Preconcentration of Ag and Pd ions using graphite oxide and 2,6-diaminopyridyne from water, anode slime and catalytic converter samples

Abstract

In this work, graphite oxide was used for the first time as an effective adsorbent for the separation/preconcentration of Ag and Pd ions in various samples prior to flame atomic absorption detection. 2,6-diaminopyridyne was used as a chelating reagent. Analytical parameters affecting the solid phase extraction of Ag and Pd such as pH, adsorption and elution contact time, centrifugation time, reagent amount, eluent concentration and volume, sample volume and matrix ions were investigated. The recovery values for Ag and Pd were found to be ≥ 95%. The adsorption and elution contact times were 60 s. The preconcentration factor of the method was 120 for a 600 mL sample using 100 mg of the graphite oxide. The elution was easily performed using 5 mL of 2.0 mol L⁻¹ HCl. The graphite oxide was reusable for 150 cycles. The detection limits of Ag and Pd were 0.39 μg L⁻¹ and 0.94 μg L⁻¹, respectively. The relative standard deviations (RSD, %) were ≤2.5%. The proposed method was validated by analysing the certified reference materials SRM 2556 (used auto catalyst pellets) and TMDA-70 lake water, and spiked real samples. The optimized method was applied for the preconcentration of Ag and Pd ions in various water (tap water, mineral water and wastewater), anode slime and catalytic converter samples.

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1 Introduction

Heavy metals are highly toxic at low concentrations and can accumulate in living organisms, causing several disorders and diseases. Among them, silver and palladium are both valuable metals and pollutants. The silver ion is one of the most toxic heavy metal species, surpassed only by mercury, and thus has been assigned to the highest toxicity class, together with cadmium, chromium(VI), copper, and mercury.¹ Silver can be assimilated into the lungs, gastrointestinal tract, mucous membranes, and skin.² However, it is a lustrous noble metal with the highest electrical and thermal conductivity of all elements. As a consequence of these characteristics, it has a wide variety of applications in electronics, batteries, conductive pastes, silverware, jewelry etc. It is also used in photography, as brazing alloys and solders, to disinfect drinking water and as an antibacterial agent. Many medical devices and implants are treated with silver to reduce the risk of infection. The growing application of silver in medicine and industry has resulted in its increased release into the environment.^3–5

Palladium is a metal of economic importance due to its extensive use in metallurgy, various chemical syntheses, the production of dental and medicinal devices and jewellery. One of the most important applications of Pd is for the production of catalytic converters for car engines.^6,7 The increasing use of platinum group elements in vehicle exhaust catalysts, in addition to some other applications, cause their anthropogenic emission and spread in the environment.⁸

The separation and recovery of Ag and Pd ions from base metal ions and the development of selective and accurate methods for the determination of precious metals in waste and in environmental compartments are important research areas.^6,9–12 Solid phase extraction (SPE) is the most widely used method for these purposes. It is a very effective and versatile tool for the separation and preconcentration of Ag and Pd. Several important features of the sorbents define their applicability and efficiency – high preconcentration factors, fast complex formation, high stability constants of the complexes formed and low column backpressures.^9,10,13

Several studies on sorbents and their use for preconcentration and separation of Ag and Pd ions have been reported. Some examples include: Sepabeads SP207,¹⁴ chelating resins,^15,16 cysteine modified silica gels,¹³ agarose coated magnetic nanoparticles,⁷ silica gels,¹⁷ a chloromethylated polystyrene polymer modified with 2-mercaptobenzothiazole,¹⁸ magnetic nanoparticles coated by 3-(trimethoxysilyl)-1-propantiol and modified with 2-amino-5-mercapto-1,3,4-thiadiazole,¹⁹ imprinted polymers,⁶ and a dithiocarbamate coated fullerene C₆₀.²⁰

New discoveries in materials science may provide new tools for analytical sample preparation. An example is the wide use of carbon nanomaterials in SPE.²¹ Graphite oxide (GO) is a layered-structure carbonaceous material and is obtained through the reaction of graphite with strong oxidants such as potassium permanganate in concentrated sulfuric acid. It is known to consist of randomly distributed regions of unoxidised (aromatic) graphite and regions of aliphatic six-membered rings, rich in oxygen-containing functional groups, including epoxys, hydroxyls and carboxylic groups.^22,23 Recently, a few investigations have been reported on GO applications for adsorption of humic acid,²² tetracycline antibiotics,²⁴ aromatic organic contaminants,²⁵ methylene blue²⁶ and U(VI).²⁷ The reported results show that GO exhibits strong and high adsorption capacities.

