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

Abstract

The use of lithium-intercalated transition metal dichalcogenides, Li x ES 2, as redox-recyclable ion-exchange materials for the extraction of the aqueous heavy metal ions Hg 2+, Pb 2+, Cd 2+, and Zn 2+ was investigated (0.25 ≤ x ≤ 1.9; E = Mo, W, Ti, Ta). For Li x TiS 2 and Li x TaS 2, hydrolysis produced S 2 - (aq) ions, which precipitated Hg(II) as HgS(s). In contrast, the materials Li x MoS 2 and Li x WS 2 did not undergo hydrolysis to form S 2 - ions. Instead, ion-exchanged materials such as Hg 0.50 MoS 2 and Pb 0.15 MoS 2 were isolated. The selectivity of Li x MoS 2 for the heavy metal ions was Hg 2+ > Pb 2+ > Cd 2+ > Zn 2+ . The affinities for the latter three ions but not for Hg 2+ increased when the extractions were performed under anaerobic conditions. When Hg y MoS 2 was heated under vacuum at 425 °C, an entropy-driven internal redox reaction resulted in deactivation of the extractant, producing essentially mercury-free MoS 2 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 Hg 2+ (aq) (primary waste) was 1.5 × 10 - 4 . Samples of MoS 2 produced by heating Hg y MoS 2 were reactivated to Li x MoS 2 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 Hg 2+ (aq) in 0.1 M HNO 3 with no loss in ion-exchange capacity. When the Mo/Hg molar ratio was 5.0 and the initial [Hg 2+ (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 Li x MoS 2 for Hg 2+ (aq) was 580 mg of mercury/g of Li 1.9 MoS 2 .