Magnetic Fe₃O₄-Ag⁰ Nanocomposites for Effective Mercury Removal from Water

Abstract

In this study, magnetic Fe₃O₄ particles and Fe₃O₄-Ag⁰ nanocomposites were prepared by a facile and green method, fully characterized and used for the removal of Hg²⁺ from water. Characterizations showed that the Fe₃O₄ 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 Hg²⁺ removal rate was very fast followed by a slow increase and the maximum solid phase loading was 71.3 mg/g for the Fe₃O₄-Ag⁰ and 28 mg/g for the bare Fe₃O₄. The removal mechanism is complex, involving Hg²⁺ adsorption and reduction, Fe²⁺ and Ag⁰ 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 Fe₃O₄-Ag⁰ nanocomposites attractive materials for the removal of Hg²⁺ from water.

1. 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 Hg⁰ or Hg²⁺ 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 Hg²⁺ 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, Ag₂Hg₃, Ag₃Hg₄, Ag₄Hg₅ and Ag₁₀Hg₁₃ [14]. The amalgamation reaction can be greatly enhanced by utilizing Ag in the form of nanocomposites. Such nanocomposites based on silica, magnetite, titanium oxide and alumina have been studied for the removal of heavy metals and mercury from water [15,16,17,18,19]. Among them, magnetite-based nanocomposites are being broadly studied for use in water purification owing to their low cost, simple application, and absence of toxicity towards the environment [20,21]. Furthermore, magnetite nanoparticles are easily separable from the aqueous solution when a magnetic field is applied and can be reused several times [22]. Heavy metals can be bound to the surface of magnetite by complexation, precipitation and adsorption mechanisms. Various types of magnetic nanomaterials are being investigated for the extraction of hazardous pollutants from water. In particular, magnetite-based nanocomposites show high efficiency in the removal and recovery of copper, zinc, nickel and mercury ions from industrial wastewater [23,24].

Fe₃O₄@SiO₂ magnetic nanoparticles modified by grafting poly(1-vinylimidazole) oligomer were used to remove Hg²⁺ from water reaching a maximum capacity of 346 mg/g [21]. Fe₃O₄ nanoparticles coated with silica shells functionalized with dithiocarbamate groups were used for mercury removal from seawater and quantification of mercury in natural waters [25,26]. Nanocomposites based on Fe₃O₄ nanoparticles, chitosan nanoparticles and polythiophene were used for Hg²⁺ removal from aqueous solutions reaching a loading of about 50 mg/g [27]. Thiol-functionalized Fe₃O₄ nanoparticles have shown a high removal capacity for Hg²⁺ reaching 345 mg/g [28]. Fe₃O₄ nanoparticles coated with amino organic ligands and yam peel biomass reached a loading of about 60 mg/g [29]. Dithiothreitol functionalized Fe₃O₄ nanoparticles showed a capacity of 6.3 mg/g and activated carbon doped with Fe₃O₄ nanoparticles reached a capacity of 38.3 mg/g [30]. Dithiocarbamate surface functionalized Fe₃O₄ particles reached a loading of 122–246 mg/g [31]. Zeolite-magnetite composites were used to remove Hg²⁺ from water reaching a maximum loading of 26.2 mg/g [32]. Fe₃O₄ particles have been also used as core covered with a silica shell [33,34].

As the literature review demonstrates the direct surface interactions of mercury ions with bare Fe₃O₄ and Fe₃O₄-Ag⁰ nanocompositeshave not been studied so far. An exception is the work of Dong et al. [35] who used Fe₃O₄-Ag⁰ particles but for the removal of Hg⁰ from flue gas. On the synthesis part, great attention is paid to the development of green processes with minimal use of toxic substances [36]. Plant extracts utilization as reducing and stabilizing agents have drawn considerable attention for the synthesis of metallic nanoparticles as it is considered an eco-friendly method [37,38]. Furthermore, the synthesis process should be low-cost and easily scalable for mass production. Such a green synthesis of Ag nanoparticles on magnetic iron oxide modified by a herbal tea extract has been studied for antibacterial activity and 4-nitrophenol reduction [37]. In this study, we synthesized magnetic Fe₃O₄-Ag⁰ nanocomposites by a facile method using green tea extract. The nanocomposite was then used as a magnetically separable adsorbent for efficient mercury removal from water. The mechanism of mercury removal is discussed in detail and verified by advanced characterization methods.

