Magnetic Fe 3 O 4 -Ag 0 Nanocomposites for E ﬀ ective Mercury Removal from Water

: In this study, magnetic Fe 3 O 4 particles and Fe 3 O 4 -Ag 0 nanocomposites were prepared by a facile and green method, fully characterized and used for the removal of Hg 2 + from water. Characterizations showed that the Fe 3 O 4 particles are quasi-spherical with an average diameter of 217 nm and metallic silver nanoparticles formed on the surface with a size of 23–41 nm. The initial Hg 2 + removal rate was very fast followed by a slow increase and the maximum solid phase loading was 71.3 mg / g for the Fe 3 O 4 -Ag 0 and 28 mg / g for the bare Fe 3 O 4 . The removal mechanism is complex, involving Hg 2 + adsorption and reduction, Fe 2 + and Ag 0 oxidation accompanied with reactions of Cl − with Hg + and Ag + . The facile and green synthesis process, the fast kinetics and high removal capacity and the possibility of magnetic separation make Fe 3 O 4 -Ag 0 nanocomposites attractive materials for the removal of Hg 2 + from water.


Introduction
Mercury and its compounds are considered to be extremely hazardous pollutants. Contamination of the environment with mercury has become a global problem and mercury polluted areas have been identified worldwide [1]. In most cases, the release of Hg 0 or Hg 2+ into the environment occurs due to industrial emissions, transportation, waste treatment or technological accidents [2]. Therefore, the development of efficient methods for the removal of mercury from water is imperative. Several removal and immobilization methods are available, such as membrane separation, reduction, precipitation, physical and chemical adsorption, ion exchange and bioremediation [3,4]. Of these methods, adsorption exhibits several advantages in terms of process design, operation and cost and it is the most studied one [4]. A number of materials have been used as adsorbents for the removal of Hg 2+ from water, including activated carbons [5], zeolites [6,7], resins and other polymers [8][9][10][11] and silver-modified materials [7,12,13].
Silver is an important metal that can form various amalgam compounds with mercury such as AgHg, Ag 2 Hg 3 , Ag 3 Hg 4 , Ag 4 Hg 5 and Ag 10 Hg 13 [14]. The amalgamation reaction can be greatly enhanced by utilizing Ag in the form of nanocomposites. Such nanocomposites based on silica, Green tea extract (GTE) was prepared by boiling 0.2 g of dried green tea leaves in 20 mL of water for 5 min. The GTE was then filtered using a Whatman filter paper N1 to obtain an aqueous extract of green tea. To prepare the Fe 3 O 4 -Ag 0 nanocomposites, 100 mg of magnetite spheres were dispersed in 10 mL of water and dispersed for 20 min. To the above solution, 500 µL of GTE was added and the solution was stirred at room temperature for 24 h. Finally, AgNO 3 (20 mg) was added to the solution and kept under stirring for 24 h. The as-prepared composite was separated by the magnet, washed with water/ethanol and then dried at 30 • C.

Mercury Removal Efficiency
The Hg 2+ removal efficiency of Fe 3 O 4 particles and Fe 3 O 4 -Ag 0 nanocomposites was studied in HgCl 2 solutions. A stock solution of Hg 2+ (100 and 200 ppm) was prepared by dissolving HgCl 2 in deionized water. The Hg 2+ solution volume was 20 mL and the solids mass 50 mg. All adsorption experiments were performed without any stirring at room temperature (23 ± 2 • C) without pH adjustment. The mercury concentration in the solutions was measured by a mercury analyzer (Lumex RA-915M) until no concentration changes were observed, i.e., until equilibrium was attained. All experiments were performed in duplicate and the average standard deviation was 2%.

Characterization
The crystalline phase and the structure of the synthesized Fe 3 O 4 particles and Fe 3 O 4 -Ag 0 nanocomposites before and after mercury adsorption were performed using an X-ray diffractometer (XRD) (RigakuSmartLab, Tokyo, Japan). The surface of the materials was studied by Scanning Electron Microscopy (SEM) using a Zeiss Auriga Crossbeam 540. Chemical analysis was carried out using an Energy-Dispersive X-ray spectrometer (Aztec, Oxford Instruments, Abingdon, UK). The nanoscale analysis was done with a high-resolution JEOL JEM-1400 Plus transmission electron microscope (TEM), operating at 120 kV.

