Elsevier

Chemosphere

Volume 282, November 2021, 131128
Chemosphere

Simultaneous adsorption of mercury species from aquatic environments using magnetic nanoparticles coated with nanomeric silver functionalized with l-Cysteine

https://doi.org/10.1016/j.chemosphere.2021.131128Get rights and content

Highlights

  • Total and simultaneous removal of all mercury forms from aqueous samples.

  • New adsorbent: Fe3O4 nanoparticles coated with silver functionalized with l-Cysteine.

  • 100% removal efficiency achieved in 30 s of contact time at a pH of 6.2.

  • The adsorbent can be recovered and reused without loss of its adsorption capacity.

  • The presence of other ions does not interfere with the adsorption process.

Abstract

We introduce a novel, efficient and fast method for the total and simultaneous removal of monomethylmercury, dimethylmercury, ethylmercury and Hg (II) from aquatic environments using magnetic core nanoparticles, coated with metallic nanomeric silver and functionalized with l-Cysteine. As far as the authors know, simultaneous removal has not been achieved previously. The experimental design was based on exploring a wide range of experimental conditions, including pH of the medium (2−12), contact time (up to 20 min), adsorbent dose (50–800 μL) and temperature (293–323 K), in order to achieve the highest adsorption efficiency. The results show that, for a pH equal to 6.2 at room temperature, 400 μL of nanoparticles is sufficient to achieve 100% adsorption efficiency for all the studied Hg species after a contact time of 30 s. The adsorbent was characterized by means of Scanning Electron Microscopy, Energy Dispersive X-ray Analysis, Fourier-Transform Infrared Spectroscopy and a BET test. Moreover, the procedure allows the total recovery and recycling of the nanoparticles using 50 μL of 0.01 M KI. As regards reuse, the adsorbent exhibits no loss of adsorption capacity during the first three adsorption cycles. Thermodynamics reveals that adsorption is of a physicochemical nature, the equilibrium isotherms being described by a Langmuir model for all the Hg species. The ability of the method to simultaneously adsorb all species of mercury present in water, achieving full adsorption in just a few seconds, along with the simple experimental conditions and its cost-effectiveness, strongly support the approach as an alternative to current procedures.

Introduction

Mercury pollution of water, air and soil is considered a major environmental problem, because of damaging effects for both the ecosystem and humans (Perez-Sirvent et al., 2007; Pirrone et al., 2009). All mercury species are toxic, but monomethylmercury (MeHg), which is considered a neurotoxin, is the most harmful (Clarkson and Magos, 2006). Diet is the major source of mercury in the human body, mainly through the intake of fish or shellfish, and methylmercury constitutes 60–90% of the mercury present in the species (Alonso et al., 2008). Prolonged exposure to mercury can damage the nervous system permanently, leading to a variety of symptoms such as ataxia, paresthesia, sensory disturbances, tremor, blurred vision, difficulty speaking, hearing problems, deafness, blindness and even death. In addition to neurotoxicity, all forms of mercury can sequentially affect other systems, resulting in adverse effects for the immune, cardiac and renal functions (Ullrich et al., 2001).

Five chemical forms of mercury are found in the environment: elemental mercury, divalent inorganic mercury (Hg2+), dimethylmercury (Me2Hg), methylmercury (MeHg) and monoethylmercury (EtHg). In aquatic and terrestrial environments, Hg2+ is the predominant species, while Hg (0) is the most common in the atmosphere. However, MeHg takes on greater relevancy due to its high toxicity, its accumulation throughout the food chain and its constant threat to human health and wildlife. Even though EtHg is not as frequent in nature as MeHg, its incidence has also been reported, especially in some wetland systems (Mao et al., 2010).

Research carried out into the elimination of mercury from water samples has mainly focused on Hg (II) (Mudasir et al., 2020; Shukla et al., 2020), including adsorbents such as carboxymethyl cellulose, carbon nanotubes, graphene oxide, chitosan and other resins (Kumar et al., 2016; Wang et al., 2018; Chen et al., 2019; Elbadawy, 2019; Jiang and Wang, 2019). Additionally, the use of functionalizing reagents is credited with improving adsorption efficiency. More specifically, l-Cysteine, which presents exceptional ability to chelate metals, has been used as a functionalizing reagent for different materials in order to adsorb Hg (II) (Bansal et al., 2018; Li et al., 2019b; Tabarinia et al., 2019; Srikhaow et al., 2020) and MeHg (Zhang et al., 2021). Recent investigations have revealed that magnetic particles exhibit high adsorption efficiency when used as adsorbent for Hg species, with the additional benefit of permitting cost-effective methods to be used (Azari et al., 2017; Fu and Huang, 2018; Fan et al., 2019; Ma et al., 2019a, 2019b; Naushad et al., 2019). They are therefore regarded as an outstanding alternative to traditional materials.

Since all mercury species present high toxicity, even at very low concentrations, and considering that organic species of mercury are the most dangerous for life, it would be desirable to have a method to simultaneously remove all mercury species from water. However, to the best of our knowledge, that goal has not been achieved yet, constituting an important gap in the literature.

The novelty of the present work lies in using a method that allows both organic and inorganic mercury to be simultaneously eliminated from aquatic environments, very quickly and easily. It uses Fe3O4 core nanoparticles coated with metallic nanomeric silver and functionalized with l-Cysteine (Fe3O4@Ag@Cys), an adsorbent that has not been used previously to remove pollutants, and which brings together all the benefits of the above-mentioned adsorbents. The key point is that Fe3O4@Ag@Cys permits the adsorption of all Hg species at once. The results show that 100% removal efficiency is achieved for all mercury forms after 30 s of contact time, at a pH of 6.2. In addition, the adsorbent can be recovered and reused, increasing the cost-effectiveness of the method.

Section snippets

Materials and instrumentation

An atomic absorption spectrometer (PerkinElmer, model AS-800) and a mercury hollow cathode lamp (PerkinElmer), working at 6 mA were used to carry out the measurements.

HgCl2 (Sigma, St. Louis, MO, USA) was used to prepare the solutions of Hg (II). MeHg, Me2Hg and EtHg used were purchased from Sigma Aldrich. An aqueous solution (0.05 M) of l-Cysteine was prepared from the products provided by Sigma. FeCl3.6H2O, FeCl2·4H2O and AgNO3 were obtained from Merck (Darmstadt, Germany). The NaBH4 used as

Characterization of the adsorbent

Scanning electron microscopy (SEM) was conducted to characterize the adsorbent, concretely Fe3O4@Ag. Fig. 1 shows the presence of Ag in the adsorbent, as seen from the SEM image (top) and its associated Energy Dispersive X-Ray Analysis (EDX, bottom), where the characteristic Ag and Fe peaks are clearly visible. The lighter structures in the image are associated to Ag, due to its higher atomic number. Transmission electron microscopy (TEM) detected the presence of l-Cysteine in the Fe3O4@Ag@Cys.

Conclusion

This work presents a simple and novel procedure using magnetic Fe3O4@Ag@Cys nanoparticles as adsorbent for the simultaneous adsorption of organic and inorganic Hg species in aqueous media, a task not previously achieved. The total removal of all Hg species is achieved in 30 s of contact time, at room temperature and pH equal to 6.2. The adsorbent can be recycled using a small volume of KI solution. Moreover, the nanoparticles can be reused for up to three adsorption cycles without loss of their

Credit author statement

All authors: Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Project administration; Resources; Software; Supervision; Validation; Visualization; Writing - original draft; Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors want to thank the University Centre of Defence at the Spanish Air Force Academy, MDE-UPCT, for financial support.

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