In this study, GO was used as an adsorbent and 2,6-diaminopyridyne (dap) as a complexing reagent for the removal of Ag and Pd from aqueous solutions. 2,6-diaminopyridine is a simple and commonly used organic dye which contains two amino groups and one pyridine ring. Experimental conditions such as the pH, adsorption and elution contact time, reagent concentration, eluent type and concentration, and sample volume were investigated in detail. To our knowledge, there has not yet been a report on the use of GO and dap for the separation and preconcentration of Ag and Pd ions from various samples.

2 Experimental

2.1 Instrument

Silver and palladium determinations were performed using a Perkin Elmer AAnalyst 800 model (Waltham, MA, USA) flame atomic absorption spectrometer in an air-acetylene flame (2.0–17 L min⁻¹). Ag and Pd hollow cathode lamps were used as the radiation source for absorbance measurements with wavelengths of 328.1 nm and 244.8 nm, respectively. A WTW pH315i digital pH meter equipped with a combined pH electrode was used for checking the pH of solutions. A Wiggen Hauser VM model vortex and Annita ALC PK120 model centrifuge were used in the work. Characterization of the GO was performed using a LEO 440 model scanning electron microscope (SEM) with an accelerating voltage of 20 kV and a BRUKER AXS D8 Advance model X-ray diffractometer using Cu Kα radiation (λ = 0.15406 nm) in the range 2θ = 10–90°. Fourier Transform Infrared (FT-IR) spectra were recorded on a Perkin-Elmer Spectrum 400 FT-IR spectrometer on samples in the form of KBr pellets.

2.2 Reagents and solutions

All chemicals were of analytical grade and were used without further purification. All solutions were prepared using ultra-high purity water from a Milli-Q system (18.2 MΩ cm, Millipore). Ag and Pd stock solutions (1000 μg mL⁻¹) were purchased from Sigma-Aldrich. Working standard solutions of Ag and Pd were prepared daily by dilution of their stock solutions. The 1.0% (w/v) solution of 2,6-diaminopyridyne (dap) was prepared daily by dissolving 1.0 g of reagent (Sigma-Aldrich) in 100 mL of ethyl alcohol. The following solutions were used for various pHs: diluted acid for pH 1, H₃PO₄–NaH₂PO₄ for pH 2 and 3, CH₃COOH–CH₃COONa for pH 3.5–6, CH₃COONH₄ for pH 7, and NH₃–NH₄Cl for pH 8 and 9.

2.3 Synthesis of graphite oxide

The GO was synthesized from natural graphite (Merck) by a Hummers method as described elsewhere.^28,29 Before use, in order to remove adsorbed impurities, the graphite oxide was washed with 2 mol L⁻¹ HCl and then ultra high purity water (see ESI†).

2.4 Solid phase extraction procedure

The pH of 20 mL of model solution including 20 μg Pd, 10 μg Ag and 100 mg of the GO was adjusted to 4. 1 mL of reagent (dap) (1% (w/v)) was added to the solution. The solution was mixed for 1 min by vortexing to facilitate the adsorption of Ag and Pd on the GO and was then centrifuged at 4000 rpm for 5 min. The supernatant was removed using a micropipette. 5 mL of 2 mol L⁻¹ HCl was used as an eluent. After vortexing for 1 min and centrifuging for 5 min, the Ag and Pd concentrations of the eluate were determined by flame atomic absorption spectrometry (FAAS).

For the optimization of the sample volume, a glass column (100 mm in length and 10 mm in diameter) was used. 100 mg of the GO was made into a slurry in water and then poured into the column. A small amount of glass wool was placed on the disc to prevent the loss of the adsorbent. The column was preconditioned using a pH 4 buffer solution. A model solution prepared at pH 4 was passed through the column at a flow rate of 2 mL min⁻¹. The adsorbed Ag and Pd ions were eluted with 5 mL of 2 mol L⁻¹ HCl. Ag and Pd concentrations of the eluate were determined by FAAS.