Calculations
The kinetics of mercury removal from water was studied in order to obtain information about the adsorption mechanism of the pure Fe 3 O 4 particles and Fe 3 O 4 -Ag 0 nanocomposites. The percentage of mercury removal (R) was calculated using as follows: where C i and C f (mg/L) are the initial and final concentrations of Hg 2+ , V (L) is the volume of the solution and m (g) is mass of the adsorbent.

Results and Discussion
SEM analysis was used to investigate the morphology of as-prepared bare Fe 3 O 4 particles and Fe 3 O 4 -Ag 0 nanocomposites. Figure 1A shows that the bare Fe 3 O 4 particles were quasi-spherical and had a mean diameter of 217 ± 76 nm. Figure 1B shows that the surface of the Fe 3 O 4 -Ag 0 nanocomposites became rougher because of Ag nanoparticle (23-41 nm) deposition on the surface of the Fe 3 O 4 particles. The TEM image ( Figure 1C) and EDX analysis ( Figure 1D) confirmed that Fe 3 O 4 particles were decorated with Ag nanoparticles. In particular, main elements such as Fe, O and Ag were clearly detectable in the EDX spectrum of Fe 3 O 4 -Ag 0 nanocomposites. Figure 1E shows that Fe 3 O 4 -Ag 0 nanocompositesweremagnetic and could be conveniently extracted by the use of a permanent magnet.   Figure 3a shows the adsorption kinetics results. It was found that Fe3O4-Ag 0 removed more than 80% of the mercury within the first hour followed by a slow approach to an equilibrium point with a maximum solid phase loading of 71.3 mg/g. On the other hand, the bare Fe3O4 removed less than 10% of mercury after the first hour and less than 40% at equilibrium, reaching a solid phase loading of about 28 mg/g. Qualitatively similar trends were observed for the removal of Hg 0 from flue gas by using bare Fe3O4 and Fe3O4-Ag 0 [35]. Some studies argue that magnetite either does not remove Hg 2+   Figure 3a shows the adsorption kinetics results. It was found that Fe3O4-Ag 0 removed more than 80% of the mercury within the first hour followed by a slow approach to an equilibrium point with a maximum solid phase loading of 71.3 mg/g. On the other hand, the bare Fe3O4 removed less than 10% of mercury after the first hour and less than 40% at equilibrium, reaching a solid phase loading of about 28 mg/g. Qualitatively similar trends were observed for the removal of Hg 0 from flue gas by using bare Fe3O4 and Fe3O4-Ag 0 [35]. Some studies argue that magnetite either does not remove Hg 2+  Figure 3A shows the adsorption kinetics results. It was found that Fe 3 O 4 -Ag 0 removed more than 80% of the mercury within the first hour followed by a slow approach to an equilibrium point with a maximum solid phase loading of 71.3 mg/g. On the other hand, the bare Fe 3 O 4 removed less than 10% of mercury after the first hour and less than 40% at equilibrium, reaching a solid phase loading of about 28 mg/g. Qualitatively similar trends were observed for the removal of Hg 0 from flue gas by using bare Fe 3 O 4 and Fe 3 O 4 -Ag 0 [35]. Some studies argue that magnetite either does not remove Hg 2+ or removes only up to 1.14 mg/g [21,41]. As mentioned in the introduction, there are no studies on the removal of Hg 2+ from water by the use of this material and for comparison representative published studies are presented in Table 1. As is evident, capacity depends on the materials and conditions used. An important advantage of Fe 3 O 4 -Ag 0 is the ease of separation of the solid phase after the adsorption process.
Sustainability 2020, 12, x FOR PEER REVIEW 5 of 10 or removes only up to 1.14 mg/g [21,41]. As mentioned in the introduction, there are no studies on the removal of Hg 2+ from water by the use of this material and for comparison representative published studies are presented in Table 1. As is evident, capacity depends on the materials and conditions used. An important advantage of Fe3O4-Ag 0 is the ease of separation of the solid phase after the adsorption process.  Cryogels 240-742 [11] Additional experiments for short time demonstrated that reaction on the surface of Fe3O4-Ag 0 particles was rapid and the majority of mercury ions are removed within the first 10 min (Figure 3b). Almost the same trend was observed for two different concentrations of Hg 2+ .
