduction in adsorption

4. Conclusion In this work, the Iranian clinoptilolite zeolite was used as a natural and economical adsorbent. In order to enhance the reproducibility and 506

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selectivity of the method and to probe the possibility of being coupled with other experimental methods, while reducing the costs and consumption of the chemical materials especially harmful organic solvents, the process of analytical solid-phase extraction (SPE) was employed as an efficient method of extraction, pre-concentration and measurement. For ease of extraction and pre-concentration and to increase its efficiency, the adsorbent was modified and magnetized by cationic surfactant CTAB and dithizone ligand (MSPE). Subsequently, the adsorbent was characterized using different identification techniques and finally, this synthetic adsorbent was investigated using design of experiment procedure so as to optimize the effective parameters on the process of simultaneous pre-concentration and extraction of lead and cadmium ions, which was eventually measured by flame atomic adsorption spectrophotometer. The combination of sonication with magnetic adsorption method was turned out to be very efficient in shortening the time and enhancing the efficiency of the adsorbent, Pb (II) and Cd (II) ions in solution as well as the effective interactions among them. In the following, under the optimal conditions, a series of experiments were one to obtain the suitability criteria of the method including linear range, coefficient of determination, limit of detection, relative standard deviation and enrichment factor were evaluated for evaluating the desired implementation method. The effect of interferences was investigated, too. Under the applied conditions, no significant interference was observed with the simultaneous extraction and pre-concentration of lead ions and cadmium ions, and review of retrieval studies showed that no significant reduction in adsorbent capacity was created. In the end. in order to perform the proposed method, this method was applied in other samples including spring water, well water, mineral water and petrochemical water under optimal conditions. The results showed that this method is suitable for analysis of the actual samples.

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