Fast, Cost-effective and Energy Efficient Mercury Removal-Recycling Technology

We herein present a novel and sustainable technology for mercury recycling, with the maximum observed uptake capacity. Facile synthesis of the most efficient (~1.9 gg−1) nano-trap, made of montmorillonite-Fe-iron oxides, was performed to instantaneously remove mercury(II) ions from water. Elemental Hg was recovered from the adduct, by employing Fe granules, at ambient conditions. Varied pHs and elevated temperatures further enhanced this already highly efficient recycling process. The reduction of Hg(II) to Hg(I) by the nano trap and Hg(I) to Hg(0) by Fe granules are the main driving forces behind the recycling process. Facile sustainable recycling of the nano-trap and Fe granules require no additional energy. We have further developed a recyclable model for Hg nano-trap, which is inexpensive (<$5 CAD), and can remove mercury in a few seconds. This technology has multiple applications, including in the communities exposed to mercury contamination.

Driving force of Hg adsorption. The highly facile redox interaction was the driving force for efficient and fast adsorption of Hg(II) on MtFe. Thus, we could not recover adsorbed Hg(II) in its +2 oxidation state from MtFe-Hg [Hg adsorbed MtFe] by shaking with water/aqueous Na 2 -EDTA solution (that makes strong chelate with Hg 2+ ) 11 , further indicating chemisorption type of behavior (confirmed from UV-visible spectra, Supplementary Fig. S5).
Aqueous Na 2 -EDTA treatment produced a broad band at lower energy region (~60 nm), indicating the formation of iron hydrosol and Hg(I)-EDTA complex. In brief, we were able to decontaminate aqueous solution from Hg within a few seconds using the MtFe nano-trap efficiently.
MtFe: An exclusive nano trap of mercury. To understand the high efficiency of MtFe for Hg(II) removal, we replaced MtFe by several commercially available iron oxides (namely, hematite, magnetite and maghemite) and iron dust. We also prepared montmorillonite passivated iron nanoparticles, TEFe using green tea, as reported by KSV 12 . Neither hematite, magnetite, maghemite & TEFe nor iron dust could adsorb Hg(II), present in an aqueous system. Furthermore, we synthesized BHFe, iron oxides, synthesized using similar protocol of MtFe, without using montmorillonite. BHFe was also able to adsorb Hg(II) from water. Yet, the efficiency per unit gram of iron is better for MtFe, as mentioned in the following section.
Montmorillonite not only enhanced the surface to volume ratio of studied iron nanoparticles, but also helped to accommodate adsorbed Hg owing to its sheet structure (two silica tetrahedral sheets with a central alumina octahedral sheet combine to produce a common layer 13 ). Only montmorillonite was found to be an insignificant Hg adsorbent ( Supplementary Fig. S6).
During synthesis, we also replaced NaBH 4 by other well-known reducing agents namely hydrazine 14 and LiAlH 4 15 , and produced MtFeHz and MtFeLi. Note that for the synthesis of MtFeLi, we replaced water/ethanol with THF, due to high reactivity of water & ethanol with LiAlH 4 . MtFeHz and MtFeLi could not absorb Hg(II) similar to MtFe, and was not used in the development of this nanotechnology (Fig. 3(A) and Supplementary  Fig. S7).
Effect of counter anions on Hg adsorption. HgCl 2 and Hg(NO 3 ) 2 could be removed by employing MtFe with equal efficiency. However, HgBr 2 changed its chemical identity in water with some yellow precipitate. The effect of MtFe on HgBr 2 was not understood with confidence, although peak intensity at ~230 nm (related to HgBr 2 ) was decreased significantly.
HgCl 2 was previously used as an antiseptic. It is still used as wood preservative, dry battery depolarizer, photographic intensifier, tanning agent for leather, catalyst in the chemical manufacture industry (vinyl chloride, disinfectants etc.), separating lead from gold, and others. In the felting industry, mercuric nitrate is usually used. The use HgBr 2 is quite rare in industrial scale. Drinking water is mostly contaminated with HgCl 2 [16][17][18] . We can speculate that MtFe shall be quite useful as novel technology for decontamination of water ( Fig. 3(B) and Supplementary Fig. S8).