The interaction of Hg 2+ with bare Fe3O4 and Fe3O4-Ag 0 was further investigated using SEM, EDX and XRD. Figure 4a shows the SEM analysis of the bare Fe3O4 after contact with Hg 2+ for 12 h. It was clear that the Fe3O4 particles still retained the quasi-spherical shape. An EDX survey (Figure 4b) revealed that a small quantity of Hg and Cl were adsorbed on the surface of Fe3O4 particles. Analysis of the Fe3O4-Ag 0 after contact with Hg 2+ for 12 h was also performed for comparison. Figure 5a shows that the morphology of the Fe3O4-Ag 0 particles was not changed significantly. However, EDX analysis revealed that the quantity of adsorbed Hg and Cl significantly increased. The detected amount of Hg (wt.%) became five times higher, while the detected amount of Cl (wt.%) became eight times higher. These results demonstrated that the addition of Ag 0 wasbeneficial in terms of Hg 2+ removal.  Cryogels 240-742 [11] Additional experiments for short time demonstrated that reaction on the surface of Fe 3 O 4 -Ag 0 particles was rapid and the majority of mercury ions are removed within the first 10 min ( Figure 3B). Almost the same trend was observed for two different concentrations of Hg 2+ .
The interaction of Hg 2+ with bare Fe 3 O 4 and Fe 3 O 4 -Ag 0 was further investigated using SEM, EDX and XRD. Figure 4A shows the SEM analysis of the bare Fe 3 O 4 after contact with Hg 2+ for 12 h. It was clear that the Fe 3 O 4 particles still retained the quasi-spherical shape. An EDX survey ( Figure 4B) revealed that a small quantity of Hg and Cl were adsorbed on the surface of Fe 3 O 4 particles. Analysis of the Fe 3 O 4 -Ag 0 after contact with Hg 2+ for 12 h was also performed for comparison. Figure 5A shows that the morphology of the Fe 3 O 4 -Ag 0 particles was not changed significantly. However, EDX analysis revealed that the quantity of adsorbed Hg and Cl significantly increased. The detected amount of Hg (wt.%) became five times higher, while the detected amount of Cl (wt.%) became eight times higher. These results demonstrated that the addition of Ag 0 wasbeneficial in terms of Hg 2+ removal.  An XRD analysis was performed to elucidate the adsorption pathways on the surface of bare Fe3O4 and Fe3O4-Ag 0 ( Figure 6). Upon contact of Fe3O4 particles with Hg 2 +, new peaks at 24° and 32° appeared due to the formation of HgO [44] and a peak at 44° appeared due to the formation of Hg2Cl2 [45]. The reaction mechanism between mercury and magnetite is still not well understood. However, a recent report suggested that Hg 2+ couldbe adsorbed on the surface of magnetite from a HgCl2 solution and then reduced to volatile Hg 0 by Fe 2+ [46].The formation of volatile Hg 0 is difficult to confirm but if it happens it obviously gives no trace on the XRD. Another study on magnetite found that in the absence of chloride ions, Hg 2+ is reduced to Hg 0 , while in the presence of chloride ions it is reduced to Hg + resulting in Hg2Cl2 [47], which is in agreement with the results of the present study.The interaction of Fe species with Hg 2+ and the redox reactions resulting in Hg2Cl2, Hg 0 and HgO are discussed in other studies as well [41]. The possible reactions are the following: 2Fe 2+ + Hg 2+ →2Fe 3+ + Hg 0 (1) Fe 2+ + Hg 2+ → Fe 3+ + Hg + (2) Hg 2+ + 0.5O2→ HgO (3) 2Hg + + 2Cl -→ Hg2Cl2 (4)  An XRD analysis was performed to elucidate the adsorption pathways on the surface of bare Fe3O4 and Fe3O4-Ag 0 ( Figure 6). Upon contact of Fe3O4 particles with Hg 2 +, new peaks at 24° and 32° appeared due to the formation of HgO [44] and a peak at 44° appeared due to the formation of Hg2Cl2 [45]. The reaction mechanism between mercury and magnetite is still not well understood. However, a recent report suggested that Hg 2+ couldbe adsorbed on the surface of magnetite from a HgCl2 solution and then reduced to volatile Hg 0 by Fe 2+ [46].The formation of volatile Hg 0 is difficult to confirm but if it happens it obviously gives no trace on the XRD. Another study on magnetite found that in the absence of chloride ions, Hg 2+ is reduced to Hg 0 , while in the presence of chloride ions it is reduced to Hg + resulting in Hg2Cl2 [47], which is in agreement with the results of the present study.The interaction of Fe species with Hg 2+ and the redox reactions resulting in Hg2Cl2, Hg 0 and HgO are discussed in other studies as well [41]. The possible reactions are the following: 2Fe 2+ + Hg 2+ →2Fe 3+ + Hg 0 (1) Fe 2+ + Hg 2+ → Fe 3+ + Hg + (2) Hg 2+ + 0.5O2→ HgO (3) 2Hg + + 2Cl -→ Hg2Cl2 (4) An XRD analysis was performed to elucidate the adsorption pathways on the surface of bare Fe 3 O 4 and Fe 3 O 4 -Ag 0 ( Figure 6). Upon contact of Fe 3 O 4 particles with Hg 2+ , new peaks at 24 • and 32 • appeared due to the formation of HgO [44] and a peak at 44 • appeared due to the formation of Hg 2 Cl 2 [45]. The reaction mechanism between mercury and magnetite is still not well understood. However, a recent report suggested that Hg 2+ couldbe adsorbed on the surface of magnetite from a HgCl 2 solution and then reduced to volatile Hg 0 by Fe 2+ [46]. The formation of volatile Hg 0 is difficult to confirm but if it happens it obviously gives no trace on the XRD. Another study on magnetite found that in the absence of chloride ions, Hg 2+ is reduced to Hg 0 , while in the presence of chloride ions it is reduced to Hg + resulting in Hg 2 Cl 2 [47], which is in agreement with the results of the present study.The interaction of Fe species with Hg 2+ and the redox reactions resulting in Hg 2 Cl 2 , Hg 0 and HgO are discussed in other studies as well [41]. The possible reactions are the following: Hg 2+ + 0.5O 2 → HgO (5) 2Hg + + 2Cl − → Hg 2 Cl 2 (6) In the case of Fe3O4-Ag 0 nanocomposites, the appearance of a new peak at around 17º probably indicated the formation of an Hg-Ag amalgam (moschellandbergite phase, Ag2Hg3) [48]. The absence of literature on the removal of Hg 2+ from aqueous solutions by the use of Fe3O4-Ag 0 nanocomposites is difficult to support this conclusion. However, there are papers presenting the removal of Hg from a gas phase by the use of Fe3O4-Ag 0 nanocomposites [35] where the formation of Hg-Ag amalgams is offered as the best explanation for the efficiency of the nanocomposite in comparison to the bare magnetite. Additional peaks at around 27° and 46° were indexed to the AgCl [49] structure, which appeared due to the reaction between the Ag + and Cl -. Furthermore, a peak at 53º appeared due to the formation of monoclinic AgO [50]. The formation of Ag2Hg3 andHg2Cl2 and the effect of Hg 2+ speciation on the reaction mechanismsare discussed in more detail on different Ag 0 nanocomposites elsewhere [19,42]. Thus, in addition to reactions 1-4, in the presence of Ag 0 the following reactions can occur: 2Ag 0 + Hg 2+ → 2Ag + + Hg 0 (5) Ag 0 + Hg 2+ → Ag + + Hg + (6) Ag 0 + 0.5O2→ AgO (7) Ag + + Cl -→ AgCl (8) 2Ag 0 + 3Hg 0 → Ag2Hg3 (9) The results suggest that the interactions on the surface of Fe3O4 and Fe3O4-Ag 0 are complex and there is a competition between several reactions, which govern the removal rate of Hg 2+ from water. As it is clear, XPS analysis should be conducted in order to further investigate the possible redox reactions.

