Carbon nanotubes magnetic hybrid nanocomposites for a rapid and selective preconcentration and clean-up of mercury species in water samples

Corals-inspired magnetic absorbents for fast and efficient removal of microplastics in various water sources

Microplastics (MPs) as the formidable pollutants with high toxicity and difficult degradation may threaten the aquaculture industry and human health, making it highly necessary to develop the effective removal methods. In this article, Fe3O4 nanoparticles (NPs) were initially fabricated with mesoporous structure, but showing undesirable adsorption efficiencies for the adsorption of MPs (lower than 70%). Inspired by the reefs-rebuilding corals acting as the sinks for various marine pollutants like plastic, Fe3O4 NPs were coated further with adhesive polymerized dopamine (PDA) yielding Fe3O4@PDA absorbents. Unexpectedly, it was discovered that the corals-mimicking absorbents so formed could allow for the removal of MPs with dramatically enhanced efficiencies up to 98.5%, which is over about 30% higher than those of bare Fe3O4 NPs. Herein, the PDA shells might conduct the increased adhesion to MPs, presumably through the formation of hydrogen bonding, π-π stacking, and hydrophobic interactions. A fast (within 20 min) and stable adsorption of MPs can also be expected, in addition to the PDA-improved environmental storage of Fe3O4 NPs. Subsequently, the Fe3O4@PDA adsorbents were utilized to remove MPs from different water sources with high efficiencies, including pure water, suburban streams, village rivers, lake water, inner-city moats, and aquaculture water. Such a magnet-recyclable adsorbent may provide a new way for rapid, effective, and low-cost removal of MPs pollutants from various water systems.

Thermal Fabrication of Magnetic Fe3O4 (Nanoparticle)@Carbon Sheets from Waste Resources for the Adsorption of Dyes: Kinetic, Equilibrium, and UV-Visible Spectroscopy Investigations

Thermal treatment is applied for the direct conversion of palm stalk waste to Fe₃O₄ (np)@carbon sheets (Fe₃O₄ (np)@CSs). The effect of conversion temperature was investigated. The TEM examination of the prepared magnetic Fe₃O₄ (np)@CSs showed the formation of Fe₃O₄ (np) in a matrix of carbon sheets as a coated layer with surface functional groups including carbonyl and hydroxyl groups. Removal of dyes such as methyl orange, methylene blue, and neutral red was achieved using fabricated Fe₃O₄ (np)@CSs which were prepared at 250 °C, 400 °C, and 700 °C in a weak acidic medium. By studying the contact time effect for the adsorption of methylene blue, neutral red, and methyl orange, using the fabricated Fe₃O₄ (np)@CSs which were prepared at 250 °C and 400 °C, equilibrium occurred between 120 min and 180 min. In addition, the first-order and second-order kinetic models were applied to the adsorption data. The results revealed that the adsorption data fit better with the second-order kinetic model. Furthermore, the Freundlich model was found to be more suitable for describing the process of the separation of the dyes onto Fe₃O₄ (np)@CSs which were prepared at 250 °C and 400 °C, suggesting heterogenous surfaces and multi-layer adsorption.

Recruiting chemical grafting method for surface modification of stainless steel to fabricate a selective sorbent for solid phase microextraction of mercury metal ion

Keeping selectiveness and efficiency in view with solid-phase microextraction (SPME) of metal ions, this work was aimed at synthesis of a novel modified sorbent on a stainless-steel surface to fabricate a selective and efficient fiber for SPME of mercury ions from real food and biological samples. After the confirmation of sorbent structure grafted on the stainless-steel surface, by different techniques, the synthesized fiber was utilized for extraction and preconcentration of mercury before its measurement by an inductively coupled plasma-optical emission spectroscopy (ICP-OES). For optimizing the efficiency, the influences of various factors on the extraction of Hg (II) ion were scrutinized. The optimized values used for extraction were pH 7.0, adsorption time 8 min, desorption time 5 min, 5 mL of eluent solvent containing nitric acid with concentration of 0.5 mol L^-1, and stirring rate of 300 rpm. Underneath optimum condition, the relative standard deviation for 30 extractions, done by one synthesized fiber, was calculated to be 2.89% and for five extractions, done by 5 synthesized fibers, was calculated to be 1.78%. The high performance of the synthesized fiber was checked with high recoveries obtained from 30 successive sorption-desorption cycles, using a unique synthesized fiber. Finally, the suggested procedure was triumphally exploited for extraction and pre-concentration of Hg (II) ion in real food and biological samples.

