J. C.
García-Mesa
,
P.
Montoro-Leal
,
S.
Maireles-Rivas
,
M. M.
López Guerrero
* and
E.
Vereda Alonso
*
Department of Analytical Chemistry, Faculty of Sciences, University of Malaga, Campus of Teatinos, 29071 Málaga, Spain. E-mail: eivereda@uma.es; mmlopez@uma.es
First published on 31st March 2021
Mercury is a non-essential trace element that is toxic to humans due to the bioaccumulation effect. In this work, a ferrofluid based on Fe3O4@graphene oxide nanospheres together with an ionic liquid was used to develop a magnetic dispersive solid-phase extraction (MDSPE) method for the extraction of the complex formed between the chelating agent methyl thiosalicylate (MTS) and mercury. This MDSPE methodology was combined with an automatic analysis by flow injection-cold vapour-graphite furnace atomic absorption spectrometry (FI-CV-GFAAS). The developed semiautomatic method was applied to the determination of ultra-trace amounts of Hg(II) in biological and environmental samples. Several analytical parameters for MDSPE and FI-CV-GFAAS, such as pH, MTS concentration, eluent composition, extraction time, etc., were optimized by uni and multivariate methodologies. Under the optimized conditions, the %RSD, detection limit and determination limit were 2.9%, 0.25 ng L−1 and 4.9 ng L−1, respectively. The achieved preconcentration factor with the MDSPE methodology was 250. The accuracy of the proposed method was verified using a Standard Reference Material (Mussel Tissue SRM 2976) and by determining the analyte content in spiked seawater and tap water samples collected from Málaga and Cádiz (Spain). The determined values were in good agreement with the certified values and the recoveries for the spiked samples were close to 100% in all cases. The results showed that the proposed method is simple, rapid, environmentally friendly, highly sensitive and accurate for determination of mercury in biological and environmental samples.
The SPE methods present excellent properties such as rapid phase separation, high selectivity, low cost, less use of organic solvents, simple extraction, high recovery, high preconcentration factor and automation of more detection techniques.16–19 Magnetic SPE (MSPE) is a new type of SPE developed by Šafaříková and Šafarík for enriching pollutants with magnetic materials.20 In MSPE, a magnetic adsorbent is added to the solution containing the target analytes. After adsorption of the analytes, the adsorbent is separated from the solution using an external magnetic field. Thus, filtration and centrifugation processes are avoided.21–23
The exploration of new magnetic nanomaterials by combining magnetic inorganic materials and non-magnetic adsorbents is an active research area in MSPE. The best adsorbents are nanomaterials due to their main advantages such as high surface-to-volume ratio, easy derivation procedures and unique properties. Among non-magnetic adsorbents, graphene oxide (GO) is characterized by being cheap and easily scalable to a high-volume production. In addition, GO is well-suited for chemical modification and subsequent processing.24 This sorbent possesses a large surface area and a high density of oxygen-containing polar functional groups on the surface (epoxy, carboxylic acid, carbonyl and hydroxyl groups), as well as a rich delocalized π–π electron systems that make it interact strongly with organic compounds with benzene rings.25 GO can be modified with magnetic nanoparticles (MNPs) for use in MSPE, which can reduce the equilibrium time due to the fast mass transfer. Different magnetic nanomaterials based on graphene have been used successfully as adsorbents for preconcentration and determination of mercury, such as Fe3O4/GO26 and graphene/ZnFe2O4.27 On the other hand, nowadays, the preconcentration steps tend towards miniaturization, so solid-phase microextraction (SPME),28 dispersive liquid–liquid microextraction (DLLME),29 single-drop microextraction,30 and so on are being widely used as extraction techniques.7
Magnetic dispersive solid-phase extraction (MDSPE) was applied for the first time in 2013 by Farahani et al.31 In this technique, an adequate volume of a ferrofluid is quickly injected into an aqueous sample using a syringe. The large contact surface between the two phases accelerates the mass transfer processes and improves the extraction kinetics; in addition, the phase separation is facilitated with the aid of an external magnetic field. Ferrofluids are stable colloidal suspensions of magnetic nanomaterials in an ionic liquid, showing both magnetic and fluid properties. Ionic liquids (ILs) are solvents with unique physicochemical properties including negligible vapour pressure and ability to be miscible in water and organic solvents, and have attracted a lot of attention for use as extractants in microextraction techniques. ILs can bind to the carbon network structures of GO via π–π electronic interactions causing strong connections by physical crosslinking.32 Metal ions tend to stay in the aqueous phase, but their hydrophobicity will be increased upon complexation with a suitable ligand. The complex formed can be quickly extracted in the ferrofluid.