2.5 Sample pretreatment and dissolving procedures

Tap water, mineral water, and wastewater samples were obtained from our research laboratory, a local market, and the Organized Industrial Region of Kayseri, Turkey, respectively. The wastewater samples were filtered through a 0.45 μm pore size membrane filter to remove the suspended particulate matter. 250 mL of tap water, 100 mL of mineral water and 50 mL of wastewater were used for the analyses of Ag and Pd.

0.020 g of a catalytic converter sample, 1.00 g of anode slime and 0.025 g of standard reference material 2556 (Used Auto Catalyst Pellets) were used for the analysis. The SRM 2556 and converter samples were calcined for 2 h at 500 °C prior to analysis to assure a stable weighing form.

The digestion of the anode slime was performed using 2 × 10 mL of aqua regia. The evaporation procedure was carried out on a hot plate at approximately 100 °C. The insoluble parts were removed by filtration through a blue band filter paper using ultra high purity water.^14,15 The filtrate was diluted to a volume of 20 mL using ultra-high purity water, and the preconcentration procedure described above was applied to the sample solutions.

The catalytic converter sample and standard reference material 2556 were placed separately into 100 mL teflon beakers. 10 mL of aqua regia was added to the beaker. The mixture was evaporated to near-dryness on a hot plate at about 100 °C. 10 mL of aqua regia was added again to the residue and the mixture was again evaporated to near-dryness. 3 mL of HF was then added to the residue.³⁰ After evaporation, the interior surface of the beaker was washed using ultra-high purity water and filtered through blue band filter paper. The filtrate was adjusted to pH 4 and the solution was diluted to 20 mL with ultra-high purity water. The preconcentration procedure described above was used to separate and preconcentrate Ag and Pd.

3 Results and discussion

3.1 Characterization of GO

The XRD patterns of pristine natural graphite and GO are shown in Fig. 1. As shown in Fig. 1b, the disappearance of the native graphite peak, between 2θ of 25° and 30°, reveals the complete oxidation of graphite. In addition, Fig. 1 shows that the most intense peak 2θ = 12° corresponds to the (001) reflections of GO, and that the inter layer spacing (0.77 nm) is much larger than that of natural graphite (0.36 nm) due to the introduction of oxygen containing functional groups onto the graphite sheets.

Fig. 1 XRD pattern of graphite (a) and graphite oxide (b).

The morphology and microstructure of graphite and GO are shown in Fig. S1.† Single flakes of GO may be observed. GO flakes have relatively large surfaces (with the edges of the sheets on the micrometer scale) and their morphology resembles a thin curtain.

Fig. 2 shows the FTIR spectra of graphite and GO. The characteristic vibrations of GO include a broad and intense O–H peak at 3186 cm⁻¹, a strong C=O peak due to carboxylic acid and carbonyl moieties at 1715 cm⁻¹, a C–OH peak at 1372 cm⁻¹, a C–O–C peak at 1224 cm⁻¹, a C–O stretching peak at 1038 cm⁻¹, and an aromatic C–H peak at 589 cm⁻¹. The peak centered at 1621 cm⁻¹ can be assigned to the vibrations of the adsorbed water molecules, but may also contain components from the skeletal vibrations of unoxidized graphitic domains.^31,32

Fig. 2 FTIR spectra of graphite and graphite oxide.

3.2 Effect of pH

The acidity of the sample solution is usually the most critical parameter for SPE studies of metal ions because the pH value affects the adsorption efficiency.^33,34 The effect of the sample pH on the adsorption of Ag and Pd on the GO was studied at different pH values from 1 to 9 in model solutions containing 1 mL of reagent (1% (w/v)), 20 μg Pd and 10 μg Ag. As can be seen in Fig. 3, 90–95% of the Ag and Pd was adsorbed in the pH range 3.0–4.5. At pH 4, the recovery values for both metal ions were quantitative (95%). At lower pHs (≤3), the recovery values of Ag and Pd decreased, due to the competition between the protons and analytes for the occupation of active sites. At higher pH values, the hydrolysis of cations may occur. Therefore, pH 4 was selected for all of the subsequent studies. The described method for Au, Co, Cu, Fe, Mn, Ni, Cr, Pb, Cd and Zn at pH 4 was also applied. The recovery values varied in the range 14–88%.

Fig. 3 Effect of the sample pH on the recovery of Ag and Pd.