Conclusions
Fe3O4particles and Fe3O4-Ag 0 nanocomposites were successfully synthesized, characterized and used for the removal of Hg 2+ from water. The results showed that micron-sized magnetite particles are formed on which Ag 0 nanoparticles are anchored. The mercury removal experiments showed that Fe3O4-Ag 0 nanocomposites are more effective than Fe3O4 particles. XRD analysis revealed the formation of several compounds on the surface of the materials, including HgO, Hg2Cl2, AgCl, AgO and possibly Ag2Hg3. The formation of these compounds is a strong indication of surface redox reactions between Fe 2+ , O2, Ag 0 and Hg 2+ . Thus, several reactions can occur at the same time and further characterizations, such as XPS, are needed in order to draw safe conclusions. The facile synthesis, the fast removal and the magnetic properties render the Fe3O4-Ag 0 nanocomposite a promising material for Hg 2+ removal from water.
Author Contributions: V.I., conceptualization, methodology, validation, writing-review and editing, supervision, project administration, A.K., methodology, data curation, writing-original draft preparation, In the case of Fe 3 O 4 -Ag 0 nanocomposites, the appearance of a new peak at around 17º probably indicated the formation of an Hg-Ag amalgam (moschellandbergite phase, Ag 2 Hg 3 ) [48]. The absence of literature on the removal of Hg 2+ from aqueous solutions by the use of Fe 3 O 4 -Ag 0 nanocomposites is difficult to support this conclusion. However, there are papers presenting the removal of Hg from a gas phase by the use of Fe 3 O 4 -Ag 0 nanocomposites [35] where the formation of Hg-Ag amalgams is offered as the best explanation for the efficiency of the nanocomposite in comparison to the bare magnetite. Additional peaks at around 27 • and 46 • were indexed to the AgCl [49] structure, which appeared due to the reaction between the Ag + and Cl − . Furthermore, a peak at 53 • appeared due to the formation of monoclinic AgO [50]. The formation of Ag 2 Hg 3 and Hg 2 Cl 2 and the effect of Hg 2+ speciation on the reaction mechanismsare discussed in more detail on different Ag 0 nanocomposites elsewhere [19,42]. Thus, in addition to reactions (3)-(6), in the presence of Ag 0 the following reactions can occur: 2Ag 0 + Hg 2+ → 2Ag + + Hg 0 Ag 0 + Hg 2+ → Ag + + Hg + Ag 0 + 0.5O 2 → AgO Ag + + Cl − → AgCl (10) 2Ag 0 + 3Hg 0 →Ag 2 Hg 3 (11) The results suggest that the interactions on the surface of Fe 3 O 4 and Fe 3 O 4 -Ag 0 are complex and there is a competition between several reactions, which govern the removal rate of Hg 2+ from water. As it is clear, XPS analysis should be conducted in order to further investigate the possible redox reactions.

Conclusions
Fe 3 O 4 particles and Fe 3 O 4 -Ag 0 nanocomposites were successfully synthesized, characterized and used for the removal of Hg 2+ from water. The results showed that micron-sized magnetite particles are formed on which Ag 0 nanoparticles are anchored. The mercury removal experiments showed that Fe 3 O 4 -Ag 0 nanocomposites are more effective than Fe 3 O 4 particles. XRD analysis revealed the formation of several compounds on the surface of the materials, including HgO, Hg 2 Cl 2 , AgCl, AgO and possibly Ag 2 Hg 3 . The formation of these compounds is a strong indication of surface redox reactions between Fe 2+ , O 2 , Ag 0 and Hg 2+ . Thus, several reactions can occur at the same time and further characterizations, such as XPS, are needed in order to draw safe conclusions. The facile synthesis, the fast removal and the magnetic properties render the Fe 3 O 4 -Ag 0 nanocomposite a promising material for Hg 2+ removal from water.
Author Contributions: V.J.I., conceptualization, methodology, validation, writing-review and editing, supervision, project administration, A.K., methodology, data curation, writing-original draft preparation, A.M., writing-review and editing, A.A.Z., data curation, validation, writing-review and editing T.S.A., conceptualization, methodology, validation, writing-review and editing. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by Nazarbayev University Grant Number 110119FD4536 and the APC was funded by the same project.