Porous Amorphous Chalcogenides as Selective Adsorbents for Heavy

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Porous Amorphous Chalcogenides as Selective Adsorbents for Heavy Metals Zohreh Hassanzadeh Fard, Saiful M. Islam, and Mercouri G. Kanatzidis* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States S Supporting Information *

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 10, 2015 | http://pubs.acs.org Publication Date (Web): September 9, 2015 | doi: 10.1021/acs.chemmater.5b02805

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stability and are stable in air and water. Ion-exchange ability was not investigated for compound 3 because of air and moisture sensitivity. We synthesized the porous amorphous chalcogenides of 1 and 2 by adding inert soluble inorganic salts into the presynthesized compounds, flame melting/water quenching of the mixture and finally liquid extraction of the salt (Figure 1).24

morphous chalcogenides are of great interest for various optical and photonic applications because of their transparency in the second atmospheric window from 8 to 14 μm.1−7 They are being studied mostly for application as passive devices such as lenses, windows, and fibers.8 Chalcogenide glasses that are doped with ions of rare-earth elements also enable active devices such as laser fiber amplifiers and nonlinear optical components.9−12 Here we introduce another attractive functionality, namely as robust inorganic heavy metal adsorbents. We describe three chalcogenides with the nominal compositions of K2 SnSb2 S 6 (1), Cs 2 SnSb 2S 6 (2), and K2SnSb2Se6 (3) featuring amorphous structures. As we have discussed previously, alkali-metal carbonates and sulfur can provide an in situ, useful polysulfide flux.13 A general equation for this reaction in which alkali metal polysulfides (A2Sx) are produced along with A2S2O3 and CO2 byproducts is given in eq 1.14,15 The title compounds can be synthesized according to the examples eqs 2 and 3.16,17 3A 2 CO3 + (2x + 2)S → 2A 2Sx + A 2S2O3 + 3CO2

Figure 1. Schematic for the preparation steps leading to amorphous porous chalcogenides.

(1)

It is important that a homogeneous liquid be achieved during melting. As the temperature drops upon quenching solidifying the system, the two phases segregate via a phase separation process. UV−vis/near-IR (NIR) spectroscopy indicates a room temperature band gap of 2.0 eV for 1, 1.8 eV for 2, and 1.4 eV for 3, which are in accordance with their colors (Figure 2, top). The powder X-ray diffraction (PXRD) patterns of all

3A 2 CO3 + 14S + 2Sn + 4Sb → 2A 2 SnSb2S6 + A 2S2O3 + 3CO2

(A = K, 1; Cs, 2)

(2)

A 2Q + 5Q + Sn + 2Sb → A 2SnSb2 Q 6 (A = K, 1 and 3; Cs, 2; Q = S, 1 and 2; Se, 3)

(3)

The K2S2O3 byproduct (detected in powder X-ray diffraction pattern of the products) can then be washed away by soaking the product in water. The scanning electron microscopy (SEM) image of the product after soaking in water showed macroporosity (Figure S1). This observation and the following three reasons motivated us to make porous glasses of 1 and 2 and test them as selective heavy metal ions adsorbents. First, sulfide based materials have very high affinity for soft Lewis acid ions, as we have shown in previous reports for crystalline A2xMxSn3−xS6 (A = alkali metal; M = Mn, Mg; x = 0.5−0.95; KMS-1 and KMS-2, respectively).18−22 In principle, chalcogenides are superior as heavy metal ion absorbents over any other classes of materials. Second, the glasses we report here can be made in almost any user defined shape and texture because they are melt processable. This property is attractive in ion-exchange column applications where the use of small submicron particle sizes is inappropriate. For example, particles that are too small can often pass through filters (in batch method ion-exchange processes) thus hindering remediation efforts by allowing the solids into the effluent. Furthermore, small particles can cause clogging of columns.23 Third, 1 and 2 show high thermal © XXXX American Chemical Society

Figure 2. UV/vis/NIR absorption spectra and PXRD patterns for (1) K2SnSb2S6 (A, B), (2) Cs2SnSb2S6 (C, D), and (3) K2SnSb2Se6 (E, F), respectively. Received: July 21, 2015 Revised: September 3, 2015

