Alkali-metal-intercalated transition-metal chalcogenides as solid-state redox-recyclable extractants for heavy metal ions. Synthesis and characterization of group IVB metal bis(hydrogen monothiophosphate): heavy metal selective ion-exchange materials

Abstract

The use of lithium-intercalated transition-metal dichalcogenides, LixES2, as redox-recyclable ion-exchange materials for the extraction of the aqueous soft-heavy-metal ions Hg2+, Ag+, Au3+, Cu2+, Tl+, Pb2+, Cd2+, and Zn2+ from aqueous acidic solutions was investigated (0.80 ≤ x ≤ 1.9; E = Mo, W, Ti, Ta, Sn). For Li1.5TiS2 and Li1.2TaS2, and LixSnS2 hydrolysis produced S2-(aq) ions which precipitated Hg(II) as HgS(s). In contrast, the materials LixMoS2 and LixWS2 did not undergo hydrolysis to form S2- ions. Instead, ion-exchanged materials such as Hg0.50MoS2 and Pb0.16MoS2 were isolated. The selectivity of LixMoS2 for metal-ion removal was Hg2+ ≈ Ag+ > Cu2+ > Tl+ = Pb2+ > Cd2+ > Zn2+ ≈ Ni2+ > Co2+ ≈ Mn2+ ≈ Na+ ≈ Ba2+. This trend represents the first known example of heavy metal selective intercalation of MoS2. The affinities for the latter ions, but not for Hg2+or Ag+, increased when the extractions were performed under anaerobic conditions. When HgyMoS2 was heated under vacuum at 425°C, an entropy-driven internal redox reaction resulted in deactivation of the extractant, producing essentially mercury-free MoS2 and a near-quantitative amount of mercury vapor (collected in a cold trap). The ratio of the volume of metallic mercury (secondary waste) to the volume of 10.0 mM Hg2+(aq) (primary waste) was 1.5 x 10-4. This is the maximum allowable secondary volume reduction possible and represents a breakthrough in heavy metal remediation technology. Samples of MoS2 produced by heating HgyMoS2 were reactivated to LixMoS2 by treatment with n-butyllithium. Some samples were used for five complete cycles of extraction, deactivation/recovery. and reactivation under anaerobic conditions with a primary waste simulant consisting of 10 mM Hg2+(aq) in 0.1 M HNO3 with no loss in ion-exchange capacity. When the Mo/Hg molar ratio was 5.0 and the initial [Hg2+(aq)] = 1 mM, only 0.033(2) μM mercury (6.5 ppb) was detected in the filtrate after the extraction step. This level is very close. but slightly higher than the acceptable limit for mercury in drinking water. The highest observed capacity of LixMoS2 for Hg2+(aq) was 580 mg mercury per g Li1.9MoS2. In addition, the compound LixMoS2 was shown to be an effective extractant for the removal of mercury and silver ions from some complex waste simulants. Heat-treatment of other MxMoS2 (M = Ag+, Pb2+) compounds led to internal redox reactions to give the native metal and deintercalated MoS2. All of the MxMoS2 materials were characterized by powder X-ray diffraction (XRD). differential scanning calorimetry (DSC). thermalgravimetric/mass spectral (TG/MS). scanning electron microscopy (SEM), transmission electron microscopy (TEM), and elemental analyses. results of which will be discussed. In addition, the mercury- and silver-intercalated MoS2 compounds and their heat-treated analogs were characterized using X-ray absorption spectroscopic (XAS) techniques. According to the XAS analyses, both the mercury and silver guest species are present in an ionic form and in a sulfur-rich coordination environment. After heating the silver species was shown to be present in its elemental form. These studies suggest that MxMoS2 compounds undergo internal redox-reactions when heated. Molybdenum XAS analyses indicate that the host material exists as a mixture of both the IT- and 2H-phases of MoS2. The XAS results are used to rationalize some inconsistencies in the literature concerning the XAS of MxMoS2 compounds. The mechanism of soft heavy metal ion removal by Li1.3MoS2 was investigated through detailed mass balance experiments. According to these experiments, the metal ion removal appears to occur through an ion-exchange process. This mechanism had previously only been speculated upon in the literature. In the course of these experiments the compound Li0.10H0.72MoS2 was discovered and isolated. This material has been shown to effectively and selectively remove soft heavy metal ions from solution. The material accomplishes this without the production of hydrogen in the extraction step, which occurs when Li1.3MoS2 is used. Finally, both mass balance studies and Ag+-intercalation reactions indicate that Schöllhom's well-known IT-MoS2 is actually a proton-intercalated form of MoS2. This material was also shown to be an effective and selective extractant for the Ag+(aq) ion. The reaction between dissolved aqueous solutions of M4+ (M = Zr, Hf) and PO3S3- resulted in the precipitation of a white gel that could be dried to a powder. Elemental analysis results for the white polycrystalline product yielded a stoichiometry of H2M(PO3S)2. These new compounds were characterized by thermal analysis (DSC, TGMS), vibrational spectroscopy (FT-IR, FT-Raman), 31P MAS NMR spectroscopy, energy dispersive spectroscopy (EDS), and powder X-ray diffraction (XRD). Based on the characterization and the results of trialkylamine intercalation experiments it appears that the H2M(PO3S)2 compounds have a layered structure that is likely similar to that of the β phase of H2Zr(PO4)2. The interlayer spacing, determined by XRD, for both H2M(PO3S)2 compounds is ~9.4 Å. Characterization results suggest that the S atom of the PO3S3- group is preferentially pointed into the interlayer region of the compound and is protonated. One of the many potentially interesting properties of H2Zr(PO3S)2 its ion-exchange capacity and selectivity, was investigated. The compound H2Zr(PO3S)2 was demonstrated to be an effective and recyclable ion-exchange material for both Zn2+(aq) and Cd2+(aq) resulting in the new compounds, H0.2Cd0.9Zr(PO3S)2 and H0.50Zn0.75Zr(PO3S)2. The extraction of metal ions was monitored by XRD, vibrational spectroscopy, and elemental analysis. The compound H2Zr(PO3S)2 was shown to reversibly intercalate Zn2+(aq) ions through three complete cycles of intercalation and deintercalation without any loss of ion-exchange capacity. Mass balance experiments indicate that the removal of Cd2+(aq) and Zn2+(aq) ions by H2Zr(PO3S)2 is occurring by an ion-exchange process. The compound H2Zr(PO3S)2 has a higher capacity for the removal of the heavy metal ions Zn2+(aq) and Cd2+(aq) ions than the well-known α-H2Zr(PO4)2 ion-exchange material Presumably this is due to the unique soft Lewis basic character of the interlayer galleries of H2Zr(PO3S)2.