2. Materials and Methods

2.1. Chemicals

High purity iron (III) chloride hexahydrate (FeCl₃·6H₂O, 99%), anhydrous sodium acetate (CH₃COONa, 99.0%), anhydrous ethylene glycol (C₂H₆O₂, 99.8%), silver nitrate (AgNO₃, ≥99%), mercury (II) chloride (HgCl₂, 99.8%) were used as received. Green tea was purchased at the local market.

2.2. Synthesis of Fe₃O₄

Magnetite particles were synthesized according to previously published protocols [39,40]. In a typical synthesis process, FeCl₃·6H₂O (2.16 g) and CH₃COONa (6 g) were dissolved in 15mL ethylene glycol. The prepared mixture solution was then transferred to a Teflon-lined stainless-steel autoclave and then heated at 200 °C for 8 h, with heating rate of 10 °C per 1 min. The black product was washed by magnet decantation several times with water/ethanol and then dried at 40 °C. 2.3. Synthesis of Fe₃O₄-Ag⁰.

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 Fe₃O₄-Ag⁰ 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, AgNO₃ (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.

2.3. Mercury Removal Efficiency

The Hg²⁺ removal efficiency of Fe₃O₄ particles and Fe₃O₄-Ag⁰ nanocomposites was studied in HgCl₂ solutions. A stock solution of Hg²⁺ (100 and 200 ppm) was prepared by dissolving HgCl₂ in deionized water. The Hg²⁺ 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%.

2.4. Characterization

The crystalline phase and the structure of the synthesized Fe₃O₄ particles and Fe₃O₄-Ag⁰ 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.

2.5. Calculations

The kinetics of mercury removal from water was studied in order to obtain information about the adsorption mechanism of the pure Fe₃O₄ particles and Fe₃O₄-Ag⁰ nanocomposites. The percentage of mercury removal (R) was calculated using as follows:

R (%) = ((C_i − C_f)/C_i) × 100

(1)

q (mg/g) = (C_i − C_f) × V/m

(2)

where ${\mathrm{C}}_{\mathrm{i}}$ and ${\mathrm{C}}_{\mathrm{f}}$ (mg/L) are the initial and final concentrations of Hg²⁺, V (L) is the volume of the solution and m (g) is mass of the adsorbent.

3. Results and Discussion

SEM analysis was used to investigate the morphology of as-prepared bare Fe₃O₄ particles and Fe₃O₄-Ag⁰ nanocomposites. Figure 1A shows that the bare Fe₃O₄ particles were quasi-spherical and had a mean diameter of 217 ± 76 nm. Figure 1B shows that the surface of the Fe₃O₄-Ag⁰ nanocomposites became rougher because of Ag nanoparticle (23–41 nm) deposition on the surface of the Fe₃O₄ particles. The TEM image (Figure 1C) and EDX analysis (Figure 1D) confirmed that Fe₃O₄ particles were decorated with Ag nanoparticles. In particular, main elements such as Fe, O and Ag were clearly detectable in the EDX spectrum of Fe₃O₄-Ag⁰ nanocomposites. Figure 1E shows that Fe₃O₄-Ag⁰ nanocompositesweremagnetic and could be conveniently extracted by the use of a permanent magnet.

XRD analysis of the bare Fe₃O₄ and Fe₃O₄-Ag⁰ confirmed the successful deposition of Ag nanoparticles on the surface of Fe₃O₄ particles. Figure 2 shows that diffraction peaks at 30.1°, 35.5°, 43.1°, 53.7°, 57.3° and 62.6° couldbe indexed to the (220), (311), (400), (422), (511) and (440) planes of the face-centered cubic structure of the Fe₃O₄(JCPDS # 19-629) and the four peaks located at 38.2°, 44.3°, 64.2° and 73.9° corresponded to the characteristic (111), (200), (220) and (311) reflection planes of the face-centered cubic Ag (JCPDS # 04-0783). It should be noted that the strong diffraction peaks indicated the formation of particles with good crystallinity and purity since no other peaks were detected.

Figure 3A shows the adsorption kinetics results. It was found that Fe₃O₄-Ag⁰ 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 Hg⁰ from flue gas by using bare Fe₃O₄ and Fe₃O₄-Ag⁰ [35]. Some studies argue that magnetite either does not remove Hg²⁺ or removes only up to 1.14 mg/g [21,41]. As mentioned in the introduction, there are no studies on the removal of Hg²⁺ from water by the use of this material and for comparison representative published studies are presented in

Table 1. Published studies on the removal of mercury from aqueous solutions.