3.3 Effect of contact time on the adsorption and elution and the effect of centrifugation time

The effect of contact time on the adsorption and elution of the Ag and Pd ions was examined for different vortexing times (10, 30, 45, 60, 90 and 120 s). The results are shown in Fig. 4. The vortexing time of 60 s was enough for both adsorption and elution of Ag and Pd ions. Therefore, 60 s was used for both the adsorption and elution contact times in the subsequent experiments. The short time required to reach equilibrium shows that the GO–metal interaction is rather rapid.

Fig. 4 Effect of contact time on the adsorption and elution of the Ag and Pd ions.

Also, the effect of centrifugation time after the adsorption of the Ag and Pd ions was studied for durations of 2, 3, 4, and 5 min at 4000 rpm. Quantitative recovery values for both Ag and Pd were obtained after 5 min. Therefore, 5 min was selected as the centrifugation time.

3.4 Effect of the amount of reagent (dap)

The effect of the amount of 2,6-diaminopyridine on the adsorption of Ag and Pd was studied using different reagent volumes from 0 to 2 mL (20 mg) of a 1% (w/v) solution. The effect of the amount of reagent is shown in Fig. 5. The recoveries of Ag and Pd were quantitative (95%) for reagent amounts of 1.0 mL (10 mg). Without the reagent, the recovery values for both Ag and Pd were found to be 73%. Therefore 1.0 mL of reagent was chosen for the subsequent experiments.

Fig. 5 Effect of the amount of 2,6-diaminopyridine on the recovery of Ag and Pd.

3.5 Effect of type, volume and concentration of eluting reagent

The most important factors that affect the preconcentration technique are the type, concentration and volume of the eluent used for the release of the Ag and Pd ions from the adsorbent.³⁵ Thus, various concentrations of HNO₃ and HCl (5 mL eluent) were tested. The method was optimized to achieve quantitative recovery using minimal concentrations and volumes of the eluent. The results (Table 1) revealed that 2 mol L⁻¹ HCl solution was sufficient for quantitative elution of the adsorbed Ag and Pd. HCl forms anionic chloro complexes with Ag and Pd ions and strips them from the sorbent. The effect of the volume (1–6 mL) of 2 mol L⁻¹ HCl on the recovery of Ag and Pd was also studied. Quantitative recovery values could be obtained with 5 mL of 2 mol L⁻¹ HCl. So, an optimum eluent volume of 5 mL was used.

Table 1 Effect of the eluent concentration on the recovery (R) of Ag and Pd, eluent volume: 5 mL, n = 3, where n is replicate number

Conc. of HNO3 (mol L^−1)	R (%) ± s^a		Conc. of HCl (mol L^−1)	R (%) ± s^a
	Ag	Pd		Ag	Pd
a s = standard deviation.
1.0	25 ± 1	37 ± 1	1.0	70 ± 1	69 ± 2
1.5	29 ± 1	37 ± 2	1.5	72 ± 1	76 ± 1
2.0	33 ± 2	41 ± 2	2.0	95 ± 1	95 ± 2
3.0	40 ± 1	48 ± 2	3.0	95 ± 1	96 ± 0

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3.6 Effect of sample volume

In solid phase extraction studies, the sample volume is one of the most important parameters affecting preconcentration factors. The effect of the sample volume on the recovery of Ag and Pd was investigated using model solutions of 100–750 mL and the column method given in Section 2.4. The recovery values of Ag and Pd were found to be in the range 91–96% for a sample volume of 600 mL (Fig. 6). Thus, the preconcentration factor for Ag and Pd was found to be 120, based on a 5 mL eluent volume.

Fig. 6 Effect of sample volume on the recovery of Ag and Pd.

3.7 Reusability of the GO

In order to examine the stability and potential regeneration of the GO, the adsorbent was subjected to several adsorption–elution cycles. The recovery values for Ag and Pd were monitored. A 100 mg sample of the adsorbent was reused after being regenerated with 5 mL of 2 mol L⁻¹ HCl and then 5 mL of ultra pure water. The cycle results show that the adsorbent is stable for up to 150 runs without a decrease in the recoveries of Ag and Pd, indicating that it can be reused.

3.8 Effects of coexisting ions

Investigating the matrix ion effects of the real samples is an important point for preconcentration/separation studies.³⁶ The influence of matrix ions such as Na(I), K(I), Ca(II), Mg(II), Fe(III), Zn(II), Cu(II), Al(III), Ni(II), Cd(II), Mn(II), Cr(III), Pb(II), Au(III), Co(II), SO₄²⁻ and PO₄³⁻ on the adsorption of 10 μg of Ag and 20 μg of Pd from model solutions was studied under the optimum experimental conditions. The recovery values are shown in Table 2. It can be seen that the developed method is useful for determining the amount of Ag and Pd in the various water, anode slime and converter samples.