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DOI: 10.1021/acs.chemmater.5b02805 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials three compounds confirmed their amorphous nature (Figure 2, bottom). The composition and chemical stability of A2SnSb2S6 (A = K and Cs) were confirmed by energy dispersive microprobe spectroscopy (EDS) (Figure S2) and X-ray photoelectron spectroscopy (XPS) (Figures S3 and S4). Thermal properties of the materials were studied with differential thermal analysis (DTA). The thermograms (Figure S5) show that compounds 1 and 2 are not phase-change materials (i.e., they do not crystallize) and preserve their amorphous structures upon heating. Compound 3 crystallizes at 305 °C to give K4Sn3Se8 as determined with PXRD (Figure S6). The amorphous K2SnSb2S6 and Cs2SnSb2S6 phases present an intimate combination of strong covalent bonding (i.e., Sb/ Sn−S bonds) and weak ionic K···S bonding in the structure. Presumably, this mixed bonding represents a type of balance in the structure that frustrates the system’s ability to crystallize when cooled rapidly from the melt. Raman spectra of both phases show a characteristic peak at ∼327 cm−1 that can be attributed to vibrations involving only the [SnSb2S 6 ]2− framework (Figure 3).

Figure 3. Raman scattering of K2SnSb2S6 and Cs2SnSb2S6 at room temperature.

The porous samples can form using various inert but soluble inorganic salts such as NaCl, NaI, KCl, and KI with different volume ratio to the chalcogenides. The critical condition for this process is to obtain a homogeneous melt of the chalcogenides and inorganic halide salts followed by rapid cooling to solidification. The latter creates rapid phase separation because the inorganic salts are immiscible in the solid state with the chalcogenides thus forming an interpenetrated 2-phase composite. Best results in terms of porosity were obtained for mixtures of Cs2SnSb2S6+KI (1:5 vol %) and for K2SnSb2S6+NaI (1:2 vol %). This procedure led to the formation of Cs2−xKxSnSb2S6 and Na2−xKxSnSb2S6 phases, respectively, which were confirmed by EDS (Figure S7) and XPS (Figures S8 and S9). The Sb 3d peaks (∼529, 538 eV) and Sn 3d peaks (∼486, 494 eV) in XPS confirm the Sb3+ and Sn4+ oxidation states both in the amorphous and their porous versions.25,26 SEM images show that the porous chalcogenides exhibit pores with a diameter ranging from several hundred nanometers to a few microns. The pore wall thicknesses vary from ∼100 to 300 nm (Figure 4). However, N2 sorption isotherms at 77K show mesoporosity as well, with a wide pore size distribution between 2 and 18 nm. Both porous phases show a type 2 isotherm (with a hysteresis loop in the case of Na2−xKxSnSb2S6) with a maximum uptake of 102.1 cm3/g for Cs2−xKxSnSb2S6 and 21.8 cm3/g for Na2−xKxSnSb2S6. The associated BET surface areas are 49 m2/g for Cs2−xKxSnSb2S6 and 18 m2/g for Na2−xKxSnSb2S6.

Figure 4. Nitrogen adsorption/desorption isotherms and SEM images for porous Cs2−xKxSnSb2S6 (A, C), and porous Na2−xKxSnSb2S6 (B, D), respectively.

It is remarkable that the porous version of 1 is stable in water with pH ranging from 0 to 12.27 At low pH the materials undergo proton exchange to form (H3O)+ derivatives which can be converted to H2SnSb2S6 upon heating to remove the water, eqs 4 and 5) (Figure S10). The S−H stretching vibration is then observed at 2430 cm−1 (Figure S10.28−30 Consistent with the partial substitution of the alkali ions by hydronium ions, (H3O)+ EDS analysis of the materials confirms that in an acidic solution, the fraction of the alkali metals is significantly reduced compared to the pristine porous chalcogenides. The ion-exchange causes a red shift in the electronic absorption edge of the material (Figure S11), in a similar fashion observed previously in the K2MnSn2S629 and (NH4)4In12Se2030 materials. Na 2 − xK xSnSb2 S6 + 2(H3O)+ → (H3O)2 SnSb2S6 + (2 − x)Na + + x K+

(4)

(H3O)2 SnSb2S6 + heat → H 2SnSb2 S6 + 2H 2O

(5)

To evaluate the ability of the porous versions of the amorphous chalcogenides, 1 and 2, to remove heavy metal ions B


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Chemistry of Materials from aqueous solutions, we performed ion-exchange experiments using the batch method.31 The initial and final concentrations of the metal ions were determined by inductively coupled plasma-mass spectrometry (ICP-MS), which is capable of identifying elements at ppt−ppb levels. At a molar ratio of M2+:Na2−xKxSnSb2S6 1:1, ∼73% of Pb2+, ∼84% of Cd2+, and 99.99% of Hg2+ ions were removed with enormous distribution coefficient values, Kd for Hg2+ is 7.2 × 108, (for definition, see and refs 31 and 32). Cs2−xKxSnSb2S6 showed higher percent removal for Pb2+ (∼84%), Cd2+ (∼90%), and the same 99.99% of Hg2+ removal with high distribution coefficient values, Kd (6.8 × 107). Selected results are presented in Table 1. More experimental details are provided in the

Figure 5. Kinetics for adsorbing Hg2+ from an aqueous solutions by porous glass of 1.