Material	Capacity (mg/g)	Reference
Dithiothreitol functionalized Fe3O4 nanoparticles	6.3	[30]
SiO2-Ag0 nanocomposites	7.8–8.3	[19]
Synthetic zeolites	20.5–22.3	[42]
Zeolite-magnetite composites	26.2	[32]
Activated carbon doped with Fe3O4 nanoparticles	38.3	[30]
Nanocomposites based on Fe3O4 nanoparticles, chitosan nanoparticles and polythiophene	50	[27]
Fe3O4 nanoparticles coated with amino organic ligands and yam peel biomass	60	[29]
Dithiocarbamate surface functionalized Fe3O4 particles	122–246	[31]
Mesoporous silica-ammonium (4-chloro-2-mercaptophenyl) carbamodithioate	164	[43]
Thiol-functionalized Fe3O4 nanoparticles	345	[28]
Fe3O4@SiO2 magnetic nanoparticles modified by grafting poly(1-vinylimidazole)	346	[21]
Cryogels	240–742	[11]

. As is evident, capacity depends on the materials and conditions used. An important advantage of Fe₃O₄-Ag⁰ is the ease of separation of the solid phase after the adsorption process.

Additional experiments for short time demonstrated that reaction on the surface of Fe₃O₄-Ag⁰ 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 Hg²⁺.

The interaction of Hg²⁺ with bare Fe₃O₄ and Fe₃O₄-Ag⁰ was further investigated using SEM, EDX and XRD. Figure 4A shows the SEM analysis of the bare Fe₃O₄ after contact with Hg²⁺ 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-Ag⁰ after contact with Hg²⁺ for 12 h was also performed for comparison. Figure 5A shows that the morphology of the Fe₃O₄-Ag⁰ 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 Ag⁰wasbeneficial in terms of Hg²⁺ removal.

An XRD analysis was performed to elucidate the adsorption pathways on the surface of bare Fe₃O₄ and Fe₃O₄-Ag⁰ (Figure 6). Upon contact of Fe₃O₄ particles with Hg²⁺, 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 Hg²⁺couldbe adsorbed on the surface of magnetite from a HgCl₂ solution and then reduced to volatile Hg⁰ by Fe²⁺ [46]. The formation of volatile Hg⁰ 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, Hg²⁺ is reduced to Hg⁰, 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 Hg²⁺ and the redox reactions resulting in Hg₂Cl₂, Hg⁰ and HgO are discussed in other studies as well [41]. The possible reactions are the following:

2Fe²⁺ + Hg²⁺ → 2Fe³⁺ + Hg⁰

(3)

Fe²⁺ + Hg²⁺ → Fe³⁺ + Hg⁺

(4)

Hg²⁺ + 0.5O₂ → HgO

(5)

2Hg⁺ + 2Cl⁻ → Hg₂Cl₂

(6)

In the case of Fe₃O₄-Ag⁰ 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 Hg²⁺ from aqueous solutions by the use of Fe₃O₄-Ag⁰ 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-Ag⁰ 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 Ag₂Hg₃ and Hg₂Cl₂ and the effect of Hg²⁺ speciation on the reaction mechanismsare discussed in more detail on different Ag⁰ nanocomposites elsewhere [19,42]. Thus, in addition to reactions (3)–(6), in the presence of Ag⁰ the following reactions can occur:

2Ag⁰ + Hg²⁺ → 2Ag⁺ + Hg⁰

(7)

Ag⁰ + Hg²⁺ → Ag⁺ + Hg⁺

(8)

Ag⁰ + 0.5O₂ → AgO

(9)

Ag⁺ + Cl⁻ → AgCl

(10)

2Ag⁰ + 3Hg⁰→Ag₂Hg₃

(11)

The results suggest that the interactions on the surface of Fe₃O₄ and Fe₃O₄-Ag⁰ are complex and there is a competition between several reactions, which govern the removal rate of Hg²⁺ from water. As it is clear, XPS analysis should be conducted in order to further investigate the possible redox reactions.

4. Conclusions

Fe₃O₄ particles and Fe₃O₄-Ag⁰ nanocomposites were successfully synthesized, characterized and used for the removal of Hg²⁺ from water. The results showed that micron-sized magnetite particles are formed on which Ag⁰ nanoparticles are anchored. The mercury removal experiments showed that Fe₃O₄-Ag⁰ 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 Fe²⁺, O₂, Ag⁰ and Hg²⁺. 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-Ag⁰ nanocomposite a promising material for Hg²⁺ removal from water.