Table 2 Effect of matrix ions for the determination of Ag and Pd ions, n = 3, where n is replicate number

Ion	Concentration (μg mL^−1)	Salt	Recovery ± s^a (%)
			Ag	Pd
a s = standard deviation.
Na(I)	1000	NaCl	94 ± 1	90 ± 1
	1500		93 ± 1	94 ± 3
K(I)	1000	KNO3	98 ± 4	91 ± 2
	1500		93 ± 0	91 ± 0
Mg(II)	1000	Mg(NO3)2·6H2O	94 ± 1	95 ± 3
	1500		91 ± 4	92 ± 5
Ca(II)	1000	Ca(NO3)2	97 ± 2	91 ± 2
	1500		92 ± 5	91 ± 4
Fe(III)	25	Fe(NO3)3·9H2O	97 ± 2	93 ± 3
Zn(II)	25	Zn(NO3)2·6H2O	97 ± 3	97 ± 0
Cu(II)	10	Cu(NO3)2·3H2O	94 ± 1	92 ± 1
Al(III)	25	Al(NO3)3·9H2O	96 ± 1	93 ± 1
Ni(II)	10	Ni (NO3)2·6H2O	93 ± 1	92 ± 2
Cd(II)	5	Cd(NO3)2·4H2O	96 ± 1	95 ± 2
SO4^2−	100	Na2SO4	95 ± 0	92 ± 1
PO4^3−	50	Na3PO4	96 ± 1	94 ± 1
Mn(II)	10	Mn(NO3)2·2H2O	97 ± 0	89 ± 1
Cr(III)	10	Cr(NO3)3·9H2O	94 ± 1	93 ± 1
Pb(II)	10	Pb(NO3)2	95 ± 6	93 ± 1
Au(III)	10	Au standard solution	95 ± 1	93 ± 1
Co(II)	10	Co(NO3)2·6H2O	97 ± 1	89 ± 2

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Matrix components of the anode slime sample, determined by FAAS without using the described separation/preconcentration method, and the metal concentrations found in the eluate solution after applying the described method, are shown in Table 3. The concentration of Ag in the anode slime sample was not detected by FAAS measurement without using the separation/preconcentration step, but was found to be 22 μg g⁻¹ when the described method was applied. The method successfully separated the Ag ions from the anode slime matrix. These results show that a separation/preconcentration method is required for the determination of Ag in anode slime. In our previous study, it was described that a separation method for the determination of Pd in street sediment and catalytic converter samples is necessary.¹⁷

Table 3 Concentrations of Ag and Pd in the anode slime sample without and with application of the preconcentration method

Ion	Concentration found without applying the method (μg g^−1)	Concentration found in eluate after applying the method (μg g^−1)
a x̄ ± s: mean concentration ± standard deviation, n = 3, where n is replicate number.b nd: not detected.
Ca(II)	76 ± 2^a	4.4 ± 0.3
Mg(II)	20.6 ± 0.1	1.5 ± 0
Co(II)	120 ± 5	2.3 ± 0
Cu(II)	650 ± 32	81 ± 2
Mn(II)	13 ± 0.3	nd^b
Fe(III)	2606 ± 99	142 ± 3
Zn(II)	11.5 ± 0.2	1.5 ± 0
Pb(II)	1.40 ± 0.05	nd
Cr(III)	34 ± 1	1.8 ± 0.3
Cd(II)	nd	nd
Ni(II)	37540 ± 877	9160 ± 298
Au(III)	nd	nd
Pd(II)	nd	nd
Ag(I)	nd	22 ± 1

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3.9 Adsorption isotherms and adsorption capacity