Ultrasound-assisted dispersive solid phase extraction for promoting enrichment of ng L-1 level Hg2+ on ionic liquid coated magnetic materials

Herein, we investigated the enrichment behavior of inorganic mercury (Hg²⁺) on magnetic adsorbent with different ultrasound (US) energy field input. The enrichment rate of 0.10 μg L^-1 mercury is increased by 4.5 times after US instead of stirring as dispersion mode. The input of higher frequency and power ultrasound can accelerate the enrichment of magnetic ionic liquid adsorbent and reduce the Hg²⁺ residue, importantly, which has not been reported. The positive correlation between cavitation effect and acoustic frequency and power in imaging experiments documents that US parameters are the key factors affecting the magnetic solid phase extraction. In addition, in-situ desorption and detection of adsorbate and recovery of adsorbent can be realized by slurry vapor generation (SVG) technology. The recovery of Hg²⁺ in four cycles is more than 90%, which indicates that the structure and properties of the material are not affected by the application of US. Hence, the degradation of adsorption properties caused by agglomeration of magnetic materials can be improved by introducing dispersion methods such as US. At the same time, 95% enrichment efficiency and 0.01-1.0 μg L^-1 linear calibration range corresponding to 150 mL sample documents that magnetic ionic liquid adsorbent combined with US and sensitive spectral detector can meet the needs of ng L^-1 level Hg²⁺ analysis in natural water samples.

Magnetic solid-phase extraction based on sulfur-functionalized magnetic metal-organic frameworks for the determination of methylmercury and inorganic mercury in water and fish samples

A novel magnetic metal-organic frameworks (Fe₃O₄@UiO-66-SH) was successfully prepared by coating Fe₃O₄ nanospheres with sulfur-functionalized UiO-66. The Fe₃O₄@UiO-66-SH possesses both the magnetic properties of Fe₃O₄ and the diverse properties of metal-organic framework (MOF) in one material, which has the superiority of high surface area, easy-operation and strong adsorb ability with mercury, is used for the magnetic solid-phase extraction of methylmercury (MeHg⁺) and inorganic mercury (Hg²⁺) in water and fish samples. The analyzes were conducted by high performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS). The different pretreatment conditions influencing the extraction recoveries of Hg²⁺ and MeHg⁺, including adsorbent amount, pH, extraction time, elution solvent, elution volume, desorption time, co-existing ions and dissolved organic materials were investigated. Under the optimized conditions, the limits of detection (LODs) of Hg²⁺ and MeHg⁺ for water samples were 1.4 and 2.6 ng L^-1, and the limits of quantification (LOQs) of Hg²⁺ and MeHg⁺ for water samples were 4.7 and 8.7 ng L^-1. The enrichment factors (EFs) were 45.7 and 47.6 fold for Hg²⁺ and MeHg⁺, respectively. The accuracy of the proposed method was demonstrated by analyzing the certified reference material of fish tissue (GBW10029) and by determining the analyte content in spiked water and fish samples. The determined values were in good agreement with the certified values and the recoveries for the spiked samples were in the range of 84.5-96.8%.

Sensitive Mercury Speciation Analysis in Water by High-Performance Liquid Chromatography-Atomic Fluorescence Spectrometry Coupling with Solid-Phase Extraction

An efficient method based on high-performance liquid chromatography coupled with atomic fluorescence spectrometry (HPLC-AFS) was successfully developed for the simultaneous determination of four mercury species including Hg²⁺, methylmercury (MeHg), ethylmercury (EtHg), and phenylmercury (PhHg) in water. Samples were enriched and cleaned up with a solid-phase extraction (SPE) pretreatment using a thiol cartridge. Some key parameters including the selection of a SPE cartridge, eluent type, eluent volume, and interference factors were systematically investigated. Chromatographic separation was achieved on a C₁₈ column using a mobile phase consisting of methanol and 60 mmol L^-1 ammonium acetate with 10 mmol L^-1 L-cysteine by gradient elution. Under the optimized conditions, good linearity (r ≥ 0.9991) was observed between 0.20 to 10.0 μg L^-1. The limits of detection were in the range of 0.001 - 0.002 μg L^-1. High recoveries (87.2 to 111%) and good reproducibility (1.1 - 6.5%) were obtained. Such a method is sensitive, selective and accurate, which can be applied to the quantification of mercury species in water samples.