Table 1. Selected Data for Pb2+, Hg2+, Cd2+ Ion-Exchange Experiments Using Porous Glasses (PGs) of 1 and 2 PGs

V/m (mL/g)

[conc.]o (ppm)

[conc.]f (ppm)

% removal

Kd (mL/g)

1

952.4(Pb)

641.4(Pb)

175.4(Pb)

72.6(Pb)

943.4(Hg)

541.0(Hg)

0.000(Hg)

99.9(Hg)

961.5(Cd)

329.5(Cd)

53.5(Cd)

83.8(Cd)

892.9(Pb)

641.7(Pb)

102.5(Pb)

84.0(Pb)

884.9(Hg)

541.0(Hg)

0.007(Hg)

99.9(Hg)

884.9(Cd)

342.5(Cd)

34.3(Cd)

89.9(Cd)

2.5 × 103 (Pb) 7.2 × 108 (Hg) 4.9 × 103 (Cd) 4.7 × 103 (Pb) 6.8 × 107 (Hg) 7.9 × 103 (Cd)

2

a

that these materials are highly promising as heavy metal adsorbents.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b02805. Experimental details and spectroscopic data (PDF).

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AUTHOR INFORMATION

Corresponding Author

*M. G. Kanatzidis. E-mail: [email protected].

b

Reaction time: 15 h; room temperature. Samples were prepared in triplicate and the average is reported here.

Funding

We thank the National Science Foundation for funding this work via grant DMR-1410169.

Supporting Information. The Kd values of Hg2+ removal by porous Na2−xKxSnSb2S6 and Cs2−xKxSnSb2S6 are much higher than the previously reported values for crystalline A2xMxSn3−xS6 (A = alkali metal; M = Mn, Mg; x = 0.5−0.95; KMS-1, and KMS-2, respectively).18,23 The best Kd values observed for KMS-1 and KMS-2 were 3.5 × 104 and 5.35 × 104, respectively. To understand better the ion exchange behavior of porous Na2−xKxSnSb2S6, we investigated the kinetics of the Hg2+ ionexchange using the batch method (pH = 7, V/m ∼ 1000 mL/ g). The maximum initial concentrations of the ions used were just high enough to saturate the exchanged sites of Na2−xKxSnSb2S6. Within ∼2 h of solution/Na2−xKxSnSb2S6 contact, Hg2+ exchange reached its equilibrium with more than 99.5% of its initial amounts removed from solution (Figure 5). The high affinity of the soft Lewis basic framework for soft Lewis acids is the driving force for the fast Hg2+ removal. In conclusion, the amorphous chalcogenides K2xSnxSb3−xS6, Cs2xSnxSb3−xS6, and K2xSnxSb3−xSe6 (x = 0.8−1; 1, 2, 3, respectively) are stable and have no crystalline counterparts. The previously reported crystalline A2Sb2Sn3S10 (A = K, Rb, Cs) have not been observed in this work.33 However, it is possible that the two families of materials share structural features. By flame melting mixtures of presynthesized amorphous sulfides of 1 and 2 and soluble inorganic salts, followed by water quenching and exsolution of the salts, porous materials can be prepared which exhibit porosity at all length scales including microporosity. These materials are effective in the selective adsorption of Pb2+, Cd2+, and Hg2+ metal ions from their solutions. The strong tendency of S atoms to bind with heavy metal ions acts as a driving force.34,35 We postulate

Notes

The authors declare no competing financial interest.