The adsorption isotherms and adsorption capacity of GO for Ag and Pd were studied under optimal experimental conditions. For model solutions of 20 mL containing 100 mg of GO and 5–500 μg mL⁻¹ of Ag and Pd adjusted to pH 4, the described method was applied. The eluent was diluted by 10 or 20 fold. As shown in Fig. 7a for Ag and Fig. 7b for Pd, the adsorption data were fitted according to the linear form of the Langmuir isotherm model based on the following equation:³⁶
where q_e and C_e are the Ag and Pd amounts adsorbed (mg g⁻¹) and the Ag and Pd concentrations in solution (μg mL⁻¹), respectively, at equilibrium. K_L is the Langmuir constant (L mg⁻¹) and q_m is the maximum adsorption capacity of the adsorbent (mg g⁻¹). The Langmuir isotherm was used to determine the q_m and K_L values from the linear coefficients obtained by plotting C_e/q_e as a function of C_e. The adsorption capacities were found to be 1.82 mg g⁻¹ for Ag and 6.39 mg g⁻¹ for Pd.

Fig. 7 (a) Linearized Langmuir adsorption of Ag on GO. (b) Linearized Langmuir adsorption of Pd on GO.

3.10 Analytical figures of merits

The detection limit (DL, 3s/b, n = 13) of the solid phase extraction method for Ag and Pd was calculated under optimum experimental conditions after application of the preconcentration method to blank solutions. In the calculation of DLs, a preconcentration factor of 120 was used.³⁷ The DL of the method, calculated as three times the standard deviation of the blank solutions (s) divided by the slope of the calibration curve (b), was found to be 0.39 μg L⁻¹ for Ag and 0.94 μg L⁻¹ for Pd. The limit of quantifications (10s/b) for Ag and Pd were 1.32 μg L⁻¹ and 3.15 μg L⁻¹, respectively. The relative standard deviations (RSD, %, n = 10) were found to be 1.6% for Ag and 2.5% for Pd, which indicated that the described method has a good precision.

3.11 Validation and applications of the method

In order to validate the accuracy of the method, certified reference materials (SRM 2556 used auto catalyst and TMDA-70, lake water) were analysed. The results are listed in Table 4. The obtained results were in good agreement with the certified values. The described method was applied to the determination of Ag and Pd in tap water, mineral water, wastewater, anode slime and converter samples. In addition, recovery experiments on spiked amounts of Ag and Pd were carried out. The results obtained are given in Tables 5 and 6. The recovery values of Ag and Pd were found to be in the range 95–109%.

Table 4 The analysis results of certified reference materials

Element	TMDA-70 Lake water			SRM^a 2556 (Used auto catalyst pellets)
	Certified^b (μg L^−1)	Found^c (μg L^−1)	R (%)	Certified^b (μg g^−1)	Found^c (μg g^−1)	R (%)
a The main component of the matrix is Al (40%), Ca (0.1%), Ce (1%), Fe (0.8%), La (0.7%) and Si (0.2%) are present at relatively high concentrations, as well as Ba (100 mg g^−1), Zn (600 mg g^−1), Zr (300 mg g^−1), Pb (6228 mg g^−1), Pt (697.4 mg g^−1), and Rh (51.2 mg g^−1).b At 99% confidence level.c x̄ ± s: mean concentration ± standard deviation, n = 3, where n is replicate number.
Ag	10.9 ± 0.13	10.6 ± 1.0	97
Pd				326.0 ± 1.6	330 ± 24	101

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Table 5 Determination of Ag in water, anode slime and catalytic converter samples

Sample	Unit	Added	Found^a	R (%)
a x̄ ± s: mean concentration ± standard deviation, n = 3, where n is replicate number.b nd: not detected.
Tap water	μg L^−1	—	0.20 ± 0.03	—
	μg L^−1	10	10.6 ± 0.7	104
	μg L^−1	20	21.0 ± 1.3	104
Mineral water	μg L^−1	—	5.4 ± 0.3	—
	μg L^−1	25	29.5 ± 1.1	96
	μg L^−1	50	53.4 ± 0.4	96
Wastewater	μg L^−1	—	nd^b	—
	μg L^−1	50	50 ± 3	100
	μg L^−1	100	106 ± 3	106
Anode slime	μg g^−1	—	21.8 ± 0.9	—
	μg g^−1	25	46 ± 3	97
	μg g^−1	50	71 ± 1	98
Catalytic converter	μg g^−1	—	162 ± 7	—
	μg g^−1	125	288 ± 11	101
	μg g^−1	250	434 ± 11	109

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Table 6 Determination of Pd in water, anode slime and catalytic converter samples

