Efficient Recovery of Elemental Mercury from Hg(II)-Contaminated Aqueous Media Using a Redox-Recyclable Ion-Exchange Material

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Abstract

The use of lithium-intercalated transition metal dichalcogenides, LixES2, as redox-recyclable ion-exchange materials for the extraction of the aqueous heavy metal ions Hg2+, Pb2+, Cd2+, and Zn2+ was investigated (0.25 ≤ x ≤ 1.9; E = Mo, W, Ti, Ta). For LixTiS2 and LixTaS2, 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 Pb0.15MoS2 were isolated. The selectivity of LixMoS2 for the heavy metal ions was Hg2+ > Pb2+ > Cd2+ > Zn2+. The affinities for the latter three ions but not for Hg2+ 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 × 10-4. Samples of MoS2 produced by heating HgyMoS2 were reactivated to LixMoS2 by treatment with n-butyllithium. Some samples were used for three cycles of extraction, deactivation/recovery, and reactivation 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. The highest observed capacity of LixMoS2 for Hg2+(aq) was 580 mg of mercury/g of Li1.9MoS2.

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