Simultaneous adsorption of mercury species from aquatic environments using magnetic nanoparticles coated with nanomeric silver functionalized with l-Cysteine
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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2022, Environmental ResearchCitation Excerpt :Other relevant adsorbents for nanomeric and ionic silver have been activated sludge (Oh et al., 2015), activated carbon (Gicheva and Yordanov, 2013), metal-organic framework-based materials (Gicheva and Yordanov, 2013) and nanoporous silica (Sim et al., 2014). Adsorption processes are attractive because of their ability to remove different pollutants and the selectivity provided by the modification of the adsorptive surfaces (Venkatraman and Priya, 2022; Vicente-Martinez et al., 2021a). However, most of the adsorption procedures presented above focus separately on either silver nanoparticles or silver ions as single species, not as a mixture, and the works that study the simultaneous elimination of both species do not achieve total elimination and require very long times to remove them (Pongkitdachoti and Unob, 2018).