Ultra-thin graphene oxide membrane deposited on highly porous anodized aluminum oxide surface for heavy metal ions preconcentration

Analyte extraction using graphene oxide (GO) can be challenging owing to the stochastic behavior of the permeation of water molecules and heavy metal ions, imperfect pore structures, and irregular distribution of multi-layer sheets. We prepared a free standing, through-hole graphene oxide membrane deposited on porous anodic aluminum oxide (AAO) substrate. The hydrophilicity of the GO membrane was improved via oxygen plasma treatment. The resulting AAO-GO membrane was used as novel adsorbent for the heavy metal ions preconcentration prior to their determination using inductively coupled plasma optical emission spectroscopy. This sub-micrometer-thick membrane allowed unimpeded permeation of water molecules via two-dimensional capillaries formed across the pores and in between the closely spaced GO sheets. The proposed method shows good detection limit of 1.2 ng L^-1, and the co-existing ions did not affect the extraction efficiency of the adsorbent. The accuracy of the method was assessed by analyzing standard reference materials, where the Student's t-test values were less than the critical Student's t-value of 4.303 (95% confidence level). Good precision was achieved, as coefficients of variation ranged between 4% and 5%. The developed SPE adsorbent is a promising alternative for bulk adsorbents owing to the wide variety of available 2D materials and deposition methods.

Magnetic nanomaterials as sorbents for trace elements analysis in environmental and biological samples

This review focuses on magnetic nanomaterials as sorbents for trace elements analysis in environmental and biological samples. The design and preparation of magnetic nanomaterials with specific functional groups for trace elemental analysis are summarized, along with relevant adsorption mechanism. The application of these magnetic sorbents in different operation modes for the quantification of trace elements and their species in environmental and biological samples are discussed. The trend of development in this field is also prospected.

Development of highly efficient and reusable magnetic nitrogen-doped carbon nanotubes for chlorophenol removal

Nitrogen-doped carbon nanotubes (N-CNTs) were synthesized via a hydrothermal method and further modified with magnetic Co_0.5Cu_0.5Fe₂O₄ nanoparticles following a one-pot solvothermal method. The characterization data show that the distribution of the magnetic materials and the adsorption characteristics of the CNTs can be tailored as a function of the N doping amount. The N-CNT adsorption isotherms as a function of N content and chlorophenol uptake show that a N doping level of 6% is optimum. After loading the N-CNTs with the magnetic Co_0.5Cu_0.5Fe₂O₄ nanoparticles (M-N-CNTs), the resulting materials were easily dispersed in aqueous media with specific surface area reaching 95.64 m²/g. The M-N-CNTs exhibit high affinities toward the adsorption of different chlorophenols following the order: Pentachlorophenol (PCP) > 2,4,6-trichlorophenol (2,4,6-TCP) > 2,4-dichlorophenol (2,4-DCP) > 2,4-dichlorophenoxyacetic acid (2,4-D) > phenol. Additionally, the M-N-CNTs exhibit good microwave absorption performance and can be regenerated by microwave irradiation with high efficiencies (> 90%) maintained with high stabilities.

Preparation of environmental samples for chemical speciation of metal/metalloids: A review of extraction techniques

Chemical speciation is a relevant topic in environmental chemistry since the (eco)toxicity, bio (geo)chemical cycles, and mobility of a given element depend on its chemical forms (oxidation state, organic ligands, etc.). Maintaining the chemical stability of the species and avoiding equilibrium disruptions during the sample treatment is one of the biggest challenges in chemical speciation, especially in environmental matrices where the level of concomitants/interferents is normally high. To achieve this task, strategies based on chemical properties of the species can be carried out and pre-concentration techniques are often needed due to the low concentration ranges of many species (μg L^-1 - ng L^-1). Due to the significance of the topic and the lack of reviews dealing with sample preparation of metal (loid)s (usually, sample preparation reviews focus on the total metal content), this work is presented. This review gives an up-to-date overview of the most common sample preparation techniques for environmental samples (water, soil, and sediments), with a focus on speciation of metal/metalloids and determination by spectrometric techniques. Description of the methods is given, and the most recent applications (last 10 years) are presented.