Sample	Unit	Added	Found^a	R (%)
a x̄ ± s: mean concentration ± standard deviation, n = 3, where n is replicate number.b nd: not detected.
Tap water	μg L^−1	—	5.6 ± 0.1	—
	μg L^−1	20	25.2 ± 2.2	98
	μg L^−1	40	43.6 ± 1.8	95
Mineral water	μg L^−1	—	nd^b	—
	μg L^−1	50	53 ± 4	106
	μg L^−1	100	107 ± 3	107
Wastewater	μg L^−1	—	nd^b	—
	μg L^−1	100	109 ± 2	109
	μg L^−1	200	217 ± 18	108
Anode slime	μg g^−1	—	nd^b	—
	μg g^−1	5	5.2 ± 0.6	104
	μg g^−1	10	10.3 ± 0.9	103
Catalytic converter	μg g^−1	—	728 ± 40	—
	μg g^−1	750	1492 ± 40	102
	μg g^−1	1500	2146 ± 26	95

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3.12 Comparison with other solid phase extraction methods

Comparative data from some recent papers on solid phase extraction of trace Ag and Pd on various adsorbents are given in Table 7. The time to attain equilibrium for several adsorbents was reported to be long. In this work, the short contact time (60 s) for both adsorption and elution showed that the GO–metal interaction is rather rapid. The reusability of the adsorbent is good. The detection limit, preconcentration factor of the method and precision are comparable and/or better than those of the other methods. The adsorption capacity of GO for Ag was generally higher than the other adsorbents.

Table 7 Comparison of published methods with the method described in this work for Ag and Pd preconcentration

Element	Adsorbent/technique	pH	AC^a (mg g^−1)	PF^b	DL^c (μg L^−)	RSD (%)	Adsorption contact time (min)	Reusability	Adsorbent amount (mg)	Sample	Ref.
a AC: adsorption capacity.b PF: preconcentration factor.c DL: detection limit. QAHBA: 3-(8-quinolinylazo)-4-hydroxybenzoic acid. PDR: 5(p-dimethylaminobenzylidene)rhodanine. EAC: ethyl-3-(2-aminoethylamino)-2-chlorobut-2-enoate.
Pd	Activated carbon modified with EAC/ICP-AES	1	92	125	11	2.6	5	10	40	Smelter and road dust	10
Pd	Magnetic Fe3O4 nanoparticles/FAAS	10.5	27.2	150	2.9	1.9	2		20	Platinum–iridium alloy, road dust	38
Pd	Fe3O4 nanoparticles/ICP-AES	2.5	10.96				30				39
Ag, Pd	Nanometer sized alumina modified by QAHBA/ICP-OES	4.5–6.5	5.1, 7.6	10	0.12, 0.44	1.6, 2.3	4	30	50	Certified reference materials, natural water	40
Pd	PDR on silica gel–polyethylene glycol/FAAS	1.5	99	125	0.54	2.5–3.8			200	Water, dust, ore	41
Ag	2,4,6-Trimorpholino-1,3,5-triazin bonded on silica gel/FAAS	3.5	0.384	130		3.03		6	100	Spring, tap water	42
Ag	2-mercaptobenzothiazole bonded on silica gel/FAAS	6.2	0.343	300		2.04		4	70	Lake water	43
Ag	Di(n-propyl) thiuram disulfide bonded on silica gel/FAAS	5.8	0.330	100	24	1.43			200	Photographic waste, lake water	44
Ag, Pd	Graphite oxide-2,6 diaminopyridyne/FAAS	4	1.82, 6.39	120	0.39, 0.94	1.6, 2.5	1	150	100	Tap, mineral, wastewater, anode slime, catalytic converter samples	This work

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4 Conclusion

In this study, a new, simple, selective, accurate, rapid, low cost, and environmentally friendly solid phase extraction method using GO as an adsorbent was developed for the first time for the preconcentration of Ag and Pd ions in various water, anode slime and catalytic converter samples. The adsorbent exhibited good stability. It could be used for 150 cycles. The described method showed high tolerance to matrix ions in real samples. The preconcentration factor of the method was 120. The adsorption and elution of Ag and Pd ions were successfully achieved within 1 min. The acidic working pH (4), good precision (≤2.5%) and low detection limits (0.39 and 0.94 μg L⁻¹) are the other important properties of the described method. The method can be used for Ag and Pd preconcentration from various water, anode slime and catalytic converter samples in ordinary analysis laboratories.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra48028c

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