To apply ferrofluids to the extraction/preconcentration of mercury and increase the efficiency towards mercury extraction during the preconcentration treatment, thiolate ligands can be used as mercury chelating agents. Thiosalicylic acid is an interesting heterodifunctional ligand. Combination of both hard (O) and soft (S) donor atoms and the ability of both carboxylate and thiolate groups to bridge two metal centres33,34 provide a multitude of bonding opportunities to metals in either their mono- or doubly deprotonated states.35,36 It is well known that thiosalicylic acid and its derivatives are organic ligands commonly used for medical purpose to treat mercury poisoning.37 The formed organic mercury complex can be easily extracted from the matrix during the MDSPE process. In this work, a magnetic sorbent material was fabricated by coupling magnetic iron nanoparticles (MNPs) and graphene oxide (GO), resulting in shell structured Fe3O4@graphene oxide nanospheres called magnetic graphene oxide (MGO). The material was suspended in the ionic liquid (IL) 1-n-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4], resulting in a ferrofluid with excellent adsorbent properties. Thus, a MDSPE/CV-GFAAS method was optimized for the determination of ultra-trace amounts of Hg in environmental water and biological samples, using the ferrofluid described and the chelating agent methyl thiosalicylate (MTS). The preconcentration efficiency of the developed method, due to MDSPE and CV, resulted in excellent detection and determination limits compared with other similar methods reported in the bibliography.
A pH meter and a conductivity meter Hatch (Loveland, CA, USA) were employed for pH and ionic strength adjustments, respectively.
An ultrasonic bath VWR (West Chester, PA, USA) Unique, model USC 2800, 40 kHz, and a Vortex VWR (West Chester, PA, USA), model UV-2500, multi tube vortex mixer were also employed.
For the evaluation of the accuracy of the proposed method, a reference material was digested in a Milestone ultraWAVE microwave oven (Sorisole, Italy) equipped with 25 mL PTFE/TFM vessels.
Hg stock standard solution, 1000 mg L−1, from Merck (Darmstadt, Germany) was used. Standards of working strength were made immediately prior to use by appropriate dilution as required. In order to adjust the pH of standards and samples, a 1 M solution of hydrochloric acid was prepared from hydrochloric acid, 37% wt/wt, Merck (Darmstadt, Germany), art. number 113386. Finally, a 0.2% (wt/v) sodium tetrahydroborate(III), Acros Organics (Geel, Belgium), solution prepared in 0.1% (wt/v) NaOH, Sigma Aldrich Chemie (Steinheim, Germany), was used as a reductant for the generation of Hg cold vapour.
For the synthesis of MGO, ferric chloride hexahydrate (FeCl3·6H2O), ferrous chloride tetrahydrate (FeCl2·4H2O), ammonium hydroxide 30% (wt/wt), methanol, sodium chloride and H2SO4 98% were purchased from Merck (Darmstadt, Germany) and H2O2 35% from Scharlab (Barcelona, Spain). 3-Aminopropyltriethoxysilane was obtained from Fluka (Buchs, Switzerland). Brij 76C18EO10, tetraethoxysilane (TEOS), N,N′-dicyclohexylcarbodiimide (DCC), graphite, NaNO3 and KMnO4 were acquired from Aldrich Chemie (Steinheim, Germany). Ethanol was supplied by Carlo Erba (Milano, Italy).
For the Hg-complex (Hg–MTS), methyl thiosalicylate from Sigma Aldrich Chemie (Steinheim, Germany) was employed. The ionic liquid 1-n-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) was purchased from Merck (Darmstadt, Germany).
The Standard Reference Material (SRM 2976) analysed to determine the accuracy of the proposed procedure was from the National Institute for Standard & Technology (NIST): SRM 2976 Mussel Tissue. Seawater and tap water samples were collected in glass bottles (previously cleaned by soaking for 24 h in 10% (wt/wt) nitric acid and finally rinsed thoroughly with ultrapure water before use). Samples were immediately filtered by using a membrane of 0.45 mm pore size cellulose nitrate filters from Millipore (Bedford, MA, USA). After that, the samples were stored at 4 °C as recommended by Method 3010B from the Environmental Protection Agency (USA), for less than 3 days until analysis. Nitric acid 65% (wt/wt) was supplied by Merck (Darmstadt, Germany), art. number 100452.
For seawater and tap water samples, aliquots of 5 mL and 20 mL of sample, respectively, were placed in a 50 mL volumetric flask, then 5 mL of 1 M HCl (pH 1), 5 g of NaCl, and 350 μL of 1% MTS (v/v) were added, and de-ionized water was added up to the mark.
All samples were analysed immediately after preparation.
Mercury was eluted from the sorbent by adding 2 mL of eluent (HNO3 0.5% and thiourea 0.5%) and stirring by vortexing for 1 min. Finally, the sorbent was separated using a magnet and the supernatant was poured into a polyethylene tube for mercury determination by FI-CV-GFAAS.