Graphene oxide lamellar membrane with enlarged inter-layer spacing for fast preconcentration and determination of trace metal ions

We report a graphene oxide (GO) lamellar membrane with increased inter-layer spacing for efficient permeation of water molecules and heavy metal ions through nanoporous graphene oxide. The inter-layer spacing of the GO sheets in the lamellar structure was increased by introducing poly-aminophosphonic acid (APA) in between the GO sheets. We demonstrate experimentally, the use of a prepared membrane (GO–APA) by a SPE technique for the preconcentration and extraction of heavy metal ions by chelate formation and their determination by ICP-OES. We found that this sub-micrometer-thick membrane allows unimpeded permeation of water molecules through two-dimensional capillaries formed across the pores and by closely spaced graphene sheets. Compared to the bulk GO sorbent, GO–APA membrane offers enhanced sensitivity and permeability for heavy metal ions due to relatively large inter-layer spacing and high surface area (extraction phase) with a high number of active functional groups. The potential of this technique for the preconcentration and extraction of Pb(ii), Cd(ii) and Cu(ii) is illustrated with the contaminated ground water and industrial waste water analysis. The detection limit achieved for studied ions was 1.1 ng L⁻¹, under optimized experimental conditions. The co-existing ions did not hinders the extraction of trace heavy metal ions. Accuracy of the developed method was assessed by analyzing Standard Reference Materials. The Student's t test values were found to less than the critical Student's t value of 4.303 at the 95% confidence level. The method shows good precision as coefficients of variation for five replicate measurements were found to be 4–5%.

A porous graphene oxide membrane with increased interlayer spacing of GO sheets was prepared by covalently introducing poly-aminophosphonic acid in between the GO sheets. The membrane was successfully employed for the extraction of heavy metal ions.

Magnetic solid-phase extraction for speciation of mercury based on thiol and thioether-functionalized magnetic covalent organic frameworks nanocomposite synthesized at room temperature

A simple and practical magnetic solid-phase extraction high-performance liquid chromatography-inductively coupled plasma mass spectrometry (MSPE-HPLC-ICP-MS) method for extraction and determination of trace mercury species, including inorganic mercury (IHg), monomethylmercury (MeHg) and ethylmercury (EtHg), was developed. The MSPE adsorbent, urchin-like thiol and thioether-functionalized magnetic covalent organic frameworks (Fe₃O₄@COF-S-SH), was synthesized by coating covalent organic frameworks (COFs) on the surface of Fe₃O₄ nanoparticles at room temperature and then easily grafting 1,2-Ethanedithiol on the COFs. The as-prepared Fe₃O₄@COF-S-SH has strong adsorption capacity for IHg, MeHg and EtHg, with excellent static adsorption capacity: 571, 559 and 564 mg g^-1, respectively. The parameters influencing the extraction and enrichment had been optimized, including pH, adsorption and desorption time, composition and amount of the eluent, co-existing ions and dissolved organic materials etc. Under the optimized condition, the limit of detection (3δ) of the proposed method were 0.96, 0.17 and 0.47 ng L^-1 for IHg, MeHg and EtHg, and the developed method has high actual enrichment factors of 370, 395, 365-fold for IHg, MeHg and EtHg based on 200 mL samples, respectively. The high accuracy and reproducibility has been proved by the spiked recoveries (96.0‒108 %) in real water samples and determination of the certified reference material. Both the adsorption and desorption process can be completed within 5 min. The proposed method with simple operation, short pre-concentration time and high sensitivity has been successfully applied to mercury speciation at trace levels in the samples with complicated matrices, including underground water, surface water, sea water and fish samples.