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REFERENCES

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Chemistry of Materials (11) Lezal, D.; Pedlikova, J.; Zavadila, J. Chalcogenide glasses for optical and photonics applications. Chalcogenide Lett. 2004, 1, 11−15. (12) Bureau, B.; Zhang, X. H.; Smektala, F.; Adam, J. L.; Troles, J.; Ma, H. L.; Boussard-Plèdel, C.; Lucas, J.; Lucas, P.; Le Coq, D.; Riley, M. R.; Simmons, J. H. Recent advances in chalcogenide glasses. J. NonCryst. Solids 2004, 345&346, 276−283. (13) Hassanzadeh Fard, Z.; Kanatzidis, M. G. Phase-Change Materials Exhibiting Tristability: Interconverting Forms of Crystalline α-, β-, and Glassy K2ZnSn3S8. Inorg. Chem. 2012, 51, 7963−7965. (14) Kanatzidis, M. G. Molten alkali-metal polychalcogenides as reagents and solvents for the synthesis of new chalcogenide materials. Chem. Mater. 1990, 2, 353−363. (15) Gobeltz, N.; Demortier, A.; Lelieur, J. P. Identification of the Products of the Reaction between Sulfur and Sodium Carbonate. Inorg. Chem. 1998, 37, 136−138. (16) Compound 1 was synthesized using three different methods: method (a) K2CO3 (0.03 mol, 4.146 g), Sn (0.03 mol, 3.561 g), Sb (0.06 mol, 7.306 g), and S (0.19 mol, 6.092 g) were combined and loaded in a 50 mL grinding jar under nitrogen atmosphere in a glovebox. The mixture was ball-milled at 100 rpm for 1 min and at 250 rpm for 30 minutes. 3 g of the ball-milled mixture was placed in a 13 mm outer diameter (OD) carbon coated fused-silica tube under N2 atmosphere. A rubber balloon was attached at the end of the reaction tube in order to accommodate the created pressure of the CO2 evolution. The mixture was heated gradually to 200 °C where it was kept for 5 h before being successfully brought to 800 °C. It was kept at 800 °C for 8 h. A red glassy ingot was obtained by cooling at a rate of 40 °C/h to room temperature; method. (b) A mixture of K2S (4 mmol, 0.4410 g), S (8 mmol, 0.2565 g), Sn (4 mmol, 0.4570 g), and Sb2S3 (4 mmol, 1.3588 g) was sealed under vacuum (10−4 Torr) in a 13 mm (OD) carbon coated fused-silica tube and heated (80 °C/h) to 800 °C. It was kept there for 24 h, followed by cooling to room temperature at 40 °C/h; method. (c) Compound 1 was also synthesized by combining in a nitrogen-filled glovebox K2S (2 mmol, 0.2205 g), S (10 mmol, 0.3206 g), Sn (2 mmol, 0.2347 g), and Sb (4 mmol, 0.4870 g) with the same type of silica reaction vessel flamesealed under vacuum, and same temperature profile as method b. The empirical formula for the products obtained from three methods was K2SnSb2S6 based on EDS analyses. The product is a red ingot. (17) Compound 2 was synthesized with two different methods: method (a) using Cs2S (1 mmol, 0.2205 g), S (5 mmol, 0.1620 g), Sn (1 mmol, 0.1187 g), and Sb (2 mmol, 0.2435 g) in accordance to the conditions and heating profile for the synthesis compound 1; method. (b) Cs2CO3 (0.03 mol, 9.775 g), Sn (0.03 mol, 3.561 g), Sb (0.06 mol, 7.306 g), and S (0.19 mol, 6.092 g) were combined and loaded in a 50 mL grinding jar under nitrogen atmosphere in a glove box. The mixture was ball-milled at 100 rpm for 1 min and at 250 rpm for 30 minutes. 3 g of the ball-milled mixture was placed in a 13 mm (OD) carbon coated fused-silica tube under N2 atmosphere. A secured balloon was attached at the end of the reaction tube in order to absorb the created pressure of the CO2 evolution. The mixture was heated gradually to 200 °C where it was kept for 5 h before being raised to 800 °C and kept for 8 h. An orange glassy ingot was obtained by cooling at a rate of 80 °C/h to room temperature. For the synthesis of compound 3, a mixture of K2Se (2 mmol, 0.3143 g), Se (10 mmol, 0.7896 g), Sn (2 mmol, 0.2347 g), and Sb (4 mmol, 0.4870 g) was sealed under vacuum (10−4 Torr) in a 13 mm (OD) carbon coated fused-silica tube and heated (80 °C/h) to 800 °C for 24 h, followed by cooling to room temperature at 40 °C/h. The product is a dark redblack ingot. (18) Manos, M. J.; Kanatzidis, M. G. Sequestration of Heavy Metals from Water with Layered Metal Sulfides. Chem. - Eur. J. 2009, 15, 4779−4784. (19) Manos, M. J.; Kanatzidis, M. G. Highly Efficient and Rapid Cs+ Uptake by the Layered Metal Sulfide K2xMnxSn3−xS6 (KMS-1). J. Am. Chem. Soc. 2009, 131, 6599−6607. (20) Manos, M. J.; Ding, N.; Kanatzidis, M. G. Layered metal sulfides: Exceptionally selective agents for radioactive strontium removal. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 3696−3699.