To determine the extraction process efficiency, a 100 ng L−1 Hg solution was prepared and extracted under optimal conditions. The supernatant was filtered and analysed by ICP-MS, showing a Hg concentration below the LOD. Then, the extraction efficiency was considered close to 100%.
The FI system configuration is shown in Fig. 1, and it was operated as follows: during the 11 s sample loading period, with the valve in the “fill” position, a 4.1 mL min−1 flow of sample (standard or blank) at pH 1.0 was pumped (via pump P1) through the 500 μL loop located in the valve. Then, the valve position was changed to inject position and P1 was stopped, while P2 pumped water at a rate of 1.8 mL min−1 through the loop dragging the sample to the chemical vapour generator. Thus, the mercury merges with 0.6 mL min−1 flow of the reductant in the mixing coil, where direct generation of mercury vapour takes place. The gas generated and the solvent were then passed into the gas–liquid separator which separates gases from liquid. The liquid was drained, and the generated vapour was swept into the graphite furnace through the 26 cm tubing, until the tip of the autosampler arm, by a stream of 250 mL min−1 argon. In this procedure, the FI system and the GFAAS instrument were coupled and operated completely synchronously.
Fig. 1 FI system schematic diagram for the loading step (A) and elution step (B). P1 and P2, peristaltic pumps; W, waste; S, sample; R, reductant. |
Two different strategies were used: a one-at-a-time method (changing one parameter at a time while keeping the others constant) and a multivariate response surface experiment design.
Some parameters relevant to the optimization were elution of Hg in the manifold and reaction conditions for CV (reagent concentrations). For that reason, a response surface design was performed. The variables to be optimized were the concentrations of NaBH4, thiourea and HNO3. The lower and upper values given for each factor were 0.0% and 4.0% for NaBH4 concentration, 0.0% and 5.0% for thiourea concentration and 0.0% and 5.0% for HNO3 concentration. The response surface design used was a rotatable central composite design (CCD) which includes a 23 factorial design (8 experiments), a 2 × 3 star design (6 experiments) and 3 centre points (3 experiments). The resulting 17 experiments required for that design were randomly performed, and as response function, the Hg signal (peak area) was chosen. The results of the experiments were processed using the statistical software Statgraphics Centurion XVI. The significance of the effects was checked by analysis of the variance (ANOVA) and using p-value significance levels. This value represents the probability of the effect of a factor being due solely to random error. Thus, if the p-value is less than 5%, the effect of the corresponding factor is significant.
Once the concentrations of the reductant and eluent solutions were optimized, the rest of the experimental parameters for the MDSPE/CV-GFAAS were optimized by the one-at-a-time method in order to obtain the best peak area signal.
Fig. 2 Effect of pH on the extraction of 10 μg L−1 Hg(II), prepared with 0.01% (v/v) MTS, 0.5% (wt/v) NaCl, and the respective buffer (pH 1–11). |
The following parameters of the MDSPE were optimized: (a) concentration of MTS; (b) influence of ionic strength; (c) extraction time; (d) elution conditions and CV generation. Respective data and figures are given in the ESI.† The following experimental conditions were found to give the best results: (a) a MTS concentration of 0.007% (Fig. S2, ESI†), (b) a NaCl concentration of 10.0% (Fig. S3, ESI†), (c) an extraction time between MGO and MTS–Hg complex of 1 min (Fig. S4, ESI†) and (d) elution conditions and CV generation: 0.5% thiourea, 0.5% HNO3 and 0.5% NaBH4 (Fig. S6, ESI†), and a reductant flow rate of 0.6 mL min−1 (Fig. S7, ESI†).
Fig. 3 Study of the aqueous phase volume for extraction of 10 μg L−1 Hg(II), prepared with 0.007% (v/v) MTS, 10% (wt/v) NaCl, and 1 M HCl until pH = 1.0. |
The precision of the whole method was evaluated in terms of inter-day precision, using the relative standard deviation (RSD), calculated as the average of relative standard deviations of 2, 50 and 100 ng L−1 standards measured on three days. The calculated inter-day precisions were 2.9, 2.2 and 1.4%, respectively. The preconcentration factor calculated as the ratio of sample volume to eluent volume and considering that the extraction process efficiency was close to 100% (Section 2.5) was 250.