The compatibility of UV and mass spectral results. When the 4000 pg of HgCl 2 was treated with MtFe and the supernate solution was treated with KMnO 4 and SnCl 2 solution 19 , the evolved Hg was estimated by GC mass spectrometry, which indicated 94.75% removal of Hg (by UV spectrometer 96.00%). The results of mass and absorption spectra, thereby, supported each other (data not shown).
Adsorption isotherm for maximum uptake coefficient. To judge the efficiency of MtFe as a nano-trap, for Hg(II) removal, the amount of MtFe was varied with a fixed amount of HgCl 2 . The removal (%) of mercury increased monotonously with the increase of MtFe. Similarly, we gradually increased the Hg(II) concentration in steps with 2.5 mg MtFe.
We subsequently fitted our data in three adsorption isotherms, Langmuir, Freundlich & Elovich and calculated maximum adsorption capacity ( Fig. 3 and Supplementary Fig. S9). The Langmuir adsorption isotherm, assuming homogenous adsorption sites and mono-layer coverage, follows the following equation 20 . C e is the equilibrium concentration of Hg (mg L −1 ) in solution, while q e is the adsorbed Hg (mg) per gram of MtFe. From the slope of the line of C e /q e against C e , the maximum adsorption capacity (Q m ) is found. Freundlich adsorption isotherm, allowing heterogeneous adsorption sites, follows the following equation.
From the slope of the line of log(q e ) against log(C e ), the adsorption intensity (n) and Freundlich constant (K F ) are obtained. Q m can be obtained by replacing C e by C 0 (Hg concentration in solution just after 100% adsorption). Elovich adsorption isotherm, representing multilayer adsorption, follows the following equation. Effect of pH and temperature on Hg removal. For an adsorbent to be used for practical applications, the removal efficiency also has to be considered at versatile conditions, including different pH conditions. pH affects metal species in solution. It also alters the surface properties of adsorbents in terms of dissociation of functional groups and surface charge 21 .
The Q m, mentioned above was measured at neutral pH and at 20 °C. It was found that removal efficiency (as well as Q m ) was further increased at low pH as well as at high pH.
We swept the pH from 2 to 10 and removal efficiencies at fixed experimental conditions were found to be 2 (98%) > 4 (96%) > 6 (77.6%) > 8 (67%) < 10 (92.4%). At lower pH, Hg 2+ had increased binding sites for the removal of OH from the adsorbent surface, and at high pH, OH − on MTFe surface attracted Hg 2+ (Fig. 4(A) and Supplementary Fig. S10). Such pH dependent adsorption of HgCl 2 is quite interesting in our case. Note that Zhang et al. 22 . reported Fe 3 O 4 @SiO 2 -SH sorbent for mercury removal and with the increase of pH, removal efficiency slowly increased.
Adsorption on Hg 2+ was more facile at higher temperature. Removal efficiency was increased monotonously with temperature. At 60 °C, 40 °C, 20 °C and 0 °C, removal (%) of mercury was 93.8, 87.6, 77.6 and 45, respectively ( Fig. 4(B) and Supplementary Fig. S11). As the process of adsorption is fast chemisorption, the adsorption capacity was enhanced due to high Brownian monition of the Hg(II), leading to effective collision with MtFe 23 .
Recovery of Hg in +2 state. MtFe adsorbed HgCl 2 via chemisorption with high adsorption capacity ( Fig. 4(C)). It was strong adsorption. HgCl 2 was converted to Hg 2 Cl 2 . By aging MtFe-Hg with milli-Q water for long time (5 h), we did not observe any peak of Hg(II) at 200 nm. However, we sonicated MtFe-Hg in 1 mL milli-Q water for 1 min with 10 μL concentrated HNO 3 (strongly oxidizing) and we recovered 99.1% of adsorbed Hg(II) from HgMtFe.