(21) Mertz, J. L.; Hassanzadeh Fard, Z.; Malliakas, C. D.; Manos, M. J.; Kanatzidis, M. G. Selective Removal of Cs+, Sr2+, and Ni2+ by K2xMgxSn3−xS6 (x = 0.5−1) (KMS-2) Relevant to Nuclear Waste Remediation. Chem. Mater. 2013, 25, 2116−2127. (22) Hassanzadeh Fard, Z.; Malliakas, C. D.; Mertz, J. L.; Kanatzidis, M. G. Direct Extraction of Ag+ and Hg2+ from Cyanide Complexes and Mode of Binding by the Layered K2MgSn2S6 (KMS-2). Chem. Mater. 2015, 27, 1925−1928. (23) Mertz, J. L. Synthesis and ion-exchange of framework and layered chalcogenide compounds for environmental remediation. Ph.D. thesis, Northwestern University, Evanston, IL, 2012. (24) General procedure: presynthesized A2xSnxSb3‑xS6 system (A = K, Cs) and soluble inorganic salts (e.g., NaI, KI etc, at different volume fractions) were mixed inside a silica tube and flame melted under flow of nitrogen. When a congruent melt is observed it is then quenched in room temperature water. The product was then soaked in water and then in ethanol for 24 h to completely dissolve the halide salts. The product was then washed with water and acetone and dried under vacuum overnight. EDS analyses were used to confirm the total removal of the halide salts (data provided in theSupporting Information, Figure S7). (25) Vasquez, R. P.; Grunthaner, J. Chemical composition of the SiO2/InSb interface as determined by X-ray photoelectron spectroscopy. J. Appl. Phys. 1981, 52, 3509−3514. (26) Morgan, W. E.; Van Wazer, J. E. Binding energy shifts in the xray photoelectron spectra of a series of related Group IVa compounds. J. Phys. Chem. 1973, 77, 964−969. (27) The required pHs (2, 4, 6, 8, 10, and 12) were achieved by diluting the commercial standards (1000 ppm) with HCl or NaOH solution to 6 ppm. (28) Patai, S. The chemistry of the thiol group; Wiley: London, 1974. (29) Manos, M. J.; Petkov, V. G.; Kanatzidis, M. G. H2xMnxSn3‑xS6 (x = 0.11−0.25): A Novel Reusable Sorbent for Highly Specific Mercury Capture Under Extreme pH Conditions. Adv. Funct. Mater. 2009, 19, 1087−1092. (30) Manos, M. J.; Malliakas, C. D.; Kanatzidis, M. G. Heavy-MetalIon Capture, Ion-Exchange, and Exceptional Acid Stability of the Open-Framework Chalcogenide (NH4)4In12Se20. Chem. - Eur. J. 2007, 13, 51−58. (31) A typical ion exchange experiment of porous glass with Pb2+, Hg2+, or Cd2+ is as follows: In a 20 mL scintillation vial, measured amounts of appropriate salts (0.1 mmol) were dissolved in water (10 mL) and the glass compound (0.1 mmol) was added. The mixture was kept under magnetic stirring for 12−15 h. Then, the glass material was centrifuged and isolated by filtration, washed several times with water and acetone, and dried under vacuum. The initial and final concentrations of the metal ions in solutions were determined by ICP-MS. The distribution coefficient, Kd, used for the determination of the affinity and selectivity of porous amorphous chalcogenides for Pb2+, Hg2+, and Cd2+ is given by the equation: Kd = (V/m)[(C0 − Cf)/ Cf] where C0 and Cf are the initial and equilibrium concentration of a given ion (ppm), V is the volume (mL) of the testing solution, and m is the amount of the ion exchanger (g) used in the experiment (32) Fryxel, G. A.; Lin, Y.; Fiskum, S.; Birnbaum, J. C.; Wu, H. Actinide Sequestration Using Self-Assembled Monolayers on Mesoporous Supports. Environ. Sci. Technol. 2005, 39, 1324−1331. (33) Yohannan, J. P.; Vidyasagar, K. Syntheses and characterization of one-dimensional alkali metal antimony(III) thiostannates(IV), A2Sb2Sn3S10 (A = K, Rb, Cs). J. Solid State Chem. 2015, 221, 426−432. (34) Ding, N.; Kanatzidis, M. G. Permeable Layers with Large Windows in [(CH3CH2CH2)2NH2]5In5Sb6S19·1.45 H2O: High IonExchange Capacity, Size Discrimination, and Selectivity for Cs Ions. Chem. Mater. 2007, 19, 3867−3869. (35) Ding, N.; Kanatzidis, M. G. Selective Incarceration of Caesium Ions by Venus Flytrap Action of a Flexible Framework Sulfide. Nat. Chem. 2010, 2, 187−191.

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Porous Amorphous Chalcogenides as Selective Adsorbents for Heavy

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