Sample | Added (μg L−1) | Found ± standard error (μg L−1) | Recovery (%) |
---|---|---|---|
Tap water | — | — | — |
Spike 1 | 0.02 | 0.020 ± 0.002 | 100.0 |
Spike 2 | 0.06 | 0.056 ± 0.003 | 93.3 |
Tarifa seawater | — | 0.018 ± 0.006 | — |
Spike 1 | 0.04 | 0.053 ± 0.006 | 87.5 |
Spike 2 | 0.12 | 0.140 ± 0.007 | 101.7 |
Malaga seawater | — | 0.0075 ± 0.0002 | — |
Spike 1 | 0.04 | 0.042 ± 0.003 | 86.3 |
Spike 2 | 0.12 | 0.1297 ± 0.0004 | 101.8 |
NIST 2976 Mussel Tissue | Certificate total value | ||
61.0 ± 3.6 μg kg−1 | 63 ± 5 (μg kg−1) | 103.3 |
Hg(II) was found in seawater samples due to the high sensitivity of the method, and the concentrations (0.018 ± 0.006 and 0.0075 ± 0.0002 μg L−1 in Tarifa and Málaga seawater, respectively (Table 1)) were below the allowed limits by Spanish legislation (RD 817/2015)6 and within the normal concentration ranges.
For comparison purposes, the analytical performance data of similar methods reported in the literature are listed in Table 2. A direct comparison of the figures of merit for the developed method with results from other workers is difficult due to the different experimental conditions. All the methods presented in Table 1 consist of preconcentration and determination procedures combined with AAS for the determination of Hg(II). As can be seen, the analytical performances, such as LOD, RSD, and PF, of the method reported in this work are the best. The preconcentration method on the ferrofluid was easy and the Hg preconcentration required only one minute and another minute for elution, being a very efficient procedure, with relative recoveries between 86 and 103%. Besides the preconcentration on the ferrofluid, another preconcentration occurs on the graphite tube thanks to the Ir cover. To our knowledge, this is the first reported method that combines MDSPE and CV-GFAAS for Hg(II) determination. The use of CV generation and the preconcentration in the Ir permanent modifier explain the better results in the analytical performance of the method compared with recent literature methods.
Method | Solid phase/reusability | Samples | Linear range (μg L−1) | Analytical performance | Relative recoveries (%) | Ref. | ||
---|---|---|---|---|---|---|---|---|
LOD (μg L−1) | RSD (%) | PF | ||||||
a DLLME: dispersive liquid–liquid microextraction. b G/ZnFe2O4: graphene/ZnFe2O4 nanocomposite adsorbent. c AFS: atomic fluorescence spectroscopy. d g-C3N4/Fe3O4: magnetic graphitic carbon nitride nanocomposites. e AMA: advanced mercury analyser. f μ-SPE: micro solid phase extraction. g UV-PVG/μ-CCP-OES: ultraviolet photochemical vapor generation/capacitively coupled plasma microtorch optical emission spectrometry. | ||||||||
DLLMEa/GFAAS | — | Blood | 0.3–60 | 0.1 | 3.7 | 112 | 90–109 | 14 |
DLLME/GFAAS | — | Biological | 0.5–50 | 0.1 | 6.2 | 68 | 90.5–108.0 | 15 |
MDSPE/CV/AAS | MGO/— | River water, cow milk, omega 3 and lipstick | 1–200 | 0.57 | 6.5 | 21 | 86–105 | 8 |
MDSPE/CV/AAS | MGO/— | Seafood | 1–85 | 0.025 | 4.0 | 17 | 85 | 26 |
MDSPE/CV/AAS | G/ZnFe2O4b/50 cycles | Biological and well, tap and wastewater | 0.25–10 | 0.01 | 2.7 | 30 | 91–107 | 27 |
MDSPE/CV/AFSc | g-C3N4/Fe3O4d/4 cycles | Natural water | 0.01–0.6 | 0.0014 | 4.6 | 40 | 85.0–116.7 | 28 |
DSPE/AAS/AMAe | Graphene/— | Environmental water including seawater | 0.00038–1.038 | 0.00038 | 3.0 | — | — | 40 |
MDSPE/CV/AFS | Au NP–Fe3O4/— | Environmental water | 0.005–0.2 | 0.0015 | 3.7 | 80 | 92.5–108.7 | 41 |
DLLME/CV/AAS | — | Blood | 0.15–85 | 0.03 | <4 | 6.6 | >97 | 42 |
μ-SPEf/CV/AFS | IL–3D graphene–Ni foam/250 | Environmental water, tap water, mineral water | 0.01–8 | 0.0036 | 4.1 | 180 | 101–105 | 43 |
UV-PVG/μCCP OESg | — | Tap water, pool water, well water, bottled water, food | 0–1 | 0.0001 | 2.6–12.7 | 41 | 82–108 | 44 |
MDSPE/CV/GFAAS | MGO/— | Biological and environmental water including seawater | 0.002–0.200 | 0.00025 | 2.9 | 250 | 86–103 | This work |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ja00516a |
This journal is © The Royal Society of Chemistry 2021 |