Vigorous washing with copious amount of water detached Hg 2 Cl 2 from iron oxides, making room to further adsorb Hg(II) from solution. However, after four cycles, the adsorption was not practically possible. With the increase of the number of recycling cycles, Hg removal capacity of MtFe was decreased, due to permanent oxidation of Fe(0) in MtFe to Fe(III)-oxide.
Experiments with environmental samples. We investigated removal of Hg from aqueous environmental samples and water samples from tap water (McGill University, Canada). Environmental samples included those taken from Saint Laurent river (Montreal, Canada), as well as melted snow and the rainwater, taken in downtown. Montreal 24 .
Each aqueous sample of 25 mL was examined. Various amounts of Hg (II) were added to them in order to study the removal process upon interactions with MtFe. Removal of Hg, for all the samples, was satisfactory (92-99%), under our experimental conditions, revealing the effectiveness of this method for aqueous environmental samples (Supplementary Table S1). Immobilization of nano trap on paper and sintered funnel. We immobilized MtFe on the Whatman filter paper and sintered glass Buchner funnel (used in gravimetric analysis 25 ) (Fig. 2(D)). The Hg(II) contaminated water was passed through it and the filtered water was found to be free of mercury.
The Hg soaked filter paper or precipitate on the sintered funnel could generate elemental mercury just by adding water and Fe granules, as mentioned below.

Recovery of elemental mercury by Fe granules.
The MtFe-Hg solution (obtained by adding 0.0020 g MtFe in 1 mL 10 −2 M HgCl 2 ) was kept in a 40 mL EPA vial (without any further treatment) and 0.07 g of Fe(0) granules was added in it. The solution was then put on an automatic shaker and evolved Hg gas (confirmed from isotopic distribution of mass spectral data), generated inside the EPA vials, was measured using a GC mass spectrometer (Fig. 5).
The Hg evolution was very fast. Within 16 min, the saturated vapor pressure of Hg (2.031558 × 10 −7 MPa at 295.15 K) 26 was reached (with mass ~5000 pg in the flask).
The rate of Hg evolution is directly proportional to the mass of employed Fe granules. Saturated vapor pressure was obtained after 20 min, 23 min and 31 min, when the employed Fe granules were 0.05 g, 0.03 g, 0.01 g, respectively. 0.07 g of TEFe as well as different commercially available iron oxides (hematite, magnetite, maghemite) could not produce elemental Hg in 16 minutes under similar experimental conditions, unlike the Fe granules ( Fig. 5 (A,B)).
MtFe adsorbed substantial amount of Hg, during the evolution of Hg from MtFe-Hg, by Fe granules. Adsorption of elemental Hg by iron oxide is well known 20 . We have used large Fe granules (~0.01 g each) for easy separation from the reaction mixture to recycle the Fe granules.
As the surface of the Fe(0) granules usually contain iron oxides due to aerial oxidation 27 , Fe granules (that adsorbed elemental Hg) were kept overnight at 40 °C in a vacuum oven after being washed with milli-Q water to remove adsorbed Hg before performing cycle 2, 3 and 4 to eliminate any confusion.
The rate of removal of elemental Hg was almost unchanged for different cycles with the Fe granules, qualifying the criteria of the cheapest Hg recycling agent (Fig. 5(C)). Moreover, pH did not significantly influence the elemental Hg evolution. Saturated vapour pressure of Hg was obtained at 16 min, 14 min, 16 min, 16 min and 13 min, when the pH of the solution was 2, 4, 6, 8 and 10, respectively (Fig. 5(D)).
XRD spectra depicted that Hg(II) was converted to Hg(I) in MtFe-Hg. Such conversion actually helped for fast recycling of elemental mercury in our case. We treated Fe granules with HgCl 2 and Hg 2 Cl 2 separately and found that Hg 2 Cl 2 could generate ~5000 pg elemental Hg vapor (a fixed value was reached in mass spectrometer due to the saturation of elemental Hg in the container) within 16 min (Supplementary Fig. S12), unlike HgCl 2 .