The synergistic effect of ultrasound power and magnetite incorporation on the sorption/desorption behavior of Cr(VI) and As(V) oxoanions in an aqueous system

Though abundant studies have targeted the sorption of Cr(VI) and As(V) anions by organic polymers or magnetic metal oxides, there is no research literature on the sorption characteristics of Cr(VI) and As(V) by thiourea-formaldehyde resin (TF) and its magnetic derivative (MTF). TF resin is a strong chelating agent, which has several practical applications. This paper reports on the removal of Cr(VI) and As(V) oxoanions by TF and MTF sorbents. The sorbents were characterized by Fourier transform infrared (FTIR), nuclear magnetic resonance (NMR) spectroscopy, elemental analysis, zetametry, Brunauer-Emmett-Teller (BET) analysis, and differential light scattering (DLS). The synergistic influence of magnetite incorporation and ultrasonic power on sorption kinetics, isotherms, and oxoanion desorption were investigated, including the analysis of the uncertainty in the study results. The relationship between kinetic and equilibrium constants of the two sorbents under normal shaking and ultrasound shaking was analysed. Ultrasound power improved the mass transfer and makes the sorption ultra-fast while magnetite enhanced the sorption capacity. The MTF particles sorbed 4.28 and 1.97 mmol g^-1 of Cr(VI) and As(V), respectively, under optimum conditions. Further, desorption kinetics and efficiency of Cr(VI) and As(V) were estimated using normal shaking and ultrasonic agitation. Ultrasound power reduced the time and the concentration of NaCl required for the stabilization of desorption efficiency.

Magnetic graphitic carbon nitride nano-composite for ultrasound-assisted dispersive micro-solid-phase extraction of Hg(II) prior to quantitation by atomic fluorescence spectroscopy

A nanosheet structure of Fe₃O₄/g-C₃N₄ magnetic composite has been prepared and utilized as adsorbent for the ultrasound-assisted dispersive magnetic micro-solid-phase extraction of the ultra-trace inorganic mercury [Hg(II)]. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Fouries transform infrared spectroscopy images manifested that the hydrothermal systhesis promoted the binding of Fe₃O₄ particles with g-C₃N₄. The enrichment performance of composites depended on their compositions, and the recovery of Hg(II) on C-m30 (with Fe₃O₄/g-C₃N₄ mass ratio 2:3) was higher than that on other ratios. Recorded data showed that ultrasound wave was an effective method for reducing the agglomeration of magnetic materials and enhancing their adsorption properties. The Hg signal obtained by 80% power input is 3.2 times stronger than the signal by strring mode. X-ray diffraction characterization of the recovered adsorbent showed that g-C₃N₄ needed to be updated after repeated use for four times. In addition, competitive adsorption may be main factor affecting the inhibitory effect of high concentration methylmercury on inorganic mercury. Under the optimized conditions, the detection limit of the proposed method was 1.4 ng L^-1 and relative standard deviation of 4.6% for 0.05 μg L^-1 Hg(II) was obtained. The linear calibration range was 0.005-0.4 μg L^-1. The accuracy of the method was verified through analysis of the certificated reference materials. The proposed method has been applied in the determination of inorganic Hg in natural water samples.

Development of a titanium dioxide-assisted preconcentration/on-site vapor-generation chip hyphenated with inductively coupled plasma-mass spectrometry for online determination of mercuric ions in urine samples

In this study, a novel automatic analytical methodology using a titanium dioxide (TiO₂)-assisted preconcentration/on-site vapor-generation (VG) chip hyphenated with inductively coupled plasma-mass spectrometry (ICP-MS) for online determination of mercuric ions (Hg²⁺) was developed. Interestingly, the TiO₂ nanoparticle (nano-TiO₂) coating on the channel surface acted not only as a sorbent for preconcentration but also as a catalyst for photocatalyst-assisted VG. Under optimum operation conditions, the developed method was validated by analyzing the certified reference material (CRM) Seronorm™ Trace Elements Urine L-2 (freeze-dried human urine). Based on the obtained results, the dramatic reduction of "hands-on" manipulation and the elimination of hazardous materials (e.g., sodium borohydride (NaBH₄) and stannous chloride (SnCl₂)) from the process enabled a simple and ultraclean procedure with an extremely low detection limit of 0.75 ng L^-1 for Hg²⁺ in urine samples. To the best of our knowledge, this is the first study to report the direct exploitation of a nano-TiO₂-coated microfluidic device for online sample preconcentration and on-site VG prior to ICP-MS measurement.

Zeolites and zeolite-based materials in extraction and microextraction techniques

This review presents an overview of the current status of zeolites and zeolite-based materials used in extraction and microextraction techniques with reference to recent applications and highlight some of the novel advances.