Formal reduction potential 28 allows iron(0) to be oxidized to iron(III), reducing Hg(I) to elemental mercury. The control experiment of Hg 2 Cl 2 explains the mechanism of Hg(0) production via MtFe with Fe granules. Importantly, the Fe in MtFe could not generate elemental Hg due to passivation of Fe with iron oxides and clay material, such as montmorillonite, under our experimental conditions. Advantage of the boron compounds in the filtrate during synthesis of MtFe. During the reduction by NaBH 4 , NaBO 2 is usually produced 29 . The filtrate during the synthesis of MtFe is not a useless by-product. We can also use such boron compounds for the confirmation of the alcohol in water.
Boric acid ester, which forms between the reaction of boron compounds and alcohol, burn with a green flame. It indicates a potential application of boron compounds in flame photometric study and pyrotechnics for both military and civilian fireworks [30][31][32] .
An instantaneous and efficient nano-trap technique has been designed to remove Hg(II) from water and recovery of elemental mercury by Fe granules by simple redox reactions (Fig. 6). The nano-trap and Fe granules are recyclable. Our protocols are applicable to different experimental conditions (pHs, temperatures etc.) indicating the versatility of this technique. Artisanal and small-scale mining, including gold, leads to a sever mercury contamination. The developed model nano-trap impregnated sintered funnel, and filter paper for water decontamination may be a valuable kit for small-scale use.
As Fe/iron oxides/montmorillonite are natural materials, and major components of atmospheric mineral dust, our Hg recycling protocol can also be used for waste management of this toxic compound.

Methods
Synthesis. 0.026 moles of FeCl 2 .4H 2 O were fully dissolved in a mixture of 48 mL ethanol and 12 mL water.
To it, 3 g of montmorillonite was added and sonicated for 2 h to obtain a muddy brown mixture (X). 1M NaBH 4 solution was prepared by dissolving 6.1 g of solid in 200 mL of ice-cold milli-Q water. The newly prepared NaBH 4 solution was added drop-wise into X, showing an instant change of color from brown to black.
After the addition of the entire NaBH 4 solution, the reaction mixture was shaken for 10 min and aged for ~7 h. The supernate was subsequently removed and the solid black precipitate was washed thoroughly with milli-Q water once and twice with 200 mL of ethanol at the end. The synthesized iron-montmorillonite composite was aged at 60 °C to dry completely. The dry black solid was crushed with a mortar to obtain fine powder MtFe.
In the X solution, Whatman 1 filter paper was dipped during sonication and then dried and dipped in NaBH 4 solution and finally dried to obtain black MtFe impregnated filter paper. Water suspension of MtFe was filtered by glass sintered funnel to obtain MtFe impregnated funnel.

Analytical Techniques for Physical and Chemical Characterization. UV-Vis spectroscopy. All UV-
Vis absorption spectra were recorded from a VARIAN CARY 50 Bio UV spectrophotometer. The solutions were put into a 1 cm, well-stoppered quartz cuvette to record after base-line corrections with milli-Q water. Absorption spectra were measured after dilution of supernate for 31 times of each sample. where C 0 and C e are the initial and equilibrium concentrations of Hg 2+ in a solution, respectively 16 . C 0 and C e can be replaced by A 0 (initial absorbance) and A e (equilibrium absorbance), when removal was measured via UV-visible spectroscopy.
High resolution Transmission Electron Microscopy (TEM). Droplets of 10 μL of MtFe (0.0025 g in 10 mL of milli-Q water), MtFe-Hg (0.0025 g MtFe in 1 mL of 10 −2 M HgCl 2 ) and Fe-MtFe-Hg (0.0025 g MtFe in 1 mL of 10 −2 M HgCl 2 and 0.05 g Fe granules) were deposited on glow-discharged carbon film coated copper electron microscopy grids. The droplets were kept on the grids for 1 day to dry on it. Then the samples were imaged using a FEI Tecnai 12 Biotwin TEM microscope (FEI Electron Optics) equipped with a tungsten filament at 120 kV, containing an AMT XR80C CCD Camera System.